Effects of Noncondensible Gases on Fluid Recovery in Fractured Geothermal Reservoir
- G.S. Bodvarrson (Lawrence Berkeley Laboratory) | Scott Gaulke (Lawrence Berkeley Laboratory)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- August 1987
- Document Type
- Journal Paper
- 335 - 342
- 1987. Not subject to copyright. This document was prepared by government employees or with government funding that places it in the public domain.
- 4.1.5 Processing Equipment, 5.8.6 Naturally Fractured Reservoir, 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements, 5.9.2 Geothermal Resources, 5.1.1 Exploration, Development, Structural Geology, 5.6.4 Drillstem/Well Testing
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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.
Most geothermal fluids contain significant amounts of noncondensible gases. The concentrations of gases in the steam at separator conditions are typically in the range of 2 to 8 wt%. Generally, CO2 constitutes more than 90% of the gas; other gases include H2S. Gas concentrations often vary considerably across a geothermal field; hence, they can be useful tools in inferring flow patterns within the reservoir. 1 More important, however, the gases patterns within the reservoir. 1 More important, however, the gases can affect the thermodynamic conditions within the reservoir that must be considered when gas-rich systems are evaluated. Various studies have been published on the effects of noncondensible gases on well behavior and fieldwide response to exploitation. Zyvoloski and O'Sullivan and Grant applied reservoir models including CO2 to the Ohaaki reservoir in New Zealand. Atkinson et al. and Pruess et al. applied their models to two gas-rich Italian fields, the Bagnore reservoir and Larderello, respectively. Pritchett et al. and O'Sullivan et al. performed generic Pritchett et al. and O'Sullivan et al. performed generic studies of the effects of CO2 on well testing results and overall reservoir performance. All these studies assumed the porous medium behavior of the reservoir systems considered.
In this paper, we investigate the effects of CO2 during exploitation of fractured geothermal reservoirs. of particular interest are the matrix/fracture flow characteristics in such systems and the overall fluid recovery. Note that gas saturation is defined here as the PV fraction occupied by water vapor and CO2 gas. The numerical studies were performed with the simulator MULKOM and O'Sullivan et al.'s equation of state for H2O/CO2 mixtures. In this work, we neglect effects of gaseous diffusion, capillary pressure, and chemical reactions involving CO2.
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