6. Matrix Acidizing

6. MATRIX ACIDIZING

TUNIO, May' 2011,,, Courtesy AP Aung

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LESSON OUTCOMES • At the end of this section, the students will be able to : • Understand different acid solutions. • Design acidizing treatment. • Identify selection criteria for acidizing.


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TYPES OF ACIDIZING METHODS • Acid is used to remove damage near the wellbore in all types of wells. • In carbonate formations, acid may be used to create linear flow systems by acid fracturing. • Acid fracturing is not applicable to sandstone. • The two basic types of acidizing are characterized through injection rates and pressures. • Injection rates below fracture pressure are termed matrix acidizing, while those above fracture pressure are termed acid fracturing. TUNIO, May' 2011,,, Courtesy AP Aung

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1. Matrix Acidizing

• It is applied to remove skin damage caused by drilling, completion, workover or well-killing fluids, and by precipitation of deposits from produced water. • Due to the extermely large surface area contacted by acid in a matrix treatment, spending time is very short. • Therefore, it is difficult to affect formation more than a few feet from the wellbore.


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1. Matrix Acidizing

• Removal of severe plugging in sandstone, limestone, or dolomite can result in a very large increase in well productivity. • If there is no skin damage, a matrix treatment in limestone or dolomite could stimulate natural production no more than one and one-half times.


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1. Matrix Acidizing • In matrix acidizing,acid flow is confined to the formations, natural pores and flow channels at a bottom pressures less than the fracturing pressures.(Fig.5.1) • The purpose is to increase the permeability and porosity of the producing formation. • During the matriz acidizing job, the contact area between the acid and the formation is very large. Therefore, friction pressure increases rapidly with increased pumping rates. • Due to high friction pressures, matrix acidizing must be conducted at low injection rates. TUNIO, May' 2011,,, Courtesy AP Aung

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1. Matrix Acidizing

Fig. 5.1 Matrix Acidizing

A matrix acidizing treatment consists of slowly injecting acid into the formation so that it penetrates into the pore spaces of the rock without fracturing the formation. TUNIO, May' 2011,,, Courtesy AP Aung

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1. Matrix Acidizing

Fig. 5.1 Matrix Acidizing

Matrix acidizing is used primarily in sandstone formations to dissolve unwanted materials that have invaded the rock pores during drilling, cementing and completions operations. TUNIO, May' 2011,,, Courtesy AP Aung

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2. Acid Fracturing • The reservoir is hydraulically fractured and then the fracture faces are etched with acid to provide linear flow channels to the welbore. • Two major problems involved in acid fracturing: – fracture closure after etching relatively homogeneous carbonates, – plugging of the fracture if appreciable undissolved fines are released by the acid. TUNIO, May' 2011,,, Courtesy AP Aung

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2. Acid Fracturing

• To combat fracture closure in uniformly soluble carbonates, hydraulic fracturing and propping should be considered, as well as special acidizing techniques designed to provide flow channels. • If the release of excessive fines is the problem, suspending agents are used to reduce settling and bridging of fines in the fracture during clean-up after acidizing. TUNIO, May' 2011,,, Courtesy AP Aung

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ACID ADDITIVES • The use of acid can create a number of well problems; 1. Release fines that plug the formation 2. Form emulsions 3. Corrode steel • Additives are available to correct these and a number of other problems. TUNIO, May' 2011,,, Courtesy AP Aung

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RETARDATION OF ACIDS Retardation of Acid • To achieve deeper penetration in acid fracturing, it is often desirable to retard the acid. • This can be done by emulsifying, gelling, or chemically retarding the acid. • Another approach is to use naturally retarded acetic or formic acid. • Emulsified acid has primary application in fracture acidizing to retard reaction rate of HCl on limestone and dolomite within the temperature range of 80oF to 300oF. TUNIO, May' 2011,,, Courtesy AP Aung

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Retaradation of Acid • Retardation of HCl with Calcium Chloride (CaCl2) – Calcium chloride is beneficial as a retarder when acidizing formation containing anhydrite, because CaCl2 greatly decreases the solubility of anhydrite. • Retardation of HCl with CO2 – Carbon dioxide retards HCl acid by cooling and by changing the kinetics of reaction. CO2 expands and provides additional clean-up following acidizing esepcially in low-pressure wells. TUNIO, May' 2011,,, Courtesy AP Aung

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Retaradation of Acid

• Retardation of HCl with Acetic Acid – Acetic acid reacts with limestone to form calcium acetate Ca (C2H3O2), which acts as a buffer to HCl acid.


