Simulation Of Micro-flow In Tight Gas Reservoirs

As typical unconventional gas reservoirs,tight sandstone gas and shale gas have become the most important alternative resources of conventional natural gas due to its wide distribution and abundant reserves.Because of tight lithology and complicated pore-throat structure,it is hard to obtain flow parameters accurately by experiment methods.In addition,due to the complex storage and transport mechanisms,gas flow in tight sandstone and shale is strongly non-linear and can not be accurately described by the traditional Darcy percolation theory.So,in the development of unconventional gas reservoirs,it is urgent to study the percolation characteristic and migration law of gas flowing in tight reservoirs.For the limitations of current experiments and percolation theory,the pore structure data from different sources was chosen,and pore network models were established to represent the complex pore structure of unconventional reservoirs.Then,based on the pore network models,the multiple flowing mechanisms were coupled to construct simulation platform of tight porous media.The research was carried out to study the transport mechanisms and migration law of different gas reservoirs as well.The major work and conclusions of this dissertation can be summarized as follows:1)According to the characteristics of pore-throat distribution of in low permeable and tight sandstone,a novel method to construct equivalent pore network model for low permeability and tight sandstone is presented by coupling CT scanning images and mercury porosimetry data.The newly-established pore network models are capable of representing the topology of void space in sample as well as capturing the distribution of tiny throats and connection of pore space.2)Based on the nitrogen adsorption experiment result,a novel random algorithm is presented to construct pore network representing nanoscale pore space in shale matrix.The good agreements in porosity,specific surface and pore-throat distribution are observed between the pore network models and real samples,which verify that the pore network models are suitable for gas flow modeling.3)A multi-mechanism mathematical model taking consideration of complex transport mechanisms(i.e.adsorption-desorption,surface diffusion,viscous flow,slip flow and Knudsen diffusion)of both free gas and adsorbed gas in nano-and micro-pores is developed.Next,the solution of the gas flow model in pore network is derived by discretizing the mathematical model and computer programming with suitable numerical solution methods.This methodology can be use to study and simulate gas flow in micro-scale.4)The characteristics of gas flow in nano-and micro-pores at different conditions are analyzed,and type curves describing the relationship between Knudsen number and transport mechanisms are established.Conclusions can be drawn:it is Darcy flow when Knudsen number is smaller than 0.006;gas flow is characterized by strong nonlinear due to the Knudsen diffusion and slip flow when Knudsen number is bigger than 0.006;the contribution of surface diffusion to gas flow is more than 5 percent when Knudsen number is over 0.3.5)According to the relation between macroscopic physical parameters and microscopic pore structure,the evaluation criteria of gas migration mechanisms which takes absolute permeability/porosity as index is given,and the transformation process of gas flow mechanisms during the gas reservoir development are analyzed.The simulation method proposed in this paper will offer some ideas and approaches to study the gas seepage mechanism in tight reservoirs.What's more,the research results will contribute to our understanding of the gas transport rules in unconventional gas reservoir,providing a theoretical support for rational development plans.