Air Drilling Manual
AirComp An Allis-Chalmers Energy Group
Percussion Drilling Manual “The Percussion Prof essionals” San Angelo, Texas
Farmington, New Mexico Wilburton, Ok lahoma
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Air Drilling Manual
Preface
This Percussion Drilling Manual is a guide to the oil and gas driller, supervisor, and/or engineer who is using or planning to use the down-the-hole, (DTH), air hammers and hammers bits. The objective of this manual is to give the reader some ideas, suggestions and Rules-of-Thumb that we at Marquis Bit and Diamond Air Drilling, a business unit of AirComp, have found helpful in planning and utilizing air hammer equipment and the bits that drill medium-hard to hard formations at rates of penetration, (50-150 feet per hour), that are normally reported for the softest formations. Although there are some formulas and theories described in this manual, the major focus is to give our customers basic understanding or air drilling techniques, suggested air volume requirements, air hammer designs, hammer bit designs, and other operational guidelines that we have found useful. Also found within this manual are sections that address trouble shooting, factors that affect Rate of Penetration and stabilization considerations for deviation control. The Company
equipment. As a unit of AirComp is the world’s 2nd largest provider of air compression equipment. the Allis-Chalmers Energy Group, we provide our customers with the widest range of equipment and and services in the industry. During the last year, Diamond Air Drilling, Drilling, Marquis Bit and Lone Star Air have become a part of the AirComp group. Operators have specified AirComp equipment and personnel on 90% of the geothermal wells and thousands of oil and gas wells in North America. With AirComp on location, you have the industry’s most experienced personnel with the highest capacity, highest pressure equipment on the market. The results consistently speak for themselves. For over ten (10) years , years , Diamond Air Drilling Services has been providing a quality air drilling service to the oil & gas drilling industry across the Western United States. Targeting the oil & gas industry throughout the Western United States, Diamond Air’s goal is to be the premier provider of quality downhole tools and hammer bit products. Our reputation has been built on performance and honesty that we provide to all of our valued customers. Providing quality tools and rig site personnel is the backbone of this business. Bit is to manufacture a “premium” hammer bit for all air The mission of Marquis Bit is hammer markets. Our goal is to be the leading leading company in hammer hammer bit technology and performance that clearly separates Marquis Bit from the competition. We believe believe that during our existence, we have exceeded expectations as proven by our performance. Marquis Bit is based in Carlsbad, New Mexico.
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Air Drilling Manual
Preface
This Percussion Drilling Manual is a guide to the oil and gas driller, supervisor, and/or engineer who is using or planning to use the down-the-hole, (DTH), air hammers and hammers bits. The objective of this manual is to give the reader some ideas, suggestions and Rules-of-Thumb that we at Marquis Bit and Diamond Air Drilling, a business unit of AirComp, have found helpful in planning and utilizing air hammer equipment and the bits that drill medium-hard to hard formations at rates of penetration, (50-150 feet per hour), that are normally reported for the softest formations. Although there are some formulas and theories described in this manual, the major focus is to give our customers basic understanding or air drilling techniques, suggested air volume requirements, air hammer designs, hammer bit designs, and other operational guidelines that we have found useful. Also found within this manual are sections that address trouble shooting, factors that affect Rate of Penetration and stabilization considerations for deviation control. The Company
equipment. As a unit of AirComp is the world’s 2nd largest provider of air compression equipment. the Allis-Chalmers Energy Group, we provide our customers with the widest range of equipment and and services in the industry. During the last year, Diamond Air Drilling, Drilling, Marquis Bit and Lone Star Air have become a part of the AirComp group. Operators have specified AirComp equipment and personnel on 90% of the geothermal wells and thousands of oil and gas wells in North America. With AirComp on location, you have the industry’s most experienced personnel with the highest capacity, highest pressure equipment on the market. The results consistently speak for themselves. For over ten (10) years , years , Diamond Air Drilling Services has been providing a quality air drilling service to the oil & gas drilling industry across the Western United States. Targeting the oil & gas industry throughout the Western United States, Diamond Air’s goal is to be the premier provider of quality downhole tools and hammer bit products. Our reputation has been built on performance and honesty that we provide to all of our valued customers. Providing quality tools and rig site personnel is the backbone of this business. Bit is to manufacture a “premium” hammer bit for all air The mission of Marquis Bit is hammer markets. Our goal is to be the leading leading company in hammer hammer bit technology and performance that clearly separates Marquis Bit from the competition. We believe believe that during our existence, we have exceeded expectations as proven by our performance. Marquis Bit is based in Carlsbad, New Mexico.
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Air Drilling Manual Air Drilling Techniques Introduction
Underbalanced Drilling (UBD) is a technique in which oil, gas or geothermal wells are drilled using pressures pressures lower than the reservoir pressure. The result is an an increase in rate of penetration (ROP), reduced formation formation damage and reduced drilling costs. Air drilling provides an efficient system in terms of operations costs and environmental safety benefits. “Air Drilling” refers to the use of air in the circulating system. The purpose for using an air drilling method is to drill low-pressure formations. During the last 20 years, airdrilling techniques have been applied worldwide, successfully drilling for energy. The five general classifications of air drilling techniques are as follows: 1. “Dust” Drilling 2. “Mist” Drilling 3. “Foam” Drilling 4. “Aerated Fluid” Drilling 5. “Nitrogen” Drilling Advantages
Air drilling techniques offer the following advantages, when compared to the use of conventional mud systems. • • • • • • • • • •
Faster Rates of Penetration. (Especially in harder formations) Improved Bit performance. (More footage per bit) Detection of low-pressure zones. Effective pressure control through loss circulation zones. Lower mud material costs. Fast return of uncontaminated cuttings for geological evaluation. Minimized formation damage. Improvements in Deviation Control. (Due to less weight on bit). Operating conditions are cleaner. Overall costs for drilling operations are lower.
Disadvantages •
•
•
Formation pressure control is minimal and, therefore, drilling is limited to geological regions where reservoir pore pressures are low. Drilling is limited to geological regions where the rock formations are mature and competent because there is little or no fluid pressure to support the borehole wall and prevent sloughing. There is limited ability to cope with significant volumes of water entering the annulus from water producing formations.
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Air Drilling Manual
•
•
•
•
The bit gage can be appreciably reduced during drilling (with exception when using Diamond Enhanced Inserts- D.E.I.) The drill pipe can experience rather high wear due to sand blasting characteristics of annular stream flow. There is little or no drill string cushion effect, which results from fluid in the borehole during drill string handling mishaps. There is danger of downhole fire.
This increase in R.O.P. may be 2-5 times the drilling rate of a conventional mud drilled hole. This can reduce the number of days required to complete a well, and reduce drilling costs. Rate of Penetration – (R.O.P.)
