Impact of coal seam mechanical properties on horizontal wellbore shrinkage during drilling

The stability of horizontal wellbores in coalbed methane reservoirs is governed by a combination of geological factors, including the complex mechanical properties of coal seams and in-situ stress conditions. This study presents a finite strain-based theoretical framework to analyze horizontal wellbore shrinkage during drilling operations. The proposed model accounts for both individual and combined effects of key mechanical parameters on wellbore deformation. Input parameters are derived from conventional triaxial compression tests and published data, using coal samples from the Huainan coalfield, China, as the baseline. The influences of burial depth and wellbore pressure on shrinkage behavior are also systematically examined. The results indicate that elastic modulus exerts the most significant control on wellbore shrinkage, followed by uniaxial compressive strength, internal friction angle, and Poisson's ratio, while cohesion shows a negligible effect. Specifically, wellbore shrinkage decreases exponentially with increasing uniaxial compressive strength, elastic modulus, and internal friction angle, increases linearly with Poisson's ratio, and remains virtually unchanged with cohesion. Compared with intact coal seams, tectonic coal seams exhibit substantially greater shrinkage, which intensifies markedly with burial depth but can be partially mitigated by increasing wellbore pressure. Based on these findings, a multi-step reaming drilling strategy is proposed for horizontal wells in tectonic coal seams as a potential means to alleviate the practical challenges associated with excessive wellbore deformation.