Evaluation of Fault Re-activation Potential during Offshore Methane Hydrate Production in Nankai Trough, Japan

Abstract

Abstract Large accumulations of methane hydrates are known to exist in the Nankai Troughlocated off the southern coast of Japan. Due to its enormous potential as anenergy source, there is growing interest in the production of methane gas fromthese deposits. However, gas hydrate production may cause significantcompaction of unconsolidated formations and trigger reactivation of faultsduring production. Compaction can damage well infrastructure, and reactivationof faults could provide potential leakage pathways for methane gas, which couldendanger offshore operations. This paper presents a study in which the potential risk of fault reactivationwas evaluated in the Nankai Trough. In the study, laboratory test data obtainedfrom core samples with methane hydrates were analyzed to determine the effectof gas hydrates on the mechanical properties of formations. By integrating logand drilling data, one-dimensional (1D) mechanical earth models wereconstructed to determine in-situ stresses and mechanical properties along wellsin the study area. Subsequently, a three-dimensional (3D) geomechanical modelof the field was constructed using Bayesian inversion of seismic and well data. In addition, faults were mapped from 3D seismic data using an advancedautomated fault mapping technique. These faults, the mechanical propertiesobtained from Bayesian inversion, and estimates of post-production gas hydratesaturation and pore pressure generated by a gas hydrate reservoir simulatorwere incorporated into a 3D finite-element reservoir geomechanics simulator. The potential risk of fault reactivation that may cause production problem suchas communication between a reservoir and seafloor was evaluated by analyzingthe deformation and slip potential of faults for a given productionscenario. This study revealed that a major fault could be reactivated during methanehydrate production if the production well is located close to the fault. Thesimulations provided critical information pertaining to the selection ofproduction sites and the minimization of potential risks associated with faultreactivation. Additionally, as the prospect of commercial production of methanehydrate becomes more promising, the workflow developed in this project willbecome one of the key technologies for exploiting this new energy resource inthe long term. Introduction Methane hydrates are methane-bearing, ice-like materials that occur inabundance in permafrost areas and in offshore continental margin environmentsthroughout the world (Collett et al., 2009). The estimated methane hydrateaccumulation worldwide is 700,000 Tcf or about twice the total carbon in coal, oil, and conventional gas in the world (Thomas, 2001). In Japan, there is alarge amount of methane hydrate accumulated in the Nankai Trough (Tsuji et al.,2009) that is of great strategic importance to Japan. Japan Oil, Gas and MetalsNational Corporation (JOGMEC) is planning to conduct the first offshoreproduction test in 2013 to explore the technical viability of producing methanehydrate offshore. One major obstacle is the operational risk associated withborehole failure, completion failure, and loss of well integrity, and faultreactivation that could lead to gas leakage to the surface (Yamamoto, 2008). Topave the way to successful production of methane hydrates offshore, anintegrated reservoir geomechanics study was carried out for two sites (Alphaand Beta) located in the Nankai Trough to assess these risks.