Introduction to Acidizing

Learning Objectives After completing this topic “Introduction to Acidizing“, you will be able answer to: What is acidizing process?What is acidizing in drilling?What is the difference between hydraulic fracturing and acidizing?What are the differences between matrix acidizing and fracture acidizing?Which acid is used in acidizing process?What is iron control agent?How is HCl used in fracking?What is SandStone acidizing?What is in hydrochloric acid?What is stimulation in oil and gas?What is acid job in oil industry?How do you make hydrofluoric acid?What are the objectives of matrix stimulation? By completing this topic and then applying the learnings in your job, you will be able to attain Basic Application Competence Level in Acidizing and Other Chemical Treatments. Background and Definitions Acidizing is a chemical process that dissolves unwanted substances in the formation, pipe, surface equipment, or flow-lines. In the case of treating formation damage, acid removes flow-restricting particles, scale deposits, and minerals in the reservoir or in the immediate wellbore vicinity. The acid mixture holds the dissolved substance in solution until it is either removed from the well system or overflushed a safe distance into the formation; then, the well can produce at its natural potential. Little, if any, increase in productivity will result unless formation damage actually exists. When a fracture is created during an acidizing operation in carbonate reservoirs, increased flow capacity can result from acid etching of the fracture faces. Here, large increases in productivity are possible by creating a highly conductive flow path through the formation. Acidizing is among the oldest techniques in well stimulation processes, having been introduced in the late nineteenth century. Table 1 (below) lists the major events in the early history of acidizing. YearEvent1894Acid was used to stimulate oil production in limestone formations in Lima, Ohio.1896First patent related to acid treatments of wells was issued to Herman Frasch.1928Acid was used in Glen Pool, Oklahoma by Gypsy Oil Company, a subsidiary of Gulf Oil Company.1929Subsidiary of Gulf Oil Corporation injected uninhibited hydrochloric acid under pressure into a well in Lee County, Kentucky.1930Acidizing experiments were discontinued.1932Dow Chemical Company and Pure Oil Company used an acid treatment of 15% hydrochloric acid plus arsenic as an inhibitor on a well in the Greendale Pool of Midland County, Michigan.1932Dow Chemical Company and Pure Oil Company used an acid treatment of 15% hydrochloric acid plus arsenic as an inhibitor on a well in the Greendale Pool of Midland County, Michigan. -Patents were issued to Grebe and Sanford of Dow Chemical Company for treatments of oil wells with chemicals containing a corrosion inhibitor. -R.H. Carr of the Pure Oil Company was issued a patent on the technique of chemically treating an oil well by the balanced fluid column method. -Dowel I Chemical Company formed Dowel I Inc. (now Dowell-Schlumberger from its Well Service Group — whose original main responsibility was stimulation of Dow's brine wells. -Carey K. West formed the Chemical Process Company, a predecessor to Byron Jackson, Inc. (now BJ-Hughes).1933J.R. Wilson with Standard Oil Company filed a patent on a technique for treating sandstone formations with hydrofluoric acid.1935Halliburton Oil Well Cementing Company (now Halliburton Services) began acidizing oil wells.1940Dowell (now Dowell-Schlumberger) began the first commercial use of mixtures of hydrochloric and hydrofluoric acid.Table 1: Early History of Acidizing Types of Acid Treatments https://youtu.be/4wo5RzxLDf8 Acid Treating Solutions Whatever their specific formulation, all acids have hydrogen ions that react with carbonate rock (principally limestone and dolomite) to form water and carbon dioxide, and with siliceous minerals (sandstone and clays) to form fluosilicic acid and water. Mineral Acids https://youtu.be/mODeOhSi8Oc Hydrochloric Acid Hydrochloric acid (HCl) is the workhorse of the stimulation business, finding extensive use in both carbonate and sandstone acidizing. Chemically, hydrochloric acid is hydrogen chloride in aqueous solution. Composed of hydrogen and