Understanding Shale Gas: Recent Progress and Remaining Challenges

Because of a number of technological advancements, unconventional hydrocarbons, and in particular shale gas, have transformed the US economy. Much is being learned, as demonstrated by the reduced cost of extracting shale gas in the US over the past five years. However, a number of challenges still need to be addressed. Many of these challenges represent grand scientific and technological tasks, overcoming which will have a number of positive impacts, ranging from the reduction of the environmental footprint of shale gas production to improvements and leaps forward in diverse sectors, including chemical manufacturing and catalytic transformations. This review addresses recent advancements in computational and experimental approaches, which led to improved understanding of, in particular, structure and transport of fluids, including hydrocarbons, electrolytes, water, and CO2 in heterogeneous subsurface rocks such as those typically found in shale formations. The narrative is concluded with a suggestion of a few research directions that, by synergistically combining computational and experimental advances, could allow us to overcome some of the hurdles that currently hinder the production of hydrocarbons from shale formations

Fluid transport through heterogeneous pore matrices: Multiscale simulation approaches

Fluids confined in nanopores exhibit several unique structural and dynamical characteristics that affect a number of applications in industry as well as natural phenomena. Understanding and predicting the complex fluid behavior under nano-confinement is therefore of key importance, and both experimental and computational approaches have been employed toward this goal. It is now feasible to employ both simulations and theoretical methods, the results of which can be validated by cutting-edge experimental quantification. Nevertheless, predicting fluid transport through heterogeneous pore networks at a scale large enough to be relevant for practical applications remains elusive because one should account for a variety of fluid–rock interactions, a wide range of confined fluid states, as well as pore-edge effects and the existence of preferential pathways, which, together with many other phenomena, affect the results. The aim of this Review is to overview the significance of molecular phenomena on fluid transport in nanoporous media, the capability and shortcomings of both molecular and continuum fluid modeling approaches, and recent progress in multiscale modeling of fluid transport. In our interpretation, a multiscale approach couples a molecular picture for fluid interactions with solid surfaces at the single nanopore level with hierarchical transport analysis through realistic heterogeneous pore networks to balance physical accuracy with computational expense. When possible, comparison against experiments is provided as a guiding roadmap for selecting the appropriate computational methods. The appropriateness of an approach is certainly related to the final application of interest, as different sectors will require different levels of precision in the predictions

Molecular Dynamics Simulation Of Gas Transport And Adsorption In Ultra-Tight Formations

Kerogen, which plays a very important part in reservoir characterization for ultra-tight formations, is also involved in the storage and production of hydrocarbons in shale. In this work, we study the kerogen structure and its interaction with insitu hydrocarbons to fully understand the fluid flow and adsorption mechanisms in the shale. Also the advancement in pore network modelling has greatly helped the understanding of mesoscale fluid flow. In this work, transport of methane in a type II marine environment kerogen model is studied using molecular dynamics simulations. Non Equilibrium Molecular Dynamics Simulations (NEMDS) using GROMACS code and Grand Canonical Monte Carlo (GCMC) using the RASPA code have been applied to simulate the adsorption and transport of ethane, carbon dioxide and methane in nanoscale environment. In this work, we used the kerogen and silica pore models to represent an organic and inorganic nanopore channels, respectively. The initial configuration models are then energy minimized, and both constant-temperature constant-volume (NVT) simulations and then constant-temperature constant-pressure (NPT) simulations are performed to obtain the final structure. For our pore network model, we used the Delaunay triangulation method to build a network model and then employed capillary pressure simulations. The simulation results from molecular simulations transport diffusivities show that as pressure increases the transport diffusion coefficients increase. Methane has a higher diffusivity in kerogen than ethane at the same temperature and pressure conditions. For adsorption, results show that CO2 has the largest adsorption capacity for both organic and inorganic pores, hence, a good candidate for enhanced gas recovery and carbon sequestration in depleted shale gas reservoirs. The amount of adsorption is more in organic pores for all studied gases, which implies that shale reservoirs with higher total organic carbon (TOC) will turn to trap more gases restricting flow and production

Molecular Dynamics Simulation Of Gas Transport And Adsorption In Ultra-Tight Formations

Kerogen, which plays a very important part in reservoir characterization for ultra-tight formations, is also involved in the storage and production of hydrocarbons in shale. In this work, we study the kerogen structure and its interaction with insitu hydrocarbons to fully understand the fluid flow and adsorption mechanisms in the shale. Also the advancement in pore network modelling has greatly helped the understanding of mesoscale fluid flow. In this work, transport of methane in a type II marine environment kerogen model is studied using molecular dynamics simulations. Non Equilibrium Molecular Dynamics Simulations (NEMDS) using GROMACS code and Grand Canonical Monte Carlo (GCMC) using the RASPA code have been applied to simulate the adsorption and transport of ethane, carbon dioxide and methane in nanoscale environment. In this work, we used the kerogen and silica pore models to represent an organic and inorganic nanopore channels, respectively. The initial configuration models are then energy minimized, and both constant-temperature constant-volume (NVT) simulations and then constant-temperature constant-pressure (NPT) simulations are performed to obtain the final structure. For our pore network model, we used the Delaunay triangulation method to build a network model and then employed capillary pressure simulations. The simulation results from molecular simulations transport diffusivities show that as pressure increases the transport diffusion coefficients increase. Methane has a higher diffusivity in kerogen than ethane at the same temperature and pressure conditions. For adsorption, results show that CO2 has the largest adsorption capacity for both organic and inorganic pores, hence, a good candidate for enhanced gas recovery and carbon sequestration in depleted shale gas reservoirs. The amount of adsorption is more in organic pores for all studied gases, which implies that shale reservoirs with higher total organic carbon (TOC) will turn to trap more gases restricting flow and production

