Transient Pressure Modeling for CO2 Miscible Flooding Processes

Date
2016-05
Authors
Li, Jianli
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Publisher
Faculty of Graduate Studies and Research, University of Regina
Abstract

Since CO2 flooding has been one of the most successful techniques for enhancing oil recovery, analyzing the pressure responses of CO2 injector is very important to understand the CO2 flooding performance and optimize the design and operation. The existing well test models of CO2 flooding have seen their limitation in accurately characterizing a miscible zone, e.g., negligence or over-simplification of the transient viscosity in the miscible zone. This study aims at establishing a comprehensive well testing model for a CO2 flooding process by incorporating a convection–diffusion mass transfer model with a transient pressure model. This thesis first compares four existing analytical methods with different pressure and time formation to a gas reservoir and determines the most suitable model for CO2 bank during a miscible flooding process. Then, a transient pressure model is developed for an entire CO2 flooding reservoir including CO2 bank, miscible zone, and untouched oil zone. Subsequently, a convection–diffusion mass transfer model is built and coupled with the transient pressure model to account for the dynamic change of properties in the miscible zone. At last, sensitivity analyses are conducted to examine the effect of reservoir properties, fluid properties, and operating conditions on dimensionless pressure and pressure derivative curves. It is found that for a typical pressure range (P > 13 MPa) for CO2 miscible flooding, the pressure and time method is simpler in form and sufficient in accuracy compared with the other methods. In addition, it is necessary and important to consider the miscible zone in the pressure modeling of a CO2 flooding process. Four flow regimes can be identified in the dimensionless pressure and pressure derivative profiles: the first radial flow regime, transition flow regime, pseudo radial flow regime, and boundary-dominated flow regime. The size of CO2 bank strongly influences the length of the first and the pseudo radial flow regimes. A larger radius of the CO2 bank leads to a longer first radial flow and a shorter pseudo radial flow. A trivial radius of CO2 bank can cause the missing of the first radial flow regime but a much longer pseudo radial flow period. Moreover, it is also found that the total compressibility in miscible zone mainly affects the transition and the pseudo radial flow regimes. A larger total compressibility contributes to a shorter transition regime, followed by a longer lasting pseudo radial flow regime. At last, the effect of CO2 injection rate on the dimensionless pressure and dimensionless pressure derivative is straightforward: A larger injection rate contributes to a larger viscosity reduction, a longer pseudo radial flow, and a later appearance of boundary effect.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Petroleum Systems Engineering, University of Regina. xvii, 140 p.
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