Hydraulic fracturing can be used in low-permeability heavy oil reservoirs for improving the production rate. It is challenging to achieve a high degree of fracturing fluid flow-back in low-permeability heavy oil reservoirs. This research investigates the application of a novel hyper-branched polymer fracturing fluid in low-permeability heavy oil reservoirs.

A hyper-branched polymer, which has numerous end groups and possesses a special highly branched three-dimensional spherical molecular structure, is characterized by Fourier Transform Infrared (FTIR) spectrophotometer. Laboratory tests are conducted to evaluate the stability of viscosity at a given shear rate, temperature and shearing resistance, reversible cross-linking property, filtration property, and proppant-carrying performance of this hyper-branched polymer fracturing fluid. The degrees of core's formation damage are compared through conducting coreflood experiments, where the cores are treated with two fluids: the hyper-branched polymer fracturing fluid and guanidine gel fracturing fluid.

The laboratory evaluation shows that the hyper-branched polymer fracturing fluid has good rheological characteristics, a high solubility in water, as well as good temperature and anti-shearing resistance. After being sheared for 90 minutes with the shear rate of 170 s−1 at 150°C, it can still be reversed to gel with high strength because of its reversible cross-linking effect. This fracturing fluid has a good antioxidant ability, which allows its basic fluid to be exposed to air for 48 h without viscosity reduction. Therefore, the basic fluid can be prepared as spare fluid in a large amount, significantly improving job efficiency on site. Meanwhile, field applications in some wells in China show that: the friction of this fracturing fluid during fracturing is about 30%-50% to that of the normal guanidine gum fracturing fluid. Thus it has an ultra-low friction feature. The flow-back rate of the fracturing fluid can reach as high as 80%. Besides, its cost is about 75% to that of the normal guanidine gum fracturing fluid, making it a clean and cost-effective system. The fracturing fluid prepared with hyper-branched polymer possesses outstanding technical advantages as well as good cost effectiveness, implying a promising application in the large-scale fracturing stimulation of low-permeability heavy oil reservoirs.

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