Evaluation of Fault Reactivation Potential During Offshore Methane Hydrate Production in Nankai Trough, Japan

摘要

Large accumulations of methane hydrates are known to exist in the Nankai Trough located off the southern coast of Japan. Due to its enormous potential as an energy source, there is growing interest in the production of methane gas from these deposits. However, gas hydrate production may cause significant compaction of unconsolidated formations and trigger reactivation of faults during production. Compaction can damage well infrastructure, and reactivation of faults could provide potential leakage pathways for methane gas, which could endanger offshore operations. This paper presents a study in which the potential risk of fault reactivation was evaluated in the Nankai Trough. In the study, laboratory test data obtained from core samples with methane hydrates were analyzed to determine the effect of gas hydrates on the mechanical properties of formations. By integrating log and drilling data, one-dimensional (1D) mechanical earth models were constructed to determine in-situ stresses and mechanical properties along wells in the study area. Subsequently, a three-dimensional (3D) geomechanical model of the field was constructed using Bayesian inversion of seismic and well data. In addition, faults were mapped from 3D seismic data using an advanced automated fault mapping technique. These faults, the mechanical properties obtained from Bayesian inversion, and estimates of post-production gas hydrate saturation and pore pressure generated by a gas hydrate reservoir simulator were incorporated into a 3D finite-element reservoir geomechanics simulator. The potential risk of fault reactivation that may cause production problem such as communication between a reservoir and seafloor was evaluated by analyzing the deformation and slip potential of faults for a given production scenario. This study revealed that a major fault could be reactivated during methane hydrate production if the production well is located close to the fault. The simulations provided critical information pertaining to the selection of production sites and the minimization of potential risks associated with fault reactivation. Additionally, as the prospect of commercial production of methane hydrate becomes more promising, the workflow developed in this project will become one of the key technologies for exploiting this new energy resource in the long term.