In-situ determination of heat flow in unconsolidated sediments

Abstract

Abstract Subsurface thermal measurements are the most effective, least ambiguous tools for locating geothermal resources. Measurements of thermal gradient in the upper few tens of meters can delineate the major anomalies, but it is also desirable to combine these gradients with reliable estimates of thermal conductivity, to provide data on the energy flux and to constrain models of the heat sources responsible for the anomalies. Problems associated with such heat flow measurements include the economics of casing or grouting holes, the long waits and repeated visits necessary to obtain equilibrium temperature values, the possible legal liability arising from disturbance of aquifers, the hazards presented by pipes protruding from the ground, and the security problems associated with leaving cased holes open for periods of weeks to months.We have developed a technique that provides reliable real-time determinations of temperature, thermal conductivity, and hence, of heat flow in unconsolidated sediments during the drilling operation. Temperature, gradient, and thermal conductivity can be measured in one operation in 1 hour using a long (1.5 to 2 m) thin (6-mm diameter) probe containing three thermistors 0.5 or 0.15 m apart in its lowermost 1.2 m. The probe is driven hydraulically through the bit up to 1.65 m into the formation, and a 20- to 25-minute temperature record is obtained for each thermistor, allowing calculation of the equilibrium temperature gradient. A line source heater is then switched on and a 10- to 15-minute temperature record is obtained, allowing calculation of thermal conductivity. Two or three such experiments over the depth range of 50 to 150 m provide a high-quality heat flow determination at costs comparable to those associated with a standard cased gradient hole to comparable depths. The hole can be backfilled and abandoned after drilling, thereby eliminating the need for casing, grouting, or repeated site visits.