chlorine gas, it is gaseous in the chemically combined state. Hydrogen chloride gas is readily soluble in water. Concentration is described as percent by weight of HCl gas dissolved in water. HCl acid is commercially available; its concentration has been standardized at 20° Be, i.e., 31.45% acid, and it has a specific gravity of 1.160. Many acidizing treatments employ HCl to some extent. Usually, it is used as a 15% solution when treating carbonate formations. This concentration, commonly referred to as regular acid, was originally chosen because of inadequacies in early inhibitors and difficulty of preventing corrosion of well tubulars by more concentrated solutions. Fifteen-percent HCl is used most frequently because it: costs less per unit volume than stronger acids and is less costly to inhibit;is less hazardous to handle than stronger acids;retains larger quantities of dissolved salts in solution after spending. Although acid concentrations higher than 15% may also be used in carbonate formations, lower strengths are more suitable where acid dissolving power is not the sole consideration. In sandstone acidizing, for example, 5 to 7.5% HCl is often used ahead of hydrochloric-hydrofluoric acid mixtures to prevent precipitation of formation-plugging reaction products. The principal disadvantage of HCl is its corrosive action on wellbore tubular goods. HCl reacts with limestone (CaCO3) to form calcium chloride (CaCl2), water (H2O) and carbon dioxide (CO2): As shown, 2 moles of hydrochloric acid reacts with 1 mole of CaCO3 to produce 1 mole each of calcium chloride, water and carbon dioxide. The molecular weights of these compounds are as follows: CompoundMolecular WeightHCl36.47CaCO3100.09CaCl2110.99H2O18.02CO244.01 Thus, to dissolve one pound of limestone, we would need [latex]\left [ \left ( 1\ lb\ CaCO_3 \right )\cdot \left ( \dfrac{mole}{100.09\ lb} \right ) \right ]\cdot \left [ \left ( \dfrac{2\ mole\ HCl}{mole\ CaCO_3} \right )\cdot \left (36.47\dfrac{lb}{mole} \right ) \right ]=0.729\ lb\ HCl[/latex] On a volumetric basis, for the case of a 15 percent HCl solution (specific gravity = 1.076 at 60°F; density = 8.96 [latex]\tfrac{lb}{gal}[/latex]), one gallon of acid will dissolve [latex]\dfrac{8.96\cdot 0.15}{0.729}=1.84\ lb\ CaCO_3[/latex]. The reaction of HCl on dolomite is very similar to its reaction on limestone, except for the presence of magnesium. The chemical reaction of HCl and dolomite CaMg(CO3)2 is: [latex]4 HCl + CaMg(CO_3)_2 \Rightarrow CaCl_2 + MgCl_2 + 2H_2O + 2CO_2[/latex] [latex]\quad[/latex](1) Hydrofluoric Acid Hydrofluoric acid is the primary dissolving chemical used in sandstone acidizing. In these applications, HF is usually mixed as a dilute solution with HCl, or an organic acid. Its principal use is to dissolve siliceous minerals. Within the chemical industry hydrofluoric acid (HF) is available commercially as a relatively pure material in anhydrous form (where it is fuming and corrosive) or as a concentrated (40 to 70%) aqueous solution. Anhydrous HF is not used in the field because its low boiling point of 66.9° F is often exceeded by ambient temperatures. HF reacts with silica and silicates, such as glass and concrete. It also attacks natural rubber, leather, cast iron, and many organic materials. HF is the only acid that reacts either siliceous minerals such as sand and clays. The reaction of HF on quartz (SiO2), a primary component of sand, is: [latex]4HF + SiO_2 \rightarrow SiF_4 + 2H_2O[/latex] [latex]SiF_4 + 2HF +\rightarrow H_2SiF_6[/latex] [latex]\quad[/latex] (2) Here, the end reaction product, fluosilicic acid (H2SiF6), is soluble in water, but its potassium and sodium salts are insoluble. HF contact with formation water containing CaCl2, MgCl2, NaCl or KCl thus must be avoided near the wellbore. The only salt solution compatible with HF is ammonium chloride. The reaction of HF on bentonite clay (Al2(Si4O10)(OH)2) is: [latex]36HF + Al_2\left ( Si_4O_{10} \right )\left ( OH \right )_2 \rightarrow 4H_2SiF_6 + 2H_3AlF_6 + 12H_2O[/latex] [latex]\quad[/latex] (3) Hydrofluoric acid also reacts with carbonates; its reaction with limestone is: [latex]2HF + CaCO_3 \rightarrow CaF_2 \downarrow + H_2O + CO_2[/latex] [latex]\quad[/latex] (4) HF will react with calcium ions