Application of bottom-hole temperature corrections in geothermal studies

Abstract

Bottom-hole temperature (BHT) data measured in oil and gas wells constitute a large, low quality set of temperature observations commonly used in geothermal studies. Raw BHT data are, on the average, cooler than true formation temperatures. To estimate true formation temperatures, corrections must be applied. Empirical BHT corrections may be applied to BHT data for which only depths of measurement are known, but may not be valid outside of the area for which they are calibrated. If multiple BHT measurements from successive logging runs are available, the Horner plot correction procedure can be used. The accuracy of the Horner plot is limited by simplifying assumptions made in its derivation, and by the common lack of information on parameters such as duration of mud circulation. More detailed and complete treatments provide insight into the borehole equilibration process, but their application is similarly limited by a common lack of data regarding borehole thermal properties. A new type of empirical correction procedure may be derived in some areas and allows a correction to be made for BHTs for which only a depth and time of measurement are known. Noise is invariably present in the BHT data found on well log headers from oil and gas wells, and the consequence of noise in these data should be considered when choosing and applying a correction. Although the average nature and magnitude of error in raw BHT data may be approximately known, application of all BHT correction procedures to the typical data available from oil and gas well log headers usually involves an unknown amount of error. Reliable estimates of average geothermal conditions can be made only from suites of BHT data, and are condemned to be imprecise at best.