一种评价页岩储层压裂液吸收的新方法

压裂液在毛细管力作用下自发吸入地层是导致页岩储层体积压裂返排率普遍低于30%的主要原因之一,有效评价页岩储层对压裂液的吸收是体积压裂优化设计、返排制度建立以及水锁伤害程度评估的基础,目前国内外尚未发现有效的评价方法。针对渝东北下寒武统鲁家坪组典型页岩和三种不同的致密储层岩石开展自发渗吸对比实验,并对比研究了3种常用的渗吸表征方法,提出了评价页岩储层压裂液吸收的方法和参数。通过实验研究发现,无因次法和Olafuyi法可用于评价页岩储层压裂液吸收能力,在表征吸收速率和规律方面尚有不足;Makhanov法可用于表征压裂液吸收速率和规律,在评价储层吸收能力方面尚有不足。新方法采用自吸能力、初期自吸速率和后期自吸速率三参数对压裂液的吸收进行定量表征,很好地反映出压裂液吸收规律。压裂液吸收可分为三个阶段:初期渗吸段、过渡段和后期渗吸段,其中初期渗吸段是压裂液吸收的主要阶段;压裂液吸收的驱动力主要为毛细管力和黏土吸附力;黏土含量越高,压裂液自吸能力越强,伊蒙混层和蒙脱石的存在可以大大提高页岩自吸能力。新方法能够评价页岩压裂液吸收的快慢、总量和规律,可以用于不同地层之间的对比评价,且直观可靠,有利于在页岩气或致密气现场推广应用。

A new method for evaluating intake of fracturing fluid in shale formations

The spontaneous imbibition of the fracturing fluid induced by capillary pressure is one of the main causes for the low flow-back rate of multi-stage fracturing in shale formations, which is generally lower than 30%.The assessment of the fracturing fluid intake in shale formations contributes to the optimization of fracturing design, the establishment of flow-back regime and the evaluation of water-blocking damage degree. At present, the evaluation method has not been discovered at home and abroad. The comparative experiments were conducted on the shale from the lower Cambrian Lujiaping formation in north-east Chongqing and three different kinds of tight gas rocks. In addition, three kinds of imbibition characterizing methods are comparatively analyzed. A new evaluation method and parameters were proposed. The results show that Dimensionless method and Olafuyi method are available for the evaluation of the imbibition capacity, but the characterization of imbibition rate and rule are inadequate. The Makhanov method can be used to characterize the imbibition rate and rule, but it cannot evaluate the imbibition capacity. The new method adopted three parameters of the imbibition capacity, imbibition rate and diffusion rate to make quantitative characterization for the intake of fracturing fluid, which clearly reflect the imbibiiton rule. The curve of the fracturing fluid intake can be divided into three regions: linear imbibition region, transition region and diffusion region, among which the imbibition region is the main region. The driving forces for imbibition are capillary pressure and clay absorption force. The imbibition capacity was enhanced by the clay content positively, and the illite and smectite can greatly enhance the imbibition capacity of shale. The new method can evaluate the imbibition rate, amount and rule, and make comparative assessments between different stratums visually and reliably, which is worthy of promotion and application on shale gas and tight gas field.