from any source to form the insoluble precipitate calcium fluoride (CaF2, which can cause sever plugging problems in the formation. The portion of the reaction that creates calcium fluoride is: [latex]Ca^{++} + 2F\,^- \rightarrow CaF_2[/latex] [latex]\quad[/latex] (5) CaF2 formation can be avoided through the proper use and optimal placement of HCl in the acid treatment. Hydrochloric-Hydrofluoric Acid Mixtures Hydrochloric-hydrofluoric acid mixtures are the chief solutions used in matrix acidizing of sandstone. These acids are very often used as mixtures because HCl ties up and displaces anions, which in turn prevents reaction of these anions (e.g., Ca++, Mg++, etc.) with HF. Although HF is the only acid effective on siliceous minerals, it is ineffective as a stimulation agent on carbonates because its reaction forms insoluble CaF2. The usual HF solution used in the field contains 3% HF and 12% HCl. This solution is commonly called regular mud acid. However, concentrations of HF in HCl solutions range from 0.5 to 9%, and some operators prefer 1-1/2% HF and 6% HCl. These mixtures may be formed from the dilution of concentrated solutions of HF or, more frequently, from the reaction of ammonium bifluoride (NH4HF2) or ammonium fluoride (NH4F) with HCl. Usually, 15% HCl is used, and enough NH4HF2 or NH4HF is added to create a solution containing 3% HF. Consumption of hydrogen chloride by these reactions leaves 12% HCl remaining in solution. Similarly, 1.5% HF can be prepared from 7-1/2% HCl solutions where the final HCl concentration is 6%. Corrosion characteristics of HCl-HF mixtures are comparable to those of HCl alone, and similar corrosion inhibitors are required. Both HCl and HCl-HF acid mixtures are stronger and more reactive than organic acids. Organic Acids https://youtu.be/b46pxisdbKs Organic acids offer the benefit of relatively low corrosivity and easy inhibition at elevated temperature. They are particularly applicable, therefore, to high-temperature well applications. Acetic (HC2H3O2) and formic (HCHO2) acids are the two principal organic acids used in acidizing. These acids are weaker than mineral acids; they react more slowly and are also less damaging. Acetic, for example, is the only acid available that will not damage chrome plating or aluminum and magnesium surfaces. In some cases, organic acids are mixed with HCl or HF to provide deeper acid penetration into the formation. Acetic Acid Acetic acid was the first organic acid to be used in appreciable volumes in acidizing. It is easy to inhibit against corrosion and commonly used as a 10% by weight solution of acetic acid in water. At this concentration, the reaction products are generally soluble in spent acid. Acetic acid functions well as a perforating fluid or low-corrosivity acid when in contact with metals that corrode easily. Commercially available "pure" acetic acid is approximately 99% acetic acid. It is called glacial acetic acid because icelike crystals form at temperatures around 60° F and it solidifies near 48° F. When glacial acetic acid is mixed with water, a contraction occurs. For this reason, the amounts of acetic acid and water normally total more than the required volume when making dilute solutions. Acetic acid reacts with limestone to form calcium acetate (Ca(C2H3O2)2), water and carbon dioxide: [latex]2HC_2H_3O_2 + CaCO_3 \rightarrow Ca(C_2H_3O_2)_2 + H_2O + CO_2[/latex] [latex]\quad[/latex] (6) As shown, 2 moles of acetic acid reacts either 1 mole of CaCO3 to produce 1 mole each of calcium acetate, water and carbon dioxide. The molecular weights of these compounds are as follows: CompoundMolecular WeightHC2H3O260CaCO3100.09CaCl2110.99H2O18.02CO244.01 Thus, to dissolve one pound of limestone would require: [latex]\left[ (1 lb.\, CaCO_3) \cdot \left( \dfrac{mole}{100.09\, lb.} \right) \right] \cdot \left[ \left( \dfrac{2\, mole\, HC_2H_3O_2}{mole\, CaCO_3 }\right) \cdot \left( \dfrac{60\, lb.}{mole} \right) \right] = 1.2\, lb. \,HC_2H_3O_2[/latex] On a volumetric basis, for the case of a 15 percent HC2H3O2 solution (specific gravity = 1.037 at 60º F; density 8.64 [latex]\tfrac{lb.}{gal}[/latex]), one gallon of acid dissolves [latex]\tfrac{8.64 \ \cdot \ 0.15}{1.2}= 1.08\, lb.