The gas flow in shale reservoirs is controlled by gas desorption diffusion and multiple flow mechanisms in the shale matrix. The treatment of hydraulic fracturing injects a large amount of fracturing fluids into shale reservoirs, and the fracturing fluids can only be recovered by 30~70%. The remaining fracturing fluid invades the reservoir in the form of a water invasion layer. In this paper, by introducing the concept of a water invasion layer, the hydraulic fracture network is di-vided into three zones: major fracture, water invasion layer and stimulated reservoir volume (SRV). The mathematical model considering gas desorption, the water invasion layer and gas–water two-phase flow in a major fracture is established in the Laplace domain, and the semi-analytical solution method is developed. The new model is validated by a commercial simulator. A field case from WY shale gas reservoir in southwestern China is used to verify the utility of the model. Several key parameters of major fracture and SRV are interpreted. The gas–water two-phase flow model established in this paper provides theoretical guidance for fracturing effectiveness evaluation and an efficient development strategy of shale gas reservoirs.

After the development of high-porosity, high-permeability, medium-high-viscosity oil reservoirs enters the high-water-cut stage, the remaining oil is highly dispersed on the microscopic scale, which leads to a change in the oil-water-flow law. If the enrichment and mobilization laws of the microscopic remaining oil cannot be truly and objectively described, it will ultimately affect the production of oil fields. At present, few studies have directly revealed the microscopic water flooding mechanism of high-porosity, high-permeability, medium-high-viscosity oil reservoirs and the main controlling factors affecting the formation of remaining oil. Starting with micro-physical simulation, this study explores the water flooding mechanism on the microscale, the type of remaining oil and its evolution law, and analyzes the main controlling factors of different types of remaining oil so as to propose effective adjustment and development plans for different types of remaining oil. It is found that this type of reservoir has a serious jet filtration phenomenon in the early stages of water flooding and is accompanied by the penetration of injected water, detouring flow, pore wall pressing flow, the stripping effect, and the blocking effect of the rock skeleton. The remaining oil is divided into five types: contiguous flake shape, porous shape, membrane shape, striped shape, and drip shape. Among them, the transformation of flake-shape and porous-shape remaining oil is greatly affected by the viscosity of crude oil. The decrease effect of crude oil viscosity on contiguous residual oil was as high as 33.7%, and the contiguous residual oil was mainly transformed into porous residual oil. The development of membrane-shape, striped-shape, and drip-shape remaining oil is more affected by water injection intensity. The decrease in water injection intensity on membrane residual oil was as high as 33.3%, and the membrane residual oil shifted to striped and drip residual oil. This paper classifies remaining oil on the microscopic scale and clarifies the microscopic water flooding mechanism, microscopic remaining oil evolution rules, and the main controlling factors of different types of remaining oil in high-porosity, high-permeability, medium-high-viscosity oil reservoirs.

Carbonate reservoirs usually have strong heterogeneity, with complex pore structure and well-developed natural fractures. During reservoir development, when the formation pressure is lower than the bubble point pressure of crude oil, the fluid undergoes phase change and degassing. This leads to the subsequent waterflooding displacement under the oil–gas two-phase condition, also followed by a secondary phase change of oil and gas caused by the increase in formation pressure. In this paper, the glass etching model is used to carry out microfluidic experiments. The porous carbonate model and the fractured porous carbonate model are designed to simulate the process of depletion development and waterflooding development. In the process of depletion development, it can be observed that the crude oil degassing and gas phase occurrence areas of the porous model are in the order of the large pore throat area first, followed by the small pore throat area. And the crude oil degassing and gas phase occurrence order in the fractured porous model is as follows: fractures, large pore throat area and, finally, small pore throat area. In the process of converting to the waterflooding development, the early stage of the replacement reflects the obvious characteristic of “displace oil but not gas”; with the replenishment of formation energy, the gas redissolution area expands from the mainstream to other areas, and the waterflooding mobilization increases. The characteristics of oil, gas and water flow in different stages of carbonate reservoirs with different pore-fracture characteristics that are clarified, and the characteristics of fluid migration and the distribution under the condition of oil and gas coexisting and water flooding after crude oil degassing are explored, and the water displacement mechanism of volatile carbonate reservoirs with different pressure levels is revealed.