EXPERIMENTAL EVALUATION OF STRESS SENSITIVITY IN TIGHT SANDSTONE RESERVOIRS UNDER HIGH STRESS AND WATER COUPLING ENVIRONMENT

The stress sensitivity evaluation of tight sandstone reservoirs under high stress and water coupling environment is a hot spot in the evaluation of the properties of the underground rock mass. In this paper, taking the tight gas sandstone reservoirs in the Lower Permian Shanxi Formation in the Ordos Basin as an example, a stress-water coupled simulation environment system is used to study the stress-sensitive variation characteristics of tight sandstone reservoirs under different stress and water environments. The results show that there is a good power-exponential coupling relationship between the confining pressure and the permeability of the rock sample. With the increase of the confining pressure, the gas permeability of the rock sample decreases rapidly at first, and then decreases slowly at the end. The gas permeability of the core under a certain effective stress during the loading process is different from the gas permeability measured during the unloading process under the same effective stress, that is, the permeability has a hysteresis phenomenon. The permeability damage rate of rock samples is mainly distributed between 48% and 75%, and the stress sensitivity of the permeability of rock samples under high pressure is medium-strong sensitivity. Compared with other components such as feldspar, the rock chip component has higher hardness, so its content has a significant negative impact on the stress sensitivity of the rock sample. In addition, there is also a difference in the stress sensitivity under the loading and unloading conditions of the stress, that is, the stress sensitivity under the stress loading environment is stronger. For rock samples with different permeability, the low permeability rock samples are more stress sensitive. For the samples saturated with formation water, the stress sensitivity of the rock samples showed an increasing trend with the increase of water saturation. Therefore, the combined effect of water saturation and effective stress will damage the gas layer more seriously. The stress sensitivity coefficient of the water-bearing tight sandstone during pressure relief is greater than that during pressurization, which is related to the mineral composition structure of fractures, quartz and debris in the reservoir.