Recent Advances in Sorption Isotherm Measurements

Mavor, Matthew J.¹, Robert C. Hartman², Timothy J. Pratt^2
1 Tesseract Corporation, Park City, UT
² TICORA Geosciences, Inc, Arvada, CO

Sorption isotherm data relate sorbed gas storage capacity to pressure and predict the production behavior coal seams and shale gas reservoirs. We have constructed new isotherm measuring equipment that measures sorption behavior as a function of time, pressure, and temperature rather than just end-point stabilized conditions. We have found that as greater care is taken in measuring and interpreting the data, that the data is more accurately fit with a Langmuir isotherm relationship.

Storage capacity data are not directly measured. When using volumetric equipment, temperature and pressure data are measured and gas storage capacity is computed. Interpretation of the raw isotherm pressure and temperature data requires accurate estimates of sorbed and free gas density. The use of different equations of state (which relate gas density to temperature and pressure) result in significantly different storage capacity data. Measurement labs report insufficient data to compute isotherm end points or to check the accuracy of their interpretation. The errors caused by inaccurate equations of state will be discussed.

We measure isotherm data by recording time dependent temperature and pressure data with highly sensitive pressure transducers and thermocouples. These data are interpreted for estimates of diffusivity and gas storage capacity. While the diffusivity estimates are for crushed samples, they are corrected to in-situ conditions by relating the isotherm measurements to reservoir temperature canister gas desorption measurements conducted on whole core samples. The relative diffusivity of different gases such as methane, nitrogen, and CO2 on crushed samples will be reported.