Geomechanical Characterization of Unconventional Source Rocks Using Rebound-Hardness Test and Mineral Mapping

Abstract

Summary Rock strength is seldom considered important by the hydrocarbon industry to help monitor the drilling progress, evaluate wellbore stability, guide well completions, or predict the success of hydraulic fracturing in unconventional reservoirs. With respect to hydraulic fracturing, rock-elastic moduli are routinely used to evaluate stress anisotropy. However, rock strength may also be a crucial parameter to evaluate when designing hydraulic-fracturing strategies to stimulate production from the rock volume. However, because of the density of horizontal laminae comprising the rock structure within a vertical whole core, the true geomechanical anisotropy for both properties is difficult to measure from extracted core plugs. Often, core-plug drilling and extraction techniques do not produce intact core plugs to measure these parameters consistently because of the laminated rock fabric. The integrity of the rock makes it is difficult to interpret both elastic moduli and rock-strength measurements using the recovered plugs extracted from horizontal, vertical, and diagonal azimuths. To address that challenge, a new, nondestructive method is demonstrated in which rebound-hardness measurements taken across a specifically gridded, slabbed rock surface can provide an estimate of the rock strength. The collected rebound-hardness numbers (RHNs) are converted into unconfined-compressive strengths (UCSs) using an empirical algorithm. The empirical algorithm was developed using UCSs measured from core plugs correlated to RHNs measured from the face of those same core plugs. The derived UCSs are then used to represent the source rock's mechanical characteristics, which can be presented as a contour map across the surface. These results have been correlated to the mineralogy of the rock surface, quantified, and mapped using micro-X-ray-fluorescence (µ-XRF) elemental maps. Differences in UCS can then be correlated to the changing mineral content of the rock surface. This nondestructive estimation of rock strength was conducted to address the challenge of relating core-scale measurements to reservoir-scaled analysis to improve hydraulic-fracturing designs in unconventional source rocks.