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Controlling inorganic sulphate and carbonate scales with polymer, phosphonate, and phosphate ester scale inhibitors is commonplace in the oilfield services industry. It is well understood in what environments induvial inhibitor types work best; for example, sulphonates are very effective for sulphate scale control in low temperatures (SPE 80229) whereas phosphonates are much less effective under these same conditions but improve at higher temperatures (SPE 179889).

Less well understood is the potential for scale inhibitors utilizing synergistic interactions with blends of polymers, phosphonates, and phosphate esters to reduce chemical cost, treatment rates, and transport logistics, resulting in a more effective scale management program with a reduced operational footprint.

To evaluate performance of a selected range of blended inhibitors, ChampionX performed a trial on a North Sea produced water system, which was applying monoethanolamine (MEA) phosphonate-type scale inhibitor as well as novel cleaning programs to counter a high carbonate saturation ratio in the heater (SPE 204365). The objective was to find an improved scale inhibitor formulation that would outperform MEA phosphonate to control the high calcite saturation ratio brine. In this application, produced fluids pass through a heater with a skin temperature between 90°C and 105°C.

Identifying the Scale Challenge

Studies of synergistic properties of phosphonates and polymer scale inhibitors show there is potential to create blends of existing chemicals to make a formulation that shows a performance greater than either inhibitor component on its own. Four generic scale inhibitors that could effectively prevent scale at 105°C were considered.

1. A poly aspartate acid, generally found to be thermally stable to 120°C

2. The incumbent MEA phosphonate chemical, which is widely used for this type of scale inhibition at elevated temperature

3. A phosphate ester, found to be thermally stable at temperatures of 90°C

4. Phosphonate-functionalized biopolymer, which showed good carbonate inhibition properties and excellent environmental properties

The goal was to develop a synergistic blend that would mitigate the legislative (cost of REACH registration) and economic (cost of new raw material product set up within the supply chain system) issues associated with the development of new classes of scale inhibitor for a relatively small market.

There are two primary methods of scale inhibition in produced water. The first is crystal nucleation inhibition, which prevents the onset of scale formation itself by keeping the ions in solution. This mechanism of inhibition is best evaluated via dynamic scale loop (DSL) tests. The polymer-type scale inhibitors (such as carboxylic acid functionated homo and copolymers, for example VS-Co) work well within this test, as they prevent deposition at low treatment rate.

The other principal inhibition mechanism is crystal-growth inhibition. This prevents the continued growth of microscale crystals as the inhibitor interacts with the scale crystal surface to prevent further addition of sulphate/barium ions. This is best evaluated via static bottle tests. Phosphonate-type scale inhibitors, for example, diethylenetriamine penta (methylene phosphonic acid), work well within this type of test.

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