Summary A series of experiments was conducted in which foam quality, foam reactivity, diverting-stage volume, core permeability, and treatment design were varied. The goal was to determine how these parameters affect diversion in competing limestone cores. Situations involving natural permeability differences and diversion to damaged cores were considered. Experiments indicate that the minimum foam quality required for useful resistance to acid flow is 60%; foams of 70% or 80% quality were the most effective for diversion. Both acid and brine foams were used. Acid foams provide good diversion and can be included easily in a treatment, while in high-porosity, low-permeability limestones like chalk, brine foams appear to be more effective than acid foams. One can expect to maintain good diversion up to a permeability ratio of 10. Results also show that both types of foam effectively divert acid to damaged cores. Introduction Diverting agents have been used in stimulation treatments for many years to improve coverage across a zone of interest. Traditionally, these diverting agents have consisted of solid materials that are soluble in the produced fluids. Common diverting agents of this type include salt, oil-soluble resins, and benzoic acid. The use of foam slugs, proposed more than 20 years ago as a diverting agent, became an accepted treatment method during the mid-1980's.The slugs originally were used to divert HF acid in sandstone formations with gravel-pack completions. In such cases, salt was not viable because it is incompatible with HF acid. Benzoic acid was used occasionally but was difficult to obtain in a particle size that would easily pass through the sandpack. Oil-soluble resins have been used successfully but must be used where oil production exists. Hence, oil-soluble resins cannot be used in dry-gas wells or in wells with a bottomhole static temperature above the resin softening point. The use of foam in other applications showed that foam could pass through a gravel pack easily but would have difficulty flowing into a formation. Thus, foam was introduced and has found use as a successful diverting agent in sandstone acidizing for almost any type of completion or production.
Summary The purpose of this work was to determine the solubilities of sodium and potassium fluosilicate over a broad range of HCl concentrations, ionic strengths, and temperatures. These data were required for determining optimum or safe HCl/HF treating-fluid compositions for acidizing sandstone formations containing feldspars. Solubilities of sodium and potassium fluosilicate were determined over a temperature range of 150 to 300°F. Analysis of the solubility data indicated that the solids formed were not simply Na2SiF6 and K2SiF6. Regression analyses indicated that the sodium salt could best be described as NaHSiF6 · 0.52HCl (or Na0.48H1.52SiF6· 0.52NaCl). Regression analyses indicated that the potassium salt could best be described as KHSiF6 · 0.41KCl (or K1.41H0.59SiF6 · 0.41HCl). The solubility data were then included in a HF-spending simulator to determine regions where possible precipitation of the fluosilicates could occur. Optimum HCl/HF concentrations were then determined for specific mineral and temperature conditions. Introduction The results from a comprehensive study discussed the analysis of HF returns from an acid treatment. Of particular interest was the absence of excess acid and silicon accompanied by a high abundance of both aluminum and sodium in essentially undiluted spent HF returns. This was unexpected because the primary reaction of HF with feldspar predicts that most of the fluoride is spent on silicon.
The standard description of HF acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alumino-silicates. Field experience has taught our industry that possible precipitation during the secondary reaction can adversely affect treatment success. This statement has been particularly true in formations with high K-feldspar content or formations having temperatures above 300°F. Recent work has also reported the existence of a third, or tertiary reaction of HF with alumino-silicates. This paper reports how the rate law and kinetics for this tertiary reaction are determined on kaolinite and feldspar over a broad temperature range. This document also discusses the discovery of how most clays were thermally unstable to HCl at temperatures above 250°F. These findings were made possible by recently applied experimental techniques including F NMR Spectroscopy, fractional pore volume flow experiments, and an accurate knowledge of the HF stoichiometry.
This paper (SPE 52397) was revised for publication from paper SPE 38597, first presented at the 1997 SPE Annual Technical Conference and Exhibition held in San Antonio, Texas, 5-8 October. Original manuscript received for review 17 October 1997. Revised manuscript received 8 July 1998. Revised manuscript approved 5 August 1998. Summary With the intent of encouraging and helping the reader to design a radial model tailored to a particular use, this paper discusses and describes radial models that have proved useful in exploring and simplifying the application of the kinetics of sandstone acidizing. With the recent development of a comprehensive theory for the complete chemistry and kinetics of sandstone acidizing, it was appropriate to develop useful radial models for exploring and simplifying the application of the new technology. These radial models were designed to accommodate both the positive and negative aspects of the recent discoveries. These discoveries included ion-exchange transformation of brines, decomposition of clays in hydrochloric acid, precipitation of fluosilicates, removal of carbonate to prevent the precipitation of complex aluminum fluorides (AIFx's), silica-gel filming, colloidal silica-gel precipitation, precipitation of the various complex AIFx's, mixing between the various stages of the treatment, damage removal, permeability modeling, and skin evolution. Calculations were initially performed with a comprehensive workstation-based computer model so that the magnitude of the various effects could be measured. These calculations showed that many negative effects could be minimized or eliminated by special design considerations. A PC-based model was then written that could help design engineers avoid serious problems when acidizing sandstone formations and provide them with visual guidance to some of the other issues that were relevant. This avoiding problems approach for the PC-based radial model provided a powerful basis for introducing advanced designs for sandstone acidizing that have dramatically improved the success rates of field treatments. In addition to choosing the correct HF fluid to avoid secondary precipitations, considerations for choice of preflushes to condition the formation proved essential. Proper choice of preflush is based on acid stabilities of the clays, carbonate removal, and the avoidance of deep clay swelling. Appropriate acid-preflush volumes help ensure carbonate removal for at least 24 in. and provide sufficient fluid spacing between formation brine and the spent HF fluids at the end of the treatment. P. 272
