Water-soluble silicate gelants: Comparison and screening for conformance control in carbonate naturally fractured reservoirs

This work compares and screens green, water-soluble silicate gelants for possible use in isolating high conductivity water paths in fractured carbonate formations. The impact of cation, K+, Na+, Li+, Al3+ in the structure of a silicate is investigated with regards to gelation time, gel strength, and gel shrinkage. Several activators, such as NaCl, HCl, HCl + NaCl, HCl + NaCl + CaCl2, SAAP (sodium acid pyrophosphate) and CitAc (citric acid), are tested with the silicate systems at 40 degrees C, 60 degrees C and 80 degrees C and evaluated based on their ability to form rigid gels and on gelation time. The ability of the silicate gelants to form gel and the gelation time are determined mainly by the type of silicate, the SiO2 to M2O ratio (M denotes a metal ion), the type and concentration of the activator, and temperature. Some silicates do not always form gels; when gels are formed for a given silicate gelant, the gelation time is a strong function of the activator type and concentration, and temperature. Other parameters tested include pressure, ethylenediaminetetraacetic acid (EDTA) (chelating agent), and formaldehyde (bactericide for added xanthan). The impact of elevated pressure on the designed gelation time of silicate gelants is very important for field applications in the oil and gas industry. Results obtained from experiments conducted at high pressures (150 and 300 bar) and ambient temperature conditions showed that applied high pressures delay gelation of a given silicate gelant compared to gelation times observed at standard pressure and desired temperature conditions. Silicate gelants containing divalent ions display shorter gelation times at given conditions (silicate, activator, pressure and temperature) compared to ones of the same salinity without divalent ions. The use of EDTA as complexing agent to neutralize or partially control the impact of divalent ions on gelation time may not be efficient; therefore, the impact of these ions on gelant's gelation and gel properties must be considered. The addition of formaldehyde as bactericide to prevent long-term degradation of added xanthan was found to reduce gelation times and decrease gel strength. Formed gels from diluted Silicate A and Silicate C gelants (these gelants are defined in the next section of this paper) showed a similar behavior (gelation time) as the one of gels formed from non-diluted gelants. Gel shrinkage is a function of the gelant type and SiO2 to M2O ratio, temperature, and storage time. Silicate A gels display a low degree of shrinkage; Silicate C gels appear to be affected significantly by high temperatures (60 degrees C and 80 degrees C) with the gels displaying up to 80% shrinkage at long post-gelation storage times; Silicate C gels are less affected (lower shrinkage) at low temperatures (40 degrees C).