Discussion of "SAGD and Geomechanics"

Introduction As a researcher studying the role of geomechanics in the SAGD Process, it was great to read Mr. Carlson's Distinguished Authors Series paper entitled. "SAGD and Geomechanics" in the June 2003 issue of the JCPT. In response to this paper the following sections provide a discussion on SAGD-geomechanical issues raised by Mr. Carlson and presents additional observations concerning the role geomechanics my play ill the SAGD process. Particulate Nature of Oil Sands Due to the particulate nature of oil sands, its volumetric behaviour will differ depending on the loading conditions. When oil sand material is subjected to isotropic loading (Stress change equal in all directions), individual particles will generally deform in an elastic manner, with the possibility of grain crushing at high stresses, but the bulk mass of oil sands will undergo both elastic and irreversible volumetric changes with little re-orientation of the grains relative to each other. Figure 1 provides the nature of this isotropic loading relationship for oil sands and is generally similar to the conventional reservoir engineering approach to rock compressibility. When oil sands are subjected to shear Stress loading (non- isotropic or anisotropic), individual groins also deform elastically but grain crushing can become more prominent. With respect to the volume change behaviour all the mechanisms outlined in the "Interlocked Structure" section of the paper can occur. The main difference with isotropic loading, however is that shear loading can result in substantial re-orientation of the grains relative to each other. The result is that the pore volume shape and distribution (and by association, the permeability) under these loading conditions can be quite different. During the SAGD process, both isotropic and shear stress loading occur simultaneously. In the SAGD process, saturated steam with high pressure and temperature is continuously injected into the reservoir. Steam injection pressure results in in increase of pore pressure. So, effective stress is decreased and clastic deformation (expansion) occurs, which is the result of isotropic unloading. In a certain distance ahead of the.steam chamber surface, total stress increases due to the thermal expansion of the oil sand material inside the steam chamber. This Stress increase is anisotropic and shearing will probably occur in this area. Clearly the impact of porosity changes due to these two geomechanical mechanisms will create different pore geometry and it seems reasonable to expect that they will influence reservoir properties such as absolute permeability differently(1). The issue of grain crushing should also not be overlooked. Oil sand grains consist of different minerals. If the hardness of these minerals is high enough grain crushing may not occur. In contrast, if these minerals are relatively weaker, grain crushing can be an issue and must be taken into account when treating the reservoir permeability variations. For example. oil sands in the Athabasca deposit arc predominantly line 10 medium grained and uniformly graded sand whose mineralogy consists of approximately 95% quartz, 2% to 3% feldspar grains, 2% to 3% mica and clay minerals, and traces of other minerals(2,3).

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