Focused Free Gas Flow Forms Concentrated Methane Hydrate in Coarse-Grained Layers in Geological Systems

Abstract:

We present an integrated conceptual model to explain methane hydrate formation in complex geological systems. We describe microbial methanogenesis, methane migration and hydrate formation as the sediment is deposited at the seafloor, passes through the methane hydrate stability zone (HSZ) and is eventually buried to below the base of hydrate stability zone (BHSZ). We illustrate our conceptual model with two-dimensional numerical simulations constructed using a commercial basin-scale simulator, PetroMod^Ò. We describe the kinetics of methanogenesis with the Middelburg model. Methane generation rate is highest near the seafloor and decreases exponentially with depth. We emphasize the role of two-dimensional focused vapor flow in forming methane hydrate. As methane is generated, methane vapor forms in fine-grained, muddy sediments within the HSZ. However, no methane hydrate forms. Instead, vapor flow and diffusion within water focus methane into coarse-grained intervals where methane hydrate forms. When these hydrate-bearing sediments pass through the BHSZ, focused lateral vapor flow commences along the coarse-grained layers. In addition, vertical vapor flow originated from below the BHSZ initiates when the trapped gas overcomes the capillary sealing capacity of a gas conduit. Both the lateral and vertical flow concentrates methane into structural closures where highly concentrated hydrate forms. Our approach expands upon previous models. Previous models focus on only one or two one-dimensional methane transport processes. In contrast, our model integrates all the two-dimensional, relevant physical, chemical and microbial processes with sedimentation to investigate hydrate formation. Our model can explain most of the hydrate deposits in geological systems. It indicates that there is little to no hydrate in muddy sediments; instead, methane hydrate fills in coarse-grained intervals; highly concentrated hydrate is most likely located at structural closures. This knowledge is essential to understand the role of methane hydrate in carbon cycle, climate change and as an energy resource.