Effect of Mixed Mineralogy on Etching Profile and Conductivity in Acid Fracturing in Naturally Fractured Carbonate Reservoirs

Abstract The naturally fractured carbonate gas reservoir of Majiagou formation in Ordos Basin is characterized by mixed mineralogy. Since mineralogy determines acid-rock reaction rate, mineral distribution has significant effect on the fracture surface etching profile. Therefore, it is necessary to investigate effect of mixed mineralogy on etching profile and fracture conductivity. In this paper we conducted the research from two aspects: experiment and numerical modeling. In the experiment, we firstly measured mineral distribution by hyperspectral scanning on the core slabs, then did acid flooding, next did 3D scanning to get etching profile, and finally measured acid fracture conductivity, based on which an acid fracture conductivity correlation was built. In numerical modeling, based on mass conservation principle, acid-rock reaction kinetics, and momentum theorem, a 3D acid flow, acid-rock reaction, surface etching model was developed. Mineral distribution on the surfaces was coupled as boundary conditions. Experimentally measured mineral distribution on the slab surface are coupled into the numerical simulation. The model is validated by the experimental results. Based on the model, extensive numerical simulation was conducted to analyze mineral distribution, acid-rock contact time, and temperature on the surface etching pattern and acid concentration distribution. By combining the experimental results and numerical simulation, how the mineral distribution affect etching profile, facture conductivity, and acid concentration distribution is analyzed. The study shows that for mixed mineralogy carbonate, the distribution of mineral is strongly spatially correlated instead of random distribution. Mineral stripes are observed from the mineralogy scanning of core slabs. Due to reaction rate contrast of different minerals and strong spatially correlated distribution, the surface etching profiles are rough, and the channel is obvious. The channels resulted from multiple mineral distribution contributes remarkably to the fracture conductivity. With the similar amount of rock dissolved, the fracture with channels has a much higher conductivity. Temperature has remarkable effect on etching profile. At a high temperature (e.g. 90°C), the difference of overall reaction rate for limestone and dolomite is small, and the etching discrepancy for calcite and dolomite is less. At a low temperature (e.g. 60°C), the difference of overall reaction rate is large, so the etching discrepancy is more distinct. Dolomite surface has an apparent higher acid concentration than limestone at a low temperature, while surface acid concentration is close for calcite and dolomite at a high temperature. The impurities such as quartz, clay, gypsum, etc. are not dissolved by the acid. Even small amount of impurities contributes to the differential etching on the surfaces. In the lab scale, the acid concentration inside the fracture has identifiable decrease from the inlet to the outlet.