Effect of flow mechanism with multi-nonlinearity on production of shale gas

Abstract

Different from oil reservoirs, variations of gas properties (such as viscosity, Z-factor and gas compressibility) under different pressures is strongly nonlinear and non-Darcy effect is significant in fractures due to high rate. Shale gas reservoirs are extremely tight with nanopores, where Darcy's law breaks down and the flow behavior is significantly influenced by pore scale and pressure. 20%–80% of the shale gas in place (in-situ) is adsorbed to organic matters and the desorption is a nonlinear process varying with pressure. Furthermore, hydraulic fractures and natural fractures close gradually, as the production proceeds, resulting in a non-linear relationship between permeability and pressure. However, the multi-nonlinear flow mechanism, as well as its effect on gas production, in the process of shale gas development is always overlooked by both laboratories and industrial analyses.Based on the five-region model, finite difference method is applied to get numerical solution in this paper. Afterwards, the effect of nonlinear mechanism on production is analyzed, according to which, the enhanced ultimate recovery (EUR) schemes are proposed. The results show that the effects of compressibility, multi-scale flow, stress sensitivity and non-Darcy flow in fractures on production are significant during early stage and should be considered in well testing model. For middle and late production stages, the effects of compressibility, multi-scale flow, stress sensitivity in natural fractures should be considered in the Rate Transient Analysis (RTA) model and long-term production prediction model. The negative effect of stress sensitivity and non-Darcy flow can be reduced or mitigated by optimizing schedule and controlling early pressure drawdown. Furthermore, some nonlinear factors can be used positively by refracturing, which reduces formation pressure and consequently leading to the increase of gas compressibility, desorption compressibility and apparent permeability.