Chapter 29 - In-situ Stress in the Western Canada Sedimentary Basin
Authors: J.S. Bell - Geological Survey of Canada, Calgary P.R. Price - National Energy Board, Calgary P.J. McLellan - Shell Canada Ltd., Calgary
This chapter summarizes the current understanding of the state of in-situ stress in the Western Canada Sedimentary Basin. The information compiled here includes specific in-situ stress measurements, interpretations of well logs, inferences from hydraulic fracture records, inferences from drilling procedures and well production behavior, and measurements of geological indicators. These data combine to give a coherent regional picture of principal stress orientations, but are not as successful in providing a comprehensive appreciation of stress magnitudes.
We use the term in-situ stress to refer to the present-day active natural stresses within the basin - in other words, how much and in what directions are the rocks being compressed at a specific location today? We are not concerned here with any stress regimes of the past, or paleostresses, which might be indicated by permanent changes in rock fabric (e.g., Hobbs and Talbot, 1966).
Buried sediments respond to pressure and temperature and to fluids that pass through them. For example, the effects of temperature in terms of organic metamorphism are widely recognized and studied; the changes initiated by migrating fluids underpin the fields of diagenesis and metamorphic petrology. Less attention has been paid to the effects of pressure. The importance of fluid pressures in determining the effective strength of rocks is addressed in structural geology and its role in the entrapment and production of hydrocarbons is recognized. However, rock pressure (stress) has often been regarded as a vertically-acting phenomenon, synonymous with the overburden load. Horizontal stresses are commonly tacitly assumed to be equal to the overburden stress, yet most rocks in sedimentary basins are not compressed isotropically. Geological evidence of past conditions, together with numerous contemporary measurements, clearly document that buried rocks are generally under anisotropic stress conditions, where the principal stresses are of different magnitudes. As this situation exists in the Western Canada Sedimentary Basin, many geological and petroleum engineering investigations would benefit from considering the contemporary stress regime.
Appreciation of the stress regime can help us understand the recent tectonic evolution of a basin. From an engineering standpoint, how a rock behaves mechanically is determined by its own inherent physical properties and by the stress field to which it is subject. In-situ stress information can predict in which directions production-enhancing hydraulic fractures will propagate, can suggest within which formations they are likely to be contained, and can determine the optimum orientation for inclined and horizontal wells. Thus, stress data are valuable for plannning how best to recover hydrocarbons from reservoirs.
To fully describe the state of stress at a point in the subsurface, it is necessary to determine the magnitudes and orientations of three orthogonal principal stresses (Fig. 29.1), referred to as the largest, intermediate and smallest principal stresses. Stress is thus a tensor quantity (Jaeger and Cook, 1979). Consideration of a stress field is simplified if a "free surface" is present. This is a surface where the acting tangential forces are negligible, as occurs at the interface between the Earth's topographic surface and the atmosphere. One of the principal stresses will be oriented perpendicular to this interface, or "free surface". To a first order approximation, the present-day topography across the Western Canada Sedimentary Basin is horizontal, therefore one of the principal stresses will be close to vertical (Fig. 29.2). Even where there is some relief, its effect on deflecting stresses will be minimal below depths of a few hundred metres, so the generalization that one principal stress is vertical will hold (Hoek and Brown, 1980). If one principal stress is vertical, the other two principal stresses must be oriented horizontally. In this chapter, the vertical principal stress is referred to as SV, and the larger and smaller horizontal principal stresses are designated SHmax and SHmin respectively (Fig. 29.2), and we use the convention that compression is positive.
Stress Magnitude Measurements
Measurements and estimates of stress magnitudes at various locations within the Western Canada Sedimentary Basin are listed in Tables 29.1 and 29.2. These measurements and estimates have been obtained in the following ways. In some cases, the overburden pressure or vertical stress, SV, was obtained by integrating a density log from surface to the depth of interest (e.g., Gronseth, 1990). In other cases, SV was estimated assuming a mean density of 2400 kg/m3. This approximation was used because no direct measurements have yet been made of vertical stress at great depth in the Western Canada Sedimentary Basin. There are, however, two overcoring measurements of SV that were made at shallow depth at the Kipp Mine in southern Alberta (Kaiser et al., 1982).
