(613c) 4D Synchrotron X-Ray Imaging to Understand Porosity Development in Shales during Exposure to Hydraulic Fracturing Fluid

Unconventional or low permeability shale reservoirs have emerged as the most important source of petroleum resources in the United States and represent a two-fold decrease in greenhouse gas emissions compared to coal. While individual plays differ significantly in lithology, P-T conditions and depositional settings, unconventional reservoirs are united by necessity of hydraulic fracturing for economically viable production. The proliferation of modern fracturing techniques, specifically horizontal drilling and multi-stage fracturing, has generated a significant increase in oil and gas production rates in the last decade alone. Despite recent progress, further optimization of hydraulic fracturing operations presents substantial technical, economic, and environmental challenges. These include inefficient recovery, wastewater production and disposal, contaminant and greenhouse gas pollution, and induced seismicity.

A relatively unexplored facet of hydraulic fracturing operations is the fluid-rock interface, where hydraulic fracturing fluid (HFF) contacts the shale matrix along pre-existing and stimulated faults and fractures. Widely used, water-based fracturing fluids (slickwater) contain additives not found in shale, including oxidants and acid. Consequently, this creates an environment significantly out of equilibrium with the shale minerals and trapped hydrocarbons. Injection and soaking during hydraulic fracturing induces a host of fluid-rock interactions, most notably the dissolution of carbonate and sulfide minerals, which results in enhanced or â??secondaryâ? porosity networks, as well as mineral precipitation. The competition between these mechanisms determines how HFF affects reactive surface area and permeability of the shale matrix. The resultant microstructural and chemical changes may also affect the formation of capillary barriers for trapped hydrocarbons and facilitate the release of heavy metal contaminants from organic matter. At present, few of these processes have been investigated, specifically how they impact production and the composition produced waters. Developing a mechanistic understanding of the microstructure and chemistry of the shale-HFF interface enables the design of new methodologies and fracturing fluid compositions to improve estimated ultimate recovery (EUR) while minimizing the potential for contaminant release from produced water.

Synchrotron-based X-ray imaging techniques provide unique advantages for studying dynamic processes in space (3D) plus time (4D). With a high brilliance synchrotron light source, tomographic datasets can be collected in minutes to seconds (depending on the instrument) and non-destructively capture time-dependent phenomenon. In addition, 2D projection images can also be collected at higher temporal frequency to visualize more rapid processes. By employing various configurations, optics can be applied in the visible and X-ray regimes to magnify the object to measure microscale to nanoscale features. X-ray microtomography provides submicron voxel size which can resolve the secondary pore network which is forming while X-ray nanotomography can further magnify regions of the sample and examine the nanoporous structure. This flexibility in different synchrotron-based microscopes provides a unique capability to quickly image and reconstruct samples at various length scales relevant to shales exposed to HFF.

To assess the effects of HFF on shale reservoir rock, imaging experiments were carried out at two synchrotron facilities, Pohang Accelerator Laboratory (PAL) and Stanford Synchrotron Radiation Lightsource (SSRL). Shale samples were collected from both outcrops (Green River, Marcellus) and vertical wells (Barnett, Eagle Ford) and span a range of lithologies from siliceous to calcareous to organic-rich. Sample were imaged before, during, or after exposure to HFF to characterize the initial 3D structure. The subsequent microstructural evolution was tracked in time using time resolved 3D imaging. By capturing time series, the 3D reconstructions reveal how the secondary porosity networks advance into the shale matrix. By testing shales of different lithologies we have obtained insights into the mineralogic controls on secondary pore network development and the morphologies obtained at the shale-HFF interface and the ultimate composition of produced water from different facies. Utilizing the information gained from the beamline imaging experiments provides the information about thickness of reacted â??skinâ? around fractures need to develop new methodologies and fracturing fluid compositions aimed to enhance EUR and minimize environmental impact.