Insight into the Sealing Capacity of Mudrocks determined using a Digital Rock Physics Workflow

Mudrocks are primarily composed of grains of silt- and clay-size and serve as geological seals for hydrocarbon accumulations or CO₂ storage. While mudrock seals have high capillary entry pressure preventing leakage of underlying nonwetting fluids, they can fail below the fracture pressure if the buoyant pressure of the trapped fluid overcomes the threshold pressure of the seal. The threshold pressure is strongly influenced by the grain size distribution. Microstructural observations of mudrocks have shown a silt bridging effect, whereby silt-size grains present in sufficient abundance can create a connected stress chain through the rock matrix, allowing ductile clay grains to bend around the rigid grains leading to preservation of large pore throats even under compaction. This effect can reduce the capillary threshold pressure of a mudrock, and thus reduce its sealing capacity by more easily permitting flow of the nonwetting phase.

We used SEM images of uncemented muds obtained at various depths (< 1.1 km burial) in the Kumano Basin offshore Japan to study this phenomenon. The image mosaics were filtered and segmented using conventional and machine-learning techniques to identify the pore space, silt, and clay grains. Statistical analysis of the images showed that while the porosity decreased with increasing depth, larger pores were preserved when surrounded by larger grains, confirming the influence of the silt-bridging effect.

We applied a 3D stochastic technique for pore space reconstruction from the SEM images and simulated capillary drainage in the resulting 3D volumes using the lattice Boltzmann method (LBM). The LBM results showed that the porosity and permeability generally decreased with depth, while the tortuosity and capillary threshold pressure values increase. However, increasing the silt content of a mudrock at a particular depth counteracted this behavior, due to better preservation of the larger pores and throats. We found that increasing the silt content above 40%, can reduce the threshold pressure as much as by 50%, thereby promoting early seal failure. The applied digital rocks workflow thus improved mudrock characterization by providing a better understanding of the impact of grain concentrations, sizes and spatial distribution on sealing capacity.