Abstract

Knowledge of the stress regime is important for reservoir engineering. This paper presents stress magnitudes in Alberta and NE British Columbia at a regional scale. Overburden stresses (SV) and smaller horizontal principal stresses (SHmin) are mapped at the tops of the Viking Formation, Banff Formation and Wabamun Group, and at the base of the Cretaceous section. SV values are based on integration of more than 100 density logs. SHmin estimates are derived from microfracs, mini-fracs, leak-off tests and hydraulic fracture data from several hundred wells. Averaging techniques are used to achieve compatibility between SHmin data sources.

The maps display regional southwestward increases in stress magnitudes that relate to depth of present and past burial. They also portray unanticipated local variations suggestive of low stress lineaments that may be sites of enhanced reservoir permeability. Although reconnaissance in nature, the results will be of value to petroleum engineers engaged in borehole stability prognoses, reservoir development and subsurface gas injection. Such information has not been available previously on a regional scale.

Introduction

Over the past few years, rock mechanics has been invoked increasingly in drilling prognoses and in engineering programs related to the recovery of hydrocarbons. Predicting and assessing the physical behaviour of subsurface rocks helps achieve significant savings in drilling and production costs, as well as enhancing reservoir performance.

In order to apply rock mechanical analyses, it is necessary to specify the mechanical properties of the rocks and the physical conditions that confine them. The major confining force is subsurface stress; the lateral and vertical pressures applied to buried rocks. Predicting the conditions under which rocks will fail during drilling or during fluid injection requires principal stress magnitudes (Fjaer et al., 1992; Stjern et al., 2003). All other factors being equal, in situ stresses can control permeability (Brighenti, 1989; Enever et al., 1994; Sparks et al., 1995), so explorationists, as well production personnel, have an interest in their magnitudes.

In the Alberta Basin of western Canada there are a limited number of direct stress magnitude measurements and a plethora of data from which estimates can be derived.

Stress Magnitude Measurement and Estimation

The state of stress at a point in the subsurface can be fully defined by the orientations and magnitudes of three orthogonal principal stresses: σ1, σ2, σ3. Since, by definition, principal stresses intercept free surfaces at right angles, and the Alberta Basin has a nearly flat topographic surface, it is reasonable to infer that one principal stress will be vertical (SV) and the other two will be horizontal (SHmin, SHmax).

The magnitude of the vertical stress, SV, at depth is equal to the pressure exerted by the overlying rocks. Continuous values for specific sites were obtained by integrating density logs. Average densities of the uppermost unlogged intervals, and of any obviously erroneously recorded sections, were estimated from linear extrapolations of adjacent intervals (Fig. 1).

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