The downhole circulating density of an air drilling system is low, compared to a typical mud system. The decreased circulating fluid pressure exerted on the wellbore increases the relief of the vertical and axial stresses residual in the formations. This results in a “reverse pressure” gradient that increases the drill ability of the rock. As the down hole circulating fluid pressure is lowered below the formation pressure, the rock tends to “explode” at the bit tooth. This result in faster penetration rates provided there is sufficient circulating fluid volume to clear the hole of cuttings. Bit Performance
An air drilling system provides sufficient fluid turbulence to ensure proper cleaning of the cutting structure. Abrasive cuttings are carried away from the bit and into the annulus, faster than a conventional mud system. This lessens the regrinding of drilled cuttings, increasing the removal efficiency of the solids control equipment, and improving bit performance. (R.O.P.) Elevated formation temperatures are common when drilling a geothermal well. One of the main factors affecting the performance of a bit is bearing life. As high formation temperatures are encountered, bearing life can be decreased. An air drilling system supplies the bit with a cool stream of air that flows around the bearings, reducing the bearing temperature and increasing bit performance. Decreasing the bearing temperature and reducing the regrinding of drilled cuttings, increases the footage that can be drilled for a given bit. This can result in fewer bits required to complete a well, reducing well costs. Drilling through Loss Zones
Once “loss” or production zones are encountered, drilling may continue through and beyond these low-pressure formations. The operator may increase production from each well, by drilling deeper and encountering new production zones.
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Air Drilling Manual
The existing air circulating system may or may not have to be changed to maintain full circulation. A properly engineered air drilling system will permit a rapid conversion from one technique to another, without any excessive delay. Minimize Formation Damage
The use of air drilling techniques can minimize formation damage and enhance production from low-pressure wells, when compared to a conventional mud drilled well. If the circulating fluid pressure is less than the formation pressure, there is little chance that circulating fluids and cuttings will invade and damage producing zones. Some geothermal operators have indicated that wells completed with conventional drilling fluid systems have less geothermal production, when compared to wells dried with air drilling systems in the same area. These operators feel that low bottom hole circulating pressures decrease invasion and “Baking off in the high temperature fractures of drilling fluid and cuttings. “DUST” Drilling
Compressed air is injected into the standpipe and circulated through the drill string in much the same way as conventional mud. The “Dust” technique is used when drilling dry formations, or where any water influx is slight enough to be absorbed by the air stream. The name “Dust” was chosen because cuttings return to the surface as a cloud of dust. The drilling fluid is used to cool the drill string and bit. The temperature of the air injected into the hole should be slightly higher than the temperature at ambient conditions. As the air travels down the drill string the air is heated to the temperature of the surrounding formation. When the air passes through the jet nozzles, the air expands and the velocity increases to supersonic flow. This expansion occurs because of the large pressure drop between the bottom hole and the above bit pressures. This causes the temperature to decrease and cool the bit and the bit bearings. As the air travels up the annulus, the air is then reheated to the temperature of the surrounding formation. If lubrication is desired of the drill string and bit, a lubricant must be added to the air stream. There are several products that perform this function. The application of a lubricant decreases torque and increases bit life. If soap is used, there will be an increase in the carrying capacity of the circulating fluid. “Dust” drilling is the ultimate progression from a high to a low density drilling fluid. Bottom hole pressures slightly exceed the value of the air column pressure head plus the weight of the entrained cuttings. This allows for maximum relief of the vertical and axial stresses residual in the formations. This procedure offers the fastest drilling rates and best economy.
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Air Drilling Manual Hole Cleaning
The lifting power of an air drilling system is proportional to the circulating density, and to the square of the velocity. The density, and thus the suspension properties, of an air stream is much lower than a conventional mud system. Therefore, the annular velocity is the primary factor in transporting the cuttings to the surface. Air volumes that generate annular velocities of 3,000 ft/min. are normally adequate to “Dust” drill. However, when penetration rates exceed 60-ft/hr. or when cuttings are large or wet, higher annular velocities may be required to effectively clean the hole. “MIST” Drilling
This technique is used where the amount of water-influx is high enough to prevent “Dust” drilling, but not enough to cause hole-cleaning problems. The name “Mist” was chosen because a pretreated drilling fluid is injected with the air, and the combination returns to the surface as a mist. A small quantity of water containing a foaming agent (soap) is injected into the “air” stream at the surface, with the water mist being carried in the air in what is a continuous air system. This technique offers increased drilling rates and economy over that of a conventional mud drilled hole. The lower bottom hole circulating pressure exerted on the well bore allows for greater relief of the vertical and axial stresses residual in the formations. Like dry air drilling, this system relies on the annular velocity of the air for cuttings transport out of the hole. Air mist drilling is used when the amount of water influx is high enough to preclude air dust drilling, but not so high as to cause hole cleaning problems. Essentially, the equipment for a successful “dust” and “mist” drilling applications is the same. The principle difference being an increase in the air volume requirements 30%, and the injection of a pretreated drilling mud. Hole Cleaning
Switching to a “mist” drilling technique requires an increase of at least 30% in the air volume. The additional volume is needed to overcome higher frictional losses caused by wet cuttings adhering to the drillstring and hole, higher slip velocities of larger wet cuttings, and transportation of the heavier wet air column. The mud is injected with the air stream to disperse the cuttings and inhibit them from adhering to the drillstring and hole. Although injection pressure of 100 to 200 p.s.i.g. are normally enough for “dust” drilling, pressures exceeding 350 p.s.i.g. can be encountered while “mist” drilling. Pressures of 1250 p.s.i.g. may be requires when large amounts of fluids are present in the annulus. The rate of fluid intrusion will dictate the amount of air and fluid that must be injected to efficiently clean the hole. Formation fluid entries of up to 150 bbl/hr (100gpm) have been successfully “mist” drilled.
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Air Drilling Manual
The addition of a foaming agent reduces the interfacial tension of the water and cuttings in the hole and allows small water/cutting droplets to be dispersed as a fine mist in the returning air stream. This allows the cuttings and water to be removed from the hole without formation of mud rings and bit balling. Proper amounts of water and soap must be added to achieve a nominally continuous flow of foam and cuttings and adequate separation of the cuttings. Obtaining the proper combination of water and soap is a trial and error process. Good starting points are: 1. 1.5 – 2.0 bph water/per inch of hole diameter (8.75” bit = 14 – 18 bph /water) 2. 2 – 3 GPH soap – fresh water 3. 2 – 4 GPH soap – Brine Water The above totals are based on experience with the air hammer and may need to be adjusted according to hole conditions and bit selection. The requirements are a function of the type and volume of influx water. Many produced brines are effective defoamers, requiring use of additional soap. Produced oil requires a special type of soap. To determine the proper amount of water and soap to be injected, several “rules of thumb” are helpful: 1. Air volumes for mist must increase by 30% as compared to dust drilling. 2. Pressures generally run at 300 - 600 psi for mist. 3. Insufficient air/soap leads to hole surging. Corrosion Control
The fluid properties required for “Mist” drilling are lower than a conventional mud system. Chemical treatment is needed to minimize corrosion caused by the additional fluid and air. Basic corrosion control is provided by maintaining the pH of the mud system above 10.5, and treating any hardness or carbonates with the appropriate chemical. Hydrogen sulfide and carbonate scale are treated in much the same way as in a conventional mud system, To reduce carbonate corrosion, lime is used to treat out the carbonates, and some excess is maintained to buffer against this type of corrosion. Oxygen corrosion is the most difficult to combat in an air drilling system, because of the air supplies large quantities of oxygen to the wet circulating system. There are several types of chemicals that can be used to minimize this type of corrosion. Scale is a common problem with some type of fluids. Using an alkaline fluid, and treating the carbonates and hardness with the appropriate chemicals will greatly reduce
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Air Drilling Manual
the tendency of scale to occur. If the fluid circulating system is correctly pretreated, corrosion can be maintained at an acceptable level. Stiff Foam Drilling
