Siriwardane, H.J. , B.D. Bowes, G.S. Bromhal, R.K. Gondle, A.W. Wells, B.R. Strazisar, “Modeling of CBM Production, CO2 Injection, and Tracer Movement at a Field CO2 Sequestration Site,” International Journal of Coal Geology 96, 120-136, 2012.
Sequestration of carbon dioxide in unmineable coal seams is a potential technology mainly because of the potential for simultaneous enhanced coalbed methane production (ECBM). Several pilot tests have been performed around the globe leading to mixed results. Numerous modeling efforts have been carried out successfully to model methane production and carbon dioxide (CO2) injection. Sensitivity analyses and history matching along with several optimization tools were used to estimate reservoir properties and to investigate reservoir performance. Geological and geophysical techniques have also been used to characterize field sequestration sites and to inspect reservoir heterogeneity. The fate and movement of injected CO2 can be determined by using several monitoring techniques. Monitoring of perfluorocarbon (PFC) tracers is one of these monitoring technologies. As a part of this monitoring technique, a small fraction of a traceable fluid is added to the injection wellhead along with the CO2 stream at different times to monitor the timing and location of the breakthrough in nearby monitoring wells or offset production wells.
A reservoir modeling study was performed to simulate a pilot sequestration site located in the San Juan coal basin of northern New Mexico. Several unknown reservoir properties at the field site were estimated by modeling the coal seam as a dual porosity formation and by history matching the methane production and CO2 injection. In addition to reservoir modeling of methane production and CO2 injection, tracer injection was modeled. Tracers serve as a surrogate for determining potential leakage of CO2. The tracer was modeled as a non-reactive gas and was injected into the reservoir as a mixture along with CO2. Geologic and geometric details of the field site, numerical modeling details of methane production, CO2 injection, and tracer injection are presented in this paper. Moreover, the numerical predictions of the tracer arrival times were compared with the measured field data. Results show that tracer modeling is useful in investigating movement of injected CO2 into the coal seam at the field site. Also, such new modeling techniques can be utilized to determine potential leakage pathways, and to investigate reservoir anisotropy and heterogeneity.
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