Large Scale Testing & Modelling for Cement Zonal Isolation of Water Injection Wells

Water injection into soft sand is a global industry challenge due to 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 from well construction to post abandonment. It contributes to ensuring no out of zone water injection occurs due to flow behind casing. If water does go out of zone, new drilling hazards due to water breaching and 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 retain its isolation capability during well life. Cement integrity simulators (CIS) provide different answers, mainly because they all assume a d ifferent 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 the cement sheath is submitted to are simulated; 3) The elasticity, plasticity, and failure of materials is taken into account; 4) The simulations are fast enough to facilitate sensitivity analysis; 5) The model outputs allow visualization of cement integrity across the entire length of the cement sheath; adjacent to both reservoirs and seals. In parallel to the modelling 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. It also showed that cooling due to water injection had a bigger impact on cement integrity than increasing pressure. Additionally, it showed that micro-annulus generation had more effect than tensile cracking in terms of cement barrier permeability increase. Based on this work, cementing design workflow for zonal isolation of water injectors has been established. As injection pressure and temperature, rock properties, confinement pressure and completion type vary from well to well, zonal isolation for water injection wells must be designed on a case-by-case basis. New cementing workflows for water injection wells involve rigorous cement mechanical property testing and cement stress modelling.