Molecular dynamics investigation of conversion methods for excess adsorption amount of shale gas

页岩气过剩吸附量转换方法的分子动力学研究

Journal of Natural Gas Science and Engineering, Volume 49, January 2018, Pages 241-249

Wenbin Jiang, Mian Lin

Abstract：In investigations of shale gas found within shale formations, the conversion of the experimentally determined excess adsorption amount to the absolute amount is crucial to the interpretation of experimental adsorption data. In our study, the excess isotherms of a shale sample from the Longmaxi Formation in China are converted to absolute ones using existing methods. Keeping in mind that the results differ widely across methods and that the accuracies of these methods are poorly understood, we take a first step towards revealing and evaluating the possible discrepancies. We construct a molecular dynamics (MD) model considering methane adsorption in nanometer channels of different widths, and the model is able to calculate the excess and absolute adsorption amounts. Our simplified MD model is proved to be able to reproduce the excess adsorption isotherms of the shale sample through comparison with experimental data. We also evaluate existing conversion methods for calculation of the excess adsorption amount based on MD results. The methods that use predetermined adsorbed-phase density values usually underestimate the absolute amount. Further, the modified Langmuir and Töth equation methods underestimate results under most conditions. The relative error is large at higher pressure, higher temperature, and in smaller channels, and it can be as large as45%. Our study will serve to remind researchers about uncertainty of the results these methods, which can be useful in application.

Pore characterization of Lower Silurian shale gas reservoirs in the Middle Yangtze region, central China

长江中游地区下志留统页岩气藏的孔隙特征

Marine and Petroleum Geology, Volume 89, Part 1, January 2018, Pages 14-26

Qingtao Wang, Hong Lu, Taoli Wang, Dayong Liu, Xianqing Li

Abstract：Organic-rich shales from Lower Silurian are widely distributed in the Middle Yangtze region, central China. However, the lack of fundamental data for shale gas reservoirs increases the difficulty of gas exploration. In this study, 34 core samples were collected to characterize the shale pore structure and conduct a preliminary evaluation of the shale gas reservoir. The TOC (total organic carbon) content of the successively-deposited black shales range from 1.6% to 5.9%, while the total porosity range from 0.5% to 4.2%. The positive correlation between TOC and porosity indicates that TOC is the key factor determining porosity. The major component of the mineral matrix is quartz (content of 21.4%–69.2%), followed by clay minerals (content of 16.7%–44.5%). Field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) results illustrate that organic matter, mixed with clay minerals, can form an organo-clay complex containing many nanopores. Furthermore, larger organic pores are developed in organo-clay complexes with higher clay content than in those with lower clay content.

Correlational analyses between pore volume (or pore surface area) and TOC (or clay content) demonstrate that micropores are associated with organic matter, while mesopores and macropores are probably associated with clay minerals. Many of the clay-related nanopores are organic in nature and are developed in organo-clay complexes containing both organic matter and clay minerals. Overall, the TOC content controls development of nanopores in the shale pore structure, followed by clay content. The DFT-derived PSD indicates that the pore volume is comprised primarily of pores having widths larger than 10 nm, while the surface area is comprised primarily of micropores. When considering the gas in place model and mechanisms of shale gas storage, further shale gas exploration in central China should aim to the deep (>1000 m) and well preserved Longmaxi Shales.

Multiscale model reduction for shale gas transport in poroelastic fractured media

裂隙孔弹介质中页岩气运移的多尺度模型简化

Journal of Computational Physics, Volume 353, 15 January 2018, Pages 356-376

I. Yucel Akkutlu, Yalchin Efendiev, Maria Vasilyeva, Yuhe Wang

Abstract：Inherently coupled flow and geomechanics processes in fractured shale media have implications for shale gas production. The system involves highly complex geo-textures comprised of a heterogeneous anisotropic fracture network spatially embedded in an ultra-tight matrix. In addition, nonlinearities due to viscous flow, diffusion, and desorption in the matrix and high velocity gas flow in the fractures complicates the transport. In this paper, we develop a multiscale model reduction approach to couple gas flow and geomechanics in fractured shale media. A Discrete Fracture Model (DFM) is used to treat the complex network of fractures on a fine grid. The coupled flow and geomechanics equations are solved using a fixed stress-splitting scheme by solving the pressure equation using a continuous Galerkin method and the displacement equation using an interior penalty discontinuous Galerkin method. We develop a coarse grid approximation and coupling using the Generalized Multiscale Finite Element Method (GMsFEM). GMsFEM constructs the multiscale basis functions in a systematic way to capture the fracture networks and their interactions with the shale matrix. Numerical results and an error analysis is provided showing that the proposed approach accurately captures the coupled process using a few multiscale basis functions, i.e. a small fraction of the degrees of freedom of the fine-scale problem.