Research and practice of seawater-based fracturing fluids

Abstract

The Dagang Oilfield, with 146 kilometers of coastline, is rich in seawater resources. In order to satisfy the demand of large-scale fracturing operation in low-permeability offshore reservoirs and to solve such technical difficulties as high-cost conventional fracturing fluid, incompatibility of seawater with conventional fracturing additives, and limitation of offshore operational space, which result in the inability to achieve industrial fracturing operation, fracturing thickening and cross-linking agents that can be formulated with seawater were studied and selected to form a seawater-based fracturing fluid system with temperature resistance of 80–180°C. The system does not need to shield or remove calcium and magnesium ions as seawater can be directly used after filtration. The thickening agent swells so quickly that it imparts viscosity to seawater to reach 92% of the maximum viscosity in 3 minutes, and can achieve continuous mixing. Additionally, it is possible to reuse the seawater-based fracturing fluid after processing such as removing sand and filtering suspended matter. Using it to prepare slippery water and new fracturing fluid, only a fewer additives are required, and the comprehensive cost is lower. In field test, the fracturing scale and process were adaptively optimized through integration of engineering and geology. For example, Well ZH50-16 produced 9.9 tons of oil per day through a 4 mm choke before fracturing operation, and after fracturing, the liquid produced increased to 77.5 m3/d, including 57.6 tons of oil, and 14800 m3 of gas through a 6 mm choke. Simple field preparation of seawater-based fracturing fluid and fracturing operation provided stable production performance.