Experimental study on creep characteristics of coal-bearing gas source rocks under grading loading

Abstract：The creep characteristics of coal-bearing gas source rocks are the key geological factors affecting fracturing mining in low permeability gas reservoirs.In order to explore the creep characteristics of coal-bearing gas source rocks, based on the characteristics of thin coal seam gas reservoirs, large sequence, deep buried depth and complex stress conditions in Yubei area, Henan province, three specimens of siltstone, sandy mudstone and mudstone in the reservoir were collected, and the triaxial grading loading creep tests under different stress levels were carried out.The results showed that three source rocks exhibited obvious brittle characteristics under triaxial grading loading and no obvious acceleration creep characteristics before their failure.The steady creep phase lasted longer, and the deformation recovery was faster during loading and unloading.The siltstone of the source rocks had the longest creep time and the largest deformation, and the maximum creep deformation was 0.356 3%;the creep time of the mudstone was the shortest, the deformation was the smallest, and the minimum creep deformation was 0.206 8%.The creep of rock mass in coal-bearing gas reservoir under grading loading had its particularity.Based on the creep test characteristics of siltstone, the“six-element nonlinear viscoelastic rheological model”was found to describe the creep process.The fitting results indicated the theoretical model agreed well with the experimental data.The creep model could identify the source rocks creep parameters more completely, which could provide reliable geological parameters for theoretical calculations in coal-bearing gas-pressure fracturing mining and a certain reference for the development of coal-bearing gas in Yubei area.

Keyword：coal-bearing gas source rock;hierarchical loading;triaxial creep;creep characteristic of source rock;creep model;