Stuckpipe Course by IPM Schlumberger.pdf

1 Introduction Stuck pipe is a continuing industry problem for drilling operations that results in very significant nonproductive time and financial losses. Stuck pipe incident investigations have shown that all stuck pipe incidents can be prevented. As the world’s leading oilfield drilling services company, Schlumberger has a key role to play in helping to prevent and mitigate the risk of stuck pipe through the application of technology and good well planning and drilling practices. It is acknowledged throughout the industry that stuck pipe is an awareness issue. This Fundamentals of Stuck Pipe Prevention training package is intended to increase basic knowledge and awareness across the entire Schlumberger drilling population. It is not a replacement for the excellent stuck pipe prevention classroom training that already exists in Schlumberger. The package draws on many existing documents and best practices, including the Schlumberger Stuck Pipe Prevention manual, and is the result of a collaborative project between IPM and Drilling and Measurements (DM). On completion of this training you should be able to         

list common causes of stuck pipe identify the different stuck pipe mechanisms describe appropriate preventive actions that can be taken to avoid stuck pipe incidents understand the importance of well planning and stuck pipe risk identification identify the parameters that influence hole cleaning in vertical and deviated wellbores understand the importance of keeping the wellbore stable understand the significance of drilling parameter trends and how they can be used to identify stuck pipe events be familiar with the recommended practices for avoiding stuck pipe while drilling, circulating, logging, running casing, making connections, tripping, surveying, and backreaming list the different actions for freeing stuck pipe.

How to use this training package Users can move quickly between sections and sub-sections using the menu on the left of the screen. For navigation within sections we recommend using the previous/next buttons at the bottom of each page. You can jump to different sections of the training package at any time, but we suggest you follow the order outlined in the Table of Contents. Each section includes links that explain key concepts through graphics or animations. Some of the links will provide additional information on a particular topic. We hope that you find the material useful and can apply the knowledge gained in your daily work. You have a key role to play in avoiding stuck pipe!

2 Definitions, units, and conversions 2.2 Units and conversions Units used in this CD Oilfield unit

Symbol

SI unit

depth

foot

ft

meter

hole or pipe diameter

inch

in

millimeter

bit size

inch

in

millimeter

weight on bit (WOB)

pounds force

lbf

decanewton

nozzle size

32nds of an inch

–

millimeter

drilling rate (rate of penetration, ROP)

feet/hour

ft/h

meters/hour

barrel

bbl

cubic meter

US gallon

galUS

cubic meter

gallons/minute

bbl/min

cubic meter/minute

barrels/minute

ft/min

cubic meters/minute

annular velocity or slip velocity

feet/minute

ft/min

meters/minute

pressure

pounds force/ square inch

psi

Quantity and property

volume

flow rate

kilopascal megapascal particle size

micron

micrometer

temperature

degrees Fahrenheit

°F

degrees Celsius

mud density

Pounds force/gallon

ppg

kilograms per cubic meter

mud gradient

psi/foot

psi/ft

kilopascals per meter

funnel viscosity

second/quart

s/qt

seconds per liter

apparent and plastic viscosity

centipoise

cP

millipascal-second

yield point

pounds force/100 square inch

lbf/100 in

gel strength

pounds force/100 square inch

lb/100 in

pascal

cake thickness

32nds of an inch

–

millimeter

filter loss

millimeter or cubic centimeter

cm

2

2

3

pascal

millimeter or cubic centimeter

torque

foot-pound

ft.lb

shear rate

reciprocal seconds

s

-1

newton-meter reciprocal seconds

Conversion factors Symbol

Multiply by

To obtain

Symbol

barrels

bbl

0.158984

cubic meters

m

barrels

bbl

5.6148

cubic feet

ft

barrels

bbl

42

gallons (US)

