Borehole Stability of Natural Gas Hydrate Reservoirs

Abstract The decomposition of natural gas hydrate (NGH) due to long-term drilling may decrease the strength of reservoirs and increase the borehole failure risk. Meanwhile, the creep properties of NGH reservoirs also may give rise to engineering problems including borehole shrinkage and jamming of drilling tools in the late stage of drilling, which seriously impairs the drilling safety. A thermal-fluid-solid multi-field coupling model considering the decomposition kinetics of NGH and creep properties of reservoirs was established to explore the influence mechanism of NGH decomposition and creep properties of hydrate-bearing sediments on mechanical behaviors of boreholes. In addition, influences of drilling parameters and geomechanical properties of reservoirs on borehole stability in the drilling process of NGH reservoirs were analyzed. Research results show that creep properties of NGH deposits causes homogenization of stress distribution in different directions of NGH reservoirs around boreholes, transition of plastic zones around boreholes to a round shape, and possibly overall borehole collapse. As the horizontal geostress becomes more inhomogeneous, the borehole instability risk increases; the higher the initial pore pressure of formation is, the lower the effective geostress and the lower the borehole instability risk. The research results can provide theoretical support for safe drilling of NGH reservoirs in South China Sea and promote the early commercial exploitation of NGH resources in China.