Barite Dissolution/Precipitation Kinetics in Porous Media and in the Presence and Absence of a Common Scale Inhibitor

Abstract

Summary Field engineers and reservoir modelers often question if equilibrium conditions prevail downhole, and when do super(sub)saturation conditions exist. This questioning is especially critical in designing seawater injection for pressure maintenance caused by serious barite scale problems in barium-containing formation water. This paper: (1) discusses the experimental research on rock-brine interaction to determine if equilibrium conditions and kinetics prevail, and (2) develops realistic seawater/inhibitor injection schemes for scale control during seawater injection. We have examined the question from three points of view: (1) thermodynamic equilibrium, (2) mass transport kinetics, and (3) experimental testing of flow through columns with or without scale inhibitors. If the reaction direction is from undersaturated toward saturation, then equilibrium normally occurs rapidly, being observed within approximately 5 minutes. When the reaction is going from supersaturated to the equilibrium direction, such as during seawater injection into a barium-containing formation, the precipitation reaction is often slow (i.e., equilibrium is not observed after 10 minutes of contact time. Both barite dissolution and precipitation rates on clean core material are consistent with those reported in literature with a second order rate constants for dissolution (≈30,172 L2·mol−1·m−2·sec−1, 100°C) and precipitation (= 938 L2·mol−1·m−2·sec−1, 100°C). The kinetics of barite formation can significantly slow down in the presence of scale inhibitors, and the sulfate tolerance can be increased. The kinetics of both barite dissolution and precipitation are poorly understood at the present time. Combining sulfate reduction and scale-inhibitor application in intelligent engineering design can significantly reduce the problems and costs associated with seawater injection. Equations for the engineering design of such treatment were derived from nucleation kinetics, inhibition efficiency, and inhibitor adsorption and transport. Sulfate tolerance in the presence of scale inhibitors is measured and compared with the prediction from nucleation inhibition theory with excellent agreement. An innovative inhibitor treatment by way of coreflood has been done as proof-of-concept and is discussed herein.