Large-Scale Testing and Modeling for Cement Zonal Isolation in Water-Injection Wells

Abstract

Summary Water injection into soft sand is a global industry challenge because of the complex problem of maintaining sustained water-injection rates into the desired reservoir. Drilling, cementing, and completion engineers are addressing each technical and operational aspect of water injectors, including cement isolation. Cement serves as a barrier during well construction through to post-abandonment. It contributes to ensuring that no out-of-zone water injection occurs because of flow behind casing. If water does go out of zone, new drilling hazards that are a result of water breaching and a loss of reservoir management will occur. At present, as far as we know, the industry does not have a systematic methodology for defining and verifying the required physical and mechanical properties of the cement to endure water-injection service and to retain its isolation capability during well life. Cement-integrity simulators (CISs) provide different answers, mainly because they all assume a different initial stress-state in the cement after hydration. As a consequence, a new CIS model that computes this stress state has been developed, along with a large-scale testing setup to validate its predictions. The new model incorporates key-design parameters of effective CIS models: (1) The initial stress state after cement hydration is computed; (2) varying loadings that the cement sheath is submitted to are simulated; (3) the elasticity, plasticity, and failure of materials are taken into account; (4) the simulations are fast enough to facilitate sensitivity analysis; and (5) the model outputs allow the visualization of cement integrity across the entire length of the cement sheath, adjacent to reservoirs and to seals. Parallel to the modeling work, a large-scale test apparatus was built to evaluate cement zonal isolation under water-injection pressure and temperature conditions. Its objective was to generate pressure and temperature cycles inside sections of cemented casing assemblies to replicate the conditions of pressure and temperature variations in a water-injection well. The results of the test confirmed the accuracy of the new CIS model. They also showed that cooling because of water injection had a bigger impact on cement integrity than increasing pressure. In addition, the results showed that microannulus generation had more effect than tensile cracking in terms of cement-barrier-permeability increase.