Investigation of Hydraulic Fracture Behavior near a CBM Wellbore: Evidence from in situ Observation in Coal Seam

In situ observations are a suitable method for studying the hydraulic fractures in a coal reservoir. The cement sheath and propped fractures can be directly observed by the naked eye. In addition, water samples collected in the roadway can be used to analyze the infiltration of the coal reservoir by fracturing fluid. The results indicate that the thickness of the cement sheath in coalbed methane (CBM) well M-1 is extremely uneven. Three failure modes have been observed on the cement sheath, namely, radial cracking, plastic deformation, and debonding between the cement and formation. The poor integrity of the cement sheath is related to the circumferential stress. When the casing pressure is large and is greater than the formation stress and the thickness of the cement sheath is small, the circumferential stress can generate tensile stress in the cement sheath. Tensile failure can occur in the cement sheath when the tensile stress is greater than the tensile strength of the cement. Based on in situ observations, natural fractures around the wellbore can induce the initiation of hydraulic fractures. The ability of different natural fractures to induce hydraulic fracture extension in order from strongest to weakest is as follows: exogenetic fractures > horizontal fractures > cleats. The flow capacity of the fracturing fluid is the best in exogenetic fractures, followed by horizontal fractures, and it is the worst in small-scale channels. This study provides evaluation evidence of the failure behavior caused by hydraulic fracturing.