Formation of Water-Soluble Silicate Gels by the Hydrolysis of a Diester of Dicarboxylic Acid Solublized 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. Introduction Waterflooding sometimes can be improved by "blocking" permeable zones to force injected water through less permeable strata. permeable zones to force injected water through less permeable strata. Near-wellbore treatments that use crosslinked polymers as blocking agents have been carried out, but larger treatments may be prohibited for both economic and technical reasons. In such cases, prohibited for both economic and technical reasons. In such cases, treatments that use gelled sodium silicate solutions appear to offereater potential because of their lower cost. Although a number of potential because of their lower cost. Although a number of patents propose the application of aqueous silicate solutions, patents propose the application of aqueous silicate solutions, their actual use has been limited compared with polymer solutions. Many sodium silicates form strong gels, so understanding why they have not been more widely applied is difficult. Possible reasons arethe mechanism of gelation is poorly understood,methods to retard gelation are not effective or controllable,the penetration of sodium silicate solutions into the formation is limited by the buffering capacity of the rock, orthe blockage did not last. This paper addresses all these questions. In our study, we used esters to initiate gelling. In the presence of an alkaline silicate solution, the esters hydrolyze to produce an acid and an alcohol, which are the gelling agent and the activator of gelation, respectively. We postulate a mechanism for the gelling process that satisfies the observation that gelation occurs without process that satisfies the observation that gelation occurs without a change in solution pH. We also consider the problem of controlling gelling time so that an ungelled silicate solution may be placed in the more permeable regions as a low-viscosity solution. With the proper ester, gelation time can be controlled to accommodate reservoir temperatures up to 367 K [200deg.F]. Using the products of an ester hydrolysis reaction to trigger gelation is not a new idea. In many cases, the best ester (with a suff ciently retarded hydrolysis reaction) will have limited solubility in water. We demonstrate that these esters can be dissolved in sodium silicate as a stabilized microemulsion. This means that the ester concentration can substantially exceed the solubility limit, thus ensurg an adequate supply of the acid and alcohol products. According to this work, any organic material can now he used as a gelling agent regardless of its water solubility. Because we demonstrate a principle rather than develop a commercial system, we made no attempt to optimize the microemulsion system. Nor are the diesters used here believed to be the only or even the best gelling agents.