Numerical Simulation Of Low Permeability Abnormal High Pressure Gas Reservoirs

Low permeability and abnormally high pressure gas reservoirs are widly distributed in China, such as Moxi ka2and Xujiahe gas reservoir of Sichuan Basin, Dabei2and Dina2gas reservoir of Tarim Basin. In the development process of such a gas reservoir, the matrix system shows the start-up pressure gradient phenomenon and the fracture system is easy to appear closed with the drop of formation pressure. Meanwhile, using the existing numerical simulation softwares of gas reservoirs, stress sensitivity and start-up pressure gradient can not be fully considered and this can cause the dynamic prediction of gas well inaccuracy in the low permeability and abnormally high pressure gas reservoirs. So it has improtant practical significence for productivity evaluation and devlopment plan design of low permeability and abnormally high pressure gas reservoirs to do "numerical simulation study on low permeability and abnormally high pressure gas reservoir"In this paper, based on the analysis of geological and flow characteristics of abnormally high pressure gas reservoirs, the caculation methods of gas Z-factor, viscosity, gas compressibility and the volume factor were studied. And using seepage mechanics and fluid thermodynamics synthetically, considering matrix start-up pressure gradient and fracture stress sensitivity, a three-dimensional gas-water two phase dual medium model was established. At the same time, the corresponding programs were compiled by the use of Matlab. And compared with the commercial numerical simulation software of Eclipse, the reliability and accuracy of the model were verified. On this basis, the sensitivity analysis of stress sensitivity, start-up pressure gradient, the shape factor of channeling between the matrix and fracture and fracture conductivity of gas well fracturing was made. Furthmore, the effect of stress sensitivity and start-up pressure gradient on gas well fracturing was analyzed.Main research results obtained in this article are as follows:1. The calculation methods of gas Z-factor were summarized and the computational model was optimized, when0.2≤ppr<15, DPR method was used to calculate Z-factor, and while15≤ppr≤30, Zhang Guodong method was applied. According to the optimized methods, Z-factor plate was drawn for abnormally high pressure gas reservoir and the relationship between gas viscosity, gas compressibility and the volume factor and pressure was studied.2. A three-dimensional gas-water two phase mathematical model of low permeability and abnormally high pressure gas reservoir was built. In this model, start-up pressure gradient of matrix and stress sensitivity of fracture were considered. The corresponding numerical simulation programs were established by the use of Matlab. The data of a low permeability and abnormally high pressure gas reservoir was used to make a simulation, and a comparison was made between these results and Eclipse simulation results to verify the reliability of the model.3. Simulation and analysis were done using Matlab programs. Stress sensitivity and start-up pressure gradient had greater impacts on gas well productivity. The effect of stress sensitivity on well productivity was shown at the beginning of producing, Non-linear inverse relationship was between production and stress sensitivity coefficient and the drop level of gas well productivity was slow down with the increase of stress-sensitive coefficient. The start-up pressure gradient was displayed later and linear inverse relationship was between production and start-up pressure gradient. Meanwhile, the effect on gas well productivity, which was generated considering stress sensitivity and start-up pressure, was greater than when considering separately.4. Gas well prodution was little affected by the shape factor calculated using Kazemi, Coats, He Yongming, improved method of Kazemi and Warren&Root method.5. According to the grid size, permeability of fractured grid was converted by fracture conductivity. Gas production increased with increasing fracture permeability, but the growing extent decreased gradually. And the impact of gas well fracturing was sharply affected by stress sensitivity and start-up pressure gradient.