High Temperatures Predicted in the Granitic Basement of Northwest Alberta - an Assessment of the Egs Energy Potential

PROCEEDINGS, Thirty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 24-26, 2014 SGP-TR-202 HIGH TEMPERATURES PREDICTED IN THE GRANITIC BASEMENT OF NORTHWEST ALBERTA - AN ASSESSMENT OF THE EGS ENERGY POTENTIAL Jacek Majorowicz1*, Greg Nieuwenhuis1, Martyn Unsworth1, Jordan Phillips1 and Rebecca Verveda1 1University of Alberta Department of Physics, Canada *email: [email protected] Keywords: Heat flow, EGS, Alberta, Canada ABSTRACT Northwest Alberta is characterized by high subsurface temperatures that may represent a significant geothermal resource. In this paper we present new data that allows us to make predictions of the temperatures that might be found within the crystalline basement rocks. In this region the Western Canada Sedimentary Basin (WCSB) is composed of up to 3 km of Phanerozoic sedimentary rocks with low thermal conductivity, which act as a thermal blanket. Commercial well-logging data was cleaned of erroneous data and corrected for paleoclimatic effects to give an average geothermal gradient of 35 K per km, and maximum geothermal gradients reaching 50K per km. These gradients, along with a thermal conductivity model of sedimentary rocks, were then used to estimate heat flow across the unconformity at the base of the WCSB. The calculations assumed a heat generation of 0.5 µW/m3 within the sedimentary rocks. Estimation of temperatures within the crystalline basement rocks requires knowledge of the thermal conductivity (TC) and heat generation (HG) of these rocks. These are mainly granitic Precambrian rocks. Thermal conductivity (TC) and heat generation (HG) of the basement rocks were measured on samples recovered from hundreds of wells that sampled the Pre-Cambrian basement rocks. TC values were corrected for pressure and temperature variation. Using these data, we have developed a temperature model of Northern Alberta which predicts temperatures in the 3-6 km depth range. Analysis of the temperature variations in NW Alberta has resulted in the discovery of the Rainbow Lake high temperature anomaly. The heat flow below the sedimentary cover (> 2 km) at this location represents the highest heat flow in Alberta. This is the hottest spot found in Alberta at these depths. The temperature model predicts high temperatures (>170°C at 4 km depth, 200°C at 5 km depth, and as high as 230oC at 6 km) within the Precambrian basement rocks in part of Northwest Alberta under 2.5-3km thermal blanket of sedimentary rocks. Since this temperature anomaly is located within crystalline basement rocks, an Engineered Geothermal System (EGS) would be required to utilize this heat. The development of an EGS in this location could be facilitated by the presence of naturally occurring fractures within the Great Slave Lake Shear Zone. 1. INTRODUCTION The goal of this study is the assessment of the EGS potential in Northwest Alberta, which has been known for its high heat flow. NPHFA - Northern Plains Heat Flow Anomaly (see: Majorowicz, 1996) extends from north-eastern British Columbia and Northwestern Alberta to the southern Northwestern Territories between the Precambrian shield (Slave Province) and the Cordillera (Mackenzie Fold Belt) as seen in this more recent heat flow data compilation shown in Fig. 1 from Majorowicz and Weides, (2013). Figure 1: Regional heat flow map in WCSB and location of the study area (modified from Majorowicz and Weides 2013). 1 Majorowicz et al. We focus on two goals: 1) determining the spatial and depth variations of temperature within the Precambrian basement rocks that underlie Northern Alberta, and 2) determining how much geothermal energy is available and whether electrical production could be economic. In order to calculate how temperature changes with depth beyond the depths where measurements are available in boreholes, we need to know (a) the properties of the rocks such as thermal conductivity (TC) and heat generation (A), and (b) the heat flow (Q) through the subsurface. Since we have access to many temperature measurements made by oil companies in wells drilled into the overlying sedimentary rocks, we are able to calculate the heat flow. Assuming that the heat flow is one dimensional, and estimating the amount of heat generation that occurs within the sedimentary basin, it is possible to calculate the heat flow at the top of the Precambrian basement rocks. To calculate temperatures within the Precambrian basement rocks we must know the heat generation and thermal conductivity of these rocks. In summary, to calculate the temperature change with depth below the top of the Precambrian basement, we need accurate estimates of three values, all of which change across Alberta: 1) heat flow at the surface, 2) heat generation of the sedimentary rocks in the basin and underlying Precambrian basement rocks, and 3) thermal conductivity of the basement rocks. 