非常规致密砂岩油藏超临界CO2驱CO2-原油-微纳米级孔喉缝系统的相互作用机理研究

Supercritical CO2 flooding in unconventional tight sandstone reservoir ischaracterisetd as the complex changeable and irregular effects on the propertiesof crude oil and the system of pore-throat-fracture in formation. In the light of the above problems, being aimed at a typical unconventional tight sandstonereservoir, such a new thinking and methods as the methods of combining multipleexperiment and test measurements, the methods from qualitative analysis to quantitative evaluation, the methods from phenomenon observation to essence revelation, and the methods from data statistics and analyzing to representation model setting up are putforwarded and used to study those problems comprehensively. A lot of typical core samples of reservoir are selected to classify and evaluate the parameters of the pore-throat-fracture system more quantitatively and with different classification level. Through analyzing and comparing the diffusion coefficient, solubility and driven-out -gas-fluid composition differences of CO2 in supercritical CO2 flooding, analyzing the variation of phase state and composition and filtration characteristic in the system of crude-oil and supercritical CO2 in different experimental conditions,and quantitatively evaluating the influences of asphalt deposition and acidization on the pore-throat-fracture system, physical properties and oil displacement effects, the interaction mechanism of CO2-oil-pore-throat-fracture system are worked out and the master factors are also determined. After all, the couple-relationship and characteristic models of the system of CO2-oil-micro-nanometer-pore-throat-fracture which are suitable to unconventional tight sandstone reservoir are established with proper classifications and levels. All those results will be used as the more accurate and scientific foundation in designing program of supercritical CO2 flooding. And there will also be great theoretical and practical significance in a wide application use of supercritical CO2 flooding technology.

针对非常规致密砂岩油藏超临界CO2驱对原油性质、孔喉缝系统影响复杂、多变，且随机性强的问题，以典型非常规致密砂岩油藏为研究对象，提出将多种实验测试方法有效融合，由定性分析到定量评价，由现象观察到本质揭示，由数据统计到表征模型进行综合研究的新思路、新方法。选取典型油藏样品，多手段定量分类分级别评价孔喉缝系统参数。通过对比CO2驱过程中CO2在原油中的扩散系数、溶解度和驱出气液相组分差异，分析不同条件下超临界CO2-原油体系在孔喉缝系统中的相态、组分变化和渗流特征，定量评价沥青沉淀和酸化作用对孔喉缝系统、物性和驱油效果的影响，揭示CO2-原油-孔喉缝系统的相互作用机理，确定主控因素，据此，分类分级别构建适合于非常规致密砂岩油藏的CO2-原油-微纳米级孔喉缝系统的耦合关系及表征模型，对致密砂岩油藏超临界CO2驱技术的应用具有重要的理论价值和实际意义。

CO2驱作为目前提高采收率的主要方法之一，已广泛应用于各类常规油藏，并取得了较好效果。在CO2注入地层驱替原油的过程中会发生一定程度的储层伤害。因此，揭示CO2-原油-微纳米级孔喉系统相互作用机理，对于提高非常规致密砂岩油藏的CO2驱替效果，具有重要意义。本项目针对致密砂岩储层孔喉尺度小、结构复杂、非均质性强的特征，在前期通过常规测试技术和微纳米CT等先进技术对实验岩心进行孔喉结构特征分析后，分类选取岩心进行室内物理模拟实验，确定沥青质沉积量及矿物溶解程度。最后，采用核磁共振技术对比CO2驱替前后岩心孔喉中的流体分布规律，定量评价不同尺度微米、纳米孔喉在驱替实验中的堵塞程度，明确孔喉的堵塞机理。通过对实验结果的分析得到以下结论：.（1）在混相驱阶段，CO2-原油相互作用产生的沥青质沉积量明显高于非混相驱阶段；沥青质沉积量与注入压力及CO2注入量呈正相关，而岩心渗透率伤害率则与沥青质沉积量呈正相关；在CO2-原油体系达到混相前后，沥青质的沉积速率最大。.（2）CO2-原油相互作用造成的孔喉堵塞率随沥青质沉积量的增加而增加，纳米级孔喉的堵塞程度最严重。在驱替初始，纳米级孔喉（10nm~1000nm）与微米级孔喉（1μm~1000μm）均发生少量堵塞；注入压力达到最小混相压力后，CO2与原油形成混相产生大量沥青质沉积，导致纳米孔喉出现55.13%堵塞，微米孔喉堵塞19.58%；在驱替压力继续增大时，纳米孔喉堵塞程度加重，最高可达72.32%。.（3）在CO2-孔喉相互作用实验后，岩心孔喉发生堵塞，纳米级孔喉的堵塞程度较严重，且孔喉堵塞程度与实验时间呈正相关。在驱替时间为60h，纳米级孔喉（100nm~1000nm）有5.58%-7.74%被堵塞；驱替时间为120h，纳米孔喉平均堵塞程度达到30.98%；而孔喉半径在（1μm~100μm）内的微米级孔喉始终未发生明显堵塞，平均堵塞程度3.05%。.（4）CO2-原油-微纳米级孔喉系统相互作用对渗透率低、孔隙结构差的岩心的堵塞程度严重。CO2-原油-孔喉系统相互作用产生的沥青质颗粒和矿物颗粒会对孔喉结构产生严重的堵塞作用，但是由于颗粒尺度有限，对岩心的孔隙体积无较大影响，主要是其桥塞在孔隙喉道的狭窄处导致孔喉堵塞和渗流能力降低，岩心渗透率下降，影响CO2驱替效率及最终原油采收率。