Ultra-deep sandstone gas reservoirs have great potential for exploration and development and have become a major focus worldwide. Under the influence of ultra-high temperature, pressure and strong in-situ stress, the mechanical properties of rocks change significantly, resulting in the coexistence of diverse pores and fractures. This rock deformation, coupled with severely non-uniform water invasion, fundamentally hinders efficient reservoir development by promoting preferential flow channeling and early water breakthrough, which reduce sweep efficiency and increase remaining gas retention. In this paper, a microfocus CT scanning experiment is conducted on ultra-deep rock under in-situ stress loading. Based on the acquired scans, three-dimensional digital cores representing the deformed multiscale fracture-pore media are reconstructed under various stress conditions. Dynamic pore network simulations are performed to systematically investigate gas-water migration and distribution characteristics under the effects of stress, displacement pressure difference and fracture conductivity. The research shows that stress is the most critical factor governing the water invasion process. In the absence of applied stress, water preferentially invades well-connected pores, exhibiting a regular front and fast migration rate, while gas remains in small, poorly connected blind pores. When the stress is loaded to the linear elastic deformation stage, the core exhibits increased radial deformation, and pronounced dynamic changes occur during the water invasion process. Remaining gas easily accumulates in isolated pores at corner or dead-end regions, and a large amount of remaining gas appears in regions with low driving force. Upon reaching the plastic deformation stage, localized fracturing occurs, which increases the number of throats and enhances connectivity. This leads to a transition of the water invasion mode from uniform to non-uniform. In the fracture stage, the water phase preferentially moves along the fracture surface rapidly, thereby accelerating the water invasion process. The local pressure distribution between the fracture and the matrix is uneven, inducing the wetting phase fluid to continuously seep from the fracture into the matrix, expanding the swept range. Fracture conductivity directly affects the migration speed and morphology of the water invasion front, but its impact on the final water invasion effect is weak.

Low salinity water flooding is a new technology for enhancing oil recovery by adjusting the ion composition or concentration of injected water. However, the applicable reservoir conditions and enhanced oil recovery mechanism of low salinity water flooding have not yet reached a consensus. In this paper, a series of laboratory experiments of wettability control-based low salinity flooding are carried out with plunger rock samples from marine carbonate reservoirs in the Middle East as the research object. Based on the theory of Derjaguin-Landau-Verwey-Overbeek theory (DLVO), an interfacial reaction model of a typical crude oil/brine/rock system is established, and the contact angle and total separation pressure are calculated simultaneously with the augmented Young-Laplace formula. The reliability of the model is verified by the literature experimental data, and the effects of ion concentration and ion type on the separation pressure curve and contact angle are clarified. The results show that in low salinity environments, the pore surface of carbonate rock is more water-wet under the action of fluid flushing, the oil displacement efficiency is higher, and the low salinity water improves the crude oil recovery by 3.2%; under the assumption of constant charge, the mathematical model established based on the DLVO theory for the crude oil/brine/rock system can accurately predict the change of contact angle; compared with the ion concentration, ion type has a greater impact on separation pressure and contact angle. Among divalent ions, Mg²⁺ ions exhibit a more pronounced influence on wettability control compared to Ca²⁺ ions. When the water film thickness is minimal, van der Waals force is the main force affecting the separation pressure. As the thickness of water film increases, the electric double layer force gradually becomes the main force. This study contributes to a deeper understanding of the wettability control mechanism of low salinity water flooding for enhanced oil recovery.

Two actual rocks drilled from a typical ultra-deep hydrocarbon reservoir in the Tarim Basin are selected to conduct in-situ stress-loading micro-focus CT scanning experiments. The gray images of rock microstructure at different stress loading stages are obtained. The U-Net fully convolutional neural network is utilized to achieve fine semantic segmentation of rock skeleton, pore space, and micro-fractures based on CT slice images of deep rocks. The three-dimensional digital rock models of deformed multiscale fractured-porous media at different stress loading stages are thereafter reconstructed, and the equivalent fracture-pore network models are finally extracted to explore the underlying mechanisms of gas–water two-phase flow at the pore-scale. Results indicate that, in the process of in-situ stress loading, both the deep rocks have experienced three stages: linear elastic deformation, nonlinear plastic deformation, and shear failure. The micro-mechanical behavior greatly affects the dynamic deformation of rock microstructure and gas–water two-phase flow. In the linear elastic deformation stage, with the increase in in-situ stress, both the deep rocks are gradually compacted, leading to decreases in average pore radius, pore throat ratio, tortuosity, and water-phase relative permeability, while the coordination number nearly remains unchanged. In the plastic deformation stage, the synergistic influence of rock compaction and existence of micro-fractures typically exert a great effect on pore-throat topological properties and gas–water relative permeability. In the shear failure stage, due to the generation and propagation of micro-fractures inside the deep rock, the topological connectivity becomes better, fluid flow paths increase, and flow conductivity is promoted, thus leading to sharp increases in average pore radius and coordination number, rapid decreases in pore throat ratio and tortuosity, as well as remarkable improvement in relative permeability of gas phase and water phase.