A. Zschocke

Correction of non-equilibrated temperature logs and implications for geothermal investigations

Optimum data for any geothermal investigation are densely sampled, high precision temperature logs from boreholes. They should represent the formation temperature as accurately as possible. Often, temperature measurements influenced by the drilling process and the circulation do not have the quality which is required for geothermal methods. Horners method is a widely used correction method that is based on analysis of a time series of temperature observations. Mostly, these data are not available and a correction is therefore not possible. The approach presented here enables us to correct entire temperature logs even if no repeat measurements were carried out. This is achieved by combination of the theory of Horners method with estimates for radial heat flow. Given a large number of technically perturbed logs this new approach enlarges considerably the potential database suitable for geothermal methods. The implications for two important fields of application are also discussed. These are fluid flow analysis and analysis of the ground surface temperature history on temperature logs.

Estimating Darcy flow velocities from correlated anomalies in temperature logs

Fluid flow in reservoirs can be identified by its characteristic signatures in temperature logs. The method presented here is based on the great sensitivity of the subsurface temperature field with respect to heat advected by fluid flow. This requires that all other temperature perturbations are eliminated. These may be caused, for instance, by heterogeneous thermal properties or paleoclimate transients, both of which may distort an otherwise linear temperature log. We propose a simple geothermal technique for quantifying regional strata-bound flow based on a detailed analysis of borehole temperatures. Depending on the spatial data distribution and the geothermal situation, this method yields results of varying quality. In a case study with data from southern Germany, the method yields a lower bound for the flow magnitude. Another case study from northern Germany yields the magnitude of the regional flow owing to a larger data set and information of the direction of flow from additional independent information. Under optimum conditions, this method also yields an estimate of the direction of regional strata-bound groundwater flow as well as the uncertainty in magnitude and direction. Finally, the full potential of the method is illustrated using a synthetic data set.