Establishing Geothermal Gradient Using a New Static Temperature Analysis Method

Abstract

This work presents a new methodology for determining the static formation temperature (Tei) by using transient well-test data. We show how a semianalytic method, involving the rectangular hyperbola technique for obtaining Tei, was used for establishing a region's geothermal gradient. Insights into heat-transfer processes were applied to develop methods of data collection and analysis. Several options were enacted to gather valid transient temperature data. For instance, sensor placement above the test interval ensured that the produced fluid had the opportunity to cool during shut-in periods, thereby creating useful perturbations. Tests accompanied by large pressure drawdowns caused Joule-Thompson heating, leading to subsequent cooling during well shut-in, even when the sensor was at the midpoint of a producing interval. Transient temperature data were gathered during pressure buildup tests in various boreholes ranging from 2,200 to 14,500 ft, encompassing different geologic horizons in Kuwait. Data collected from traditional open- and cased-hole logging were used and compared with the new approach. Statistical analyses clearly showed the superiority of the proposed procedure. Results of the new approach established Kuwait's geothermal gradient (gG) at 0.012 F per ft with a mean surface temperature (MST) of 87.23 F. Introduction Temperatures in the subsurface are related to a region's MST and the depth-dependent temperature or the geothermal gradient. Actually, the temperature of each subsurface geologic formation is dependent upon the composition, thermal properties of formation constituents, and supply of heat from the earth's interior. Therefore, Tei at each of many geologic horizons are needed to be established and combined to determine the geothermal gradient of a region. Accurate knowledge of geothermal gradient is required for many oilfield applications. Some of these applications include evaluating open- and cased-hole logs, designing cementing programs, basin modeling for discerning source rock, modeling steady- and unsteady-state fluid and heat flows in the wellbore designing thermal recovery projects, to name a few. Despite the diversity of needs, very few reliable methods exist for obtaining the true static formation temperature at a given depth, en route to establishing a region's geothermal gradient. Common oilfield practices for obtaining Tei rely on discrete temperature measurements, usually during borehole geophysical logging operations. Such data are frequently obtained in newly drilled wellbores following mud circulation. Complications arise because fluid circulation induces significant cooling to the near-wellbore region, requiring extrapolation of discrete measurements to Tei. In contrast, temperature data acquired while logging cased boreholes, under well shut-in conditions, are improved because measurements are not necessarily preceded by cooling or heating owing to mud circulation. However, no procedures exist for confirmation of thermal equilibrium between borehole fluids and formations. Therefore, understanding heat-transfer mechanisms between the borehole fluids and the formation constitutes the first step in developing reliable modes of data gathering and interpretation methods. Methods are available for estimating the static formation temperature in an openhole situation. These methods include those of Edwardson et al., Tregasser et al. and Dowdle and Cobb. Of these, the method of Dowdle and Cobb, developed in analogy to the pressure analysis or the Horner method, gained wide acceptance because of its simplicity. However, Hasan and Kabir pointed out some of the limitations of the Dowdle-Cobb approach and presented several simplified graphical methods for the Tei evaluation of early-time data. P. 267^