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Geothermal Resource Characterization of the Middle Devonian Slave Point Formation at Clarke Lake Field, Fort Nelson, B.C., Canada

Abstract

Clarke Lake field in northeastern British Columbia displays anomalously high reservoir temperature and strong water drive, making it suitable to investigate the potential of repurposing the field as a source of geothermal electrical power. The field occurs in carbonate sediments of the Slave Point Formation, which were deposited within a rimmed carbonate platform environment flanking the Horn River Basin during Givetian time. The development of porous and permeable reservoir resulted from hydrothermal alteration of parent limestone to dolomite due to the movement of halite- and gypsum-saturated brines through aquifers toward the reef margin.

We describe depositional and diagenetic facies and relate them to porosity and permeability measurements to develop a flow model for the reservoir. Nine depositional facies and two diagenetic facies were described. Dolomitized units of back-reef and reef margin facies show enhanced porosity and permeability at the top of shoaling upward cycles where stromotoporoid bioclasts have been dissolved leaving mouldic and vuggy porosity. Average porosity and permeability in back-reef facies are 6.4 % and 124 md. Diagenetic facies show high permeability but reduced porosity from increased precipitation of dolomite, fluorite, and sulphide minerals occluding mouldic, vuggy and fracture porosity. Average porosity and permeability for these facies are 5.1 % and 183 md. High quality reservoir zones exist at the reef margin due to hydrothermal alteration preferentially occurring in more porous and permeable sediments that are stratigraphically trapped by shales of the Horn River and Muskwa formations. High quality reservoir zones also extend some distance into the back-reef within porous and permeable carrier beds of back-reef facies.

Clarke Lake field has shown water production rates upward of 2800 m3/day in two wells, with an average reservoir temperature from DST measurements of 98.2°C. Applying these values of temperature and water rates and an appropriate range of porosity and permeability values, we use simple flow simulations to assess the viability of hot water production over a 25 year time span. Initial simulations show the rock properties inherent to the reservoir at Clarke Lake field are capable of sustaining 25 years of hot water production. Estimates of geothermal power potential are 300 kW using a doublet well model and 2400 kW using a four injector and eight producer well model.