A convenient protocol for screening the potential efficacy of scale inhibitors is described. All of the techniques used require relatively inexpensive equipment. Two types of scale are addressed in this paper, calcium carbonate and calcium oxalate, but the general procedures are expected to be applicable to inhibitor screening for others as well.
Scale and corrosion inhibitors are commonly used in many oil and gas production systems to prevent inorganic deposition and to protect asset integrity. Scale inhibitor products are based on organic compounds with phosphate or carboxylic functional groups such as amino phosphonates, phosphate esters, phosphino polymers, polycarboxylate and polysulfonates,1 as shown in Figure 1. These anionic groups have strong affinity to alkaline earth cations and can adsorb on the active growth sites of scale crystal (Figure 2), resulting in stopping or delaying the scale formation process.
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.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 formation of mineral scale is an undesirable phenomenon which is as a result of the disturbances in thermodynamics and chemical equilibria of the water system. CaCO3 scale is one of the major flow challenges in the oil industry and the crystallization process starts from thermodynamically unstable hydrated form to anhydrous polymorphic stable forms1,2 The transformation involves a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy of the system where the crystallization of the dehydrated amorphous material lowers the enthalpy the most. There are two theories regarding the polymorphic transformation of a solid structure. The first suggests the transformation occurs through a direct solid transition in which the metastable phase exhibits a rearrangement of its molecules or atoms to a more stable form3. The second is valid in the presence of a solvent which allows the dissolution and the re-nucleation and growth of the stable phase4.
Calcium carbonate precipitation in the presence of pollutants is a carrier of importance for their transport to the sediments and for their subsequent release depending on the local microenvironment conditions. Zinc is often present in industrial waters (heat exchangers and boilers) mainly for corrosion protection. Depending on the alkalinity and calcium concentration of waters used in water intensive processes, calcium carbonate fouling is common. Moreover, among other metals, is present in natural waters together with iron and copper, where the concentration of Zn(II) may reach at levels of several ppm.