Diffusion Behavior of Methane in 3D Kerogen Models

As global energy demand increases, natural gas recovery from source rocks is attracting considerable attention since recent development in shale extraction techniques has made the recovery process economically viable. Kerogens are thought to play an important role in gas recovery; however, the interactions between trapped shale gas and kerogens remain poorly understood due to the complex, heterogeneous microporous structure of kerogens. This study examines the diffusive behavior of methane molecules in kerogen matrices of different types (Type I, II, and II) and maturity levels (A to D for Type II kerogens) on a molecular scale. Models of each kerogen type were developed using simulated annealing. We employed grand canonical Monte Carlo simulations to predict the methane loadings of the kerogen models and then used equilibrium molecular dynamics simulations to compute the mean square displacement of methane molecules within the kerogen matrices under reservoir-relevant conditions, that is, 365 K and 275 bar. Our results show that methane self-diffusivity exhibits some degree of anisotropy in all kerogen types examined here except for Type I-A kerogens, where diffusion is the fastest and isotropic diffusion is observed. Self-diffusivity appears to correlate positively with pore volume for Type II kerogens, where an increase in diffusivity is observed with increasing maturity. Swelling of the kerogen matrix up to a 3% volume change is also observed upon methane adsorption. The findings contribute to a better understanding of hydrocarbon transport mechanisms in shale and may lead to further development of extraction techniques, fracturing fluids, and recovery predictions

Carbon Dioxide Utilization and Sequestration in Kerogen Nanopores

Carbon dioxide (CO2) has been injected into oil reservoirs to maximize production for decades. On the other hand, emitted CO2 from industrial processes is captured and stored in geological formations to mitigate greenhouse gas effects. As such, greater attention is drawn to the potential of utilizing the captured CO2 in EOR processes. A significant portion of the injected CO2 remains trapped due to capillary forces and through dissolution in residual liquids. In organic-rich shales, the presence of isolated kerogen nanopores add to the sequestration process due to the adsorptive nature of the surface and its preference to CO2 over methane (CH4), in addition to the sealing capacities of these formations. This work summarizes the latest findings of the literature with the purpose of defining further areas of investigation to fully capitalize on the potential of CO2 sequestration and utilization in kerogen nanopores

Stability analysis of the water bridge in organic shale nanopores: A molecular dynamic study

In the last decades, shale gas development has relieved the global energy crisis and slowed global warming problems. The water bridge plays an important role in the process of shale gas diffusion, but the stability of the water bridge in the shale nanochannel has not been revealed. In this work, the molecular dynamics method is applied to study the interaction between shale gas and water bridge, and the stability can be tested accordingly. CO2 can diffuse into the liquid H2O phase, but CH4 only diffuses at the boundary of the H2O phase. Due to the polarity of H2O molecules, the water bridge presents the wetting condition according to model snapshots and one-dimensional analyses, but the main body of the water bridge in the two-dimensional contour shows the non-wetting condition, which is reasonable. Due to the effect of the molecular polarity, CO2 prefers to diffuse into kerogen matrixes and the bulk phase of water bridge. In the bulk of the water bridge, where the interaction is weaker, CO2 has a lower energy state, implies that it has a good solubility in the liquid H2O phase. Higher temperature does not facilitate the diffusion of CO2 molecules, and higher pressure brings more CO2 molecules and enhances the solubility of CO2 in the H2O phase, in addition, a larger ratio of CO2 increases its content, which does the same effects with higher pressures. The stability of the water bridge is disturbed by diffused CO2 , and its waist is the weakest position by the potential energy distribution.Cited as: Liu, J., Zhang, T., Sun, S. Stability analysis of the water bridge in organic shale nanopores: A molecular dynamic study. Capillarity, 2022, 5(4): 75-82. https://doi.org/10.46690/capi.2022.04.0

Doctor of Philosophy

dissertationShale resources provide a tremendous opportunity for a long-term viable energy source, but the lower hydrocarbon recovery rates are hindering the economic development of shale reservoirs. One of the main reasons for the lower hydrocarbon recovery rates is the inadequate understanding of the fate of various injected fluids and the recovered hydrocarbons during various stages of exploration and production. As Darcy's law is limited in describing the multiphase fluid transport in shale, a comprehensive simulation framework is necessary, enabling the replication of the nanometer and subnanometer pores found in organic and inorganic matrices, and the simulation of the multiphase fluid flow in these nanopores, thus improving the comprehension of the pore-scale fluid transport process in shale reservoirs. A molecular dynamics simulation-based framework is developed in present research to address the above-defined challenges. The applications of various open-source molecular modeling tools are integrated to develop molecular pore structures found in the organic and inorganic matrices. An application of the general-purpose DREIDING force field is extended to simulate the kerogen. A gas-liquid (methane and water) transport is simulated in nanopores confined in the organic and inorganic matrices, and various dynamic transport properties of fluids (subjected to confinement) are determined to gain the qualitative and the quantitative understanding of the fluid flow. The present research provides a powerful molecular dynamics simulation-based framework that will enable the development of more complex models of nanoporous shale structures and address numerous challenges encountered in hydrocarbon recovery from shale reservoirs