Seawater-based fracturing fluids provide greater flexibility when designing stimulation treatments for offshore environments by reducing delays in stimulation vessel scheduling and using an, essentially, unlimited supply of seawater, opposed to fresh water. Because seawater has a higher ionic strength than traditional fresh water, lower viscosity is observed from guar-based polymers. By comparison, the seawater can produce scaling precipitates when sulfate ions contact the calcium, barium, and strontium ions in the formation. The formed sulfate precipitates could lead to formation damage in the form of scaling, pore throats being clogged, and an overall reduction in production capacity.

This paper presents an innovative polymer-based scale inhibitor (SI) to mitigate scaling issues from seawater with 4,000 mg/L sulfate when mixed with 200,000 mg/L total dissolved solids (TDS) formation water. From static bottle tests with seawater, 74% strontium sulfate inhibition was achieved at 300°F. To eliminate sulfate scaling issues, a nanofiltration (NF) membrane-based technique was used to filter seawater. The SI performance was tested by both static bottle tests at different mixing ratios and dynamic tube blocking tests at 300°F. Following static bottle tests, the key ion concentrations were determined using inductively coupled plasma (ICP) spectroscopy. Zero scaling was achieved by adding 300 mg/L SI. The fracturing fluid was prepared using a zirconate-crosslinked derivatized guar. The rheology of the fracturing fluid was measured with and without SI to investigate its influence on crosslinking and proppant carrying capacity at 300°F. High magnesium in the seawater consumes hydroxide ions and affects pH control. To obtain a stable fracturing fluid, the pH was optimized to minimize magnesium precipitation. Coreflooding experiments confirmed the addition of SI does not cause additional formation damage.

Using NF-seawater in conjunction with proven SIs can provide effective treatment of seawater-based-fracturing fluids. The separation technique (NF) and chemical solution (SI) complement each other to neutralize scaling issues completely. This technique minimizes freshwater usage in fracturing, which can enable significant savings for offshore operations time and cost.

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