The total gas-in-place (TGIP) in a gas-shale resource is a direct measure of the reservoir quality of the play. Knowledge of TGIP as a function of depth enables the identification of gas-bearing zones and aids in the determination of sweet spots for landing horizontal drainholes. We describe a new method to determine TGIP directly from nuclear magnetic resonance (NMR) logs, and demonstrate how this can be further improved with multisensor analysis.
Gas-shale resources are more difficult to evaluate than traditional gas reservoirs, which require an estimate of gas volume, the product of porosity and saturation, and direct application of natural gas tables for gas hydrogen index. In gas shales, the hydrocarbon exists not only as pore-filling free gas but also as adsorbed gas on high-surface-area kerogen, with different densities and their downhole NMR signals cannot be separated and the effective hydrogen index of the hydrocarbon phase cannot be determined. The current method of evaluating TGIP, based on the use of the Langmuir isotherm to estimate adsorbed gas, requires laboratory investigations on core, which are rarely performed in a time frame appropriate for production decisions, and the limited number of measured isotherms are often insufficient to characterize the heterogeneity of the resource.
We introduce the TGIP-NMR method that converts NMR measurements into TGIP by counting hydrogen nuclei directly. The method circumvents the requirement to know the hydrogen index, pore sizes, pore pressures and formation temperature in order to use natural gas tables. We show examples comparing gas-in-place estimates, which use the current model (free and adsorbed gas) with TGIP-NMR, demonstrating 20% more gas than the free and adsorbed gas method, most likely because organic shales have very small pore sizes and as a consequence the free gas may be denser than predictions from natural gas tables.