Use of Well Log Temperatures to Evaluate Regional Geothermal Gradients

Abstract

Abstract A previously untested technique of using maximum recorded temperatures while logging to total depth is used to study regional geothermal gradient variations in Oklahoma. A theoretical analysis of the mud temperature build-up is made to support this new method which shows that maximum logged temperatures, when properly recorded, provide better basic data than previously realized. Data obtained from searching through "More than 50,000 well logs are presented in the form of a regional geothermal map. A second-order polynomial surface is fitted to the data to study regional trends and deviations. Geological and other correlations are included only to establish the validity of the technique. Results of the study enable estimation of geothermal gradients in areas which are not presently covered by existing geothermal gradient maps. Introduction The thermal condition existing within the early is of fundamental importance to geological and geophysical theory. Knowledge of subsurface temperatures is also of interest to the oil industry, particularly in recent years because of higher temperatures associated with deeper drilling and the drilling of geothermal wells. Subsurface temperature information is useful when considering potential oil and gas exploration areas, drilling mud, lost circulation and cementing materials, casing design, logging programs, optimum location of geothermal wells and in reservoir engineering applications. A basic hypothesis of geophysics is that a correlation should exist between heat flow and major geologic and topographic features. This theory, while generally adhered to, has not been adequately tested because of the limited number of reliable determinations of heat flow through earth strata. The problem is manifested by the directional conductivity exhibited by shales, slates and some sandstones as shown by the observation that in situ conductivity determined in a single borehole through anisotropic material is not the conductivity parallel to the borehole. An alternate and useful approach in studying earth temperatures has been to determine the average rate of increase of temperature with depth (although locally temperatures may differ by a factor of 10 in different places and the gradients may differ at different depths along any given vertical). This technique has been used in studying local temperature anomalies over salt domes and anticlines and in proposed theories on the accumulation of hydrocarbons. It has been effectively applied to oil and gas exploration, showing that temperature variations usually conform to the subsurface structure at greater depths. Studies associated with geothermal gradients have shown thatthe gradient as determined from boreholes does not necessarily extrapolate back to the mean surface temperature;correlations of decreasing porosity with higher thermal gradients can be explained by differential compaction of sediments by tectonic forces;the spatial distribution of aquifer temperatures may be used to determine its flow characteristics;reservoirs with high original pressures are usually associated with geothermal highs; andlow geothermal gradients may exist over basin or platform-located oil reservoirs. Good subsurface temperature data are not readily available. Nichols and Moses selected temperatures measured in conjunction with pressure build-up surveys in preparing the only published regional geothermal gradient maps. Lyons extended this work in a map that is now available. Temperatures obtained while logging were excluded in these earlier works because it was expected that temperatures measured after circulation of drilling fluids would not be representative of true formation temperatures. Several authors have consistently discouraged the use of bottom-hole temperatures recorded when logging, on the assumption that the temperatures would not be correct because the wells were not in thermal equilibrium. An expedient solution to the problem of obtaining fairly reliable data over extensive areas is available in the logging files if logged temperature data are restricted to the maximum recorded temperatures in those wells logged to total depth. This technique has been shown to give bottom-hole temperature data that are representative of the true geothermal gradient. JPT P. 667ˆ