Controlling the gelation time of sodium silicate gelants for fluid management in hydrocarbon reservoirs

Abstract

An efficient gel-based treatment of fractures and high-permeability layers requires the precise placement of the gelant within the formation, which is highly dependent on the gelation time. This study proposes an optimal method that reduces the uncertainties associated with the exact placement of sodium silicate gelants in a formation. This method is based on the control of the gelation time, so that these potent and eco-friendly blocking agents can be used more effectively in water shut-off, profile modification, and carbon storage projects. We proposed a new pre-flush operation and gelation protocol for sodium silicate gelants; gelation time experiments were conducted using a rheometer and through characterization of gel strength codes. To improve the pre-flush operation, the masking potential of citric acid is examined in this study. We show that citric acid is capable of complexing formation water ions, thus small amounts of citric acid can delay the gelation time by up to 8 h. In previous studies, silica nanoparticles were used as nucleation promoters in sodium silicate gelants, which required large amounts of surfactant for their stabilization. As an alternative, we propose a new gelation mechanism that is based on the adsorption tendency and nucleation ability of silica nanoparticles. We show that, based on their adsorption tendency, silica nanoparticles initially form an H+ layer around themselves and do not participate in the gelation reaction. At the same time, they can reduce the reaction rate of silicates, which postpones the gelation time by up to 14% (at 0.15 wt% of silica nanoparticles in defined concentrations of sodium silicate and acid). However, we observed that adding silica nanoparticles at a concentration less than 0.15 wt% or greater than 0.3 wt% accelerates gelation. But when the H+ layer breaks down, the nanoparticles intensify the gelation process through nucleation. The mechanistic insights are derived from an experimental study in which the concentrations of silica nanoparticles, sodium silicate, and gelling agent are varied. Also, experiments in a wide range of temperatures and concentrations of silica nanoparticles, sodium silicate, and citric acid showed that shielding silica nanoparticles with H+ ions released from the gelling agent keeps the nanoparticles stable in the gelant without the use of other additives. Accordingly, this mechanism, in addition to having the ability to decrease the gelation rate, increases the nucleation capacity, and reduces the complexity and cost of the system. The results of this work provide guidelines for preparing sodium silicate gelants and optimizing their gelation process.