Core samples from the subsurface can provide unambiguous direct information to guide operator decisions. Core may be acquired with drilling equipment (full-bore core) or by post-drill wireline methods (sidewall core). Both approaches have distinct profiles of cost, risk, sample type and value, and an operator must select the most appropriate to progress business in an informed way. The option to selectively core after drilling and perceptions of lower cost and risk might indicate that sidewall coring will always be the best approach. Recent developments to increase the size and quality of rotary sidewall samples would only add weight to this view. It's not all good news for sidewall core, however. Individual sample size and total volume delivered per run are tiny; weak rock or high overbalance pressure may cause poor recovery and biased datasets; time between drilling and logging allows mud invasion and borehole relaxation so samples are often broken, and pore fluids contaminated. Sidewall sample sets therefore leave a higher degree of uncertainty when compared to full-bore core. It is this operator's view that both approaches have a role to play in reducing subsurface uncertainty, and cost, risk and value should be carefully considered when deciding which to apply.

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 analysis 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. Six sandstones of known properties were used in the testing program to cover a wide range of particularly low and medium, unconfined compressive strengths (UCS). Reference plugs were cut from all samples. The sandstone samples 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 reference plugs and test plugs were imaged with high-resolution X-ray micro-Computed Tomography (micro-CT) at resolutions between 2 and 11 microns, and digital rock analysis was conducted on all samples. Using pre- and post-test images, damage could be identified and petrophysical properties including porosity and permeability could be determined and directly compared. Digital rock physics provide 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 a 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.

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.

Extensive operator experience obtaining rotary sidewall cores on wireline showed the recovery and quality dropped significantly in lower unconfined compressive strength (UCS) formations, making standard analysis results for petrophysical calibration unreliable. This paper discusses a series of physical tests used to quantify the quality and condition of cores obtained using a novel adaption of existing percussion sidewall coring (PSWC) using high-resolution X-ray micro-Computed Tomography (microCT) scan data. The methodology demonstrates how the combination of low technology percussions cores and high technology microCT scans can provide an efficient input into digital rock analysis for formation evaluation. Within defined UCS ranges the quality of the data can be higher than alternative methods. Several sandstones of known properties were first microCT-scanned and then percussion cored in a laboratory environment. PSWC barrels of traditional and novel designs were tested. A range of testing methods were used including sophisticated fixtures that applied hydrostatic pressure with constraining pressures and overbalance. The resulting sidewall cores were then microCT-scanned to evaluate the effects of the percussion test, the quality of the samples and to provide high-resolution images for digital rock analysis. Further information was obtained from finite element modelling, high-speed cameras and sensors to determine impact velocity and deceleration. This data was used to iterate improvements to the PSWC barrel design, modifying variables such as core diameter, sample length and barrel cutter profile. Around seventy-five different combinations of environment and barrel design were successfully acquired. This included repeatability tests and allowed conclusions to be made on the value of the data from different attributes obtained from the microCT scans. The testing allowed the barrel design to be optimized to reduce damage induced by the percussion coring process. Following the testing the range of UCS where this technique is practicable has been determined. Analysis of digital rock physics provides great benefits but requires representative samples as an input. In lower UCS formations acquiring sidewall cores can be challenging. Traditional percussion sidewall cores have long been considered of limited value, however with hardware modifications this paper demonstrates that they can provide high quality inputs into digital rock analysis for a defined range of lower and even moderate UCS formations. Acquiring percussion sidewall cores typically requires significantly less rig time per core than rotary sidewall coring making it practical to acquire a larger number of cores for a given cost.