Characterizing Lower Eagle Ford Shale for Hydrocarbon Gas
Huff-n-Puff Simulation from Pore-Scale to Core-Scale
Sherifa Cudjoe1, Qinwen Fu 1, Reza Barati1, Jyung-Syung Tsau 1,
Robert Goldstein2, and Craig Marshall2
1) KICC, Department of Chemical & Petroleum Engineering,
University of Kansas, Lawrence, KS
2) KICC, Department of Geology, University of Kansas, Lawrence,
KS
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Key Findings
Detailed heterogeneity of Lower Eagle Ford shale through SEM-EDS
and FIB-SEM characterization.
A workflow is developed detailing pore network extraction and
calculation of transport properties controlling molecular
diffusion.
Migrated organic matter (MOM) is investigated in-situ with Raman
spectroscopy.
Effect of injected hydrocarbon gas for huff-n-puff on
extractible MOM/hydrocarbon.
A workflow for developing a core-scale model using
high-resolution images and photomicrographs for gas
huff-n-puff.
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Introduction
Lower Eagle Ford (LEF) shale is characterized as a dark-gray
argillaceous marlstone with laminated foraminiferal depositional
facies, which falls within the black oil window (0.98 – 1.03 %Ro).
The LEF shale is regarded as a productive shale interval with an
average of porosity of 6.5%, total organic carbon (TOC) > 5 wt.
%, and pulse decay permeability of 0.0016 mD [1]. However, owing to
the complex microstructure of shales, primary recovery despite
advances in multi-stage hydraulic fracturing and horizontal wells
remains low (5 – 10% of original oil-in-place (OOIP)).
MOM occupies the chambers of foraminifer tests (“forams”) in
addition to interparticle pores in the matrix. Within the black oil
window, MOM in LEF shale develops a network of nanopores to aid in
fluid transport. Usually, thermal maturity is attributed to the
development of these nanopores but the effect of organic matter
(OM) source is yet to be determined.
Core models developed for gas huff-n-puff simulation are mostly
assumed to be homogeneous and isotropic, which fails to capture the
heterogeneity that impacts the diffusion-controlled huff-n-puff
process. Therefore, this study seeks to 1) understand the complex
heterogeneity of LEF shale at the pore-scale and estimate related
transport properties to improve gas huff-n-puff simulation; 2)
study the effect of injected gas on hydrocarbon/bitumen; 3)
understand what other than thermal maturity leads to porosity using
Raman spectroscopy; and 4) develop a core-scale model of the LEF
that captures detailed heterogeneity for gas huff-n-puff
simulation.
Methods
Using high-resolution imaging methods (SEM/BSE, EDS, FIB-SEM), a
digital workflow is developed at the pore-scale to characterize the
multicomponent grains and pore types in the LEF shale and to
estimate transport properties for rock chips from similar depths
(Figure 1). In addition, thin sections prepared from LEF shale of
similar depths produced photomicrographs and were used to
investigate MOM under Raman spectroscopy.
Figure 1. Workflow developed for pore-scale characterization and
Raman analysis [1,3]
Finally, combining details from high-resolution imaging and
photomicrographs as well as micro-CT imaging, a core-scale model of
the LEF shale, detailing core morphology, multiple pore space, and
micro-fractures is developed for gas huff-n-puff simulation.
Results & DiscussionPore-scale characterization
High-resolution images show a predominant calcite matrix with
common grain types such as detrital quartz, clay minerals,
microfractures, microfossils and fragments, depositional kerogen as
wispy seams and discrete particles, diagenetic calcite, pyrite, and
MOM infilling the chambers of foraminifer tests (“forams”) (Figure
2). Multi-scale pores were found be either between different
minerals or within a mineral framboid or both in addition to pores
developed within depositional kerogen and MOM, respectively [1,2].
