6. matrix acidizing

6. MATRIX ACIDIZING6. MATRIX ACIDIZING

1TUNIO, May' 2011,,, Courtesy AP Aung

LESSON OUTCOMES

• At the end of this section, the students will be able to :

• Understand different acid solutions.

• Design acidizing treatment.• Design acidizing treatment.

• Identify selection criteria for acidizing.


TYPES OF ACIDIZING METHODS

• Acid is used to remove damage near the wellbore in alltypes of wells.

• In carbonate formations, acid may be used to create linearflow systems by acid fracturing.flow systems by acid fracturing.

• Acid fracturing is not applicable to sandstone.

• The two basic types of acidizing are characterized throughinjection rates and pressures.

• Injection rates below fracture pressure are termed matrixacidizing, while those above fracture pressure are termedacid fracturing.


1. Matrix Acidizing

• It is applied to remove skin damage caused by drilling,completion, workover or well-killing fluids, and byprecipitation of deposits from produced water.precipitation of deposits from produced water.

• Due to the extermely large surface area contacted by acidin a matrix treatment, spending time is very short.

• Therefore, it is difficult to affect formation more than afew feet from the wellbore.


1. Matrix Acidizing

• Removal of severe plugging in sandstone, limestone, ordolomite can result in a very large increase in wellproductivity.productivity.

• If there is no skin damage, a matrix treatment inlimestone or dolomite could stimulate natural productionno more than one and one-half times.


1. Matrix Acidizing

• In matrix acidizing,acid flow is confined to the formations,natural pores and flow channels at a bottom pressures lessthan the fracturing pressures.(Fig.5.1)

• The purpose is to increase the permeability and porosity• The purpose is to increase the permeability and porosityof the producing formation.

• During the matriz acidizing job, the contact area betweenthe acid and the formation is very large. Therefore, frictionpressure increases rapidly with increased pumping rates.

• Due to high friction pressures, matrix acidizing must beconducted at low injection rates.


1. Matrix Acidizing

Fig. 5.1 Matrix Acidizing

A matrix acidizing treatment consists of slowly injecting acidinto the formation so that it penetrates into the pore spacesof the rock without fracturing the formation.


1. Matrix Acidizing

Fig. 5.1 Matrix Acidizing

Matrix acidizing is used primarily in sandstone formations todissolve unwanted materials that have invaded the rockpores during drilling, cementing and completions operations.


2. Acid Fracturing

• The reservoir is hydraulically fractured and then thefracture faces are etched with acid to provide linear flowchannels to the welbore.

• Two major problems involved in acid fracturing:

– fracture closure after etching relatively homogeneouscarbonates,

– plugging of the fracture if appreciable undissolved finesare released by the acid.


2. Acid Fracturing

• To combat fracture closure in uniformly solublecarbonates, hydraulic fracturing and propping should beconsidered, as well as special acidizing techniquesconsidered, as well as special acidizing techniquesdesigned to provide flow channels.

• If the release of excessive fines is the problem,suspending agents are used to reduce settling andbridging of fines in the fracture during clean-up afteracidizing.


ACID ADDITIVES

• The use of acid can create a number of well problems;

1. Release fines that plug the formation

2. Form emulsions2. Form emulsions

3. Corrode steel

• Additives are available to correct these and a number ofother problems.


RETARDATION OF ACIDS

• To achieve deeper penetration in acid fracturing, it is oftendesirable to retard the acid.

• This can be done by emulsifying, gelling, or chemicallyretarding the acid.

Retardation of Acid

retarding the acid.

• Another approach is to use naturally retarded acetic orformic acid.

• Emulsified acid has primary application in fracture acidizingto retard reaction rate of HCl on limestone and dolomitewithin the temperature range of 80oF to 300oF.


Retaradation of Acid

• Retardation of HCl with Calcium Chloride (CaCl2) –Calcium chloride is beneficial as a retarder when acidizingformation containing anhydrite, because CaCl2 greatlydecreases the solubility of anhydrite.

• Retardation of HCl with CO2 – Carbon dioxide retards HClacid by cooling and by changing the kinetics of reaction.CO2 expands and provides additional clean-up followingacidizing esepcially in low-pressure wells.


Retaradation of Acid

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


Matrix Acidizing

• Primary purpose of matrix acidizing is to remove orbypass damage due to scale, mud, clay, or hydrocarbondeposits, and to restore natural formation permeability.

• Matrix treatments are usually performed by soaking,• 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 fewfeet, volume of acid needed is relativelly small.


Acid Fracturing

• Acid Fracturing is to prop the fracture faces open withsand or glass beads.

• The choice between acid fracturing and conventional• The choice between acid fracturing and conventionalhydraulic fracturing is often a difficult decision.

• If both systems appear equally feasible to obtain desiredfracture flow capacity, then the decision may be based oncomparative costs.


• All carbonate formations can be candidates for acidfracturing

• More suitable candidate wells are as follows:

CANDIDATE SELECTION

• More suitable candidate wells are as follows:

– Poorly performing wells due to low reservoirpermeability

– Wells with restrictions due to damage near the wellbore


POTENTIAL SAFETY HAZARDS IN ACIDIZING

• Hydrogen sulfide, a poison gas, may be produced fromthe reaction of acid on sulfide scale.

• High concentrations can paralyze the olfactory nerves.

• High concentrations can also paralyze other nerves inthe respiratory system.the respiratory system.

Note: (Source Wikipedia)

The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It isinstrumental in the sense of smell.


• Arsenic inhibitor is poisonous if swallowed.

• Contact of arsenic with aluminium or magnesium mayproduce arsine gas in dangerous concentrations.

• Arsine gas is an inhalation hazard and is very deadly.

