基于闪电模拟的油藏压裂裂缝网络扩展计算方法

Calculation of reservoir fracture network propagation based on lightning breakdown path simulation

油藏压裂产生高导流能力水力裂缝,并形成复杂次生裂缝网络,为非常规油气藏经济效益开发提供了支撑.描述压裂裂缝网络形态是压裂效果评价及生产动态模拟的基本前提,本文通过分析闪电击穿路径模拟方法,采用相似性原理综合考虑储层地质参数、地应力分布及压裂施工参数,采用周向应力和临界破裂应力确定次生裂缝扩展范围,引入随机函数及分形概率指数表征裂缝扩展随机分布特征,提出了一种新的压裂裂缝网络扩展计算方法.基于该方法分析了地应力差梯度、初始注入压力、分形概率指数等参数对裂缝形态的影响,研究表明地应力差梯度越小、压裂液注入压力越大、分形概率指数越小、压裂液黏度越小,越易形成多分支裂缝网络体系.同时,对具有微地震监测资料的实际生产井压裂缝网形态进行模拟,结果表明本文模拟方法可以精确刻画裂缝网络形态,且与微地震数据点所覆盖范围具有很高的匹配程度.最后,将本文方法应用到某页岩油藏两口井压裂裂缝网络扩展模拟,结果表明反演裂缝形态与微地震数据分布范围较吻合.同时对X2井32段94簇裂缝进行模拟,研究表明该井压裂分支裂缝发育较为明显,整体呈现复杂裂缝网络形态,单翼缝网横向沟通近200 m,纵向沟通近20 m.基于闪电模拟的油藏压裂裂缝网络扩展计算方法可以考虑裂缝扩展方向不确定性以适用储层地质特征认识不清等情况,为水力压裂裂缝扩展模拟提供了新思路.

Hydraulic fracturing produces fractures with high conductivity and forms a complex fracture network,which supports the economic development of unconventional reservoirs.Describing a fracture network is the basic premise for fracturing evaluation and production simulation.This study analyzes the lightning breakdown path simulation method and adopts the principle of similarity to comprehensively consider the reservoir geological parameters,in situ stress distribution,and fracturing construction parameters;the results are used to develop a new fracture simulation method.In the proposed method,circumferential stress and critical fracture stress are used to determine the extension range of secondary fractures,and random function and fractal probability index are introduced to characterize the random distribution of the fracture extension.Then,an actual production well fracture network with microseismic monitoring data is simulated.The results show that the proposed simulation method can accurately describe the fracture network with a high degree of matching in the range covered by the microseismic data.In addition,the influences of the in situ stress difference gradient,initial injection pressure,and fractal probability index on the fracture morphology are analyzed.This research shows that a smaller in situ stress difference gradient relates to greater fracturing fluid injection pressure and smaller fractal probability index and fracturing fluid viscosity,which facilitates the formation of more branch fracture network systems.Finally,the proposed method is applied to the simulation of a fracture network in two wells.The results show that the inverted fracture morphology is consistent with the distribution range of microseismic data.The simulation of 94 clusters of fractures in 32 sections of well X2 shows that the fracture branches of the well are obviously developed,which indicates a generally complex fracture network.The transverse communication of the single wing fracture network is about 200 m,and the longitudinal communication is about 20 m.Therefore,the calculation method of fracture network propagation based on lightning simulation can consider the uncertainty in the fracture propagation direction,particularly when the reservoir geological characteristics are unclear.Moreover,it provides a new method for simulating hydraulic fracture propagation.