Characterisation of Natural Fractures and Fracture Zones of the Basel EGS Reservoir Inferred from Geophysical Logging of the Basel-1 Well

Abstract

The development of a geological model for the reservoir of an Enhanced Geothermal System (EGS) provides an essential framework for geomechanical models that simulate reservoir behaviour during the stimulation and production phases. The geological model describes the spatial distribution and scaling of discontinuities within the reservoir as well as lithological variations. In this paper we analyse logging data from the 5 km deep well, Basel-1, located in Switzerland to investigate the natural fractures and zones characterised by high fracture frequency in the crystalline basement. The logs extend from 2.6 km depth, about 100 m below the weathered palaeo-surface of the granite, to a depth of 5.0 km, and include acoustic televiewer (UBI), density and p-wave velocity. The results of drill cuttings analysis were also available. Two previous analyses of the UBI log have been made. Considerable differences in the distributions of natural fractures in the crystalline basement were found from the three analyses. The differences in large part reflect the difficulty in distinguishing natural from drilling-induced fractures. Poor quality images in the open-hole section below 4.7 km resulting from stick-slip motion of the UBI sonde were radically improved by applying a novel correction method using accelerometer data. This led to fewer natural fractures in the open section than recognised in earlier studies. Fracture frequency decreases with depth from 3.1 fractures/m near the top of the logged section to 0.3 fractures/m below 3.0 km. Orientation cluster analyses revealed a complex pattern of up to 6 potential fracture sets along the well, some of which may be conjugate pairs. Only set 1 (steeply dipping to W–SW) is present along the entire imaged borehole, the other sets occurring over limited sections of the hole. The mean orientation of set 1 does not coincide with prominent NNE-striking Rhenish lineaments (faults) of first- and second-order in the Basel area, but strikes subparallel to the maximum principal horizontal stress. Fractures belonging to set 1 are spatially clustered and form localised zones of high fracture frequency. Zone lengths ranged up to 100 m, but were more typically tens of metres, and below 4.0 km the zones consisted predominantly of fractures belonging to set 1. Zones of high fracture frequency did not necessarily coincide with low density or low p-wave velocity anomalies, as might be expected from fracture zones with damage or higher porosity.