The smaller principal stress, SHmin, has been measured with accuracy by hydraulic micro-fracturing and by overcoring. Lower limits on its magnitude are provided by mini-frac tests, and upper limits have been estimated from selected formation leak-off tests.
The larger horizontal principal stress, SHmax, has been directly measured by overcoring methods at two sites in the Kipp Mine in southern Alberta (Kaiser et al., 1982). Elsewhere, it has been estimated from hydraulic fracture pressure data and pore pressure estimates using the following relation:
SHmax = SHmin - Fracture Reopening Pressure - Pore Pressure
This equation presumes that the rocks involved are relatively isotropic, impermeable, and exhibit linear elastic behaviour (Hubbert and Willis, 1957). For the hydraulic fracture cases, no other direct measurements of SHmax were made, hence there is no independent confirmation of these values.
The largest body of stress magnitude data in the Western Canada Sedimentary Basin comprises SHmin estimates that have been derived from several types of hydraulic fracturing tests, which have different levels of reliability.
The most reliable data come from measurements made by micro-fracture tests and by overcoring (Table 29.1, Figs. 29.3, 29.4). In the micro-fracture tests, hydraulically induced fractures were reopened several times until their closure pressures stabilized to give repeatable measures of SHmin (Kry and Gronseth, 1983; McLennan et al., 1982, 1986; McLellan, 1988a). The overcoring program involved precise monitoring of strain which, coupled with laboratory measurements of the geomechanical properties of the rocks involved, yielded reliable magnitudes of all three principal stresses (Kaiser et al., 1982). Both these techniques give accurate repeatable measurements of SHmin.
By contrast, mini-frac records and formation leak-off tests (Table 29.2) give less accurate estimates of SHmin. This is unfortunate, because the largest amount of potentially useful stress magnitude data for the Western Canada Sedimentary Basin is provided by these two sources. In earlier studies, the fracture closure stresses interpreted from mini-frac records were equated with SHmin (Woodland and Bell, 1989; Bell and McCallum, 1990). We now recognize that this procedure is rarely likely to give regionally representative values. Mini-fracs are performed to propagate small fractures in reservoirs that are candidates for subsequent larger acid or propped fracture treatments (massive fracs). The data gathered help design the final fracture treatments. In Alberta, reservoirs that are mini-fraced typically fall into two categories. They are either shallow oil sands or heavy-oil-bearing formations that are low to moderate producers, or they are deeper sandstone formations that may have already been produced successfully for some time and are now judged to need fracturing to boost their performance (McLellan, 1988b).
The shallow units (0-1200 m) with poor production expectations are, typically, fractured early in their production history before the virgin formation pressures have been reduced to any significant degree. On the other hand, by the time the deeper sandstone reservoirs (1200 m +) are fractured, they have usually been producing for some years, their fluid pressures have been reduced and, in some cases, sand has been produced, indicating that the reservoir has been "damaged" in the area adjacent to the wellbore. We have plotted depth-normalized reservoir pressures against depth-normalized fracture closure pressures and there is some degree of correlation, though not enough to clearly demonstrate that a reduction in reservoir pressure alone modifies mini-frac measurements of SHmin. Reduction in fluid pressures due to hydrocarbon production may be a factor, as was demonstrated by Salz (1977), but clearly there are other processes at work. The net result is that the closure stresses obtained from mini-fracs run in such sandstones tend to be lower than if the measurements had been made in the virgin reservoirs. This means that much of the mini-frac data will not be suitable for documenting natural stress magnitudes accurately, because the fracture closure pressures will underestimate SHmin in many cases.
The above comments are generalizations and they apply with varying degrees to the data plotted in Figure 29.5. Production history information, geomechanical aspects of any reservoir disruption and reservoir pressure would be required to assess individual closure stresses as measures of SHmin. What can be stated is that most of the shallower estimates of closure stress (above -1200 m) are likely to be close to virgin SHmin magnitudes, whereas most of the deeper ones are likely to underestimate SHmin.
