With increasing global energy consumption, oil/gas drilling has gradually expanded from conventional shallow reservoirs to deep and ultra-deep reservoirs. However, the harsh geological features including high temperature and high salinity in ultra-deep reservoirs have become a critical challenge faced by water-based drilling fluids (WDFs), which seriously deteriorate the rheology and fluid loss properties, causing drilling accidents, such as wellbore instability and formation collapse. In this study, a novel temperature- and salt-resistant micro-crosslinked polyampholyte gel was synthesized using N,N-dimethylacrylamide, diallyldimethyl ammonium chloride, 2-acrylamido-2-methylpropanesulfonic acid, maleic anhydride and chemical crosslinking agent triallylamine through free radical copolymerization. Due to the synergistic effect of covalent micro-crosslinking and the reverse polyelectrolyte effect of amphoteric polymers, the copolymer-based drilling fluids exhibit outstanding rheological and filtration properties even after aging at high temperatures (up to 200 °C) and high salinity (saturated salt) environments. In addition, the zeta potential and particle size distribution of copolymer-based drilling fluids further confirmed that the copolymer can greatly improve the stability of the base fluid suspension, which is important for reducing the fluid-loss volume of WDFs. Therefore, this work will point out a new direction for the development of temperature- and salt-resistant drilling fluid treatment agents.

Liquid condensation near the wellbore region may cause cessation of gas production in gas-condensate reservoirs when pressure drops lower than its dew point. It is believed that gas deliverability can be enhanced by altering the wettability of reservoirs to gas-wetting. In order to investigate the influence of wettability alteration on the displacement efficiency of gas-condensate, contact angle measurement and gas flood tests were conducted. Results show that core wettability can be altered from liquid-wetting to intermediate gas-wetting or preferential gas-wetting by the fluorosurfactants FG24 and FG90. The contact angles of brine and decane indicated that core wettability can be altered from liquid-wetting to preferential gas-wetting by FG24 at a concentration of 0.3%, and by FG90 at 0.5%. Surface free energy before and after core treatment was measured by Owen's two-liquid method, the results showing that core surface free energy sharply decreased from 70 to about 3 mN/m after FG24 treatment. Oil displacement efficiency, relative permeability, and gas flow in gas–oil systems can be effectively improved by FG24 treatment due to alteration of wettability, results showing that these can be enhanced by 57.21, 47.41, and >100% at a temperature of 120°C. Additionally, FG24 is thermally stable and appears to alter wettability permanently.

To investigate the mechanisms of enhancing oil recovery and the flow behaviors of foamed gel in porous media, foamed gels with characteristics of excellent strength and viscosity were prepared with polymer, crosslinking agent, foam agent, and formation water. The breakthrough-vacuum method and a rotary viscometer were used to evaluate the strength and viscosity of foamed gel. Coreflooding and pore-level visualization experiments were performed in heterogeneous reservoir models. Laboratory results illustrate that high strength and viscosity of foamed gel can be prepared by 0.15% NJ-8, 0.2% polyacrylamide solution, and 1.5% foaming agent. The strength and viscosity of the foamed gel reached 0.06 MPa and 10,000 MPa · s, respectively. The results of coreflooding experiments in heterogeneous cores show that oil recovery can be improved by approximately 36.9% after injecting 0.3 pore volume of the foamed gel, and enhanced oil recovery is mainly attributed to the improving sweep efficiency of mid- to low-permeability layers. Images of visualization flooding demonstrate that foamed gel exhibits good oil resistance and elasticity when used with crude oil. Furthermore, the new amoeba effect, Jamin effect, fluid-diverting effect, and extruding effect between foamed gel and oil in porous media can enhance oil recovery by improving sweep efficiency. GRAPHICAL ABSTRACT