Effect of Reservoir Mineralogy and Texture on Acid Response in Heterogeneous Sandstones

Abstract

Abstract A series of core flood experiments with the sequence of fluids typically used in HCl/HF stimulation (HCl pre-flush, followed by HCl/HF main flush, followed by NH4Cl post flush) have 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 X–ray diffraction (XRD). Core compositions ranged from relatively clean (quartz rich) to relatively "ratty" (clay and feldspar rich) and some cores contained abundant clinoptololite zeolite. Acid response was assessed based on permeability change and effluent chemical analysis and varied dramatically among the samples. 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 pre-flush, while HCl/HF clearly attacked alumino-silicates and was roughly half spent during transit through the core. Comparison of thin sections before and after acidization revealed changes in sample quality. In the better quality samples, near complete (greater than 80 percent) dissolution of carbonate and an extensive "cleaning" of clays and fines from the pore space was apparent. 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.