Evaluation of Cement Systems for Oil and Gas-Well Zonal Isolation in a Full-Scale Annular Geometry

Abstract

Summary Loss of zonal isolation in a wellbore can be caused by mechanical failure of the cement or by development of a microannulus. However, behavior of the sealant is driven by specific boundary conditions such as rock properties. Large-scale laboratory testing of the cement sheath in an annular geometry and a confined situation was performed to simulate various downhole stress conditions and evaluate the behavior of several sealants. Failure modes of the cement sheath were determined as a function of cement mechanical properties, loading parameters, and boundary conditions. Results were used to validate an analytical model that predicts cement-sheath failure. Introduction Interzonal communication in a wellbore may lead to loss of reserves, contamination of zones, production of unwanted fluids, or safety and environmental issues. Remedial solutions exist to repair the problems, but for technical or economic reasons, the well may be shut in or abandoned. To maximize well life, the cement sheath must be chemically and mechanically durable. Sealants resistant to aggressive formation fluids should also be designed to withstand stresses exerted during production and well operations, such as casing-pressure tests, stimulation treatments, or temperature changes during production cycles. To achieve this design goal, a better understanding of the mechanical behavior of different sealants under downhole conditions is required.1,2 According to Thiercelin et al.,3 changes in downhole conditions can cause mechanical damage (e.g., mechanical failure or creation of microannuli) to the cemented annulus, which may lead to loss of zonal isolation. Thiercelin et al.'s paper3 concludes that the complete mechanical system formed by the steel casing, cemented annulus, and formation should be considered, rather than sealant strength alone. Increase of pressure and temperature in the wellbore first expands the inner steel casing, which instantly imposes this deformation on the surrounding cement sheath. This applies imposed displacements, rather than imposed stresses, to the cement inner diameter (ID). Over the lifetime of the well, the cement sheath must withstand multiple displacement cycles. Several authors4,5have proposed numerical models to simulate sealant mechanical behavior and predict initiation of failures according to known mechanical properties of the complete system (i.e., steel, cement, and rock). A large-scale laboratory test for sealants in an annular geometry has been developed. This test simulates changes in well conditions that cause contraction or expansion of the inner casing. It can also evaluate the confining role of the formation or outer casing. Such an experiment enables the evaluation of sealant mechanical responses under wellbore conditions. The tensile and compressive stresses generated in the annulus are similar to those the sealant must withstand in a real wellbore. Loading simulated in the full-scale annular sealing test is close to real field conditions. Several cement systems exhibiting different mechanical behaviors have been tested, and the experimental results have been compared with predictions of a numerical model.

Recipe of Lightweight Slurry with High Early Strength of the Resultant Cement Sheath

“…Cement slurries are divided into the ones with a 'normal density', around 1750-1850 kg/m 3 , lightweight slurries (below 1750 kg/m 3 ) and heavy slurries (above 1900 kg/m 3 ). The slurry density is chosen based on the geological and technical conditions resulting from the geological structure and well depth [5][6][7][8][9][10].…”

An analytical solution to simulate the effect of cement/formation stiffness on well integrity evaluation in carbon sequestration projects

“…The cracks opened sufficiently for the annular flow as the decreasing of the casing temperature. Boukhelifa et al (2005) performed large scale laboratory tests on the cement sheath in an annular geometry. The influence of contraction and expansion of the inner casing were investigated.…”

Evolution of the Elastic Properties of an Oilwell Cement Paste at Very Early Age under Downhole Conditions: Characterization and Modelling

“…Over the past ten years, several papers were concerned with the long term mechanical durability of the cement sheath. However, until now there are no relevant physical and mechanical inputs that allow to evaluate the development of stress and strain states in the cement at very early age of hydration in oil or gas wells [1][2][3][4]. The monitoring of mechanical properties measurements of the cement paste at very early age under downhole conditions is not trivial due to the lack of instrumentation that can help curing and maintaining the cement paste under downhole conditions while testing for mechanical properties.…”