Experimental study of pressure measurement without sealing hole based on coal core in situ volume recovery

The non-sealing pressure measurement technology based on “state recovery” is a new experimental pressure measurement technology that compensates the gas loss, restores the coal core volume, and artificially simulates the coal reservoir temperature to restore the coal seam gas pressure under the original coal body occurrence state by transferring the gas pressure test coal sample from the original coal body occurrence state to the stripped coal sample and put it into the experimental coal sample tank. It is a feasible new technology to obtain the residual gas pressure of pre-pumped coal seam accurately and quickly, which is not limited by the location of downhole pressure measurement and the quality of hole sealing.The accurate recovery of the core volume is the key to the accurate determination of residual gas pressure in the pre-pumped coal seam by this method. In order to explore the appropriate coal core volume restoration method, relying on the no-seal coal seam gas pressure tester independently developed by the team, the coal core volume restoration experiment was carried out by loading axial compression coal core volume restoration, contact liquid injection (water/oil) coal core volume restoration, and the combination of pressure and liquid injection coal core volume restoration. The results showed that: When the loading axial pressure is set as 5, 10, 15……65 MPa in order to restore the coal core volume, due to the mutual support and resistance between the coal sample particles, the newly formed cracks caused by the sampling of broken coal body cannot be completely "healed", and it is difficult to restore the coal core volume by simply pressuring it in both theory and technology. Set the initial adsorption equilibrium pressure P0 to 0.31, 0.53, 0.74, 1.00 and 1.50 MPa for the coal core volume recovery test of contact water injection, contact silicone oil injection and compression/liquid injection combination. The experimental error of the coal core volume restoration method is 40.67%～116.13%. The experimental error of the coal core volume restoration method with contact silicone oil injection is 29.33%～87.10%, and the experimental error of the coal core volume restoration method with pressure/liquid injection is 2.67%～9.68%, and the error decreases with the increase of adsorption equilibrium pressure. Comparing the experimental results of three kinds of coal core volume restoration methods, it is finally determined that the combination of pressure in the upper cavity and injection of silicone oil in the lower cavity restores the coal core volume in situ. It provides a reliable way for the realization of non-sealed hole pressure measurement technology based on “state recovery”, provides a new method for coal mine to accurately and quickly obtain the residual gas pressure value of pre-pumped coal seam, and provides technical support for coal seam gas extraction effect inspection and extraction standard evaluation.