A Study Of Deformation And Percolation Mechanisms Of Deep Gas Reservoir And Its Application

Currently, a plenty of low permeability/tight gas reservoirs with deep buried depth have been found in western China. Gas percolation in these reservoirs does not comply with the Darcy law, which is different from those of normal gas reservoirs. The reason is that gas production of these reservoirs are effected by stress sensitivity effect, gas slippage effect and starting gradient effect. Based on experimental methods, and combined with theories of rock mechanics, material mechanics, percolation mechanics and gas reservoir engineering, non-Darcy percolation mechanism and relative influence factors of low-permeability deformable gas reservoirs are studied.First, by a great deal of core deformation experiments, the effects of deformation on rock parameters such as porosity, compressibility, permeability are studied, and the deformation characteristics of low permeability cores are obtained. Based on these results, the mechanics of stress sensitivity of deep low permeability reservoirs are studied deeply, and the evaluation standard of stress sensitivity is analyzed. A new stress sensitive coefficient is defined, and then the relationship between permeability and effective stress can be expressed by a dual-power function. By this method, core permeability in reservoir condition can be calculated conveniently, which provides a basis to establish percolation model of low-permeability deformable gas reservoir. At the same time, various influence factors of stress sensitivity are studied by experiments.Second, by experimental methods, the gas slippage effect and starting pressure gradient effect on gas production in low permeability gas reservoir are studied. Productivity formulae of gas wells of steady state flow and pseudo-steady flow are deduced respectively. And according to the effect of stress sensitivity on gas productivity, an evaluation standard of stress sensitivity for gas reservoirs is given. Taking stress sensitivity, gas slippage and starting pressure gradient effect into account, gas reservoirs are categorized into high-medium permeability gas reservoir, low-permeability gas reservoir, extra-permeability gas reservoir and tight gas reservoir.Finally, based on experimental and theoretical research, considering the effect of stress sensitivity, gas slippage and starting pressure gradient, a non-Darcy gas-water percolation numerical model is established, and 3-D two phases simulator is developed. Gas reservoir simulation result shows that the effect of gas slippage and starting pressure gradient on gas production is slight; however, stress sensitivity brings great effect on gas production. When taking permeability variation and rock compressibility variation into account, the stable production period and recovery percent of reserves are both reduced. The higher the daily gas production, the more serious the impact on gas production. In order to obtain optimum rate of gas production, stable production period and ultimate recovery factor, both economic factors and technical factors should be considered when determine the gas well daily production.The research above is useful to understand the percolation characteristics of deep deformable gas reservoirs and is helpful to the reasonable development of such gas reservoirs.