Transverse hydraulic fracturing along the horizontal well segment is the key technology for developing the low permeability shale reservoir, and the efficiency of the horizontal drilling and completion is growing. While concurrent with advancing D&C technologies, the well bore integrity, particularly on production casing deformation is worthy of study. To reveal the casing deforming mechanism, a 2D finite element model of injection-induced deformation under hydraulic fracturing is established in this study. The poro-elastic constitutive relation is employed to analyze the changes of stress and flow fields during hydraulic fracturing. The cohesive zone model is adopted to simulate fracture growth. Results indicate that hydraulic fracturing would cause formation deformation and natural fracture slippage. The casing failure mechanism is identified as shear deformation induced by the slippage of shear fractures during hydraulic fracturing. This study supplies an easy method to forecast formation/ wellbore response and casing deformation under hydraulic fracturing.
Due to the increasing energy demand, the development of unconventional resources in extremely tight formation is becoming more and more critical. The advanced drilling and multistage hydraulic fracturing technologies in horizontal well have made it possible to develop extreme low permeability unconventional formations (Yuan et al., 2017; Sharma et al., 2017). Field practices have shown that the large scale hydraulic fracturing could compromise the well integrity through casing deformation or failure, which results in the premature abandonment of well completion and low well productivity and ultimate recovery. In Pennsylvania, unconventional wells showed a six times higher integrity issue occurring rate compared to the same period conventional wells (Ingraffea et al., 2014). In another case, the failure rate of casings in shale gas wells is as high as 31.2% during hydraulic fracturing in ChangNing and WeiYuan, China (Chen et al., 2017). In these regions, the casing deformation or failures occur frequently during the multi-stage hydraulic fracturing process, which significant impacted later operations and production efficiency.
Sustaining casing integrity is one of the challenges of shale gas wells. Davies et al. (2014) gave an overview on the oil and gas wells and their integrity, especially for shale and unconventional resource exploitation based on data from all over the world. Daneshy (2005) attributed casing failure to a consequence of large formation deformation, including compressive, tensile and shear stresses in the formation. Using downhole tilt data in an observation well, Wang et al. (2015) identified that the local buckling, connection failure and shear failure are the main failure modes. Lian et al. (2015) concluded that some casing failures were the joint result of rock property change, asymmetric treatment zones and stress field redistribution using finite element modeling. The heterogeneity severity of stress field increases significantly. Dusseault, Bruno, and Barrera (2001) attributed the dominated casing-deformation mechanisms to localized horizontal shear at weak lithology interfaces and injection intervals. Last et al. (2006) studied the casing deformation in a tectonic setting situated in foothills of the Andes. Analysis of sonic caliper logs from wells showed that the deformation accompanying formation movement results in ovalisation of the pipe rather than local shearing. The above literature survey shows that the fundamental mechanisms of casing deformations are multitude and some of them are contradictory to each other. Therefore, it is important to study the casing deformation, particular in shale formation from a fundamental and integrated manner.
