Adsorbed and Free Gas

Gas in Two Places

A shale-gas reservoir stores its gas two ways. Some is free gas in the pore space and microfractures, exactly like a conventional reservoir, and its amount grows with pressure. The rest is adsorbed gas, molecules clinging to the enormous internal surface of the kerogen, held there by pressure.

The Langmuir Isotherm

The adsorbed content follows the Langmuir isotherm,

which rises steeply at low pressure and then flattens toward the capacity $V_L$, the most the kerogen can hold (more kerogen, higher $V_L$). $P_L$ is the pressure at which the rock holds half its capacity. The total gas-in-place is the free gas plus this adsorbed gas.

The Desorption Tail

The split is what makes shale production distinctive. As the well draws the pressure down, the free gas drains first. Then, as the pressure keeps falling, the adsorbed gas desorbs off the kerogen and feeds the well for years, the long, slow tail that conventional reservoirs do not have. A shale with little adsorbed gas declines fast; one rich in kerogen produces gently for a long time.

References

• Langmuir, I. (1918). The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc., 40(9), 1361-1403.
• Ambrose, R. J. et al. (2012). Shale gas-in-place calculations. SPE Journal, 17(1).