Systematic Review and Perspectives of Methane (CH₄) Classifications and Production Methods from CH₄ Hydrate Reservoirs

Methane (CH4) hydrates represent a promising yet intricate future energy source. Challenges in sustainable offshore extraction, such as fine sand production, CH4 hydrate reformation, water production, and a possible increase in the bottom well pressure due to sand formation, highlight the need for ongoing research. This study provides a systematic review of CH4 hydrate classification, evaluating its deposit composition, formation, stability, and extraction potential through various experimental, simulations, and field trial tests and highlighting the factors impairing their effectiveness. Critical factors, such as the kinetic behavior of CH4 hydrate formation, the impact of geological structures on CH4 migration and accumulation, and the environmental and technical challenges, are illustrated. The effects of sediment specific and sediment modification at varied temperatures, pressures, and salinity for developing efficient methods for CH4 recovery from hydrate reserves and optimizing conditions for hydrate storage and transport technologies are also presented. The unique characteristics of Class 1, Class 2, Class 3, and Class 4 CH4 hydrate reservoirs along with production methods, production factors like the injection rate, temperature, and pressure drop, as well as reservoir parameters such as the permeability, porosity, and surface area are revealed to influence gas production significantly. It is revealed that depressurization is widely recognized across all class types for its effectiveness due to the low economic cost and feasibility of implementation, particularly highlighted in Class 1 and Class 3 reservoirs, where it facilitates the dissociation of CH4 hydrates for CH4 recovery, indicating potential extraction rates of up to 75% over two decades. Thermal stimulation and CO2 swapping also stand out, especially for Class 1 reservoirs, as viable methods contribute to CH4 extraction by directly heating the reservoir to destabilize hydrates or injecting CO2 to replace CH4 in the hydrate structure, simultaneously sequestering CO2. Class 2 reservoirs, characterized by low permeability, often require combining depressurization with thermal methods or innovative approaches like CO2 injection to enhance CH4 extraction efficiency, indicating potential extraction rates of up to 87.80%. Furthermore, hydraulic fracturing emerges as essential for Class 3 reservoirs by improving the permeability and facilitating gas flow, indicating potential extraction rates of up to 80.60%, thus enhancing CH4 extraction. Additionally, this review emphasizes the current challenges and suggests potential interventions. The concise synthesis of findings, encompassing both experimental evidence and simulation deductions, as presented in this review, will enhance comprehension regarding screening, designing, and formulating CH4 extraction strategies.