The removal of drilled cuttings from low-pressure formations is a serious problem, particularly when permeability is high and the rocks are unconsolidated. When conventional methods are used, solids can be forced back into the formation. Since the reservoir pressure is extremely low, these contaminants stay in place and restrict permeability with a subsequent reduction in productivity. A low density circulating fluid minimizes contamination of the producing zoned by mud materials and cuttings. Decrease circulating pressures allow the drilling of severe loss zones with minimum fluid loss. A proven method for lowering the hydrostatic head is to use the “stiff foam” drilling technique. The “stiff foam” technique is a stable air in mud emulsion. Stiff foam is a mixture of water, foamer, appropriate mud additives, and compressed air. The foam is generated at the surface and injected in the drillstring as the circulating fluid. This technique can be used when “dust” or “mist” drilling techniques would not be practical because of economic, mechanical, or other reasons. When to use Stiff Foam Drilling
1. To drill severe loss circulation zoned, particularly those with wet or weeping formations. 2. To drill water sensitive shales which tend to slough when mist drilled. 3. To drill unconsolidated formations or producing zones. 4. To cope with situations where not enough air is available for large diameter holes, remoteness of location, or economics. Advantages
1. Low hydrostatic pressure is exerted against the wellbore which limits the invasion of drilling fluids and cuttings thereby minimizing formation damage. 2. Full circulation can be obtained in holes where it has been impossible to maintain returns with conventional fluids. 3. Analyzing the foam returns at the blooie line can readily identify oil and water zones. 4. Lower annular velocities by approximately 100 to 300 ft/min. permit the drilling of unconsolidated formations without the problems associated with hole washout. 5. Lower fluid volumes enhance the ability to drill in gauge, large diameter holes. The mud products are used to provide hole stability, carrying capacity, foaming characteristics, and corrosion control. They also decrease the tendency of the air to
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Air Drilling Manual
“break out” of the foam in the annulus. The air is used to reduce the down hole circulating pressure. Properly mixed foam weighs 2 to 4 lbs/cu. ft. When the foam is compressed down hole it exerts minimum hydrostatic pressure with little or no fluid loss to the formation. Foam has excellent carrying capacity for cuttings, about 7 – 8 times that of water. The foam mixture cannot be recirculated. It must be transferred to a sump, dispersed into liquid form, and then hauled off to a dumpsite. Environmental concerns may result when large amounts of foam are present in the surface sumps. “Stiff Foam” drilling does not work well when large formation flows are encountered. As the formation fluid enters the wellbore the foam strength is reduced, decreasing the ability of the circulating system to clean the hole of cuttings. To combat this problem an increase in the circulating rate and material concentration is required to strengthen the foam, which increases the drilling costs. There are many types of foaming products on the market that will perform well in fresh water, slat, water, oil, and gas producing zones. It is better to purchase a quality foamer rather than a bargain foamer. When drilling problems are encountered the additional cost of the quality foamer will be offset by better performance down hole. Cuttings removal is strictly dependent on the stability of the foam. If the foam returns and cuttings are watched constantly, they give adequate warning of impending trouble. When the foam is muddy, or when carvings begin to appear it is apparent that the hole is enlarging. As the hole enlarges the annular velocity drops and the foam ceases to lift the cuttings efficiently. As long as an adequate air volume and foam mix is available to lift drill solids and fluid entries, hole integrity is the only limiting factor in successful foam drilling of lowpressure reservoirs. Stiff Foam Mix for Air Drilling With Hammer Initial pre-mix
1. 2. 3. 4. 5.
3 sacks of Soda Ash 1 sack of Caustic 4 sacks of KCL 10 gallons of 105 Polymer – (screened) 1-2 sacks of Pac R for maintaining 37-40 viscosity
Secondary mixture:
1. 2 sacks of Soda Ash 2. 1 sack of Caustic 3. 2 sacks of KCL
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Air Drilling Manual
4. 5 gallons of 105 Polymer – (screened) 5. 1-2 sacks of Pac R for 37-40 viscosity Nitrogen Drilling
Because nitrogen is inert and inflammable, it is the preferred gas for underbalanced drilling. Nitrogen can be supplied for oil field use by three different methods: cryogenic liquid separation, pressure swing absorption, and hollow fiber membranes. The selection of nitrogen supply from one of these methods depends on the cost of delivered nitrogen, the required flow rates and pressure, the required nitrogen purity, and the reliability of the equipment for nitrogen generation. Total underbalanced drilling system costs are also a primary consideration. Until mid 1994, nitrogen for underbalanced drilling in western Canada was supplied from cryogenic plants not central to the major drilling areas and was subject to seasonal demand pressures. Thus, the cost of cryogenic nitrogen was higher and a major component of total well costs. In 1994, the first hollow fiber membrane units were shipped to Canada and placed in service to supply gaseous nitrogen for underbalanced drilling. More than 120 wells have been drilled in Canada using nitrogen supplied from these hollow fiber membrane units. The projected amount of required nitrogen may be estimated through the use of advanced computer simulation models that analyze multiphase flow of the combined drilling fluid, injection gas, produced formation fluids, and hole cuttings through the drillstring, bottom hole assembly, and surface production equipment. Although this type of simulation modeling is extremely complex and subject to many uncertainties, it has proven effective on numerous jobs when applied by experienced drilling engineers. Critical data for this type of modeling include bottom hole pressure, flow rates, and temperatures. Nitrogen Supply
Nitrogen for oil field operations can be supplied on site in liquid form or can be produced on site by extracting nitrogen from compressed air. Cryogenic nitrogen is obtained when air is super cooled to a point where the density differences between nitrogen and oxygen allow for high purity separation. Generally, liquid nitrogen is obtained from this process, and the liquid nitrogen is stored and transported to the well site for gasification and injection. Pressure swing absorption (PSA) equipment can generate gaseous nitrogen from compressed air. PSA units usually have two or more tanks containing a loose or granulated carbon material that absorbs oxygen. Compressed air is pumped into the tank, the oxygen absorbs onto the carbon material, and nitrogen flows through to the outlet. When the carbon material in the tank approaches an oxygen saturation point,
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Air Drilling Manual
oxygen sensors on the outlet detect the increasing oxygen content in the nitrogen, and then valves open and close to redirect the compressed air flow to a second tank. The tank containing the oxygen-saturated carbon material is then depressurized to release the oxygen, and the cyclic process continues. PSA units can provide nitrogen at purities up to 99%. Because of the cost, bulkiness, weight, and mechanical complexity of these units, however, they are not widely used in the oil field, particularly at the high nitrogen flow rates required for drilling. Hollow fiber membrane nitrogen-generated systems also produce gaseous nitrogen from a supply of compressed air. They use special polymers configured into very small diameter fibers that are bundled together in tubes called modules. Compressed air flows down the inside of each hollow fiber. Oxygen and water vapor molecules diffuse through the walls of the fibers at a much faster rate than the nitrogen, thus providing separation to produce nitrogen at the outlet. The oxygen and water vapor are vented to the atmosphere. Multiple modules can be manifold together to achieve the desired nitrogen flow rates. These units have few moving parts; can be configured onto smaller skids, and have a large range of nitrogen production rates. The selection of nitrogen supply for underbalanced drilling is usually based on both cost and technical, specifications (amount of nitrogen needed, required flow rates and pressures, and nitrogen purity) of the drilling operation. Nitrogen supply costs can vary substantially and depend on factors such as distance from supply points, trucking costs, job duration, and total quantity required, and purity. Oxygen corrosion can be a concern if membrane-generated nitrogen is used to drill wells in conjunction with water containing chlorides or if significant amounts of water containing chlorides are produced during the operation. Oxygen corrosion is the only significant corrosion concern with membrane-generated nitrogen. Oxygen corrosion can occur in the presence of water containing significant amounts of chlorides and pH levels lass than 7. In these conditions, only a few parts per million oxygen can cause corrosion. There have been instances of oxygen corrosion with membrane-generated nitrogen, and these have occurred when good practices were not followed. Nitrogen production units
Membranes used in nitrogen production units (NPU’s) are made of a special polymer extruded into very small diameter hollow fiber has a circular cross section and uniform wall thickness and is so small that several may fit through the eye of a needle. The small fiber size provides a huge surface area to separate large quantities of nitrogen.
