Driven by its wide range of matrix permeability, the Clair field, located on UK Continental Shelf, requires high proppant concentrations and fracture conductivity for economical production uplift. Borate crosslinked guar-based fracturing fluids were selected because of their reliability, environmentally friendly nature, good recovery with the ability to carry proppant and maintain viscosity after mechanical shearing. In addition, these fluids have a strong historic record in the North Sea and a relatively low cost. Fresh water is typically used as a base for this fracturing fluid type. However, with novel fracturing completions that allow fracturing multiple zones continuously, fresh water becomes the logistically limiting factor. Seawater can be considered as an enabler for such completions. However, seawater presents a technical and economic challenge for such fluids due to cation precipitation at high pH and the need for an elevated concentration of a high-cost scale inhibitor. Due to strict environmental regulations in the North Sea, availability of approved scale inhibitors is limited.
A North Sea approved scale inhibitor additive has been identified to prevent the precipitation of divalent cations at high pH. However, the volume required for this scale inhibitor was relatively large, leading to prohibitively high costs of the subject fluid. A strong relationship between the final fluid pH and the needed amount of scale inhibitor was observed. Therefore, this study aims to find the balance between scale inhibitor concentration, fluid pH, and viscosity stability to enable an economical seawater based borate fracturing fluid.
A dynamic scale loop test was conducted to confirm the scale precipitation and the need for scale inhibitors. Thereafter, several visual compatibility tests were conducted at different loading of scale inhibitors and fluid pH. Results show a positive correlation between the concentration of high-pH buffer and the corresponding amount of scale inhibitor. Initially, a high loading of scale inhibitor was required due to the concentration of high-pH buffer to keep the fluid stable. However, reducing the high-pH buffer to a lower value can significantly reduce the required concentration of scale inhibitor. A series of rheological tests were conducted using a high-pressure high-temperature (HPHT) viscometer to confirm that the new amount of high-pH buffer is enough to keep the fluid stable at downhole conditions. Finally, a series of coreflood tests were conducted to measure the regained permeability using outcrop cores and formation cores to confirm no damage due to the use of seawater.
The study presents a solution that meets technical and economic requirements to enable efficient hydraulic fracturing operations and minimize scale risks where adequate fresh water supply is not readily available. Rheology, coreflood, and compatibility test results demonstrate that seawater can be used as a base fluid for fracturing treatments.