USgal

barrels

bbl

158.984

liters

L

bars

bar

14.5038

pounds-force per square inch

psi

bars

bar

100,000

pascals

Pa

bars

bar

100

kilopascals

kPa

bars

bar

0.1

megapascals

MPa

centimeters

cm

0.3937

inches

in

centimeters

cm

0.03280839

feet

ft

feet

ft

0.3048

meters

m

feet

ft

12

inches

in

cubic feet

ft

0.17811

barrels

bbl

gallons (US)

galUS

0.02380952

barrels

bbl

gallons (US)

galUS

3.7854

liters

L

horsepower

hp

0.7457

kilowatts

kW

inches

in

2.54

centimeters

cm

inches

in

0.0833333

feet

ft

square inches

in

6.4516

square centimeters

cm

kilograms (force)

kgf

2.20462

pounds-force

lbf

kilograms (force)

kgf

9.81

Newtons

N

Multiply

3

2

3

3

2

kilopascals

kPa

0.01

bars

bar

megapascals

MPa

145.038

pounds-force per square inch

psi

meters

m

3.28084

feet

ft

Newtons

N

0.224809

pounds-force

lbf

pounds-force

lbf

4.44822

Newtons

N

pounds-forece per sqaure inch

psi

0.06894745

bars

bar

pounds-forece per sqaure inch

psi

6.894745

kilopascals

kPa

pounds-force/gallon

lbf/gal

0.1198

kilogram-force/liter

kgf/L

quart

qt

0.94635

liter

L

3 Stuck pipe mechanisms

3.1 Introduction and section objectives On completion of this section, you should be able to   

describe the three main stuck pipe mechanisms and their causes describe the measures to prevent sticking recognize the warning signs of stuck pipe on the rig site.

3 Stuck pipe mechanisms 3.2 Stuck pipe mechanisms: Causes, occurrence, and prevention Experience has shown that stuck pipe incidents can occur during any stage of the drilling process, for example, drilling, tripping in and out, backreaming, logging, or running casing. But some operations, such as tripping, are more prone to sticking incidents. The chart below shows the distribution of IPM stuck pipe incidents by activity.

Figure 3.1: IPM stuck pipe incidents by activity in 2009. This wide distribution range is why field personnel must remain vigilant at all times if they want to prevent stuck pipe. There are three categories of sticking mechanism: solids-induced packing off, differential sticking, and wellbore geometry and mechanical issues. Each category is associated with different preventive measures and freeing techniques. In each category, you will encounter several specific causes that can lead to sticking. The following sections illustrate these causes and group them under the relevant sticking mechanism. A good understanding of the causes and the preventive measures should help to minimize the occurrence of stuck pipe.

3.2.1 Solids-induced packing off Packing off occurs when debris, such as cuttings, cavings or junk, accumulates between the drillstring and the wellbore. Large pieces of debris can easily create bridges and can cause pipe to stick. Pieces of debris that are smaller than the annular space can build up on the low side of the wellbore and make movement of the string difficult. If the right action is not taken, the string can become tightly packed. Historically, packing off is the most common stuck pipe mechanism. Packoffs occur most frequently when tripping and/or backreaming in deviated wells. The most common causes are related to poor hole cleaning.

3.2.1.1 Poor hole cleaning It has been observed that poor hole cleaning is responsible for many stuck pipe incidents. Some statistics show that up to one-third of the stuck pipe events recorded in nondeviated wells are related to poor hole cleaning. In high-angle wells, this figure is up to 80%. Section 5 covers the factors affecting hole cleaning in more detail. Good hole cleaning means removing enough solids from the wellbore to allow unhindered passage of the drillstring and the casing. When low annular velocities, poor mud carrying properties, and/or insufficient rotation (especially in deviated wellbores) are found, the cuttings and cavings are not transported out of the wellbore fast enough and hole cleaning becomes problematic.

Figure 3.2: Poor hole cleaning. In deviated wellbores, cuttings can settle on the low side and form accumulations called cuttings beds. Cuttings beds can form in open and cased holes. The BHA may become stuck if it is pulled into the cuttings bed. Cuttings beds that occupy just 10% of the hole diameter can cause problems. They increase the friction between the drillstring and the wellbore, and cause the hook load to increase. In wells with profiles that include inclinations of between 30 and 60°, cuttings can slide down the annulus and collect or pack around the drillstring. This sliding or avalanching effect is more likely when the pumps are switched off and/or when the drillstring vibration is high enough to destabilize the bed. At inclinations above 60–70°, the beds are usually stable, but tripping can still be difficult. Cuttings beds are easy to create but difficult to clean out. High flow rates, carefully controlled mud rheology, and pipe rotation in directional wells are critical for achieving good hole cleaning. Particular BHA tool shapes and stabilizer profiles can also reduce the risk by maximizing the flow to ease the passage of tools through any cuttings beds during trips, and allow cuttings to pass the tools during circulation.

