Quantifying the retention and release of polyphosphonates in oil and gas producing formations using surface complexation and precipitation theory

The prevention of scale is an important chemical process in many commercial applications, such as water purification, electricity generation, and chemical production. Scale prevention is especially important in the area of oil and gas production, where scale formation can restrict or completely block fluid flow in a well. One economical way to prevent scale buildup in oil and gas wells is through an inhibitor squeeze treatment, where scale inhibitor chemical is injected into the producing formation. The concentration of inhibitor returning with the brine inhibits scaling. This return concentration is currently unpredictable, because the interaction between the inhibitor and the rock forming minerals have not been quantified. Thus, treatment can neither be controlled, optimized, nor predicted. In the current research, surface complexation, solubility, and phase transformation theory are used to quantify the inhibitor-rock interactions. Solubility and adsorption equilibrium constants of the chemical components in commercial Nitrilotri (methylene phosphonic acid), NTMP, have been determined from laboratory experiments or obtained from the literature. The precipitation and solubility of calcium-NTMP salts were observed to vary with experimental conditions and with extent of aging. This observation was similar to observations made with calcium phosphate precipitation. The equilibrium constants obtained were used to evaluate the chemical conditions in the formation during an inhibitor squeeze at the Texaco N. R. Smith #4 well. Results suggested that $Ca\sb{2.5}HNTMP*2H\sb2O$ and $CaHPO\sb3*1.5H\sb2O$ were in equilibrium with the return brine. The results also suggested that the relative concentration of the chemical components in the commercial product differed in the return brine. The equilibrium constants were also incorporated in a simplified transport equation to simulate the squeeze process. The simulation results suggested that calcite availability may limit inhibitor component precipitation. The results also showed that the adsorption and calcium salt solubility of the inhibitor components could be used to estimate the shape of the return curve