Computing wave velocity of rock sample using rock chips and cuttings

Rock cuttings and chips, usually from boreholes, are considered as direct samples from the subsurface layers containing valuable information. They are rarely used in routine studies because of their imprecise depth, induced cracks during transfer, and saturation by mud fluid, however, mainly due to their small sizes. To investigate the latter, we computed P- and S-wave velocity of Ruhr and Bentheimer sandstones from 2D images using a 2D-to-3D procedure in Digital Rock Physics (DRP). We found that cuttings are problematic when: a. the size of their images is smaller than Representative Elementary Volume (REV) and b. their porosity is medium to high. To overcome these problems, we introduce two methods based on Differential Effective Medium equations (equation-based method) and Cross-Correlation Simulation (CCSIM) (simulation-based method). From both sample preparation and computational perspectives, these methods are easy to apply, inexpensive and fast relative to standard 3D DRP. Results on Bentheimer and Berea sandstone and Grosmont carbonate showed that both methods are valid and reliable. In the best cases, they could compute wave velocity of rock with a root mean square error (RMSE) less than 100 m/s, which are comparable or, in some cases, even better than 3D DRP. The size of cuttings images could also be up to 5 and 3 times smaller than REV for the equation- and simulation-based methods, respectively, which is promising for practical purposes in the petroleum industry.