Abstract: Hydraulic fracturing technology has become one of the main stimulation measures for tight oil and gas reservoirs. At present, the investigations on hydraulic fracture morphology are mainly focused on tight sandstone and shale reservoirs; due to late discovery, there is less research on the conglomerate tight oil reservoirs, represented by Mahu oilfield. The conglomerate reservoirs are characterized with a high degree of heterogeneity, and the presence of conglomerates further increases the difficulty in research of the hydraulic fracture morphology. Based on the cohesive zone finite element-based model, the differential characterization of matrix and conglomerate at the scale of large physical simulation experiments has been realized, and the feasibility of the model has been verified by the available experimental results. On this basis, a mechanism study of key factors affecting the propagation of hydraulic fracture in conglomerate reservoirs has been performed, thus clarifying the characteristic behaviors of conglomerate reservoirs, i.e., shown as gravel-bypassing and bifurcation under a low stress difference, and gravel-penetrating and turning under a high stress difference. Based on the mechanism study and the boundary element method, in combination with the on-site fracturing treatment data, finite element mechanistic model results and micro-seismic monitoring data, this study has proved that the macroscopic hydraulic fracture propagation processin conglomerate reservoirs can be simulated using equivalent natural fractures, and also obtained the understanding that the hydraulic fractures can be characterized as bifurcated fractures on a macroscopic scale, and small-scale branched fractures may exist around the main fractures. The accuracy of macroscopic fracture geometry has been verified based on fracture propagation simulations in actual fractured wells.

Key words: conglomerate reservoir, hydraulic fracture, cohesive zone model, boundary element method, multiscale simulation

摘要： 水力压裂技术已成为致密油气储层增产改造的主要措施之一。目前针对水力裂缝形态的研究主要集中在致密砂岩及页岩储层,以玛湖油田为代表的砾岩致密油储层由于其发现时间较晚,相关研究较为缺乏。砾岩储层非均质性较强,砾石的存在进一步加大了水力裂缝形态研究的难度。基于有限元中的内聚力模型实现了大型物理模拟实验尺度下基质和砾石的差异化表征,并通过现有实验结果,验证了模型的可行性。在此基础上,开展了影响砾岩储层裂缝形态扩展关键因素机理研究,明确了储层在低应力差下主要表现为绕砾和分叉,高应力差则以穿砾、转向为主的特征行为。在机理研究的基础上,结合现场压裂施工数据、有限元机理模型结果及微地震监测结果,基于边界元方法,论证了可采用等效天然裂缝的方式对砾岩宏观人工裂缝扩展过程进行模拟,并得出水力裂缝在宏观上以双翼缝特征为主,主缝周边可能存在小尺度分支缝的认识。基于现场压裂井的裂缝扩展模拟,验证了宏观裂缝形态的准确性。

关键词: 砾岩储层, 水力压裂, 内聚力模型, 边界元方法, 多尺度模拟