吸附气对气水两相流页岩气井井底压力的影响

Influence of adsorbed gas on bottomhole pressure of shale gas wells with gas-water two-phase flow

在页岩气藏中,有相当一部分页岩气以吸附气状态存在。在气藏开发过程中,随着气藏压力的不断降低,吸附气会发生解吸扩散,这一现象会影响页岩气藏的压力变化。应用Langmuir等温吸附曲线和Fick拟稳态扩散定律,对吸附气的解吸扩散特征进行了数学表征;结合气水两相渗流理论,建立了页岩气藏气水两相渗流数学模型;应用有限差分法得到相应的数值模型,编程求解得出了井底压力变化的数值解;分析了吸附气的解吸扩散对气水两相流井底压力变化的影响。研究结果表明:解吸气补充了流体供应量,提高了地层能量,使井底压力的下降变缓,压力差曲线下移;由于基质收缩及孔隙增大,储层物性和地层流动性能得到改善,压力差导数曲线前期凹陷加深。

There is a number of shale gases exsiting in the form of absorbed gas in shale gas reservoir. In the process of the development of gas reservoir, desorption and diffusion of absorbed gas will occur with the dropping of gas reservoir pressure. This will impact the pressure change process of the reservoir. Applied Langmuir isothermal adsorption curves and Fick diffusion law of quasi-steady state, the features of desorption and diffusion of adsorbed gas are charicterized mathematically in this paper. Combined with the principle of gas-water two-phase flow, a mathematieal model of gas-water two-phase flow was built up for the shale gas reservoir. The finite difference method was used to get the numerical model and the computer programming was used to work out the analytic solutions of bottomhole pressure. At last, the influences of desorption and diffusion characteristics of adsorbed gas on the variation of gas well bottomhole pressure was analyzed. The results show that the fluid is supplied with adsorption gas, which increases the formation energy in turn. Thus, the decreasing of the bottomhole pressure becomes slower and pressure difference curve moves down. Due to matrix shrinkage and porosity increasing, physical property and formation flow property are improved,which leads to more obvious prophase depression of the pressure difference derivative curve.