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