Hydraulic fracturing has become an important stimulation technique for low/ultralow permeability reservoirs. Supercritical CO₂ (SC-CO₂), as a no water phase material of fracturing fluid, receives wide attention. Current researches on SC-CO₂ fracturing fluid has mainly focused on its viscous properties and superiorities. However, little attention has been devoted to the interaction between SC-CO₂ fracturing fluid and the oil reservoir during the fracturing process. Besides, in a low permeability reservoir, the wettability determines the oil recovery by imbibition, which is a main way to explore oil for a matrix. Therefore, it is crucial to study the wettability alteration on a low permeability oil reservoir introduced by SC-CO₂ fracturing fluid. In this study, a contact angle goniometer was introduced to characterize the wettability alteration on low permeability cores by SC-CO₂ fracturing fluid. Meanwhile, nuclear magnetic resonance T2 spectra, a scanning electron microscope, and a energy dispersive spectrometer were used to explain the mechanisms of wettability alteration and the adsorption of fracturing fluid was analyzed. In addition, spontaneous imbibition tests were conducted to definite the impact of wettability alteration on oil production. The results showed that the thickener in SC-CO₂ fracturing fluid, fluid filter loss, and reservoir permeability were all responsible for wettability alteration on the core surface. With the increasing thickener contents and filter loss of fracturing fluid, cores turned to be intermediate and slightly water-wet from initial strongly water-wet, which was unfavorable for oil production.Comparatively low permeability cores which consist of more micro-small pores were more likely to make treated cores oleophilic, as well. Thickener adsorption was confirmed to be the main mechanism on wettability alteration by SC-CO₂ fracturing fluid, which resulted in different pore surfaces, and thus a more oleophilic cores surface. On the basis of the results of spontaneous imbibition, strong lipophilicity cores corresponded to lower oil recovery, which illustrated that wettability alteration caused by SC-CO₂ fracturing fluid was not favored for oil flow.

中文翻译：

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低渗透油藏超临界CO ₂压裂液润湿性变化研究

水力压裂已成为低/超低渗透率油藏的重要增产技术。超临界CO ₂（SC-CO ₂）作为压裂液的无水相材料受到广泛关注。目前对SC-CO ₂压裂液的研究主要集中在其粘性和优越性上。然而，在压裂过程中很少关注SC-CO ₂压裂液与储油层之间的相互作用。此外，在低渗透油藏中，润湿性决定了通过吸油采收率，这是勘探基质油的主要方法。因此，研究SC-CO引入的低渗透油藏的润湿性变化至关重要。₂压裂液。在这项研究中，引入了接触角测角仪来表征低渗透岩心在SC-CO ₂压裂液中的润湿性变化。同时，利用核磁共振T2谱，扫描电镜和能谱仪解释了润湿性变化的机理，并对压裂液的吸附进行了分析。此外，还进行了自吸实验，以确定润湿性变化对石油生产的影响。结果表明，SC-CO ₂中的增稠剂压裂液，滤液损失和储层渗透率都是造成岩心表面润湿性改变的原因。随着增稠剂含量的增加和压裂液过滤损失的增加，岩心从最初的强水润湿变为中等偏水，这对采油不利;相对低渗透率的岩心则由更多的微小孔隙组成。也可能使处理过的核心具有亲油性。确认增稠剂的吸附是SC-CO ₂改变润湿性的主要机理压裂液，导致不同的孔隙表面，从而形成更亲油的岩心表面。根据自吸结果，较强的亲脂性岩心对应较低的采油率，这说明由SC-CO ₂压裂液引起的润湿性变化不利于油流。