The hydration and swelling of shale caused by water in water-based drilling fluids is one of the most important problems that causes wellbore instability. The development of high-performance shale inhibitors is an important prerequisite for ensuring the drilling of shale formations. In this study, four deep eutectic solvents (DESs) based on choline chloride (urea-DES, Gly-DES, Oxa-DES, and Cit-DES) were synthesized and introduced as promising shale inhibitors. Fourier transform infrared spectroscopy (FT-IR) and hydrogen nuclear magnetic resonance spectroscopy (1H NMR) were used to characterize the structural information on DESs. The inhibition ability of DESs was evaluated by a bentonite inhibition test, a linear swelling test, and a settlement test. Compared with the traditional inhibitors, such as potassium chloride (KCl) and polyether amine, the four DESs can effectively inhibit the hydration and swelling of shale. Five wt % DESs can maintain low rheological parameters when the content of sodium bentonite (Na-bent) was as high as 40 wt %. Among them, Gly-DES not only exhibited the most efficient inhibition capability but also had the advantage of not affecting the basic performance of the drilling fluid. Inhibitory mechanism analysis showed that the electrostatic interaction and hydrogen bonding between DESs and sodium bentonite (Na-bent) are the most important factors to inhibit clay hydration. Besides, the reduction of the surface tension of the liquid by DESs effectively weakened the driving force for water to penetrate the Na-bent layer. Finally, it is important to note that, although DESs exhibited strong inhibitory capabilities, some DESs, such as Oxa-DES and Cit-DES, are prone to adversely affect the basic performance of drilling fluids. Therefore, a careful selection is required.
The wellbore instability caused by the penetration of drilling fluids into the formation is a vital problem in the drilling process. In this study, we synthesized a polymer/graphene oxide composite (PAAN-G) as a fluid loss additive in water-based drilling fluids. The three monomers (acrylamide (AM), 2-acrylamide-2-methyl-1-propane sulfonic acid (AMPS), N -vinylpyrrolidone (NVP)) and graphene oxide (GO) were copolymerized using aqueous free radical polymerization. The composition, micromorphology, and thermal stability properties of PAAN-G were characterized by Fourier transform infrared (FT-IR) spectroscopy and thermogravimetric analysis (TGA). According to the American Petroleum Institute (API) standards, the influence of PAAN-G on the rheological and filtration properties of bentonite-based mud was evaluated. Compared with PAAN, PAAN-0.2G has more stable rheological properties at high temperatures. The experimental results showed that even at a high temperature of 240 °C, PAAN-G can still maintain a stable fluid loss reduction ability. In addition, PAAN-G is also suitable for high-salt formations; it can still obtain satisfactory filtration volume when the concentration of sodium chloride (NaCl) and calcium chloride (CaCl 2 ) reached 25 wt %. Besides, we discussed the fluid loss control mechanism of PAAN-G through particle size distribution and scanning electron microscopy (SEM).
One of the biggest difficulties with water-based drilling fluids for shale horizontal wells is its relatively high friction, especially for long horizontal wells. Improving the lubricating ability of drilling fluids is very important for expanding the extension range of horizontal wells in unconventional oil and gas wells. In this work, three kinds of lubricants, graphene oxide (GO), choline chloride/glycerol deep eutectic solvent (Gly-DES), and GO/Gly-DES, were synthesized to investigate their ability to improve the lubricity of water-based drilling fluids. These synthesized lubricants were all environmentally friendly. The tribological test and the filter cake adhesion coefficient test were used to compare the lubricity of these fluids. On the other hand, the traditional commercial lubricant solid graphite (HY-202) was taken as the reference sample. The results indicated that GO and Gly-DES can exert a more obvious lubricating effect at much lower concentrations compared to the reference sample. GO (0.25 wt %) can reduce the adhesion coefficient of filter cake by 93.33% and the friction coefficient of drilling fluid by 48.47%. The complex of GO and Gly-DES can further increase the reduction rate of adhesion coefficient and friction coefficient. Whether it was in reducing the adhesion coefficient of the filter cake or the friction coefficient of the drilling fluids, the combined lubrication effect of GO and Gly-DES was significantly better than using GO or Gly-DES alone. GO and Gly-DES decreased the coefficient of friction due to their good ability in producing a strong protective film on the contacting surfaces of the metal and filter cake. GO and Gly-DES can slightly reduce the filtration volume of drilling fluids and are suitable for shale water-based drilling fluids.
Drilling fluids with high suspending capacity without high viscosity are required in drilling operations, such as horizontal wells and highly deviated wells, with the “cutting bed”. Non-viscosifying rheological modifiers that can enhance the rheological properties of polymer drilling fluids are urgently needed. This study reports the application of a choline-chloride-based deep eutectic solvent (Gly-DES) as a rheological modifier for polymeric water-based drilling fluids. The rheological effects of Gly-DES on bentonite/polymer fluids were evaluated by high-temperature rheological testing, rheological model fitting, and rheological parameter (yield point, fluidity index, gel strength, etc.) analysis. The results showed that 2–4 wt % Gly-DES can improve the rheological properties of polymer drilling fluid in the temperature range of 150 °C. The addition of Gly-DES significantly improved the rheological parameters of drilling fluids, such as the yield point, gel strength, low shear rate viscosity, etc. At the same time, the viscosity of drilling fluids was always stable at different temperatures, which means that Gly-DES can help improve the suspension and cutting carrying capacity of drilling fluids without significantly increasing the viscosity of drilling fluids. Mechanistic analysis revealed that Gly-DES was connected to the polymer and bentonite through hydrogen bonds and ionic bonds, which enhanced the cross-linked network structure of the polymer at high temperatures and promoted the formation of weak gels in Na bentonite suspensions. Gly-DES can simultaneously enhance rheology, inhibit clay hydration, and improve filter cake lubricity and has the potential to be a multifunctional drilling fluid treatment agent.
With the depletion of conventional resources, it is necessary to enhance the recovery of remaining oil. The tertiary oil recovery chemical flooding is the most promising for enhanced oil recovery (EOR). The use of alkali in chemical flooding produces fouling with the formation, which brings new challenges to chemical flooding EOR. So, alkali-free surfactant-polymer (SP) flooding is used as a new solution for EOR. The polymer makes the displacing fluid have a high viscosity and increases the swept volume. The surfactant can decrease the interfacial tension and emulsify the crude oil while the displacing fluid reaches the corresponding area. The synergistic effect of the two is more conducive to the recovery of the remaining oil. As a result, SP flooding is a more practical approach to enhance oilfield EOR. The research status of several surfactant and polymer types for EOR is reported in this work. According to the study, complex and anionic surfactants are better for EOR. For difficult formations, SP flooding is more suited because of the presence of polymers. Additionally, study analysis demonstrates that SP flooding and alternative polymer injection, when employed at the same cost, can enhance EOR and assist in resolving issues brought on by low oil prices.
The chemical structure of the choline chloride/graphene oxide composite.
Graphene is a material formed with carbon atoms connected by sp2 hybridization.
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