基于液膜非线性阻尼作用的裂缝网络内颗粒微观动力学行为研究

To clarify the micro-dynamic behavior of particles is the key to accurately recognize the macroscopic transport and placement of proppants in a fracture of supercritical carbon dioxide fracturing. However, the present numerical simulation based on a dry particle contact model is inefficient to accurately reveal the inherent interaction mechanism. Because particle transport derives from the interactions of solid-liquid and solid-solid, the key forces acting on particles is first investigated. Based on the accurate characterization of non-Newtonian rheological properties of supercritical carbon dioxide fracturing fluid, taking the lubrication resistance of liquid film between solid surfaces and the nonlinear damping effect caused by liquid bridge force into consideration, a wet particle contact model is established to accurately represent solid-solid interaction. According to a two-way coupled CFD-DEM approach, the key processes of proppant transport in a fracture during supercritical carbon dioxide fracturing is studied, the micro-dynamic behavior of particles is investigated and its transport mechanisms are revealed, and the retardation effect of rough fracture walls on particle transport is quantitatively studied. In addition, particle transport in a fracture network is simulated, the diversion mechanisms of solid and liquid phases are investigated, and the diversion behavior is quantitatively represented. This project can develop a prediction theory for quantitatively evaluating the micro-dynamic behavior and macroscopic transport of particles in a fracture, which can provide guidance to supercritical carbon dioxide fracturing.

明确固相颗粒的微观动力学行为是准确认知超临界二氧化碳压裂裂缝内支撑剂宏观运移与铺置的关键，而现有基于干颗粒接触模型的数值模拟难以精确揭示固相颗粒之间内在交互作用机制。基于固相颗粒运移是固-液与固-固交互作用具体体现的事实，本项目以固相颗粒所受关键作用力为研究出发点，在对超临界二氧化碳压裂液非牛顿流变特性精确表征的基础上，考虑固-固表面液膜润滑阻力和液桥力导致的非线性阻尼作用，建立准确描述固-固交互作用机制的湿颗粒接触模型，利用CFD-DEM双向耦合方法模拟超临界二氧化碳压裂裂缝内支撑剂运移，从本质上阐释固相颗粒微观动力学行为，揭示其运移机理，并定量描述粗糙裂缝壁面阻滞运移规律；最终探索固相颗粒在裂缝网络内的运移规律，揭示交叉缝口分流机理，定量描述分流行为。本项目可形成一套定量评价固相颗粒在裂缝内微观动力学行为和宏观运移的预测理论，指导超临界二氧化碳压裂。

超临界二氧化碳压裂过程中，支撑剂在裂缝内的运移规律直接影响压后产能，明确支撑在裂缝内的水动力学行为对于提高超临界二氧化碳压裂的认识具有重要的指导意义。然而，支撑剂运移涉及复杂的固-固和固-液交互作用，压裂液的存在进一步加剧了固-固之间的交互作用，尽管CFD-DEM数值模拟能够精确跟踪计算支撑剂颗粒的运移轨迹，现有软球接触模型难以精确揭示液相环境中固相颗粒之间内在交互作用机制。基于此，本课题首先研究了超临界二氧化碳稠化交联混合体系流变性，优选了Giesekus本构模型对其流变性进行表征。其次，借助自行搭建沉降装置，探索了液相环境中固-固交互作用，明确了间隙流体对固-固交互作用的影响，并通过CFD仿真揭示了沉降颗粒在近壁面附近的水动力学行为。研究表明，当固-固相互逼近时，间隙流体导致两者之间产生非接触作用，壁面粗糙度诱导的刹车效应增强了该非接触作用，此外颗粒周围流场会演变出旋涡，进而加剧间隙流体的作用。对实验结果进行定量表征，建立了作用高度和非接触作用力模型，并将该模型耦合到CFD-DEM数值模拟中，探究了裂缝壁面阻滞的颗粒沉降与运移规律，分析了裂缝壁面粗糙度的影响，定量评价了壁面阻滞作用。随后，探究了三维平板裂缝内支撑剂运移与展布规律，揭示了平板裂缝内支撑剂运移机理，明确了砂堤展布形态，并对其平衡高度进行了定量表征，建立了无量纲平衡高度方程。最后，探索了次级和三级裂缝影响的水力裂缝网络内流量和支撑剂分布规律，明确了交叉缝口处的分流行为，评价了逼近角、注入参数、流体性质等因素的影响。并基于此，开展了裂缝网络内支撑剂运移规律研究，通过实验和CFD-DEM模拟分析了裂缝网络内支撑剂展布形态。总之，本课题通过实验和数值模拟揭示了超临界二氧化碳压裂裂缝内支撑剂颗粒微观动力学行为和宏观展布形态，所得结论有助于提高现场工作人员对超临界二氧化碳压裂过程中支撑剂运移的认识。