水力裂缝与天然裂缝相交作用准则研究

Research on the Acting Criteria When Hydraulic Fracture Intersects with Natural Fracture

从弹性力学理论出发,建立水力裂缝与天然裂缝相交作用准则。该准则涵盖了水力裂缝与天然裂缝相交作用下的各种模式,以及相交点流体压力从小到大变化过程中可能出现的各种破裂方式。利用该准则,通过模拟计算进行定量分析,探讨了水力裂缝与天然裂缝相交的剪切破裂机制:1在较小范围的低逼近角区域和较大范围的高逼近角区域内,天然裂缝将不发生剪切破裂;2在发生剪切破裂的逼近角范围内,逼近角越大,剪切破裂发生具有"难—易—难"的趋势;3存在一个最小临界剪切破裂应力差,当实际水平应力差小于该值时,无论任何逼近角下,天然裂缝都不发生剪切破裂;4天然裂缝内聚力和内摩擦系数越大,发生剪切破裂的逼近角范围越小;5在低逼近角情况下,无论是高水平应力差还是低水平应力差,水力裂缝都将沿天然裂缝发生转向延伸;6在高逼近角情况下,当应力差大于岩石的抗张强度时,高应力差有利于穿过延伸发生;7在高逼近角情况下,当应力差小于岩石的抗张强度时,水力裂缝在天然裂缝处于张开状态下才会直接穿过天然裂缝延伸;8在应力差极小的情况下,无论为任何逼近角,都可能将会发生穿过和转向的混合模式;9天然裂缝内流体压降对转向延伸和穿过延伸的交界逼近角具有重要影响。

Acting criteria when hydraulic fracture intersects with natural fracture is established based on the elasticity theory.Each mode of the intersection acting and every failure mode caused by fluid pressure change at intersection point are all known in these criteria.Mter the criteria are applied,quantitative analysis may be made through analog calculation and shear fracture mechanism of the intersection is discussed： （1） the shear fracture will not appear in the natural fractures in the region of small-range of low approximating angle and large-range of high approximating angle ;（2） at the range of the approximating angle of the shear fracture ,the trend of the shear fracture is from difficult to easy and then to difficult at the larger angle; （3） a minimum differential stress of critical shear fracture exists ,and the shear fracture will not occur in the natural fracture at any approximating angle when the actual horizontal differential stress is less than this value; （4） larger co- hesive force and coefficient of internal friction factor of the natural fracture will bring smaller range of the approximating angle of the shear fracture;（5） the hydraulic fracture will cross over and extend along with the natural fracture at either high horizontal differential stress or low horizontal differential stress on the condition of low approximating angle; （6） high differential stress will be favorable to occur across the extension when the differential stress is larger than the tensile strength of the rocks on the condition of high approximating angle; （7） the hydraulic fracture will extend across the natural fracture while the natural fracture is open and the differential stress is less than the tensile strength of the rocks on the condition of high approximating angle; （8） mixed pattern of crossing and turning around will occur at any approximating angle when the dif- ferential stress is very little; （9） the fluid pressure drop in the natural fracture will have important impact on the interface approximating angle between the extension of crossing and turning around.