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Matrix Acidizing • Primary purpose of matrix acidizing is to remove or bypass damage due to scale, mud, clay, or hydrocarbon deposits, and to restore natural formation permeability. • Matrix treatments are usually performed by soaking, jetting or agitation, or circulation below fracture pressure. • Fifteen percent HCl is normally used. • Since the depth of damage is seldom more than a few feet, volume of acid needed is relativelly small.


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Acid Fracturing

• Acid Fracturing is to prop the fracture faces open with sand or glass beads. • The choice between acid fracturing and conventional hydraulic fracturing is often a difficult decision. • If both systems appear equally feasible to obtain desired fracture flow capacity, then the decision may be based on comparative costs.


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CANDIDATE SELECTION • All carbonate formations can be candidates for acid fracturing • More suitable candidate wells are as follows: – Poorly performing wells due to low reservoir permeability – Wells with restrictions due to damage near the well bore TUNIO, May' 2011,,, Courtesy AP Aung

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POTENTIAL SAFETY HAZARDS IN ACIDIZING • Hydrogen sulfide, a poison gas, may be produced from the reaction of acid on sulfide scale. • High concentrations can paralyze the olfactory nerves. • High concentrations can also paralyze other nerves in the respiratory system.

Note: (Source Wikipedia) The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It is instrumental in the sense of smell. TUNIO, May' 2011,,, Courtesy AP Aung

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POTENTIAL SAFETY HAZARDS IN ACIDIZING • Arsenic inhibitor is poisonous if swallowed. • Contact of arsenic with aluminium or magnesium may produce arsine gas in dangerous concentrations. • Arsine gas is an inhalation hazard and is very deadly. • Arsenic inhibitors should generally be avoided because of their toxicity and the environmental protection problem. Note: (Source Wikipedia) Arsine is the chemical compound with the formula AsH3. This flammable and highly toxic gas is one of the simplest compounds of arsenic. TUNIO, May' 2011,,, Courtesy AP Aung

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DESIGN AND OPTIMIZATION PROCESS Matrix Acidizing • Acid is injected into the pores and flow channels of carbonate rocks at a bottom-hole pressure considerably less than the fracturing pressure, the purpose being to increase uniformly the permeability of the formation. • Under these conditions, it is assumed that the acid enters only the natural pores and flow channels, and reacts with the walls of the pores, dissolving the rock and enlarging the pores. TUNIO, May' 2011,,, Courtesy AP Aung

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Matrix Acidizing • This reaction slows down as the acid is spent, until finally additional radial penetration produces no additional benefit. • Maximum penetration is attained when the first increment of injected acid is completely spent. • Additional acid will only enlarge the cross-sectional area.


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Matrix Acidizing • In evaluating this type of acidizing, 1. 2. 3. 4.

The formation is homogeneous. The pores are of uniform size. The acid penetrates uniformly and radially. The reaction rate declines uniformly with decreasing acid concentration. 5. The weight of limestone dissolved per increment of distance declines uniformly until the acid is completely spent.

