Visualization simulation experiments and porosity evolution mechanisms of deep to ultra-deep carbonate reservoirs

Abstract

To address the challenges in studying the pore formation and evolution processes, and unclear preservation mechanisms of deep to ultra-deep carbonate rocks, a high-temperature and high-pressure visualization simulation experimental device was developed for ultra-deep carbonate reservoirs. This unit comprises four core modules: an ultra-high temperature, high pressure triaxial stress core holder module (temperature higher than 300 °C, pressure higher than 150 MPa), a multi-stage continuous flow module with temperature-pressure regulation, an ultra-high temperature-pressure sapphire window cell and an in-situ high-temperature-pressure fluid property measurement module and real-time ultra-high temperature-pressure permeability detection module. Using the new experimental device and the carbonate rock samples from the Sichuan Basin and Tarim Basin to simulate the dissolution-precipitation process of deep to ultra-deep carbonate reservoirs in an analogous geological setting, the geological insights were obtained in three aspects. First, the pore-throat structure of carbonate is controlled by lithology and initial pore-throat structure, and fluid type, concentration and dissolution duration determine the degree of dissolution. The dissolution process exhibits two evolution patterns. The dissolution scale is positively correlated to the temperature and pressure, and the pore-forming peak period aligns well with the hydrocarbon generation peak period. Second, the dissolution potential of dolomite in an open flow system is greater than that of limestone, and secondary dissolved pores formed continuously are controlled by the type and concentration of acidic fluids and the initial physical properties. These pores predominantly distribute along pre-existing pore/fracture zones. Third, in a nearly closed diagenetic system, after the chemical reaction between acidic fluids and carbonate rock reaches saturation and dynamic equilibrium, the pore structure no longer changes, keeping pre-existing pores well-preserved. These findings have important guiding significance for the evaluation of pore-throat structure and development potential of deep to ultra-deep carbonate reservoirs, and the prediction of main controlling factors and distribution of high-quality carbonate reservoirs.