Well Performance Evaluation For Shale Gas Production With Varying Flow-rates And Pressures

Shale gas has now become an important part of unconventional hydrocarbon resources all around the world.In order to improve the production capacity,horizontal wells with multiple hydraulic fractures are the most popular stimulation technique for shale gas reservoirs.The application of multi-fractured horizontal wells is expected to create a complex sequence of flow regimes.In addition,the existence of pressure-sensitive effects,gas adsorption/desorption,and diffusion cannot usually be ignored in shale gas reservoirs.Consequently,fractured shale gas wells exhibit a complex production behavior.In this situation,conventional well performance analysis is not applicable.To ensure that shale gas wells produce efficiently and stably,new well performance analysis methods are essential during field production.Production data analysis is an important approach for well performance analysis.It is widely used to analyze the long-term production with variable flow rate and pressure data of a single well,and provide a full-life analysis for oil and gas production.In this work,we develop an analytical model to study gas flow mechanisms in shale formations.Based on the model,we perform type curve matching and production data analysis,and then the precision of the production predicting is improved.The main contents are described as follows:1.The mathematical model coupling flow mechanisms of shale gas is proposed and the analytical solution is provided.Considering adsorption,diffusion,pressure-sensitive effects and the state equation with high pressure and temperature,we establish the mathematical model of multi-fractured horizontal wells in a fully closed shale gas reservoir with a variable flow-rate.Duhamel's principle,Laplace transform and inversion,and Newman's method are employed to solve the model.Then type curves for multi-fractured horizontal wells in shale gas reservoirs are plotted.To verify the proposed method,we conduct analytical and numerical validations.The effects of relevant parameters are analyzed as well.The storage ratio ?,interporosity flow coefficient A,and dimensionless constrained axial modulus MD affect the log-log type curves differently.2.The definition of virtual equivalent time is proposed to address the issue caused by abrupt changes during production data analysis.For most shale gas wells,frequent shut-ins and nozzle size changing often occur,leading to significant discontinuities in production data.To address the problem caused by abrupt changes in production data,this study proposes the definition of the virtual equivalent time,which is calculated based on the average formation pressure.The virtual equivalent time can transform the abrupt production data into continuous data.By use of the virtual equivalent time,the matching qualities will become better,and the issue caused by abrupt changes can be satisfactorily addressed.Moreover,the history matching and productivity forecasting of a single well are efficiently improved.Fewer times of superposition principle calculations and smaller cumulative errors are owing to the virtual equivalent time.3.The method to calculate the adsorbed gas ratio in production is presented and the dynamic reserves of adsorbed and free gas are provided for the first time.During shale gas production,both adsorbed gas and free gas play important roles.Thus the contributions of free and adsorbed gas to the shale gas production are hot,significant,and challenging problems in petroleum engineering.With the normalized formation pressure distribution obtained by solving the mathematical model of multi-fractured horizontal wells in shale gas reservoirs,the expressions of the amounts of adsorbed gas,matrix free gas,and fracture free gas in shale versus the producing time are presented on the basis of Langmuir adsorption model.To verify the proposed expressions,we conducted a simple case study to calculate the amounts of adsorbed and free gas in production.The comparison between the calculated flow-rates and the measured flow-rates shows a good agreement.4.The large software for shale gas production data analysis is developed independently,and applied to evaluate the well performance of shale gas wells in China.Field case studies of four multi-fractured horizontal shale gas wells in China are carried out with the independent developed software.The software can provide the formation parameters and predict the well production performance effectively and practically.Also,pressure distribution in the formation,average formation pressure,adsorbed to total ratio during production,water saturability around the well are offered by the software,which are significant to the filed shale gas production.