基于有限体积方法的页岩气多段压裂水平井数值模拟

Numerical simulation on multi-stage fractured horizontal wells in shale gas reservoirs based on the finite volume method

为了实现页岩气在多尺度介质中的流动模拟,考虑页岩气在基质—天然微裂缝和人工大尺度压裂缝中的流动特征,建立页岩气多段压裂水平井不稳定渗流数学模型,针对模拟区域采用非结构四面体网格进行网格剖分,基于有限体积方法离散建立页岩气三维渗流数值模型,然后通过顺序求解的方法进行求解,进而模拟页岩气多段压裂水平井的生产动态和储层压力分布变化,并对模拟结果进行分析。研究结果表明:(1)采用所建立的数值模拟计算方法与商业数值模拟软件Eclipse计算的多段压裂水平井产气量基本一致,证实该模型正确、可行;(2)分别采用顺序求解方法和全隐式求解方法计算得到的页岩气水平井产气量虽然在生产初期存在着差异,但随着计算的推进,二者迅速趋于一致,进一步验证了该模型的正确性;(3)尽管解吸气对地层压力具有补充作用,但作用有限,对产气量的影响不大,随着生产时间的延长,解吸气量在产气量中所占比例逐渐上升;(4)确定合理的压裂段数且获得较长的压裂缝长,是页岩气水平井增产改造的核心。结论认为,该研究成果有助于页岩气储层体积压裂的设计以及多段压裂水平井生产动态的预测。

In order to simulate the flowing of shale gas in multi-scale media,we established a mathematical model for the unsteady seepage of multi-stage fractured horizontal wells in shale gas reservoirs in consideration of the flowing characteristics of shale gas in matrix,natural fractures and large-scale artificial fractures.Grid division in the simulation region was carried out by means of nonstructural tetrahedral grid.Then,a3D numerical model for the seepage of shale gas was established discretely using finite volume method and solved using sequence solution method.Finally,the production performance of multi-stage fractured horizontal wells in shale gas reservoirs and the reservoir pressure distribution were simulated,and the simulation results were analyzed.And the following research results were obtained.First,the gas production rates of multi-stage fractured horizontal wells calculated by this newly established numerical simulation method are basically consistent with the calculation results by the commercial numerical simulation software Eclipse,which proves that this new model is accurate and feasible.Second,the gas production rates of horizontal wells calculated by the sequential solution method are different from those calculated by the fully implicit solution method in the early production stages,but as the calculation progresses,both of them tend to be consistent,which further verifies the accuracy of this new model.Third,desorbed gas plays a supplementary role to reservoir pressure,but its function is limited,and its effect on gas production is little.As the production goes on,the percentage of desorbed gas increases gradually.Fourth,the key to the stimulation of shale-gas horizontal wells is to determine the number of fractured sections rationally and create longer artificial fractures.In conclusion,the research results are conducive to the design of stimulated reservoir volumes(SRVs)of shale gas reservoirs and the prediction of production performance of multi-stage fractured horizontal wells.