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Air Drilling Manual
Compressed air is directed to the NPU equipment, where it is first filtered to remove particulates and entrained water and oil droplets down to about 0.1µ diameters. The clean compressed air, at 120-350 psig, enters the inlet of the membrane module and travels down the fiber tubes. Oxygen and water vapor molecules permeate the wall of the fiber membrane and are vented to atmosphere. The product nitrogen remains inside the fiber and is collected at the outlet connection, where it is metered. Permeation of the oxygen and water vapor through the membrane wall is governed by Fick and Henry’s laws and is not a matter of molecule size, per se. The hollow fiber membranes can produce nitrogen purities up to 99.9%. The total flow rate of nitrogen product from the membrane is inversely proportional to the purity. For most drilling applications, in which 92-95% nitrogen purity is sufficient, the membrane efficiency is generally 50-67%, depending upon the particular NPU equipment used. Membranes have intrinsic performance characteristics that are a function of temperature and pressure, and thus different membranes may have optimum operating pressures and temperatures at selected purities. Equipment Configuration
The NPU receives compressed air from one or more primary compressors at pressures ranging from 100 to 350 psig. The product nitrogen is pumped, with about a 20-40 psig pressure drop, to the suction of a booster compressor where its pressure is increased to that required for injection into the drillstring. NPU’s have three major components: an air filtration system, an array of air separation modules, and a control panel. The air filtration system usually consists of a scrubber, coalescing filler, and a particulate filter. Some NPU’s also include an activated carbon bed filter and possibly a refrigerated air dryer. The activated carbon bed filter removes aerosol-sized and smaller oil droplets down to a concentration of a few parts per billion. The refrigerated air dryer reduces the relative humidity into the carbon bed to improve oil droplet filtration. The arrays of hollow fiber modules are manifold together to accept the clean compressed air feed and to collect and deliver the nitrogen product. The oxygen and water vapor permeate stream is also collected from each membrane module and piped at near atmospheric pressure to the outside of the NPU skid, where it can quickly and harmlessly dissipate into the atmosphere. The control panel on the NPU allows monitoring and control of the operation. Control panel design and function vary greatly depending on the manufacturer. Some panels measure flow rates, temperatures, purity, and pressure drops across the NPU precisely, yet others only provide simple output of flow rate and nitrogen purity. Nitrogen purity is controlled by varying the back pressure on the membrane modules.
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Air Drilling Manual
Nitrogen purity
To date, for nearly all horizontal underbalanced drilling operations using nitrogen, the nitrogen purity requirement has been based on the concern of hydrocarbon combustion or flame propagation as the hydrocarbons produced from the formation came into contact and mix with the drilling fluids. The nitrogen purity required to prevent combustion is a function of pressure, temperature, and composition of hydrocarbons encountered. Previous work by the U.S. Bureau of Mines and others has indicated that methane may combust in oxygen concentrations as low as 12-14%, but this lower explosive limit of oxygen content decreases to 8% or less as pressure and temperature increase. Subsequent studies by Canadians Operators and one Canadian University have indicated that the minimum amount of oxygen required to initiate and propagate a flame may be as low as 6% in the presence of hydrogen sulfide and under high temperature, high pressure conditions. Other drilling applications
In addition to its use in horizontal underbalanced drilling, nitrogen is often used to drill oil and gas wells under other conditions. The most common practice is to drill with straight dry nitrogen instead of compressed air or misted compressed air. Although leasing NPU equipment increases the total drilling costs slightly, drilling with straight dry nitrogen can reduce total well costs by eliminating concerns about downhole fires or explosions, increasing penetration rated, improving bit life, reducing the number of drilling trips, reducing or eliminating corrosion, and improving safety. The use of NPU generated nitrogen in some traditional air drilling areas (West Texas, eastern Oklahoma, and the San Juan basin, for example) has become commonplace and often the method of choice both to cut costs and improve drilling operations. Nitrified mud drilling is also becoming popular, particularly in areas where water based muds cannot be used, rates of penetration with liquid muds are slow, there are lost circulation zones, aeration of mud may cause excessive corrosion, or drilling mud invasion and permeability damage are concerns, In these applications, nitrogen is injected into the drilling fluid at rates as low as 300 scfm or as high as 3,000 scfm, depending on operating conditions. This technique has been applied successfully in East Texas, eastern Oklahoma, the Texas panhandle, the Permian basin; the Over thrust belt, and the San Juan basin.
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LAYOUT # 1
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LAYOUT # 2
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Air Drilling Manual AIR VOLUMES FOR PERCUSSION DRILLING
Air Compressors
There is two basic types of compressor equipment used in air drilling, the screw and piston type air compressors. During air drilling operations the compressor uses local atmospheric air. The compressor unit intakes a specific volumetric rate of atmospheric air, compresses the volume to the required pressure level (or it pressure capability limit) and injects this air into the standpipe manifolds. A booster may be required to increase the pressure of the airflow from the compressors. The piston type compressor is the general type used for air drilling operations. This type of compressor has the important characteristics of responding to pressure variations without appreciably altering the volumetric flow rate from the machine. Increased pressure requirements are met with increased power to produce a higher pressure at the exit. In air drilling the volumetric flow rate is very important to hole cleaning and the field equipment must have the capability of producing a relative constant volumetric flow rate under a variety of pressure conditions. There have been many disputes in the past regarding the manner in which the volumetric output of a compressor unit is reported.
