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

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The combination of matrix acidizing experiments with visualization techniques is commonly used to elucidate the details of wormhole networks formed during matrix acidizing of carbonate reservoir rock. Previous experimental studies of wormhole growth have focused mainly on small linear core plugs, with only a limited number of radial flow studies published in the literature. Results from these conventional experiments have provided extensive information on linear wormhole growth in one dimension (1-D) along with some basic insights into radial growth mechanisms in 2-D. However, larger-scale test systems must be considered if 3-D wormhole characteristics are to be understood. Toward that end, a new methodology has been developed which integrates (1) acidizing experiments on carbonate rock samples up to 14 ft3 in volume, (2) high-resolution nondestructive imaging and analysis, and (3) computational modeling to extend the results of experiments to field applications. This article highlights the experimental and imaging components of the methodology.

Recent studies in sandstone acidizing have shown the importance of HCl in maintaining the solubility of the reaction products of HF acid. However, HCl can cause such damage as corrosion, crude oil sludging, and instability of formation minerals. These problems can become severe at elevated temperatures. HF fluids based on acetic acid and formic acid can successfully overcome these problems. Despite the successful application of the organic-based HF fluids, very little work has been done to understand and optimize these systems. Fluid reactivity studies, flow tests, and flowback analyses following field treatments were undertaken as a means of studying acetic-HF and formic-HF fluids. Severe, damaging precipitation of aluminum fluorides during the HF reactions was discovered with these fluid systems. This paper will discuss these results and the development and application of a new organic-HF fluid system that overcomes the precipitation problems and maintains all the advantages of acetic-HF and formic-HF fluids. Laboratory flow tests, returns analyses, and case histories of the new system will be also be presented. Introduction Over the years, the need for alternatives to HCl-based acidizing fluids has been realized. Common problems associated with HCl-based fluids include high reactivity, high corrosivity, sludging tendencies when the fluids contact crude oils, and the sensitivity of clay minerals and zeolites. These problems are aggravated at higher temperatures. Organic acids, such as acetic acid, formic acid, and combinations of the two have overcome many of these problems. Ion Exchange and Clay Swelling. One problem with organic acids that has been overlooked until recently is ion exchange. Organic acids are weak acids that have a very low degree of ionization. Therefore, an organic acid by itself cannot ion-exchange or prevent clay swelling or fines migration. Pumping plain organic acids into a formation would be like treating it with fresh water. As a result, 5% NH4Cl must be added to all organic acid fluids used in sandstone formations to overcome these problems. HCl Sensitivity of Clays. It has long been known that a select group of minerals will decompose in the presence of HCl or any strong acid. The most common minerals in this group are chlorite and zeolites. Their instability in strong acid is directly related to their structure. Alumina layers are attacked by the strong acid, which causes their structures to collapse. This collapse results in a residual silica gel mass that can be severely damaging. Therefore, the use of organic acids in place of HCl in acidizing formations containing these minerals is imperative. Recent research has shown that all clays (with their layered structures) have a temperature above which they are no longer stable in HCl. Fig. 1 (Page 7) plots the stabilities of clays in HCl with respect to temperature. For example, a formation at 225 F containing 10% illite would be considered HCl-sensitive; therefore, organic acid-based fluids should be used instead of HCl-based fluids to prevent formation damage. Aluminum Precipitation. Despite the successful application of the organic-based HF fluids, researchers have done little to understand and optimize these systems. The chemistry of HC/HF fluid systems, however, has recently been studied in much detail. This paper is an extension of the earlier work, focusing on organic-HF fluid systems. Fluid reactivity studies, flow tests, and flowback analyses following field treatments were performed to study the currently used acetic-HF and formic-HF fluids. A somewhat surprising observation from these tests was the nearly complete precipitation of aluminum fluoride complexes during the HF reactions. This paper will discuss these results and the development and application of a new organic-HF fluid system that overcomes the precipitation problems and maintains all the advantages of acetic-HF and formic-HF fluids. Laboratory flow tests, returns analyses, and case histories of the new system will be also be presented. P. 327

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Abstract Matrix acidizing experiments combined with visualization techniques commonly are used to study the details of wormhole networks formed during matrix acidizing of carbonate reservoir rock. Previous experimental studies of wormhole growth focused mainly on small linear core plugs, with only a limited number of radial-flow studies published in the literature. Results from these conventional experiments provided extensive information on linear 1D wormhole growth along with some basic insights into 2D radial growth mechanisms. However, larger-scale test systems must be considered if 3D wormhole characteristics are to be understood. Toward that end, a new method was developed that integrates acidizing experiments on carbonate rock samples up to 14 ft3 in volume, high-resolution nondestructive imaging and analysis, and computational modeling to extend the results of experiments to field applications. This article highlights the experimental and imaging components. Introduction Substantial hydrocarbon volumes have been and will continue to be produced from carbonate formations, which hold nearly half of the world's reserves. Because these formations are highly soluble in acid, matrix acid stimulation provides a cost-effective means to enhance well productivity. Effective acid stimulation can be critical to achieving the desired longterm production rates from targeted reservoir layers. Interaction of acids with carbonate rock has been studied extensively using quarried, outcrop, and formation core samples. Linear flow tests on core plugs are conducted to determine the optimum conditions to generate wormholes (i.e., highly conductive flow channels that connect the nearwell region to the completion). At a given temperature, the ability of a particular acid to generate wormholes is largely dependent on the acid injection rate or ’acid flux,?? as illustrated in Fig. 1. The figure shows an example of a ’wormhole efficiency curve?? developed for core plugs of quarried limestone. As the curve indicates, there is a certain optimal acid flux for which wormholes will most efficiently propagate along the main axis of the core plug.