Unravelling the Mystery of Inhibitor Resistant Calcite Impacting Onshore and Offshore Operations

Abstract

Magnesian calcite has been identified as the main constituent of solids fouling several bp facilities in the North Sea and Azerbaijan and in partner operations in other locations. These solids were, for many years, thought to be calcite scale for a number of reasons. The deposits formed compacted solids and thus looked similar to scale, they effervesced and dissolved in acid and calcium was detected as the main constituent. XRD analysis, however, revealed the deposits were magnesian calcite. The inclusion of magnesium into the crystal cannot be co-precipitated with calcite scale under typical oilfield conditions and timelines and their deposition cannot be controlled by conventional scale inhibitor injection. These solids, transported from the reservoir with the produced fluids can act as scavenger sites, reducing the effective concentration of scale inhibitors available to manage conventional scale deposition. The fact that these solids, in many ways, present themselves as a conventional scale has perhaps hidden the extent of their occurrence in offshore and onshore operations. Magnesian calcite has been identified as the main constituent of retrieved solids that were (1) adhered to tubing and restricting production in a North Sea field (2) impacting operation of subsurface safety valves in both North Sea and Caspian Sea operations (3) reducing the efficiency of heaters and coolers offshore North Sea and onshore in the Caspian and (4) plugging strainers in the produced water handling systems offshore North Sea and onshore Caspian. This paper will describe how this ‘pseudo scale’ was detected and its prevalence, using field case examples from several bp operated and non-operated assets. It describes the theories developed to rationalise magnesian calcite deposition onto production surfaces including the possible role of ‘sticky’ molecules and the technology bp is evaluating to holistically manage this fouling phenomena. It will share dispersant chemical field trial performance data and the deployment of electromagnetic fields to determine the impact of changing zeta potential on the attraction of fine magnesian calcite particles to the surface and thus the fouling rate.