Changes in pore structures and porosity-permeability evolution of saline-lacustrine carbonate reservoir triggered by fresh water-rock reaction

Salt dissolution occurs frequently during the drilling, water-flooding development and other operations of saline-lacustrine carbonate reservoirs, greatly changing the petrophysical properties of rocks. To understand the salt dissolution behavior and its effect on the rock structures, core-flooding and crushed sample soaking experiments are conducted on saline-lacustrine carbonate samples. Results indicate that ion concentration from core flooding experiments increases first and then decreases fast with time proceeding, while it has a positive e-based logarithmic relation with soaking time from sample soaking experiments. According to ion analysis, the main dissolved salt minerals include halite, anhydrite and glauberite. With the size increase of crushed samples, the dissolved mass is decreased, having a positive logarithmic relation with sample specific surface, and sample specific surface is logarithmically increased with soaking time. Rock capillary pressure curves get lower, and pore throat size distributions become wider than those before dissolution of salt minerals. Salt dissolution mainly affects the pore microstructure with size ranging from 0.01 to 10 mu m, while having little effect on nanopores < 0.01 mu m. Salt dissolution could make the clay swelling and fine migration, inhibiting the increment in porosity and permeability. In addition, rock permeability has a power relation with porosity during the dissolution, and a permeability-growth-controlling threshold porosity is observed, which indicates the transformation point of dissolution area from pore bodies to throats. Furthermore, a mathematical model for characterization of rock porosity and permeability evolution relationship is derived, which allows for rock microstructure, salt composition and dissolution, and clay composition and swelling behavior.