分段压裂裂缝扩展规律及其对导流能力的影响

Fracture propagation laws in staged hydraulic fracturing and their effects on fracture conductivities

基于前人的研究并充分考虑裂缝间的相互干扰以及各种摩擦效应对排量分配的影响,提出了模拟水平井分段压裂中裂缝同步扩展的全耦合有限元方法,利用该方法研究了裂缝扩展规律及其对导流能力的影响。模拟结果表明,虽然各裂缝的排量分配率均沿平均裂缝分配率上下波动,但缝间干扰导致裂缝扩展速度各不相同,部分裂缝甚至短暂地停止扩展。短裂缝内的流体净压力普遍比长裂缝高,但压力梯度更小。缝间流体净压力的差异使长裂缝在远离井口的位置发生颈缩,在井口和颈缩处长裂缝可能发生砂堵。分段压裂裂缝扩展规律将对导流能力产生不利影响:(1)短裂缝因支撑剂沉降较快弱化了自身的导流能力;(2)长裂缝因在井口发生砂堵丧失导流能力;(3)长裂缝在颈缩处发生砂堵降低了导流能力。

Completely taking into account the interferences between fractures as well as the friction effects on injection allocations, a fully coupled finite element method inherited from a verified one is proposed to discuss fracture propagation laws and analyze their impacts on fracture conductivities. Simulations show that although fractures have similar injection allocations that fluctuate around the allocation averaged by fractures, interferences between them lead to their different propagation rates and some fractures even stop propagating for a while. Shorter fractures generally have higher pressure and smaller pressure gradients than longer ones. The pressure differences between fractures result in long fractures having bottlenecking zones far away from the wellbore, and make them vulnerable to screen-out at the inlets and the bottlenecking zones. The effects of the propagation laws on fracture conductivities include：（1） the conductivities in short fractures are weakened by rapid proppant settlement in them;（2） long fractures may lost their conductivities due to screen-out near the wellbore;（3） the conductivities in long fractures decrease because of screen-out at the bottlenecking zones.