Experimental study on the effect of tight gas fracturing flowback fluid composition on the performance of modified polyacrylamide viscosity reducing and slippery water system

Tight gas belongs to unconventional gas reservoirs, and to obtain high gas production, it needs to be fractured and reformed. After the construction is completed, a large amount of flowback fluid is produced. In order to reduce the cost of flowback fluid treatment and avoid environmental pollution, a slippery water system with flowback fluid is usually used on the site of hydraulic fracturing. Almost all flowback fluids contain high-order cations such as calcium ions, magnesium ions, iron ions, and a large amount of suspended solids. Some flowback fluids contain incomplete gel-breaking micelles and unseparated condensate oil. The performance of the viscosity-reducing and slippery water system with flowback fluid as the base fluid is poor, usually manifested as low viscosity of the slippery water, low friction reduction rate, and high formation damage. To study the effect of tight gas fracturing flowback fluid on the performance of modified polyacrylamide viscosity-reducing and slippery water system, this paper found through cryo-electron microscopy scanning experiments that the flowback fluid made the modified polyacrylamide molecular chain unable to form a complex network structure and showed the phenomenon of reduced strength and aggregation. The effects of mineralization degree, calcium and magnesium ions, divalent and trivalent iron ions, suspended solids, pH value, and oil content on the viscosity, friction reduction performance, and rock damage of the viscosity-reducing and slippery water system were analyzed experimentally. It was found that the viscosity of the viscosity-reducing and slippery water system was mainly affected by divalent iron ions, calcium and magnesium ions and other divalent cations; the friction reduction rate was mainly affected by the content of suspended solids and oil content; and the rock damage was mainly affected by the pH value and the content of suspended solids. Based on this, recommended parameters for reusing tight gas fracturing flowback fluid are given to guide the smooth progress of hydraulic fracturing construction on-site.