Li Qiang,Wang Fuling,Forson Kobina,Zhang Jinyan,Zhang Chenglin,Chen Juan,Xu Ning,Wang Yanling
The fracturing property of liquid CO 2 fracturing fluid varies greatly due to the rheology of fracturing fluid during fracturing process. The main objective of this investigation is to study the rheology property of thickened liquid CO 2 by measuring the viscosity of thickened liquid CO 2 in different physical parameters of this prepared thickener and explain the causes of rheological changes. The results show that thickener content, branching content, and molecular weight of a thickener for all could significantly improve the rheology of liquid CO 2 ; the consistency coefficient K increased as they rose, but the rheological index n presented a decreased trend. Meanwhile, the mesh structure is proposed as a model to explain the rheological changes, and the large wetting angle means an excellent backflow, low reservoir damage, and low adsorption property. These results herein provide a basic reference to improve the CO 2 fracturing technology and molecular design of CO 2 thickener.
Xin Sun,Caili Dai,Yongpeng Sun,Mingyong Du,Tao Wang,Chenwei Zou,Jiayuan He
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.
Reducing the adsorption of CO 2 thickeners in soil particles of shallow shale is a pathway to protect the shale environment and water cycle. In this work, we first synthesized a siloxane-based thickener named AOB based on ring-opening polymerization and hydrosilylation reaction and discussed the influence of geological factors (including contents, pressure, temperature and shear rate) on the adsorption capacity of this thickener. The soil surface adsorption behavior of AOB was characterized by a self-assembled high-pressure soil adsorption device and the adsorption capacity was analyzed by Attenuated Total Reflectance Infrared Spectroscopy (ATR-FTIR). The results showed that AOB molecule belongs to the monolayer adsorption type on the soil surface by measuring the contents. Moreover, an inverse relationship was showed between pressure and adsorption capacity for AOB, while the significantly proportional relationship was presented for temperature and shear rate, respectively. It is found that the combined force formed by soil repulsion (originated from aromatic groups in AOB) and grid drag weakens the adsorption of AOB on the soil surface, resulting in a much lower adsorption capacity than the commercial PDMS. This work not only paves a new avenue for synthesizing siloxane-based polymer as CO 2 thickener, but also provides an efficient measure to reduce the adsorption of thickener on the shallow soil.
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