CO₂封存是实现“双碳目标”、抵消碳排放量的有效途径。CO₂在储层中的封存为一热门话题，CO₂在储层中的注入有效地解决了碳排放的问题。同时，CO₂的注入也会导致储层物性的改变，从而降低储层的封存效果。砂岩作为一种常见的储层沉积岩，其驱替过程对研究气体与液体的驱替特性及储层物性的改变有重要意义。本文对层理砂岩进行不同相态CO₂驱替实验，通过改变驱替过程中的驱替压力及驱替温度等因素，观测驱替过程中样品的核磁共振T₂谱、分层含水率、核磁共振成像结果，明确驱替过程中样品内部的孔隙水运移规律，结合驱替前后孔隙率、渗透率和核磁共振T₂谱等测试结果，阐明CO₂驱替对于砂岩孔隙结构改造的机制。此外，结合毛细渗吸实验过程的参数观测计算样品的渗透率，评价砂岩内部孔隙的连通性。研究结果表明：

       （1）毛细渗吸过程中，吸水质量、吸水率及吸水锋面高度等变化随吸水时间的增加划分为不同的阶段。结合水以水汽的形式扩散并吸附在孔隙表面，其到达稳定状态的时间远快于吸水质量稳定时间，毛细渗吸主要以毛细水迁移为主；根据分层含水率测试结果将毛细渗吸过程划分为水分运移阶段和水分分布稳定阶段两个阶段，且由不同层理样品由分层含水率得到的吸水锋面高度与实测吸水锋面并无显著差异。结合毛细渗吸过程的参数观测，平行层理砂岩的渗透率计算值略低于垂直层理砂岩。

       （2）在气态CO₂驱替过程中，驱替效率随驱替时间的增加而不断增加。平行层理砂岩T₂谱信号幅值的演化规律较垂直层理砂岩更加显著，且信号幅值以毛细水和自由水的信号损失为主；样品的分层含水率信号幅值随驱替时间和驱替压力的增加而不断降低，及锋面后退距离不断增加；核磁共振成像结果表明在层理因素的影响下气态CO₂驱替的水分运移规律并无显著差异。

       （3）在超临界CO₂驱替过程中（改变温度），随着驱替温度的增加，不同层理样品的核磁共振T₂谱信号幅值均呈现下降趋势，以毛细水含量的显著下降为主；分层含水率信号幅值及成像结果的变化规律与样品的层理结构有显著关联，层理砂岩内部水分沿层理方向逸散，锋面后退距离不断增加，垂直层理样品的锋面后退距离可到达样品高度，样品视为干燥。驱替结束，平行层理样品的剩余水饱和度是垂直层理样品的1.99倍。

       （4）在超临界CO₂驱替过程中（改变压力），不同层理样品在驱替过程中的水分迁移规律并无显著差异。核磁共振T₂谱信号幅值呈现降低趋势。驱替过程中样品内部水分的损失主要是由于毛细水含量的降低。不同层理样品的分层含水率信号幅值和成像结果均呈现先整体下降，后沿气体驱替方向信号幅值下降的规律。不同层理样品的锋面后退距离与计算水饱和度无显著差异。

       （5）CO₂气驱实验对砂岩内部的连通性呈积极作用。驱替后样品的孔隙率、渗透率及核磁共振T₂谱信号幅值均呈现增加趋势，且平行层理样品参数的增长值显著高于垂直层理样品。结合渗透率计算模型，毛细渗吸过程也可用于评价储层连通性。一方面，CO₂的压缩性、溶解性、界面张力和黏度随相态改变而改变，对应驱替过程中的气体运移路径及速率发生变化。另一方面，驱替后孔隙连通性增加的主要原因为矿物颗粒的溶解及运移以及CO₂性质的改变。在不同的实验条件下，不同层理样品在驱替过程中气体运移路径存在显著差异。

