Experimental Simulation of Mineral Dissolution and Permeability Variation in Unconsolidated Sandstone Formations During Steam Flooding Process

Heavy oil reservoirs are mostly characterized by unconsolidated sandstone formations, and steam (hot water) injection at high temperature is currently the dominant recovery method for such reservoirs. However, the injected high-temperature steam (hot water) can dissolve formation minerals, resulting in variations in formation permeability. Based on the analysis of actual field conditions in oilfields, sand-packed tube experiments were conducted to simulate steam flooding in unconsolidated sandstones in this study. By analyzing the dried residues in produced water and testing the formation sand before and after the experiments, the influence laws of steam injection temperature and steam quality on the surface morphology, elemental composition and particle size distribution of formation sand were investigated. An evaluation method for mineral dissolution rate was established, and the effects of steam injection temperature and steam quality on mineral dissolution rate and reservoir permeability variation were quantitatively assessed. The results indicate that the rock dissolution amount increases and permeability rises significantly with the increase in steam injection temperature; the total mineral dissolution amount reaches the maximum at a steam injection temperature of 250 ℃, while a further increase in temperature leads to a decrease in water enthalpy and thus a reduction in rock dissolution amount. Under the same other conditions, the rock dissolution amount decreases and the magnitude of permeability increase diminishes with the rise in steam quality. Additionally, the rock dissolution amount increases with the increase in injected pore volume (PV); it basically remains unchanged after the injected PV reaches 200. The research findings can provide theoretical support for the reservoir engineering research and development plan design of steam flooding in heavy oil reservoirs.