Gravimetric measurements of high pressure and temperature C1, C2 and C3 adsorption isotherms on Beetaloo Sub-basin shales.

Exploration in Australia's Beetaloo Sub-basin focuses on the liquids, wet-gas, and dry-gas mature shales of the Kyalla and Velkerri formations. Gas storage in these reservoirs occurs as free gas and additionally through adsorption to the internal pore surfaces of organic matter and clay minerals. The adsorbed gas component may represent a significant proportion of total gas-in-place for shales, and in the Beetaloo the co-production of wet gas components (e.g. ethane, propane) is also expected to improve the economics of gas production. Accurate characterisation of the adsorption behaviour of these hydrocarbons is critical for developing accurate resource estimates and understanding production behaviour. A gravimetric isotherm rig and measurement methodology was developed enabling accurate high pressure high temperature (HPHT) adsorption measurements on shales. Such measurements present a challenge due to the low sorption capacity of shale, as well as the requirement to approximate in-situ conditions. For example, an interlaboratory comparison by Gasparik et al. (2014) observed discrepancies of around 25% in CH4 sorption measurements in the high pressure range, and significant variation in C2H6 measurements above 5 MPa. Gravimetric isotherm measurements avoid cumulative errors of the volumetric method, and are less sensitive to the presence of leaks, an important consideration for HPHT measurements. Equipment design allowed the CSIRO gravimetric rig to overcome reported shortcomings of the method at high pressures due to sample buoyancy and low sample mass, and a calibration methodology using material with a known adsorption isotherm validated rig performance. The method was applied to measure CH4, C2H6 and C3H8 adsorption isotherms at multiple temperatures of four Beetaloo Sub-basin shales with a range of petrological properties. Measurements were also undertaken using a volumetric isotherm method to enable comparison. The measured isotherms present the first published C2 and C3 isotherms for Australian shales.