Heat flow, depth temperature variations and stored thermal energy for enhanced geothermal systems in Canada

Transcription

1 IOP PUBLISHING JOURNAL OF GEOPHYSICS AND ENGINEERING J. Geophys. Eng. 7 (2010) doi: / /7/3/002 Heat flow, depth temperature variations and stored thermal energy for enhanced geothermal systems in Canada Jacek Majorowicz 1,2 and Stephen E Grasby 3 1 NGC, Edmonton, T6R 2J8, AB, Canada 2 Department of Geology and Geological Engineering, University of North Dakota, Grand Forks, ND , USA 3 Geological Survey of Canada, Calgary, AB, Canada Received 27 June 2009 Accepted for publication 20 April 2010 Published 9 June 2010 Online at stacks.iop.org/jge/7/232 Abstract In order to help assessment of enhanced geothermal energy potential in Canada, we constructed a new series of heatflow and depth temperature distribution maps (down to 10 km). We focus on high-temperature resources (>150 C) capable of electrical production. Maps presented show large temperature variability, related mainly to heat flow patterns. The highest temperatures occur in western and northern Canada. Here temperatures greater than 150 C, required for enhanced geothermal systems (EGS), can be reached at reasonable drilling depths of <5 km. Heat flow, by itself however, is not a sufficient tool to predict areas of high energy content. A combination of thick low thermal conductivity sedimentary blankets and moderate to high heat flow areas can generate targets that are as favorable as regions with high conductivity and high heat flow. Some moderate heat flow areas in the deeper parts of the Western Canada Sedimentary Basin have heat content comparable to high heat flow zones of the the Canadian Cordillera. The magnitude of in-place thermal energy available for future heat mining/farming was esitmated throughout Canada by calculating heat released through cooling a defined rock volume through a fixed temperature change. These estimates show the first-order appoximation of available geothermal heat content. The fraction of true heat energy available will be as low as 0.02 of these values. However, even this more limited energy production could be large enough to be a considerable future renewable energy resource for Canada. Keywords: 1. Introduction heat flow, geothermics, geothermal energy, Canadian heat content Previous studies of geothermal energy potential across Canada assessed mainly low enthalpy systems (see Jessop et al (1991) for references). Specific focus was given to geothermal energy potential in western Canada (e.g. the Western Canadian Sedimentary Basin (WCSB)) (Anglin and Beck 1965, Lam and Jones 1984, Lamet al 1985, Jones et al 1985, Majorowicz and Jessop 1981a, 1981b, Majorowicz et al 1985, 1999, Majorowicz and Moore 2008). Temperature profiles up to 3 km in depth were studied to assess heat extraction for district heating in sedimentary basins. In addition, a pilot project to assess the feasibility of electrical generation in a hightemperature recent volcanic complex, Meager Mountain Hot Springs in British Columbia, was also performed (Ghomeshei and Stauder 1989). Recent work examined national shallow heat exchange potential (Majorowicz et al 2009). However, the potential of the enhanced geothermal system (EGS) concept, as outlined in the MIT report (Tester et al 2006, Blackwell 2006, Blackwell et al 2007) has not yet been adressed. Enhanced (or engineered) geothermal systems (EGS) are reservoirs that have been created to extract economical amounts of heat from low permeability and/or low porosity geothermal resources (see Tester et al (2006) for details). The best resources defined for /10/ $ Nanjing Geophysical Research Institute Printed in the UK 232

2 the USA are less than 7 km deep with temperatures greater than 150 C(Testeret al 2006). Practical use of such resources has been demonstrated elsewhere in the world, including Soultzsous-Forêts, Alsace (Genter et al 2009, Namiet al 2008, Cornet et al 2007, Gerard et al 2006), hot dry rock (HDR) in Europe (Rybach 2008) and the Cooper Basin in Australia (Tester 2006). While the Soultz-sous-Forêts, Alsace project is in an extensional tectonic environment that has issues with induced seismicity, the Cooper Basin project located in a stable craton does not. Additional research on EGS in Europe is underway (Huenges 2008, Baujard et al 2008, Clauser 2006). The goal of this study is to provide an initial evaluation of EGS geothermal heat content as a potential renewable energy supply for Canada. New maps were constructed that provide detailed heat flow and depth temperature distribution as a first step in determination of the geothermal resource base in conduction dominated systems (sedimentary basins and crystalline basment) across Canada. The quantity of thermal energy (heat content) available from deep hot rocks was also determined. Thermal energy availability for three depth slices, that are consistent with Tester et al (2006) (3 4 km, 6 7 km and km), was then calculated. As a result we are able to provide the first estimate of EGS energy potential in Canada, and define EGS target areas that should be a high priority for future more detailed regional to local studies, that could eventually establish the scientific basis for future drilling and EGS demonstration projects. 