Teresa Jordan, Cornell University
The Principal Investigator and a graduate student conduct this study. The graduate student in particular is mastering a variety of laboratory analytical methods, utilizing advanced microscopy capabilities of the Cornell Center for Materials Research. Both PI and student are learning to relate macroscopic properties of shale to micro- to nano-scale constituents. Within this first year of the project, both PI and student have interacted with engineers who range from academics who work with micromechanics of solids to industry personnel who design and operate hydraulic fracture operations. Through those interactions and hands-on research they improved their knowledge of attributes of the rocks that are of critical importance to engineering practice.
Prior to beginning field observations in the summer season, the graduate student conducted experiments indenting and fracturing Marcellus samples, with the supervision of PI and faculty from materials science and from mechanical engineering. The initial mechanics studies were conducted using a microindentor, which tests the properties of the composite of all the components in a shale. The samples, from the quarry at which the new field observations would begin, were chosen to represent a range of concentrations of organic matter. Using Scanning Electron Microscopy and Electron Backscatter Microscopy, the graduate student characterized the samples before and after the indentation experiments. Thus the field-based examination of sedimentary properties at multiple locations in the quarry was informed by a preliminary knowledge of the microscopic characteristics of the materials.
The study site is a series of rock quarries extending in an east-west direction across central New York State. Preparatory work included determining which of 22 large quarries indeed expose Marcellus shale. There appear to be only five quarries in which the Union Springs Formation is completely exposed, and not all of these are accessible. In the second year of study, fractures are also being documented in several cores through the Union Springs Formation, owned by the New York State Museum.
Scanning electron microscopy (SEM) surveys conducted on an array of samples documented the distribution and characteristics of microfacies within the rock. Fractures were induced in mounted and polished samples using a microindentation hardness tester on polished surfaces, at an orientation perpendicular to bedding. A cube corner indenter tip was found to be the most effective for the purpose of stable crack initiation, and numerous indents were performed across each sample using the maximum load achievable by the microindentation apparatus.
Imaging after the indentation experiments allowed characterization of the path of cracks and the mineral or organic properties adjacent to the fractures. The most clay and organic-rich microfacies were found to fracture less readily, and with smaller fracture widths than in microfacies which contained higher carbonate mineral content. Ongoing experiments seek to characterize relationships between crack properties and the degree of dispersion of the organic matter and other mineral constituents.
Field observations in the quarries have included three types of information: primary sedimentary textures and types of particles, and their organization into microfacies; documentation of fracture orientations, lengths, apertures, and spacing, and their variability between differing microfacies; collection of samples. Prior work had documented the primary sedimentary properties and provided samples for a single vertical column of the Union Springs Formation in the Marcellus shale in one quarry. During June-August 2013, descriptions and sampling were completed for three additional vertical columns. These occur at distances of 160 m, 700 m, and 39,000 m from the original Marcellus location. Based on the availability of Marcellus exposed in quarries, we believe that we can later complete study of one other site, at a distance of 110 km from the original site. Use of cores during the second project year will enable us to extend the database south of the outcrop zone, and to clarify which of the fractures occur under burial conditions rather than only in near-surface exposures.
Comparing the microfacies of four detailed stratigraphic sections, variability is observed from section to section, although the degree of variability is not constant. Two sections located only ~160 m apart have a high degree of similarity, of both sequence of microfacies and their thicknesses. The section 700 m distant from the original site has considerable similarity of microfacies properties and sequence, but more differences than exist between the two 160-m-distant sections. The Union Springs section located at a distance of 39 km from the original differs significantly from the other sections, and these differences are currently being catalogued and described. To be completed in the second project year is the development of a numerical description of the variability that could be communicated clearly to reservoir engineers or environmental engineers.
The majority of laboratory analyses remain to be completed during the next year. Ongoing work includes nano-indentation testing to determine the mechanical properties of individual components within the composite material. Additional micro-indentation studies of the shale composites are planned, which will enable compiling data sets for the relationships between fracture paths, rock structure, and composition. For the study of microfacies and natural fractures, also underway are microscopic examination and relevant chemical analyses, such as elemental concentrations and Total Organic Carbon.