(129f) Supercritical Fluid Interaction Between Kerogen and Gas in Shale

Recently, there has been widespread activity to develop natural gas resources available in various shale formations throughout the United States and beyond.  The resources available in these formations represent a long-term source of abundant natural gas.

There is a significant difference between the reservoir matrix materials in shale formations and those in sandstone or carbonate reservoirs in that the shale matrix contains substantial amounts of heavy organic materials such as kerogen.  Some of these organic materials have the potential for dissolution into the gas phase, especially in at higher temperatures and pressures.  The concentrations of heavier hydrocarbons in the gas phase are small, but, as large quantities of gas are produced, the accumulated amount of heavy hydrocarbon can become significant, possibly leading to flow assurance challenges.  The solubility effects may also be exacerbated by the use of light hydrocarbon fluids such as propane in the fracturing process.

The flow assurance challenges arise because the solubility of heavy hydrocarbons in methane gas is strongly influenced by the gas pressure.  As the pressure of the gas decreases as it flows through the reservoir matrix and then through the production tubing to the surface, the decreased solubility may lead to precipitation of solids or viscous liquids that would impede flow through the reservoir or production tubing.  The effect is similar to the precipitation of asphaltenes and waxes that provide flow assurance challenges in the production of more conventional reservoirs, especially in deep-water production systems.

This paper uses phase-equilibrium thermodynamics to explore the potential impact in the shale-gas production environment.  There is little information available about the details of the shale matrix, which is composed of kerogen and associated geological decomposition products.  In addition, the composition will differ for each reservoir.  As a result, although this paper will draw upon knowledge gained through previous applications of supercritical fluid extraction processes, a detailed and accurate representation of the shale-gas behavior also will require experimental results that currently are not available in the public domain.