Some strategies for estimating present day heat flow from exploration wells, with examples

Abstract

Techniques for estimating heat flow from downhole temperature data are discussed, with application to 3 exploration wells in Australia. The thermal resistance method, or Bullard Plot, is straightforward to use, and allows rapid visual assessment of the consistency of temperature and thermal conductivity. Calculation of interval and apparent heat flow can be useful to reveal depth trends in heat flow, although the interval heat flow is sensitive to errors in the temperatures. A generalised least squares method relies on the specification of a priori, or input, values and uncertainties for the temperature and thermal conductivity data, as well as the heat flow, to obtain maximum likelihood model estimates of all parameters. The dependence of these estimates to the input values can be examined with sensitivity matrices. A well from the Eromanga Basin (Graham 1) in southwestern Queensland with temperature data obtained during a downhole pressure survey and thermal conductivity estimates based on measurements from nearby well has a heat flow of 116±14mWm?2. Another well from the Gippsland Basin (Volador 1) shows inconsistencies in the temperature observations and assumed thermal conductivities. The heat flow is calculated to be 110±20mWm?2, although this is not considered to be a reliable estimate as the thermal conductivities are poorly constrained. The final example from Olympic Dam (RD21) has 65 accurate temperature observations and 18 laboratory thermal conductivity measurements. The data is shown to be consistent with a deep heat flow of 82±15mWm?2, and an additional 45mWm?2 from internal heat production, provided the measured thermal conductivity for one formation is accepted to be an overestimate of the in situ value.