Carbon Dioxide Storage Capacity of Organic-rich Shales

Abstract This paper presents an experimental study on the ability of Barnett shale core samples to store carbon dioxide. An apparatus has been built for psrecise measurements of gas pressure and volumes at constant temperature. A new analytical methodology is developed allowing interpretation of the pressure-volume data in terms of measurements in total porosity and Langmuir parameters of core plugs. The method considers pore volume compressibility and sorption effects and allows small gas leakage adjustments at high pressures. Total gas storage capacity for pure carbon dioxide is measured at supercritical conditions as a function of pore pressure under constant reservoir confining pressure. It is shown that, although widely-known as an impermeable sedimentary rock with low porosity, organic shale has the ability to store significant amounts of gas permanently due to trapping of the gas in adsorbed state within its finely-dispersed organic matter, i.e., kerogen. The latter is a nanoporous material with micropores (< 2 nm) and mesopores (2-50 nm). Storage in organic shale has the added advantages because the organic matter acts as molecular sieve allowing carbon dioxide —with linear molecular geometry— to reside in small pores that the other naturally-occurring gases cannot access. In addition, the molecular interaction energy between the organics and carbon dioxide molecules is different which leads to its enhanced adsorption. Hence, affinity of shale to carbon dioxide is due to partly steric and thermodynamic effects similar to those of coals that are being considered for enhanced coalbed methane recovery. Mass transport paths and the mechanisms of gas uptake are unlike coals, however. Once at the fracture-matrix interface, the injected gas faces a geomechanically strong porous medium with dual (organic/inorganic) pore system, therefore, has choices of path for its flow and transport into the matrix: the gas molecules (i) dissolve into the organic material and diffuse through a nanopore-network, and (ii) enter the inorganic material and flow through a network of irregularly shaped voids. Although the gas could reach the organic pores deep in the shale formation following both paths, the application of the continua approximation to the percolation threshold is not known. Here, using gas permeation experiments and history-matching pressure pulse decay, we show that a large portion of the injected gas reaches the organic pores through the inorganic matrix. This is consistent with SEM images that do not show connectivity of the organic material on scales larger than tens of microns. It indicates an in-series coupling of the dual continua in shale. The inorganic matrix permeability is therefore predicted less, typically in the order of 10 nD. More importantly, transport in the organic pores is not due to flow but mainly pore and surface diffusion mechanisms.