6. MATRIX ACIDIZING
6. MATRIX ACIDIZING
LESSON OUTCOMES
• At the end of this section, the students will be able to : • Understand different acid solutions.
• Design acidizing treatment. • Design acidizing treatment.
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.
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.
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.
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
1. Matrix Acidizing
• Removal of severe plugging in sandstone, limestone, or dolomite can result in a very large increase in well productivity.
productivity.
• If there is no skin damage, a matrix treatment in limestone or dolomite could stimulate natural production
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 • 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
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
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
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,
2. Acid Fracturing
• To combat fracture closure in uniformly soluble carbonates, hydraulic fracturing and propping should be considered, as well as special acidizing techniques 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.
ACID ADDITIVES
• The use of acid can create a number of well problems; 1. Release fines that plug the formation
2. Form emulsions 2. Form emulsions 3. Corrode steel
• Additives are available to correct these and a number of other problems.
RETARDATION OF ACIDS
• 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.
Retardation of Acid
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.
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.
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.
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, • 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
Acid Fracturing
• Acid Fracturing is to prop the fracture faces open with sand or glass beads.
• The choice between acid fracturing and conventional • 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
• All carbonate formations can be candidates for acid fracturing
• More suitable candidate wells are as follows:
CANDIDATE SELECTION
• More suitable candidate wells are as follows:
– Poorly performing wells due to low reservoir
permeability
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. 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.
• 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.
POTENTIAL SAFETY HAZARDS IN ACIDIZING
• 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)
DESIGN AND OPTIMIZATION PROCESS
• 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
Matrix Acidizing
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.
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. increment of injected acid is completely spent.
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 acid 4. 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.
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 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 s = (cm2/ cm3).
acid, here denoted as specific area sΦ = (cm2/ cm3).
• Specific surface area can be obtained from the Kozeny
equation as modified by Pirson.
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.
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 is necessary to either decrease the reaction rate or increase the rate of injection of the acid into the formation.
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.
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.
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. 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.
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
Acidizing through Preexisting Fractures
• If qi is expressed in barrels per minute, t in seconds, and W in inches.
(2.5) (2.5)
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.
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.
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. 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
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 • 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
Example (1)
• Design an acid fracture, given:
fracture radius rf =110 ft
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
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 total
Example (1)
• The injection rate,
• The acid density is • The acid density is
Example (1)
• The bottom-hole treating pressure is
• And the velocity in the tubing is • And the velocity in the tubing is
Example (1)
• The surface injection pressure is