[/latex] of limestone. An identical concentration of HCl dissolves 1.84 lb. of limestone. (see Table 1). Formic Acid Formic acid is the simplest of the organic acids. It is stronger than acetic, yet weaker than HCl, and is most frequently used in combination with HCl as a retarded acid system for high-temperature wells. Formic acid can be easily inhibited, but not as effectively as acetic acid at high temperatures and long contact times. The percentage of formic acid generally used is 8% to 10% by weight. A 10% solution of formic acid will dissolve as much limestone as an 8% solution of HCl. The chemical reaction of formic acid and limestone is: [latex]2HCHO_2 + CaCO_3 \rightarrow Ca(CHO_2)_2+ H_2O + CO_2[/latex] Acid typeConcentration %CaCO3 dissolved per gal. acid- lb.CO2 formed per gal. acid cu. Ft.CaCl2 formed per gal. acid- lb.HCl*15*1.846.992.04 202.509.472.75 253.2212.203.57Acetic*15*1.084.091.71 201.435.412.25 251.806.822.84Formic*15*1,425.381.84 201.907.202.47 252.409.093.12Reaction of three acids on limestone at various concentrations. * The most commonly used strength for HCl is 15% and 10% for acetic and formic acids. In this table, however, the HCl reaction rate on limestone is compared to the equivalent strength rates for acetic and formic acid rather than to the latter's commonly used strengths. Formic acid is frequently used in reservoirs with extreme temperatures or low injection rates. At high temperatures, organic inhibitors work more effectively on formic acid with HCl than on HCl alone. This property minimizes the danger of hydrogen embrittlement of steel associated with HCl treatment in high-temperature formations. Organic acids have less dissolving capability than HCl and must be used in greater volumes for comparable results, making such treatments more expensive. The dissolving power of various concentrations of HCl, acetic, and formic acids is shown in Table 1. The cost of acetic acid per unit based on dissolving power is more expensive than either HCl or formic acid. With the exception of fracturing acidizing treatments, dissolving power is not often the primary consideration in acid selection. The most commonly used strength for HCl is 15% and 10% for acetic and formic acids. In Table 1, however, the HCl reaction rate on limestone is compared to the equivalent strength rates for acetic and formic acid rather than to the latter's commonly used strengths. Hydrofluoric-Organic Acid Mixtures Acid mixtures of HF and either acetic or formic acid are used to slow the reaction of the acid on sand and clay, and to reduce corrosiveness. These acid mixtures can be effectively inhibited for up to 16 hours at 400° F. Mixing HF with organic acids can provide deeper penetration and, therefore, effective removal of deep formation damage. Organic HF mixtures are recommended at temperatures of 200° F and above; below 200° F they can cause the formation of undesirable reaction products. Specialty Acids Specialty acids are designed to deal with specific formation conditions, such as deep clay damage, paraffin blockage in the reservoir, and situations requiring a retarded acid. Powdered Acid Powdered acids (sulfamic and chloroacetic) are used in applications where logistics are of primary concern - for example, in remote locations where bulk transport of liquids is impractical or very expensive. These crystaline powders are readily soluble in water. Sulfamic acid (HSO3NH2) is a nonvolatile, nonhydroscopic, white crystalline or granular solid. Its reaction products with carbonates are water soluble, and it is highly ionized in water. Sulfamic acid is less corrosive than HCl, yet its strength is about the same. At 77° F a saturated solution of sulfamic contains about 18% hydrochloric acid equivalent. Sulfamic acid reacts with limestone to form calcium sulfamate cvs, water and carbon dioxide [latex]2HSO_3NH_2 + CaCO_3 \rightarrow Ca(SO_3NH_2)_2 + H_2O + CO_2[/latex] [latex]\quad[/latex] (7) One advantage of sulfamic acid is its solid physical state. Solid sulfamic acid can be transported and stored without special equipment or tanks, and mixed with water at or near the wellsite as needed. Sometimes it is cast into "acid sticks" for easy introduction into the wellbore. The disadvantages of sulfamic acid include its decomposition at around 180° F, making