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The AAPG/Datapages Combined Publications Database

Indonesian Petroleum Association


23rd Annual Convention Proceedings (Volume 1), 1994
Pages 33-43

Empirical Models for Predicting CO2 Concentrations in North Sumatra

C. M. Reaves, A. Sulaeman


Compositional analyses of reservoir fluids from oil and gas accumulations in North Sumatra have been compiled to describe and predict the occurrence of CO2. Carbon isotope compositions of CO2 demonstrate that it is primarily derived from inorganic sources. A set of simple empirical models was developed which utilize reservoir lithology, temperature and pressure to calculate upper and lower limits for CO2 concentrations.

The initial estimation in all of the models is based on a least squares linear regression of CO2 partial pressure and reservoir temperature derived from a study of sandstone reservoirs in the Gulf of Mexico (Smith and Ehrenberg, 1989). This correlation provides an accurate prediction of CO2 concentrations, +/- 15%, for 90% of the clastic reservoirs examined in North Sumatra. CO2 concentrations in carbonate reservoirs show a much wider positive variation from the regression values. The variation between calculated and observed CO2 concentrations shows a systematic relationship to reservoir temperature. Reservoirs at temperatures less than 280° F have observed CO2 concentrations 15% to 30% higher than the calculated values. At temperatures in excess of 280° F, the range of observed CO2 concentrations expands to 0% to 30% higher than the calculated values. These concentration limits contain 85% of the observed CO2 values from carbonate reservoirs in North Sumatra.

The principal mechanism controlling CO2 concentrations in clastic reservoirs is the interaction of silicate mineral transformations and carbonate mineral dissolution. CO2 concentrations in gas and oil accumulations reflect local equilibrium between gas, pore water, and mineral phases within the reservoir. In carbonate reservoirs the absence of silicate minerals allows CO2 to be transported within the carbonate aquifer system with little or no alteration. Carbonate reservoirs which are exposed to significant up dip fluid flow will possess CO2 concentrations that are representative of the base or entry point of the regional flow system.

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