Solving the Challenge of Acquiring Low UCS Cores for Quantitative Digital Rock Physics

Abstract

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.