Over the past decade, operators in offshore fields such as the Gulf of Mexico and West Africa have increased their investment in deepwater projects. The trend is expected to continue in the future. For frac and pack operations in deep water, the industry is facing some particular challenges that limit completion options. A major challenge is the need to reduce the surface treating pressure to stay within current equipment operating limits. One solution is to increase the fluid density, thereby allowing for a reduction in the surface pressure while generating the same bottomhole pressure. However, higher fluid densities can also lead to higher friction pressures that may overcome the hydrostatic benefits. In addition, as the water depth increases, the travel time to the mudline increases, resulting in the need for extended crosslink delay times. Therefore, fluids that provide flexibility in density and extended crosslink delay times are required for effective treatment designs in deep water.
This paper describes a weighted, delayed crosslink polymer (WDC) fluid and engineering guidelines for its use in frac and pack applications. Sodium bromide is used as the weighting agent to provide higher densities (8.3 to 12.5 ppg) compared to conventional fluids. Additionally, the fluid has a delayed crosslink that can be controlled from 2 to 20 minutes, which helps in limiting the tubing friction.
Extensive laboratory testing was performed to ensure the technical feasibility and effectiveness of the WDC fluid. Tests included polymer hydration in brine, crosslink delay time measurements, rheology at bottom-hole static temperatures, shear sensitivity, fluid stability at mudline temperatures, breaker design, leakoff characteristics and matrix cleanup, proppant-pack conductivity, and friction pressure tests. Results demonstrate that the WDC fluid meets all performance requirements. It is stable at mudline temperatures down to 40 °F, and effective at bottom-hole temperatures of at least 250 °F.
An engineering simulator was developed based on completion characteristics to select the weighted fluid in cases where it can be beneficial. Example case histories are presented that demonstrate conditions under which weighted fluids provides a reduction in treating pressure. The WDC fluid has shown the ability to reduce the treating pressure by at least 20% depending on the wellbore configuration. The strong dependence of treating pressure on completion geometry, fluid density, and crosslink delay time necessitates evaluation of each treatment on a well-by-well basis to determine effective applications of weighted fluids.