EXPERIMENTAL EVALUATION OF RESERVOIR SENSITIVITY AND MICROSCOPIC DAMAGE MECHANISM OF TIGHT SANDSTONE GAS RESERVOIRS

The Ordos Basin in western China is currently the largest oil and gas exploration basin in China. The Upper Paleozoic in the basin contains multiple sets of favorable hydrocarbon-bearing strata. Exploration in recent years has shown that the Permian Shiqianfeng, Shihezi, Shanxi, Taiyuan Formation have good natural gas displays and formed a certain scale of exploration, mainly tight sandstone reservoirs. For this type of unconventional reservoir, the drilling fluid, fracturing fluid, completion fluid, etc. invade the reservoir during the drilling and completion process, which will bring certain pollution to the reservoir, resulting in a decrease in the porosity and permeability of the reservoir, affecting the flow capacity of the reservoir. Therefore, in order to analyze and identify reservoir sensitivity characteristics and the microscopic damage mechanism of reservoir physical properties, this paper takes the Shanxi Formation natural gas reservoir in the Ordos Basin as an example, using scanning electron microscopy, cast thin sections, X-ray diffraction clay minerals and other experimental methods to analyze the reservoir characteristics in the study area, the reservoir sensitivity and damage mechanism. Experimental studies have shown that the physical properties of the reservoirs in the Shanxi Formation are poor, belonging to low porosity and low permeability reservoirs, with an average porosity of 11.25% and an average permeability of 0.82x10(-3)mu m(2). Through the analysis and evaluation of sensitivity experiments, the reservoirs of Shanxi Formation are shown as moderate to weak water sensitivity, weak velocity sensitivity, moderate to strong acid sensitivity, moderate to strong alkali sensitivity, and moderate to weak salt sensitivity. Analysis of the microscopic damage mechanism found that salt sensitivity can be regarded as the reverse process of water sensitivity. These causes of reservoir damage can be summarized as the physical and chemical reactions between external fluids and clay minerals, causing clay mineral particles to hydrate, expand, and swell. The clay minerals migrate, chemically react with fluids to produce precipitation, and finally block the pore throats, resulting in a decrease in reservoir permeability and damage to the reservoir.