This material has been extracted from:

Taylor, A.E., Burgess, M.M., Judge, A.S., Allen, V. and Wilkinson, A., 1998. Canadian Geothermal Data Collection - Deep permafrost temperatures and thickness of permafrost; Global Geocryological Database, CAPS version 1, CD-rom. Published by the International Permafrost Association and the National Snow and Ice Data Centre, Boulder Colorado.

Introduction

Over the past thirty years, precise ground temperature logs to depths greater than 125 m have been obtained from many resource exploration holes in and adjacent to the permafrost regions of northern Canada. Most of these data were acquired by the Geothermal Service of the Earth Physics Branch, Energy Mines and Resources Canada (later amalgamated with the Geological Survey of Canada, Natural Resources Canada), in cooperation with the exploration industry and the regulatory agencies. This web site presents the entire digital data set for the area north of 60 degrees north latitude. Data are also included for some holes less than 125 m depth.

Acquisition of data

The data reported in this collection were acquired over three decades of field work, starting in the late 1960's and extending across arctic Canada. Permission to make ground temperature measurements at these sites was obtained through arrangements with the lease-holder or well operator and the regulatory agencies. Logistics of access to sites was a particular problem in this remote area. Helicopters were used generally in the western arctic (Mackenzie Delta and Valley) and fixed-wing aircraft in the high arctic (Arctic Archipelago), where a greater range was required. The remoteness of site locations and the lack of infrastructure precluded the use of conventional industry downhole well logging systems or of truck-mounted units for temperature measurement.

In the deep (greater than 125 m) wells considered here, the thermal disturbance due to drilling through porous, frozen unconsolidated sediments or rock may take years to dissipate (Lachenbruch and Brewer, 1959) and temperatures must be monitored over several years in order to calculate an equilibrium temperature profile, or to estimate undisturbed temperatures. Simple techniques were developed to facilitate the measurement of well temperatures in permafrost for several years after drilling was completed (e.g. Jessop, 1970; Judge, 1973; 1974). In mining exploration holes (e.g. site 114), a multi sensor cable was often allowed to freeze into the drillhole. In petroleum exploration wells (e.g. site 197), such cables rarely survived freezeback in the large, cased holes and a different approach was needed. Before the drill-rig left the site, the drilling mud above the regulatory cement plug at the base of the permafrost casing was displaced with diesel fuel. This allowed subsequent re-entry of the well for an indefinite period, generally giving access through the permafrost section, and tens of metres to a few hundred metres into the unfrozen horizons below. Canadian regulatory agencies assisted with the development of these procedures.

These completion arrangements permitted the periodic logging of the drillhole or well with light-weight, portable precision equipment. Where a permanent cable was installed measurements were made at the cable termination. Elsewhere, a light-weight portable probe was lowered in stages down the well. In the early 1990s, several wells were logged with a quasi-continuous, portable downhole logging system. Temperatures were derived to an accuracy of 0.02 degrees Celsius and a resolution of several millidegrees (thermistors used in the program were calibrated in-house to an accuracy of 0.01 degrees Celsius, traceable to standards at the National Research Council of Canada).

Estimating undisturbed downhole temperatures

Temperature logs can reflect the considerable thermal disturbance caused by drilling, depending on how soon after drilling they were recorded. This is particularly apparent in petroleum exploration wells because of the much longer times involved in drilling these wells to total depth (months), compared to mining exploration holes (days). The disturbance also persists for a much longer time in holes in permafrost than in unfrozen areas, or in the permafrost section of a hole compared to the unfrozen formations below (see example in figure below). Temperatures taken periodically following completion of a well may provide insight into the thermal dynamics of the well (Jessop, 1970; Taylor, 1979; Taylor and Judge, 1981).

Temperature logs taken over several years at dates indicated, at site 193, the Ikhil I-37 petroleum exploration well, Mackenzie Delta. These logs illustrate the thermal effects of drilling and may be used to estimate the undisturbed, or equilibrium temperature Teq. drilling took place between April 10. 1973 and December 12, 1973. (From Taylor et al., 1982) larger image [JPEG, 33.2 kb, 433 X 360, notice]

An estimate of undisturbed ("equilibrium") temperatures may be made from a series of downhole temperature logs taken over a period of time following completion of the hole. A model suggested by Lachenbruch and Brewer (1959) may be used.

The slope of the line through the temperatures logged at several times following drilling, plotted against a logarithmic time function (ln [1+t1/t2]) yields the temperature intercept of Teq, where:

t1 = drilling duration at depth of interest (s)

t2 = time between completion of drilling and taking of temperature log (s)

Teq = equilibrium or undisturbed temperature at the depth of interest

This is shown in the figure below.

Temperature measured over several years at Ikhil I-37 exploration well, plotted versus a logarithmic time function to illustrate a method that may be used to calculate equilibrium temperatures. The dates of the individual logs are given along the lower margin. larger image [JPEG, 29.1 kb, 376 X 434, notice]

The logarithmic method, strictly speaking, is valid only if no phase change affects the return to equilibrium. Only data from below the permafrost can be used to determine the base of permafrost unless the permafrost has very low porosity or has refrozen. However, as additional temperature logs are taken (in this data set, usually annually), the accuracy of the equilibrium estimate increases. For ratios t2/t1 greater than 25, calculated equilibrium temperatures are generally within a few tenths of a degree of undisturbed temperatures. Such estimates converge more quickly for Arctic Archipelago wells, and generally five years of annual data are sufficient to attain this accuracy.

References

Jessop, A.M., 1970. Depth of permafrost. Oilweek, Jan. 12, p. 22-25.

Judge, A.S., 1973. Deep temperature observations in the Canadian North. in, Permafrost, Second International Conference, National Academy of Sciences, Washington, DC. p. 35-40.

Judge, A.S., 1974. Geothermal measurements in northern Canada. in, Proceedings, Symposium of geology of Arctic Canada. Geological Association of Canada - Canadian Society of Petroleum Geologists, Saskatoon. p. 301-311.

Lachenbruch, A.H. and Brewer, M.C., 1959. Dissipation of the temperature effect in drilling a well in arctic Alaska. U.S. Geological Survey Bulletin 1083-C, p. 73-109.

Taylor, A.E., 1979. The thermal regime modelled for drilling and producing in permafrost; Journal of Canadian Petroleum Technology 18, no. 2, 59-66.

Taylor, A.E., Burgess, M.M., Judge, A.S., and Allen, V.S, 1982. Canadian geothermal data collection - Northern Wells 1981; Earth Physics Branch, Geothermal Series 13, 153 p.

Taylor, A.E. and Judge, A.S., 1981. Measurement and Prediction of Permafrost thickness, Arctic Canada; Technical Papers, 51st Annual Meeting, Society of Exploration Geophysicists, v. 6, p. 3964-3977.