Figure 29.6 plots formation leak-off test pressures that were measured in 35 wells at depths greater than -500 m. These are the pressures at which fluid begins to be lost to the formation when the drilling fluid pressure is raised by pumping to test the integrity of the strata below a string of cemented casing. Traditionally, this pressure is interpreted as recording fracture initiation, or injectivity, into the rocks that surround the borehole at the top of the open interval, below the lowest casing shoe. The 35 formation leak-off test records were checked for interpretational accuracy in this study and, as can be seen, there is a considerable scatter to the pressures (Fig. 29.6). The values below a gradient of 12 kPa/m, close to hydrostatic pressure levels (c. 10 kPa/m), possibly simply record the injection of drilling mud into permeable rocks without any induced fracturing occurring. Hence, they are likely to be indicative of fluid injectivity, not in-situ stresses, and should be ignored. Values near 24 kPa/m are close to the typical overburden stress levels (SV) for depths greater than 1200 m. Possibly, such rocks resisted easy fracturing because their tensile strengths were high. It is suspected that what has been measured in these anomalous cases are "formation breakdown" pressures that are likely to be significantly larger than the closure stresses (SHmin) for the fractures, hence they too should be ignored. The median population (14 kPa/m - 24 kPa/m) are likely to record fracture initiation in rocks with low tensile strengths and, therefore, these leak-off pressures will overestimate SHmin by a small amount. They can be used to place an upper limit on SHmin magnitudes (Breckels and Van Eekelen, 1981; Ervine and Bell, 1987; Bell, 1990).
Implications of Stress Magnitudes in Western Canada
A number of shallow-depth micro-frac measurements of SHmin have been made in the oil sands and heavy oil deposits of northeastern Alberta (Table 29.1, Figs. 29.3, 29.4). Some of the results suggest that the vertical stress may be the smallest principal stress at depths of less than 500 to 600 m (e.g., Settari and Raisbeck, 1978), while others imply that the vertical stress and the smaller horizontal principal stress are of closely similar magnitudes (e.g., Proskin et al., 1990a,b). It is not clear how representative these shallow measurements are for the rest of the basin. The few reliable deep measurements from micro-fracturing (Fig. 29.4) are all from the western flank of the basin (Fig. 29.3). They point to a stress regime with SHmax > SV > SHmin in the Elmworth area (Kry and Gronseth, 1983), in the Caroline area (McLennan et al., 1982), and possibly around the Kidd Mine (Kaiser et al., 1982), where overcoring measurements support this relationship. On the other hand, a thrust regime, with SHmax > SHmin > SV, may possibly exist in the Wapiti area (McLellan, 1988a). In Saskatchewan (McLennan et al., 1986, McLellan et al., 1992), SV may be the largest principal stress. In other parts of the basin, the stress magnitude indicators are not accurate enough to unequivocally identify the regime.
In earlier interpretations (Woodland and Bell, 1989; Bell and Price, 1989; Bell and McCallum, 1990), it was suggested that the mini-frac closure pressures yielded magnitudes of SHmin that were precise enough to use for identifying the relative magnitudes of the principal stresses at the site. With this reasoning, Woodland and Bell (1989) concluded that much of the central part of the Western Canada Sedimentary Basin lay within a normal fault regime, where SV > SHmax > SHmin was widely present. Later, Bell and Price (1989) and Bell and McCallum (1990) suggested that mini-frac closure stresses could also be used to map lateral variations of SHmin across the basin, and drew conclusions from the apparent configurations. These procedures can no longer be considered valid in light of the apparent inaccuracies in the closure stress magnitudes that have been measured in productive sandstone reservoirs, as discussed above.
To obtain a reliable three-dimensional picture of stress magnitudes in the Western Canada Sedimentary Basin we need more data. Until these become available, we shall not have a clear idea of relative and absolute magnitudes of principal stresses in most parts of the basin, nor will we be able to speculate on the evolution of its stress regime, or on the implications of any regional variations in stress magnitudes. Today's data give an idea of expected SHmin magnitudes locally, but little more.
