Feasibility of gas production from a gas hydrate accumulation at the UBGH2-6 site of the Ulleung basin in the Korean East Sea

摘要

We investigate the feasibility of production from a marine hydrate accumulation that has the properties and conditions of the UBGH2-6 site at the Ulleung basin in the Korean East Sea. The 20 m-thick system is in deep water (2160 m) but close to the ocean floor (with its top at 140 mbsf), and is characterized by alternating mud (near hydrate-free) and sand (hydrate-rich) layers. The layered stratigraphy and the presence of mud layers preclude the use of horizontal wells and necessitate vertical wells. The analysis indicates that production from such a hydrate accumulation is feasible, but the production rates are generally modest. The production rate Q_p peaks at about 1.45 ST m~3/s=4.4 MMSCFD at about t= 1 year, and continuously declines afterward. Sensitivity analysis indicates that cumulative production increases with a declining initial hydrate saturation, an increasing intrinsic permeability of the sand layers and an increasing thermal conductivity of the porous media, while the effect of porosity is non-monotonic: production initially increases with a decreasing porosity, but the trend is later reversed. However, the sensitivity to these parameters is limited, and does not alter the overall predictions of modest production potential. The geomechanical situation appears challenging, as significant subsidence (exceeding 3.5 m at a depth of 20 m below the sea floor, and 1.5 m at the top of the hydrate deposit) is estimated to occur along a large part of the wellbore, and yielding and failure within the 20 m-thick system are possible early in the production process. However, there is significant uncertainty in the predictions of the geomechanical system behavior because they are not based on measured system properties but only on estimates/assumptions from analogs.