Numerical experiments of geyser eruption caused by ascent-driven decompression boiling, using the wellbore-reservoir simulator T2Well/ECO2N

The eruption processes of geysers, particularly those involving a two-phase flow of water and vapor, remain insufficiently understood due to the inherent difficulties in conducting direct observations within the conduit. To address this, we conducted numerical experiments focusing on geysers caused by ascent-driven decompression boiling, using the T2Well/ECO2N wellbore-reservoir coupled two-phase flow simulator. Our simulations successfully reproduced the periodic eruption driven by the decompression boiling of water and CO₂ under specific conditions. The typical characteristics of the simulated eruption cycles were consistent with those commonly observed in natural geysers situated in and around volcanic fields. The CO₂ dissolved in water initiated boiling at deeper depths due to its partial pressure, though it had a limited impact on the explosivity of the eruptions. Sensitivity tests, conducted by varying conditions within the aquifer, reservoir, and atmosphere indicated that permeability and hydraulic head control the average flow rate to the wellbore. In contrast, factors such as the CO₂ mass fraction in the liquid phase, temperature, and barometric pressure significantly influence the boiling process during eruptions. Furthermore, we described a physical mechanism underlying the eruption process based on the spatiotemporal variation in physical parameters within the conduit. Specifically, the initiation and termination of eruptions may be governed by a self-enhancing process occurring at shallow depths with large temperature gradients and a self-limiting process at greater depths with small temperature gradients. Although we assumed simplified geometry and used limited thermodynamic conditions, our results may provide valuable insights into the dynamics of geyser eruptions.