Experimental Study on Water‐rock Reactions with CO2 Fluid in a Deep Sandstone Formation under High Temperature and Pressure

Abstract Qiongdongnan Basin has a tectonic geological background of high temperature and high pressure in a deep reservoir setting, with mantle‐derived CO 2 . A water‐rock reaction device was used under high temperature and high pressure conditions, in conjunction with scanning electron microscope (SEM) observations, to carry out an experimental study of the diagenetic reaction between sandstone at depth and CO 2 ‐rich fluid, which is of great significance for revealing the dissolution of deep clastic rock reservoirs and the developmental mechanism of secondary pores, promoting deep oil and gas exploration. In this study, the experimental scheme of the water‐rock reaction system was designed according to the parameters of the diagenetic background of the deep sandstone reservoir in the Qiongdongnan Basin. Three groups of single mineral samples were prepared in this experiment, including K‐feldspar samples, albite samples and calcite samples. Using CO 2 as a reaction solution, a series of diagenetic reaction simulation experiments were carried out in a semi‐closed high temperature and high pressure simulation system. A field emission scanning electron microscope (SEM) was used to observe the microscopic appearance of the mineral samples after the water‐rock reaction, the characteristics of dissolution under high temperature and high pressure, as well as the development of secondary pores. The experimental results showed that the CO 2 ‐rich fluid has an obvious dissolution effect on K‐feldspar, albite and calcite under high temperature and high pressure. For the three minerals, the main temperature and pressure window for dissolution ranged from 150°C to 300°C and 45 MPa to 60 MPa. Scanning electron microscope observations revealed that the dissolution effect of K‐feldspar is most obvious under conditions of 150°C and 45 MPa, in contrast to conditions of 200°C and 50 MPa for albite and calcite. Through the comparative analysis of experimental conditions and procedures, a coupling effect occurred between the temperature and pressure change and the dissolution strength of K‐feldspar, albite and calcite. Under high temperature and high pressure, pressure changed the solubility of CO 2 , furthermore, the dissolution effect and strength of the sandstone components were also affected. The experiment revealed that high temperature and high pressure conditions with CO 2 ‐rich fluid has a significant dissolution effect on aluminosilicate minerals and is conducive to the formation of secondary pores and effective reservoirs. Going forward with the above understanding has important implications for the promotion of deep oil and gas exploration.