Predicting Shale Gas Content and Productivity Based on Isotope Fractionation

Carbon and hydrogen isotopic composition belongs to the most important information to determine natural gas reservoir properties; however the conventional interpretation methods on gas isotopes may not work well for shale gas due to the special isotope fractionation of gas in the micropore systems in the organic-rich shale. Based on the field observation, experimental simulation and theoretical modeling, we developed a systematic approach to evaluate shale gas content and productivity based on gas isotope properties.

Our interdisciplinary research revealed that the isotope “rollover”, which describes the reversal trend of ethane carbon isotopic composition against maturity, may be largely attributed to the mixing of gas generated from the cracking of oil and condensates. We solved the “rollover” curves based on the gas end members derived from our kinetic isotope fractionation and gas expulsion models, and established the method to derive the contribution of oil-cracking gas. This deconvolution based on isotopes helps to improve the reliability in the estimation of gas content, oil/gas ratio, gas wetness and condensate amount in shale.

We evaluated gas isotope fractionation during production based on a continuum flow model with coupled adsorption/desorption and diffusion, to describe the isotope fractionation due to gas flow through low permeability organic-rich shale rocks. With this method, we can derive reliable information for shale plays during production:

1. Determine the shale preservation condition based on the methane isotopic composition profile;

2. Determine the reservoir properties with well-controlled isotope measurement of cuttings under laboratorial conditions;

3. Predict gas productivity based on the variation of methane isotopic composition;

4. Diagnose engineering problems based on the variation of methane isotopic composition.

These approaches may remarkably increase the beneficiary of gas isotope applications.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California