Caprock integrity is essential to the success of Steam Assisted Gravity Drainage (SAGD) operations. This issue is especially critical in the case of shallow SAGD operations where the in situ stresses are relatively low and, by necessity, the steam pressure has to be limited in order to ensure the safety of the process. Limited information is available on changes in caprock material properties that occur when temperature and stress conditions change in areas of the caprock affected by SAGD. By including these changes in caprock properties in the calculation of maximum operating pressure (MOP), it is possible that the allowable MOP could increase. This would allow the sealing capability of the caprock to be exploited more fully and provide an avenue for improving the economics of shallow SAGD operations.
This paper presents the results of experiments measuring geomechanical properties of Clearwater caprock, at both ambient temperature (25°C) and elevated temperature (115°C and 200°C). The shale samples used in the tests were cut from two sets of shale cores that had been obtained from the Wabiskaw Member of the Clearwater Formation at Athabasca oil sands SAGD projects. The first set of core samples originated from a depth of approximately 200 m. The second set was obtained from a different site and a shallower depth, approximately 150 m. The purpose of these experiments was to study the effect of temperature on geotechnical properties such as the elastic parameters (bulk compressibility, Young’s modulus, Poisson’s ratio) and shear strength parameters (cohesive strength, friction angle), and to investigate the behaviour of shale permeability in a stress, pressure and temperature environment similar to one that might exist during SAGD operations.
The experiments were performed in a triaxial cell, under conditions in which the pore pressure and axial and confining (radial) stress were controlled. The triaxial compression tests indicated that the elastic properties and shear strength of the shale samples were sensitive to temperature, for the range of effective stresses used in the tests. While the change in bulk compressibility was negligible, the samples became much stiffer (higher Young’s modulus) and stronger (higher cohesion, lower friction angle) at elevated temperature. The measured increases in permeability caused by a reduction in effective stress were relatively small, usually not greater than a factor of ten over the initial permeability. In one exceptional case, at elevated temperature, the permeability increased by a factor of nearly one million, but even in this case the maximum permeability measured was never larger than 10 md. A substantial portion of the change in permeability occurred after the sample failed in shear. The increase in permeability that was measured on the Clearwater shale samples at elevated temperature was somewhat greater than at ambient temperature, particularly at larger volumetric strains.
At present, only a limited number of tests have been carried out to measure changes in caprock properties under temperature, pressure and stress conditions that prevail during SAGD. Further tests would be required to establish a better quantitative understanding of these changes in caprock properties.