页岩气藏分段压裂水平井不稳定渗流模型

针对页岩气藏中吸附气和游离气共存的储集方式,基于双重介质模型建立考虑吸附解吸过程的页岩气藏不稳定渗流数学模型,并定义新的参数来表征基质中吸附解吸气量与游离气弹性释放量的比值,利用Laplace变换计算页岩气藏点源解,通过叠加原理得到定产量生产时水平井分段压裂改造后井底压力解,对考虑井筒和表皮系数的影响以及定井底流压生产时水平井动态产能进行分析。结果表明:在开采过程中页岩吸附解吸气量所占比例较大,且考虑吸附解吸后,定产量生产所需压差小,压力波传播到边界时间晚,压力导数曲线凹槽更加明显,同时定井底流压生产时压裂水平井产量更大,稳产时间更长;Langmuir吸附体积越大,压力波传播越慢,所需压差越小,压力导数曲线凹槽越深,同时页岩气藏稳产时间越长,产量越大,但产量的增幅越小。

Transient flow model of stage-fractured horizontal wells in shale gas reservoirs

Considering the coexistence of adsorbed gas and free gas in shale gas reservoir, a mathematical transient flow model incorporating gas desorption process was established based on dual porosity gas reservoir model. A new parameter was defined to represent the ratio of gas production from desorbed gas to that from free gas in matrix system. The analytical point source solution of shale gas reservoir was solved in Laplace space, and the bottom-hole pressure of multi-stage fractured horizontal well with constant rate was obtained by superposition. Then the impacts of wellbore storage and skin factor on well production performance were evaluated and the rate decline response at constant bottom hole pressure was investigated. The results show that a large portion of gas production comes from desorbed gas. When gas desorption in shale matrix is considered, lower draw down pressure is required to achieve constant production rate for those fractured horizontal wells, it takes longer time for the pressure front reaching boundary, and the pressure derivative curve shows more obvious groove. If the fractured well is produced under constant bottom hole pressure condition, higher gas production rate and lower decline rate can be achieved. The results also show that the bigger the Langmuir absorption volume is, the slower the pressure front propagates, and the deeper the groove shows on the pressure derivative curve. But the Langmuir absorption pressure impact must be analyzed with both matrix pressure and Langmuir adsorption isotherm curve.