Chromatographie Transport of Alkaline Buffers Through Reservoir Rock

Summary Use of relatively low-pH alkaline buffers, such as sodium carbonate or sodium silicate, is explored as a means for overcoming sodium/hydrogen ion-exchange delay in alkaline waterflooding. A local-equilibrium chromatographic model is outlined to describe the concentration velocities for injection of alkaline buffers into a linear porous medium that exhibits reversible sodium/hydrogen exchange. The theory predicts a buffer ion-exchange wave that is substantially faster than that for equivalent-pH sodium hydroxide solutions. New experimental displacement data are presented for NaOH over a pH range from 11 to 13 and for 0.1, 0.5, and 1.0 wt% Na2CO3 flowing through a 1 wt% NaCl brine saturated Berea sandstone core at 50°C [122°F]. To permit a complete description of the system, column effluent concentrations are measured for sodium ions, hydroxide ions, H3-tagged water, and 14C-tagged carbonate. The experiments confirm that Na2C03 propagates through the Berea sand at a higher rate than NaOH. For example, at pH=11.4, Na2CO3 migrates with a velocity that is 3.5 times faster than NaOH. Comparison of experiment with the ion-exchange chromatography theory shows good agreement. We successfully model the concentration histories of tritium-labeled water, total carbon, sodium, and hydroxide, all with no adjustable parameters. This work establishes with both theory and experiment that buffered alkali significantly increases the propagation speed of hydroxide in reservoir sands in comparison with unbuffered alkali at equivalent sodium and hydroxide concentrations. Because lower-pH buffered alkali can also protect against rock dissolution loss, the validated reduction of buffer ion-exchange lag considerably improves the promise of the alkaline flooding process for field application.