Characterization of gas shale pore topology and composition supports efforts to produce methane as well as assess the feasibility of using depleted organic-rich shale reservoirs for carbon dioxide sequestration. Imaging performed with Full-field Transmission X-Ray Microscopy technology to characterize gas shale structure and heterogeneity was conducted on a set of Barnett and Haynesville gas shale samples to help determine how the physical and chemical processes associated with CO2 in organic-rich shales affect injectivity and storage capacity (over long periods of time), and the ability of the shale to sequester CO2 (as both a free and adsorbed phase). The images show the presence of density-differentiated areas that might be an indication of predominantly organic and inorganic matter matrices, along with lowest-density lineal pathways that point to the potential presence of micro cracks. Using an innovative imaging technique at the nanoscale, high X-ray contrast gas was applied to the sample and the images obtained show visual enhancement of particular rock features, such as available porous space and microcracks.
Since the industrial revolution, carbon dioxide (CO2) emissions from fuel combustion have increased to about 30 billion tons of CO2 per year. Fossil fuels account for approximately 90% of emissions, of which coal comprises 41%. Carbon capture and sequestration is one strategy that could potentially mitigate gigatons (Gt) of CO2 emissions per year; however, technical obstacles have thus far hindered wide-scale deployment of this strategy. One potential geological repository for CO2 storage is depleted gas shale reservoirs, but the need remains to understand the chemical and physical properties of CO2 and its interaction with its local surroundings.
Gas shale samples exhibit structural and chemical features across a broad range of length scales to order 10 nm or less. Gas shale reservoirs have been largely overlooked as a viable energy resource. Lack of motivation to produce them in the fossil fuel energy market in the wake of rich and overflowing oil reservoirs, and their inherent heterogeneous and anisotropic nature, pushed shale and their gas to the far background of the energy resources inventory. Shales have been responsible for the majority of drilling problems. They cause seismic problems and directly affect AVO hydrocarbon interpretations (Sondergeld et al., 2010).
The advent of new drilling and completion technologies such as horizontal drilling and hydraulic fracturing (Curtis et al., 2011), a growing network of natural gas pipelines and specialized ships to carry LNG, depleting oil reserves, and strict anti-flaring regulations have made natural gas production easier and more attractive. Gas shale is also emerging as a viable option for subsurface sequestration of anthropological carbon dioxide because of their capability of storing free but mostly adsorbed gas, presumably within their organic component. It has been demonstrated, that CO2 is adsorbed in organic-rich shales preferentially to CH4 (Nuttall et al., 2005). A similar process occurs in coal and is the basis for enhanced coal bed methane recovery and geologic sequestration of CO2 in unmineable coal seams. In the case of shales, adsorption occurs on both the organic constituents and the clays. Development for natural gas utilizes wells and other infrastructure that is being constructed for shale gas production.
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