Formation of cisphultic sludge during acid ,stinlli[a~itjtt Iurs been a serious probletn in many areas for' several years. Recent studies have shown that sludge ntay ako aflect resu]ts in many areas where it has not yet been recognized as such, These studies indicate that: (1) sludge is a precipitate of colloidal materiqls present in crude oil;(2) the precipitates occur due to changes in environ- mental" conditions of ihe crude by addition of material.r sucIi as acid; (~) on~e formed, sludge is insoluble in most treating chetniral,s; and (4] sludge can be prevented or controlled by use of stabilizing agents in treating fluid or by use of certain solvents as the outer phase Of acid-in-.. oil enndsions, 1The purpose of this paper will be to show how and why sludge is fotwed and ito w it can be prevented or controlled. Simple laboratory tests to determine tile probability of sludge formation prior to treatment are discussed.

Studies were performed over a temperature range of 100 to 400°F [38 to 204°C] to evalute the effectiveness of commonly used additives for preventing precipitation of ferric hydroxide from spent treating acid. Citric acid. ethylenediaminetetraacetic acid (EDT A), nitrilotriacetic acid (NT A), and erythorbic acid were all shown to be effective iron (Fe) stabilizers at temperatures up to at least 400°F [204°C]. In most cases, efficiency increased with elevated temperature. In contrast, acetic acid performed poorly above 125°F [52°C]. When compared on a weight basis, erythorbic acid was the most efficient agent tested, stabilizing nine times as much iron as citric acid. Unlike the other iron stabilizers, which function by complexation, erythorbic acid prevents ferric hydroxide precipitation primarily by reducing ferric Fe(lll) iron to the ferrous Fe(II) form.

Summary A matrix acidizing system employing fluoboric acid (HBF4) has been developed to stimulate problem sandstone formations. The fluoboric acid hydrolyzes to generate hydrofluoric acid (HF), thus achieving deeper live-acid penetration. Laboratory tests also show that spent fluoboric acid reacts with undissolved clay to reduce cation exchange capacity greatly and render the clay insensitive to incompatible fluids. Introduction Sandstone matrix acidizing long has been used as a means of improving production of oil. and gas by removing formation damage and increasing permeability of the zone immediately around the wellbore. Laboratory and field studies1–4 have demonstrated the effectiveness of this type of treatment. In spite of widespread use, however, many formations do not respond satisfactorily to conventional HCl/HF treatments. This normally is attributed to rapid spending of HF near the wellbore. Some wells initially show good stimulation but later experience an unusually rapid decline in production rate. Such production declines commonly are observed in wells producing from both consolidated and unconsolidated sands. The declines usually are attributed to plugging by migratory clays and other fines. Formation plugging by migratory fines was demonstrated by Krueger et al.5 in both laboratory and field studies. Core test results revealed that high flow rates caused dislodgment of fines with resulting loss of permeability. Krueger also reported field studies in which the production decline rate was accelerated by flowing wells above an optimal rate. As the production rate was increased, a corresponding increase in inorganic solids content of the produced oil also was observed. These observations strongly support the theory of formation damage from fines migration. Migratory fines apparently are released first by exposure to strong acid and later by mechanical forces resulting from the increased flow of produced fluids. Various treatments and production techniques6 have been devised in an effort to control or minimize this problem. Krueger et al.5 and Templeton et al.7 described treating techniques in which wells, following stimulation, were returned to production at a gradually increasing rate to minimize fines migration. Various clay stabilizing agents8,9 also have been applied in an effort to control movement of fines. Also, delayed acting acidizing systems7,10 have been developed to provide deeper live-acid penetration and, thus, remove damaging fines some distance from the wellbore.

