A new novel crosslinker with space structure for low-polymer-loading fracturing fluid
Abstract
Boron-based crosslinkers are used commonly to increase viscosity and to improve fluid-loss control and proppant transportability of guar and its derivative fluids. Boron crosslinkers are usually preferred because of their ability to reheal after shearing and their favorable environmental properties. In order to reduce both the formation and the proppant-pack damage from polymer residues and to reduce over-all fluid cost, more-efficient crosslinkers capable of crosslinking fluids with reduced polymer loading is of great interest. Previous studies demonstrated that polymer solutions have critical overlap concentration (C*), below which no intermolecular crosslinking leading to increased viscosity can occur. However, recent studies demonstrated that increased crosslinker size or length can lead to the crosslinking of polymer solutions well lower than the C* and can reduce polymer loading without compromising the rheology of the fracturing fluid.
This paper shows the effect of new crosslinkers with space structure capable of interacting with multiple poly-saccharide strands to form crosslinking networks at lower polymer loadings than conventional guar fluids. The crosslinker is formed by the reaction of boric acid and a Polyamine with six-member ring to improve the spatial structure. The formation procedures of the new crosslinker by Boric and six-member Polyamine is very simple, and the synthetic condition is also very mild. The concentration of guar fluid with this new crosslinkers can be reduced by 30%. In addition, the crosslinker reacted with glucose to improve the control of the fluid-viscosity buildup can make the product fit into broader applications. The time of the fluid-viscosity buildup ranges from 15 to 180s. The pH value is very important to the heat resistance characteristics of the fracturing liquid system, the heat resistance characteristics increase with the increase NaOH loading. The polymer residual of the new fracturing system with new crosslinkers is much lower than that with conventional crosslinkers because of the low polymer loading. The cost of new crosslinkers is low because of the low price of raw material and the simple formation procedure and the mild synthetic condition, which is very important for extensive used in the oil field.
Keywords
Download Options
Introduction
Wells drilled into low-permeability reservoir need to be treated with hydraulic fracturing to increase the conductivity and thereby to obtain the economic production. Hydraulic-fracturing cracks in the zone are created by forcing a fluid at a pressure higher than the parting pressure of the rock. The fractures expanded as the continued injection of fracturing liquid with proppants. When the predetermined amount of liquid is injected into the formation, the pressure in the surface is released, and the fracturing gel will break and retreat from the formation to the surface leaving the proppants in the fractures to inhibit the fractures from closing. The viscosity of the fluid is typically generated by polymers, such as polysaccharides and its derivatives. Hydrated guar and derivatives create linear gels that do not achieve the required viscosity for proppant transport at elevated temperatures [1]. Thus, crosslinkers, such as boron, zirconium or titanium compounds, are used to significantly increase the viscosity of the fluid system. Boron-Based crosslinkers are used to increase viscosity, fluid-loss control and proppant transportability of guar fluid. Boron crosslinkers can reheal after shearing, so they are used very commonly in the oil field. Guar grum contain protein species, insoluble even after the breaking of the fluid that is damaging to the formation, so that less polymer usage will result in less damage to the subsurface formation. More efficient crosslinkers capable of crosslinking fluids with reduced polymer loading have always been of great interest to reduce formation and proppant pack damage from polymer residues, and to reduce overall fluid cost [1] .
Conclusion
A new chemistry was developed on the basis of the condensation reaction of boric acid with a polyamine scaffold of tripolycyanamide to yield a polyaminoboronate crosslinker with multiple boron sites. The molecular size of new crosslinker enables crosslinking guar or its derivatives at lower concentration, thus reducing the loading of polymer required. New chemistry was developed in which glucose was incorporated into the crosslinker to delay the crosslink rate at the boronate sites. The NaOH has a great influence on the crosslink rate and thermal endurance, at the same time, the pick-up time increase with the increase of crosslinker loading. With the new chemistry, the fracturing gel can endure 120℃ with 0.3wt% guar loading, which is lower than conventionla guar loading by 30%. The new novel crosslinker also has good performance of gel breaking, gel residuals and fluid loss.