• On the basis of the above assumptions, the radial distance the acid will penetrate before being totally spent depends on the equation. TUNIO, May' 2011,,, Courtesy AP Aung

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Matrix Acidizing • Volume injected, ft3 = Pore volume invaded, ft3

(2.3)


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Matrix Acidizing • The only unknown factor in Eq (2.3) is the spending time t, which must be measured for the particular acid in the laboratory. • The spending time for an acid depends upon the ratio of the area of the rock exposed to the acid to the volume of acid, here denoted as specific area sΦ = (cm2 / cm3). • Specific surface area can be obtained from the Kozeny equation as modified by Pirson. (2.4) TUNIO, May' 2011,,, Courtesy AP Aung

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Matrix Acidizing

• The formation resistivity factor is related to the porosity by • Where cementation factor m varies from 1.3 for unconsolidated sands and oolitic limestone to 2.2 for dense limestone. Note: Source (Wikipedia) Oolite (egg stone) is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. Ooids are most commonly composed of calcium carbonate (calcite or aragonite), but can be composed of phosphate, chert, dolomite or iron minerals, including hematite TUNIO, May' 2011,,, Courtesy AP Aung

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Matrix Acidizing • It was found that in matrix acidizing sΦ is so great that the spending time for most acids is less than 15 sec. • To obtain greater penetration during matrix acidizing, it is necessary to either decrease the reaction rate or increase the rate of injection of the acid into the formation.


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Matrix Acidizing


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Acidizing through Preexisting Fractures • Treatments may be conducted in formations containing natural fractures. • Purpose of this type of treatment is to remove secondary deposition or loose particles in the fracture. • The injection rate is controlled during such treatment so as to not exceed the formation fracturing pressure. • Maximum penetration of the acid into the fracture is dependent on the spending time of the acid under reservoir temperature and pressure and so on the injection rate. TUNIO, May' 2011,,, Courtesy AP Aung

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Acidizing through Preexisting Fractures • The evaluation of these treatments requires following basic assumptions: 1. The fractures are horizontal and of uniform width and extend radially from the wellbore. 2. The acid leak-off into the formation is considered negligible. 3. The rate of reaction of the acid is proportional to its concentration, and the quantity of rock dissolved form the fracture face decreases with increased acid penetration until the acid is spent.


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Acidizing through Preexisting Fractures • At an injection rate qi, the radial distance the acid will penetrate a horizontal fracture until it is spent at time t is found from the equation Volume of fractures = Volume of acid injected


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Acidizing through Preexisting Fractures • If qi is expressed in barrels per minute, t in seconds, and W in inches.

(2.5)


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Acidizing through Preexisting Fractures • For natural fractures, it is safe to assume a fracture width of 0.1 mm or less. • Some investigators found that the spending time for most acids is less than one minute. • Maximum penetration cannot be calculated accurately because it is impossible to determine the number of fractures exposed to the wellbore.


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High Pressure Acidizing through Fractures • In evaluating this type of treatment the following assumptions are made: 1. A single fracture is created which is either vertical or horizontal. 2. The major portion of the acid enters the fracture, and therefore acid entering the matrix from the wellbore is negligible. 3. The acid solutions do not contain propping agents. The high pressure acid-fracturing technique is used to increase the producitivity of reservoirs of extermely low permeability.


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ACID-FRACTURING DESIGN • An acid fracture treatment is designed in the same manner as the hydraulic fracturing treatment, with the additional condition for the spending time of the acid. • Also, as in fracturing, the design is controlled by economic factors. • The main difference between fracturing and acidizing design is the absence of propping agents in the latter case. • For this, an acidizing productivity ratio will be calculated considering the capacity of the fracture to be infinite. TUNIO, May' 2011,,, Courtesy AP Aung

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Example (1) • Design an acid fracture, given:

fracture radius rf =110 ft frictional pressure drop is 1657 psi


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Example (1) • Solution: The well before acidizing is producing 20 bbl of oil per day; it is desired to increase the production to 86 bbl per day. The fracture area required is


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Example (1) • The fracture width to be used in area calculations, is

• The term x is

• Then the fracturing efficiency is 29 percent, and the total volume of acid required is TUNIO, May' 2011,,, Courtesy AP Aung

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Example (1) • The injection rate,

• The acid density is

• And the hydrostatic pressure,


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Example (1) • The bottom-hole treating pressure is

• And the velocity in the tubing is

• The value of Reynolds number is


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Example (1) • The surface injection pressure is

• And the hydraulic horsepower required is


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Thank You TUNIO, May' 2011,,, Courtesy AP Aung

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6. Matrix Acidizing

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