Abstract
Abstract. Electrical properties of rocks are important parameters for well-log and reservoir interpretation. Laboratory measurements of such properties are time-consuming, difficult, and impossible in some cases. Being able to compute them from 3-D images of small samples will allow for the generation of a massive amount of data in a short time, opening new avenues in applied and fundamental science. To become a reliable method, the accuracy of this technology needs to be tested. In this study, we developed a comprehensive and robust workflow with clean sand from two beaches. Electrical conductivities at 1 kHz were first carefully measured in the laboratory. A range of porosities spanning from a minimum of 0.26–0.33 to a maximum of 0.39–0.44, depending on the samples, was obtained. Such a range was achieved by compacting the samples in a way that reproduces the natural packing of sand. Characteristic electrical formation factor versus porosity relationships were then obtained for each sand type. 3-D microcomputed tomography images of each sand sample from the experimental sand pack were acquired at different resolutions. Image processing was done using a global thresholding method and up to 96 subsamples of sizes from 2003 to 7003 voxels. After segmentation, the images were used to compute the effective electrical conductivity of the sub-cubes using finite-element electrostatic modelling. For the samples, a good agreement between laboratory measurements and computation from digital cores was found if a sub-cube size representative elemental volume (REV) was reached that is between 1300 and 1820 µm3, which, with an average grain size of 160 µm, is between 8 and 11 grains. Computed digital rock images of the clean sands have opened a way forward for obtaining the formation factor within the shortest possible time; laboratory calculations take 5 to 35 d as in the case of clean and shaly sands, respectively, whereas digital rock physics computation takes just 3 to 5 h.
Highlights
Electrical formation factor (FF) refers to the ratio of the electrical resistivity of a saturated medium to that of the saturating fluid (Guéguen and Palciauskas, 1994). This is an important parameter in exploration geophysics as, contrary to the electrical resistivity of reservoirs that is dependent on the resistivity of the saturating fluid, the formation factor is an intrinsic property of the rock independent of fluid salinity
The values of the formation factors obtained using the flow cell are higher than those obtained using the static cell for both the Scarborough (8.2) and Cottesloe (8.5) Beach samples, whereas for Scarborough Beach, formation factors have close values at high porosities and depart from each other at lower porosities
We have successfully combined the scientific approach of laboratory measurements with micro-CT scan computational images
Summary
Electrical formation factor (FF) refers to the ratio of the electrical resistivity of a saturated medium (sediment or rock) to that of the saturating fluid (Guéguen and Palciauskas, 1994). This is an important parameter in exploration geophysics as, contrary to the electrical resistivity of reservoirs that is dependent on the resistivity of the saturating fluid (and the same type of reservoir can exhibit high or low resistivities; Constable and Srnka, 2007; Jinguuji et al, 2007; Mitsuhata et al, 2006), the formation factor is an intrinsic property of the rock independent of fluid salinity. Results are affected by current leakage problems (especially at high frequencies) and electrode polarization (emphasized at low frequencies).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