2. Background EGS can extract economical amounts of heat from low permeability and/or porosity geothermal resources. In Switzerland it is referred to more precisely as geothermal heat mining (GHM), whereas historically it has been referred to as HDR. In Australia, the term heat farming is used to recognize that after heat is extracted from a rock body, temperatures will recover if left undisturbed, and the same unit can be farmed for heat again at a future date. An EGS requires introduction of water into rock of limited permeability (either tight sedimentary or crystalline rocks) in a controlled fracture setting so that this water can be withdrawn in other wells for heat extraction. An EGS project has several stages. (1) Drilling an injection well to the depth required to reach the desired temperature. (2) Fracturing the rock by hydraulic stimulation. (3) Creating and testing of the storage capacity (3D seismic). (4) Drilling a production well for a doublet system or two production wells for a triplet system (one injection + two production wells). Directional drilling technology is required to create some 600 m distance from the injection well to allow a large enough induced heat exchange system in the subsurface. (5) Creating fracture connectivity between the injection and the production wells. (6) Extracting thermal energy from the rock by injecting cool water through the injection well and producing hot water and/or steam from the production wells. Heat flow, depth temperature variations and stored thermal energy for EGS in Canada 3. Methods Determining the geothermal heat available to be farmed (mined) with an injection/producing well requires, among other parameters, information on deep geothermal conditions, surface temperature distribution and geology. High precision depth temperature logs, in addition to variable quality point measurements (bottom hole temperatures, drill stem test temperatures, etc), provide valuable information on the temperature distribution in the main geological provinces of Canada: sedimentary basins, the Canadian Shield and Canadian Cordillera. These data can be used to derive information on geothermal gradient, heat flow, geothermal energy potential and prediction of temperatures beyond the depth range of observation. Temperatures of C required for EGS are largely found only at significant depths and typically below sedimentary basins Heat flow map Locations shown in figure 1 were used for derivation of the Canadian portion of the Heat Flow Map of North America (Blackwell and Richards 2004a). This was based on initial heat flow compilations completed by Jessop et al (1984a, 2005), data compiled for the WCSB (Majorowicz and Jessop 1981a, 1981b, Majorowicz 1996, Majorowicz et al 1985, 1999), the Mackenzie and Beaufort Basins (Majorowicz et al 1988, 1996), Canadian Arctic (Majorowicz and Embry 1998), Canadian Cordillera (Lewis 1991, Lewiset al 2003) and the Canadian shield and eastern Canada (Geotop 2009, Jessop 1990a, Jessop et al 1984a, 1984b, 2005). Figure 1 illustrates that there are large regions of Canada with sparse or no data. To better honor this data distribution, our map does not extrapolate contours through areas with no available data as was done by Blackwell and Richards (2004b). In addition, new smoothing and averaging techniques were used which filter out anomalous data. Averaging of values was done using the averaging program of M Webring (see Cordell et al (1992)) for a 48 km radius on a 4 4 km grid. The heat flow, depth temperature and heat content maps have a uniform Transfer Mercator projection with a central meridian at 95 W Depth temperature maps Our new heat flow map is the basis for modelling of temperatures for different heat flow heat generation provinces at depths of 3.5, 6.5 and 10 km. These depths are chosen for consistency with depth temperature maps for the United States (Tester et al 2006) and also discussed by Blackwell (2006) and Blackwell et al (2007). For depths where no measured data are available, heat flow maps are used for calculation of temperature as discussed below Temperature depth (T(z)) estimations There is an established statistical relationship between heat flow (Q o ), heat generation (A) and thermal conductivity (K) that can be used to calculate the temperature at depth 233

