Unlike intergranular sand methane hydrate (MH), shallow-type methane hydrate (SMH) in Sea of Japan is characterized by the massive state at seafloor and/or the massive interbedding state at the unconsolidated formation down to 100m below seafloor. Therefore, the previous field-tested production methods such as depressurization in MH cannot be applied because of the permeability of sediments and thermal supply for dissociation. It requires developing new production technologies for operation, which would not cause any instability of the formation, when the solid hydrate volumes are removed.
To maximize the recovery with secured operational and environmental safeties threatened by the natures of SMH bearing formation, the understanding of the formation strength is essential. The formation strength is governed by soil and rock mechanics which gradually shift with depth at water saturated formation, and crystalized MH behaves geochemically before dissociation that is governed by geothermal condition and pressure at depth. Geotechnical and geomechanical properties are acquired and compensated by both core analysis and logging which have advantages on more detail laboratory examinations with real samples and on in-situ continuous measurements respectively.
Although some key properties of mechanical studies are derived from acoustic measurements such as dynamic Young modulus, conventional procedures of E&P wireline logging tool was not suitable to acquire dipole shear measurement at unconventional formations based on the investigations of the acquisition results and parameters at Jyoetsu Knoll and Umitaka Spur in 2022. Therefore, new parameters were designed to record longer shear waveforms for the acquisition in 2023 to mitigate the quality risks affected by washouts and relatively shorter recording time length to the slower and more dispersive waveforms.
As the results in the parameter updates, the coverages of shear waveform recording were improved for all receiver stations and enable to extract the slower shear slowness at shallower interval of the new well. Thus, the customized solution proposed in this study achieved to extract the shear slowness around 2,700us/ft up to 33m below seabed under 1000m water column. The shear slowness is around 960us/ft slower and around 12m shallower than the existing well. The missing interval just below seabed had borehole enlargements where the shear signal strength was too weak. Therefore, the acoustic properties such interval need to be compensated by the other measurements like PS logging.