it inappropriate for applications where formation temperatures are above 160° F. Moreover, HCl offers more dissolving power and comparable reactions at lower cost. Chloroacetic acid is generally preferred to sulfamic acid when use of a powdered acid is appropriate, because it is stronger and more stable. Retarded Acids Retarded HF acid can penetrate deeper into a formation than conventional HF to remove siliceous solids. Retardation of HF achieves deeper penetration of unspent acid, can aid more complete formation damage removal, and further increase production. A number of retarded HF acid systems are commercially available several of which are: SGMA (self-generating mud acid): The first retarded sandstone-acidizing system to be used extensively was developed by Shell Oil Company. It involves pumping ammonium fluoride and an organic ester, methyl formate, into the formation. (Methyl formate has a very low flash point and should be pumped with caution.)In time, ester hydrolysis produces formic acid. This acid reacts with ammonium fluoride to form HF, which then dissolves clays or any siliceous minerals it contacts.Claysol retarded HF(Halliburton): A retarded clay-dissolving system which utilizes ion-exchange properties of clay minerals to generate HF on clay in situ. Since HF is formed on clay surfaces, little sand is dissolved by this process. HF is created by sequentially injecting a volume of 3% ammonium fluoride followed by an equal volume of 5% HCl. This process dissolves clay in the formation as deep as a set of sequences can be pumped without completely mixing together.Clay acid(Dowell-Schlumberger): A retarded acidizing fluid using fluoboric acid (HBF4) for matrix acidizing of sandstone formations. HBF4 is usually applied as an afterflush to an HF treatment. Upon entering the formation, HBF4 slowly hydrolizes to generate HF. Deep acid penetration to remove fines is possible because of the slow generation of HF. Unlike clay-control agents, HBF4 also produces a chemical fusion of both fines and clay platelets, which provides fines/clay stabilization.RHF acid solution(Nowsco): A retarded HF system for treating sandstone formations suffering deep damage caused by migration and/or swelling of siliceous minerals. This single-stage treatment does not require sequencing or "shut-in" time for hydrolysis reactions. Addition of aluminum chloride(AlCl_3) to an HF acid solution forms aluminum fluoride complexes, similar to those formed in spent regular mud acid, which retards the reaction rate of HF with siliceous materials. Retarded Hydrochloric Acid The HCl reaction rate on carbonate formations can be retarded by gelled acid, oil-wetting formation solids, or emulsifying acid with hydrocarbon. Gelled Acid Systems: Gelled acids are used to retard acid reaction rate during fracture acidizing. Retardation occurs because increased fluid viscosity reduces the rate of acid contact with the fracture face. Other related advantages include: reduced leak-off ratedeeper penetrationbetter cleanup of fines or solids transport Viscosifying agents normally associated with gel led acid systems consist of natural polymers, synthetic polymers, cross-linking agents, and surfactant gelling chemicals. Chemically Retarded Acids: When it is desirable to extend the spending time of an acid system, a chemical retarding agent may be used. Most chemical retarders are anionic surfactants (such as sulfonates or sulfates). These oil-wetting surfactants adsorb onto a carbonate to create a physical barrier to acid contact with the rock surface. The adsorption mechanism on a calcium crystal may look like the illustration in Figure 1 (absorption mechanism). FIGURE 1 Emulsified Acids: Emulsified acids, obtained by emulsifying acid and a hydrocarbon, are effective over a wide range of bottomhole temperatures. The system may be either oil or acid external, but the most common form is an oil-external emulsion. Oil-external emulsion is composed of hydrocarbon-base fluid (refined or crude) as the continuous phase and HCl as the internal phase. This acid system retards the reaction of acid with the formation by decreasing the amount of acid in contact with the rock. As the acid spends, the emulsifier reacts with the resulting CaCl2 solution, releasing