Photo of a Clark CFB 4 Product Description • •
4 Stage Design High Volume, High Pressure
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Air Drilling Manual
• • •
Dust, Mist, Foam and Aerated Mud Drilling Trailer Mounted Sufficient annular velocities for hole cleaning
Product Specifications
Compressor
4 – Stage Reciprocating
Driver
Caterpillar D398TA V-12, Turbocharged Diesel Engine
Driver Rating
Rated 750 HP @ 900 RPM
Volume Output
1250 SCFM @ 1150 PSIG discharge w/ booster; 1200 SCFM @ 2100 PSIG discharge w/ booster
Cylinders
Cylinder 1: 21” bore Cylinder 2: 13 ½” bore Cylinder 3: 8 ¼” bore Cylinder 4: 4 ¾” bore
Compressor Weight
Dimensions
Booster Dimensions / Weight
/
39’ L x 13’11” H / Weight = 84,000 lbs. 39’ L x 10’W x 13’11” H / Weight = 84,000 lbs.±
Photo of a GHH 246 - G Product Description •
Screw / Booster Design
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Air Drilling Manual
• • • • •
Twin Screw, Oil-Flooded Compressor Two Stage Booster Trailer Mounted Air, Mist, Foam and Aerated-Mud Drilling Ideally suited for Geothermal applications
Product Specifications
Screw
CF246G Single – Stage
Booster
Cylinder 1: 6” bore Cylinder 2: 3.625” bore
Driver
Caterpillar 3412 E-TA V-12, twin turbo-discharged diesel engine
Driver Rating
Rated 750 HP @ 1800 RPM
Volume Output
1250 SCFM @ 1700 PSIG 1450 SCFM @ 600 PSIG
Dimensions / Weight
33’ L x 8 ½” W x 13’3” H – 46,000 lbs.±
Free Air (acfm)
Free air is the actual amount of air delivered by the compressor unit without correction for temperature, pressure, and humidity (acfm). A value reported in free air (acfm) must be accompanied with the pressure, temperature, and humidity for that reading to give any significance to the value. Standard Air (scfm)
Standard air (scfm) is the amount delivered by the compressor adjusted for pressure, temperature, and humidity variations for standard conditions. Correcting free air to Standard air allows the use of a set of standard conditions as the reference point for comparison between various locations. If a value is reported in comparison between various locations. If a value is reported in Standard air (scfm) the pressure, temperature, and humidity are known and does not have to be provided when reporting the flow rate. The compressor unit compresses its rated intake volumetric flow rate at the intake pressure and temperature conditions to some higher pressure and temperature. This results in a smaller airflow rate at the exit of the unit. Particle Dynamics
A particle falling under the influence of gravity will accelerate until the drag force on the particle just balances the gravitational force. The particle will continue to fall at a constant rate known as the “terminal velocity”.
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Air Drilling Manual
The terminal velocity of a typical cutting in air will be very high compared to the terminal velocity of the same cutting in mud. The velocity of the air must exceed the terminal velocity of the cutting to move upward in the annulus. To use air as the circulating fluid requires a high annular velocity to successfully clear the hole of cuttings. As cuttings are generated at the bit, they will rise rapidly past the drill collars. Once the cuttings clear the top of the collars and enter the annulus between the drill pipes and the well bore, the fluid velocity decreases considerably because the cross sectional area increases. Due to this reduction in fluid velocity, the point of most difficult lift is located at the top of the drill collars. Particles that are larger than the critical diameter capable of being lifted by the circulating fluid will tend to accumulate in this section. They will fall back and be reground between the drill pipe and the well bore, drill collars, and the well bore, the bit, or by collisions with other particles. This process will continue until the larger particles are broken into a size such that their terminal velocity is less than the fluid velocity above the collars. When air drilling has failed, very often the reason has been an insufficient air volume, or velocity, to clean the hole at the fast drilling rates. As the well deepens, more air is necessary to maintain the velocity needed to bring the cuttings to the surface. The increase in the air requirements stem from higher friction losses due to the lengthening fluid column, and density increases due to the consequently lower bottom hole velocities. If no additional compressor capacity is immediately available, air requirements may be reduced by decreasing the area of the annulus by either decreasing the hole size or increasing the drill pipe diameter. A smaller annulus imparts higher velocities for a given injection rate.
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Air Drilling Manual MINIMUM ANNULAR VELOCITY IN FEET PER MINUTE 528 (√DC) D = Density of material in pounds per cubic foot C = Chip diameter in inches 528 = constant (increases 5% compounded by 1000’) 528 (/165x .5) (165 average rock density x .5 – ½” diameter of rock) 528 (/ 82.5) 528 x 9.08 4,794 Feet Per Minute at 1000’
TO INCREASE CONSTANT BY 5% PER 1000’ 2000’ – 528.0000 3000’ - 554.4000 4000’ – 582.1200 5000’ – 611.2260 6000’ – 641.7873 7000’ – 673.8766 8000’ – 707.5700 9000’ – 742.9400 10000’ – 780.0870
X X X X X X X X X
1.05 = 554.4 1.05 = 582.12 1.05 = 611.226 1.05 = 641.7873 1.05 = 673.8766 1.05 = 707.570 1.05 = 742.94 1.05 = 780.087 1.05 = 819.091
CUBIC FEET PER MINUTE - MINIMUM REQUIREMENTS CFM = AV (H² - D²) 183.3 CFM = Cubic Feet per Minute AV = Annular Velocity H = Hole Diameter D = Drill Pipe Diameter 183.3 = Constant CFM = 4794 (8.75² - 4.5²) 183.3 CFM = 4794 (76.56 – 20.25) 183.3 CFM = 4794 X 56.31 183.3 CFM = 269,950 183.3 CFM = 1,473
21
Air Drilling Manual DETERMINE BAILING VELOCITY AND SCFM REQUIREMENTS
Let’s determine the recommended bailing velocity of a 7 7/8” hole dusting with 4 ½” drill pipe at 6000’ when drilling sand. (Sand weight is 165 lbs/ft³) First we use a simplified formula to get an estimate> V = 528 D¹ ⁄ ² C¹ ⁄ ² (U.S. Units) Where V = Bailing Velocity in Feet per minute D = Rock Density in pounds per cubic feet (see appendix) C = Diameter of Rock cuttings in inches We assume a large chip size of ½”, so that we are assured of cleaning small ones from hole. V = 528 ⁰⁃⁵C⁰⁃⁵ V = 528 (165) ⁰⁃⁵ (0.5) ⁰⁃⁵ V = 4795 FT/MIN (1000’ Depth.) = 6119 ft/min (6000’ Depth) A good usable rule of thumb is to add 5% compounded per 1000’ to compensate for depth and compressibility changes. Now that we have the recommended bailing velocity the following formula will be used to determine the CFM required. CFM = V (D ²н-DDP²) 183.3 Where: V Dн DDP 183.3 CFM
= Bailing Velocity In ft/min = Diameter Hole in Inches = Diameter Drill Pipe in Inches = Constant Conversion Factor (U.S. Units) = Standard Cubic Feet Per Minute
CFM = V (D ²н-DDP²) = 6119 (7.875² - 4.5²) = 6119(62.01 – 20.25) = 1394 cfm 183.3 183.3 183.3
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Air Drilling Manual
Recommended Air Requirements for Dust Drilling SCFM Hole Size
Drill Pipe
Depth/Feet
1000 16”–17 ½” 14 ¾ - 15 12 ¼” 11” 9⅞ 8 ½-8 ¾ 7⅞ 6¾
4½ 4½ 4½ 4½ 4½ 4½ 4½ 3½
7481 5356 3395 2635 2021 1473 1092 871
2000 7854 5624 3565 2766 2121 1546 1147 914
4000 8658 6201 3929 3049 2339 1704 1264 1008
6000 SCFM 9544 6837 4332 3362 2578 1879 1393 1111
8000
10000
12000
10520 7537 4775 3706 2842 2071 1536 1225
11599 8310 5264 4086 3133 2283 1693 1351
12786 9162 5803 4504 3454 2517 1867 1489
* Recommended additional 30% for mist drilling and directional wells.
