Numerical simulation of gas production from Class III hydrate reservoirs using low-frequency electric heating-assisted depressurization with horizontal wells

The global clean energy shortage has become a significant concern along with environmental pollution caused by the burning of conventional fuels. Natural gas hydrates are considered to be a potential source of clean energy with vast resources widely distributed in permafrost and marine sediments. In order to improve the gas pro-duction efficiency from Class III hydrate reservoirs, an innovative production method using low-frequency electric heating-assisted depressurization with the dual-horizontal well mode is first proposed. This method takes advantage of the in-situ heat generated within the hydrate layer through a low-frequency electric field and the wide range of fluid flow and uniform heating of horizontal wells. To better understand the production mechanisms of the proposed method and evaluate its feasibility for increasing gas production, its production performance is analyzed in this work using numerical simulations. Based on the geological data of the Shenhu Area in the South China Sea, a numerical simulation model is first established. Then, the gas production and energy recovery performance are studied. Finally, the influence of production parameters on production per-formance is analyzed. The results show that additional heat input at the early stage of production can have a synergistic effect with depressurization in the gradient decreasing voltage heating mode, thus resulting in better production performance compared to the gradient increasing voltage and constant voltage modes. The maximum energy efficiency ratio can reach 17.65, which implies that the energy utilization efficiency of the proposed method has great potential for hydrate recovery. Horizontal wells have a larger wellbore contact area with the hydrate reservoir compared to vertical wells; therefore, they exhibit an expanded range of depressurization and hydrate dissociation. When the horizontal wells are positioned in the upper part of the hydrate layer, the negative impact caused by gravity can be reduced. Cumulative gas production increases with the initial voltage, but the energy efficiency ratio decreases, highlighting the need for optimizing the initial voltage. The proposed method and the findings of this work provide a useful reference for the efficient development of gas hydrate reservoirs.