Author's Response to Discussion of "When Is It Important to Consider Geomechanics in SAGD Operations?"
Authors: Ito Y. , Li P. 
Published: March 2010

Congratulations to Rick and Pingke on their excellent paper(1), which was very impressive and enjoyable. Many technical papers were presented on the reduction of effective stress through elevating pore pressure and forcing oil sands to shear failure. However, the role of heating was not discussed in other papers. I now clearly understand the effect of reservoir heating and associated thermal expansion on stress-strain behaviour. For a long time, I have believed that heating should assist sands to reach shear failure in the reservoir. The contents of their paper are the latest and highest quality information on the geomechanical properties of oil sands. I have had different experiences with the geomechanical properties of oil sands and I would like to introduce my thoughts and recommendations for future study. I define geomechanical behaviours as changes in fluid flow properties due to a change in pore pressures and temperatures. Most changes are detectable through the analysis of temperature variations at observation wells and can be confirmed by rigorous numerical simulation studies. Some examples of geomechanical behaviours are as follows:Steam chambers are detected to stop rising or shrinking when injection pressure is reduced.Steam chambers resume rising when pressure is increased.Steam injection rates sharply increase when pressure is increased during a cyclic steam stimulation process.Other phenomena at high pressure and high temperature operations that are hard to understand through the examination of reservoir properties determined from core and log analysis.

These phenomena are difficult to explain by assigning 10 to 20﹪ changes in absolute permeability. When the effective stress is reduced in geomechanical laboratory tests, the sands start to dilate and absolute permeability increases. When effective stress is reduced by injecting high mobility fluid (such as steam) into the reservoir, the dilated space becomes occupied by the injection fluid (steam and water) and generates wormhole type flow paths. Under these conditions, the mobility of the injection fluid is enhanced but that of the resident fluid (bitumen) is not changed.This speculation could be confirmed by analyzing actual field performance data.

The first example in demonstrating the geomechanical behaviour of oil sands is the characteristic fluid flow of a cyclic steam stimulation process. Injectivity of steam under the native reservoir pressure is extremely low but can be dramatically improved when ore pressure is increased. According to the Sand Deformation concept(2), this is due to the creation of wormhole type flow channels by increasing injection pressure (Figure 1). Steam injection provides rising reservoir pressure thus lowering effective stress and resulting in the dilation of the formation. The generated space is then occupied by steam and its condensate. This is how the enormous increase in steam injectivity under high-pressure operation can be explained. Injection fluids (steam and condensate) gain huge increases in mobility but resident fluids do not undergo significant changes in the region where the failure zone is developed. This is the basic premise behind the Sand Deformation concept.