Static Formation Temperature From Well Logs - An Empirical Method

Abstract

Reliable values of static temperature are important for a number of reasons. For example, they are required in designing deep-well cementing programs and in analyzing reservoir fluid properties. Though many engineers hate discouraged the use of recorded log temperatures, these log temperatures can be used to predict static temperature. Introduction Static formation temperature should be determined as accurately as possible for a number of reasons. In-situ sauration distributions computed from resistivity logs require accurate formation water resistivities that depend on temperature. Reliable estimates of bottom-hole temperatures are important in designing deep-well cementing programs and in evaluating reservoir-fluid formation volume factors. Also, the determination of static temperature is necessary for establishing geothermal gradients that can be used to estimate the temperatures of deeper zones. More recently, new exploration techniques have used temperature as a mappable proximity parameter. proximity parameter.Unfortunately, the temperatures recorded during logging operations frequently are not static temperatures. The recorded values are too low. These low temperatures result because the circulating mud temperature frequently is much less than the formation temperature. Schoeppel and Gilarranz reported that early investigators had discouraged the use of bottom-hole temperatures obtained from logs under the assumption that the temperatures would not be correct because the mud and the formation were not in thermal equilibrium. More recently, however, Timko and Fertl suggested that the temperatures recorded while running a series of logs can be interpreted to estimate static formation temperature. They recommend use of a Horner temperature plot, similar to the conventional pressure buildup method, plot, similar to the conventional pressure buildup method, for estimating static formation temperature. Timko and Fertl demonstrate the apparent applicability of the technique with an example. Significantly, we have been unable to find any theoretical justification for an analysis of this type. However, we have seen field cases where the technique gives satisfactory results. The accuracy of this analysis was especially surprising when it was concluded that the two methods - pressure build-up and temperature buildup - are not completely analogous. Thus, the chief objective of the present study was to determine under what conditions the Horner temperature plot can be used to estimate static temperature. Comparison of Temperature And Pressure Buildup Pressure Buildup Pressure Buildup The equation that describes pressure behavior at each point at any time in the well drainage area is point at any time in the well drainage area is(1)2p 1 p c p----- + ---- ---- = ------ ------r2 r r k t Eq. 1 frequently and referred to as the "diffusivity" equation because of its similarity to the diffusivity equation in the heat-transfer literature. Subject to the constraints of an initial condition and a set of boundary conditions, Eq. 1 can be solved. Consider the case of a well producing at a constant rate and located in an infinitely large reservoir. JPT P. 1326