(286g) Study on Flow Behavior of CO2-in-Water (C/W) Emulsion in the Porous Medium Under Hydrate Stability Conditions

CO₂-in-Water (C/W) emulsion injection method (a method of injecting CO₂ droplets mixed in water as an emulsion state) is expected to be a useful technique for recovering methane gas from methane hydrate (MH). In this technique, CO₂ hydrate particles are formed from C/W emulsion injected into aquifer layers in the MH zone. The heat of CO₂ hydrate formation can be used to increase the temperature of sediment and supply energy to dissociate MH. In this work, we have developed a numerical program for predicting flow behavior of C/W emulsion with hydrate formation in the porous medium. After comparing the calculated pressure and temperature behavior with experimental data of C/W emulsion injection into the core, injection schemes of C/W emulsion to make the heat of CO₂ hydrate formation maximum were investigated by numerical case studies.

In the simulation program, five governing equations are solved: Three continuity equations of H₂O component, liquid CO₂ component, and total CO₂ component (liquid and dissolved CO₂) in C/W emulsion; one equation of describing transport of CO₂ hydrate particles; and one energy balance equation considering heat convection/conduction and the heat of hydrate formation/CO₂ dissolution into water. These equations are formulated based on the following assumptions: (1) C/W emulsion flows as a homogeneous phase before hydrate formation. (2) CO₂ droplets are converted into hydrate particles at a given rate after induction time (thin films are formed around CO₂ droplets). (3) Hydrate particles are transported in the pore by emulsion flow and dispersion, where some particles are trapped on the pore surface and the pore throat of the porous medium. (4) The permeability of the porous medium decreases with increasing saturation of trapped hydrate particles. In the numerical procedure, five independent variables of pressure, temperature, concentration of H₂O and liquid CO₂ in C/W emulsion, and saturation of flowing hydrate particles are solved by five governing equations.

To examine the validity of numerical calculation, a number of experiments were conducted to inject C/W emulsion (CO₂ vol. concentration of 10-30%) into 0.1 mm glass-bead sintered water-saturated cores under controlled pressure and temperature conditions of 7 MPa, 7°C and 5 MPa, 2°C. The length of the core was 10 cm, and injection pressure and core surface temperatures at the locations of 2.5 cm, 5 cm, and 7.5 cm from inlet were sequentially recorded during the experiment. In most experiments, we observed that inlet pressure rose steeply to lead the core plugged a certain time after the increase in temperatures by hydrate formation.

After confirming that simulation could reproduce the tendency of pressure and temperature behavior during the experiments, numerical case studies were carried out to find appropriate injection schemes of C/W emulsion maximizing the heat of CO₂ hydrate formation until the plug of the core. The following conclusions were obtained. (1) Heat of 10-16 MJ/m³ of the porous medium is generated during C/W emulsion until the plug of the core. This corresponds to the heat value that can dissociate about 10 % of MH. (2) The lower the CO₂ concentration of C/W emulsion, the smaller the heat generation rate of hydrate formation, but the time to core plugging is much delayed compared with the case of high CO₂ concentration. As a result, the use of lower CO₂concentration generated the maximum heat. This study showed that C/W emulsion injection method would be effective to increase gas recovery efficiency from MH if we select an appropriate injection scheme.