Hydrogeology Journal DOI 10.1007/s10040-013-1061-5
The geothermal potential of the basal clastics of Saskatchewan, Canada Grant Ferguson & Stephen E. Grasby Abstract The Winnipeg and Deadwood formations form deep clastic reservoirs in Saskatchewan, Canada, with temperatures exceeding 40°C over most of southern Saskatchewan and reaching 100°C in southwestern Saskatchewan. At these temperatures, the formations have geothermal potential for development of direct use and electricity generation systems. Numerous disposal wells operating at rates of 30L/s or more are currently installed in these formations, suggesting that electricity could be generated at rates exceeding 2 megawatts of electrical output (MWe) from individual wells. These basal clastic units, thus, could provide significant energy supply over a broad region of Saskatchewan. Keywords Canada . Thermal conditions . Sedimentary rocks . Injection wells . Heat flow
Introduction Inability to predict geothermal reservoir performance is one of the greatest obstacles to expansion of geothermal energy development (Goldstein et al. 2011). Preliminary characterizations of high temperature environments often rely on interpretation of surface expressions of geothermal activities along with shallow wells and geophysical surveys prior to drilling exploration wells at the depth of interest (Barbier 2002). These exploration strategies can be inadequate because of the misleading nature of many Received: 30 January 2013 / Accepted: 26 September 2013 * Her Majesty the Queen in Right of Canada 2013 Published in the theme issue “Hydrogeology of Shallow Thermal Systems”
G. Ferguson ()) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada e-mail:
[email protected] S. E. Grasby Geological Survey of Canada, Natural Resources Canada, 3303-33 Street North West, Calgary, AB T2L 2A7, Canada
surface expressions (Duffield and Sass 2003; Ferguson and Grasby 2011) and difficulties associated with extrapolating shallow temperature measurements (Gosnold et al. 2011; Smith and Chapman 1983). Sedimentary basins offer a different set of challenges and opportunities for geothermal development. While these environments are less attractive because of the lower temperatures, in many cases, large databases are available from the oil and gas industry that can reduce exploration costs as well as uncertainty surrounding subsurface conditions (IEA 2011; US Deparment of Energy 2010). Over 700,000 wells of various types have been completed in the Western Canada Sedimentary Basin (WCSB; Fig. 1; IHS Energy 2012). The geological and geophysical information collected during the drilling and subsequent testing of these wells can provide data for geothermal assessments that are not always readily available in high enthalpy systems. Sedimentary environments also tend to contain reservoirs that allow for large fluid production and injection rates without stimulation, which may not be the case in other higher heat-flow environments. The high level of development in some sedimentary basins can provide some basis for estimating realistic fluid production rates, which may not be possible in other settings. For example, Tester et al. (2006) based many of their projections on fluid production rates of 80 L/s in their assessment of potential enhanced geothermal systems (EGS) in the United States. However, testing from pilot projects completed as of 2006 were only able to produce between 10 and 30 L/s over long periods (Tester et al. 2006), indicating that further advances in reservoir stimulation techniques are required to meet those projections. Conversely, there is less need for speculation on injection and production rates in sedimentary basins because of a wealth of experience with fluid production and injection at higher rates that could be used to support geothermal assessments. The combination of higher fluid production rates and reduced geological uncertainty may allow for development of geothermal resources in areas with only moderate heat flow. In Saskatchewan, Canada, the focus of this study, (Fig. 1) over 5,000 injection and disposal wells have been completed over a range of depths (IHS Energy 2012; Fig. 2). Many of these wells have been operating for decades, providing insight into the sustainability of fluid production and injection. The high permeabilities of these environments
Fig. 1
Map showing study area (dark grey area) along with location of the Western Canada Sedimentary Basin (white area) and Saskatchewan
may allow for projects to proceed with little to no stimulation and can avoid problems, both perceived and real, associated with such activity. These concerns include induced seismicity (Giardini 2009) and, similar to the oil and gas industry (Baihly et al. 2010), there is a lack of certainty on long-term reservoir behaviour following stimulation. The Winnipeg and Deadwood formations of Saskatchewan, often referred to as the basal clastics (Fig. 3), offer an intriguing geothermal opportunity. These are the deepest sedimentary formations in Saskatchewan, reaching depths of over 3,000 m (Fig. 4), and also happen to form one of the more permeable reservoirs in WCSB. Here the reservoir formed by the Winnipeg and Deadwood formations is examined to determine the feasibility of geothermal energy development using data available from exploration and injection wells from both the petroleum and mining industries in Saskatchewan.
