Publications

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Methane (CH4) and carbon dioxide (CO2), the two major components generated from kerogen maturation, are stored dominantly in nanometer-sized pores in shale matrix as (1) a compressed gas, (2) an adsorbed surface species and/or (3) a species dissolved in pore water (H2O). In addition, supercritical CO2 has been proposed as a fracturing fluid for simultaneous enhanced oil/gas recovery (EOR) and carbon sequestration. A mechanistic understanding of CH4-CO2-H2O interactions in shale nanopores is critical for designing effective operational processes. Using molecular simulations, we show that kerogen preferentially retains CO2 over CH4 and that the majority of CO2 either generated during kerogen maturation or injected in EOR will remain trapped in the kerogen matrix. The trapped CO2 may be released only if the reservoir pressure drops below the supercritical CO2 pressure. When water is present in the kerogen matrix, it may block CH4 release. However, the addition of CO2 may enhance CH4 release because CO2 can diffuse through water and exchange for adsorbed methane in the kerogen nanopores.

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Shale is characterized by the predominant presence of nanometer-scale (1-100 nm) pores. The behavior of fluids in those pores directly controls shale gas storage and release in shale matrix and ultimately the wellbore production in unconventional reservoirs. Recently, it has been recognized that a fluid confined in nanopores can behave dramatically differently from the corresponding bulk phase due to nanopore confinement (Wang, 2014). CO₂ and H₂O, either preexisting or introduced, are two major components that coexist with shale gas (predominately CH₄) during hydrofracturing and gas extraction. Note that liquid or supercritical CO₂ has been suggested as an alternative fluid for subsurface fracturing such that CO₂ enhanced gas recovery can also serve as a CO₂ sequestration process. Limited data indicate that CO₂ may preferentially adsorb in nanopores (particularly those in kerogen) and therefore displace CH4 in shale. Similarly, the presence of water moisture seems able to displace or trap CH₄ in shale matrix. Therefore, fundamental understanding of CH₄-CO₂-H₂O behavior and their interactions in shale nanopores is of great importance for gas production and the related CO₂ sequestration. This project focuses on the systematic study of CH₄-CO₂-H₂O interactions in shale nanopores under high-pressure and high temperature reservoir conditions. The proposed work will help to develop new stimulation strategies to enable efficient resource recovery from fewer and less environmentally impactful wells.

One critical issue related to shale oil/gas production is a rapid decline in wellbore production and a low recovery rate. Therefore, maximizing wellbore production and extending the production life cycle are crucial for the sustainability of shale oil/gas production. Shale oil/gas production starts with creating a fracture network by injecting a pressurized fluid in a wellbore. The induced fractures are then held open by proppant particles. During production, oil and gas release from the shale matrix, migrate to nearby fractures, and ultimately reach a production wellbore. Given the relatively high permeability of the induced fractures, oil/gas release and transport in low-permeability shale matrix are likely a limiting step for long-term wellbore production. This project is aimed to (1) fundamentally understand the disposition and release of complex hydrocarbon mixtures in nanopore networks of shale matrix and their transport from low-permeability matrix to hydrofracking-induced fractures under various reservoir conditions ranging from black oil to dry gas and (2) use machine learning to upscale and integrate the nanometer-scale understanding into reservoir-scale model simulations. The work will help develop new stimulation strategies to enable efficient resource recovery from fewer and less environmentally impactful wells.

Experimental hydrology data from the Mizunami Underground Research Laboratory in Central Japan have been used to develop a site-scale fracture model and a flow model for the study area. The discrete fracture network model was upscaled to a continuum model to be used in flow simulations. A flow model was developed centered on the research tunnel, and using a highly refined regular mesh. In this study development and utilization of the model is presented. The modeling analysis used permeability and porosity fields from the discrete fracture network model as well as a homogenous model using fixed values of permeability and porosity. The simulations were designed to reproduce hydrology of the modeling area and to predict inflow of water into the research tunnel during excavation. Modeling results were compared with the project hydrology data. Successful matching of the experimental data was obtained for simulations based on the discrete fracture network model.

