| Description |
Periodic temperature measurements in the DOI/GTN-P Deep Borehole Array on the western Arctic Slope of Alaska have shown a strong near-surface permafrost warming over the last 40 years, particularly since ∼ 1990. Due to the manner in which these deep wells were drilled, the portion of the observed permafrost warming caused by climate change has remained unclear. Other factors that have strongly influenced temperatures near the wellbores include the heat deposited into permafrost during drilling and local-landscape changes associated with drilling operations (creation of reserve pits and drill pads). Multidimensional heat-transfer models capable of assessing the magnitude of the drilling and local-landscape disturbances near the wellbores have not been available. For the western Arctic Slope, such models must be capable of simulating heat-transfer processes in layered fine-grained mudrocks whose thermal properties are highly nonlinear due to the occurrence of unfrozen water at temperatures well below 0°C. An assessment of the drilling and landscape-change effects also requires knowledge of the specific thermophysical properties occurring at the well sites. Little information has been available about these properties on the western Arctic Slope. To establish the portion of the observed permafrost warming related to drilling and landscape-change effects, multidimensional (2-D cylindrical, 3-D cartesian) numerical heat-transfer models were created that simulate heat flow in layered heterogenous materials surrounding a wellbore, phase changes, and the unfrozen water properties of a wide range of fine-grained sediments. Using these models in conjunction with the borehole temperature measurements, the mean thermophysical properties of permafrost rock units on the western Arctic Slope were determined using an optimization process. Incorporation of local meteorological information into the optimization allows a more refined estimate of the thermal properties to be determined at a well site. Applying this methodology to the East Simpson #1 well on the Beaufort Sea coast (70°55.046'N, 154°37.286'W), the freezing point of permafrost is found to be -1.05°C at this site and thermal diffusivities range 0.22-0.40 × 10 -6 m2 s-1. Accounting for the drilling and landscape-change effects, tundra adjacent to East Simpson is found to have warmed 5.1 K since the mid-1880s. Of this, 3.1 K (60%) of the warming has occurred since 1970. |