Geology of the Winnipeg and Deadwood formations The deep Phanerozoic strata of Saskatchewan consist of a basal clastic sequence overlain by a thick sequence of platform carbonates and evaporites with minor clastics (Kreis et al. 2004). This basal clastic sequence is comprised of the Deadwood Formation, which was deposited during the Late Cambrian, and the Winnipeg Formation, which was deposited during the Middle Ordovician. The Deadwood Formation consists of fine-bedded sandstones, interbedded siltstones and shales, along with minor carbonates (Slind et al. 1994; Vigrass 1971). Fine interbeds of limestone are present in southwestern Hydrogeology Journal
Saskatchewan. The Deadwood Formation is thought to have been deposited in a shallow to moderately deep inshore basin (Slind et al. 1994). During the Middle Ordovician, a major sea level fall occurred, resulting in a subaerial exposure of this formation and erosion. The Deadwood Formation is uncomformably overlain by the Winnipeg Formation in eastern Saskatchewan and by the Red River Formation where the Winnipeg Formation is absent in the western portion of Saskatchewan. The Winnipeg Formation consists of sandstones, siltstones and shales deposited during a transgression during the Middle Ordovician (Norford et al. 1994). Separation of the Winnipeg Formation into members is somewhat controversial (Kreis et al. 2004). In ascending order, the Black Island Member, Icebox Member and Roughlock Member are often recognized. The Roughlock Member, which consists primarily of siltstone (Carlson 1958), is more commonly found in the southern extent of the Winnipeg Formation in the United States and has not been found extensively in
Fig. 2 Distribution of injection well depth in Saskatchewan DOI 10.1007/s10040-013-1061-5
Fig. 3 The Paleozoic stratigraphy of Saskatchewan (Saskatchewan Ministry of Energy Resources 2011)
Saskatchewan (Kreis et al. 2004). Both the Black Island Member and the Icebox Member are found over large areas of Saskatchewan. The Black Island Member consists of quartz arenites and quartz wackestones, while the Icebox Member consists of shales (Paterson 1971). The depth of the top of the basal clastics ranges from a few hundred metres north of Saskatoon, Saskatchewan to over 3,000 m in the southeastern reaches of the province (Fig. 4). The thickness of the basal clastics increases from east to west across southern Saskatchewan. The thicknesses of the basal clastics is less than 50 m present near the Manitoba border and greater than 550 m near Lloydminster, on the Saskatchewan/Alberta border (Fig. 5).
