Pore-Scale Characterization of Eagle Ford Outcrop and Reservoir Cores with SEM/BSE, EDS, FIB-SEM, and Lattice Boltzmann Simulation
Authors: Cudjoe Sherifa , Liu Siyan , Barati Reza , Hasiuk Franciszek , Goldstein Robert , Tsau Jyun-Syung , Nicoud Brian , Bradford Kyle , Baldwin Amanda , Mohrbacher David 
Published: September 2019

The objective of this work is to conduct pore-scale analysis of the pore systems in an Eagle Ford (EF) outcrop sample and a Lower Eagle Ford (LEF) sample from a producing interval in the subsurface. After characterization, we estimate bulk transport properties (such as tortuosity and permeability) within each pore network model (PNM) using the lattice Boltzmann method (LBM). Comparing the two will evaluate the degree to which outcrop samples of the EF are or are not applicable analogs to the subsurface for laboratory-scale "huff-n-puff’ enhanced oil recovery experiments.

Grain types and pore systems of both samples were visualized and quantified at the micro- and nanoscale using scanning electron microscopy/backscattered electron microscopy (SHM/BSH), energy- dispersive X-ray spectroscopy (EDS), and focused ion beam-scanning electron microscopy (FIB-SEM). These methods measure mineral content, elemental (mineral) analysis, size distribution of pores and pore throats in addition to serving as the basis to develop pore network models (PNMs) for simulation. The LBM was then applied to the extracted pores to estimate permeability for each medium.

The 2D SEM/BSE/EDS images of the EF outcrop sample showed that the microstructure of finegrained inorganic matrix was modified by calcite neomorphic and passively precipitated microspar, spar, and pseudospar altering the texture of the depositional matrix, low clay content with some of feldspar, solution-enlarged microfractures, compactional fractures, coccolith debris, and calcite deformation (solution-enlarged cleavages). There are abundant microfossils including foraminifer tests ("forams") filled with either diagenetic calcite, quartz, organic matter or a mixture of these minerals; the organic matter in the foram chambers mostly show cracks/shrinkage pores or lack pores in organic matter.

On the other hand, the LEF reservoir sample showed significantly different diagenetic alteration with localized phosphate diagenesis, less calcite neomorphism, and better developed pores within the organic matter infilling the foraminiferal tests as well as in depositional kerogen embedded within the inorganic matrix. In addition, 3D FIB- SEM volumes showed the variation in tortuosity of each extracted PNM and its impact on the diffusion coefficient during gas huff-n-puff recovery. The LBM enabled the estimation of permeability at the molecular level from each extracted PNM.

Not surprisingly, the textural and compositional differences between the outcrop and subsurface samples lead to different PNM and different behavior in huff-n-puff experiments. This work bridges a gap in the literature by comparing and revealing the pore-scale heterogeneities of an outcrop sample to that of a subsurface sample to measure the impact of the underlying mechanisms associated with gas huff-n-puff recovery at the laboratory-scale, while estimating permeability within extracted pores with the LBM.