Determination of Total Gas in Place for Gas Shale Reservoirs using NMR Logs
Presented by Rick Lewis, Schlumberger
The total gas in place (TGIP) in gas shale reservoirs is a direct reflection of the reservoir quality of the play, making it one of the most important properties to quantify. 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 horizontals. We will present a novel methodology for the direct determination of TGIP in gas shale reservoirs from nuclear magnetic resonance (NMR) logs and demonstrate how multi-tool petrophysics can aid this analysis.
Gas shale reservoirs are more difficult to evaluate than conventional reservoirs. Deterministic evaluation of conventional gas resources depends simply on reservoir volumetrics and the direct application of natural gas tables. In gas shales, the hydrocarbon exists not only as pore-filling free gas but also as adsorbed gas on high surface area kerogen. Because the pore-filling free gas and the surface-adsorbed gas have different densities and exhibit fast exchange in the magnetic resonance time scales, they cannot be separated nor their effective hydrogen indices determined. Corrections to conventional methods based on the Langmuir isotherm can be applied, but this depends on laboratory investigations of cores that are rarely performed in a time frame relevant for development decisions, and the limited number of isotherm analysis may not account for heterogeneity of the reservoir rock.
Gas in place calculated with the traditional free plus adsorbed gas model requires inputs of effective porosity, effective water saturation, total organic carbon (TOC), Langmuir isotherm, pore pressure, formation temperature, and gas gravity. We present the TGIP-NMR method requiring only the measured NMR porosity, gas gravity, and bulk volume water, without assumptions about average in situ gas density and knowledge of pore pressure, as are necessary with the natural gas tables. Multiple examples will be presented.
About the presenter
Rick is a Schlumberger Fellow and the developer of the gas-shale evaluation workflow that was initially fielded 16 years ago and has been applied to thousands of wells in North America and abroad. In his current position, located in Denver, Colorado, USA, Rick manages a group responsible for the continual improvement for this workflow, for its introduction and application to the international market, and for the development of workflows for the evaluation of tight oil reservoirs. Rick is also the interface to the Schlumberger research and engineering groups for the development of evaluation technologies for unconventional reservoirs. Rick has also worked for Shell Oil and the U.S. Geological Survey. Rick received a BS degree from UCLA and MS and PhD degrees from Caltech, all in geology.
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