Summary Recent studies have shown that the reaction of clay withhydrofluoric (HF) acid is more complex than was earlier believed. It hasbeen found that fluosilicic acid, generated during initial dissolution ofclay, reacts with additional clay, extracting aluminum and depositing hydrated silica. It has been suggested that this silica precipitation may result in significant formation damage. The paper describes studies designed to investigate further the silicadeposition phenomenon as it relates to potential formation damage. Possible beneficial effects resulting from clay stabilization also areexplored. Long core tests show that in spite of extensive silica precipitation, there is little evidence of actual formation damage. Scanning electronmicroscope (SEM) examination of clays situated beyond the zone ofpermeability improvement shows major modification of clay surfaces as aresult of secondary reactions. Sand grains appear clean, however, withsilica deposition apparently confined to the clays. Energy-dispersive X-ray (EDX) analysis of modified clays indicates complete loss of alumin. Bulk analysis of core material, however, reveals that much undissolved aluminum remains through the entire length of the acidized core. Water-sensitivity testing of acidized cores shows almost completestabilization of clays both in the zone of permeability improvement and fora considerable distance beyond. Long-term flow testing reveals no increasein the migration tendency of fines as a result of the acid treatment. Introduction Precipitation of insoluble reaction products from spent HFacid has long been recognized as a major problem insandstone-matrix acidizing. The reaction of HF acidwith clays, feldspars, and other minerals can result in theformation of various insoluble precipitates. Failure toconsider this problem can lead to poor treatment results and, in some cases, severe formation damage. HF acid is usually used in sandstone-matrix acidizing because of its ability to dissolve a variety of siliceousminerals, including quartz, clay, and feldspar. Althoughquartz (SiO2) constitutes the bulk of most sandstoneformations, the greatest portion of the HF acid is consumed by reaction with clays and feldspars because of their muchfaster reaction rate. It has been estimated that thereaction rate of HF acid with clay is 100 to 200 times fasterthan with quartz. As a result, the reaction with quartzis so limited that it is usually ignored, and thegeneralized initial reaction of HF acid with clays in theformation is expelled as Actually, this is only the initial stage of a complexreaction sequence. Depending on free fluorideconcentration, silicon fluorides can exist as SiF4, SiF5 -, andSiF6 --, while the aluminum fluorides are present as Al+++, AlF++, AIF2+... AlF6. Because aluminumhas a greater affinity for fluorine than silicon, the siliconfluorides and more-fluoride-rich aluminum species reactwith undissolved clays, extracting aluminum andprecipitating hydrated silica. Thus the aluminumconcentration in the acid increases with a corresponding decreasein silicon content. As a result, most of the silicon initially dissolved by the HF acid precipitates within the matrixof the rock. That this can be a major cause of formationdamage has been suggested. In addition to Si(OH)4 Precipitation, other insolublereaction products can also form. Among these areNa2SiF6, K2SiF6, and CaF2. All these materials arerelatively insoluble. Damage resulting from theirprecipitation, however, can be minimized by use of an HClpreflush, ahead of the HF acid, to remove any CaCO3present and by avoidance of brines containing Ca, Na, or K during the treatment. However, there is no knownway to prevent precipitation of Si(OH)4, exceptpossibly by returning the spent acid before precipitation occurs. The main purpose of this study was to investigate thedegree of damage caused by Si(OH)4 precipitation and, if necessary, to propose methods of minimizing thisdamage. Formation Damage FromSilica Precipitation Precipitation of hydrated silica from spent HF acid hasbeen the subject of extensive investigation. Theconditions under which Si(OH)4 precipitation occurs havebeen established, and there is little doubt that most of the silicon dissolved by HF acid during acidizing ofargillaceous sandstone is precipitated as hydrated silica beforethe spent acid can be recovered from the formation. Themanner in which the precipitate forms and the magnitude of the resulting formation damage, however, have notbeen thoroughly investigated. JPT P. 1234^

Iron sequestering agents frequently are misused and overused as acid additives. The attitude "it won't hurt and it might help" is no proper basis for choosing an iron control agent because many agents themselves can cause damage following an acid treatment. Introduction Recently there has been increased interest in using chemical additives in acid to prevent secondary precipitation of iron compounds following the acidizing precipitation of iron compounds following the acidizing treatment. Acid readily dissolves iron scale in pipe and also attacks iron-containing minerals in the formation under treatment. This dissolved iron will remain in solution in the acid until the acid is spent. As pH of the spent acid begins to rise, the iron loses its solubility and precipitates. The precipitation of ferric hydroxide or other iron-containing compounds can seriously damage the flow channels recently opened by the acid reaction in the formation. Simple calculations can be made to show that if the acid dissolved the rust in only 5,000 ft of old pipe, this would be enough to deposit more than 100 lb of damaging precipitant in the formation. While there is no question of the stoichemiometry of this calculation, it is an oversimplification of the conditions under which acid is used, and it does not account for the electrochemical behavior of the multiple valence states of iron under oxygen-free, or anaerobic conditions. Iron is a definite hazard to successful acidizing in some areas, but this does not mean sequestering agents should be routinely used unless there is positive evidence of their need. Like many chemical positive evidence of their need. Like many chemical additives for acid, iron-control agents can be misused and overused with damaging results. Some agents precipitate if the expected down hole sources of iron are not present. In some cases the iron actually keeps the sequestering agent in solution in spent acid. Thus, the effective use of iron sequesting agents depends upon the chemical conditions existing down hole during acid reaction. Since it is obviously impossible to know exactly what conditions will be encountered during an acid treatment, it is doubly important that care be used in selecting acid additives based on the best information available. In the case of iron sequestering agents there are several factors that dictate whether or not an agent should be used, and, if so, which one. Several iron-control agents are available at varying costs and each has limitations. There is no single iron-control agent available today that can economically prevent secondary iron precipitation at temperatures from 100 degrees to 250 degrees F without danger of agent precipitation if iron is not found down hole in the precipitation if iron is not found down hole in the expected quantity. This does not mean that iron sequestering agents are ineffective or not needed. It simply indicates some study should go into the decision to use an iron control agent. The thoughtless and excessive use of sequestering agents, like many other acid additives, can ruin an otherwise successful acid treatment. The Electrochemical Nature of Iron Underlying the need of an iron-sequestering agent is the chemical behavior of iron down hole when contacted with acid. JPT P. 1121