煤岩水力压裂裂缝形态实验研究

Experimental Study on the Hydraulic Fractures′ Morphology of Coal Bed

水力裂缝形态的正确判断是压裂施工设计和产能预测的重要部分,针对煤岩进行真三轴水力压裂,研究水平裂缝、垂直裂缝和复杂裂缝之间的转换条件,得出判断依据。实验证实:应力差(最小水平地应力减去垂向应力)为4～6MPa时,水力裂缝形态在垂直裂缝和水平裂缝间转变;在等应力差状态下,高围压状态会使水力裂缝形态趋于复杂;天然裂缝和割理对水力裂缝起裂与延伸过程产生不同影响;煤岩应力状态主导水力裂缝走向,当应力差异系数Kv在0.6～0.7之间时,煤岩内部天然裂缝和割理对水力裂缝形态有显著影响;并且在不考虑井眼附近天然裂缝时,需要苛刻的应力条件使水力裂缝的起裂阶段表现为水平裂缝。

The correct judgment of the hydraulic fractures＇ morphology is very important part for fracturing operation design and productivity prediction. This paper studied the true tri-axial hydraulic fracture experi- ment of coal and obtained the judgment criterions for the conversion conditions between horizontal, vertical and complicated fracture. The result shows that the hydraulic fracturesr morphology will be changed be- tween vertical and horizontal fracture when the stress difference （the minimum horizontal stress minus ver- tical stress） span from 4MPa to 6MPa. High confining pressures will make the hydraulic fractures＇ mor- phology complicated when the stress difference is stable. The nature fractures and cleats have different effects on the initiation and extending process of hydraulic fracture. The stress states of coal bed decide the hydraulic fracture strike. When the stress difference coefficient, i. e. Kv, belongs to 0.6 to 0.7, the nature fractures and cleats have significant influence on the hydraulic fracturesr morphology. From the Kv value, we can know that it need harsh stress condition to make the hydraulic fractures horizontal at the stage of initiation without near wellbore nature fracture.