Summary
Gas content and storage capacity are the key parameters for determination of
the gas resources and reserves in unconventional reservoirs. These parameters
must be obtained from laboratory experiments in core samples such as desorption
canister tests and adsorption isotherm experiments. Desorption canister testing
is performed to determine the total adsorbed gas content, gas composition, and
the total desorption time. Adsorption isotherm experiments are conducted to
determine the gas storage capacity with pressure and for CO2
sequestration purposes. Other analyses of coals include proximate analysis and
bulk-density measurements of all samples. Shales are commonly analyzed for
total organic carbon in lieu of proximate analysis.
The gas content is estimated by placing selected freshly cut reservoir
samples in airtight sealed canisters and measuring desorbed gas volume as a
function of time at atmospheric conditions. Total gas content is the summation
of three components: "lost gas," desorbed gas, and "residual gas." "Lost gas"
is the volume of the gas that desorbs from the sample during the recovery
process at the wellsite, before the core sample can be sealed in a desorption
canister. "Residual gas" is the gas that remains sorbed on the sample at the
completion of the canister desorption test.
A disadvantage of this procedure is the estimation of "lost gas." The volume
of the "lost gas" is usually estimated by extrapolation of desorbed data to
time zero using linear and/or polynomial curve-fit to the plot of cumulative
desorbed gas vs. square root of time. The differences between both methods can
become more pronounced, especially in high-gas-content reservoirs.
In this paper a new method, which is based on nonlinear regression of
measured gas content, is presented. This technique offers an accurate
estimation of lost gas, which, coupled with sorption isotherm, has an impact on
the calculation of gas in place, the recoverable reserves, and production
profiles.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
7 August 2008
- Meeting paper published:
22 September 2008
- Revised manuscript received:
17 January 2009
- Manuscript approved:
1 September 2009
- Published online:
19 April 2010
- Version of record:
20 April 2010
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