Rock Mechanical Characterization of Unconventional Source Rocks

Lee, Ji Soo (Aramco Services Company: Aramco Research Center - Houston) | Jacobi, David (Aramco Services Company: Aramco Research Center - Houston) | Alnajar, Zainab (Stony Brook University)

OnePetro 

Abstract

Rock strength is an important property to measure for determining its effect on drilling, wellbore stability, and potential well completions associated with hydraulic fracturing of unconventional reservoirs. The industry traditionally relies on elastic moduli measured from core plugs to determine the stress anisotropy to predict the extent of hydraulic fractures. This provides some estimate of the expected stimulated rock volume in unconventional reservoirs. Rock strength however based on the finding of this study could also be a factor that needs to be considered for designing hydraulic fracturing plans to stimulate production from the rock volume. However, rock strength is difficult to measure in highly laminated source rocks comprising unconventional reservoirs. The existence of weak, horizontal bedding planes within the laminated rock fabric creates anisotropy that influences the rock strength values obtained. Moreover, drilling and extracting intact horizontal, vertical, and diagonal core plugs to test the effects of anisotropy on the rock strength is difficult to achieve. Often, the plugs fracture during extraction due to the laminated fabric. To compensate for the challenge of extracting intact core plugs from these lithofacies, this study proposes that rock strength can be estimated without the need of extracting core plugs. Instead, a new method is demonstrated where non-destructive rebound hardness measurements are collected across a specifically gridded, slabbed rock surface to provide an estimate of the rock strength. The collected rebound hardness values are converted into unconfined compressive strength values using an empirical algorithm. The empirical algorithm was developed using unconfined compressive strength values measured from core plugs correlated to rebound hardness numbers measured from the face of those same core plugs. The derived unconfined compressive strength values 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 elemental maps. Differences in unconfined compressive rock strength can then be correlated to the changing mineral content of the rock surface. This non-destructive estimation of rock strength was conducted to address the challenge of relating core scale measurments to reservoir scaled analysis to improve hydraulic fracturing designs in unconventional source rocks.