3 J Majorowicz and S E Grasby Table 1. Key parameters used to estimating depth temperature relationships for main geological regions of Canada. Region D (km) Q r (mw m 2 ) A (μw m 3 ) K (W m 1 K 1 ) Cratonic basin Shield Atlantic Cordillera (Jessop 1990b, Lachenbruch 1971, Pollack and Chapman 1977, Blackwell 2006, Drury 1988). Heat flow values are available for locations shown in figure 1, and estimated average values of thermal conductivity for sediments and crystalline rocks are derived from Canadian and US compilations (Jessop 1990b, Beach et al 1987, Blackwell 2006). Knowledge of near-surface temperatures (derived from depth temperature logs) and calculated temperatures at the top of Precambrian (in the case of areas overlain by sedimentary cover) permits calculation of temperature profiles for the upper parts of the crystalline crust below sedimentary basins. The variation of temperature with depth in the crystalline crust can be calculated by the Lachenbruch model of an exponential decrease in crustal heat generation with depth. This model (Lachenbruch 1970, 1971) is the only possible model able to explain the heat flow/heat generation relationship in the cases of significant upper crustal erosion. The relationship is in the following form (Roy et al 1972): Q o = Q r + DA o (1) where the basement heat flow Q o is correlated statistically with heat generation in the basement A o. Temperature (T) versus depth (z) was calculated from T(z)= T b + Q r zk 1 + A o D 2 K 1 (1 exp( z/d)) (2) where Q r is the reduced (deep) heat flow, A o is heat generation at the top of the basement, T b is the temperature at the crystalline basement surface and D is the slope in km. This model was applied to the Canadian land mass because good heat flow heat generation statistical relationships were established for all of the major Canadian provinces, similar to that in the United States (see review in Jessop (1990b)). The thermal conductivity (K) for the crust and upper mantle of Canada is derived from Jessop (1990b). Typically, the thermal conductivity for crystalline rocks is 3.2 W m 1 K 1, whereas a lower value of 2 W m 1 K 1 is used for sedimentary rock. The typical heat generation values for the study area are A o = 1to5μWm 3 for the low and high heat flow regions respectively (based on existing data from the study area (Burwash and Burwash 1989, Drury1986, Jones and Majorowicz 1987, Jessop 1990b, Lewiset al 2003). The parameters used to construct maps of temperature fields at 3.5, 6.5 and 10 km for major heat flow provinces of Canadian are provided in table Thermal energy potential To estimate the thermal energy or heat content of a rock mass a 4 4 km unit that is 1 km thick (16 km 3 total 234 volume) was considered, which is at an initial temperature of T o ( C). If this rock mass of volume V and density ρ is cooled through a temperature difference of T T o ( C) to a reinjection temperature of C (achievable if thermal water is used in heating, drying, balneology or other processes before reinjection (Barbier 2002)), then the heat removed is given by Q = C p ρ V (T T o ) (3) where reasonable average values are 2550 kg m 3 and 1000 J kg 1 C, for the density (ρ) and heat capacity (C p ) of the rock, respectively. This is consistent with parameters used by the MIT report on EGS potential in the USA (Tester et al 2006, Blackwell 2006). The quantity of thermal energy released from the 16 km 3 rock volume with initial temperature 150 C was then calculated. If this rock mass is cooled through a temperature difference of 120 C to a final temperature of 30 C, then the heat removed Q (in joules (J)) is given by Q = (2550 kg m 3 ) (1000 J kg 1 C) (16 km 3 ) ( C) = J. (4) Using this approach, the quantity of thermal energy which could potentially be released from a volume of deep-seated rock (termed here in-place resource) can be mapped to see patterns of highest energy content (primary target zones). The total Canadian energy content was also calculated based on these parameters. The size of the accessible resource is much smaller than implied by this simplistic approach and it can be as low as 2% of the total in-place resource (conservative), 20% (midrange) and 40% (upper limit) (Tester et al 2006). 