Enhancing gas production in low-permeability natural gas hydrate reservoirs by the axial water jet slotting and grouting in vertical wellbores

Natural gas hydrate (NGH) is considered as clean energy in the 21st century, and most of the hydrates in the world exist in low permeability reservoirs. However, the exploitation of low permeability hydrate reservoir is not easy to transport gas and water to wellbore, which seriously restricts the production. To enhance production and increase efficiency, an axial water jet slotting and grouting (AWJSG) technology is put forward for the first time, that is, the near-well reservoir is slotted by high-speed water jet along the axis direction, and the slot is filled with foamed mortar material, forming the high-permeability production channel. In this study, the feasibility of AWJSG is analyzed by a numerical production model based on the NGH reservoir in Shenhu area of South China Sea. The results show that with AWJSG operation, the hydrate dissociation efficiency, the cumulative gas pro-duction, and the average ratio of gas to water production can be increased by 145.0%, 139.2% and 52.4%, respectively, indicating the efficiency of the proposed method. Furthermore, the yield-increasing effect of AWJSG mainly depends on the number, length, width, height, and permeability of slots. For the stratum and well type in this study, the recommended slotting parameters are a number of 4, a length of 5 m, a width of 0.1 m, a height of 15 m, and a permeability of 500 mD, respectively.