Mudrock images from Nankai Trough>
Bihani, A..
Grain-scale controls on seal integrity in mudrocks: Capillary entry pressure and permeability prediction.
Doctoral dissertation, University of Texas at Austin.
2020.
Abstract —
Mudrocks serve as geological traps and seals for carbon sequestration or for hydrocarbon formation, where mudrock capillary seals having high capillary entry pressure prevent the leakage of underlying fluids. However, they can fail if the buoyant pressure of the trapped fluid overcomes the threshold pressure of the seal. Mudrocks are composed primarily of silt-size and clay-size grains in various fractions. Microstructural observations of mudrocks have shown a silt bridging effect, whereby sufficiently abundant silt-size grains will create a stress chain across the rock matrix to preserve large pores and throats. At shallower depths, this effect can create a dual-porosity system, consisting of larger pores and throats near the coarser grains, and smaller pores and throats existing only between the finer clay grains. If the preserved larger pores and throats are connected across a mudrock, it may increase the absolute permeability, and reduce the capillary threshold pressure and tortuosity, thereby decreasing its sealing capacity.
Using pore-network modeling, artificial bidisperse grain packs (packings of two sizes) were generated, with and without the effect of gravity, to understand the effects of deposition and compaction on the petrophysical properties. It was observed that when the fraction of larger grains reaches about 40 - 60 % of the total volume of the grain pack, the capillary threshold transitions to a lower value and permits fluid percolation across the grain pack.
An image analysis workflow consisting of multiple filtering and user-guided segmentation steps was used to identify pores, silt grains, and clay from scanning electron microscope (SEM) images of sediments from the Kumano Basin offshore Japan. Statistical analysis showed that larger pores are better preserved when surrounded by detrital, silt size grains, and the presence of a higher fraction of silt-size grains led to a higher concentration of larger pores. The distributions of pore characteristics at different depths showed that larger pores are observed in samples with higher silt fractions despite being deeper.
Since the images only offer a two-dimensional view of the three-dimensional rock structure, a digital rocks workflow was applied to reconstruct the mudrock pore space. Lattice Boltzmann simulations were run on the reconstructed grain packs to simulate capillary drainage using high-performance computing. The results showed that at all depths, the capillary threshold pressure for the grain packs with a higher silt fraction was lower than those with a lower silt fraction. Capillary threshold pressure also increased with depth, likely due to larger pores and throats preserved as a result of the silt bridging.
Thus, using a combination of pore-network modeling, image analysis, and lattice Boltzmann simulations, I found that there is a significant dependence of the grain concentration and texture on the petrophysical properties. Improving our understanding about the influence of grain concentrations, spatial positions, and sizes on the fluid flow behavior is an important step towards a better characterization of mudrock seals and can help improve risk management efforts in anthropogenic waste storage and estimates of the reserve capacity of petroleum reservoirs.
