Preparation and Evaluation of a Novel Branched Polymer as Thickener for Calcium Chloride-Based Drilling and Completion Fluids

Calcium halide-based fluids are often used in drilling and completion operations due to their high density, clay inhibition and low solid content. However, there is a lack of thickeners to promote gel strength, which improves the fluid’s capacity to carry and suspend cuttings. To solve this problem, the branched polymer (hereafter abbreviated as PAD-B) was prepared by the copolymerization of N,N-dimethylacrylamide (DMAM) and 2-acrylamide-2-methylpropane sulfonic acid (AMPS), using polyethylenimine as a branching agent and cerium ammonium nitrate as the initiator. Compared with linear polymer (PAD-L), PAD-B has better shear strength at the same low viscosity. The experimental results indicated that the increase in shear strength of PAD-B is due to the interactions between branched PAD-B molecules, which lead to the formation of a network structure. The effect of calcium chloride (CaCl2) on the rheological performance of PAD-B was investigated at 25 °C and 50 °C. Compared with PAD-L, PAD-B shows better thermal stability and calcium resistance. Its high gel strength provides technical support for addressing issues such as low yield point, gel strength and difficulty in controlling the rheological parameters of calcium halide-based fluids during the drilling and completion of complex wells.