Effects of Long-Term Exposure to Ultrahigh Temperature on the Mechanical Parameters of Cement

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98896, "Effects of Long-Term Exposure to Ultrahigh Temperature on the Mechanical Parameters of Cement," by D. Stiles, SPE, ExxonMobil Development Co., prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21–23 February. Long-term cement-sheath mechanical durability is critical to maintaining zonal isolation for the life of a well. Wells subjected to cyclic steam stimulation (CSS) undergo extreme temperature changes that cause substantial stresses on the cement sheath. Analysis models that are useful in predicting the failure potential of a cement sheath require input of cement mechanical parameters. The full-length paper presents a test methodology for measuring the mechanical parameters of five cement formulations after exposure to 645°F. Introduction The Cold Lake development in eastern Alberta, Canada, has been in operation since the mid-1980s. It is the largest in-situ heavy-oil recovery operation in Canada and one of the largest thermal heavy-oil operations in the world. Production of the heavy oil is by CSS, in which steam is injected into the reservoir above fracture pressure and a mixture of bitumen, gas, and water is produced from the same wells. Historically within the oil and gas industry, the only concern with cement formulations at ultrahigh temperatures was chemical stability. The loss of compressive strength in Portland cement and increase in permeability at temperatures greater than 235°F was discussed in the literature as early as 1935. This phenomenon, commonly referred to as strength retrogression, is the result of morphological changes in the crystalline structure of cement when it is exposed to elevated temperatures. Preventing strength retrogression by addition of finely ground crystalline silica at a 35 to 40% concentration by weight of cement (BWOC) was discovered nearly 50 years ago and has been the industry standard ever since. All cement formulations used throughout the Cold Lake project have contained the recognized silica concentration. Cement-sheath failure resulting from wellbore stresses received little attention before 1992. While there are distinctive differences in the mathematical models that have been developed, they all agree that sheath failure is a function of the stress states imposed on the wellbore and the mechanical parameters of the cement sheath and the surrounding rock. The stress states imposed on the wellbore by large temperature increases, such as those encountered during steam injection, have been recognized as sufficient to cause tensile cement-sheath failure. The key mechanical cement parameters responsible for whether or not sheath failure will occur are Young's modulus, Poisson's ratio, and tensile strength. No measurements of these parameters ever had been performed on cement samples that had undergone long-term exposure to the ultrahigh temperatures encountered during CSS at Cold Lake.

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