The Cement-to-Formation Interface in Zonal Isolation

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 88016, "The Cement-to-Formation Interface in Zonal Isolation," by H.K.J. Ladva, SPE, B. Craster, T.G.J. Jones, SPE, G. Goldsmith, and D. Scott, Schlumberger Cambridge Research, prepared for the 2004 IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition, Kuala Lumpur, 13-15 September. During the completion and production phases of the well, temperature and pressure variations can cause stresses at the cement-to-formation interface. The ability of the casing cement system to maintain a seal at the cement-to-formation interface depends on the condition of the formation surface before slurry placement. The condition of a shale will depend upon the nature of the drilling fluid used, whereas the condition of a permeable rock will depend upon the presence and nature of the filter cake deposited during drilling. The full-length paper presents an improved understanding of chemical interactions at the cement-to-formation interface and the factors that determine bond strength and the position of the plane of failure. Introduction Maintaining zonal isolation for the lifetime of oil and gas wells is critical. Leakage behind casing can reduce the cost-effectiveness of the well and cause health and safety risks from pressure buildup and contaminated aquifers. In the petroleum industry, many studies have been performed on the cement-to-casing interface and bulk cement to improve the isolation efficiency of the cement in the wellbore. Less attention has been given to the cement-to-formation interface. Ideally, samples of set cement and the adjacent formation should be recovered from oil wells to give a better understanding of the chemical nature of the interface under downhole conditions during the lifetime of the well. Low-level radioactive waste sometimes is encased in cement blocks and lowered into clay beds. There is concern that, over time, alkaline fluids near the clay surface will alter its effectiveness as a barrier to the encased nuclear waste. Studies by the nuclear industry to increase their understanding of the cement-to-formation interface are ongoing. At an underground laboratory in Belgium, plugs of cement were precured and then placed in cored-out holes in Boom clay. Samples were maintained between 65 and 85°C. After 12 to 18 months test time, the samples were cored out of the clay. Sections of the interface were analyzed by microprobe and X-ray diffraction. Observations from the study using ordinary Portland cement are summarized as follows.