Equilibrium Modelling of Interactions in DETPMP-Carbonate System

Abstract

Abstract Scale inhibitor (SI) squeeze treatments are widely used for the prevention of inorganic scale deposition in oil and gas production operations. This may be an expensive operation and its efficiency depends on the degree of SI retention in the formation. Carbonate formations are known to be highly reactive, where the SI retention is driven by both adsorption and precipitation of SI and SI-Ca/Mg complexes. To design and carry out this type of squeeze treatment, a comprehensive model capable of simulating SI retention in carbonate formations is required. In this study, a model has been developed to fully characterize the retention of DETPMP in a carbonate system. This model considers all the equilibrium reactions coupled with precipitation and adsorption processes to simulate the equilibrium of a DETPMP-Calcite-brine system containing free Ca2+ and Mg2+ cations. To equilibrate this system, the following coupled reactions were considered: (i) the full aqueous carbonate system, (ii) SI speciation by dissociation (SI is considered as a weak n-poly acid, HnA), (iii) SI impurities and their reactions, (iv)SI-Ca and SI-Mg complexation, (v)the associated adsorption and precipitation of the SI/Ca/Mg complexes. These reactions may be coupled together through the equilibrium equations, the mass balance of base species and the system charge balance. After some algebra, the system of equations is reduced and solved by Newton Raphson to find the concentration of key primary species from which the concentration of all other species is calculated and the equilibrium of the entire couple system is characterized. In the entire system, there may be up to ~100 species involved in the chemical equilibrium equation set. The adsorption process is characterized by an adsorption isotherm, Γ(C), which can be a reversible process. In the examples presented, the adsorption is considered to proceed in both directions of adsorption and desorption. Precipitation (denoted Π) is coupled with the adsorption (and the rest of the system) to satisfy the SI solubility by removing further SI from the solution, if required, through the complex SI species that may physically precipitate. Finally, the proposed model was validated against coupled adsorption-precipitation experiments. The results showed very good agreement between the model and experiments and confirmed the reliability and validity for various conditions and DETPMP concentrations.