Emergence of Delamination Fractures around Casing and Its Stability

The cement sheath failures and nearby wellbore failures may lead to upward flow of drilling fluid or formation fluid, which may have significantly adverse consequences like loss of reserve and environmental hazards. In order to maintain wellbore integrity in the long term, it is expedient to examine the causes of failures around the wellbore and propose suitable numerical models to predict annulus cracks around the casing. The complex failure behavior of cement/rock interfaces observed in the laboratory experiments does not look like the behavior of linear or simple nonlinear mechanical interfaces. Cohesive zone method (CZM) with BK-form bilinear traction separation law can be a good candidate to reproduce the complicated failure behavior around the casing. Then fracture critical energy, cohesive strength, and the deformability can be derived for cohesive zone constitutive equations by reproducing the loading-displacement curves from laboratory and inverse analyses. In this work, the comprehensive analysis for microannulus formation is presented by utilizing axisymmetric or three-dimensional poroelastic finite element models with CZM. This dissertation investigated these aspects: 1) Two and three-dimensional analysis of cement sheath integrity around wellbores due to presence of a leakage point; 2) Stimulation multi-zone fracturing and its cement sheath integrity during hydraulic fracturing. In this research, the physical mechanism of the loss of wellbore integrity is explained by the combined effects of fluid pressure, tensile and shear stresses, as well as failures. The excessive fluid pressure induced by leakage or hydraulic fracturing fluid acts as the drive for failures. The intensified tensile stress and shear stress occur at the crack tip initiate failures if they satisfy with the failure initiation criterion. Moreover, Lab and field scaled sensitivity analysis extract the influential parameters involved in failure development. Furthermore, the matching between failure patterns from numerical analysis and real field measurements using radioactive tracer-logs provides a comparison basis for model accuracy. Additionally, micro-annulus cemented systems are further analyzed by considering interface strength heterogeneity, anisotropic in situ stresses, wellbore inclination and eccentricity. The proposed approach provides a tool for more accurate predictions of cement integrity in the subsurface conditions to quantify the risk of wellbore integrity issues