4. Results 4.1. Canadian heat flow The regional variability of average heat flow in Canada, based on the existing database of 3085 locations, is shown in figure 2. Based on our methodology, estimated heat flow values are available for only 40% of Canada s landmass, whereas the remainder in white is unknown due to the lack of data. Based on the heat flow pattern shown in figure 2, the national average is calculated here to be 64 mw m 2 ± 16 mw m 2. However, regional heat flow varies from values as low as mw m 2 in the Canadian Shield of central Canada to highs in the northern Canadian Cordillera reaching >100 mw m 2. High variability in heat flow values is driven by variability in heat generation of the upper crust (highest in the crystalline rocks like granites) and mantle heat from below. The amount of so-called reduced heat flow (Q r ) characterizing heat under the upper crystalline crust is lower for the cratonic areas, and especially in the Canadian Shield. Much higher vertical heat flow (from the mantle) occurs in younger orogenic belts of the Canadian Cordillera (according to Jessop (1990b) that difference is 30 to 60 mw m 2, respectively). The highest heat flow values known in Canada occur in the volcanic belt around Mt

4 Heat flow, depth temperature variations and stored thermal energy for EGS in Canada Figure 1. Distribution of heat flow data points (red dots) across Canada. Figure 2. Contour map of heat flow (in mw m 2 ) in Canada with projection of values restricted to a 50 km grid centred on data points. White areas represent regions of no data. Garibaldi (>200 mw m 2 ); however, these systems are not conductive and available temperature logs are disturbed by movement of hot water or vapour (see Jessop et al (1991) for references) and are characteristic of a local heat flow zone related to a back-arc system. While recognizing that these local high heat flow areas of great geothermal potential exist, our methods avoid having local anomalies give an apparent high regional value. As such these highest values do not appear on our maps. Overall, the heat flow values in the Canadian Cordillera are similar to the western USA Basin and Range areas (Blackwell 2006). This is especially true for extensional areas like the Ominica Belt in southern British Columbia. Other regions of very high heat flow (>70 mw m 2 ) occur in parts of the WCSB, particularly in NW Alberta, NE British Columbia, southwestern Northwest Territories and the southern Yukon. Heat flow in these areas is inthe70to120mwm 2 range. High heat flow values of >70 mw m 2 are also found in the Mackenzie Corridor, in the foreland basins of the Northwest Territories, and in the eastern part of the Tuktoyaktuk Peninsula in the 235

5 J Majorowicz and S E Grasby (a) (c) Figure 3. (a) Range (low and high estimates) of modelled synthetic temperature depth profiles based on different assumed parameters (as explained in the text) for major Canadian heat flow provinces. (b) Observed temperature data from deep wells in the northern part of the WCSB versus range of possible values from modelled synthetic geotherms. (c) Observed temperature data in deep wells in the Intermontane basins of the Canadian Cordillera versus range of possible values from modelled synthetic geotherms. (b) Beaufort Mackenzie area. High heat flow is also observed in shallow parts of the WCSB in southeastern Saskatchewan (Weyburn area) and southwestern Manitoba, and also in the Lac LaBiche area (heat flow >70 mw m 2 ). Elevated heat flow is also observed in the Appalachian region in Eastern Canada (>60 mw m 2 ). Passive margin basins of the Atlantic are of lesser interest for geothermal development as the high heat flow areas are under water. In general, high heat flow regions in Canada s High Arctic are in areas with little or no population, and temperatures are depressed due to thick ice bearing permafrost. The base of permafrost, which constrains temperatures to 0 C, extends from 0.2 to 1 km. However, there are some isolated communities (e.g. Resolute Bay) near high heat flow areas that are dependant on diesel for all their energy needs that may benefit from a local geothermal resource base. 236

6 Heat flow, depth temperature variations and stored thermal energy for EGS in Canada 4.2. Depth temperature profiles and temperature patterns at depth As described above, heat flow patterns are used to calculate depth temperature profiles at depths beyond which physical measurements have been made. There are uncertainties in heat flow (here we show the calculations for a 10 mw m 2 error). There is also uncertainty in the near surface temperature (temperature of the basement in case of the WCSB and north of 60 N basins). Large uncertainties for the WCSB are due to the fact that the heat flow heat generation relationship for the crystalline basement underlying the sedimentary basin had Figure 4. Modelled synthetic temperature at 3.5 km (in C). Figure 5. Modelled synthetic temperature at 6.5 km (in C). to be derived from equivalent Precambrian-Archean rocks exposed at surface to the east. Surface temperature control is well known and mapped for all of Canada (Majorowicz et al 2009, Smith and Burgess 1998). The analysis of geotherms in figures 3(a) (c) shows that the thermal blanketing effect of relatively low thermal conductivity sediments (relative to higher thermal conductivity of crystalline basement rocks) makes thick sedimentary basins into relatively high heat flow regions of the WCSB attractive for geothermal development. These regions have geotherms that are comparable with very high heat flow areas of the 237

