南海荔湾区域水合物水平井降压开采模拟研究

南海荔湾区域赋存有大量的天然气水合物资源,储层具有薄层和厚层水合物互层分布特征,极具开发潜力。因此,基于该区域多层水合物地质资料,利用TOUGH+HYDRATE建立了该区域水平井降压开采数值模型,井底开采压力取值为0.2倍的第三层水合物底部初始压力,主要研究了储层渗透率对水合物开采的影响规律。结果表明:降压法能够有效促进井周水合物分解,但受到地层低渗透率的制约,水合物有效分解范围主要局限于井周有限的圆形区域内,且在分解前缘面附近易形成少量的二次水合物;增大地层渗透率对水合物增产效果影响最为显著,当地层渗透率增大至20 mD时,水平井布设在最下部水合物层中间时,可以同时开采三层水合物,从而极大提高了开采效率,但同时也会导致地层流体涌入井眼,产水量急剧增大。

Numerical simulation on depressurization-induced gas production from hydrate reservoirs in the Li Wan area, South China Sea using horizontal well

According to the geological data of hydrate reservoirs in the Li Wan area, South China Sea, a production model using horizontal well by depressurization was established in TOUGH+ HYDRATE. The bottom hole pressure is assigned to be 0.2 times the initial pressure of the third hydrate layer. This paper mainly investigates the effects of reservoir permeability on gas recovery behaviors. The results show that: (1) depressurization can effectively promote hydrate dissociation around the well; however, the hydrate dissociation range is primarily distributed in the vicinity of the wellbore, which is dependent on the low formation permeability. In addition, a small amount of secondary hydrate forming near the dissociation front can be observed. (2) Increasing the formation permeability presents the most significant enhancement effect on gas recovery. When the formation permeability increases to 20 mD, the horizontal well deployed in the middle position of the lower hydrate layer is able to extract all hydrates in each layer simultaneously, which sharply improves the production efficiency. However, the formation fluid can also invade into the borehole, resulting in the dramatic increase in water production.