Rethinking Methodologies of Characterizing Gas in Place in Gas Shales
R. Marc Bustin, Daniel Ross, and Gareth Chalmers
University of British Columbia, Vancouver, BC
Producing gas shales range from true organic rich shales (i.e. Antrim shale) with a significant gas in the adsorbed state to siltstone and fine grained sandstones with gas storage almost entirely in the free state (i.e. Lewis Shale). Based on new analytical methodologies and production data, it is evident that the adsorbed gas content of most over mature gas shales has been overestimated and the free capacity underestimated because of the application of coalbed methane and conventional core analyses techniques for characterising gas in place. In contrast the total reservoir capacity has been underestimated in many tight sands in which the adsorbed gas component has not been considered in reservoir evaluation. The use of He to measure effective porosity to methane or heavier hydrocarbons in microporous rocks results in an over estimate of effective porosity and over estimation of gas in place.
Simple numerical models, pore size and permeability analyses and laboratory experiments show that diffusion rates and pressure driven flow in the shale matrix occur at the same time scale such that in many fine grained rocks it may be impossible to differentiate free gas from sorbed gas using standard methods. Standard reservoir assessment techniques thus tend to over estimate gas in place in true shales where a component of the free gas is assigned to the adsorbed state. A production isotherm provides a better assessment of total reservoir capacity, flow characteristics of the strata and production prediction than standard adsorption or desorption data or matrix permeability.
Shales such as the Barnett, which have a relatively high reservoir temperature and pressure, have low sorbed gas capacities and the adsorption isotherm is nearly flat at initial reservoir conditions. Hence not until late in the production life of the reservoir will the adsorbed gas component be produced.