Numerical Inversion Method For Estimating SRV After Fracturing In Shale Gas Reservoirs

Stimulated reservoir volume(SRV)in shales is generated by horizontal drilling with multi-stage hydraulic fracturing,which governs shale gas production.Micro-seismic data has been used to evaluate the SRV,but it is high-priced,and likely overestimates the SRV.Thus,we aimed to use shale gas well production data to characterize subsurface fracture networks.To achieve this aim,we developed a practical analytical model to predict stimulated reservoir volume(SRV)whilst obtaining effective fracture parameters on the basis of flowback data.Then,the dynamic two-phase relative permeability function is derived on the basis of gas production.Moreover,the analytical model of early flowback stage is derived by using numerical method which includes the material balance equations and the diffusion equations describing the early gas-water two-phase flow in SRV.By establishing two kinds of equations in the effective fracture volume and solving them simultaneously,the function curve of pseudo-pressure with pseudo-time is acquired to calculate SRV of shale gas wells.According to the numerical relationship between slope and intercept of the curve,the effective fracture volume and relevant fracture parameters are solved.Moreover,we verified the analytical model against micro-seismic data.Our results show that the new model predicts SRV using early flowback data,only using measured gas-water flow data in a period of time after well opening and production.The variable SRV model described here has the following significant consequences.The two-phase flowback data of fracturing horizontal wells in the shale gas reservoir has a v-shaped gas-water ratio.The abnormal overpressure in the Longmaxi Formation located in southern Sichuan province makes it necessary to consider the stress sensitivity of permeability in the study area,thus the gas-water two-phase permeability is more accurate.So the results obtained from this computational process are more convincing,and the differences between model calculation and the micro-seismic data can be negligible which provides reliability of the model.The final work was that we verified the proposed model by numerical simulation model.So it is believed that this mathematical model is accurate and useful.The presented approach and obtained results can significantly enrich the production performance analysis methods for shale gas reservoirs.