Research on special seepage mechanism and prediction method for stable productivity of low permeability gas pools

The low permeability gas pools, different from conventional gas reservoirs, are characterized by poor reservoir physical properties and high water saturation, thus its seepage mechanism is even more complex. This paper sets forth in detail the special seepage mechanism of low permeability gas pools based on low permeability reservoirs. Meanwhile, in line with inadequate consideration factors for productivity forecast of low permeability gas pools and inaccuracy of the result of productivity assessment, this paper inferred a general equation for prediction of stable gas productivity, according to gas pool productivity equation under stable seepage conditions and activating pressure gradient model, and taking into full consideration the gas slippage effect and multiple non-linear effects like low-rate non-darcy effect, high-rate non-darcy effect, pressure sensitivity effect, in combination with seepage mechanic fundamentals. The result shows that the low-rate non-darcy effect, high-rate non-darcy effect and pressure sensitivity effect of low permeability gas pool all cause the gas pool productivity to become smaller. If these effects were neglected, then optimistic assessment would be made on productivity. Among which, the activating pressure gradient and pressure sensitivity coefficient (particularly the anomalous high pressure reservoir) have a significant effect on gas well productivity. The slippage factor plays a second role, and the high-rate non-darcy effect has the least effect. The examples confirmed the correctness of the productivity model, which can be used for actual field development work.