With ongoing environmental concerns and increasingly stringent regulations, the traditional topic of wellbore integrity is more important than ever before. A great deal of importance is attached to the cement sheath because it is necessary to provide zonal isolation and well integrity during the life of a well. Debonding at the casing/cement or cement/formation interfaces, which may result in substantial flow channels and fluid leakage, is often responsible for the loss of wellbore integrity. A three-dimensional finite-element model is developed in this paper to simulate debonding fracture propagation at cement interfaces. Debonding is driven by pressure build-up at the casing shoe due to fluid leakage. The model can be used to quantify the length, width, and circumferential coverage of the debonding fracture. The simulation results show dependence of debonding fracture width and circumferential coverage on in-situ stress conditions, initial cracks around the casing shoe, and cement and formation properties. For example, with initial cracks in the cement interface, debonding fractures tend to develop vertically along the axis of a vertical well, rather than circumferentially around it. The method proposed herein presents a useful step towards prediction of well integrity and provides improved guidance for cement selection and completion optimization.
Well integrity, always of importance, is becoming more so because of increasing environmental concerns and regulatory activities. Moreover, increasingly hostile environments such as HTHP fields, ultra-deep-water fields, and geothermal fields, along with more complicated development operations, such as those encountered in gas producing wells, gas storage wells, water injectors, and cuttings/waste injectors, all present new challenges for well integrity. Failure of well integrity can lead to costly remedial operations or total loss of the well, severe environment contamination, and even fatal accidents to operating personnel.
The cement sheath is the heart of well integrity. It is expected to ensure well integrity by providing zonal isolation and support for the casing throughout the life of a well, from well construction, hydrocarbon production, to post-abandonment (Gray, et al., 2009; Feng, 2016). A successful cementing job is expected to result in complete zonal isolation, without leaving any leakage pathway in the annulus between casing and formation. Unfortunately, this goal is not always achieved, and failure of cement sheath commonly occurs during the life of a well (Fourmaintraux et al., 2005).