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.

Leakages Through Radial Cracks in Cement Sheaths: Effect of Geometry, Viscosity, and Aperture

“…Thus, we have used this equation to estimate a hydraulic aperture of the crack for Cases 1-5 which would best resemble the magnitude of the volumetric flow found from simulations (shown in Figs. [8][9][10][11][12]. The resulting curves are shown in Figs.…”

“…Nevertheless, defects in the cement might occur, caused by chemical degradation, shrinkage, or pressure/temperature variations. As shown by several experimental studies [6][7][8][9][10][11][12][13] such defects in the cement could lead to loss of zonal isolation and thus an increase in the effective permeability. Even though these abovementioned studies document loss of zonal isolation, they provided limited information about the exact location, shape, and size and magnitude of the flow path.…”

Numerical modeling to evaluate tensile mechanical and shear failure of cement in the casing-cement interface

“…Recently, [10] developed a thermo-poroelastic numerical model to evaluate the integrity of cement according to its thickness. [11] presented a 3D poroelastic finite-element (FE) model coupled with cohesive regions to represent fracture propagation during hydraulic fracturing. [12] used a simulation tool based on a thermo-chemo-poro-elastoplastic approach to estimate the evolution of the stress state, shrinkage and pore pressure of cement versus the degree of hydration.…”

Digital Cement Integrity: A Methodology for 3D Visualization of Cracks and Microannuli in Well Cement

“…This casing contraction leads to increased tensile stress in the surrounding cement sheath, and cement 2 of 14 debonding subsequently occurs if the induced tensile stress exceeds the cement's tensile bond strength towards the casing and/or formation. Both radial cracks and microannuli can thus form as a result of normal production operations that involve temperature and pressure cycling, such as repeated shut-downs/start-ups, pressure tests and fluid injections [15]. Such dynamic loading scenarios could be particularly severe for CO 2 injection wells [16], and for example, in a well after 30 years of CO 2 injection, prominent leak paths at both the cement-casing and cement-formation interfaces were found after coring [17].…”

“…Consequently, several laboratory experiments have been performed to study and understand these cement sheath failure modes and the resulting loss of zonal isolation [15,[18][19][20][21][22]. Furthermore, other authors have focused on studying microannuli formation and the resulting flow through these leakage pathways [23][24][25][26].…”