Methane Hydrate Formation in Ulleung Basin Under Conditions of Variable Salinity: Reduced Model and Experiments

Abstract

In this paper, we present a reduced model of methane hydrate formation in variable salinity conditions, with details on the equilibrium phase behavior adapted to a case study from Ulleung Basin. The model simplifies the comprehensive model considered by Liu and Flemings using common assumptions on hydrostatic pressure, geothermal gradient, and phase incompressibility, as well as a simplified phase equilibria model. The two-phase threecomponent model is very robust and efficient as well as amenable to various numerical analyses, yet is capable of simulating realistic cases. We compare various thermodynamic models for equilibria as well as attempt a quantitative explanation for anomalous spikes of salinity observed in Ulleung Basin.

Reduced Numerical Model for Methane Hydrate Formation under Conditions of Variable Salinity. Time-Stepping Variants and Sensitivity

“…These are the fully comprehensive equilibrium models such as [4], and the simpler conceptual models [2,15,16], in which simplified mechanisms for fluid equilibria and/or kinetics were assumed. The model presented in [3] and discussed here falls somewhere inbetween, and is a direct simplification of the comprehensive model in [4]. The simplicity of the reduced model allows for rigorous mathematical well-posedness analysis in the case of the diffusive transport in [17], and more general analysis in [18] for advective/diffusive transport.…”

“…In the paper [3] we introduced a reduced model for methane hydrate formation in variable salinity conditions and provided details on the equilibrium phase behavior adapted to a case study from Ulleung Basin. One of the advantages of this reduced model in contrast to fully comprehensive models such as in, e.g., [4], is that the reduced model is easy to implement and to extend, and is amenable to various analyses.…”

“…The typical time scales of interest for reservoir simulation with these models are days to decades of production or environmental remediation. On the other hand, in [3] and here we are interested in long-term behavior and hydrate basin modeling, and it makes sense to assume that the pressures and temperatures are known and given by hydrostatic and geothermal distributions. Our models need only to resolve the interdependence between methane and water phase equilibria that depend on the presence of salt, and our time-stepping algorithms have different features than those for the oil-gas reservoir simulators.…”

“…The water-methane-salt equilibria are handled using the approach of nonlinear complementarity constraints, and are either tightly or loosely coupled to the salt mass conservation; their implementation is especially easy with the reduced phase behavior model adopted in [3]. We consider and compare three variants of time-stepping that realize these tight or loose couplings: the fully implicit (I), semi-implicit (SI), and sequential (SEQ) algorithms.…”

“…In Section 2 we briefly recall the model proposed in [3] including the phase behavior. In Section 3 we describe in detail the time-stepping variants and spatial discretization for that model.…”