Abstract: In order to solve the technical problems in deep shale gas development, such as great reservoir damage, poor proppant carrying and complicated variable-viscosity process of conventional fracturing fluid, an environmentally friendly variable-viscosity fracturing fluid system was developed, and laboratory performance evaluation and field application were carried out, taking into account the characteristics and construction requirements of deep shale reservoirs in Weiyuan Block. The study shows that the environmentally friendly fracturing fluid system was completely dissolved within 30 seconds, and the fracturing fluid viscosity was adjusted in real time within the range of 2 to 150 mPa·s. The drag reduction rate of low/high viscosity fracturing fluid prepared with flowback water was more than 70%, and the drag reduction rate of linear-gel fracturing fluid wais more than 65%. The reservoir damage rate of low/high viscosity fracturing fluid was less than 15.00%, and the core damage rate of linear-gel fracturing fluid was 15.47%. The environmentally friendly variable-viscosity fracturing fluid was biologically non-toxic and high in proppant-carrying performance, which was up to 65 times higher than that of clean water. The application in Weiyuan Well H21-5 in Weiyuan indicates that the environment-friendly variable-viscosity fracturing fluid actualized fast dissolution speed, excellent drag reduction performance and outstanding proppant-carrying performance, and it could changed the viscosity in real time to adapt to different fracturing conditions and satisfy the technical requirements of fracturing integrated with drag reduction and proppant carrying, with good application prospect.

Key words: shale gas, deep, variable-viscosity fracturing fluid, environmentally friendly, drag reduction, proppant carrying