Reservoir models typically utilized for desorption-controlled reservoirs such as coals and gas shales possess dual-porosity/single-permeability characteristics. In this case dual-porosity means that two in-situ locations exist that can be used for gas storage, adsorbed within the matrix and in the free form in the cleat system. Single-permeability, which refers to the cleat system, is the only permeability network that gas or water must flow through to reach the wellbore. While this approach to modeling coals and shales has become accepted practice, experience has shown that the models can frequently be in gross error when forecasting well or field performance based on limited reservoir and/or production data; gas production is usually over-predicted and water production under-predicted. The implications for economic decision-making in an exploration mode are obvious, and there are many examples of projects that have suffered from this very problem. Further, reservoir parameters derived from history-matching, when historical gas production does exist, are commonly found to be inconsistent with measured permeability and gas sorption/content data. While there has been considerable effort focused on improved data collection procedures, such as well testing and gas content measurement for example, these problems persist.
While performing reservoir studies in the Antrim shale and low-rank coal plays throughout the world, it became clear that the accepted assumption of gas desorbing directly from the coal matrix into the cleat system is not entirely valid. In practice, gas production occurs much later than the models predict, and cannot be adequately explained though the normal parameters of sorption time, permeability, relative permeability, etc. Analysis of core and other data suggests that another porosity and permeability system is required to account for this effect, specifically within the matrix blocks themselves. An advanced, triple-porosity/dual-permeability model has therefore been developed, in which gas desorbs from the internal matrix block surfaces, migrates via conventional Darcy flow through micro-permeability matrix, and into the cleat system where it then flows to the wellbore. Water can also be stored both within the matrix blocks and in the cleat system. In essence, this model requires that desorbed gas must work its way through the matrix before reaching the cleat system, and must establish a relative permeability to gas within the matrix block before it can do so. This geometry is similar to conventional dual-porosity models, with the addition of an adsorbed gas component.
Comparisons of this new model versus the historical modeling approach confirm that the new model predicts lower gas and higher water production rates, consistent with field evidence. Further, more accurate production forecasts can be achieved using measured well test information (for the cleat permeability), low cleat porosities (which are known to exist), and lab-derived porosity and permeability data for the matrix block properties. This paper presents the historical accuracy problem with reservoir simulation in desorption-controlled reservoirs, the practical theory behind the new model, comparisons between the new and conventional models, and some example applications.