Characteristics of coal seam insitu stress and its influence on the stability of coalbed methane wellbore in Bijie experimental area, Guizhou

As an important coalbed methane occurrence structural unit in Guizhou Province, the Bijie Experimental Zone has developed multiple sets of thin- medium-thick coal seams (divided into three coal seam groups). The coal ranks are mainly high-rank coal, and the coal body structure is characterized by primary and fractured structural coal. In some local thick coal seam areas, the development of tectonic coal is remarkable. The methane content of the coal seams in this area is 6.37-27.01 mL/g·daf (with an average of 11.58 mL/g·daf), accounting for more than 70% of the total gas content, thus it has good potential for coalbed methane resources. The logging analysis and in-situ stress inversion show that the in-situ stress distribution in the study area follows the law of σH⪆σv⪆σh. The maximum horizontal principal stress is close to the east-west direction. The average ratio of the maximum to the minimum horizontal principal stress is 1.317, and the average value of the relative shear stress coefficient is 0.137, revealing the risk of shear-tensile combined failure in the wellbore. In the process of well drilling and completion, special attention should be paid to the wellbore stability. Based on the mechanical experiments of coal samples (acoustic emission monitoring and digital speckle technology), it was found that the strength of the coal samples with hole was positively correlated with the lateral pressure. The crack propagation began around the hole, but it was obviously influenced by the natural fractures. The failure mode was dominated by shear and accompanied by tensile characteristics, which overall conformed to the shear-tensile combined failure mechanism. The research results show that the optimization of wellbore design should focus on reducing the horizontal stress difference, and improve the safety and efficiency of coalbed methane drainage by adjusting stress field distribution.