Precipitation of Hydrated Silica From Spent Hydrofluoric Acid: How Much of a Problem Is It? (includes associated papers 16441 and 16444 )
Authors: Crowe C.W. 
Published: December 2007

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^