Permeability Evolution Model Of Fractured Porous Adsorption Media And Its Engineering Application

China is a country with coal as its main energy source.With the development of society,it is currently facing difficulties such as energy shortage and environmental pollution.As an unconventional energy source,shale gas/coalbed methane is a clean and environmentally friendly energy source and can be used as an important strategic supplement to conventional natural gas.Therefore,accelerating the development of shale gas/coalbed methane industry has important strategic significance for alleviating energy shortage,controlling environmental pollution and optimizing energy structure.The shale and coal rock are typical matrix-fracture multiple adsorption media,and the evolution law of permeability performance controls the law of gas production in reservoirs.During the exploitation of shale gas/coalbed methane,the reservoir pore pressure will gradually decrease as the mining time increases.Generally speaking,the reduction of pore pressure brings two effects.One is that the pores are compressed by the increase of the effective stress,and the other is that the equilibrium state of the adsorption desorption is broken,which induces the shrinkage or expansion of the matrix,and then acts on the pores as well as on the fracture.These two roles ultimately lead to the dynamic evolution of reservoir porosity and permeability.Accurate description of reservoir porosity and permeability evolution law is of great significance for predicting shale gas coalbed methane production.According to the theory of elastic mechanics of porous media,the evolution law of permeability performance in shale gas/coalbed methane production process is systematically studied by means of theoretical analysis,numerical simulation and engineering data comparison verification.Mainly got the following conclusions:1)Fractures have a great influence on gas production of reservoir gas,and the most obvious influence is the early production period with the highest gas production rate.Therefore,it is very important to accurately predict the evolution of fracture permeability.Although the strain caused by adsorption/desorption is greater than that caused by the increase of effective pressure,we still need to accurately consider the difference of effective stress in different media.If the effective stresses of different media are assumed to be equal,the description of permeability evolution will produce errors,which will affect the results of numerical simulation.Through comparative analysis under different conditions,it can be concluded that although the improved fracture permeability evolution model(IPEM)is more complex in mathematical form,its application scope will be larger.Especially for fracture pore adsorption media with large compressibility,the improved fracture permeability evolution model(IPEM)can more accurately describe the evolution of fracture permeability.2)In the process of shale gas reservoir gas production,the dynamic change of reservoir permeability(migration performance)is caused by two aspects:stress action and chemical action(adsorption/desorption).But the relationship between these two roles is not always competitive.For example,in the early stage of reservoir gas production,these two effects would enhance each other's influence,because the increase of effective stress would reduce the porosity of the reservoir.However,chemical action involves both adsorption and desorption,so chemical action may increase the porosity of the reservoir.3)We found that the period of fracture permeability increase was significantly shorter and the period of inorganic matrix permeability increase,which revealed that the degree of stress effect depends on the compressibility of the medium,but the degree of chemical effect(adsorption/desorption)has nothing to do with the compressibility of the medium.In view of the different compressibility of organic matter matrix,inorganic matter matrix and fracture,it is very important to accurately describe the deformation of each medium in the process of reservoir gas production,which also reflects the significance of the model proposed in this paper.Moreover,the higher the initial permeability(migration performance)of the reservoir,the more drastic and rapid its evolution in the process of gas production.The higher the initial permeability(migration performance)of the reservoir,the more intense the competition between the effects of stress and chemistry,which not only makes the evolution behavior of permeability(migration performance)more intense,but also shortens the time of each stage in the evolution process.4)Because the Langmuir volume constant of carbon dioxide is larger than that of coal-bed methane,and the coal-rock matrix has a stronger adsorption capacity for carbon dioxide.Therefore,the expansion caused by the adsorption of carbon dioxide on the coal-rock matrix is greater than the contraction caused by the desorption of coal-bed methane.Therefore,adsorption is the main influencing factor of reservoir permeability evolution near the injection well,and the change of effective stress is the main reason of reservoir permeability evolution near the production well.Therefore,under such circumstances,the evolution trend of reservoir permeability is generally declining.The evolution of permeability is fiercest near the production and injection Wells,and moderates the further away from the wellhead.As we increase the CO2 injection pressure,the permeability evolution of the reservoir will become faster and more dramatic.