Laboratory Testing of Environmentally Friendly Sodium Silicate Systems for Water Management Through Conformance Control

The goal of this work is to screen and evaluate laboratory mixtures of existing commercial, environmentally friendly sodium silicate chemicals that can be used for water management in naturally fractured carbonate (NFC) reservoirs. A thorough evaluation of a chemical for conformance-control purposes requires the investigation of the chemical properties before, during, and after gelation. These properties are the gelant's viscosity, pH, filterability, and injectivity; the 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 xanthan biopolymer and a low-molecular-weight synthetic polymer) and a crosslinker. Measurements on the gelant systems' viscosity showed that the base sodium silicate system has a low, water-like, viscosity with a Newtonian behavior. This provides significant benefits for matrix treatment, but higher gelant viscosities may be required when sodium silicate gelants are used for conformance control of highly conductive fluid pathways to minimize potential placement difficulties caused by the lack of wellbore control with the possibility of the injected gelant reaching nondepleted formation regions, especially in cases of static gelation. This can be partially addressed by enhancing the gelant's viscosity through the addition of polymers without sacrificing, or even increasing in some cases, the resulting gel strength while at the same time lowering the gelant matrix injectivity. Gelant systems containing biopolymer resulted in a more shear-thinning behavior than other gelant systems, and the measured viscosity/shear-rate data could be wellmatched with the Carreau-Yassuda model. Traditional tube testing and dynamic oscillatory tests were performed to measure the gelation time and monitor the gelation process and viscoelastic behavior of the formed gel at various temperatures. 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 on the basis of the 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 systems, with or without crosslinker, 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, coreflood tests were also conducted in artificially fractured carbonate cores to investigate gel-isolation effectiveness and formed-gel stability and shrinkage at static gelation conditions. Although some shrinkage was observed over time in practically all core laboratory 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.