The goal of this work is to screen and evaluate laboratory mixtures of existing commercial, green sodium silicate chemicals that can be used for water management in carbonate naturally fractured (CNF) reservoirs. A thorough evaluation of a chemical for water management purposes requires the investigation of the chemical properties before, during, and after gelation. These properties are the gelant's viscosity, pH, filterability and injectivity, gelation time and kinetics of the gelation process, strength of the formed gel against applied external forces, gel stability, gel shrinkage, and post-gelation time behavior. This investigation has been conducted for different combinations (gelant systems) of sodium silicate with two types of polymers (a biopolymer and a synthetic polymer) along with a crosslinker.
Measurements on the gelant systems viscosity showed that the base sodium silicate system has a low viscosity with a practically Newtonian behavior. Therefore, potential field placement difficulties may exist due to the lack of wellbore control with the possibility of the injected gellant reaching non-depleted formation regions; this can be partially addressed by improving the gelant's viscosity through the addition of polymers without sacrificing, or even increasing in some cases, the resulting gel strength. Gelant systems containing biopolymer resulted in a more shear-thinning behavior than other gelant systems, and the measured viscosity/shear-rate data could be well-matched with the Carreau model.
Traditional tube testing and dynamic oscillatory tests, using an Anton-Paar MCR Rheometer, were performed to measure the gelation time and monitor the gelation process (initial viscous to viscoelastic state), and viscoelastic behavior of the formed gel at various temperatures. It was found that gelation time, a critical factor in the placement of the injected chemical system, can be tailored to field conditions by adjusting the gelant systems' contents and concentrations.
The strength of the formed gels of various gelant systems were compared based on results from Maximum Compressional Pressure (MCP) tests. Strength tests showed promising gel behavior mainly for two of the selected gelant systems, one of which is the base system (sodium silicate without polymer additives) and the other is a silicate/biopolymer system. Silicate/synthetic-polymer, with or without crosslinker, systems yield significantly weaker gels. More detailed investigations are needed mostly on the interpretation of the tests to be able to upscale the laboratory results to field applications.
In addition to bulk measurements, core flood tests were also conducted in artificially fractured carbonate cores to investigate gel isolation effectiveness, as well as formed gel stability and shrinkage at static gelation conditions. Although some shrinkage was observed over time in practically all core-scale experiments, the resulting core average permeabilities in post-gelation floods were significantly lower than the original fracture rock sample permeability; this was observed especially in the base sodium silicate and the silicate/biopolymer systems that were engaged to isolate the artificial core sample “fracture. Systems containing synthetic polymers, resulted in more shrinkage.