Sodium Carbonate Preflush Theoretical Analysis and Application to Chateaurenard Field Test

ABSTRACT In a previous paper1 we described the mechanisms of oil recovery by the microemulsion injected in Chateaurenard (France) micellar/polymer field tests. The calcium gradient created through the micellar slug as a result of cation exchanges between micelles, water and clays has been shown responsible for both high displacement efficiency of the microemulsion and trapping of surfactant in residual oil. The efficiency of the same microemulsion is improved if calcium is removed ahead of the micellar slug and if the calcium gradient is replaced by a salinity gradient. In this paper we describe how this can be achieved by using a sodium carbonate preflush which induces precipitation of calcium from the clays as calcium carbonate. A series of sandpack displacement tests have been carried out, both with and without oil present, to study the effect of a sodium carbonate preflush and to understand the mechanisms involved in such a process. Experimental results have confirmed that use of a 5g/l Na2CO3 preflush can increase the oil recovery rate up to twice its value without preflush, depending on preflush size and quantity of microemusion injected, and reduce correspondingly retention of the surfactant. In order to interpret these tests a numerical simulator has been elaborated. It takes into account the dispersive properties of the sandpacks and the significant interactions between minerals and fluids. A good agreement between preflush displacement tests and simulations has been achieved using measured input parameters.