Both high-resolution images (SEM/BSE) and photomicrographs acquired
for the LEF shale before gas huff-n-puff showed displaced
bitumen/hydrocarbon represented with an empty pore space after
experimentation [1,3].
Figure 2. A) BSE image of LEF shale showing distributions of
kaolinite-filled forams (white arrows), calcite-filled forams
(yellow arrows), and phosphate (blue arrows); B) MOM with pores
(white arrows) with brecciated calcite cement [2]
Raman Analysis
Furthermore, Raman spectroscopy revealed the impact of the
original maceral composition in the development of maturity
indicators other than thermal maturity [3]. Figure 3 shows Raman
spectra of two different MOM selected from the same LEF sample with
vitrinite reflectance of 0.98%. The D-band at 1347 cm¯¹ (Figure
3-left) and at 1337 cm¯¹ (Figure 3-right), respectively, give an
indication of the degree of clusters. Thus, the intensity of D-band
corresponds to the size of the aromatic clusters formed as organic
matter evolves to develop pores and produce hydrocarbons. Although,
the selected MOM are within the same sample, they show different
spectra, which is indicative of the impact of the original maceral
composition [3].
Figure 3. Raman spectra of selected MOM in LEF shale with
vitrinite reflectance (VRo%) of 0.98% [3]
Multicomponent core-scale model
A multicomponent core-scale model of the LEF shale was developed
combining the acquired high-resolution images, photomicrographs,
and micro-CT images. Estimated digital porosity and bulk transport
properties from the high-resolution images were used together with
pulse decay porosity and permeability measurements as constraints
in distributing the petrophysical properties of the core-scale
model[4]. Initial volumetric calculations presented a total pore
volume of 27.5 cc with an oil volume of 2.5 cc similar to
laboratory measurements for gas huff-n-puff experimentation.
References
[1]Cudjoe S, Barati R, Goldstein RH, Tsau J, Nicoud B, Bradford
K, et al. An Integrated Pore-Scale Characterization Workflow for
Hydrocarbon Gas Huff-n-Puff Injection into the Lower Eagle Ford
Shale. Unconv. Resour. Technol. Conf., Denver, CO: URTeC; 2019, p.
1–20. https://doi.org/10.15530/urtec-2019-442.
[2]Cudjoe S, Liu S, Barati R, Hasiuk F, Goldstein R, Tsau J-S,
et al. Pore-Scale Characterization of Eagle Ford Outcrop and
Reservoir Cores with SEM/BSE, EDS, FIB-SEM, and Lattice Boltzmann
Simulation. SPE Annu. Tech. Conf. Exhib., Calgary: SPE; 2019, p.
1–20.
[3]Cudjoe S, Barati R, Marshall CP, Goldstein RH, Tsau J-S,
Nicoud B, et al. Application of Raman Spectroscopy in Investigating
the Effect of Source and Temperature on the Maturity of the Organic
Matter Exposed to Hydrocarbon Gas Injection. Unconv. Resour.
Technol. Conf., Denver, CO: URTeC; 2019, p. 1–16.
https://doi.org/10.15530/urtec-2019-501.
[4]Cudjoe S, Fu Q, Tsau J-S, Barati R, Goldstein R, Nicoud B, et
al. A Reconstructed Core-Scale Model of the Lower Eagle Ford Shale
Through FIB-SEM, SEM-EDS, and Microscopy for Gas Huff-n-Puff
Simulation. SPE Improv. Oil Recover. Conf., Tulsa, OK, USA: SPE
IOR; 2020, p. 1–13.
[5]Fu Q, Cudjoe S, Barati R, Tsau J-S, Li X, Peltier K, et al.
Experimental and Numerical Investigation of the Diffusion-Based
Huff-n-Puff Gas Injection into Lower Eagle Ford Shale Samples.
Unconv. Resour. Technol. Conf., Denver, CO: URTeC; 2019, p.
1–18.
KICC 2020 Annual Virtual Meeting – Extended Abstracts