Occurrence   

When flow rate is inadequate (for example, large holes, formation washout, limited pump capacity) and/or the rotation is insufficient (for example, large bent housing, curved sections, sliding) Most common in deviated wells (30–60°) where unstable cuttings beds can be created When well is drilled faster than the hole is cleaned. In this case, the excess volume of solids generated modifies the flow profile of the drilling fluid and reduces its ability to carry solids.

Rig-site warnings   

  

Increasing torque and drag. Usually, a trend to increased drag is a clear indicator while pulling out. Reduced cuttings returns at the shakers, i.e., the volume of cuttings being generated is not seen at the shakers Increasing pump pressure or equivalent circulating density (ECD), as the mud in the annulus contains more solids. Note that in deviated wells, a decrease in ECD can be a sign of poor hole cleaning and solids accumulating on low side of the wellbore (a decrease in the cuttings concentration in the annulus of the lower inclination section of the well). Poor weight transfer to bit when slide drilling in directional wells Reground cuttings Difficulty in orienting the toolface

Preventive measures           

Maintain adequate annular velocity (both flow rate and annular clearance contribute to annular velocity). Maximize the drillpipe rotation to optimize cuttings bed agitation in directional wells, even when circulating or pumping sweeps. Ensure that the bottom-up circulation times are adequate before pulling out of hole. Monitor the rate of cuttings return at the shakers. Ensure that low, high, or weighted sweeps are properly used and that their efficiency is monitored. Optimize the drilling mud properties (increase low-end rheology in near-vertical wells). Design the well properly to provide an adequate flow rate. This may require using a larger drillpipe, thinner mud, a lower pressure drop bit or BHA, and/or a third mud pump. Establish an overpull limit before pulling out. Evaluate and monitor the torque and drag trends. Control the ROP, if necessary, to limit the amount of solids concentrating in the mud. Minimize the use of tools with restricted flow paths, i.e., those with a small junk slot area.

3.2.1.2 Unconsolidated formations

Figure 3.3: Unconsolidated formations. Unconsolidated formations are loosely packed rock layers having little or no bonding between the particles. As the well is drilled through such a formation, the formation becomes fragmented. Removing the supporting rock as the well is drilled causes formation collapse. This is very similar to digging a hole in the sand on the beach; the faster you dig, the faster the sand collapses. When drilled with a fluid that cannot build a mudcake, an unconsolidated formation cannot be supported by hydrostatic overbalance because the fluid simply flows into the formation. Sand or gravel then falls into the hole and packs off the drillstring. The effect can be sudden or may be seen as a gradual increase in drag with time. The problem is normally associated while drilling the surface hole sections. At depth, when mudcakes are usually present, unconsolidated sands are stable owing to their internal friction, and can be drilled easily if there is an overbalance. Causes

   

No bonding between particles in the formation Little or no mudcake present No formation support from the hydrostatic overbalance because the fluid flows into the formation Sand or gravel falling into the hole

Occurrence  

While drilling tophole sections While drilling shallower unconsolidated formations

Rig-site warnings     

Increasing pump pressure due to increased solids in the drilling mud Increasing torque and drag Overpull on connections Fill on bottom observed while running in hole Shaker blinding leading to mud losses at surface

Preventive measures             

The mud must be designed to build a cohesive, low-permeability cake. The pump must provide sufficient flow rate to clean the hole. Avoid shaker, desilter, and desander overloading by controlling drilling to the limitations of the solids control equipment. Avoid unnecessary reaming and backreaming that will destabilize the mudcake. Avoid excessive circulating time with the BHA opposite unconsolidated formations to reduce hydraulic erosion. Check and clean out any hole fill before drilling ahead. Use sweeps (usually highly viscous sweep) to help maintain hole cleanliness. Consider spotting viscous pills to minimize the fill on bottom. Control drill the suspected zone to allow time for the mudcake to build up and to minimize the concentration of solids in the annulus. Minimize annulus loading and the resultant ECDs. Keep the BHA to the minimum length. Trip carefully across troublesome formations to minimize mudcake removal, surging, or swabbing. Start the pumps slowly to avoid a pressure surge being applied to the unconsolidated formation.

