The effect of sediment interlayers on the triaxial compressive strength of gas hydrate-bearing sediments

In the transition from high CO2-emitting fuel to lower CO2-emitting fuel, natural gas hydrates (NGH) or methane hydrates (MH) provide an opportunity for further development. It is an ice-like solid trapping methane gas within its cage-like structure. It has a high energy density and a higher calorific value compared to coal, with 50% less CO2 emission. The availability of NGH globally is to such an extent that its 10% extraction can suffice the global energy demand for the next ~200 years. 

However, NGH or MH reservoirs come with challenges such as extreme environments, difficult exploration conditions, unpredictable estimation of resources, etc. Though well-explored, the complex geological formations make them very difficult to produce from. The geological matrix of gas hydrates plays a crucial role in their dissociation, strength and mass transfer behaviour. The strength and fluid flow in the hydrate reservoirs is mainly governed by the impermeable layers of fine-grained sediments, which are mud/clay dominant.  

To study the effect of interlayers on the strength of the MH sample, we conducted an experimental procedure that replicates the deep submarine environment. Unlike the conventional cylindrical core sample with homogeneous sediment distribution across the volume, the thin interlayers of fine-grain sediments (clay/mud) were introduced at different height intervals that mimicked the natural lithological conditions. The triaxial stress configurations replicate the real-world submarine environment where MH occurs. After applying confinement pressure, overburden pressure, and lowering the sample temperature to that of a hydrate-bearing zone, gas hydrates formed inside the sediment sample by injecting a mixture of methane and water. After the hydrate formation, the permeability of the sample was measured. Subsequently, the gas hydrate sample was allowed to dissociate, and the drained geomechanical test on the sample was performed. The depressurisation method was used for the dissociation of the hydrates. 

During the experiment, P-S wave velocities were continuously measured. The wave velocities increased between pre and post-hydrate formation and decreased after dissociation. It indicates the enhancement in the strength of the sample due to hydrate formation and reduction due to dissociation. Furthermore, the sample showed compliance as the number of layers or layer thickness increased. The ductile behaviour was observed in the interlayered samples compared to those with non-layered (homogeneous). Moreover, peak strength was reduced by about ~15-20% in the dissociated samples compared with the hydrate-bearing sample. This study resolves the geomechanical behaviour of gas hydrate reservoirs, which is key to developing production strategies.