Air Volume (SCFM) Correction Factor For Altitude And Ambient Temperature
Temperature F -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 ˚
0 .805 .824 .844 .863 .882 .901 .920 .939 .959 .978 .997 1.016 1.035 1.055 1.074 1.093 1.112
ALTITUDE IN FEET 1000 2000 3000 .835 .866 .898 .855 .886 .920 .875 .907 .941 .895 .928 .962 .915 .948 .984 .935 .969 1.005 .954 .990 1.026 .974 1.010 1.048 .994 1.031 1.069 1.014 1.051 1.091 1.034 1.072 1.112 1.054 1.093 1.133 1.074 1.113 1.155 10.94 1.0134 1.176 1.114 1.154 1.198 1.133 1.175 1.219 1.153 1.196 1.240
23
4000 .932 .954 .976 .999 1.021 1.043 1.065 1.087 1.110 1.132 1.154 1.176 1.198 1.221 1.243 1.265 1.287
5000 .968 .991 1.014 1.037 1.060 1.083 1.106 1.129 1.152 1.175 1.198 1.221 1.244 1.267 1.290 1.313 1.336
Air Drilling Manual AIR HAMMER DESCRITION
The hammer selection can be the most complicated option to consider since there are many designs and the details of each hammer’s operational characteristics (i.e. piston weight, frequency, actual stroke length, etc.). These tools are chosen according to area, bit size design and well design. With different types of tools, we can match the one that best fits your needs. Hammer Configuration •
•
•
Choke size should be determined to operate 350 psi differential across the hammer Hammer with heavy piston has been found to be faster in misting and/or hard rock applications Hammer with lighter piston but higher frequency has been found to be faster in medium to soft rock while dusting Conventional Air Hammer Design
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Air Drilling Manual
Air Hammer Comparison Chart
Hole Size Range 3 5/8” – 3 7/8” 4 1/4” – 4 7/8” 6” – 6 3/4” 7 7/8”
Tool Series
Air Hammer Comparison Tool API Piston Piston O.D. Conn. Weight Stroke
11 lbs.
4.00”
Air Req. (CFM) @ 350 psi 450
18 lbs.
4.00”
620
2000+
57 lbs.
4.00”
990
2040
87 lbs.
5.13”
1165
1479
7 7/8” 7.125” 94 lbs. 5.00” 9 1/2” 8 1/4” – 7.125” 112 lbs. 3.35” 9 1/2’ 9 3/4” – 9.06” 167 lbs. 5.50” 11” 12 1/4” 10.750” 314 lbs. 5.90” – 17 1/2” 17 1/2” SW 15³ 14.750” 8 5/8” 385 lbs. 4.00” – 26” Reg. Pin Information above is based on hammer manufacturers charts.
985
1580
985
1579
2000
1450
2500
1277
2100
1200 @ 250 psi
Dominator 350¹ Dominator 400¹ Dominator 600¹ Dominator 750¹ FAM-Us 8² Dominator 880¹ Dominator 1000¹ Mach 132¹
3.12” 3.75” 5.625” 6.75”
2 3/8” Reg. Pin 2 3/8” Reg. Pin 3 1/2” Reg. Pin 4 1/2” Reg. Pin 4 1/2” Reg. Pin 4 1/2” Reg. Pin 6 5/8” Reg. Pin 6 5/8” Reg. Pin
BPM @ 350 psi
2200
¹ – Hammer manufactured by Halco America ² – Hammer manufactured by Puma Tools in Chile ³ – Hammer manufactured by Drillmaster International Check Valve
The check valve assembly consists of an aluminum check valve with a spring. The check valve head is fitted with a permanently molded rubber which seats the check valve in the internal bore of the backhead, virtually eliminating potential air leakage. Rubber coated spring-loaded stem, which stops flow back through the hammer when air supply has been stopped. Piston
The only moving part of the hammer, the piston travels at very high frequency and transfers energy to the bit and formation. Usually the heavier part of the hammer, the piston may travel at 800 to 1700 beats per minute. The piston, which has a large contact
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Air Drilling Manual
area with the cylinder to minimize wear on the piston & cylinder bore, directs the airflow thereby eliminating the need for an internal cylinder. Bit Retaining Rings
Two semi circle devices which are captured between the driver sub and the top bit bearing to keep the bit engaged in the hammer and provide the freedom for the bit to travel from off bottom to on bottom position. Driver Sub – (Chuck)
Internally splined drive collar attached to hammer that aligns with splined section of bit to transit rotational force from hammer and drill string to the bit. Polymer pins between bit and driver sub splines eliminate metal-to-metal contact and help prevents galling on the 12” and 18” series bits. Description of Air Hammer Operations
Air hammer operation begins with the hammer bit off bottom. Bypassing air through the timing ports and through the piston and bit. As the hammer is lowered to bottom, the bit slides up in the hammer and moves piston up. The lower chamber air ports in the piston align with the feed tube windows and the lower chamber is charged (pressurized) starting the piston in the upward stroke. The piston lifts off the blowtube (standard hammer) or seal area (FAM-Us) allowing the lower chamber to exhaust through the bit. The feedtube windows misalign with the lower chamber ports at this time.
Bottom Chamber exhausting Conventional Air Hammers
Bottom Chamber exhausting FAM-Us Series Hammers
While traveling upwards, the top chamber is charged when the feedtube windows align with the upper ports in the piston. When the pressure in the top chamber exceeds the upward force of the piston, the lower chamber is exhausted, thus the piston stops and 26
Air Drilling Manual
begins its downward travel. The piston travels downward with great velocity until impacting the bit strike face and sending the energy through the hammer bit and into the formation. As soon as the piston impacts the strike face area of the bit, it quickly rebounds and the cycle is repeated until the bit is picked up off-bottom or is not properly in the on bottom position (drilled off and not keeping proper weight on bit).
Off Bottom
On Bottom
Air Flows Freely through the bit
Bit Set on Bottom allowing pressure in lower chamber
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Air Drilling Manual
On Bottom
On Bottom
As piston moves upward the lower chamber is exhausted
Pressure in upper chamber forces piston down
On Bottom Piston strikes bit and cycles starts again
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Air Drilling Manual
Best Drilling Practices
Along with the improved hammer and bit design, field operations have made improvements which can be documented as “Best Drilling Practices” for percussion applications. It has been noted in field operations that in hard formations, a hammer with a heavy piston and/or long stroke will usually out penetrate a hammer with a light piston and/or short stroke, especially in “misting” applications. However, it is difficult to see a major difference in the rate of penetration in softer formations and/or when “dusting”. It is also worth noting that the bit life is often extended with the lighter piston and/or short stroke hammers in soft formations. Thus, for example in Terrell County, where most of the applications have medium formation strengths and can be dusted, a hammer with a higher frequency but moderate impact strength (average piston weight; average stroke) delivers the optimum rate of penetration and bit life combination. Air Volume •
Minimum A.V. of 3000 ft/min. (Angel’s curves)
•
Optimum A.V. of 5000+ ft/min. (Field experience)
•
30% additional air volume suggested for misting and/or directional applications
Air Volume suggested for drilling and cleaning these straight air circulation holes is suggested to be enough to deliver an annular velocity of 5000 feet/min. It is also suggested that an additional 30% air volume should be used when misting or drilling a directional hole. The FAM-Us series hammers have a “by-pass” choke for allowing additional air to be circulated, (for hole cleaning) without running the hammer over its optimum operating parameters. The Halco series hammers do not have the “by-pass” choke system. Bypass Volumes for Hammer Chokes PSI 1/8” Choke 3/16” Choke 200 psi 34 76 250 psi 42 94 300 psi 50 112 350 psi 58 130
1/4” Choke 136 168 199 231
5/16” Choke 212 262 311 361
3/8” Choke 306 377 449 520
The optimum differential pressure ranges from 350 to 380 psi while “dust” drilling, therefore the choke size will be installed to allow the hammer to operate in this range. It is important to have the means to calculate accurate air flow so that the proper choke is installed.
