液态CO2/N2泡沫干法压裂液稳定机理及其携砂和滤失规律研究

Liquid CO2/N2 foam fracturing fluid is a type of water free foam fluid, composed of liquid CO2 as external phase and N2 as disperse phase. Which has an obvious advantage of elimination of formation damage associated with water in fracturing stimulation of unconventional or water-sensitive formation. This project focuses on the stabilizing mechanism, proppant carrying capacity and leakoff mechanism of liquid CO2/N2 foam. The absorption characteristic and stabilizing mechanism of foaming agent on the interface of liquid CO2 and N2 will be studied via molecular simulation. The effects of interfacial viscoelastic modulus, foam texture, temperature and pressure on CO2/N2 foam stability are investigated by interfacial rheology, microfluidics and high pressure stability test, to reveal the mechanism of CO2/N2 foam stability. This project will study the static and dynamic proppant carrying capacity and its effect factors of CO2/N2 foam, investigate the rheological model of CO2/N2 foam, model the transportation and settling of proppant in CO2/N2 foam. Through dynamic leakoff experiment and microfluidics, the project will study the leakoff characteristic of each component of CO2/N2 foam, analyze phase change and its effect in leakoff process, reveal the leakoff mechanism and its controlling factors, model the leakoff of CO2/N2 foam. The key theoretical problems of liquid CO2/N2 foam fracturing will be solved through the research of this project to promote the development of this technology.

液态CO2/N2泡沫干法压裂液是以液态CO2为外相，以N2为内相形成的无水泡沫体系，在非常规油气和水敏性地层的压裂增产中具有显著优势。本项目重点研究液态CO2/N2泡沫稳定机理、携砂规律和滤失机理。通过分子模拟，研究起泡剂在液态CO2和N2液膜上的吸附特征，通过实验探讨界面粘弹模量、泡沫结构、温度、压力等因素对CO2/N2泡沫稳定性的影响，揭示液态CO2/N2泡沫稳定机理；实验研究CO2/N2泡沫静态和动态携砂规律及其影响因素，确定CO2/N2泡沫流变模式，建立模型并分析计算支撑剂在液态CO2/N2泡沫中的输送和沉降规律；通过动态滤失实验和可视微流控实验，研究液态CO2/N2泡沫各组分滤失特征及其影响因素，分析滤失过程中的相态变化及其影响，揭示液态CO2/N2泡沫滤失机理和控制因素，建立滤失模型，分析计算液态CO2/N2泡沫滤失规律。从而解决液态CO2/N2泡沫干法压裂中的关键理论问题。

液态CO2/N2泡沫干法压裂液是以液态CO2为外相，以N2为内相形成的无水泡沫体系，在非常规油气和水敏性地层的压裂增产中具有显著优势。本项目通过实验研究，理论分析和计算等方法，重点研究了CO2/N2泡沫体系稳定机理，携砂规律和滤失机理。研究发现：.1）液态CO2/N2泡沫的稳定性与起泡剂分子在液态CO2/N2界面的吸附情况、液膜的机械强度、液态CO2/N2的界面张力和界面扩张流变性等因素紧密相关。起泡剂氟代丁烷乙基醚（HFE-6）能够吸附到泡沫液膜界面上， 两亲基团定向排列，降低液态CO2/N2体系的界面张力，增强液态CO2/N2泡沫液膜的机械强度，起到稳泡作用；.2）液态CO2/N2泡沫体系表现出剪切稀释特征，其流变模式可看作幂律流体。当泡沫质量低于83.33%时，液态CO2/N2泡沫摩阻系数随泡沫质量增加而升高，当泡沫质量高于83.33%，摩阻系数降低。摩阻系数随压力上升而升高，随温度上升而下降；.3）支撑剂临界沉降速度和临界再悬浮速度随砂比和支撑剂密度的增大而增大，随泡沫质量的增大先减小后上升。压裂液流速越大，砂堤高度越低，并且支撑剂沉降位置离裂缝入口越远。支撑剂密度越大，砂比越大，砂堤高度越高，且主要沉降集中在远离裂缝入口的位置。液态CO2/N2泡沫压裂液粘度随泡沫质量的升高先上升后降低，砂堤高度先降低后上升；.4）与纯液态CO2相比，液态CO2/N2泡沫滤失控制能力良好。滤失压降在2MPa下，泡沫质量升高（28%-80%），滤失系数降低，但继续升高泡沫质量，则不利于滤失的控制。滤失系数随着岩心渗透率增大而升高，当泡沫质量控制在50%-80%时，滤失系数维持在相对较低水平，当渗透率升高约2个数量级时，滤失系数相应增大约1个数量级。随着温度升高，滤失系数增大，特别是当温度高于临界温度后，气液界面逐渐消失，整个泡沫体系趋向于变为一相，泡沫滤失控制能力变差。随着压力的升高（15MPa-25MPa），滤失系数略有增大，但变化幅度相对较小。.本项目研究发展了CO2/N2泡沫干法压裂基础理论，为CO2/N2泡沫干法压裂在油气田高效开发中的应用提供了理论依据