In-situ stresses at a point (x) in the subsurface, represented by three orthogonal principal stresses.
Assumed orientation of the principal stresses in the Western Canada Sedimentary Basin. The vertical principal stress, SV, is presumed to parallel vertical well bores; the other two orthogonal principal stresses would therefore be directed horizontally.
Locations of stress magnitude measurements made by overcoring and micro-frac testing in the Western Canada Sedimentary Basin. The measurements are listed in Table 29.1 and plotted against depth in Figure 29.4.
In-situ stress magnitude measurements of high reliability made by overcoring and by micro-fracing in the Western Canada Sedimentary Basin. All three principal stresses can be quantified separately by overcoring, but only SHmin can be measured independently during micro-fracing. SV and SHmax are estimated as discussed in the text. Locations are shown in Figure 29.3 and details of the stress measurements are listed in Table 29.1.
Closure stresses derived from mini-frac treatments in Alberta wells. Most of the measurements underestimate SHmin because prior production of hydrocarbons has led to local reductions in in-situ stress magnitudes. Original data are listed in Woodland and Bell (1989), and are summarized in Table 29.2.
Formation leak-off test pressures from 35 wells in Alberta in which the tests were performed at depths greater than 500 m. Pressure measurements with gradients less than 12 kPa/m possibly represent fluid injection pressures with no fracturing. Tests with gradients greater than 24 kPa/m may record fractures initiated in rocks with high tensile strengths. The values between 14 and 24 kPa/m are likely to be closer to SHmin and can be used as upper limits for its magnitude. Well locations and test details are given in Table 29.2.
Vertical view of a borehole breakout in a vertical well. The long axis of eliptical caving is perpendicular to SHmax, the larger horizontal stress.
Orientations of SHmin measured in Paleozoic rocks in the Western Canada Sedimentary Basin. The majority of orientations are derived from mean well azimuths of breakouts (Table 29.3). Other stress orientation indicators (Table 29.4) are also shown.
Orientations of SHmin measured in Mesozoic rocks in the Western Canada Sedimentary Basin. The majority of orientations are derived from mean well azimuths of breakouts (Table 29.3). Other stress orientation indicators (Table 29.4) are also shown.
Horizontal stress trajectories projected across the Western Canada Sedimentary Basin. The interpretation is based on all the measured principal stress orientation indicators in Paleozoic and Mesozoic rocks, as documented on Figures 29.8 and 29.9.
Table 29.1 (as text) (as image)
Stress magnitudes measured by overcoring and micro-fracing in the Western Canada Sedimentary Basin.
Table 29.2 (as text) (as image)
Formation leak-off and mini-frac closure pressures, Western Canada Sedimentary Basin.
Table 29.3 (as text) (as image)
Mean breakout azimuths in degrees for 181 wells in the Western Canada Sedimentary Basin.
Table 29.4 (as text) (as image)
SHmax orientations measured in the Western Canada Sedimentary Basin by methods other than breakout analysis. Locations are shown on Figures 29.8 and 29.9.
