The Determination of Virgin Strata Temperatures from Observations in Deep Survey Boreholes

Summary When a survey borehole is drilled downwards from the surface of the Earth the temperature is disturbed by the circulating drilling fluid, but fluid and adjacent strata gradually attain the local virgin strata temperature when drilling is stopped. Temperature equilibrium is approached most quickly near the bottom of the hole where the time of disturbance has been the least. Observations of the fluid temperature at this position may be made over a period of a day during a normal week-end break in drilling. Cylindrical heat flow theory is used in this paper to show that, with a knowledge of the thermal properties of the fluid and surrounding strata, the virgin strata temperature at the depth reached can be calculated from such observations. New and inexpensive equipment using a thermistor temperature measuring probe has been developed for making the observations of temperature and it has been tested successfully down to depths of 1060 m (3480 ft) in Hopton Pool Borehole in the Cannock Chase area. The paper also describes the equipment and gives the virgin strata temperatures for the Hopton Pool Borehole. It also gives observations of the temperature of the borehole fluid at various depths taken on one day several days after the borehole drilling had ceased and shows that these could not be regarded as reliable measures of the virgin strata temperature.