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Kerogen plays a central role in hydrocarbon generation in an oil/gas reservoir. In a subsurface environment, kerogen is constantly subjected to stress confinement or relaxation. The interplay between mechanical deformation and gas adsorption of the materials could be an important process for shale gas production but unfortunately is poorly understood. Using a hybrid Monte Carlo/molecular dynamics simulation, we show here that a strong chemo-mechanical coupling may exist between gas adsorption and mechanical strain of a kerogen matrix. The results indicate that the kerogen volume can expand by up to 5.4% and 11% upon CH4 and CO2 adsorption at 192 atm, respectively. The kerogen volume increases with gas pressure and eventually approaches a plateau as the kerogen becomes saturated. The volume expansion appears to quadratically increase with the amount of gas adsorbed, indicating a critical role of the surface layer of gas adsorbed in the bulk strain of the material. Furthermore, gas uptake is greatly enhanced by kerogen swelling. Swelling also increases the surface area, porosity, and pore size of kerogen. Our results illustrate the dynamic nature of kerogen, thus questioning the validity of the current assumption of a rigid kerogen molecular structure in the estimation of gas-in-place for a shale gas reservoir or gas storage capacity for subsurface carbon sequestration. The coupling between gas adsorption and kerogen matrix deformation should be taken into consideration.

The Mizunami Underground Research Laboratory is located in the Tono area (Central Japan). Its main purpose is providing a scientific basis for the research and development of technologies needed for deep geological disposal of radioactive waste in fractured crystalline rocks. The current work is focused on the experiments in the research tunnel (500 m depth). The collected tunnel and borehole data were shared with the participants of DEvelopment of COupled models and their VALidation against EXperiments (DECOVALEX) project. This study describes how these data were used to (1) develop the fracture model of the granite rocks around the research tunnel and (2) validate the model.

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Molecular structures of kerogen control hydrocarbon production in unconventional reservoirs. Significant progress has been made in developing model representations of various kerogen structures. These models have been widely used for the prediction of gas adsorption and migration in shale matrix. However, using density functional perturbation theory (DFPT) calculations and vibrational spectroscopic measurements, we here show that a large gap may still remain between the existing model representations and actual kerogen structures, therefore calling for new model development. Using DFPT, we calculated Fourier transform infrared (FTIR) spectra for six most widely used kerogen structure models. The computed spectra were then systematically compared to the FTIR absorption spectra collected for kerogen samples isolated from Mancos, Woodford and Marcellus formations representing a wide range of kerogen origin and maturation conditions. Limited agreement between the model predictions and the measurements highlights that the existing kerogen models may still miss some key features in structural representation. A combination of DFPT calculations with spectroscopic measurements may provide a useful diagnostic tool for assessing the adequacy of a proposed structural model as well as for future model development. This approach may eventually help develop comprehensive infrared (IR)-fingerprints for tracing kerogen evolution.

The porosity of clay aggregates is an important property governing chemical reactions and fluid flow in low-permeability geologic formations and clay-based engineered barrier systems. Pore spaces in clays include interlayer and interparticle pores. Under compaction and dewatering, the size and geometry of such pore spaces may vary significantly (sub-nanometer to microns) depending on ambient physical and chemical conditions. Here we report a molecular dynamics simulation method to construct a complex and realistic clay-like nanoparticle aggregate with interparticle pores and grain boundaries. The model structure is then used to investigate the effect of dewatering and water content on micro-porosity of the aggregates. The results suggest that slow dewatering would create more compact aggregates compared to fast dewatering. Furthermore, the amount of water present in the aggregates strongly affects the particle-particle interactions and hence the aggregate structure. Detailed analyses of particle-particle and water-particle interactions provide a molecular-scale view of porosity and texture development of the aggregates. The simulation method developed here may also aid in modeling the synthesis of nanostructured materials through self-assembly of nanoparticles.

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