The commom occurrence of massive methane hydrate in numerous gas chimney structures, located in Joetsu basin, Sea of Japan, stimulates great interest from by academia, industry and national institutes in developing technologies to produce the potential energy resource. Unlike other deep methane hydrate deposited in formations a few hundred meters below seafloor, the hydrate chimney structures are located at the seafloor or up to 100 m below the seafloor; therefore, previously field-tested production methods such as depressurization are not applicable. The newer production method of jetting from the openhole section of a wellbore to excavate the hydrate bearing formation was proposed as a possible production method. However, jetting will create large empty chambers below the seafloor and could possibly jeopardize the stability and safety of well-heads and the production facility on the seafloor.
This paper presents a 3D geomechanical simulation study to evaluate the feasibility of the jetting method to produce methane from the hydrate chimneys in the Sea of Japan. In this study, to honor lateral and vertical variation of hydrate saturation as well as mechanical properties, different 3D geomechanical models were constructed to represent three shallow methane hydrate inhabitation types (chunk, laminated, and dispersed) by using data from various sources. Dynamic numerical simulation by using a 3D finite element simulator was conducted to simulate the jetting process to excavate 16-m diameter chamber from bottom of the borehole (about 100 m below seafloor) progressively up to the bottom of the conductor of the wellbore, about 10-m below the seafloor.
The numerical simulation shows that jetting is likely to be feasible as all simulation cases resulted in tolerable vertical displacement and equivalent plastic strain under ideal conditions, e.g., lateral homogeneous formation, constant chamber pressure (equal to formation pore pressure) and blowout preventer (BOP) weight of 20 t. In these cases, the plastic zone only extends to limited area from the sidewall. Additional complexities were considered in the numerical simulation to evaluate the operational risks during actual jetting operations, such as faulting, fluctuation of chamber pressure, and change of BOP weights.
This numerical simulation evaluated potential risks related to jetting operations of hydrate chimneys in the Sea of Japan and provided critical information for the engineering design of the proposed field test of jetting operations to produce this valuable resource in the Sea of Japan.