Abstract:
The nanoindenter has recently been expanded as a method to monitor the tensile failures of KRS, by loading and failing micro-cantilever geometries. Existing methods have not identified the micro effects of the interlacing kerogen into the overall clay and non-clay matrices. Here we demonstrate a sophisticated tool for mechanically characterizing KRS in that it accounts for contributions from both the rock and the organic matter. Micro-beams are manufactured with a focused ion beam under a scanning electron microscope (FIB-SEM) and then loaded in cantilever mode with a nanoindenter while monitoring in-situ via SEM. The force-displacement curves were analyzed in light of the high resolution images collected during fracture initiation, propagation, and ultimate failure. Under loading, the micro-beams exhibit linear elastic behavior followed by plastic yielding before complete failure. This behavior has been shown to correlate with the amount of organic matter in the failure zone of the cantilever beam.
Introduction
Mechanical characterization of source rock reservoirs such as shales is of primary interest to the oil and gas industry as it seeks to optimize hydraulic fracturing operations. This porous granular material has proven a complicated specimen for mechanical studies due to its high degree of porosity, its silt-sized quartz and other hard mineral particles, and the sea of nanoscale clay platelets that creates an encompassing matrix [1, 2]. Shale is a transverse isotropic material [3] whose bedding planes result from the mode of deposition during sedimentation.
The tensile strength of rocks is an important mechanical parameter for hydraulic fracturing design. During the fluid treatment, the pore pressure is increased by injecting fluid under high pressure into the formation. The pump pressure is raised until the formation breakdown pressure is achieved and the fracture initiates. As many laboratory experiments have demonstrated, the tensile strength influences the formation breakdown pressure [4]. Classical macroscale testing methods based on ISRM standards have been established for determining this rock tensile strength [5]. For example, the Brazilian test proposed in 1943 by Carneiro is still widely used to indirectly determine the tensile strength of rocks, concrete, etc. [6].
However, these methods were not able to capture the true tensile behavior of the kerogen-rich shales (KRS) in source rock reservoirs. Kerogen is the hydrocarbon source material which is interbedded and interlaced with the clay and non-clay mineral matrix on the micro- and nanoscales [7]. The kerogen content varies from one shale source to the next, and the trace levels present in kerogen-free shales (KFS) have no observable effect upon the mechanical properties. Rather, the anisotropic mechanical properties of KFS were determined by nanoindentation to be governed by packing density, porosity, and the stiffness of the various components [8]. Some success has been seen using the nanoindenter to determine the mechanical properties of KRS [9, 10]. These small scale measurements were able to isolate the elastic properties of kerogen from the surrounding minerals [9].