1993; Majorowicz and Jessop 1981; Majorowicz and Jones 1986). Many of these studies have focused on extrapolation of temperature logs that do not penetrate through the entire
Heat flow and temperature distribution There have been numerous studies addressing the distribution of heat flow in the WCSB (Bachu and Burwash 1991; Bachu Hydrogeology Journal
Fig. 4 Depth to the top of the basal clastics in Saskatchewan. Contours are in metres below ground surface. CAN Canada, SK Saskatchewan, AB Alberta, MB Manitoba. Black squares are towns DOI 10.1007/s10040-013-1061-5
Fig. 5 Thickness of the Winnipeg and Deadwood formations in Saskatchewan. Contours are in metres
sedimentary sequence or modeling of heat flow based on measured temperature gradients and heat generation. Heat flow at the base of the sedimentary basin ranges from less than 40 mW/m2 in southwestern Saskatchewan, to slightly more than 60 mW/m2 in the northwestern extent of the sedimentary basin, north of Yorkton, Saskatchewan. These heat flow values are unremarkable compared to the Earth’s average continental value of 65 mW/m2 (Pollack et al. 1993). Temperatures above 100 °C are present in the lower reaches of the Williston Basin in southeastern Saskatchewan (Bachu and Burwash 1991; Bachu 1993; Majorowicz and Jessop 1981; Majorowicz and Jones 1986). Heat flow from the underlying Precambrian basement is approximately 50 mW/ m2 in this area (Bachu 1993), suggesting that these high temperatures are the result of the greater depth of the basin in this region rather than elevated heat flow. Other researchers have suggested that advection could explain this anomaly (Majorowicz and Jones 1986) but subsequent analysis revealed that groundwater flow rates were insufficient to cause this type of regional heat flow anomaly (Bachu and Burwash 1991). More recent studies have examined heat flow in the WCSB from the perspective of geothermal energy development. The greatest potential exists in deeper parts of the WCSB in Alberta and in southeastern Saskatchewan (Majorowicz and Grasby 2010). Melnik et al. (2011) presented an overview of subsurface temperatures in Saskatchewan, which confirmed the general patterns presented by earlier studies (Bachu and Burwash 1991; Bachu 1993; Majorowicz and Jessop 1981; Majorowicz and Jones 1986). Melnik et al. (2011) also compared various types of temperature measurements and found that dedicated temperature logs and temperatures obtained during drill stem tests (DSTs) were more representative of in situ conditions than measurement of bottom hole temperatures (BHTs), which tend to underestimate temperatures. Correction of BHTs derived from the Winnipeg and Deadwood formations was not possible because of a lack of higher resolution temperature measurements at this depth in Saskatchewan. There are 68 temperature measurements available from DSTs conducted in the basal clastics of Saskatchewan Hydrogeology Journal
Fig. 6 Spatial distribution of drill stem test (DST) temperatures in the basal clastics of Saskatchewan. Contours are in °C. Crosses represent measurement locations
(Fig. 6; IHS Energy 2012) that show a wide range of temperatures. Temperatures are generally less than 40 °C north of 52° N latitude, while temperatures near 100 °C were only found in southeastern Saskatchewan. Temperatures exceeding 70 °C were also found in the Swift Current area. Variations in temperature are strongly influenced by depth (Fig. 7). Majorowicz and Grasby (2010) suggested that a thick cover of low conductivity sediments could result in higher temperatures in the deeper reaches of the WCSB. However, the strong correlation between depth and temperature suggests that variations in the overlying formations do not exert a strong control on the temperatures within the basal clastics.
Hydrogeology of the basal clastics There has not been extensive hydraulic testing of the Winnipeg and Deadwood formations in Saskatchewan,
Fig. 7 Temperatures obtained during DSTs in the Winnipeg and Deadwood formations of Saskatchewan versus depth. Average geothermal gradient = 28.1 °C/km (n=59) DOI 10.1007/s10040-013-1061-5
primarily because of a lack of oil and gas reserves. Hydraulic conductivities between 10−6 to 10−4 m/s have been measured in the Winnipeg Formation in Manitoba (Ferguson et al. 2007). Values from testing of the Winnipeg and Deadwood formations beneath Regina have given values between 1 × 10−6 and 3 × 10−6 m/s (Hutchence et al. 1986; Vigrass et al. 2007). There is insufficient data to comment on the spatial variability of hydraulic conductivity but the smaller combined thickness of the Winnipeg and Deadwood formations is less in southeastern Saskatchewan, suggesting that transmissivity will be lower in that region. This implies that wells will be less productive in this area if hydraulic conductivity is similar throughout the extent of these formations.
Geothermal energy development potential Geothermal energy extraction from the Winnipeg and Deadwood formation will require that a sufficient amount of fluid can be produced and injected into these formations. The produced fluid will need to be at a sufficient temperature to allow for electricity generation or direct use. Injection wells will need to be situated at a distance that will allow for long periods of operation before thermal breakthrough of cold injected water at the production well. Here these requirements are explored by examining injection wells currently operating in the basal clastics, estimating power production from injection rates and possible production and injection temperatures and analytical modeling of thermal breakthrough in geothermal doublets.
