Impact Of Gas Adsorption Induced Coal Damage And Supercritical CO₂ Fracturing On Permeability Evolution Of Coal Seam

The coal seam gas extraction is the basic method to prevent coal and gas outburst and to use gas as a clean energy.Injecting CO₂ into coal seam,on the one hand,can increase the coal seam gas drainage quantity;on the other hand,can store the CO₂ in the non mining seam to reduce the greenhouse gas emissions.During the process of injecting CO₂ into coal seam,CO₂ is likely to adsorb and dissolve into coal seam,swells them slightly,and generate absorption strain.Rearranging coal rock micro-structure to a lower free energy release status,absorption induces coal damage and affects permeability evolution.This adsorption-and dissolution-induced coal damage is usually ignored in the permeability model even though this kind of damage has been observed and detected in the past CO₂ injection experiments.The other hand,super-critical CO₂ fracturing is viewed as a very promising method for coal bed permeability improvement.However,the mechanism of the permeability evolution caused by fracture is not clear.In view of this,we study the impact of gas adsorption induced coal damage and super-critical CO₂ fracturing on permeability evolution of coal seam,and the main contents include:?1?Under the effect of adsorbed gas,the strain of the adsorption would cause coal seam damage which can affect the evolution of permeability.Based on this,taking the coal damage induced by gas adsorption into account,the dual-permeability model is established.Compared with complete elastic model,it proves that it is necessary and scientific to consider absorption damage into the permeability model.?2?Using the established dual-permeability model which considering gas adsorption-induced damage,study the impact of different gas,different injection pressure and different coal rank on the permeability curve,analyze permeability evolution mechanism,and compare with typical experimental results to verify the model correctness.?3?Based on the Span-Wagner state equation,and considering coal seam deformation,gas flow and coal seam damage,the multiphysics field coupling model of supercritical C02 fracturing is established to simulate super-critical CO₂,liquid CO₂ and hydro-fracture.And by this model,researchers can analyze crack initiation and expansion process and study the influence of dynamic viscosity on crack pressure and fracture pressure,which can verify obvious fracturing and permeability improvement advantages of supercritical CO₂.