The Automatic Well Temperature Measuring System Installed At Cape Allison C-47, Offshore Well, Arctic Islands Of Canada. Part 1 - Concept And Deployment
Authors: Judge Alan , Duguid Alan , Taylor Alan , Allen Vic 
Published: March 2010

In May 1985, an 800 m multithermistor cable and automatic data acquisition system were installed in an offshore well in the Canadian Arctic Islands. The well, Cape Allison C-47, was drilled by Panarctic Oils Ltd. in 244 m of water during the winter from an ice-strengthened platform. A novel technique was developed to run the cable and electronics package through the marine riser and to land the system in a specially designed casing hanger assembly just prior to rig release. Over most of the following year, temperature data were gathered at four-hour intervals by the subsea equipment and transmitted acoustically through the water to a recording system left on the sea ice. This paper describes the well modifications required, the automatic data acquisition system and the installation procedure.

Introduction

A long-standing challenge in marine science and offshore geotechnical engineering has been the acquisition of precise long-term, downhole data from deep sea wells. In onshore wells, long-term data such as temperature may be obtained by strapping sensor cables on casing during drilling(1), by leaving a sensor cable in the well upon completion (2, 3), or by re-entering the well at a later date with portable logging equipment(4, 5). However, in offshore wells, data acquisition may be limited to the logging and testing phase; following completion or abandonment, offshore wells may not be accessible for long-term measurements. Temperatures and other parameters may be disturbed at this time, and may be poor approximations to the equilibrium state, despite the application of corrections(6, 7).

A need exists for precise ground temperatures in offshore environments. Earth scientists use geothermal data to calculate the terrestrial heat flow in the study of the geology and tectonics of a region, while petroleum geologists use similar data to assess petroleum maturation in the quest for exploration targets. Geotechnical and production engineers use ground temperatures and thermal properties in casing design. In permafrost regions, thermal information is of crucial importance because physical properties of earth materials depend critically on whether they are frozen or unfrozen; the problem must be addressed beginning with the interpretation of seismic records(8).

For the past several years, Energy, Mines and Resources Canada has worked with industry and the Canada Oil and Gas Lands Administration (COGLA) in the development of a system to permit the acquisition of precise geophysical data for periods of years in abandoned offshore petroleum exploration wells. The design target was a subsea system that would operate in up to 1000 m of water, measuring data from 20 channels at intervals of hours to days over a 5-year lifetime. For the demonstration system, specifications called for well temperatures to be read to a resolution of 0.01 K.

In May 1985, a prototype system was installed in a well abandoned in 244 m of water offshore in the Canadian Arctic Islands (Fig. I). The well, Panarctic et al. Cape Allison C-47 (77 ° 46.1 ' N, 100 ° 17.3' W) is located between King Christian and Ellef Ringnes Islands in the central Sverdrup Basin (Fig. 2).