Injection wells in the basal clastics The greatest interest in the basal clastics has been for brine disposal, largely from the potash industry. Most of Saskatchewan’s active potash operations have injection wells that operate at average rates between 30 and 140 L/s (Figs. 8 and 9). Of particular interest are the collection of disposal wells at a solution mine west of Regina. These wells have at times injected at average monthly rates in excess of 200 L/s and have produced at an average rate of 140 L/s for over a decade (Fig. 10). There is limited data on the far field effects of these wells but a report by MDH (2008) indicated that changes in injection rates at one of the wells at the mine site did not have an impact on the pressures at wells which were approximately 1 km away. Injection rates from groups of wells from at least three other sites have exceeded 100 L/s and several injection wells have been operating at rates of 50 L/s or more since the 1970s. It is not clear if the production rates of these wells are limited by the hydraulic properties of the basal clastics. A number of operations have added additional disposal wells in the past decade, suggesting that it might be unrealistic to Hydrogeology Journal
Fig. 8 Histogram of injection rates for wells completed in the Winnipeg and Deadwood formations in Saskatchewan
expect injection rates in excess of 50 to 100 L/s from individual wells. No production wells are currently operating in the Deadwood or Winnipeg formations in Saskatchewan. For the purposes of this study, it is assumed that production rates will be equal to injection rates. Deviations from this relationship could occur for a variety of reasons. There has been a great deal of research in the petroleum industry documenting reductions in permeability of up to 20 % where porewater pressures are reduced by as little as 10 MPa (Han and Dusseault 2003; Holt 1990; Schuthjens et al. 2004). Current injection pressures within the basal clastics are typically 5 MPa (Ruse 2008) and pressure reductions of approximately 10 % of this amount occurred following a short duration pumping test at Regina conducted at a rate of 25 L/s (Vigrass et al. 2007). Reduction in permeability and productivity may be noticeable following operation of production wells at high rates in the basal clastics. There are also cases where injection rates are the limiting factor in geothermal developments, usually because of issues with precipitates that reduce permeability in the vicinity of the injection well (Regenspurg et al. 2010; Ungemach 2003). Both geomechanical and geochemical effects should be examined in greater detail for the formations in question.
Fig. 9 Location of injection wells in the Winnipeg and Deadwood formations of Saskatchewan DOI 10.1007/s10040-013-1061-5
Fig. 10 Long-term Injection rates at a potash mine east of Regina, Saskatchewan (Fig. 9) since 1992. Different colours represent different injection wells
Geothermal energy production Direct use Direct use for heating is possible where subsurface temperatures exceed 30 °C, although many applications require temperatures greater than 50 °C (Milenić et al. 2010). Potential for direct use application is high in southern Saskatchewan due to the relatively high fluid temperatures (Fig. 6). Larger projects in Regina, Saskatchewan have received serious consideration (Vigrass et al. 2007). In the Saskatoon area, the potential for direct use is less attractive due to the lower temperatures present. Developments in this region may need to consider the use of heat pumps. The amount of thermal power that can be produced from a well is described by: Qthermal ¼ Q f ρ f c f ΔT
ð1Þ
where Qthermal is the amount of power, Qf is fluid production rate, ρf is the fluid density, cf is the specific heat of the fluid and ΔT is the temperature difference between the produced fluid and discharged fluid. Assuming it possible to produce water at the average injection rate of the wells west of Regina (Figs. 9 and 10), approximately 12 megawatts of thermal output (MWthermal) could be produced with a ΔT of 20 °C, per well. These numbers are somewhat greater than a project that was proposed for the University of Regina during the 1980s (Vigrass et al. 2007). Detailed
measurements from a proposed project in Regina showed a temperature of 58–59 °C. Subsequent models using fluid production rates of approximately 10 L/s with a ΔT of 33 °C, indicated that production of approximately 1.4 MWthermal could be expected from this development. This suggests that large amounts of power for direct use applications could be generated over much of southern Saskatchewan
Electricity generation Generation of electricity is feasible in the southeastern portion of Saskatchewan, where temperatures approach 100 °C (Fig. 6). This is the only area where electricity production is possible with current geothermal technologies (Gallup 2009). Using Eq. (1) and assuming 10 % conversion efficiency from thermal power to electrical power, plants of 2 megawatts of electrical output (MWe) appear to be feasible if fluid can be produced at rates similar to those observed in the Regina area. Installation of additional production wells could allow for even larger power plants.
