This paper describes a geomechanical approach to the determination of injection pressures that will result in formation shearing within the Athabasca oil sands. The enhancement of in situ permeabilities, resulting from shearing, is a beneficial phenomenon and often an essential prerequisite of many successful in situ recovery processes.

This geomechanical analysis is based upon the general geological setting in the Athabasca oil sands, observations made from field data, published laboratory data, and geomechanical principles. The first objective was to estimate the existing stress state in the rock. This determination was based on elastic theory and tectonics, and fitted to field measurements of the minimum in situ stress from minifrac tests in the Athabasca oil sands. The methodology also calculates the maximum horizontal stress, a parameter that is extremely difficult to measure, yet is an important parameter for most stress analyses. The resultant stress anisotropy creates the potential for formation shearing; a potential that is unleashed once the effective stresses are sufficiently reduced with high injection pressures.

Lastly, the amounts by which injection pressures must be increased for shear failure are provided, assuming typical values of formation strength.

The analysis begins with a determination of the minimum in situ stress, as a function of depth. This is the fracture closure pressure that is a useful parameter in determining fracture treatment pressures, fracture containment, and fracture orientation. The associated tectonic strain is used to calculate the maximum horizontal stress. Using the maximum and minimum stresses, the injection pressure at which extensive shearing will occur is calculated. This is the geomechanical optimum operating pressure. While other considerations may favour a lower operating pressure, recovery processes that rely upon high in situ permeabilities may not be viable at lower injection pressures.

The methodology presented is applicable to other regions.

You can access this article if you purchase or spend a download.