Unconventional shales can include adsorbed gas, mostly calibrated in coal seams at low pressure for CH4 and CO2, whilst also including Natural Gas Liquids and condensates. For the Duvernay formation in Alberta, Canada, the Langmuir pressure can be above 3000psia/200bar, pressures where PVT issues such as dewpoint and liquid yield are important.
The adsorbed gas released first is likely to be enriched in the lighter components, so that the composition of the gas released may not be reflective of the free gas. With an increasing proportion of dry adsorbed gas being released with pressure decline, an alteration in liquid yield can occur, even above the dew point, and this change has been observed in Duvernay production data, As initial production and gas samples will likely be representative of the free gas, such that a conventional fluid sampling and PVT analysis for dewpoint may miss the impact. The objective of this paper is to present compositional simulation work that has been completed to describe this observed behavior with a Langmuir isotherm.
The Marcellus shale has been a good training ground for understanding the impact of adsorbed gas in a dry gas system. In the Marcellus, the adsorbed gas contribution ranges from 30-50%usinghigh pressure adsorption models such as BET. When you shift from dry gas into the more liquid rich phase windows, gas condensates can have a dew point pressure in the range of 4000psia. Adsorption isotherm analysis of the Duvernay have shown Langmuir pressures in the range of 3000psia, therefore the adsorbed gas will become active as soon as the reservoir pressure drops from initial conditions.
If the adsorbed gas released is exactly the same composition as the free gas, the composition will remain unchanged, and the adsorbed gas will merely be a source of additional support, such as in the Marcellus dry gas. However, in a system with natural gas liquids and condensates, compositional effects can be anticipated, primarily with changes in the lighter ends as it is those that liberate first. In producing Duvernay wells, above the saturation pressure, changes in the C1/C2 ratio occur along with liquid yields changes appearing to indicate that the liberation of adsorbed gas in effect.