Multi-mechanism Math-physics Models For Tight Oil And Gas Reservoirs And Their Solutions

Tight oil and gas are important parts of unconventional oil/gas resources.However,traditional geophysical theories and methods can hardly be applied to the exploration and development of tight oil/gas resources directly.So it is of great importance to develop new theories and methods.The key point is to establish math-physics models,namely the wave propagation models,which can accurately describe the characteristics of waves in tight porous media.Therefore,based on the main characteristics of tight oil/gas reservoirs,this thesis develops multi-scale,multi-mechanism wave propagation models characterizing fluid-saturated tight porous media.And we studies the dispersion and attenuation of waves.Specifically,it includes the following contents:Firstly,this thesis develops a wave propagation model based on the non-Darcy flow.The model considers the effects of the non-Darcy flow,which commonly exists in tight oil/gas reservoirs,and introduces the seepage relation of the non-Darcy flow into the poroelastic equations.After that,based on the plane wave analysis,the effects of non-Darcy flow on the dispersion and attenuation of waves are studied.The numerical results show that the power law index of the effective fluid and the initial amplitude of the wave have a significant effect on the dispersion and attenuation.Additionally,to accurately describe the dispersion and attenuation in low frequency band,this thesis combines the non-Darcy flow model with the viscoelastic model to establish a unified model and studies the effects of the two mechanisms on the dispersion and attenuation.Applying the model to the real fluid-saturated tight rocks,the results show that the model can accurately describe the dispersion in the low frequency band.Secondly,considering the mesoscopic and microscopic heterogeneities,which are the main reasons causing the energy dissipation,a multi-scale and multi-mechanism poreviscoelastic model is established.This model can simultaneously describe the dispersion and attenuation caused by the two mechanisms.In addition,comparing with the experimental data of fluid-saturated tight porous media,the new model can accurately describe the wave dispersion and attenuation.Thirdly,this thesis uses the finite element method to solve the multi-scale and multi-mechanism poreviscoelastic model.Based on the characteristics of the model,the SRM absorbing boundary condition is applied to the numerical method and the results show that the boundary condition works well.Based on the geological characteristics of tight oil/gas reservoirs,we simulate the wave propagation in macroscopic homogeneous and heterogeneous porous media.Finally,for the extremely low frequency,the wave velocity in the tight porous media may not satisfy the Gassmann relation.To solve this problem,the concept of generalized effective pressure is introduced into the multi-scale and multi-mechanism poreviscoelastic model.Then the low frequency limit predicted by the original model can be modified.The effects of the effective pressure coefficient,which is a new parameter,are studied.The results show that the effective pressure coefficient has significant effects on the velocity and amplitude of the fast P wave.