Well-Integrity Evaluation for MethaneHydrate Production in the Deepwater Nankai Trough

Large accumulations of methane hydrates are known to exist in the deepwater Nankai Trough off the southern coast of Japan. Because of its enormous potential as an energy source, there is growing interest in the production of methane gas from these deposits. An offshore production test was scheduled to test the technological viability to produce the methane hydrate with a depressurization method. However, methane- hydrate production may cause large amounts of compaction and subsidence, which can damage well integrity and cause loss of zonal isolation in the overburden. This condition could provide potential leakage pathways for methane gas to the surface and endanger offshore operations. The paper presents a study in which well integrity was evaluated for a methane-hydrate-production test well in the Nankai Trough. In this study, a new work flow was proposed and applied to honor both fieldwide formation heterogeneity and near-wellbore geometry. The geomechanics properties were determined through integration of seismic inversion data, log data, and core data from the Nankai Trough. Interface elements were installed between the casing and cement and the cement and formation to evaluate the impact of cement-bond quality on the well integrity. The numerical simulation was conducted through coupling of the simulation results produced by a methane-hydrate production simulator from a third party with a 3D finite-element geomechanics simulator. The study predicted the occurrence of 2 cm of subsidence on the seafloor generated by approximately 1.5% of reservoir compaction for a production test of 20 days. Over this period, casing would yield with a plastic strain of approximately 1%. Most findings indicated a low risk in the loss of zonal isolation that is consistent with the results of the production test conducted in March 2013. This simulation evaluated potential risks related to well integrity, and provided critical information for the preparation and execution of the pioneering offshore methane-hydrate production test in the Nankai Trough.