Comparison of several BHT correction methods: a case study on an Australian data set

Abstract

Bottom-hole temperatures (BHT) from oil exploration provide useful constraints on the subsurface thermal regime, but they need to be corrected to obtain the equilibrium temperature. In this work we introduce several BHT correction methods and compare them using a large Australian data set of more than 650 groups of multiple BHT measurements in about 300 oil exploration boreholes. Existing and suggested corrections are classified within a coherent framework, in which methods are divided into: line/cylinder source; instantaneous/continuous heat extraction; one/two component(s). Comparisons with reservoir test temperatures show that most of the corrections lead to reliable estimates of the formation equilibrium temperature within ± 10±C, but too few data exist to perform an inter-comparison of the models based on this criterion. As expected, the Horner method diverges from its parent models for small elapsed times (or equivalently large radii). The mathematical expression of line source models suffers from an unphysical delay time that also restrains their domain of applicability. The model that takes into account the difference of thermal properties between circulating mud and surrounding rocks – that is the two-component model – is delicate to use because of its high complexity. For these reasons, our preferred correction methods are the cylindrical source models. We show that mud circulation time below 10 hours has a negligible effect. The cylindrical source models rely on one parameter depending on the thermal diffusivity and the borehole radius, which are poorly constrained, but the induced uncertainty on the extrapolations remains reasonably low.