Fe3+-crosslinked alkyl phosphate ester as thickener for oil-based fracturing fluids

Abstract

To reduce damage to the formation of water locking and water sensitivity causing poor fracturing effect during the fracturing by water-based fracturing fluid in shale oil and gas reservoirs, the oil-based fracturing fluid as anhydrous fracturing fluid were prepared with dialkyl phosphate (PO(OH)(OR)(OR’)) as gelling agent, complexed iron ion solution as activating agent, n-hexane or kerosene as base fluid. The dialkyl phosphate was synthesized by two-step processes. In the first step, the optimal reaction conditions for preparing monoalkyl phosphate intermediate were the reaction time 5 h, reaction temperature 85 °C, molar ratio of triethyl phosphate to phosphorus pentoxide 3.4:1. In the second step, the optimal synthesis conditions for preparing dialkyl phosphate esters were the reaction time 4 h, reaction temperature 80 °C, molar ratio of the intermediate monoalkyl phosphate esters to the mixed alcohol 1:1.4. Complexed iron ion solution as activating agent was prepared by a consisting of 30 wt% trisodiun citrate, 10 wt% ethanediol, 30 wt% ferric sulfate (Fe2(SO4)3) and 30 wt% deionized water. The effects of gelling agent, activating agent and based fluid on oil-based fracturing fluid performance were examined. The experiments showed that with the increase of gelling agent dosage, the gelation time was significantly shortened and the viscosity gradually increased. When the amounts of gelling agent and activating agent were 2.5 wt% and 4 wt%, respectively, the gelling time and the viscosity of fracturing fluid was up to 166 s and 450 cP. The viscosities of the fracturing fluid were always greater than 100 cP after it was sheared by shear rate of 170 s−1 for 1 h at 80 °C, indicating that the fracturing fluid have good shearing resistance and temperature resistance. The viscosity of the fracture fluid with n-hexane as based fluid was 1.32 cP at 70 °C when the sodium acetate as broken gent was 0.40 wt%, thus the broken gelation effect was remarkable. And the core damage rate of prepared oil-based fracturing fluid was also much lower than that of the common water-based fracturing fluids.