Using Digital Rock Physics to Evaluate Novel Percussion Core Quality

Abstract

This paper details a benchmarking and validation workflow using digital rock physics (DRP) to evaluate the effectiveness of various percussion sidewall core (PSWC) acquisition methods. The workflow consists of obtaining digital rock properties and images of reference material to compare with material obtained from laboratory percussion sidewall acquisition, including novel designs. This workflow allows insight into the acquisition processes and potential sources of damage in the PSWC technique, and potentially other rock sampling techniques, and offers an opportunity to evaluate its appropriateness as a subsurface rock sample acquisition method. Sample cubes from six outcrop sandstone formations of known properties were used in the testing program to cover a wide range of particularly low and medium unconfined compressive strengths (UCS). Multiple control rotary plug samples were cut from each sandstone formation. The sample cubes were then used as the parent material in laboratory testing of various designs of PSWC bullets. The PSWC bullets, including novel designs, were shot in simulated downhole environments. Both control rotary plug samples and PSWC test core samples were imaged with high-resolution X-ray micro-computed tomography (micro-CT) at resolutions between 2 and 11 μm, and digital rock analysis was conducted on all samples. Using pre- and post-test images, the damage could be identified, and petrophysical properties, including porosity and permeability, could be determined and directly compared with DRP results from control samples and available routine core analysis (RCA) results. DRP provides unique insight to evaluate and quantify changes (or lack of changes) to the sample material subjected to the PSWC acquisition. Damage encountered in the test samples includes grain crushing and compaction that degrades storage and transport properties and dilatant zones that locally enhance transport properties. The presence, frequency, and distribution of these zones are dependent on experimental parameters. In all cases, undisturbed rock fabric could be identified in each sample, and intact texture was verified by comparison with reference material. A novel and efficient method for acquiring and evaluating subsurface samples was developed and benchmarked. Lab results indicate this method may be equally applicable to low- and mid-range UCS rocks. This approach enables a cost-effective reservoir characterization strategy. By optimizing PSWC bullet design and coupling this with mature, image-based digital rock technology, this work demonstrated that the samples and results obtained by this method are representative and that the controls on storage and transport properties are well understood.