Stress Orientation Measurements
In the Western Canada Sedimentary Basin, the axes of the present-day horizontal principal stresses recorded in publications are indicated by breakouts (181 wells), hydraulic fracture orientation (1 well), anelastic strain recovery (2 wells), differential strain curve analysis (1 well), waterflood breakthrough (2 locations), overcoring measurements (3 sites), near-surface stress release features (2 sites) and microseismic monitoring (1 site).
Breakouts are spalled cavities that occur on opposite sides of a borehole wall. They form because the well, being a cylindrical opening in rock, distorts and locally amplifies the far-field stresses, so as to produce caving in the borehole wall (Bell and Gough, 1979). If the horizontal principal stresses are of unequal magnitude, the wall rock is squeezed anisotropically, and caving occurs preferentially on opposite sides of the hole, perpendicular to the trajectory of SHmax (Fig. 29.7). Thus, breakouts that occur in a near-vertical well can usually be used to diagnose stress anisotropy and also to map the orientations of the horizontal principal stresses.
Breakouts have been previously identified and oriented by studying 4-arm dipmeter logs from 181 wells in the Western Canada Sedimentary Basin (Table 29.3). The total thicknesses of all breakout intervals in a single well ranged from 2 m to 2087 m. The shallowest breakout occurred at a depth of 113 m; the deepest at 5485 m. The majority reported here were identified between depths of 1000 and 3500 m. The largest number of breakout intervals measured in a single well was 84; the smallest was 1. Of the 181 wells examined, 144 contain only a single major population of breakout orientations. Thirty seven wells have secondary minor populations, commonly with mean azimuths at approximately 90° to the major breakout population. Consistency in breakout orientation with depth is widely observed, no matter what type or age of rock is involved.
Breakout orientations have been mapped for Paleozoic and Mesozoic rocks (Figs. 29.8 , 29.9) and both data sets have been combined for the horizontal stress trajectory map (Fig. 29.10). This map has been constructed chiefly from the mean breakout orientations in 181 wells (Table 29.3), but it honours the horizontal principal stress orientations yielded by the other measurements (Table 29.4). The reliance on breakouts is justified because these features have proved to be extraordinarily sensitive orientation indicators of contemporary stress. Table 29.3 lists the mean azimuths for all the breakout populations identified in the 181 wells and also separates the mean breakout directions observed in Paleozoic rocks (Fig. 29.8) and in Mesozoic rocks (Fig. 29.9).
Other Orientation Indicators
The horizontal stress orientations documented by mean azimuths of breakouts in wells are supported by measurements made using other techniques (Table 29.4).
Bell (1985) reported updip bed slip in Mississippian limestones in roadcuts near Exshaw, Alberta and in the nearby Kananaskis Valley. This phenomenon was interpreted as near-surface stress release and suggested SHmax azimuths of 049° and 032° at the two respective localities. In the same area, two overcored boreholes in Mesozoic rocks in the roof of the Wilson Mine at Canmore gave SHmax azimuths between 055° and 066° (Grant, 1970). Other overcoring measurements in Cretaceous rocks at the Kipp Mine in southern Alberta suggested SHmax orientations of 070° and 090° (Kaiser et al., 1982). Three hundred and seven overcoring stress measurements were made in seven test holes drilled into Lower Cretaceous sediments at the W.A.C. Bennett dam site in northeastern British Columbia. Based on the results of these measurements, Imrie (1991) interpreted an SHmax orientation of 076° for the central and southeastern area beneath the powerhouse and a 020° orientation for the northwestern area. The latter measurements were made within 200 m of the Peace River canyon wall, which is also aligned at approximately 020°, so they may not be representative of the regional stress signature. Other overcoring stress measurements have been made farther downstream at Dam Site C in northeastern British Columbia, in Lower Cretaceous sediments a few hundred metres from the banks of the Peace River. They yield east-west axes for SHmax but, since this orientation closely parallels the river banks and may not be regionally representative, it is not plotted on Figure 29.9.