29
Air Drilling Manual Air Hammer Operation
Air Hammers are primarily used to improve penetration rate and control deviation in medium to hard formations. Bit weight, torque loads and rotation speed can be reduced when compared to conventional rotary drilling. Air Hammer Bits require only 200 to 500 pounds of down pressure per inch of bit diameter. (ex.: 7 7/8” = 2000 – 4000 lbs.). In highly deviated areas, a lighter bit weight may be required than the rule of thumb method. The light bit weight required by the air hammer eliminates the need for additional drill collars. Excessive weight on bit will accelerate bit wear. WOB & RPM
•
WOB should be minimized but must maintain a “closed” hammer Typically 500 lbs or less per inch of bit diameter is found to be sufficient.
•
RPM should depend upon formation, hole size and frequency of the hammer
•
•
•
RPM with high frequency hammer in soft formation would utilize higher RPM (i.e. 40 – 60 RPM for 8 3/4” hole size in shale) RPM for hard abrasive formations in large hole sizes, (12 1/4” – 17 1/2”) should be as low as possible. (i.e. 10 – 20 RPM)
The short rapid blow of the downhole air hammer provides for good penetration rates while drilling straight holes. When increasing the blows per minute, (BPM), an accompanying increase in the rpm may be required. When drilling in softer formations, the rpm’s may have to be increased. Torque loads and rotation speeds are much in air hammers than rotary bits. A rotary speed of 30 to 45 rpm’s will allow the inserts to penetrate new formation after each blow. The piston drives its energy through the bit and into the formation. After fracturing occurs, the inserts are rotated to a new position. Bit rotation should also be as slow as possible to maintain a smooth operation with as little torque as possible. Drilling speed and the ability to clean the wellbore are proportional to the amount of air pressure and volume used. Air hammers require no more air volume but higher pressures than rotary drilling. Different rock types and conditions will dramatically affect penetration rates. Hammer Bit Application Procedures – Rig Site
1. Examine hammer to make sure no foreign material has entered the tool. 2. Torque ALL connections to proper requirements. Do not attempt to tighten all connections at the same time.
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Air Drilling Manual Recommended Torque for Air Hammers Hammer Series Pin Connection (ft./lbs.)
Dominator 400 Dominator 600 Dominator 750 Dominator 880 FAM-Us 8 Dominator 1000 Mach 132
6,800 9,600 19,000 19,000 19,600 47,000 47,000
Backhead to Wearsleeve (ft./lbs.) 2,600 7,900 16,700 19,400 19,400 42,000 46,000
Chuck to Wearsleeve (ft./lbs.) 2,600 7,900 16,700 19,400 19,400 42,000 46,000
It should be emphasized that the torque values shown in the table are minimum requirements. The normal torque range is from the tabulated figure of 10% higher.
3. Test fire the air hammer on the rig floor before tripping in the hole. Place the hammer and bit on a block of wood and turn the air on slowly until the air hammer “fires”. 4. While TIH, pour one quart of rock drill oil for every ten stands of drill pipe. Use the recommended rock drill oil for the air hammer. 5. Be cautious when approaching bottom. Establish the off bottom pressure before beginning drilling operations. Be sure that the rock drill oil and/or mist is on the recommended settings. Always make sure you’re off bottom pressure is normal and that you have circulation before going to bottom with the air hammer. 6. Engage the rotary table before approaching bottom. Allowing the air hammer to fire before starting rotation may cause premature insert/bit body breakage. 7. Set Rotary Torque at 22% of capacity (reviewed by rig) 8. Engage the hammer slowly. Do Not Attempt to put the required WOB all at once. 9. RPM’s and WOB should be adjusted to allow the bit to run smoothly. In most cases, RPM will be as slow as possible to maintain consistent torque reading. 10. After drilling Kelly down, always leave at least 12 inches on the Kelly, the rotary table should remain turning, the brake handle locked and the bit allowed to drill off. After the bit drills off and the hammer quits firing, raise Kelly about 2’ to 3’ off bottom and blow hole clean before making a connection. If mist drilling, allow hole to unload before making a connection. 11. Do not break the air until Kelly bushings clear the floor. Never break the air close to bottom as this may allow cuttings into the hammer, causing the hammer to plug and resulting in a trip to replace the hammer. 12. Always keep a watch on the air pressure and monitor the air chart at all times. If you have any pressure increases, pick up off bottom and monitor the pressure. Make notes showing the reason for the increase. The pressure must return to the normal off bottom pressure before resuming drilling. 13. The on bottom pressure, when the air hammer is closed, should be 80# to 100# higher than the off bottom pressure. This is based on “Dust” drilling applications. Mist drilling will carry a higher differential. This on bottom pressure should remain steady while the bit is on bottom drilling. 31
Air Drilling Manual
14. Do Not keep trying to get the hammer to drill. If the bit is broken, (shanked), this will only tend to drive the bit head deeper into the formation. This will also lead to additional failures. 15. The following may result from a “shanked” hammer bit: ROP – rate will slow or cease Pressure may fluctuate dependent upon where the bit is “shanked”. Hammer operations will be erratic 16. If any of these occur, pick up off bottom, compare the off bottom pressures to previous pressures. 17. Blow hole clean 18. Turn off air supply and set hammer on bottom where you will put several turns in the drill string “before” TOH. This procedure is to insure that the drill string and air hammer connections are tight before TOH. Note the wraps in and what you got back. Quick Rule of Thumb: “6 in and 6 out” • • •
Note: If drilling out float equipment, drill out cement and float equipment using a mist drilling system. You will drill 5’ to 10’ of new formation, you will pick up off bottom and blow hole dry. Dry up hole and continue “dust” drilling until hole conditions require changes to be made to the air drilling system. Automatic Drillers •
•
•
•
Make sure automatic driller is serviced periodically or between wells. The use of an automatic driller and appropriate braking system will keep a consistent low WOB and more uniform RPM’s. Keeping a consistent WOB and RPM is “KEY” to minimizing the downhole operating parameters which are typically significantly greater that the surface. Only qualified personnel should attempt to operate automatic driller while using the air hammer.
LUBRICATION
It is recommended that drill rigs used for downhole drilling be equipped with automatic oilers. The use of a positive displacement pump for injecting oil into the air stream is preferred to the aspirator type of pump. The rate of lubrication is a function of air hammer consumption, which is dependent on the operating pressure and the choke size being used. Recommended minimum oil requirements can be calculated for a specific application using the rule of thumb. Rate of Lubrication = 0.2 quarts per hour per 100 SCFM
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Air Drilling Manual
Over the past years, the need for environmentally safe oils has been increasingly apparent due to governmental regulations and increasing environmental awareness. For environmentally sensitive applications where the use of petroleum hydrocarbon oils is prohibited, a number of rock drill oil Suppliers are offering synthetic oils specially formulated for these applications. As with the standard rock drill oils, the viscosity grade of sympathetic oils must be proper for the climatic and operating conditions at the drill site and is, therefore, dependent upon the ambient temperature, air delivery temperature and the operating pressure. For any environmentally sensitive drilling application, the selection of the lubricant must be coordinated with the supplier of the lubricant and the regulating agency at the drill site. Prior to start up of drilling, the following lubrication practices are recommended: 1. Coat inside diameters of hammer and drill pipe with rock drill oil. 2. Make sure that all threads, stress relief grooves and make up faces are coated with thread lubricant. 3. Check the oil level in the lubricator and make sure that it is filled with the recommended grade of rock drill oil for the specific application and location. 4. Determine the hammer air consumption based on pressure and choke size. Calculate or look up the lubrication rate for the hammer in the table in the Operating and Maintenance Manual. Remember to use the next higher grade of rock drill oil when drilling with water injection. 5. Adjust the lubricator setting to the proper lubrication rate. 6. To insure that the lubricator is working, the main air valve should be partially open for a couple of minutes with the rotary headset for a low RPM. After turning the main air valve off, the air hole in the face of the bit should be checked for evidence of a thin film of oil. When drilling, it is recommended that the level of oil in the lubricator be monitored on a regular basis. Drilling with the lubricator empty is extremely hazardous. It is also recommended that air be blown through the inside of the drill pipe to remove debris when adding a new section of drill pipe and that a quart of oil be poured down each section of pipe. Most manufacturers of air hammers recommend the use of rock drill oils for lubrication. These oils are specially formulated to provide operating characteristics such as: 1. 2. 3. 4. 5.
