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Influence of Capillarity Entry Pressure from CO2 Migration in a Fractured Brine Reservoir

Castagna, Marta 1; Becker, Matthew 2
1 Geology, University at Buffalo, Buffalo, NY.
2 Geology, California State University, Long Beach, CA.

The disposal of industrial CO2 into deep reservoirs is a promising technique for reducing the CO2 emissions. The CO2 can be injected at high pressures in deep formations, where the fluid is entrapped in supercritical (liquid) phase and where water is present as brine. Although CO2 injection for sequestration is similar, in practice, to CO2 injection for tertiary oil recovery, CO2 sequestration involves injection of non-wetting phase fluid into a wetting phase fluid. As a result, the capillarity pressure of the injected CO2 must overcome the entry pressure in pore spaces. In this study, we focus on the effect of entry pressures in dual porosity systems. CO2 injected into fractured reservoirs will occupy first the fractures and then the matrix. To transfer fluid from fracture to matrix, the capillarity pressure must overcome the entry pressure of CO2 into the matrix pores. If the entry pressure cannot be overcome, the CO2 phase remains entrapped into the fracture. Laboratory studies from the literature indicate these pressures range between 0.2 and 1 MPa; thus, the hydro fracturing pressure may be exceeded before the entry pressure is overcome in some reservoirs.

The analysis of the effects of the capillarity pressures in the matrix is performed by modeling a CO2 injection in a fracture embedded into a low permeability matrix, with the simulator TOUGH2. The capillarity pressures in the matrix strongly limit the flow from the fracture to the matrix, constricting the flow of CO2 along the fracture. As a consequence, the CO2 plume spreads quickly along the fracture leading to transport CO2 far from the injection well. Transport prediction depends critically upon the geometry and extent of the fractures and could potentially lead to a large area of regulatory review. In the presence or absence of entry pressure CO2 is stored in the matrix. However, in the absence of entry pressure, the majority of matrix storage is in the immiscible gas phase and when entry pressure is significant, the majority of matrix storage is in the dissolved phase. The magnitude of entry pressure, then, has a large impact on the dominant trapping mechanism for the sequestration reservoir. In spite of the apparent influence of CO2 entry pressure at the fracture/matrix interface, few entry pressure measurements or estimates have been made in deep brine reservoirs.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009