Reservoir stability in the process of natural gas hydrate production by depressurization in the shenhu area of the south China sea

Abstract

Reservoir stability is a key factor in the production of natural gas hydrate (NGH), and also a prerequisite to ensuring safe and efficient NGH production. However, it has been rarely discussed. To analyze the reservoir stability in the process of NGH production by depressurization in the Shenhu area of the South China Sea, we established a 3D geological model of NGH production by depressurization on the basis of NGH drilling data in this area, which was then discretized by means of nonstructural grid. Then, the mathematical model coupling four fields (i.e. thermal, hydraulic, solid and chemical) was established considering the heat and mass transfer process and sediment transformation process during NGH production. The model was solved by the finite element method together with the nonstructural grid technology, and thus the time-space evolution characteristics of reservoir pore pressure, temperature, NGH saturation and stress in the condition of NGH production by depressurization were determined. Finally, reservoir subsidence, stress distribution and stability in the process of NGH production by depressurization in the Shenhu area were analyzed. The results obtained are as follows. First, the higher the reservoir permeability and the larger the bottomhole pressure drop amplitude are, the larger the subsidence amount and the higher the subsiding speed. Second, as the reservoir pore pressure decreases in the process of production, the effective stress increases and the shear stress near the well increases obviously, resulting in shear damage easily. Third, the increase of effective reservoir stress leads to reservoir subsidence, which mainly occurs in the early stage of NGH production. After the production for 60 days, the maximum reservoir subsidence reached 32 mm and the maximum subsidence of seabed surface was 14 mm. In conclusion, the NGH reservoirs in the Shenhu area of the South China Sea are of low permeability and the effect range of reservoir pressure drop is limited, so the reservoirs would not suffer from shear damage in the sixty-day-production period. Keywords: South China sea, Shenhu area, Natural gas hydrate (NGH), Natural gas hydrate production by depressurization, Effective stress, Reservoir stability, Multi-field coupling numerical simulation