Thermodynamic calculation of water-rock reaction process for sandstone reservoirs in petroliferous basins and its indicative significance for reservoir reconstructions

The essence of diagenesis is the interaction between water and rock in reservoir and the migration of matter and energy in the process. Diagenesis controls the formation and distribution of favorable reservoirs in deep oil-gas basins .This paper mainly focuses on the solubility of various minerals in sandstone reservoirs under acidic fluid environment rich in CO₂, and then defines the genetic mechanism of various secondary pores. In this study, the types and occurrence states of various substances in the H₂O-CO₂-CaCO₃-Albite-SiO₂ system were determined based on literature research, and then the Gibbs formation energy of pure or near-pure phase substances such as H₂O(l), arspar, calcite, and CO₂(g) was quantitatively calculated using the constant pressure specific heat model under different temperature and pressure conditions. The Gibbs formation energy of Na⁺(aq), CO₂(aq) and other dissolved particles was quantitatively calculated by HKF model. Furthermore, Gibbs function is used to calculate the chemical equilibrium constants of various chemical reactions in the system. Finally, based on the charge balance principle, the solubility of various substances in the system under different temperature and pressure conditions is calculated. The calculated results are compared with the contents of various diagenetic products in sandstone reservoirs, and the genetic mechanism and main controlling factors of various secondary pores in deep layers of petroliferous basins are clarified. The results show that:The contents of various particles in aqueous solution in the system are mainly controlled by temperature and partial pressure of CO₂, the influence of pressure is relatively small, and the law of different ions changing with temperature has obvious difference; The composition of Ca-containing and C-containing particles in aqueous solution changes with temperature, while the composition of Al-rich and Si-rich particles is basically unchanged with temperature. The content of secondary pores is determined by the solubility of various particles in aqueous solution, the rate of fluid migration and the duration of fluid migration. The distribution mode and range of secondary pores are determined by the changes of various particles in aqueous solution with temperature and pressure.