POTENTIAL SAFETY HAZARDS IN ACIDIZING

• Arsine gas is an inhalation hazard and is very deadly.

• Arsenic inhibitors should generally be avoided because oftheir toxicity and the environmental protection problem.

Note: (Source Wikipedia)

Arsine is the chemical compound with the formula AsH3. This flammable and highlytoxic gas is one of the simplest compounds of arsenic.


DESIGN AND OPTIMIZATION PROCESS

• Acid is injected into the pores and flow channels ofcarbonate rocks at a bottom-hole pressure considerablyless than the fracturing pressure, the purpose being to

Matrix Acidizing

less than the fracturing pressure, the purpose being toincrease uniformly the permeability of the formation.

• Under these conditions, it is assumed that the acid entersonly the natural pores and flow channels, and reacts withthe walls of the pores, dissolving the rock and enlargingthe pores.


Matrix Acidizing

• This reaction slows down as the acid is spent, until finallyadditional radial penetration produces no additionalbenefit.

• Maximum penetration is attained when the firstincrement of injected acid is completely spent.increment of injected acid is completely spent.

• Additional acid will only enlarge the cross-sectional area.


Matrix Acidizing

• In evaluating this type of acidizing,

1. The formation is homogeneous.

2. The pores are of uniform size.

3. The acid penetrates uniformly and radially.

4. The reaction rate declines uniformly with decreasing acid4. The reaction rate declines uniformly with decreasing acidconcentration.

5. The weight of limestone dissolved per increment of distancedeclines uniformly until the acid is completely spent.

• On the basis of the above assumptions, the radial distancethe acid will penetrate before being totally spent dependson the equation.


Matrix Acidizing

• Volume injected, ft3 = Pore volume invaded, ft3

(2.3)(2.3)


Matrix Acidizing

• The only unknown factor in Eq (2.3) is the spending time t,which must be measured for the particular acid in thelaboratory.

• The spending time for an acid depends upon the ratio ofthe area of the rock exposed to the acid to the volume ofacid, here denoted as specific area s = (cm2 / cm3).acid, here denoted as specific area sΦ = (cm2 / cm3).

• Specific surface area can be obtained from the Kozenyequation as modified by Pirson.

(2.4)


Matrix Acidizing

• The formation resistivity factor is related to the porosity by

• Where cementation factor m varies from 1.3 forunconsolidated sands and oolitic limestone to 2.2 fordense 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 ofphosphate, chert, dolomite or iron minerals, including hematite


Matrix Acidizing

• It was found that in matrix acidizing sΦ is so great thatthe spending time for most acids is less than 15 sec.

• To obtain greater penetration during matrix acidizing, itis necessary to either decrease the reaction rate oris necessary to either decrease the reaction rate orincrease the rate of injection of the acid into theformation.


Matrix Acidizing


Acidizing through Preexisting Fractures

• Treatments may be conducted in formations containingnatural fractures.

• Purpose of this type of treatment is to remove secondarydeposition or loose particles in the fracture.deposition or loose particles in the fracture.

• The injection rate is controlled during such treatment so asto not exceed the formation fracturing pressure.

• Maximum penetration of the acid into the fracture isdependent on the spending time of the acid underreservoir temperature and pressure and so on theinjection rate.



• The evaluation of these treatments requires followingbasic assumptions:

1. The fractures are horizontal and of uniform width and extendradially from the wellbore.

2. The acid leak-off into the formation is considered negligible.2. The acid leak-off into the formation is considered negligible.

3. The rate of reaction of the acid is proportional to itsconcentration, and the quantity of rock dissolved form thefracture face decreases with increased acid penetration untilthe acid is spent.



• At an injection rate qi, the radial distance the acid willpenetrate a horizontal fracture until it is spent at time t isfound from the equation

Volume of fractures = Volume of acid injected



• If qi is expressed in barrels per minute, t in seconds, and Win inches.

(2.5)

31

(2.5)

TUNIO, May' 2011,,, Courtesy AP Aung


• For natural fractures, it is safe to assume a fracture widthof 0.1 mm or less.

• Some investigators found that the spending time for mostacids is less than one minute.acids is less than one minute.

• Maximum penetration cannot be calculated accuratelybecause it is impossible to determine the number offractures exposed to the wellbore.


High Pressure Acidizing through Fractures

• In evaluating this type of treatment the followingassumptions 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 thereforeacid entering the matrix from the wellbore is negligible.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 producitivityof reservoirs of extermely low permeability.


ACID-FRACTURING DESIGN

• An acid fracture treatment is designed in the same manneras the hydraulic fracturing treatment, with the additionalcondition for the spending time of the acid.

• Also, as in fracturing, the design is controlled by economic• Also, as in fracturing, the design is controlled by economicfactors.

• The main difference between fracturing and acidizingdesign is the absence of propping agents in the latter case.

• For this, an acidizing productivity ratio will be calculatedconsidering the capacity of the fracture to be infinite.


Example (1)

• Design an acid fracture, given:


fracture radius rf =110 ftfrictional pressure drop is 1657 psi

Example (1)

• Solution: The well before acidizing is producing 20 bbl ofoil per day; it is desired to increase the production to 86bbl per day. The fracture area required is


Example (1)

• The fracture width to be used in area calculations, is

• The term x is• The term x is

• Then the fracturing efficiency is 29 percent, and the totalvolume of acid required is


Example (1)

• The injection rate,

• The acid density is• The acid density is

• And the hydrostatic pressure,


Example (1)

• The bottom-hole treating pressure is

• And the velocity in the tubing is• And the velocity in the tubing is

• The value of Reynolds number is


Example (1)

• The surface injection pressure is

• And the hydraulic horsepower required is


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6. matrix acidizing

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