its emulsifying properties and therefore causing the emulsion to break.Acid-external emulsion is composed of acid as the external phase, where selection of acid depends on the well conditions involved. The acid phase may account for 80 to 90% of its total volume. Ordinarily, aromatics, such as toluene or xylene, are used as the hydrocarbon or internal phase. This acid system is used primarily to remove hydrocarbon materials like paraffin, congealed oil, and other deposits so acid can contact acid-soluble materials. Foamed Acid Foamed acid has widespread application for effective fracture acidizing in carbonate reservoirs. Both oil and gas wells have responded successfully to foamed acid stimulation treatments. Foamed acid is particularly beneficial in low-pressure, low-permeability, liquid-sensitive formations because of the following advantages: High Viscosity: While often difficult to achieve with conventional gelling agents in acid, high apparent viscosities are easily developed by foaming the acid. Higher viscosities may result in wider fractures, better fluid-loss control, and better solids-carrying capabilities.Reduced Fluid Loss: The high apparent viscosity of foamed acid results in reduced fluid loss, which allows deeper acid penetration. In low-permeability reservoirs, foam bubbles may be sufficient to decrease leakoff into the formation matrix.Improved Solids Transport: In conventional acid treatments, large amounts of insoluble fines can be left in the formation because of low-viscosity spent acid. These solids can reduce fracture conductivity. Foamed acid and high viscosity suspend and remove more fines from the well during cleanup.Better Cleanup: Foamed acid offers built-in gas assist. This means faster cleanup. It may even eliminate the need to swab the well after acidizing.Less Damage: Foamed acid has a low liquid content. Normally, the volumetric gas content(referred to as foam quality) is 60 to 80 quality (i.e., 60% to 80% gas and 40% to 20% acid). Experience with foamed acid has shown that increasing acid viscosity with conventional gelling agents before foaming can help increase foam stability. The longer foamed acid, live or spent, is allowed to remain static, the easier it is for acid to drain from the foam bubbles. This foam destabilization (liquid/gas separation) allows suspended fines to settle out. Nitrogen is the most common gas used to make foamed acid. Foam quality is generally between 60% and 80%, although qualities as high as 95% have been used. The acid phase of the foam may contain 0.5% to 1.0% surfactant and 0.2% to 2.0% corrosion inhibitor. Turflo/Hydrofluoric Acid As stated earlier, HCl-HF mixtures react with both siliceous and carbonate materials. In most sandstone formations carbonate content is small, and an HCl preflush removes all carbonate material. HCl may not, however, do the job effectively in formations with high limestone or dolomite content. These carbonates can reduce the effectiveness of a HCl-HF treatment by increasing the chance of secondary reactions and by limiting the amount of clays removed. Turflo/hydrofluoric acid, a phosphoric acid-HF combination, is specifically designed for formations with high carbonate content. Tests with this system have shown its reaction rate is much slower than that of normal 12% HCl-3% HF with limestone and nearly as fast with silicas. Since Turflo/HF is slightly slower reacting than HF acid, deeper penetration of HF is possible. Turflo/HF is more expensive than conventional HF, but its limited reaction rate with carbonates allows for improved formation damage removal in sandstone formations with high carbonate content.. However, caution should be exercised because precipitation problems may subsequently cause severe permeability damage. Assessment 1. One of the benefits of organic acids is their application in high temperature reservoirs. A .True ✔B .False 2. Which of the following is an organic acid? A .SulfuricB .Acetic ✔C .HydrochloricD .Hydroflouric 3. No acid dissolves sandstone. A .TrueB .False ✔ 4. It is relatively easy to make a foam with acid for such applications as acid fracturing. A .True ✔B .False Recommended for You Hydraulic Fracturing Fundamentals Production Logging of Multiphase Flow in Horizontal Wells