裂隙展布特征对EGS采热影响的理论数值模拟研究

增强型地热系统(EGS)储层的裂隙展布特征决定了热开采的效果.基于EGS储层压裂得到的裂隙网络呈现出较强的非均匀性,本文构建EGS平行多裂隙非均匀展布模型研究裂隙展布特征对EGS采热影响.为表征裂隙展布的非均匀性,创新地引入了优势流动比的概念.研究结果表明:在体积为500 m×600 m×600 m,初始温度200℃,均匀激发热储层7条裂隙展布,流量为30 kg/s时,储层产出温度可保持储层初始温度15年,并在热开采进行50年后仍能保持较高产出温度192.3℃,电功率为2.88~3.10 MW,电功率的降幅仅为7.1%.非均匀激发热储层的采热性能受到裂隙数量和裂隙展布特征的共同影响.产出温度与裂隙数量呈不严格的正相关性,并与优势裂隙比呈负相关性.非均匀压裂的热储层中,通过封堵优势裂隙或增强储层压裂使裂隙宽度均匀化,均可增强系统的采热性能.综合来看,裂隙数目越多,裂隙宽度分布越均匀,流体产出温度越高,采热效果越好.本文对EGS地热田的储层激发,和人工热储层的构建有一定的指导意义.

Theoretical numerical simulation of the effect of fracture distribution characteristics on EGS heat extraction

The heat extraction from an Enhanced Geothermal System(EGS) strongly depends on the structure of fracture network of the reservoir. As the strong non-uniform distributed fracture networks are generally stimulated in EGS reservoir, a parallel multi-fracture non-uniformly distributed model is built to study the impact of fracture distribution characteristics on EGS heat extraction. Here, the concept of the preferential flow ratio is introduced to characterize the non-uniformity in fracture width. Theoretical simulation results show that in a reservoir of 500 m× 600 m× 600 m and initial temperature of 200 ℃ with 7 uniformly distributed fractures, with a water production rate of 30 kg/s, the production temperature can maintain at 200 ℃ for the first 15 years and remains at a higher temperature level of 192.3 ℃ in 50 years. The reservoir obtains an electric power of 2.88~3.10 MW and the drop of electric power is estimated to be 7.1%. The sensitivity analysis indicates that the electric power for a non-uniformly stimulated reservoirs, will be co-affected by the number of fracture and the distribution characteristics of the fracture. The production temperature is in a loose positive relation to the number of fractures, and in a negative correlation to the preferential flow ratio. Moreover, it can enhance the heat extraction performance by blocking the preference fracture or narrowing the gap of fracture widths. In conclusion, it is favorable to exploit the heat reservoir with the conditions of sufficient fractures and evenly distributed fracture width.