The Automatic Well Temperature Measuring System Installed At Cape Allison C-47, Offshore Well, Arctic Islands Of Canada. Part 2 - Data Retrieval And Analysis Of The Thermal Regime
Authors: Judge Alan , Taylor Alan , Allen Vic 
Published: March 2010

Precise well temperatures to 800 m below the seabed have been obtained with an automated data acquisition system from an offshore well, Canadian Arctic Islands. Temperatures show the cooling of the well following the drilling and testing phase, Analysis of the data has characterized the geothermal regime for the region. The deep temperatures appear to be in thermal equilibrium, in contrast to temperatures at many onshore wells that are in disequilibrium because of the influence of recent marine regression and climatic change. The terrestrial heat flow calculated from this data is 39 mWm-2; uncertainty in thermal conductivity suggests it may be somewhat higher, probably not exceeding 52 mWm-2. This range is supported by calculations using bottomhole and drill-stem test temperatures from deeper in the well, These results suggest a low to moderate terrestrial heat flow for the Sverdrup Basin. The analysis has demonstrated the anisotropy of shale with respect to thermal conductivity, and has clarified the interpretation 0/ the onshore geothermal regime.

Introduction

Bottom-hole temperatures (BRT) obtained during geophysical well logging, and temperatures measured during drill-stem tests (DST) afford the opportunity of reconstructing the ground thermal regime(l). Such data have been used, for instance, to predict well performance(2) and to determine the underlying thermal history for petroleum maturation studies(3). However, temperatures and other parameters may be disturbed at this time, and may not represent the undisturbed state, despite the application of corrections(4). In addition, such data do not afford the opportunity of studying long-term changes.

To overcome some of these limitations and to establish a data base of precision ground temperatures and permafrost thicknesses, temperature measurements up to 1000 m have been made at some 130 onshore petroleum exploration wells in the Canadian Arctic for periods of up to 20 years(5, 6). Figure 1 (right half) illustrates two methods used to acquire the data.

A challenge, however, has been to obtain ground temperature information from offshore areas comparable to that more readily obtained at these onshore wells; this is particularly crucial in the Canadian Arctic for two principal reasons. First, hundreds of metres of ice-bonded permafrost underlie the Beaufort Sea and no precision deep temperature data exists at present(7). Second, in the Queen Elizabeth Islands (Fig. 2), for example, ground temperatures at onshore wells reflect a complicated history of climatic change and marine regression; the offshore thermal regime, where transient environmental effects are minimal, may hold a key to the reconciliation of the o:pshore temperatures and to the wide variation recorded in onshore permafrost thicknesses(6) (Fig, 3).

Recently, precise temperatures from a deep-water offshore well in the Queen Elizabeth Islands have been obtained to provide some insight into these problems. The novel method of installing an 800 m multithermistor cable and automatic temperature logging system in this well, Panarctic et al, Cape Allison C-47 (No. 427 in Fig. 3), is described in a preceding paper(8); the configuration is shown in the left half of Figure 1.