Acidizing Concepts - Matrix vs. Fracture Acidizing
Authors: King George E. 
Published: December 2007

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology.

Summary
Acidizing involves a wide range of stimulation treatments that canyield impressive production increases in many wells if properly applied. Acidizing treatments are divided into two categories: matrix acidizing andfracture acidizing. To use either treatment properly, an understanding isrequired of what the treatments do and what is necessary to stimulate thewell.

Matrix Acidizing
In matrix acidizing, the intent is to improve or to restore the permeabilityof the region very near the wellbore (a radius of 8 to permeability of theregion very near the wellbore (a radius of 8 to 24 in. [20.3 to 61 cm]) withoutfracturing the producing formation. The increase in permeability will decreasethe pressure drop associated with the production or the injection of fluids byenlargement of the pore throats or by removal of formation permeability damagecreated by drilling or completion fluids. The permeability damage created bydrilling or completion fluids. The amount of production increase that iscreated by a matrix acid job will depend on the reservoir pressure and whetherthe formation permeability next to the wellbore is damaged. In an undamagedpermeability next to the wellbore is damaged. In an undamaged formation, theproduction increase from a matrix acidizing job is very low. However, if theformation permeability near the wellbore is reduced as a result of damage fromnatural causes or completion fluids, the production can be increasedconsiderably (i.e., 10 to 100 times) by removing the damage. In a producingwell, a zone of damaged permeability near the wellbore chokes the convergingradial flow and can decrease production severely. To determine the extent ofpermeability damage (skin damage), a pressure-buildup test must be run. Not allcauses of permeability damage are acid-soluble. For example, sulfate scales, permeability damage are acid-soluble. For example, sulfate scales, paraffin, tar, water blocks, and most emulsions are largely paraffin, tar, water blocks, and most emulsions are largely unaffected by mineral or organic acids. Theseproblems require special treatment in addition to (or instead of) acidtreatments. Also, not all damage shown by a buildup test is true formationpermeability damage. Results of inadequate or too few perforations permeabilitydamage. Results of inadequate or too few perforations will appear in the totalformation damage that is calculated from the test. If there is doubt about thequality of perforations, it is often more economical to reperforate than toacidize. Matrix acid treatments use from 15 to 200 gal [0.19 to 2.5 m3] of acidper foot [meter] of producing formation. The acid is injected at pressures lessthan the pressure that will cause the formation to fracture. Hydrochloric acid(HC1) in strengths from 5 to 15 % is commonly used to remove carbonate and ironscales and as a preflush for HC1/hydrofluoric (HF) acid. To treat clay damageand to remove drilling mud, HF at a strength of 1.5 to 3% is used with theHC1.

Fracture Acidizing
Fracture acidizing is a hydraulic fracturing treatment for carbonateformations in which acid-etched channels serve as very-high-conductivity flowpaths along the face of the fracture. HC1 of 15 to 28% strength is used in acidfracturing at volumes of 100 to 500 gal [1.24 to 6.2 m3] of acid per foot[meter] of producing formation. Hydraulic fracturing actually breaks theproducing formation. Hydraulic fracturing actually breaks the formation withpump- and hydrostatically produced pressures. The pressure pans the formationand produces a crack along which the pressure pans the formation and produces acrack along which the acid flows. The acid reacts with the carbonate, removespart of this reactive rock, and leaves channels along the face of the crack. For the channels to form, the formation must be limestone, dolomite, or chalkwith a total carbonate content of at least 60%. Productivity increases that areavailable from acid fracturing depend on many of the same conditions as waterand proppant fracturing. These conditions are permeability, pressure, viscosityof produced fluid, and length and conductivity of the fracture. In undamaged, low-permeability formations with equal reservoir pressures, fracturing willincrease production far more than pressures, fracturing will increaseproduction far more than matrix acidizing. Higher-viscosity fluids (such asoil) can flow much more easily down the high-conductivity acidizing fracturethan through a high-permeability matrix. The length of an acidized fracture islimited, however, by the rate at which acid is spent on the carbonate and bythe increasing fluid leakoff. The acid-etched fracture will extend only as faras unspent acid has penetrated. The acid reactivity is a function of severalvariables, the most important of which are temperature and the area/volumeratio (i.e., the ratio of the area of reactive formation in the matrix orfracture to the volume of acid in contact with that area). The higher thisratio (more surface area), the faster the acid will be spent (penetration oflive acid is reduced). In the formation matrix of a low-permeability carbonate, the area/volume ratio can be over 30,000: 1. In a hydraulically createdfracture, the value will be about 100: 1. This ratio is a description of theformation or fracture conditions, and very little can be done to alter it. Fluid lost from the acid in the fracture also decreases the possible fracturelength because less fluid is left to extend the possible fracture lengthbecause less fluid is left to extend the fracture. Leakoff from the acid volumein the main fracture is increased by the reaction of the acid, which increasesthe permeability of the leakoff zones. permeability of the leakoff zones. JPT
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