硼交联胍胶压裂液高压流变特性及其非氧化破胶方法

High pressure rheological characteristics of boron cross-linked guar fracturing fluid and its non-oxidative gel-breaking method

高压流变性有助于全面认识压裂裂缝内压裂液黏度变化的真实特征。应用流变学方法分析硼交联胍胶压裂液高压流变特征,考察温度、剪切速率对压裂液高压流变特性的影响,提出一种基于高压流变特性的非氧化破胶方法,以破胶液黏度为指标优选破胶剂使用质量分数;结合核磁共振硼谱(^(11)B NMR)、红外光谱(FT-IR)以及透射电镜(TEM)等手段,分析压力作用下硼交联胍胶压裂液黏度变化机制及破胶液微观形态。结果表明:高压作用下硼酸根离子与胍胶分子形成的B—O—C共价键及胍胶分子与水分子之间的氢键断裂导致压裂液内部网状结构及胍胶分子聚集体被破坏,体系黏度呈现出可逆的“压力稀释”效应,60 MPa压力下黏度损失率最高可达91.4%,且随温度与剪切速率上升,体系黏度“压力稀释”效应逐渐减弱;多羟基胺NMGA-R可促使硼交联压裂液压力释放后黏度不能恢复而实现破胶,破胶液中胍胶呈较大的连续絮状体,分子结构较为完整,回收破胶液配制压裂液80℃剪切1 h,表观黏度超过80 mPa·s,为硼交联胍胶压裂液组分再利用提供了试验依据。

It is essential to understand the rheological properties of fracturing fluid at high-pressure and temperature conditions for proppant transportation.In this study,the variations of the viscosity of boron cross-linked guar fracturing fluid under high pressure and the effects of temperature and shear rate were investigated using a rheology method.A non-oxidative gel-breaking method was proposed and the gel mass concentration of the breaker was optimized with the viscosity of the gel-breaking fluid as an evaluation index.Combining with the nuclear magnetic resonance boron spectrum(^(11)B NMR),infrared spectroscopy(FT-IR)and transmission electron microscopy(TEM),the viscosity change mechanisms of the boron cross-linked guar fracturing fluid at high pressure and the micromorphology of gel-breaking fluid were observed and analyzed.The results show that the breakage of B-O-C covalent bonds and the hydrogen bonds among borate ions,guar gum and water at high pressure can lead to the destruction of the internal network structure and the guar gum molecular aggregates of the fracture fluid,demonstrating versible"pressure dilution"effect on fluid viscosity under high pressure.The reduction rate of viscosity can reach to 91.4%at 60 MPa.With the increase of temperature and shear rate,the "pressure dilution" effect of the fracturing fluid decreases gradually.Polyhydroxylamine NMGA-R can make the viscosity of the fracturing fluid unrecoverable after pressure release,resulting in an actual gel breaking.The micromorphology of the guar gum in the gel-breaking solution was observed as large continuous floccules with relatively complete molecular structures.The apparent viscosity of the cross-linked fracturing fluid prepared with the recovered fluid after gel breaking can be over 80 mPa•s at 80℃after 1h of shearing,indicating the feasibility for the reuse of the boron cross-linked fracturing fluid components after gel breaking.