Detailed Geological Reservoir Characterisation Using an Integrated Analysis of Borehole Image Logs

For the geological characterisation of subsurface borehole image logs provide an excellent data source. Using specialised software for QC and a tailored processing chain high-resolution images of the borehole circumference are obtained, which are able to resolve sub-cm scale features. The image observations are upscaled and used for calibration of seismic scale features within the larger field area. Demonstrated by examples from hydrocarbon and geothermal exploration wells the structural analysis re-constructs palaeo-horizontal and any changes due to faulting and tilting during and after deposition of the stratigraphic sequence. Faults and fractures are characterised in great detail, including a quantification of fracture density and porosity, and are interpreted with respect to their influence on hydraulic conductivity. A correlation of fracture density with elastic properties allows the connection with seismic derived parameters, hence the field wide scale. The sedimentological analysis evaluates transport directions and unravels the depositional environment from features visible in the borehole image. The vuggy porosity is quantified using a semi-automatic work flow. Image facies stacking patterns are used for calibration of seismic-scale sequence boundaries interpreted in terms of possible reservoirs. The stress field direction and stress magnitudes are inverted from breakout and drilling-induced fracturing. The stress tensor data is then used in order to optimise mud weight and well path for future wells, as well as for evaluation of fracture permeability. Borehole image logs therefore represent a prime tool for characterisation of oil and gas, geothermal or CO2 storage reservoirs.