Experimental Study On Effective Stress Of Coal Under Gas Pore Pressure And Adsorption

Coal is a dual porous medium composed of pores and fissures.Free and adsorbed coalbed methane exists in coal seam pores and fissures.The deformation of coal body is affected by the pore pressure generated by free gas and the adsorption expansion of adsorbed gas.This paper uses a three-axis solid-flow coupling test device,use three different gases?Ar,CO₂,CH₄?as pore media,the axial deformation and radial deformation of coal body caused by different pore pressures were measured under constant temperature and constant pressure conditions.Calculate the stress and strain of coal body to study the deformation law and effective stress law of coal body caused by pore gas,and the deformation effect and effective stress law of coal body caused by gas pore pressure and adsorption are analyzed.Through systematic experimental and theoretical analysis,the following results were obtained:?1?Under stress constraints,the gas in the pores of the coal has two forms of action on the coal body:the mechanical action of the free gas pressure on the solid skeleton and the adsorption expansion of the adsorbed gas.When the pore gas pressure is the same,the deformation of the coal body under the action of the free gas is the same,and the adsorption gas causes the deformation of the coal body to be different.?2?The axial and radial deformation of the coal body caused by gas pore pressure or adsorption increases with the increase of pore pressure,and the deformation perpendicular to the bedding direction is larger than the deformation parallel to the bedding direction.?3?When the coal pores are filled with single-phase CH₄,the pore pressure causes the deformation of the coal body to be greater than the deformation of the coal body caused by the adsorption;when the pores of the coal body are filled with single-phase CO₂,the adsorption causes the deformation of the coal body to be greater than the deformation of the coal body caused by the pore pressure?4?The effective stress coefficient?of coal under the action of pore gas follows the bilinear law of pore pressure and volume stress;the effective stress coefficient?_p of coal under gas pore pressure and the effective stress coefficient?_a of coal under adsorption also follow the bilinear law of pressure and volume stress,and??28??_p?10??_a.?5?According to the experimental results,the effective stress coefficient of the coal body is calculated according to the axial deformation and the radial deformation.It is found that the effective stress coefficient of the coal body has anisotropy,and the effective stress coefficient perpendicular to the bedding direction is larger than the effective stress coefficient parallel to the bedding direction.?6?By establishing a simplified model of CO₂ flooding coal seam CH₄,it is found that the increase in the effective stress of the coal seam decreases with the increase of depth,regardless of whether the same pressure or CO₂ is used to drive the coalbed methane.The vertical deformation of the coal seam also decreases with increasing depth,and both have a good linear relationship.