Liquid CO2 Replacement and Sequestration Behavior after Depressurization Production of Methane Hydrates

Abstract

Aligned with China’s “dual carbon” strategic objectives, natural gas hydrates─recognized as a crucial source of clean unconventional natural gas─constitute a significant technological avenue for securing national energy stability and meeting carbon reduction targets through the integrated approach of efficient resource extraction and CO2 geological sequestration. The clayey silt hydrate reservoirs located in the LS36-1 block of the South China Sea exhibit pronounced heterogeneity and low permeability, presenting dual engineering challenges in field development: suboptimal methane (CH4) extraction efficiency and limited CO2 sequestration capacity. Presently, the commercial viability of these reservoirs cannot be realized through a single extraction technology alone. Consequently, a synergistic extraction framework that merges the engineering practicality of pressure-reduction techniques with the carbon sequestration benefits of CO2 displacement methods has emerged as a focal point for advanced research. Utilizing the in situ physical characteristics of the target reservoir, this investigation employed a proprietary high-pressure multigas coproduction experimental apparatus to fabricate core samples representative of reservoir conditions. Physical simulation experiments were conducted under varying parameters, including initial saturation levels, pressure reduction extents, injection sites, and injection modalities. The study elucidated the coupled mechanisms governing CH4 recovery and CO2 sequestration efficiency within the “pressure reduction extraction combined with liquid CO2 displacement” synergistic development model. Results demonstrated that, exemplified by the hydrate layer, increasing the pressure drop from 5 to 9 MPa enhanced the methane production rate from 45.86% to 84.05%, while the displacement rate declined from 40.39% to 32.73%. The CO2 sequestration efficiency exhibited a unimodal distribution, peaking at 75.37% at a pressure drop of 7 MPa. Notably, the sequestration rate in the underlying gas layer exceeded that of the hydrate layer by 5.28%. Among injection strategies, intermittent injection yields the best displacement results, while cyclic injection yields the best grouting results; sensitivity analysis ranked the factors influencing methane recovery as follows: pressure reduction magnitude > liquid CO2 injection location > liquid CO2 injection pattern > vertical pressure reduction points. For CO2 sequestration performance, the order was pressure reduction magnitude > liquid CO2 injection pattern > liquid CO2 injection location > vertical pressure reduction points. This research offers critical empirical data and a theoretical framework to support the safe and efficient exploitation of natural gas hydrates in the South China Sea as well as to advance the synergistic optimization of marine CO2 sequestration methodologies.