Mechanisms of Retention in Silicate Enhanced Dilute Surfactant Low Tension Floods

ABSTRACT Earlier work has shown that the addition of sodium silicates to dilute surfactant systems reduces surfactant retention in Berea cores better than Na2CO3, NaOH or STPP. This paper addresses the fact that in solution the sodium silicate molecule is a complex mixture of species which can exhibit a variety of properties, depending on the concentration and ratio of SiO2 to NaO2. By selecting the specific form in which the sodium silicate is added, it is possible to optimize the recovery of oil under a broad range of reservoir conditions. Specific variables which may be affected by silicate chemistry are IFT, rock wettability, surfactant retention and specific permeability reduction, leading to improved three-dimensional sweep. A series of dilute surfactant core flooding tests were carried out, both with and without oil present, to determine the concentrations of the injected chemicals in the effluents. These tests covered a range of silicate ratios, brine salinity and hardness levels, and temperature conditions. The results show that each of these variables has specific effects on the retention of the injected chemicals by the sandstone substrate. Specifically, hardness increases retention of both silicate anion and surfactant, even with a large, soft water preflush. However, the silicate anion is preferentially retained, thereby leaving more surfactant in solution. Higher temperatures decrease retention of the surfactant and silicate anions. These results relate to observations of changes in recovery efficiency and permeability. While few people consider carrying out low-tension floods at surfactant levels of 0.1 to 0.5% by weight, the results of these studies are generally applicable to both alkaline flooding and micellar flooding at much higher surfactant levels. In fact, this technology may have advantages as a low cost and technically less complex enhanced waterflood as compared to a micellar/polymer technique. Such a system would be best applied in the secondary waterflood stage. In either case, these same reactions and mechanisms control the surfactant concentrations in solution, and how well the injected fluid contacts the major part of the reservoir.