Published: 08/26/2019
Published: 08/26/2019
An operator's GIP estimates for unconventional reservoirs were consistently lower than expected, which caused a lack of confidence in reserves estimation. This mismatch was resulting from the poor constraint of resistivity-based saturation methods in shales, which was further aggravated by the presence of conductive graphite in the mature gas play, suppressing the resistivity. For free gas, quantification is challenged by assumptions of kerogen properties and pore pressure. For adsorbed gas, quantification is fraught with uncertainties stemming from insufficient Langmuir isotherm data, assumptions of pore pressure, and the model of a monolayer of adsorbed gas.
The operator wanted a timely, model-independent analysis of porosity and fluid volumes without having to resort to acquiring core for lengthy, costly laboratory analysis.
Unlike traditional NMR logging, CMR-MagniPHI high-definition NMR service is uniquely suited for the near-real-time evaluation of unconventional reservoirs. Its industry-shortest 200-us echo spacing enables quantification of the nanometer-scale pore volumes typical of shale reservoirs. Because diffusion measurements are ineffective in such tight reservoirs, CMR-MagniPHI service maps simultaneously acquired, continuous T1 and T2 measurements with a proprietary resistivity-independent analysis to resolve the porosity and fluid components within the logged interval. Pore-size distribution and permeability remain as standard deliverables.
CMR-MagniPHI service employs proton counting to leverage the sensitivity of NMR to hydrogen atoms in relation to the service's short echo spacing. This method to evaluate the GIP provides direct and continuous measurement across the shale, independent of pressure, temperature, or the other usual model parameters, irrespective of whether the gas is free or adsorbed and without the need for core.
The T1T2 distribution maps were used to quantify continuous fluid-filled porosity volumes across the logged interval. The values for the two gas source clusters 1 and 4 were combined to provide the total GIP as the gas-filled porosity, or bulk volume of gas. The clay-bound water and pore water were added to determine the total water volume, which was interpreted as nonproducible bulk water.
With the input of gas gravity (γg), CMR-MagniPHI service’s response to gas volume was converted to GIP via the proton counting method. Proton counting provides a simple, depth-by-depth evaluation of GIP, independent of the assumptions inherent in the traditional free plus adsorbed gas model.
As shown on the log, the GIP method of CMR-MagniPHI service calculates a larger total volume of GIP—by nearly 60 Bcf per section—than the Langmuir isotherm estimate. This larger volume is more accurate and in better alignment with the operator’s reserves estimates from field production.
R. Kausik et al., 2016. SPWLA 57th Annual Logging Symposium.
Challenge: Improve the accuracy of reserves estimates in unconventional reservoirs where a traditional model for free plus adsorbed gas that is based on saturation methods and Langmuir isotherm assumptions is typically underestimating the gas volume.
Solution: Deploy new CMR-MagniPHI high-definition NMR service to acquire continuous T1T2 distributions that accurately map the distribution of porosity and fluids for a continuous evaluation of gas in place (GIP), independent of resistivity-based volumetrics and Langmuir isotherms.
Results: Improved reserves estimates, adding nearly 60 Bcf per section from accurate measurement of gas volume to enable better evaluation of the total GIP.
