Effect of reservoir mineralogy and texture on acid response in heterogeneous sandstones
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SummaryA series of coreflood experiments with stimulation fluids [hydrochloric acid (HCl) preflush followed by hydrochloric/hydrofluoric acids (HCl/HF) main flush followed by amonium chloride (NH4Cl) post flush] has been conducted on a suite of cores from several Gulf of Mexico Miocene turbidite reservoirs. Using thin-section petrography and computer tomography (CT) scans, the samples were characterized as more or less heterogeneous. Samples ranged from reasonably homogeneous to highly laminated. Sample mineralogy was assessed by Xray diffraction (XRD). The quartz content of the cores ranged from 28 to 78%. One core plug contains 39% clay, and the rest of the cores contained between 5 and 18%. These cores also contained between zero and 40% clinoptilolite zeolite. All of these sands are unconsolidated and contain only detrital carbonates. Treatment with HCl was not expected to cause core failure. The only core failure observed occurred during HCl/HF treatment.Acid response was assessed on the basis of permeability change and effluent chemical analysis, which varied dramatically among the samples. Release of particles was not observed. In the poorer-quality, more-laminated samples, channeling with significant permeability increase was observed during HC1 treatment with little additional change during later stages of the treatment. Selective pore enlargement with little alteration of the matrix framework was observed in post-treatment thin sections. In more-homogeneous sand samples, more-general acid attack was observed. Core effluent indicated carbonate dissolution during the HCl preflush, while HCl/HF attacked alumino-silicates and was roughly half spent during transit through the core. In the homogeneous samples, comparison of thin sections before and after acidization revealed near-complete (greater than 80%) dissolution of carbonate and an extensive "cleaning" of clays and fines from the pore space.We used a fine-scale simulator of sandstone acidizing to match the responses observed in these experiments. The model corroborates the effects of heterogeneities in the permeability field and in the mineral distribution on the sandstone-acidizing process. These model results show how characterization of fine-scale heterogeneity in sandstone can improve the design of matrix-stimulation treatments.