7 J Majorowicz and S E Grasby Figure 6. Modelled synthetic temperature at 10 km (in C). Figure 7. Map of the quantity of thermal energy which could potentially be released from the 3 to 4 km depth based on cooling of km rock slide from in situ temperature down to 30 C through Canada (for the areas with well temperature data only). Total available energy is in J Btu = 1 Quad). Canadian Cordillera. Recorded deep well temperature data for the high heat flow zones of the northern part of the WCSB, and basins of the Canadian Cordillera, constrain T z modelled synthetic geotherms (figures 3(b) and (c) respectively). The temperatures at depths of 3.5, 6.5 and 10 km, calculated for each of the Canadian heat flow data locations shown in figure 1, are shown in figures 4 6. Data points are same for all maps. Analysis of figures 4 6 shows that temperatures suitable for EGS (>150 C) can be reached over large areas of Canada. Overall, temperatures are highest in western Canada, reaching 150 C at depths of 3.5 km in limited areas of the Canadian Cordillera, and in the southern part of the Mackenzie Corridor (figure 4). At depths of 6.5 km (figure 5), large areas of the Canadian Cordillera and parts of the WCSB show temperatures of 150 to 200 C. These represent the best target areas under the most likely limit of drilling depths, based on drilling costs and technical abilities in the foreseeable future. However, drilling to depths of 10 km is potentially feasible in the future and these depths have been already achieved (e.g. the deep drilling project on the 238

8 Heat flow, depth temperature variations and stored thermal energy for EGS in Canada Kola Peninsula). At 10 km we can expect EGS temperatures in the 150 to 200 C range across most of Canada (figure 6), excepting some areas of the Canadian Shield. At this depth, temperatures in the 200 to 300 C range are estimated for large regions of the Canadian Cordillera and the WCSB Thermal content of rocks The quantity of thermal energy present for the 3 4 km depth range was calculated and the resultant spatial distribution mapped in figure 7. Data points for this calculation are the same for all other maps and are based on the heat flow data mesh. Values for the areas outside of the data points were interpolated within a 50 km grid limit as with other maps. The results from our estimates show that in-place thermal energy potential for only one small 16 km 3 block >150 C is comparable with Canada s annual energy consumption 3 to J(= 10 Quads) per year between 1961 and 1997 (Environment Canada 2008 Energy Bulletin). Given this, the potential for geothermal energy to provide significant renewable energy supply for Canada is significant. The actual accessible and usable geothermal energy resource however will be much smaller than the in-place resource. The estimate for conservative production used by MIT (Tester et al 2006) is 0.02 of in-place thermal energy. Using this value gives us J = 0.1 Quads for the same rock volume as above. This would still provide a significant contribution towards Canadian energy consumption; requiring only 100 developments to meet Canada s current energy demand. A similar process was used to define thermal energy potential for a unit 500 m above and below each depth temperature field. The total quantity of stored thermal energy was estimated for these three depth slices. We then summed the total in-place heat content calculated for Canada at the depth intervals of 3 to 4 km, 6 to7 km and 9 to 10 km. The integrated in-place geothermal energy content is estimated to be , and J respectively. These results show that a massive energy resource exists throughout Canada, though only a small fraction of this dispersed resource can be used. The key challenge is to define what proportion of this resource is producible and economic. While there is more energy at greater depths (up to 10 km) drilling costs increase nonlinearly with depth. However, based on petroleum industry experience, it is now quite common to drill 4 to 6 km wells in deeper parts of the WCSB and technology to go to such depths is readily available. A statistical function of drilling cost with depth for EGS development, based on estimates for drilling a single EGS well as