An integrated workflow to characterize lower Eagle Ford shale for hydrocarbon gas huff-n-puff simulation

Abstract

Huff-n-puff (HnP) gas injection has proven to be effective for recovering more liquid hydrocarbons from shale oil reservoirs. However, effective simulation of HnP requires a better understanding of the complex microstructure of shales and the multi-scale pore systems. An integrated workflow is developed that identifies multicomponent grains and pores types in the Lower Eagle Ford (LEF) shale at different depths, and distinguishes depositional kerogen from migrated organic matter (bitumen). It also investigates how the injected gas interacts with the extractible bitumen, and estimates relevant transport properties including tortuosity and surface area-to-volume ratio (SAV). Common mineral components found in the LEF shale include detrital quartz, clay minerals, coccoliths, diagenetic calcite cement, and pyrite. Depositional kerogen appears as wispy seams and discrete particles, whereas bitumen commonly reduces or occludes foraminifer tests. After experimental gas HnP, high-resolution images show a significant amount of bitumen/hydrocarbon was mobilized at reservoir conditions. This translates into evacuated pore space with a wider range of pore diameter (13 nm−840 nm) compared to samples before HnP (13 nm–350 nm). The organic pore network models have a relatively higher SAV and this impacts gas adsorption and permeability. The hydraulic tortuosity is in the range of 2.28–2.85 for the organic pore network and the calculated permeability in a pore network model was observed to increase from 0.000146 mD before HnP to 0.00122 mD after HnP. This workflow provides detailed pore system and chemical information as well as transport properties controlling molecular diffusion during gas HnP.