The geomechanical response of a reservoir to the Steam Assisted Gravity Drainage (SAGD) process involves changes in stress, deformation (strain), and permeability. For example, in many SAGD operations the steam injection pressure is greater than the initial reservoir pressure. As the local pore pressure is elevated by steam injection the effective stress in the reservoir is reduced. This may bring local shear failure and dilation of the reservoir material if the change in effective stress is large enough, particularly where the initial stress environment is anisotropic. In turn, dilation can result in substantial permeability enhancement. A better understanding of the interdependence between stress, deformation and permeability could help in improving the design of SAGD operations to take advantage of the effects of dilation, particularly on vertical permeability barriers such as zones of inter-bedded shale (IBS).
This paper presents the results of experiments investigating changes in the permeability of IBS samples under pressure and temperature conditions similar to those that exist during SAGD operations. The IBS samples used in the tests were cut from cores that had been obtained from a relatively shallow depth (∼ 160 m) at the Dover site in the Athabasca region. The samples consisted predominantly of quartz (∼ 70%), with a clay content of just over 20%. Experiments were performed at both ambient temperature (∼ 22°C) and elevated temperature (∼ 200°C). Elastic properties (Young's modulus, Poisson's ratio) and shear strength parameters (cohesive strength, friction angle) for the IBS samples were also measured at both ambient temperature and elevated temperature.
The experiments were performed in a triaxial cell, under conditions in which the pore pressure and the axial and confining (radial) stress were controlled. The measured increases in permeability caused by a reduction in effective stress were significant at low effective stress. At ambient temperature, the permeability increased by as much as a factor of 15,000 to a level in the range of 1-5 millidarcies (md). At elevated temperature, the permeability increase was even larger, by another order of magnitude, to a level in the range of 10-50 md. The triaxial compression tests indicated that the elastic properties and shear strength of the IBS samples were also sensitive to temperature. The samples became much stiffer (higher Young's modulus) and stronger at elevated temperature.
The study presented here was focused on measurements of permeability change under conditions representative of SAGD in IBS samples obtained from one site, a relatively shallow one. Further tests should be undertaken to investigate permeability change in IBS samples that originate from different sites and different depths, to search for broader trends in the permeability behaviour of IBS under SAGD conditions.