Hydraulic fracturing operations require large amount of water which, in some cases, could reach six million gallons per treatment. Fresh water, from surface and subsurface sources, is usually used to formulate the fracturing fluid. The use of produced water in hydraulic fracturing has gained increased attention in the last few years, as it would solve fresh water acquisition challenges and reduces its disposal costs. However, produced water usually contains high amounts of total dissolved solids (TDS) that prevents its compatibility with fracturing fluid additives and negatively affects the fracturing fluid rheology. This paper investigates the effect of different chelating agents on the use of high-TDS produced water in high pH borate-crosslinked gels used in hydraulic fracturing fluids.
Samples of field-produced water were analyzed for different ion concentrations. Synthetic water was prepared with different amounts of Na, K, Ca, and Mg salts. The fracturing fluid was formulated using natural guar polymer, borate-based crosslinker, biocide, surfactant, clay controller, scale inhibitor, and pH buffer. Five chelating agents tested in this study included HEDTA, GLDA, di-sodium EDTA, di-ammonium EDTA, and sodium gluconate. Compatibility tests of the fluid system at 180ºF were conducted at different ion concentrations. Apparent viscosity of the fracturing fluid was measured using a high-pressure high-temperature rotational rheometer. All rheology tests were conducted at a temperature of 180ºF and were conducted according to API 13m procedure with a three-hour test duration.
Results indicated the potentiality of high-TDS water to cause precipitate and, hence, formation damage. However, the use of chelating agents has reduced the precipitation of divalent cations and enabled the formulation of fracturing fluid at higher Ca and Mg concentrations. The use of 2.5 wt% of HEDTA has enabled the formulation of the fracturing fluid using synthetic produced water that contains TDS of 101,000 ppm with Ca and Mg concentrations of 2,500 and 200 ppm, respectively. The fracturing fluid showed an average viscosity of 150 cp at 100 s-1 despite the high-TDS and high-hardness environment. The use of GLDA reduced the Ca and Mg ion precipitation; however, its chelation effect was found to be limited to certain concentrations. Some chelating agents showed the ability to chelate the boron ion and/or reduce the system's pH value. Consequently, viscosity measurements indicated the breaking of the fluid viscosity after the addition of the chelating agent.
This paper presents the use of chelating agents to maximize the effectiveness of produced water for hydraulic fracturing operations. Successful use of chelating agents would reduce produced water disposal costs and improve the environment by reducing the consumption of fresh water.