11:15 AM - 11:30 AM
[SVC31-15] Dynamics of geyser eruption based on a numerical simulation of two-phase flow in a wellbore-aquifer system
Keywords:geyser, numerical simulation
The mechanism of geyser eruptions is frequently elucidated by the following conceptual model: the eruption is triggered by the ascent-driven decompression boiling of high-temperature water flowing into the conduit (e.g., Kieffer, 1984). However, the dynamics of two-phase flow inside the conduit during the eruption remain unclear due to the challenges associated with direct observations. Previous studies have conducted numerical experiments to simulate the periodic eruption of water, employing groundwater flow simulators such as those described by Ingebritsen and Rojstaczer (1996), which approximate conduit flow using Darcy's law. However, in actual geysers, the conduit's shape generally resembles a wellbore, or a fracture with a large diameter. Consequently, Darcian flow, which assumes porous media, is inadequate for describing changes in flow regime resulting from the rapid emergence of the gas phase. Some observational studies have suggested that the presence of dissolved CO2 in the liquid phase affects the conditions of bubble nucleation due to its partial pressure (e.g., Hurwitz et al., 2016; Tsuge et al., 2023). However, the contribution of water boiling and CO2 gas bubbling to the eruption process remains poorly understood. In this study, we performed numerical experiments using a two-phase flow model within the wellbore, taking into account the generation of bubbles from both water and CO2. Our objective is to gain a thorough understanding of the physical processes underlying geyser eruptions.
In this study, we employed the T2Well/ECO2N simulator, a coupled wellbore-aquifer unsteady two-phase flow model (Pan et al., 2011). T2Well treats the wellbore and aquifer as distinct sub-domains and can simulate two-phase flow within each system using appropriate physical laws. The equation of state module, ECO2N, integrated into this simulator, can calculate the physical properties of NaCl-H2O-CO2 fluids within the pressure and temperature range of P <= 60 MPa and 10 <= T <= 110℃. This study developed a numerical domain consisting of a one-dimensional wellbore and a two-dimensional cylindrical aquifer connected to the deeper section of the wellbore. The constant pressure, temperature, and mass fraction of CO2 were assigned at the lateral boundary of the aquifer to represent the water supply condition. Through simulation, we investigated the spatiotemporal variations in the physical parameters.
As a result of the simulation, intermittent discharge driven by the decompression boiling of water and CO2 was observed under the specified conditions. The simulated eruption cycle could be divided into three phases: the recharge phase, where the water level ascends following Darcian flow in the wellbore; the overflow phase, where the liquid-dominated water discharges slightly; and the eruption phase, where the discharge rate increases rapidly accompanied by an increase in the gas phase. The characteristics of these phases were consistent with those commonly observed at geysers. CO2 dissolved in the liquid phase induced boiling at deeper depths due to its partial pressure but contributed less to explosiveness. Moreover, based on the spatiotemporal variations in each physical parameter obtained from the simulation, we proposed a conceptual model of the eruption process: a self-enhancing process in which boiling is intensified in shallow depths with high temperature gradients, and a self-limiting process in which boiling is suppressed in deeper depths with low temperature gradients, governing both the initiation and termination of the eruption. Although our simulation employed simplified conduit geometry and was conducted under limited pressure and temperature conditions, it offers valuable insights into the dynamics of geyser eruption.
In this study, we employed the T2Well/ECO2N simulator, a coupled wellbore-aquifer unsteady two-phase flow model (Pan et al., 2011). T2Well treats the wellbore and aquifer as distinct sub-domains and can simulate two-phase flow within each system using appropriate physical laws. The equation of state module, ECO2N, integrated into this simulator, can calculate the physical properties of NaCl-H2O-CO2 fluids within the pressure and temperature range of P <= 60 MPa and 10 <= T <= 110℃. This study developed a numerical domain consisting of a one-dimensional wellbore and a two-dimensional cylindrical aquifer connected to the deeper section of the wellbore. The constant pressure, temperature, and mass fraction of CO2 were assigned at the lateral boundary of the aquifer to represent the water supply condition. Through simulation, we investigated the spatiotemporal variations in the physical parameters.
As a result of the simulation, intermittent discharge driven by the decompression boiling of water and CO2 was observed under the specified conditions. The simulated eruption cycle could be divided into three phases: the recharge phase, where the water level ascends following Darcian flow in the wellbore; the overflow phase, where the liquid-dominated water discharges slightly; and the eruption phase, where the discharge rate increases rapidly accompanied by an increase in the gas phase. The characteristics of these phases were consistent with those commonly observed at geysers. CO2 dissolved in the liquid phase induced boiling at deeper depths due to its partial pressure but contributed less to explosiveness. Moreover, based on the spatiotemporal variations in each physical parameter obtained from the simulation, we proposed a conceptual model of the eruption process: a self-enhancing process in which boiling is intensified in shallow depths with high temperature gradients, and a self-limiting process in which boiling is suppressed in deeper depths with low temperature gradients, governing both the initiation and termination of the eruption. Although our simulation employed simplified conduit geometry and was conducted under limited pressure and temperature conditions, it offers valuable insights into the dynamics of geyser eruption.