Characteristics And Parametric Analysis Of Gas Migration In Tight Gas Reservoirs

Tight gas reservoirs are featured by ultra-low porosity and permeability,high adsorbed gas content,high crustal stress,and complex rock formation structures,which make tight gas migration become a multiphysics coupling complex process.It is of great significance to investigate the gas migration in tight reservoirs for accurate evaluation of production capacity,reasonable production management and utilization of tight gas.Based on the theory of linear poroelasticity,this work systematically studies the migration process of the tight gas using theoretical and numerical simulation methods.The fluid-solid coupling mechanisms of the gas migration is further clarified,the key parameters that greatly affect the gas migration are determined,and an index of migration capacity which can to some extent provide guidance to the engineering practice is proposed.The major research results include:1.More accurate porosity model and apparent permeability model are established considering the solid deformation and multiple flow mechanisms.The improvements of new models include:(1)the initial deformation state of the reservoir is more accurately determined from boundary conditions;(2)the real stress distribution instead of the constant total stress assumption is adopted;(3)the weights of the bulk and adsorbed gas in gas flow are taken into account.Moreover,the fluid-solid coupling governing equations of gas migration are derived on the basis of new models.2.Using COMSOL software,numerical simulation and systematic parameter analysis of the process of the tight gas migration is carried out under the stress confined boundary conditions and the uniaxial strain boundary conditions.The results show that :(1)the gas transport capacity depends on the competition between the decrease of the intrinsic permeability caused by the solid deformation and the increase of the permeability caused by the dynamic and adsorption effects.The initial deformation state of the reservoir has an important influence on gas migration capacity.The constant total stress assumption results in the disappearance of the instantaneous equilibrium process during a full gas migration.The introduction of weight coefficients can more accurately evaluate the contribution of different flow mechanisms.(2)when the pore bulk modulus is taken as an independent parameter,higher values of pore bulk modulus,Langmuir adsorption strain constant,surface adsorption diffusion coefficient,the confined stress,the initial porosity,and lower values of the reservoir bulk modulus,Poisson's ratio,Langmuir pressure constant,the ratio of pore adsorption strain to the reservoir adsorption strain,the initial pore pressure,and the initial intrinsic permeability can improve the gas migration capacity,while the effects of the grain bulk modulus,Langmuir volume constant,and the wellbore,pressure can be neglected.3.A new porosity model is developed when the relationship between the pore bulk modulus and other parameters is deduced in terms of the Betti-Maxwell reciprocal theorem.Based on the new porosity model,numerical simulation and parameter analysis of gas migration under the above two boundary conditions were performed.It is found that:(1)the gas migration capacity decreases as the pore bulk modulus gradually reduces with the process of gas migration.But the fluid-solid coupling mechanisms behind the tight gas migration do not change.(2)when the pore bulk modulus is not an independent parameter,higher values of the reservoir bulk modulus,the initial porosity,and lower values of the grain bulk modulus can facilitate gas migration in the reservoirs.The influence of other parameters is similar to those when the pore bulk modulus is taken as an independent parameter.4.An index of migration capacity related to the initial deformation state is suggested according to the analysis of numerical results.The qualitative correlation between the index and the migration capacity of the tight gas is verified for different porosity models.That is,the higher the index value,the stronger the gas migration capacity.Such migration capacity index may provide helpful reference to conveniently assess the migration capacity of the tight gas reservoirs in engineering applications.