       CO₂ sequestration is an effective approach to achieve the "double carbon target" and offset carbon emissions. The topic of CO₂ sequestration in reservoirs has gained significant attention, as it effectively addresses the issue of carbon emissions. However, CO₂ injection also induces alterations in the physical properties of reservoirs, thereby decreasing the efficiency of sequestration within them. Sandstone, as a widely utilized sedimentary rock for reservoirs, has an important significance for the study of gas and liquid replacement characteristics and the change of reservoir physical properties. In this study, we conducted various phase CO₂ displacement tests on layered sandstone, adjusting pressure and temperature during the process., and observed the nuclear magnetic resonance (NMR) T₂ spectra of the samples, stratified moisture distribution, and the results of nuclear magnetic resonance imaging during the displacement process. The transportation of pore water within samples during displacement was clarified, along with the mechanism of CO₂ induced modification on sandstone pore structure by comparing porosity, permeability, and NMR T₂ spectra before and after displacement. Additionally, sample permeability is calculated based on observations from capillary imbibition tests to evaluate internal pore connectivity. The results show that:

       (1) In the process of capillary imbibition, the changes in water absorption mass, water imbibition rate, and the height of the water imbibition front can be categorized into distinct stages as water imbibition time increases. The migration of capillary water primarily dominates capillary imbibition due to combined water vapor diffusion and adsorption on pore surfaces along the direction of capillary imbibition. The time required to reach a stable state for combined capillary imbibition is significantly faster than that for stabilizing the mass of absorbed water. Based on layered water content test results, the capillary imbibition process can be divided into two phases: a stage of water transport and a stage of stabilization in terms of distribution. The measured height of the imbibition front does not differ significantly from that obtained through variation in stratified moisture distribution, indicating that different stratification samples exhibit similar behavior during combined capillary imbibition processes. Considering observations made regarding parameters related to capillary imbibition, the calculated permeability of parallel to the bedding plane is slightly lower than that of perpendicular to the bedding plane.

       (2) In the process of gaseous CO₂ displacement, the efficiency of displacement increases with prolonged displacement time. The amplitude evolution pattern of the T₂ spectrum signal is more pronounced in parallel to the bedding plane compared to perpendicular to the bedding plane, and this amplitude is primarily influenced by capillary and free water signal loss. The layered water content's signal amplitude decreases with increasing displacement time, pressure, and recession distance of the fronts. Magnetic resonance imaging (MRI) results show that there is no significant difference in the water transport law of gaseous CO₂ displacement under the influence of layered factors.

       (3) During supercritical CO₂ displacement, the NMR T₂ spectra signal amplitude in varying layered samples shows a decreasing trend with increasing displacement temperature. Notably, the capillary water content showed significant variations. The pattern of variation in the signal amplitude and MRI results of the stratified water content is significantly related to the layer structure of the samples. The water inside the layered sandstone escapes in the direction of the layers, and the fronts recede an increasing distance. the front retreat distance reaches the height of the sample indicating a dry state. At the end of displacement, the residual water saturation of parallel to bedding plane was 1.99 times that of perpendicular to the bedding plane.

       (4) During the supercritical CO₂ displacement process, there is no significant difference in the water migration law of different layered samples during displacement. A decreasing trend in the amplitude of NMR T₂ spectra signals is observed for varying layered samples as pressure increase. The loss of internal water primarily contributes to the loss of capillary water during the displacement process. Overall, both signal amplitude and MRI results indicate a decrease in stratified water distribution for varying layered samples, with a subsequent decrease in signal amplitude along the direction of gas displacement. There is no significant difference observed between the frontal retreat distance and calculated water saturation for different layered samples.

       (5) The CO₂ gas displacement tests shows positive effects on the sandstone's connectivity. After the displacement, there is an increasing trend observed in porosity, permeability, and NMR T₂ spectra signal amplitude of the samples. Moreover, the increasing values of these parameters are significantly higher for parallel to the bedding plane compared to perpendicular to the bedding plane. The integration of the capillary imbibition process with a permeability calculation model can provide an effective approach for evaluating reservoir connectivity. On the one hand, the compressibility, solubility, interfacial tension and viscosity of CO₂ change with the change of phase state, which corresponds to the change of gas transportation path and rate during displacement. On the other hand, the main reason for the increase in pore connectivity after displacement is the dissolution and transportation of mineral grains and the change in the nature of CO₂. Under different experimental conditions, there are significant differences in the paths of gas transportation during displacement in different layered sandstones.