Calcium carbonate (CaCO3) scale formation is recognized as a major problem affecting production and transportation in the oil and gas industry. The phenomenon is extensively studied however; very limited work has been carried out to evaluate it in multiphase environments. This work aims to study calcium carbonate surface deposition in a multiphase environment that can replicate more accurately conditions encountered during secondary and ternary oil production. Multiphase conditions induced by introduction of a light distillate within the system were used to create oil in water (o/w) emulsions in order to reflect more accurately the scaling process in oil pipeline transportation. Using a set of bulk and surface analysis techniques such as Inductively Coupled Plasma (ICP) spectroscopy, X-ray powder diffraction (XRD), or Scanning Electron Microscopy (SEM), the results showed that the presence of an oil phase within the system retard the nucleation as well as the dissolution of vaterite, the metastable phase of calcium carbonate. This affect the growth kinetic of calcite and contribute overall to hinder mineral surface fouling. When lead sulfide (PbS) co-precipitates alongside calcium carbonate, the experimental observations show that PbS crystals provides additional seeding point for the nucleation of CaCO3 to take place. In such conditions, the mechanism of surface fouling build-up is altered and proceed via the impact and adhesion of PbS/CaCO3 particles onto the stainless-steel surfaces which result in higher mass gain.
Inorganic fouling in oilfields has resulted in millions of dollars of operating expenditure every year since the inception of offshore oil and gas drilling, where mineral scale deposition in tubing, flowlines and downhole equipment leads to significant production downtime. Calcium carbonate (CaCO3) fouling is endemic in oilfield systems, as produced water containing both bicarbonate and calcium ions is prone to form precipitates as a result of pressure changes during production.1,2,3 The release of carbon dioxide gas from the aqueous phase prompts the evolution of carbonate resulting in a rise in pH and consequent precipitation. The additional reduction of CaCO3 solubility with increased temperature dictates that precipitation can occur anywhere from the wellbore to the topside, with case studies showing precipitation of CaCO3 species both near the wellbore and higher up the production tubing.4,5 Before deploying a mitigation technique (i.e., scale chemical inhibitors) an experimental matrix is usually developed and laboratory tests are carried out to evaluate its adequacy with the oilfield under consideration.6,7 However, the match between laboratory performance and the one ultimately observed in the field can be poor. Overcoming these discrepancies requires a comprehensive understanding of the calcium carbonate precipitation process in complex multi-phase environments in which they usually occur. Indeed, oil production environments rarely contain only an aqueous phase and are usually more complex due to the presence of an oil phase. The presence of such organic phases makes prediction of surface fouling difficult and lead to the need for field measurement to refine formulation of scale inhibitors to the specific conditions prevailing in each application. This study aims to establish a better knowledge on calcium carbonate surface fouling kinetics and mechanisms in oil and gas production systems.
