Low-carbon hydrocarbon fracturing fluid with fast cross-linking rate based on n-hexane/n-octane: synthesis and properties

Abstract

ABSTRACT The use of slick-water fracturing fluids has been greatly limited owing to the shortage of water and transportation difficulties in some areas. Serious formation damage occurred in the shale gas reservoirs with low porosity and permeability during the slick-water fracturing with large amounts of water. A fast cross-linking waterless fracturing fluid using the n-hexane/n-octane low-carbon hydrocarbons was prepared in this work. Firstly, a polyphosphate intermediate was synthesized by triethyl phosphate reacting with phosphorus pentoxide, which then reacted with a mixed alcohol of n-butanol, n-octanol, and dodecanol to get the dialkyl phosphate (named as gelling agent). Ferric sulfate, ethylene glycol, sodium citrate, sodium edetate were used to prepare a composite iron-based cross linker with an excellent gelation time and gelation performance, which was composed of 30% ferric sulfate, 10% glycol, 11% trisodium citrate, 9% sodium edetate, and 30% deionized water. By optimizing the ratio of gelling agent to cross linker and the content of gelling agent, an anhydrous fracturing fluid formulation based on n-hexane/n-octane was prepared: n-hexane/n-octyl (mass percent 100%) + gelling agent (1.6%-1.8%)+cross linker (0.6%-0.8%) + gel breaker (0.6%-1.0%).The n-hexane and n-octane fracturing fluids can be used in the formation temperature of 60°C, and 100°C, and the viscosity for the fracturing fluid was 178 and 230 mPa.s under the shear rate of 170 s−1 and shear time for 90 min. The sedimentation velocity of a single ceramsite was 0.2780 cm/min, and the sedimentation velocity of the mixed ceramsite was 0.0022 cm/min, which fully satisfied the fracturing suspension performance. The crushing time of n-hexane and n-octane fracturing fluid was controllable (5–10 h), and the gel breaking effect was excellent (less than 2 mpa.s).