Abstract

This paper presents an analysis of present day initial stress states in the Upper Cretaceous formations overlying the heavy oil deposits of Northeastern Alberta. Two models based on elastic unloading of Pleistocene glacial ice sheets are presented to predict the dependence of present day initial stress ratios with depth. The results of the both models are consistent with published in situ stress data in the region.

These computed stress ratios were used to calculate the maximum slip on an inclined interface where the shear resistance has been eliminated by hydraulic fracturing. The finite element program ABAQUS was used to compute the maximum slip at the interface. The magnitude and the direction of the slip is dependent not only on the interface inclination but also on the stress ratio. The material above the fracture will slide up-dip when the stress ratio is greater than 1 and down-dip when the stress ratio is less than 1. There is no slip when the stress ratio equals to 1.

Introduction

Fluid or steam injection into bitumen oil sand reservoirs is often used to stimulate or assist viscous heavy oil production. However, fluid injection into the reservoirs is often done above the fracture pressure resulting in the formation of hydraulic fractures. Fluids may also escape into the overlying shale's, causing additional hydraulic fractures above the reservoir. The shear strength along these fractures or interfaces is effectively zero. In the cases where a non-isotropic state of stress exists in the vicinity of inclined interfaces, the removal or reduction of the shear strength will cause stress redistribution in the formation and will likely cause relative sliding along the fracture. The stress redistribution and associated deformations may induce wellbore casing failures.

The sliding along injected interfaces or natural fractures depends on the stress states at the plane before the fluid injection. More specifically, it depends on the direction and magnitude of the shear stress on the plane.

The principle directions of the present day stress fields are generally aligned with or close to vertical and horizontal directions. Elastic theory dictates that shear stresses are absent on the principle stress planes. Therefore reduction of shear strength on horizontal or vertical planes normally will not cause any significant shear deformations. However, this is not the case for inclined planes. The magnitudes of the present day shear stresses along inclined interfaces depend on the angle of inclination and the difference between the maximum and the minimum principle stresses which can be calculated from the effective horizontal to vertical stress ratio.

For normally consolidated conditions, the stress ratio is empirically defined by Jaky's rule1 and is always less than 1. However, normally consolidated conditions are rare in nature, particularly in temperate, onshore regions. There are many processes that can create over consolidation. With few exceptions in Northeastern Alberta, the Upper Cretaceous sediments are over consolidated. This state of over consolidation can be attributed to several factors. Tectonic activity during the Laramide Orogeny raised horizontal stresses.

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