Core Hardness Testing and Data Integration for Unconventionals

Stimulation of unconventionals often demands the characterization of fine-scale mechanical properties and correlations between several data sets, including well logs, core descriptions, and lab tests. This study presents a comparison of hardness data from a slabbed liquids-rich core with complementary well logs and core analysis. Hardness refers to the measure of resistance to permanent deformation from an applied force, and depends on a range of material properties, some of which are independently measured by standard well logs and core work. Leeb rebound hardness was manually measured with the portable Equotip 3 by Proceq. Within the handheld tool, a 3 mm diameter ball indenter is launched by spring against the sample; the ratio of the rebound velocity to impact velocity determines the Leeb hardness value. The Equotip was originally designed for testing metals, although recent studies have employed this tool to improve geomechanical characterization of core samples and field exposures. Hardness measurements, collected at two inch intervals, and high resolution whole-core dual energy computed tomography (CT) data elucidate the fine-scale mineralogical heterogeneity of unconventional reservoir rock. For example, thin laminations of softer calcareous marl cause anomalously low hardness measurements within hard, blocky, limestone or chalk. X-ray CT provides core images, as well as variations in density and effective atomic number (Zeff). In the tested core section, hardness trends with Zeff more closely than with other data sets, such as density, elastic properties, total organic carbon, and shale volume. This suggests that the mineralogy may be a key factor in controlling mechanical stratigraphy. The Equotip is less destructive than scratch or indentation hardness tests, is easy to use, provides instantaneous results, and has the potential to replace costly tests; however, a dependable methodology is necessary to maximize efficiency and minimize error. Reliable hardness data can then be scaled up and integrated with well logs.