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

Key words:  shale gas reservoir  Langmuir adsorption isotherms  fractured horizontal well  point source solution  transient flow

Abstract:

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.