Multiscale Simulation Of Shale Gas Reservoir

It is widely considered that the shortage of conventional oil and gas resources has become a bottleneck of global economic development.As an important class of unconventional natural gas resources,shale gas reservoirs with huge reserves significantly contribute to the growing energy demand.In the past decade,shale gas resources have received additional attention and become a major focus of the petroleum industry,as well as energy resource industries worldwide.Due to the multiscale pore spaces,special gas storage types,multiple transport mechanisms and complicated fracture networks generated by horizontal-drilling and hydraulic-fracturing technologies,the traditional fluid flow theory is no longer applicable to shale gas reservoir.Therefore,it is very necessary and urgent to establish a new method for gas flow simulation in shale gas reservoir.A multiscale simulation method combining the microscale modeling with macroscale numerical simulation is proposed in this master thesis.Firstly,both the pseudo-steady state transfer models and transient transfer model under oil reservoir condition are derived and their applicability is analyzed based on the numerical simulation study of fine grid model.In addition,considering the special gas storage types and multiple transport mechanisms in shale gas reservoir,the pseudo-steady state and transient transfer models under shale gas reservoir condition are also derived,and a corrected pseudo-pressure based transient transfer function of shale gas reservoir is finally obtained.Secondly,how to consider the effect of kerogen in shale gas reservoir simulation is investigated.Based on these complex shale gas transport mechanisms,including viscous flow,Knudsen diffusion,surface diffusion and gas adsorption/desorption on the internal kerogen grain surfaces,a kerogen – inorganic matrix – fracture triple-continuum model is established.The kerogen – inorganic matrix transfer flow is simulated by the Warren-Root pseudo-steady state transfer model,while the inorganic matrix – fracture transfer flow is simulated by the transient transfer model.The finite element method(FEM)is adopted to solve the model and the factors influencing shale gas production performance are analyzed.Then,based on a large amount of simulation studies of triple-continuum model,an improved dual-continuum model considering the effect of kerogen is further proposed.The improved dual-continuum model can deal with kerogen more precisely compared with the conventional matrix – fracture dual-continuum model.Meanwhile,compared with the triple-continuum model,this improved model has the strengths of clear physical meaning,simple mathematical model and better applicability to large scale reservoir simulation.Thirdly,considering the real gas effect and geomechanics effects of natural fractures and hydraulic fractures,two integrated reservoir models of stimulated shale gas reservoir are established.One model includes a single region and the matrix – fracture dual-continuum model is applied.In addition,the Discrete Fracture Model(DFM)is used to simulate the hydraulic fractures.The other model includes two zones: Stimulated Reservoir Volume(SRV)and the zone outside SRV.In SRV,the hydraulic fractures are also simulated by DFM and the matrix with natural fractures is simulated by dual-continuum model.The single-porosity model is applied in the zone outside SRV.FEM is used to solve the above models and a detailed sensitivity analysis is conducted.Finally,based on the above research work,a multiscale simulation method is proposed in this thesis.The microscale physical model is first established based on the pore structures of shale rocks.Different gas transport mechanisms in organic matter and inorganic matrix are considered and the microscale mathematical model is established.The microscale modeling is conducted to investigate the factors influencing the permeability of shale rocks.By this model,both the apparent permeability curve and intrinsic permeability curve can be obtained.Then,these two curves are incorporated into the macroscale numerical simulation of shale gas reservoir.For the macroscale numerical simulation,we combine MINC and the above improved dual-continuum model.By doing so,the microscale characteristics,including the organic matter distribution and different transport mechanisms in organic matter and inorganic matrix,can be reflected in the macroscale shale gas reservoir simulation.