well as the installation costs (Tester et al 2006), was adjusted to prices and shows that cost increases exponentially with depth: COST = 3.23 exp(0.236 DEPTH), where cost is in 10 6 $ and depth is in km. Only a portion of the rock volume heat content can be mined/farmed for thermal and electrical energy. This depends not only on the initial temperature, but also on the possibility of generation of enough of water flow through hydraulically enhanced cracks in the rock. To see what can be produced with the use of water as a high heat capacity carrier, the available thermal power and related electrical power were calculated (for a binary power system conversion rates given in Tester et al (2006) are used). For the feasible cooling of rock mass by 100 C, at the specific heat capacity of water of 4000 J (kg 1 C 1 ), a flow rate of 100 kg s 1 (triplet system with one well injecting water and two producing wells), for the factor-thermal to electrical of 0.15 MWth, the available power of electrical EGS production is estimated to be 6 MWe. In general, high quality targets need to be located at feasible drilling depths. Temperatures >150 C are mainly achievable at high depths except for high heat flow portions of the Cordillera, and locally below thick thermal blanket of sedimentary basins. As sedimentary rocks have usually 2/3 the thermal conductivity of crystalline rocks, the gain of temperature with depth (geothermal gradient) in crystalline rocks will be less than in sediments. Thick sedimentary blankets thus help achieving high EGS temperatures in the deeper western-central and northern parts of the WCSB. Another advantage of EGS opportunities under sedimentary basins is that drilling through sedimentary rocks is typically less expensive than crystalline rocks, potentially making targets under sedimentary basins more economically attractive. 5. Conclusions Our results show that Canada has significant potential for EGS development. Similar to the United States (Blackwell et al 2007), the best EGS prospects are in western Canada due to higher overall heat flow. The most promising targets for EGS are in the Canadian Cordillera, with especially interesting areas in southern British Columbia and in the southern part of the Yukon. In addition, the Mackenzie Basin shows promising target areas. Regions of high geothermal energy content in the WCSB occur in northeastern British Columbia and parts of northwestern Alberta. Also, interesting potential targets occur in central Alberta (including the Lac La Biche high and in Saskatchewan (Williston Basin high)). While the required depths for 200 C are all deeper than the EGS Soultz, France project (200 C at 5 km), temperatures as low as 120 C can be used for EGS (Tester et al 2006). The analysis of the geotherms shows that the thermal blanketing effect of relatively low thermal conductivity sediments (relative to higher thermal conductivity of the basement crystalline rocks) makes deep sedimentary basins in relatively high heat flow areas of the WCSB as attractive for high-temperature mining as high heat flow belts of the Canadian Cordillera. The results from our estimate of thermal energy potential for only one small 16 km 3 block at 150 C initial temperatures is comparable with Canada s annual energy consumption 3 to J = 10 Quads per year between 1961 and The total quantity of thermal energy at 6.5 km is Q = E06 Quads, where 1 Quad = J. The available heat energy to be farmed at depth is in reality only a small percentage of potential heat available due to various technical limits. Even though actual accessible and usable geothermal energy resource will be significantly smaller than the in-place potential, it can still be a very significant resource. An 239

9 J Majorowicz and S E Grasby additional issue is that energy potential can only be accessed in portions of the Canadian landmass near population centres and grid infrastructure given the high cost of connecting remote areas to the grid. This would limit the economics of some of the highest in-place resource potential. However, given the widespread distribution of geothermal energy, and the high energy content, the potential geothermal resource in Canada is significant. Acknowledgments The authors thank Maria Richards, Miriel Ko, Dr Alan Jessop and Dr J-C Mareshal for help with acquiring heat flow data. Dr Michal Moore is thanked for introducing us to the EGS theme. We would like to thank Dale Issler of GSC Calgary and the anonymous reviewers for their helpful comments. The work has been supported by Geological Survey of Canada Contribution no References Anglin F M and Beck A E 1965 Regional heat flow pattern in Western Canada Can. J. 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