Molecular dynamics simulations to study the effect of fracturing on the efficiency of CH₄ - CO₂ replacement in hydrates

Abstract

Feasible techniques for long-term methane production from naturally occurring gas hydrates are being explored in both marine and permafrost geological formations around the world. Most of the deposits are found in low-permeability reservoirs and the economic and efficient exploitation of these is an important issue. One of the techniques gaining momentum in recent years is the replacement of CH₄-hydrates with CO₂-hydrates. Studies have been performed, at both laboratory and field based experimental and simulation scale, to evaluate the feasibility of the in situ mass transfer by injecting CO₂ in gaseous, liquid, supercritical and emulsion form. Although thermodynamically feasible, these processes are limited by reaction kinetics and diffusive transport mechanisms. Increasing the permeability and the available surface area can lead to increased heat, mass and pressure transfer across the reservoir. Fracturing technology has been perfected over the years to provide a solution in such low-permeability reservoirs for surface-dependent processes. This work attempts to understand the effects of fracturing technology on the efficiency of this CH₄-CO₂ replacement process. Simulations are performed at the molecular scale to understand the effect of temperature, initial CO₂ concentration and initial surface area on the amount of CH₄ hydrates dissociated. A fully saturated methane hydrate lattice is subjected to a uniaxial tensile loading to validate the elastic mechanical properties and create a fracture opening for CO₂ injection. The Isothermal Young's modulus was found to be very close to literature values and equal to 8.25 GPa at 270 K. Liquid CO₂ molecules were then injected into an artificial fracture cavity, of known surface area, and the system was equilibrated to reach conditions suitable for CH₄ hydrate dissociation and CO₂ hydrate formation. The author finds that as the simulation progresses, CH₄ molecules are released into the cavity and the presence of CO₂ molecules aids in the rapid formation of CH₄ nanobubbles. These nanobubbles formed in the vicinity of the hydrate/liquid interface and not near the mouth of the cavity. The CO₂ molecules were observed to diffuse into the liquid region and were not a part of the nanobubble. Dissolved gas and water molecules are found to accumulate near the mouth of the cavity in all cases, potentially leading to secondary hydrate formation at longer time scales. Temperatures studied in this work did not have a significant effect on the replacement process. Simulations with varying initial CO₂ concentration, keeping the fracture surface area constant, show that the number of methane molecules released is directly proportional to the initial CO₂ concentration. It was also seen that the number of methane molecules released increases with the increase in the initial surface area available for mass transfer. On comparing the positive effect of the two parameters, the initial CO₂ concentration proved to have greater positive impact on the number of methane molecules released as compared to the surface area. These results provide some insight into the mechanism of combining the two recovery techniques. They lay the groundwork for further work exploring the use of fracturing as a primary kick-off technique prior to CO₂ injection for methane production from hydrates.