Fracture mapping with electrical images: Observations from cores and modelling

Downhole electrical images are used to assess the nature, variability and distribution of subsurface formations. The images can be used to describe and delineate sedimentary features, to provide input into depositional models, to describe and quantify fracture occurrence and orientation, to identify and distinguish breakouts and hydraulic (induced) fractures, and to provide indications of local variability and heterogeneity in terms of porosity and permeability variations (with calibration to core). In parallel to these downhole images, detailed laboratory studies have provided improved understanding of the variability of petrophysical properties at the pore scale. These variations are often related to the fine scale sedimentary (depositional and/or diagenetic) and structural (gross geometry and stress related) fabric, although stratigraphic descriptions based on visual (optical) observations do not always agree with petrophysical parameters. We have previously demonstrated the ability to image sedimentary fabric in the laboratory and to relate the electrical core images to properties such as porosity and permeability as well as to grain size and cementation. More recently we have adapted our approach to enable electrical imaging of fractures in core using similar principles to those employed in downhole electrical imaging. The results demonstrate the ability of the technique to image conductive fractures in fully saturated water-bearing core; these fractures are electrically connected from the flat measurement surface through to the outer surface of the core. Published results for numerical modelling of downhole electrical imaging tools show the electrical response to be related to fracture depth and fracture aperture. Initial experimental results on fractured core in the laboratory appear to support these numerical observations with increased current flowing into the fracture as the aperture increases. The finite size of the electrode, however, (e.g. the downhole button) means that this technique cannot theoretically distinguish between a single fracture and smaller groups of fractures adjacent to the electrode. It is impossible to determine solely from the static electrical images whether the fractures are open or closed to fluid flow. Our work with sediments, however, has demonstrated that passing a tracer fluid of different resistivity to the saturating fluid can identify fluid pathways through the core. Applying this technique to fractured core will enable the investigation of open and closed fractures and the effects of stress on fracture compressibility. © SPWLA 43rd Annual Logging Symposium 2002.