Investigation of the Antiscaling Performance of Phosphonated Chitosan for Upstream Petroleum Industry Application

Scale deposition is one of the main water-based production problems in the upstream oil and gas industry. Few environmentally friendly scale inhibitors show good thermal stability as well as calcium compatibility. We report the synthesis of phosphonated chitosan (PCH) in a two-step route via a phosphonate ester. Chitosan is made from chitin, a natural polysaccharide. In dynamic tube blocking tests, PCH showed good performance as a calcium carbonate scale inhibitor, similar to some commercial nonpolymeric aminophosphonates. Performance was not lost even after thermal aging as a 5 wt % aqueous anaerobic solution for 1 week at 130 °C. The performance as a barite inhibitor was shown to be significantly worse. PCH showed excellent calcium compatibility from 100 to 10 000 ppm Ca2+ and 100 to 50 000 ppm PCH for 24 h at 80 °C. Density functional theory (DFT) and molecular dynamics (MD) simulations are employed to gain atomic insights into the interaction of PCH with the mineral surface as well as the polymer morphology. DFT predicts that PCH interacts as strongly as commercial scale inhibitors. MD simulations reveal a conformational contraction of PCH due to its internal hydrogen bonding network, which makes the inhibition mechanism complicated. Our simulation results bring new insights into the inhibition mechanism of polymeric inhibitors compared to small molecules. For example, a polymer with a well-defined structure such as carboxymethyl inulin (CMI) performs better than random folded polymers (PCH). The structural regularity maximizes the interaction sites of the mineral particles on the polymer surface. The compact morphology of PCH and the slow barite kinetics could be the main reason for the bad performance of PCH for barite scale inhibition.