Determination of Alkalinity Losses Resulting From Hydrogen Ion Exchange in Alkaline Flooding
- Zdenka Novosad (Petroleum Recovery Inst.) | Jerry Novosad (Petroleum Recovery Inst.)
- Document ID
- Society of Petroleum Engineers
- Society of Petroleum Engineers Journal
- Publication Date
- February 1984
- Document Type
- Journal Paper
- 49 - 52
- 1984. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 1.2.3 Rock properties, 1.6.9 Coring, Fishing, 5.6.5 Tracers, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex)
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Maintaining an effective level of alkalinity during an alkaline flood is of prime importance in design of this EOR process. Alkaline solution can be consumed in the reservoir through interaction with both reservoir rocks and fluids. The rock/alkali interaction can be characterized as either chemical or ion-exchange reaction, and the magnitude of alkalinity consumption and its type must be considered when alkaline floods are designed for specific reservoirs. This is because alkalinity propagation in porous media is directly affected by the type of alkalinity loss. It has been shown previously that kinetically controlled mineral dissolution reactions may cause the produced alkalinity to plateau at a lower concentration than the injected concentration, whereas ion-exchange processes may cause characteristic delay in the produced alkalinity, which in turn delays tertiary oil production. The type and the magnitude of alkalinity loss therefore must be known to design a flood with alkaline concentration maintained at its optimum for the longest possible time. This paper concerns determining alkalinity loss resulting from hydrogen ion exchange. The model describing the release of initially rock-bound hydrogen ions into solution in the presence of alkali has been previously proposed and investigated for California Wilmington and Huntington sands. According to this approach the mechanism for reversible alkalinity uptake is described by the following equation.
Since the experiments were carried out with sand fully converted to the sodium form (i.e., sodium form at neutral pH), this implies that additional cation-exchange sites, different from those responsible for the Ca2+ -Na+ exchange, are involved in Na+ -H+ exchange. The number of these sites per unit mass was defined as the hydrogen exchange capacity (HEC) of the rock. The reaction described by Eq. 1 is reversible and implies simultaneous and equal uptake of Na+ and OH- ions. Previous experimental studies of the process described by Eq. 1 consisted of monitoring the effluent hydroxyl ion concentration during NaOH injection into a sandpack. However, mineral dissolution reactions always contribute to OH- loss, especially when minerals of high specific surface area are present. Hence, monitoring only effluent OH- concentration may exaggerate alkalinity loss resulting from hydrogen exchange. This work describes a new approach for determining the alkalinity lost as a result of hydrogen ion exchange during a coreflood. Monitoring sodium rather than hydroxyl ion concentration in the effluent of an alkaline flood circumvents the difficulty in assessing alkalinity consumption caused by hydrogen exchange alone. Effluent hydroxyl ion concentration reflects alkalinity losses resulting from chemical reactions with rock minerals and fluids as well as from any ion-exchange process; the changes in effluent sodium reflect alkalinity loss only from hydrogen exchange. Obviously, the method requires that the rock is converted to a single cationic form (i.e., sodium) before an alkaline flood.
All experiments presented were carried out with Berea sandstone at 23 deg. C [73 deg. F] in the absence of an oil phase. Two series of coreflooding experiments were performed. The first was designed to ascertain whether the Na+ uptake occurs during an alkaline flood (as suggested by Eq. 1) by performing an alkaline flood in a Berea core completely converted to sodium form and by monitoring the sodium concentration in the effluent. The sodium concentration in the NaCl brine filling the pore space prior to NaOH injection was closely matched to the sodium concentration in the injected NaOH solution to make it possible to measure small changes in the effluent sodium concentration caused by hydrogen ion exchange. The second series was designed to determine the Berea sandstone cation-exchange capacity (CEC) at alkaline pH by Na+ -Ca2 + exchange and to compare its value with that of the HEC.
Caustic Floods in Berea Core. Dry and epoxy-coated rectangular Berea cores were evacuated and saturated with either 0.5 or 1.0 N NaCl brine (2.9 or 5.6 wt% NaCl). Several pore volumes (PV) of NaCl brine were injected and the cores were allowed to age for several days to achieve a complete conversion to the sodium form.
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