Abstract:

This study presents a comparison of three alternative approaches to estimate uniaxial compressive strength (UCS) to traditional uniaxial compressive test for shale which may be incompatible with standard sample preparation. Rebound hardness and scratch tests have been employed to overcome unique sampling challenges concerning shale lithology. Both approaches estimate UCS values in shale formations where sampling is impossible. In addition, this study develops a new approach to estimate UCS of shale referred herein as the “Drill Test”. Comparison and discussion between the three alternative tests and the traditional UCS test are presented. UCS estimations of the three alternative methods showed a range of UCS values from 1933–39,299 psi, whereas uniaxial compressive tests using plugs varied from 1328–58,280 psi. Overall, the four methods showed fairly good agreement. However, the current empirical calibration equations were developed from UCS data less than 40,000 psi, but the real range of strength was up to 58,000 psi. Therefore, equations should be updated with additional samples greater than 40,000 psi UCS to represent the wider range of rock strength from this Barnett core.

Background

Uniaxial compressive strength (UCS) has widely been applied in rock engineering as a design index of rock structures, since it is one of the fundamental rock mechanical properties. In general, NX-size (2.16 inches in diameter) plugs have traditionally been used to determine UCS values of hard rocks in geological and mining engineering. UCS is also important for petroleum engineering regarding drilling, drill bit designs, wellbore stability, sanding prediction, perforation tunnels, well completions, and frac designs. The oil and gas industry, however, primarily uses smaller-diameter (1 inch) plugs due to limited core resources. Further, unconventional reservoir formations have more restrictions in preparing standardized rock specimens based on horizontal discontinuities along bedding planes such as laminations [1-2]. Therefore, other alternative methods have been developed to overcome the restrictions and estimate formation strength. This study conducted four different laboratory experiments to determine or estimate horizontal UCS values of a Barnett shale core. They are scratch [3-29], drill [30-38], rebound hardness [39-44], and uniaxial compressive tests. The first three methods are indirect estimations of UCS, and thus, they converted each test result to the corresponding UCS value using empirical calibration equations. This study introduces a new approach using drill test and compares with other results for technical review.

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