Spacing for geothermal systems Adequate spacing must also be considered in the design of any developments. Gringarten and Sauty (1975) provide
Table 1 Parameters used in application of Eq. (1). All parameters from Hutchence et al. (1986) except thermal conductivity, which is estimated from Clauser and Huenges (1992) Parameter
Value
Porosity (−) Fluid density (kg/m3) Reservoir matrix density (kg/m3) Fluid heat capacity [J/(kg K)] Reservoir matrix heat capacity [J/(kg K)] Thermal conductivity of caprock [W/(m K)]
0.15 1,065 2,650 3,770 780 3
Hydrogeology Journal
Fig. 11 Predicted thermal breakthrough time as a function of well spacing for the basal clastics of Saskatchewan at different production and injection rates DOI 10.1007/s10040-013-1061-5
an equation for estimating thermal breakthrough of cold water from the injection well at the production well: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 2QΔt D¼u u 1=2 t ρcr ccr κcr ρ ccr ρ c 2 hþ η þ ð1−ηÞ ρcrf ccrf H 2 þ 2 ρ crc ΔtÞ η þ ð1−ηÞ ρf cf f f
temperatures are lower. The characteristics of the reservoir in southeastern Saskatchewan, along with the relationship between injection and production rates, are not well understood. These shortcomings must be addressed to further reduce uncertainty and encourage development of this energy resource.
ð2Þ where D is the distance between the injection and withdrawal well, Q is the rate of withdrawal and injection, H is the thickness of the aquifer, ρf and ρcr are the densities of the fluid and the geologic unit confining the aquifer, cf and ccr are the specific heat capacities of the fluid and the confining unit, κcr is the thermal conductivity of the confining unit, η is porosity and Δt is the time prior to thermal breakthrough, also defined as reservoir lifetime. This model assumes heat flow is due to horizontal advection within the reservoir and vertical conductive heat flow in the units underlying and overlying the reservoir. Using a thickness of 100 m and expected thermal and hydraulic parameters (Table 1), reservoir lifetimes of greater than 30 years should be expected if wells are spaced at 1,000 m or greater for production and injection rates of 75 L/s (Fig. 11).
Discussion and conclusions The temperatures, depths and current injection rates in the basal clastics of Saskatchewan are remarkably similar to production and injection rates observed in the Dogger aquifer in France, which has a long history of direct use geothermal energy projects (Lopez et al. 2010). The Dogger aquifer has been exploited for direct-use geothermal energy for more than 40 years and currently supplies 29 district energy networks. The production wells supplying these systems are typically 1,500–2,000 m deep and produce water at 14–167 L/s at temperatures of 58–85 °C. These similarities suggest that under the right economic conditions direct use of geothermal energy could be quite successful in Saskatchewan. Overall, the Deadwood and Winnipeg formations of Saskatchewan, Canada have significant geothermal potential. In southeastern Saskatchewan, these formations host fluids that are sufficiently hot to generate electricity. Over a large portion of the extent of these formations, particularly south of 52° latitude, direct-use applications should be possible. The hydraulic properties of the basal clastics make it particularly attractive. Production rates in excess of 50 L/s should be possible over large areas of the province. Minimal to no stimulation of the reservoir will be necessary to provide the flow rates necessary for geothermal systems. Additional research is necessary to support development of this resource. Hydraulic testing has been quite limited and although high injection rates have been recorded in the basal clastics, the bulk of this experience comes from the northern part of the study area where Hydrogeology Journal
Acknowledgements This research was funded by a Discovery Grant from the National Sciences and Engineering Research Council of Canada to G. Ferguson. The authors are grateful to Peter Bayer, Inga Moeck and one anonymous reviewer for insightful reviews that substantially improved this manuscript.
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DOI 10.1007/s10040-013-1061-5
