Considerable interest exists for better understanding the gas storage and transport properties for shale gas reservoirs in Australia’s Beetaloo Basin. In these reservoirs, fluid transport through natural and induced fractures may be described by Darcy’s Law, whereas transport in nanopores of the shale organic and inorganic matrix can occur via diffusion (Clarkson et al., 2016). Estimation of the timescales required for primary production of gas-in-place from shale reservoirs will depend on knowledge of these diffusion and desorption rates (Haghshenas et al., 2015). Understanding the impact of reservoir pressure and temperature on diffusion behaviour of Beetaloo Basin shales enhances the ability to constrain this behaviour in subsurface numerical models and outside well control, thereby improving accuracy when considering well completions or undertaking field development planning.
In this work, a high-precision high-temperature adsorption/diffusion rig was used to characterise methane adsorption and diffusion behaviour on an intact cube-shaped sample of Beetaloo Basin shale from the Amungee C member. The fixed-volume volumetric method was used to measure across a temperature range of 35 to 150°C and a pressure range of 0.6 to 21 MPa. The adsorption was modelled using the Langmuir isotherm, and the diffusion behaviour modelled using the unipore model. The TOC, thermal maturity, mineralogy and pore structure of the shale was characterised.
Pore characterisation indicated the presence of multiple scales of porosity in the shale (micro, meso and macro). The Langmuir isotherm model was applicable to the measured adsorption data indicating that a homogeneous distribution of monolayer adsorption may predominate in the sample. The pore scales and experimental conditions indicate diffusion is the primary transport mechanism occurring in the shale. The unipore diffusion model provided a good fit to measured CH4 uptake data, and alongside the measured diffusion coefficients suggested that transport is primarily governed by the sample mesoporosity. Increases in diffusivity with respect to CH4 pressure were observed, which reflected an established direct correlation observed in shales and coals between diffusion coefficient and adsorbate density.
This study assists in developing an understanding of the relationship between adsorption and diffusion behaviour and reservoir conditions for shales in the highly prospective Beetaloo Basin. The importance of non-Darcy fluid flow behaviour to shale gas production, and the limited availability of physical samples of Beetaloo basin shales underscores the importance of developing relationships that can help to understand diffusion behaviour where existing data are sparse.