Formation of Water-Soluble Silicate Gels by the Hydrolysis of a Diester of Dicarboxylic Acid Solubilized as Microemulsions

Abstract

Summary Relatively inexpensive sodium silicate gels are attractive candidates for large-scale well conformance treatments. For a successful treatment, gelation must be delayed to permit proper placement of the silicate solution within the reservoir, to minimize rock/fluid interactions during placement, and to ensure long-term gel stability. We investigated each of these process phases, focusing on stable high-pH (≈11) sodium silicate solutions ranging up to 10 wt% SiO2 concentration. The addition of an acid or alcohol will cause these high-pH sodium silicate solutions to gel. To regulate the gelling time, an organic material that reacts with water to produce an alcohol and/or acid is often mixed with the silicate solution. An organic rather than inorganic compound is selected because the slow organic reaction rates yield a controlled gelation. The choice of the organic compound normally is quite limited because it must possess sufficient water solubility to be present in the quantities needed to cause gelling. This study shows that diesters, which hydrolize to produce both acid and alcohol, can be solubilized into silicate solutions as a microemulsion (not a macroemulsion), thus eliminating this limitation. All the rules developed with regard to selection of surfactants for EOR also apply to sodium silicate solutions. Field processes should use caustic preflushes to propagate silicate solutions deep into the formation. Long-term stability studies showed that, upon standing for several weeks, silicate gels tend to contract, expelling water. This process, called syneresis, clearly will affect the long-term effectiveness of a silicate treatment. We investigated syneresis over a wide range of silicate concentrations and temperatures. Gels formed from the high silicate concentrations exhibited the greatest degree of syneresis; up to an 80% decrease in volume. Increasing the temperature increased the syneresis. Some degree of syneresis may be desirable because all treated zones will retain residual permeability.