燃爆诱导水力压裂多裂缝耦合起裂规律

Laws of multi-fracture coupling initiation during blasting induced hydraulic fracturing

为了明确多相位裂缝组合下的应力干扰作用及其对后续水力压裂裂缝起裂压力的影响,基于原有预存单条诱导裂缝的应力场计算模型建立预存多条诱导裂缝的耦合应力场计算模型,分析诱导应力影响下的井周周向应力场变化规律,再结合断裂力学判据计算后续水力压裂预存裂缝的起裂压力,并对预存裂缝长度、相位、水平主应力差异系数及预存裂缝条数等4个因素对预存裂缝起裂压力的影响进行了分析。研究结果表明:(1)预存裂缝后近裂缝区域周向应力差明显增大,甚至出现水平主应力反转;(2)随新增预存裂缝长度增加,原预存裂缝起裂压力先增后降,在新增预存裂缝长度增加到原预存裂缝长度时,原预存裂缝起裂压力迅速降低,而后下降趋势变平缓;(3)高相位裂缝的起裂压力大于低相位裂缝的起裂压力;(4)随着裂缝条数的增加,新增预存裂缝起裂压力逐渐降低,但起裂压力差减小的趋势不明显;(5)多相位裂缝的应力干扰作用影响裂缝的起裂压力,较长高相位裂缝和较短低相位裂缝均有利于多相位裂缝的同时起裂;(6)多裂缝同步延伸会产生更为复杂的应力干扰作用,激发水力压裂复杂缝网的演化,实现均衡的压裂改造。结论认为,该研究成果可为燃爆诱导压裂射孔相位、诱导裂缝规模及后续水力压裂施工泵压等压裂施工设计提供理论指导。

For figuring out the stress interference of multiphase fracture combinations and its effect on the fracture initiation pressure of subsequent hydraulic fracturing, a calculation model for the coupled stress field with multiple induced fractures preexisted was estab-lished based on the calculation model for the stress field with single induced fracture preexisted, and the change laws of circumferential stress field around the wellbore under the effect of induced stress were analyzed. Then, the fracture initiation pressure of subsequent hy-draulic fracturing was calculated according to the fracturing mechanics criterion. Finally, the effects of the length, phase, horizontal prin-cipal stress difference coefficient and quantity of preexisting fractures on its initiation pressure were analyzed. And the following research results were obtained. First, the circumferential stress difference in the area near the fractures behind the preexisting fractures increase greatly and even the horizontal principal stress is reversed. Second, as the length of new preexisting fractures increases, the initiation pressure of initial preexisting fractures rises first and then drops. And when the length of new preexisting fractures is equal to that of the initial preexisting fractures, the initiation pressure of initial preexisting fractures drops quickly first and then slowly. Third, the initiation pressure of high phase fractures is lower than that of low phase fractures. Fourth, with the increase of fracture quantity, the initiation pres-sure of new preexisting fractures drops gradually, but the decline trend of initiation pressure difference is not obvious. Fifth, the stress interference of multiphase fractures influences the fracture initiation pressure, and high-phase long fractures and low-phase short frac-tures are both favorable for the simultaneous initiation of multiphase fractures. Sixth, synchronous propagation of multiple fractures can generate more complicated stress interference and excite the evolution of hydraulic fracture network, so as to realize uniform fracturing stimulation. In conclusion, the research results can provide a theoretical guidance for the design of fracturing operation, e.g. perforation phase of blasting induced fracturing, scale of induced fractures, and pump pressure of subsequent hydraulic fracturing.