Dissolution Simulation Experiment of Buried Carbonate Reservoir: A Casefrom the Ordovician Carbonate Rock ofthe Western Ordos Basin

The corrosion and microscopic characteristics of the Ordovician carbonate rocks in the Ordos Basin were studied by simulation of the carbonate dissolution in different fluids (0.2% acetic acid and CO₂ under partial pressure of 2 MPa respectively) under buried conditions. It is found that the dissolution rates of dolomite in above fluids are obviously higher than those of limestone. In a deeply buried environment, the difference in dissolution rates of the two increases with depth. Dolomite is more likely to dissolve than limestone to form dissolved pores; The dissolution rates for both limestone and dolomite in acetic acid increase with temperature and pressure, while the dissolution rate of the two rocks increases first and then decreases afterwards in CO₂. Under the same temperature and pressure, the dissolution rate of either dolomite or limestone is higher in acetic acid rather than in CO₂; The microscopic characteristics of the rock after the dissolution in either CO₂ or acetic acid are similar. For micritic limestone, dissolution is usually weak and easy to form corrosion pits; In granular limestone, however, corrosion pits are always found in the inner part of the grains; Intergranular pores are often formed by dissolution of dolomite; After dissolution of dolomite, the dissolution of calcite will slow down in general; Dissolved anhydrite mold pores are often observed in anhydritic limestone. The experiment results confirm that the fluid medium under burial conditions play important roles on the development of rock dissolution pores. The dissolution ability of dolomite is greater than that of limestone. The reservoir space developed by dissolution is worthy for attention in oil and gas exploration in the Majiagou Formation of the Ordos Basin.