Gas Isotherms

Generating Gas Isotherms using NMR allows for a non-destructive measurement to quantify adsorbed versus absorbed gas content while also providing additional useful information.

The total natural gas content in a shale consists of both absorbed gas in the porous spaces of the shales and adsorbed gas on the surface of the shale matrix.  Adsorbed gas can contribute a significant fraction (~50%) of the total gas in place in a shale reservoir.

The total gas in place is dependent on the pore pressure and temperature and is vital to the profitable development of a shale reservoir.  Gas companies use gas isotherms, which are a measure of total gas content as a function of pore pressure, to assess a reservoir’s profitability.  Traditionally, gas isotherms are measured by exposing the core to helium and methane at ever increasing pressure while tracking the volume of gas absorbed and adsorbed.  These experiments involve destruction of the core and provide no information on the pore size distribution.

We present a method for measuring gas isotherms in shales using NMR.  Measurements of the T₂ relaxation time were taken for 24 hours after the introduction of methane to a shale.  These measurements were then used to determine the total gas content and pore size distributions within the rock.  This NMR gas isotherm analysis proves to be advantageous over conventional techniques as it can be completed without destruction of the shale core while also providing pore size distributions.