Numerical Simulation Of Gas-water Two-phase Percolation Considering Dynamic Capillary Force

It is hard to effectively develop low-permeability gas reservoirs currently, which account more than half of China's gas reservoirs. Compared with high and moderate permeability gas reservoirs, there exist threshold pressure gradient in low-permeability gas reservoirs which can affect productivity a lot. The dynamic (non-equilibrium) capillary pressure effect is much obvious in low and special low permeability gas reservoirs. All of the gas reservoir simulation software existed doesn't consider the dynamic capillary pressure effect, thus it is impossible to compare and contrast the influence of the threshold pressure gradient effect and the dynamic capillary pressure effect on gas reservoirs'productivity. So it has important practical significance for productivity evaluation and development plan design of low-permeability gas reservoirs to conduct gas-water two-phase flow numerical simulation with the consideration of dynamic capillary pressure effect.In this paper, a detailed investigation of gas-water flow regularity in low-permeability gas reservoirs and the dynamic capillary pressure (non-equilibrium capillary pressure) effect at home and abroad is presented firstly. The 3-D flow mathematical model of gas and water is established, which simultaneously considers the threshold pressure gradient effect and dynamic capillary pressure effect. After the establishment of the differential form of this mathematical model, the pressure equation is derived and it goes under implicit processing, then the pressure matrix and saturation equation is established and solved. The underlying language of the simulator is C language, the simulator passed some fundamental tests respectively, and its computing results were in a comparison with those of commercial software ECLIPSE. After the verification of the simulator, it is used to separately study the effect of these two factors mentioned above on low-permeability gas reservoirs'productivity.As can be seen from the results, the threshold pressure gradient can significantly reduce gas reservoirs'productivity and the dynamic capillary pressure can also have an impact on it. When the dynamic capillary coefficient is small, the impact of dynamic capillary pressure on gas reservoirs'productivity is limited, ignoring the dynamic effect and directly using the static capillary pressure data obtained from experiment can be allowed. While the dynamic capillary coefficient is large, the impact of the dynamic effect of capillary pressure on gas reservoir should be considered. The influence of dynamic capillary pressure effect on gas reservoirs' productivity can be ignored if water saturation equals to irreducible water saturation at very beginning; when it is greater than irreducible water saturation, the production decline ratio increases dramatically even under the same dynamic capillary coefficient. On the other hand, the production decline ratio caused by dynamic capillary pressure effect will decrease in small increments when water saturation increases.