| Two prominent problems have emerged as a result of intensive study and understanding of shale gas reservoir development.The first problem is the ambiguous understanding of the gas-water flow that results from injection of primary water and fracturing fluid,especially the water flow law and its influence on gas production.The second issue is the complex multi-field coupling mechanism caused by changes in stress and temperature.This complex coupling exerts an unknown influence on the flow law leading to various consequences,such as the complex flow law in shale reservoirs,limitation of simulation methods,and insufficient production prediction accuracy.Therefore,this article presents a research involving numerical simulation of the gas-water multi-field coupling flow in shale reservoirs.This study was conducted via aspects such as physical model characterization,mathematical model construction,numerical solution,and simulation application.ⅰ)A heterogeneous conceptual model is established based on the characteristics of the two-phase,multi-sector,multi-field coupling of shale gas reservoirs.Based on this model,the mechanisms of multi-scale flow,gas-water flow,fluid-solid coupling,and hydration are summarized.In addition,the mathematical characterization of the evolutionary relationship between the high-pressure physical properties of the fluid and temperature-pressure system,which form a parameter library,was established.This study also creates a complex physical model of the field structure of a two-phase heterogeneous flow that includes the water saturation,relative permeability curve,and capillary force curve in different sectors in the parameters of inhomogeneous flow field distribution for the first time.In addition,this study combines the embedded discrete fracture model with the equivalent inhomogeneous continuous medium model.ⅱ)A mathematical model of a complex flow field for gas-water production of shale gas reservoirs is established.This model accounts for the coupling of thermalfluid-solid multi-physical fields and constructs control equations for the flow,stress,and temperature fields.In addition,an accurate characterization of the complex mechanism and flow field structure of shale gas reservoir development was performed after systematically considering various complex and crucial factors.The factors included the methane adsorption capacity related to the temperature-pressure system,dynamic high-pressure physical evolution of the gaswater flow,and influence of hydration on the adsorption capacity,shale physical properties,and solid mechanical properties.ⅲ)The multi-field coupling mathematical model was created to develop a numerical simulator based on the vectorization programming principle.The solution algorithm was developed using the automatic differentiation method combined with the meshless generalized finite difference method for the first time.The coefficient matrix at the left end of the linear equations and the constant term at the right end were accurately calculated using the weight matrix,so that the complete coupling of multi-physical fields was also efficiently and accurately simulated under arbitrary node distributions.In addition,the two-phase distribution and the effect of the stress-temperature field relationship were quantitatively analyzed in the shale gas reservoir development process.Thus,the decisive factors in gas and water production in shale reservoirs were determined based on geological condition factors and production system parameters.Furthermore,history matching was done with actual data of gas and water production in shale reservoirs,resulting in satisfactory fitting performance,which verifies the accuracy of the model and algorithm presented in this article.ⅳ)An electromagnetic-thermal-fluid-solid multi-field coupling simulation process was presented for the first time by establishing a multi-field coupling numerical simulator for microwave heating in shale gas reservoirs.The two-phase flow was taken into account while developing this simulation process.This study revealed the limitation of predicting the effect of in situ heating only under singlephase conditions as was done in the past.In addition,the influence of the water saturation on the effect of increasing production was also set forth in the actual production process to determine that microwave heating is not suitable for shale gas reservoirs with high water saturation.The two-phase and multi-field coupling mathematical model established for shale gas reservoirs and the simulator developed in this article can provide some technical support for predicting shale gas production in the future.This algorithm also adds a new platform for the multi-field coupling function to future reservoir numerical simulations based on an enhanced understanding of shale gas reservoir development. |
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