Integrated Carbon Sequestration-Geothermal Heat Recovery: Performance Comparison Between Open and Close Systems

In this paper, the lateral boundaries for a theoretical homogeneous, isotropic, horizontal, 3-dimensional sedimentary hot aquifer were prescribed by either unhindered open to fluid and heat flow, or compartmentalised fully close boundary conditions. CO2 injection was administered through three well patterns of varying well spacing and numbers: 3x3 (5 injection wells and 4 production wells, 750-m well spacing), 5x5 (13 injection wells and 12 production wells, 500-m well spacing), and 7x7 (25 injection wells and 24 production wells, 375-m well spacing). To assess the effects of boundary conditions and well spacing on a combined carbon sequestration-geothermal heat recovery process, number of output metrics such as net cumulative CO2 stored, volumetric storage efficiency, geothermal heat recovery and pressure build-up were quantified in 54 simulation runs. Based on the assumption made and conditions that the simulation, it was shown that for a typical and more common fewer well number cases, the open versus close boundaries have critical role in determining the effectiveness of operation. (a) Open boundary conditions may counter-intuitively lead to heat recovery relative deficiencies if the pressure gradient stays largely towards the outside of the system (pressure of system>pressure of surrounding). This, however, will benefit CO2 injectivity and storage efficiency by providing escape paths for fluids (CO2 and brine). (b) Close boundary conditions will universally be beneficial for fluid and heat production purposes; however, the excessive pressure build-up can significantly affect the operation duration length. For economically unjustifiable cases with high number of wells, the effects of boundary conditions were shown to be reduced by effective fluid extraction from the medium, thereby enhancing the geothermal heat recovery and avoiding pressure build-up. The research delineated no significant effect of boundary conditions on salt precipitation.