砂砾岩储层水力裂缝扩展规律试验研究

由于砂砾岩储层中砾石的存在,导致压裂工程中水力裂缝扩展形态复杂,施工难度大。为了研究砂砾岩储层水力裂缝扩展规律,采用真三轴压裂模拟试验系统对天然砂砾岩露头岩样开展试验研究,分析水平应力差、砾石粒径和分布规律以及压裂液黏度和排量对水力裂缝扩展形态的影响。研究表明:砂砾岩储层的压裂裂缝形态由应力状态和砾石特征共同决定。以小粒径砾石为主的岩样中,砾石对裂缝扩展形态的影响较小,形成单一主裂缝,裂缝面平直;大粒径砾石为主的岩样中,水力裂缝沿着砾石界面偏转并形成分支裂缝,裂缝面扭曲。水平应力差较低、砾石无规则杂乱排布的情况下,分支裂缝的偏转更加明显,裂缝形态更加复杂。同时,水力裂缝由砾石内部起裂时的破裂压力更高。分支裂缝的产生以及裂缝的转向扩展导致裂缝宽度变窄;采用高黏度的胍胶压裂液能够有效增加裂缝宽度,有助于加砂和避免砂堵。

Experimental study on the propagation mechanism of hydraulic fracture in glutenite formations

Due to the existence of gravels in a glutenite formation，the geometry of hydraulic fracture becomes more complex，which makes the stimulation treatment difficult. In order to investigate the propagation mechanism of hydraulic fracture，laboratory experiments were performed on six natural glutenite samples，using a large scale tri-axial fracturing simulation system. The influences of the horizontal stress difference，gravel size and distribution，fracturing fluid viscosity and pumping rate on the fracture propagation were analyzed. The experimental results show that a single planar hydraulic fracture was created when the small gravels dominated in the glutenite sample. This indicates that small gravels have no obvious influence on the fracture propagation. By contrast，the hydraulic fracture was deflected along the interface of gravels and resulted in a complex fracture geometry with multiple tortuous branches in samples with large gravels. This phenomena is more distinctive when a sample with randomly distributed large gravels was hydraulically fractured under a lower horizontal stress difference. Additionally，a higher breakdown pressure was observed in the injection pressure curve when the hydraulic fracture initiated within a high-strength gravel. The fracture branching or deflecting is likely to reduce the fracture width. The fracture width was effectively increased when the high-viscosity fracturing fluid such as guar gel was used which is helpful for preventing sand plug.