Summary The standard description of HF acidizing chemistry clearly demonstrates a primary and secondary reaction of HF with alumino-silicates.1–7 Field experience has taught us that possible precipitation during the secondary reaction can adversely affect treatment success,8 especially in formations with high K-feldspar content or temperatures above 300°F. Recent work has also reported the existence of a tertiary reaction of HF with alumino-silicates.9 This paper reports the determination of the rate law for the secondary reaction and the kinetics for that reaction on kaolinite and feldspar over a broad temperature range. In addition, the kinetics of fluosilicate precipitation during the secondary reaction were studied and found to be fast but not instantaneous. These findings were made possible by recently applied experimental techniques including 19F nuclear magnetic resonance spectroscopy,9 fractional pore-volume flow experiments,10 and an accurate knowledge of the HF stoichiometry.11 Laboratory findings have been verified based on detailed analysis of flowback samples from field treatments.12 The reaction rate for the secondary reaction is much slower on feldspars than on clays. At temperatures below 125°F, the secondary reaction rate is very slow, which makes spent HF susceptible to fluosilicate precipitation if the HF is mixed with brines as it often is for disposal or injection well treatments. The difference in the kinetics between clays and feldspar allows one to design treatments at lower temperatures based on just the clay fraction. At temperatures above 150°F, the average mineralogy, clay mineralogy, and feldspar mineralogy should each be evaluated separately for potential fluosilicate precipitation. The fluid of choice is the one that passes all three conditions. The rate law indicates that the speed of the reaction is independent of HCl concentration and would proceed to completion even in the absence of HCl. Chemically, one can accomplish this effect by using water as a proton source and by adjusting the stoichiometry based on a previously reported equilibrium.11 The kinetics for fluosilicate precipitation were fast, but not instantaneous. At very high temperatures, the secondary reaction is fast enough that fluosilicate precipitations can largely be avoided. Unfortunately, the silica gel precipitation then occurs so close to the wellbore that damage to permeability will likely occur. The equations recently developed for the HF acidizing process have been incorporated into a fully kinetic geochemical radial flow simulator. Calculations performed with this simulator have confirmed that a good knowledge of the formation mineralogy and acidizing kinetics is necessary for proper optimization of HF acidizing treatments. Introduction Fundamental research into the detailed chemistry of HF acidizing has produced critical information that renders much of the earlier geochemical and engineering modeling incomplete.13–16 Geochemical models that rely on global thermodynamic completeness of reactions ignore the effects of reaction kinetics and have been unable to reproduce the observed chemical species. Engineering models have assumed mass transfer-controlled kinetics in the absence of any real information about reaction-rate laws and surface-controlled kinetics. In addition, some of these latter models have required empirically adjustable parameters to reproduce short-core effluent analyses and are not based on realistic chemistry. Now that the chemistry of HF acidizing is more accurately understood, reaction-rate laws and kinetics are required so that engineering models can be developed that will help improve treatment design and performance. Therefore, experiments were designed that allowed the relevant reaction-rate laws to be determined. However, with the discovery of essentially "three reactions" of HF, the experiments had to isolate the individual reactions. Recent work has documented the rate law for the tertiary reaction and the importance of acid decomposition of clays.17 This paper documents the experimental determination of the rate law for the secondary reaction and the consequences of this rate law. Equipment and Method Column Flow Tests. Flow tests with packed columns were conducted with the equipment and method reported earlier.10,11 The general procedure for the tests was to flow a preflush of HCl ahead of the reactive fluid of interest. The reactive fluid was followed by an overflush of HCl and final displacement of the fluids from the column with NaCl brine. The reactive fluid was a mixture of HCl and HsiF5 that would allow the testing of chemical theories. The reactive fluid was typically about 0.9 pore volumes (PV). Kaolinite was purchased as a high-quality, finely divided powder while chunks of feldspar were ground to a powder finer than 200 mesh. Berea Core Flow Tests. Flow tests were conducted with the same equipment and method as the packed-column flow tests. However, instead of packed material, the Hassler sleeve was loaded with a Berea core 12 in. long and 1.5 in. diameter. The general procedure for the tests was to flow a preflush of HCl to remove the carbonates from the core. The reactive fluid of interest was flowed next followed by an overflush of HCl. NaCl brine was used for final displacement of the fluids from the core. The reactive fluid was a mixture of HCl and HsiF 5 that would allow chemical theories to be tested. The reactive fluid was typically about 12 PV. Analytical. Liquids collected from the column flow tests were analyzed for the various ions in solution. Inductively coupled plasma emission spectroscopy (ICP) was used to analyze for Na, K, Ca, Mg, Fe, Si, and Al. 19F nuclear magnetic resonance (NMR) spectroscopy of the samples provided the fluoride distribution on Si and Al, from which the F/Al molar ratio was calculated. 9,11 Cl was determined by AgNO3 titration, and acidity was determined either by ?H measurement, titration with NaOH (to a ?H of approximately 3), or by ionic balance.
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