3.2.1.3 Reactive shales

Figure 3.4: Reactive shales. Issues arise when water-sensitive shales or clays are drilled with a less inhibited drilling mud than required, which causes the rock to swell or weaken. As a result, chunks of shale break off and fall into the borehole, thereby restricting the annular space. The amount of swelling varies from that of highly reactive gumbo (fast absorption rate) to that of shales that absorb water very slowly. The swelling process can occur in hours or over several days. Occurrence     

Occurs more frequently with water-base mud (WBM) than with oil-base mud (OBM) The reaction is time dependent, and is determined by the mud and formation interaction Often happens while tripping Can happen while drilling Generally occurs while the BHA is passing the reactive formation

Figure 3.5: Swelling in reactive shale samples. Rig-site warnings      

Hydrated or mushy cavings Shaker screens blind off and clay balls form Increase in low-gravity solids (LGS), mudcake thickness, plastic viscosity (PV), yield point (YP) and methyl blue test (MBT) Increasing pump pressure Restricted or impossible circulation Increasing torque and drag

Preventive measures     

Use an inhibited mud system such as salt or polymer mud. If the effect is severe, use OBM. Drill and case off reactive formations as quickly as possible. Perform wiper trips to gauge the hole (the frequency of wiper trips should be based on the exposure time and warning signs). Keep the mud properties on specification. Monitor the mud properties (MBT, PV, YP).

3.2.1.4 Naturally overpressured shale

Figure 3.6: Naturally overpressured shale. A naturally overpressured shale is one with a pore pressure greater than the normal hydrostatic pressure gradient. This situation is caused by geological conditions such as undercompaction, naturally removed overburden, for example, by weathering, or tectonic uplift. Using insufficient mud weight in these naturally overpressured formations will cause the hole to become unstable and collapse. Fractured shales and cavings will then fall into the wellbore and may lead to packing off.

Multimedia 3.3: Abnormal pressure. Occurrence

   

Drilling through a formation with abnormal pressure Removal of ECD Most likely while tripping out because swabbing can contribute to hole instability by reducing the wellbore hydrostatic pressure When using an inappropriate mud weight

Rig-site warnings       

The appearance of splintery cavings at the shakers Increasing torque and drag Increased gas levels Restricted or impossible circulation once pipe is stuck Hole fill after trips An increase in ROP due to underbalanced conditions Changes in sonic or resistivity measurement trends if LWD tools are used

Preventive measures      

Monitor shale shakers for cuttings and cavings. Use sufficient mud weight to control the pore pressure. Use pore pressure analysis and confirm the results with gas readings. Plan to minimize the hole exposure time. Do not reduce the mud weight once the shale has been exposed. Employ wellsite monitoring techniques or services such as LWD and/or mud logging.

3.2.1.5 Induced overpressured shale

Figure 3.7: Induced overpressured shale. Induced overpressured shales develop when a shale formation assumes the hydrostatic pressure of the wellbore fluids. This usually happens after several days of exposure to that pressure. The shale will have now a higher internal pressure. If the hydrostatic pressure in the wellbore is subsequently reduced, the shale will collpase. The collapse process is similar to that found in naturally overpressured shales (see Section 3.2.1.4). Occurrence    

After a reduction in mud weight or a long exposure with a constant mud weight Most likely to occur with WBM but can happen with OBM While drilling or tripping In the casing rathole when the mud weight has to be decreased after drilling out the shoe

Rig-site warnings   

Splintery cavings at shakers Cuttings or cavings that show no sign of hydration Increasing torque and drag

  

Restricted or impossible circulation once pipe is stuck Hole fill after trips Tight hole in casing rathole

Preventive measures    

Control the ECD properly to minimize the induction of overpressure in sensitive formations. Use the appropriate mud weight. If cavings are observed, apply good hole cleaning practices, such as pumping combined high viscous–weighted sweeps, and controlling ROP. Minimize the casing rathole length.

