Gas adsorption and reserve estimation for conventional and unconventional gas resources

This chapter describes the traditional approaches and recently developed models for in-place natural gas resource and reserve estimation calculations suitable for reservoir rocks displaying a wide range of permeability and porosity distributions. The inclusion of gas adsorption in these models is assessed and identified as being essential for unconventional gas reservoirs (tight zones, organic-rich shales, and coal bed methane). Volumetric, material balance equation (MBE), decline curve analysis (DCA), and other numerical models, including reservoir simulations, are used for in-place resource and reserve estimation. The many technical and commercial uncertainties involved in such estimations are identified and approaches for taking them into account are addressed over the reservoir life-cycle. Formulations for material balance calculations taking into account gas adsorption are provided and their performance assessed. Factors influencing gas adsorption and alternative gas isotherms suitable for shale gas and coal bed methane reservoirs are considered in detail. Whereas the Langmuir isotherm is widely used to estimate adsorbed gas contents for shale gas and coal bed methane reservoirs, its mono-layer adsorption assumptions are found to be inaccurate. Research has shown that the more realistic adsorption assumptions of the Freundlich and BET isotherms provide improved accuracy in resource and reserve estimates. Taking into account temperature, as well as pressure, further improves the prediction performance of the gas adsorption isotherms. Total organic carbon, temperature, porosity, pore size, thermal maturity, and clay content are some of the factors identified as influencing gas adsorption. Banner headline. It is essential to consider gas adsorption when calculating the in-place resource and reserves associated with natural gas reservoirs, especially for the unconventional reservoirs including tight zones, organic-rich shales, and coal bed methane. Volumetric, material balance equations, decline curve analysis, and reservoir simulations are all used to provide resource and reserves estimates. Numerical methods applied to unconventional reservoirs use gas adsorption isotherms to distinguish the contribution to reserves made by adsorbed gas.