2. RELIABILITY OF THE INDUSTRIAL TEMPERATURE DATA IN THE BASIN The temperature data used in this study to predict the deep geothermal field within the sedimentary basin are: Annual Pool Pressure surveys (APP), Drill Stem Tests (DST) and Bottom Hole Temperatures (BHT), (Gray et al., 2012), as well as a small number of precise equilibrium logs (Garland and Lennox, 1962, Majorowicz et al., 2012). Measurement and systematic errors inherent to the APP, DST, and BHT data are significant (Hackbarth, 1978; Majorowicz et al., 1999; Gray et al., 2012) and can result in large data noise (Lam et al., 1985; Majorowicz et al., 2012, 2013a, Gray et al., 2012). To this end these data were initially cleaned to remove erroneous data as described by Gray et al., 2012 (e.g. a significant overestimation of Alberta industrial well logs from shallow depths was removed <1000m). Where possible Gray et al., (2012) also applied the standard corrections (Horner, Harrison, SMU etc..., Lachenbuch and Brewer, 1959; Harisson et al, 1983; Blackwell and Richards, 2004, respectively). The surface heat flow has previously been calculated by the geothermal group at the University of Alberta using industrial temperature data collected in the basin, as described by Gray et al, (2012), and Majorowicz et al, (2012a,2013b). A paleoclimatic correction was also applied based on a correction factor derived from the Hunt Well temperature data by Majorowicz et al., (2012, 2013b). See Gosnold et al., (2011) and Majorowicz and Wybraniec, (2010) for a description of the correction methodology. Prior to using these heat flow values, we used computer software developed at Southern Methodist University to determine which heat flow estimates can be considered reliable. This has resulted in a confirmation of our previous results, which showed that heat flow calculated from shallow boreholes in the basin are not reliable. To further constrain the heat flow in Northern Alberta we applied an algorithm which is being developed as part of the Geothermal Atlas of Alberta (Nieuwenhuis et al., 2014). This algorithm uses robust statistics to remove data which is not reliable, and resulted in the removal of approximately 25% of the measured temperature data. Here we show the newest edition of the heat flow map of the Northern area after paleoclimatic correction has been applied. Figure 2: Heat flow map of the study area. The heat flow data (Q) are based on approximately 30,000 data points from bottom hole temperatures, drill stem test temperatures, temperature measured annually in shut in wells and a small number of shallow (few hundred meters) precise logs in equilibrium. The thermal conductivity model of sediments is from 2 Majorowicz et al. measured sedimentary rock conductivity averages for common rock types (Beach et al., 1987) and abundances of the 13 main sedimentary rocks. The paleoclimatic correction developed for North Alberta has been applied (Majorowicz et al., 2012a; 2013a). 3. HEAT FLOW AT THE SURFACE VS. HEAT FLOW AT PRECAMBRIAN TOP As discussed above, in order to determine heat flow at the Precambrian surface, the heat generation in the sedimentary basin must be known. We have estimated heat generation of the sedimentary cover above the Precambrian crystalline basement from gamma logs. We have used the empirical relationship between GR (API units) and A (W/m3), equation (1), developed by Bucker and Rybach (1996): A=0.0158(GR [API]-0.8) (1) Four well logs which were measured through the entire depth of the basin at four different locations (Peace River, High Level Hinton-Edson and Edmonton vicinity were analyzed (Figure 3). The resulting estimates of A for the sedimentary basin show that the contribution in heat flow from the heat generation of the sedimentary basin is less than the error in the heat flow, confirming our previous results from the Hunt Well (Majorowicz et al., 2013a). This means that spatial and depth variations in the heat generation throughout the sedimentary basin are not significant, and an average value (0.6W/m3) has been used throughout Northern Alberta. Figure 3: Heat generation estimated in four wells in the sedimentary rocks of Northern Alberta, resulting in an average of 0.6 W/m3. The cleaned and corrected temperature database described above was used to calculate the heat flow at the surface. Heat flow at the surface reduced by the sedimentary heat generation contribution gives the heat flow at the Precambrian surface, which in the study area varies in depth from a few hundred meters down to 3500 meters. Heat flow contribution from sedimentary rocks is low and will not be higher than 3.5mW/m2, which is less than the error of heat flow determination from thermal data in sediments (15%). This heat flow was used for further calculation of temperature in the Precambrian basement. 4. THERMAL CONDUCTIVITY OF THE PRECAMBRIAN BASEMENT In 1986 a divided bar set up was used in the University of Alberta Physics Department geothermal lab, supervised by Prof. Walter Jones. It was used to measure the thermal conductivity of rocks sampled by 32 wells in North Western Alberta (mainly granites; Tempest Geophysical, 1986/GSC Open File).

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