Isotherms provide a measurement of the absorbed gas storage capacity. This value is used with other measurements to calculate gas-in-place volumes.
Isotherms are run with a crushed sample at a single temperature. Pressure is varied and the amount of gas absorbed by the sample is measured. This data can then be fit to a mathematical equation to calculate a Langmuir isotherm.
Parameters to be considered
When running an isotherm, the following parameters are important to consider:
Type of gas
Gases include helium, methane, ethane, propane, carbon dioxide, and nitrogen. Pressures can be as high as 10,000 psi, and temperatures are routinely measured up to 150°C. Recommended sample size is 250 to 300 grams for shale, but we have run rotary sidewall cores with less than 50 grams.