3.2.1.6 Overburden stress

Figure 3.8: Overburden stress. The overburden (weight of the overlying formation) or vertical stress is usually the maximum stress in a formation. When a well is vertical, the overburden stress will apply equally around the wellbore. But as the wellbore angle increases, the overburden stress applies directly on the wellbore wall. If the mud weight is not adjusted, this will lead to wellbore collapse. Cavings will fall into the wellbore and result in stuck pipe. Occurrence

 

In deviated wellbores While drilling or tripping

Rig-site warnings     

Packoffs and bridges may occur Angular cavings at the shakers Increasing torque and drag Restricted or impossible circulation Increasing volume of returns at the shakers relative to the volume of hole drilled

Preventive measures     

Use offset data and mechanical earth models (MEM) to establish the optimum inclination and azimuth. Maintain the mud weight and ECD within the planned mud weight window. Plan to case off these formations as quickly as possible. If possible, drill these formations with smaller hole sizes. Maintain hole cleanliness and be prepared for increased amounts of cuttings and cavings.

3 Stuck pipe mechanisms

3.2 Stuck pipe mechanisms: Causes, occurrence, and prevention 3.2.1 Solids-induced packing off 3.2.1.7 Stressed formations

Figure 3.9: Stressed formations. All formations are stressed to some extent. In tectonically stressed areas, the rock is compressed or stretched owing the movement of the Earth’s crust, as shown in Figure 3.10. When a hole is drilled in a formation, the stress is redistributed around the wellbore wall and concentrated at specific points. If the hydrostatic pressure is reduced, the formation can shear at these points and produce angular cavings. The hydrostatic pressure required to stabilize the wellbore may be much higher than the fracture pressure of the other exposed formations.

Figure 3.10: Geological sources of rock stress. Note that the overburden stress is only maximum in Case A. Occurrence  

Common in mountainous locations While drilling or tripping

Rig-site warnings     

Packoffs and bridges may occur Angular cavings at the shakers Increasing torque and drag Restricted or impossible circulation when stuck Increasing volume of returns at the shakers relative to the volume of hole drilled

Preventive measures     

Use offset data to establish the optimum inclination and azimuth for the well. Maintain mud weight and ECD within the planned mud weight window. Plan to case off these stressed formations as quickly as possible. Keep the hole clean and be prepared for increased cuttings and cavings. Consider pumping high-viscosity weighted sweeps to carry the cavings.

3.2.1.8 Faulted and fractured formations

Figure 3.11: Faulted and fractured formations. Natural fracture systems in rock can often be found near faults. These are common geological features that can affect all kinds of rocks, at all depths. The rock in fault and fracture zones may be broken before a well is drilled into small or large pieces that, if loose, can fall into the hole and jam the drillstring. In fractured shales, the fractures usually remain stable providing the minimum stress acting on the fractures is not exceeded. If it is exceeded, fluid begins to infiltrate the rock mass and loosen the blocks, thus releasing them into the wellbore. The symptoms are similar to those experienced when the rock shears because of low mud weight. However, increasing the mud weight in this situation will make the problem worse, as more mud infiltrates the rock mass. In addition, impacts from the BHA due to drillstring vibrations can cause the formation to fall into the wellbore. This sticking mechanism is unusual in that it may occur during drilling. Experience shows that the first sign of this stuck pipe mechanism while drilling is the string torquing up and sticking.

Figure 3.12: Field example of a fractured formation. Occurrence     

In tectonically active zones Fractured limestone Highly dipping formations As the formation is drilled During trips

Rig-site warnings     

Hole fill on connections Possible losses Fault-damaged cavings at shakers Blocky cavings (cavings with parallel faces) Instantaneous sticking

Preventive measures       

Plan the well properly to minimize the exposure time. Design the well and use drilling practices to minimize fluctuations in the annular pressure. This may require thin mud, a slim drillpipe, or a bigger casing or hole in extreme or difficult situations. Keep the annular space as clean as possible and avoid overloading. Monitor the ECDs while drilling to avoid inducing losses (losses are often difficult to detect). Limit the rotary and tripping speeds across fractured formations. Wash and ream when running in and clean out the hole before drilling ahead. If coal is present, reduce the ROP and control drill the coal section. Drill the coal in “small bites”, and pick up and circulate out every 10–20 ft.

Figure 3.13: Example of coal caving.