Feasibility of EcoGrout: Modelling of in-situ calcite precipitation and CO2 degassing in porous medium

Abstract

EcoGrout is a new method to strengthen and stabilize unconsolidated sediments in the subsurface. This method is based solely on chemically induced calcite precipitation. In situ precipitation of calcite will result in cementation of the grain particles during the EcoGrout process. Consequently, it will also decrease the porosity and thereby the permeability of the soil. The basic application of Ecogrout involves use of a solution with a designated degree of calcite saturation and CO2 pressure by the use of calcium chloride (CaCl2), sodium bicarbonate (NaHCO3) and pH adjustments to control both the amount and rate of calcite precipitation. This modelling study explores the conditions under which precipitation amounts and rates can be controlled and prevent clogging in the vicinity of the injection point, whilst ensuring sufficient radii of influence for the Ecogrout treatment. To analyze this EcoGrout process, a model for multiphase flow of multiple gas-components and water (STOMP-WNE) with a reactive batch chemistry module (ECKEchem) is used. This model is extended to account for porosity and permeability alteration over time during calcite precipitation. Also a kinetic equation for degassing of CO2 proposed by Zhao et al. (2011) is implemented. Simulation of an injection pulse with an EcoGrout solution at calcite equilibrium at a pCO2 of 5.1 atm was done to investigate the behavior of CO2 degassing in the porous medium and its effect on calcite precipitation. Several cases with different mechanical properties, kinetic properties or EcoGrout solution configurations are presented to investigate its sensitivity on CO2 degassing and calcite precipitation. Precipitation of calcite is controlled by the degassing of CO2. Due to degassing of CO2 the alkalinity and pH increase. Consequently, the injected EcoGrout solution becomes supersaturated with respect to calcite and calcite precipitation starts. Since the pore volume reduction obtained from one injection pulse is low, many injections are required to obtain the required cementation within the soil. Large injection radii can be achieved when the injection pressure is higher than the pCO2 of the solutions and no degassing occurs during the injection. In addition, the degassing rates of CO2 are sufficiently low that injection at CO2 supersaturation is also feasible.