Geothermal Resource Characterization Of The Middle Devonian Slave Point Formation At Clarke Lake Field, Fort Nelson, B.C., Canada

Abstract

Clarke Lake field is a dolomite reservoir that shows exceptional potential for geothermal development, exhibiting porosity and permeability values ranging from 5 to 15 % and 100 to 4000 md, and an average reservoir temperature of 98.2°C. In addition, a recent field experiment demonstrated the presence of an unusually strong water drive. The experiment was intended to increase gas production by depressuring the reservoir through increasing water production, but reservoir pressure decreased significantly less than expected, resulting in no increase in gas production. This was the result of an unanticipated strong water drive. We describe depositional and diagenetic facies in the Slave Point Formation and relate them to porosity and permeability measurements in order 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 stromatoporoid 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, which occlude 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 that preferentially occurred in more porous and permeable sediments, which 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. We can test the viability of hot water production from reservoir rock at Clarke Lake over a 25-year time period, using petrophysical values as inputs into flow simulations. Two sets of simple 2D flow simulations and a high resolution 3D simulation were conducted. The first set of 2D simulations used cells populated with statistical values that represent the distribution of porosity and permeability at Clarke Lake field. The second set of 2D simulations used cells populated using a Monte Carlo sampling method to represent the heterogeneity of porosity and permeability values inherent to the hydrothermal dolomite reservoir. A third flow simulation, which applies the spatial distribution of petrophysical properties at Clarke Lake, was conducted to test the field-wide potential for hot water production and the overall geothermal power capacity. Initial simulations show that the reservoir properties at Clarke Lake field are conducive to a 25-year geothermal project, with estimates of geothermal power potential of 300 kW using a doublet well model and 2400 kW using a four injector and eight producer well model.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019