High film strength Extreme pressure characteristics Resistance to shock loading Resistance to corrosion Emulsibility
These are all useful in promotion trouble free operation of the air hammer.
33
Air Drilling Manual Hammer Energy
Piston size determines the area that the air can act on to provide the force that causes the piston to accelerate and strike the bit. This energy is transferred to the bit in the form of an impact wave and travels through the bit to the formation through the inserts. This energy wave crushes the formation directly under the insert and resultant stresses cause cracks and secondary fractures in the formation. The following simplified formula can be used to understand this relationship of piston size, PSI and stroke to blow energy: Blow Energy = PSI x Piston Area x Stroke (Ft./lbs.) 24
It is evident that while stroke and PSI increases are linear the area is a squared function of the diameter thus a small increase in diameter will make a much larger increase in blow energy. Since R.O.P. is related to the energy, (work), put into the formation the following simplified formula can be used to understand the horsepower input: H.P.= Blow Energy x Beats Per Minute 33,000
From this relationship it is easy to understand that piston beats per minute, (B.P.M.), and blow energy per stroke determine the H.P. is reached. The optimum horsepower is limited depending on hole cleaning capabilities and downhole equipment design limitations. Excessive blow energy can cause piston breakage, (cracking), bit failures, and premature insert damage. The hammer size should be selected as close to the size of the hole drilled as possible and leave adequate area for the cuttings to pass. This will not only insure a large robust shank on the bit but will provide a hammer with the largest piston for efficient rock destruction. The manner in which a formation fractures and its resistance to fracturing, (compressive strength), can effect R.O.P. greatly. Any friable rock can usually be drilled with a hammer bit and the R.O.P. will vary with the compressive strength and with the porosity of the formation. Unconsolidated formations such as clay and gravel should not be drilled using a hammer and a better choice would be roller cone rock bits.
34
Air Drilling Manual BHA and STABILIZER SELECTION FOR PERCUSSION DRILLING
In General, percussion bottom hole assemblies are pendulum for straight holes and packed for directional holes. In most straight hole application, hammer operators, (rig site supervisors provided by the hammer company), will suggest a slick assembly, (pendulum), because it maintains a straight hole with minimum drag, (minimum drag reduces the chance of the hammer not closing completely). However, in some instances a pendulum assembly with the # 1 stabilizer sixty feet from the hammer bit and the # 2 stabilizer ninety feet from the hammer bit will be utilized to maximize the pendulum effect. (NOTE: 1/16” to 1/8” undergage stabilizers are recommended to minimize drag when used with the air hammer) The packed hole assembly for percussion drilling is assembled in three ways for mild, medium and severe cases of crooked hole problems. The most common BHA would be the mild, (for directional holes), with the hammer bit, air hammer, 1/16” undergage near bit stabilizer, full drill collar, and finally another 1/16” undergage string stabilizer. Other directional BHA’s may include stabilizing the hammer either on the driver sub, retainer or the hammer housing. In place of the stabilizers, roller reamers are advised when drilling with an air hammer. When planning on a bottom hole assembly for use with the air hammer, consult your hammer company or bit company to determine which bottom hole assembly is better suited for the hammer bit.
35
Air Drilling Manual HAMMER BIT DESCRIPTION
Blow Tube
Delrin tube which is pressed into the upper bore of the bit. This tube mates in the bottom ID of the piston, seals the lower chamber and starts the piston in an upward direction after it strikes the bit. The blow tube also exhausts the lower chamber after the piston clears it to start the next cycle. Bit Strike Face
Uppermost surface of bit that is perpendicular to shank which when contacted by the piston in its downward thrusts allows energy to be transferred from piston through bit and ultimately to the formation. Upper Bearing Surface
Area supported by the upper bearing which helps align the bit and keep the bit as close to parallel with the hammer as possible. Bit Retaining Area
The upper area of bit with the smallest outside diameter which allows the bit to be captured by the bit retaining rings and also allow the bit to travel in and out of the operating positions. Drive Splines
Externally splined section of bit that mates with the internally splined section of the hammer to transfer rotational force from drill string through hammer to the bit. Lower Bearing Surface
Area of the shank closest to the bit head that helps support the bit in the driver sub when on bottom. Dual sleeve retention system
A dual sleeve retainer that features a “positive lock” system. The inner halves fit over the drive sub and retention ring on the bit. The outer sleeve then slides down over the inner sleeves protecting them from the cuttings. This is a patent pending system.
36
Air Drilling Manual Marquis Hammer Bits Double Sleeve Bit Retentions System (Patent Pending)
Large Solid Retaining Shoulder
Drive Sub lower onto bit and inner sleeves placed on bit
Outer Sleeve slid down to protect the inner sleeves
Head Section
Bottom area of hammer which contains the inserts that transmit energy from the hammer to the formation. Contained in the head are the exhaust ports which allow air from hammer to travel through bit and help carry cuttings up the annulus and to the surface. US SYNTHETIC
U S Synthetic is the leading manufacturer of polycrystalline diamond inserts used in oil and gas drilling
•
Proprietary interface technology
– – •
FEA improved designs favorably manage residual stresses inherent to diamond inserts for longer life
Application-specific percussion insert properties:
– – •
Enhances diamond/carbide attachment strength
Highly impact resistant Resistant to wear (long life)
ISO 9001 certified quality program
–
Delivers defect free products
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Air Drilling Manual
–
In-house testing necessary to develop impact resistant diamond inserts. U S Synthetic facilities allow testing of multiple designs quickly to improve product “G” Feature
Diamond enhanced inserts are placed on the O.D. side of the hammer bit to stabilize and to prevent drilling an undergage hole. These inserts are set at 90 degrees to the longitudinal axis of the bit and project out to just smaller than gage diameter. It is desirable to always have the “G” feature undersize to the diameter of the gage cutters. This feature is designed to become effective when the gage wears or breaks and eliminate costly reaming of hole prior to running another bit. BIT SIZES
‘G’ TOLERANCE
API BIT GAUGE
6’ – 13 ¾” 14” – 17 ½” 17 5/8”- 26”
Nominal Minus 1/32” Nominal Minus 1/16” Nominal Minus 3/32”
+1/32” minus 0 +1/16” minus 0 +3/32” minus 0
Hammer Bit Design Blow Tube
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Air Drilling Manual
39
