Thermal Measurements and Geothermal Gradient from Deep Boreholes in the Revell Batholith, Northwest Ontario

ABSTRACT: The Revell Site, in northwestern Ontario, within the Canadian Shield, is presently being studied as one of the sites for hosting a deep geological repository (DGR) in Canada. Preliminary site investigation included the drilling and coring of six 1-km long boreholes. The thermal characterization involved obtaining temperature logs from multiple sources in boreholes and laboratory thermal properties testing of different rock types. After drilling, coring, and flushing of the borehole, downhole temperature data were collected in this order: (1) continuous logs from geophysical dynamic fluid temperature probe, (2) discrete temperature data measured during hydraulic packer testing, and (3) discrete temperature data collected post installation of a Westbay monitoring system. The hydraulic testing and Westbay data provided the most reliable estimates of in-situ temperature profile with an average geothermal gradient of 9.9°C/km. This gradient is within the lower range of the data from other study sites in the Canadian Shield. The low gradient is consistent with low concentrations of natural uranium, thorium, and potassium in the tested core samples, which results in a low radiogenic heat generation rate. This paper presents a summary of the thermal data collection in the first three boreholes and development of a thermal model for the site, which provides boundary conditions for thermal simulations of the DGR. This initial thermal property dataset and site model also provides a prediction against which data from the latter three boreholes can be expected. 1. INTRODUCTION Deep geological disposal is an internationally accepted method for the safe and long-term management of used nuclear fuel. Safe long-term performance of a Deep Geological Repository (DGR) spans over the construction, operation, and post-closure phases which includes geological processes such as earthquakes and glacial advance and retreat. The Nuclear Waste Management Organization (NWMO) is implementing Adaptive Phased Management (APM), the approach approved by the Government of Canada in 2007 for the long-term management of used nuclear fuel. The NWMO site selection process is described in NWMO (2010). As part of APM, the NWMO is conducting preliminary site assessment activities in potential siting areas in: i) crystalline rocks in the northern portion of the Revell batholith within the Canadian Shield, near the Township of Ignace in the traditional territory of Wabigoon Lake Ojibway Nation (WLON) in northwest Ontario (referred to as the Revell Site), and ii) in sedimentary rocks in the Municipality of South Bruce in the traditional territory of Saugeen Ojibway Nation (SON) in southern Ontario (referred to as the South Bruce Site).