An Integrated Shale Oil Characterization Workflow for Hydrocarbon Gas Huff-n-Puff Candidates

Abstract

The complexity of unconventional shales is inherent in the variation of mineral micro-structure and heterogeneous pore space, which contributes to the fast decline of primary oil production in shale oil reservoirs resulting in small recovery factors (< 10%). Furthermore, the implementation of hydrocarbon gas huff-n-puff has proven to be effective for enhancing the production of liquid hydrocarbons from the horizontal wells with multistage hydraulic fracturing in shale oil reservoirs. However, accurate simulation of the huff-n-puff process for optimum recovery proves challenging. Therefore, this work employs a multi-scale characterization approach to understand the complexity of the shale medium and investigate the underlying mechanisms of the hydrocarbon gas huff-n-puff process in shale oil samples. The integrated workflow comprises of large-scale and pore-scale characterization, respectively. Large-scale characterization involves regional characterization of a shale oil formation using laboratory measured petrophysical parameters, wireline log measurements, and seismic data. While pore-scale characterization makes use of the scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), focused ion beam-scanning electron microscopy (FIB-SEM). The integrated workflow considers seismic – wireline log correlations beyond the wellbore for large-scale characterization. On the other hand, the pore-scale characterization provides detailed heterogeneity information of the shale oil samples in addition to a 3D generated pore network model to approximate the geometric and transport properties. Raman spectroscopy and nuclear magnetic resonance (NMR) methods are employed to determine structural changes associated with the maturation of organic matter and to evaluate the hydrocarbon gas huff-n-puff recovery process, respectively. The NMR results also estimates core-scale petrophysical properties such as porosity and hydrocarbon saturation after a high pressure, high temperature (HPHT) system is used to subject Eagle Ford shale oil samples to hydrocarbon gas huff-n-puff. A quality index map was generated as part of the reservoir-scale characterization to identify sweet spots for well placement and optimize hydraulic fracturing for hydrocarbon recovery in the Chattanooga shale formation. Significant findings for pore-scale characterization using SEM/BSE, EDS, and FIB-SEM include the differentiation of depositional kerogen from migrated organic matter with developed nanopores and identifying the rock fabric of Eagle Ford shale oil to be dominated by carbonate with a mix of quartz, pyrite, and clay minerals. The organic pore tortuosity averaged at 2.36, 1.49, and 2.03 in the x, y, and z- directions, respectively, and an average permeability of ~ 0.00364 mD are among the estimated geometric properties of the shale samples. Furthermore, the equivalent pore diameter of the Eagle Ford shale samples are approximated from 13 nm – 580 nm for organic pores and 20 nm – 4 µm for inorganic pores. Furthermore, Raman thermal maturity measurement is shown to be dependent on the original maceral/source chemistry. NMR T2 distributions exhibited reduced amplitude to indicate oil/hexadecane recovery and a shift to the left is interpreted as remaining heavier fractions. The estimated oil/hexadecane recovery was comparable to that of mass balance estimations. This integrated workflow to shale oil reservoirs is capable of optimizing hydrocarbon recovery in shale oil reservoirs at the large-scale through sweet-spot identification as well as understanding the heterogeneity of the shale medium at the pore-scale and estimating the fluid flow properties for simulation.