页岩层理对压裂裂缝垂向扩展机制研究

RESEARCH ON INFLUENCE OF SHALE BEDDING TO VERTICAL EXTENSION MECHANISM OF HYDRAULIC FRACTURE

文章采用页岩露头开展了大型三维压裂裂缝物理模拟研究,考察了围压、排量、压裂液黏度等参数对水力裂缝垂向扩展的影响,综合分析了水力裂缝尖端应力场和远场应力作用在层理面上的应力场,建立了判断水力裂缝是否穿透层理面的穿透准则。研究表明,不同于常规砂岩储层,层状页岩压后裂缝形态复杂,水力裂缝极易沟通层理形成纺锤状裂缝网络;地应力状态是影响水力裂缝是否穿透层理的最关键因素,水平地应力差值存在一个临界值,小于该临界值不能穿透层理面;提高压裂液黏度及排量能够增加水力裂缝穿透层理面的几率,黏度过高会降低裂缝沟通体积。基于文章研究结果,取得了规律性认识,为水平井眼轨迹的部署、压裂层段的选择、压裂优化设计和压裂现场质量控制等提供了重要依据。

North America has made the success in shale gas development with the help of fracturing technology,which has greatly stimulated the enthusiasm of shale gas exploration and development in China. A national shale gas demonstration area has been especially established in China. The shale in the demonstration area shows highly developed bedding. Problems like whether the bedding can influence vertical extension of hydraulic fractures or not and what reservoir conditions and controllable parameters can impact vertical extension of hydraulic fractures are critical to the shale fracturing job. In this paper,outcrop cores in the shale state demonstration area were used to carry on large-size tri-axial hydraulic fracturing experiment,the influence of confining pressure,frac pump displacement,fracturing fluid viscosity and so on to vertical extension mechanism of hydraulic fractures was investigated. The tip stress field of hydraulic fracture and the in-situ stress field impacted on the bedding were analyzed comprehensively,and a criterion was proposed to identify whether the hydraulic fractures had penetrated through the natural fractures. The study result indicate that the geometry after fracturing is complex, the hydraulic fractures tends to communicate with the beddings to form a spindle-shaped fracture network,which is different from the geometry formed in conventional sandstone reservoir. In-situ stress condition is the most key factor influencing the penetration of hydraulic fracture through the beddings,there exists a critical value for in-situ stress to penetrate shale bedding,the hydraulic fractures can not penetrate the bedding if in-situ stress under the critical value.Higher fracturing fluid viscosity and displacement can contribute to vertical extension. However,excessive fracturing fluid viscosity will limit hydraulic fracture propagation. Based on the result of this paper,some understanding of the rules has been obtained,which provide an important basis for s horizontal well track planning,fracturing zone determination,fracturing optimization design and fracturing quality control.