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

  • Reservoir pressure

  • Reservoir temperature

  • Sample size

Weatherford capabilities

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.

Storage capacity measured with isotherms can also be correlated with total organic carbon (TOC) determinations. Once calibrated, the TOC measurements provide a more cost-effect method for looking at a large number of samples.