Flow-Through Experiments of Reactive Ba-Sr-Mg Brines in Mons Chalk at North Sea Reservoir Temperature at Different Injection Rates

Abstract

Abstract Chalk reservoirs in the North Sea of Norway contain significant amounts remaining oil and gas, and are potential candidates for enhanced recovery by modifying the injected brine composition. This work investigates how brines with divalent cations Ba, Sr and Mg interact when injected into chalk (CaCO3). Ba and Sr are often associated with mineral precipitation and occur in formation water while Mg is present in seawater, commonly injected in chalk. Relatively clean (>99% calcite) outcrop chalk cores from Mons, Belgium, were flooded at 130 °C in triaxial cells with four brines containing 0.12 mol/L divalent cations: either 0.06 mol/L Sr and Ba, 0.06 mol/L Sr and Mg, or 0.12 mol/L Ba or Sr. Each brine was injected in a separate core, with 100 to 150 pore volumes. The injection rate was varied between 0.5 and 8 pore volumes per day. Produced brine was analyzed continuously and compared with the injected brine composition. After flooding, the cores flooded with only Ba or only Sr were cut into slices and analyzed locally in terms of Scanning Electron Microscopy (SEM), matrix density, specific surface area and X-Ray diffraction. In all experiments, the produced divalent cation concentration was reduced compared to the injected value. The total reduction of injected cation concentration closely equaled the produced Ca concentration (from calcite dissolution). When flooding 0.12 mol/L Sr, the Sr concentration depleted 50%, while when flooding 0.12 mol/L Ba, 10% Ba depleted. When injecting equal concentrations of Ba and Sr, 40% Sr and 7% Ba depleted, while with equal concentrations of Mg and Sr injected, ~50% Sr was retained and almost no Mg depleted. Sr appeared to dominate and suppress other reactions. There was little sensitivity in steady state concentrations with variation in injection rate. The similar modification of the brine regardless of residence time suggests the reactions reached equilibrium. Cutting the cores revealed a visually clear front a few cm from the inlet. The material past the front was indistinguishable from unflooded chalk in terms of density, specific surface area, micro scale structure, porosity and composition (XRD and SEM-EDS). The material near the inlet was clearly altered. Images, XRD, SEM-EDS and geochemical simulations indicated that BaCO3 and SrCO3 formed during BaCl2- and SrCl2-flooding, respectively. The simulations also predicted equal exchange of cations to occur. The matrix densities, porosities and the distance traveled by the front corresponded with these minerals and suggested that the chalk was completely converted to these minerals behind the front. It was demonstrated that Ba-, Sr-, Mg-brines and their mixtures can be highly reactive in chalk without clogging the core after tens of pore volumes. This is because precipitation of minerals bearing these ions associates with simultaneous dissolution of calcite. The Ca-, Ba-, Sr-mineral reactions are effectively in equilibrium. Previous investigations with MgCl2 show rate dependent results and smoother alterations, indicating that Mg-mineral reactions at same conditions have longer time scale.