纤维暂堵人工裂缝附加压差影响因素分析

地层温度条件下自动可降解纤维能有效暂堵已形成的人工裂缝或天然裂缝,大幅提高缝内净压力,从而迫使人工裂缝发生转向,从其他方向启裂与扩展,增加油气藏改造体积。此时纤维在裂缝内形成滤饼填充带,形成一附加压差。理解纤维暂堵人工裂缝附加压差的影响因素,是优化纤维暂堵转向压裂设计关键参数的基础。由于缝内纤维滤饼引起附加压差影响因素众多,纤维转向压裂关键参数优化难度较大,本研究以纤维暂堵人工裂缝的物理模型为基础,利用经典水力压裂理论,推导并求解纤维滤饼附加压差的数学模型,模拟计算其影响因素的变化规律。模拟结果表明,较低排量、较小裂缝宽度和较高黏度有利于增大附加压差,加强纤维封堵裂缝的效果。结合裂缝延伸准则,得出井底压力增量与附加纤维滤饼长度两者成线性增加关系。

Analysis of factors influencing additional pressure drop of fiber-assisted temporary blocking of an artificial fracture

An automatically degradable fiber at reservoir temperature could be used to temporarily block artificial or natural fractures effectively. After injecting diverting clean fiber into the fracture, net pressure in the blocked fracture will be remarkably increased. Then it forces the artificial fracture to re-orientate, i.e. fractures are initiated and propagate in another direction. This stimulation technology will improve the stimulated volume of hydrocarbon reservoirs. In the blocking process, the fiber cake zone will be formed in the fracture. It will produce an additional pressure drop when hydraulic fracturing fluid is injected through the blocked fracture. It is the basis for optimizing the critical parameters of design program of fiber-assisted diverting fracturing to understand the influential factors of additional pressure drop induced by the fiber cake. Because additional pressure drop is related with many factors, it is very difficult to optimize the critical parameters of fiber-assisted diverting fracturing. In this paper, on the basis of the physical model of the blocking, the mathematical model of additional pressure drop induced by fiber cake is deduced and calculated by adopting the classical theory of hydraulic fracturing. The change rule of its influential factors is numerically simulated. The results show that low injection rate, small fracture width and high viscosity will be beneficial to increase the additional pressure drop and improve the temporary blocking effect. According to the propagation criterion of tensile fracture, the relationship between the bottom hole pressure increment and the required length of additional fiber is obtained. Excellent linearity between them is achieved.