Analysis of the Production Casing Deformation Due to Shearing of Offset Hydraulic Fractures

In a number of wells in an unconventional reservoir, access to some hydraulic fracturing stages is lost before, during or after the stage stimulation. Available caliper data show casing deformation up hole from the inaccessible stages that might be due to the same processes. While induced hydraulic fractures are abundant and certainly intersect wells, they have been previously ignored as a cause of casing deformation, because there should not be any shear stress on the tensile fracture planes. However, the stress shadowing may induce small shear stresses on the existing hydraulic fracture planes with a negligible friction resistance. The analysis is carried out to investigate the magnitudes of slip on offset fractures and of the increase in the axial force in the casing induced by active fractures and if the induced combination of the loads, in shear and compression, is sufficient to cause the observed deformation of the wellbore casing. An approach, in which the problem is analyzed using two different models on two scales, is used here. The fracture-scale model is used to analyze stress and deformation by explicitly including entire fractures formed due to fracture propagation within the active stage. An offset fracture was assumed at a certain distance from the active stage, with induced shear deformation of the offset fracture and axial force in the casing calculated as a function of stage design, assumption of the fracture geometry and net pressures, and the position of the offset fracture. The wellbore-scale model was then used to analyze details of casing deformation for the offset fracture deformation and axial forces determined from the fracture-scale models. Excessive casing deformation is typically observed in very small fraction of the wellbore length. That indicates that the probability of coincidence of conditions favorable for excessive deformation is small. It seems that the plausible mechanism of excessive casing deformation is caused by coincidence of: 1) relatively large increase in the axial force (e.g., due to presence of poor-quality rock along the wellbore), 2) some shearing required to provide perturbation and to reduce the critical buckling force, and 3) sufficiently long segment of the steel casing that can be considered unsupported because of damaged, poor quality, or completely missing cement around the metal casing. The results of the study provide possible explanation of observed excessive deformation of well casing in unconventional plays. Recommendations for improved design and operations to reduce probability of such events are provided.