Effects of Noncondensible Gases on Fluid Recovery in Fractured Geothermal Reservoirs

Abstract

Summary Numerical simulations are performed to investigate the effects of noncondensible gases (CO2) on fluid recovery and matrix depletion in fractured geothermal reservoirs. The model is of a well producing at a constant bottomhole pressure (BHP) from a two-phase fractured reservoir. The recoverable fluid reserves are shown to depend strongly on the amount of CO2 present in the reservoir system. The results obtained revealed a complex fracture/matrix interaction caused by the thermodynamics of H2O/CO2 mixtures. Although the matrix initially contributes fluids (liquid and gas) to the fractures, the flow directions later reverse and the fractures backflow fluids into the matrix. The amount of backflow depends primarily on the flowing gas saturation in the fractures; the lower the flowing gas saturation in the fractures, the more backflow. An analytic expression has been derived that allows for the determination of mass backflow on the basis of wellhead measurements of noncondensible gases and flowing enthalpy.