Exploring the role of the structural heterogeneity of fractured carbonate reservoirs in contact with dissolved CO2 based on fracture-water-rock simulation experiments

Abstract

Based on the geological background of ultra-deep fractured carbonate reservoirs in the Shunbei oil and gas field, the objective of this study was to clarify the interactions between fractured carbonate reservoirs and formation fluids (brine with dissolved CO2) under deep strata, and the potential impact of these interactions on reservoirs. In this study, high-temperature and high-pressure simulation experiments and numerical simulations based on the Yijianfang Formation in the Shunbei area were designed to investigate the influence of temperature, pressure, fluid, and structural heterogeneity on fluid transport and mass transfer, calculate the ion diffusion characteristics and the trend of mineral dissolution/precipitation along the fractures, and clarify the change in the reservoir space. The results indicated that the volume of the main fracture increased after the reaction between an acidic solution and a formation sample, the permeability of the sample increased by orders of magnitude, and its porosity increased accordingly. The overall reaction during the experimental period was dominated by the dissolution of calcium carbonate, and no calcium carbonate precipitated. The structural heterogeneity and hydraulic properties of the sample make the main flow channel a main fracture. The flow and reaction processes in the flow channel promoted each other. The effects of temperature and pressure on the overall reservoir space were insignificant. Branch fractures or microfractures can weaken the main fluid flow rate and tend to result in the accumulation of solutes, the ends of which may be potential locations for mineral precipitation. Therefore, the priority exploration location for deep and ultra-deep fractured reservoirs should be the main fractures upstream of the fluid flow.