Imperial Oil (1978) reported 045°-trending vertical hydraulic fractures in Cretaceous rocks at Cold Lake, based on heavy oil production research. McLennan et al. (1986) interpreted oriented packer impressions of induced fractures in lower Paleozoic rocks at the University of Regina well site and reported SHmax azimuths of 070° and 071°. Waterflood breakthrough in Cretaceous Cardium sandstone reservoirs at Pembina is attributed to inadvertent hydraulic fracturing. McLeod (1977) and Hassan (1982) reported connections between NE-SW-aligned wells, suggesting an approximate 045° orientation for SHmax. In a multi-facetted study of Cretaceous Cardium sandstones at Wapiti in Alberta (McLellan, 1988a), anelastic strain recovery measurements made on newly cored samples suggested that SHmax was oriented at 036°. Subsequent differential strain curve analysis of the same cores gave an SHmax azimuth of 043°. Fractures were propagated from the same well in which these cores were taken and monitored seismically. The results suggested that the induced fractures propagated along a direction of 035° (+/-15°). At Kindersley, in Saskatchewan, seismic monitoring suggested that an induced fracture in the Cretaceous Viking Formation propagated along an SHmax azimuth of 052° (Talebi et al., 1991). Anelastic strain recovery was also monitored on horizontal core samples of Mississippian carbonates recovered from a well at Midale in Saskatchewan and the measurements suggested that SHmax was oriented between 055° and 070°.
These stress directions, which have been obtained independently of breakouts, are also shown on the maps portraying the mean breakout azimuths for Paleozoic and Mesozoic rocks (Figs. 29.8, 29.9). On these maps, they are oriented to show SHmin directions.
Implications of Stress Orientations in Western Canada
On a local scale, the stress trajectories are useful for predicting the orientations of hydraulic fractures (Hubbert and Willis, 1957; Hassan, 1982).Typically, below depths of about 500 m, fractures will be vertical and will trend parallel to SHmax trajectories. Such information is helpful for planning secondary recovery programs that involve hydraulic fractures, for directed fracture operations, and for effectively directing inclined and horizontal wells (Bell and Babcock, 1986; McLellan et al., 1992).
With the recent surge in horizontal drilling of fractured reservoirs, there has been much interest in determining whether fractures trending in certain directions are more likely to remain open and productive compared to fractures trending in other directions. This information has a direct bearing on how the horizontal wells are drilled. Stress information can help predict which of several fracture sets is likely to contribute most to long-term hydrocarbon production. As yet, there are very few published case histories that demonstrate the significance of the in-situ stress orientations in fractured reservoirs. However, all other factors being equal, fractures running NE-SW, which are subparallel to SHmax in the Western Canada Sedimentary Basin, are likely to be more resistant to closure than those oriented NW-SE, parallel to SHmin trajectories. Local exceptions exist, particularly in the Rocky Mountain Foothills, where there are indications of open fractures running NW-SE, parallel to thrust planes and fold axes (Barss and Montandon, 1981).
On a continent-wide scale, the Western Canada Sedimentary Basin lies within the North American Mid-Plate Stress Province, which is characterized by NE-SW-trending SHmax orientations (Zoback and Zoback, 1980; Adams and Bell, 1991). Mean SHmax azimuths measured in the Western Canadian Sedimentary Basin, and at many locations within the Mid-Plate Stress Province (Zoback et al., 1989), coincide closely with the absolute plate motion vectors determined for the North American Plate (Minster and Jordan, 1978). This suggests that the horizontal stress anisotropy documented in the sediments may be an upper crustal manifestation of mantle drag on the base of the lithosphere. For Western Canada, the basal drag could be exerted by the lithosphere sliding southwestward across the asthenosphere (Zoback and Zoback, 1980), or by northeastward sublithospheric mantle flow, as proposed by Gough (1984). Bell et al. (1992) have noted that the directional coincidence of plate motion vectors and SHmax orientations appears to be most pronounced in the faster moving plates.
The authors thank their respective employers for permission to publish this chapter. J.D. McLennan, H.S. Chhina and L. Vandamme are thanked for providing information to supplement their published stress measurements. Unpublished formation leak-off test data were supplied by the Energy Resources Conservation Board of Alberta. The authors are grateful for the critical reviews of P.R. Kry, I. Shetsen and G.S. Stockmal, which have led to significant improvements in this chapter.
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