Effect of Ca2+, Mg2+, Ba2+ and Sr2+ cations on calcium carbonate scaling formation in oil-gas well: Based on density functional theory study and molecular dynamics simulation

The effect of different cations (Ca2+, Mg2+, Ba2+ and Sr2+) adsorption on carbonate type scaling formation was studied by using CASTEP and Forcite module based on density functional theory and molecular dynamics, respectively. By calculating the adsorption energy of cations adsorbed on four adsorption sites (top, hollow, short-range bridge and long-range bridge) of (2 x 1 x 1)CaCO3 (1 -1 0) surface, the most stable adsorption site was determined. On this basis, the banding mechanism of O atom and cation was studied by analyzing the state density of different cations adsorbed model. It has been shown that the Mg2+ cation adsorbed at top site of O atom on CaCO3(1 -1 0) surface was the most stable one according to the adsorption energy calculation results. Under the condition of multiple type of cations co-existence at the same ionic concentration, Mg2+ and Ba2+ would preferably combined with CO32- to form magnesium carbonate and barium carbonate scaling, causing the crystal growth process of calcium carbonate scaling inhibited. The converse is equally true that the existence of Sr2+ is more conducive to the calcium carbonate scaling crystal growth. The radial distribution function analysis indicated that the bonding strength between Mg2+ and O atom on CaCO3 (1 -1 0) surface is the largest, weakening followed Ba2+, Ca2+ and Sr2+ successively. Density of state study indicated that adsorption of Mg2+ cation on CaCO3 (1 -1 0) surface is mainly due to the interaction between 2p orbit of O atoms and 3s orbit of Mg2+ cation.