Water-Soluble Sodium Silicate Gelants for Water Management in Naturally Fractured Carbonate Reservoirs

Abstract

Abstract Fluid diversion gels, normally formed from polymer/crosslinker gelants, are often used to control injected water in oil-bearing formations. Injected fluid diversion becomes more important, and well treatment design requirements more challenging, in naturally fractured carbonate (NFC) formations due to the presence of highly conductive fluid pathways. The goal of this work is to screen and evaluate two commercially available, and environmentally friendly, sodium silicate gelant systems that may be used for water management in NFC reservoirs. HSE regulations may not permit the use of field-tested and successfully applied polymer-based gelants for water management. Strict environmental constraints on Norwegian Continental Shelf (NCS) require the use of "green" chemicals for any type of chemical field intervention. Several bulk- and core-based testing techniques have been engaged in this work, with the two investigated water-soluble silicate gelant chemicals having undergone through careful investigation of their filterability, injectivity, gelation time, gel strength, and gel shrinkage. The gelant impact on oil production was examined and the ability to reverse poorly deployed gel treatments in the field through gel dissolution was also investigated. Two commercial, water-soluble, green sodium silicates (Silicate A and Silicate B) have been examined. Activators used were NaCl solution, although other chemicals (e.g., Ca2+, Mg2+, HCl and HCOOH acids) have a similar effect, hydrochloric acid (HCl), nitric acid (HNO3), formic acid (HCOOH), and urea depending on the silicate used. Silicate A is a commercial product and Silicate B is a silicate system that has been used in North Sea for matrix treatment. Gelation, in either gelant, is mainly controlled by the activator type and concentration, silicate concentration and temperature. Normally, formed gel strength could be improved by increasing the injected silicate concentration but in Silicate B experiments, gelation difficulties were experienced for concentrations of approximately above 7 wt%. Filterability of Silicate B is significantly better than that of Silicate A. Fractured core tests using Silicate A gelant revealed that the average core permeability was reduced by three orders of magnitude between prior- and post-gelation conditions. Silicate B gelants exhibited "difficulties" in the gelation process and formed gels displayed a lower strength compared to ones for Silicate A gels. Unsuccessfully deployed gels may be reversed through dissolution with alkaline fluids such as NaOH or KOH; the gel dissolution rate is a function of the alkaline fluid concentration and the type of process used. Overall, Silicate A outperforms Silicate B in several tests and shows promising properties for its potential utilization in field applications to control injected water in NFC formations.