3D Visualization and Classification of Pore Structure and Pore Filling in Gas Shales

Abstract

Shale gas is a growing resource worldwide as many basins are being explored and produced. However, little is still known and understood about two key parameters in gas shales: the gas-filled porosity and permeability. Digital rock physics technique, presented in this paper, contains three basic steps: (a) 3D CT imaging at 200 nanometer resolution, and/or FIB-SEM (focused ion beam combined with SEM) imaging at 3-15 nanometer resolution (b) segmentation of the digital volume to quantitatively identify the components, including the mineral phases, organic-filled pores, and free-gas inclusions; and (c) computations of TOC (Total Organic Content), porosity, pore connectivity, and permeability in three axis.A number of gas shale samples have been used, to specifically analyze the pore systems. The characteristics including dual porosity, organics distribution, gas-filled porosity distribution, and how these properties relate to the maturity of the organic material. Pore geometries (pores filled either with organics or free gas) of these samples fall into the following categories: (a) relatively large (up to 4 micron) with poorly disconnected pores; (b) pores connected by very thin (down to 15 nanometers) conduits; (c) dual porosity system where the large pores are interconnected by large conduits and very thin conduits are interconnected and also connected to the large pores. Within each of these three categories, the pore space may be (a) completely filled with organics or (b) partially or completely filled with gas.The latter is of most interest as it is a gas source. In such systems we observe various geometries of pore space, including (a) disconnected pores floating in the organics and (b) connected pores within the organics. TOC, open pore volumes, as well as pore-space connectivity are not just qualitatively estimated from the images but quantitatively computed for a given sample. Our ongoing effort is to relate the quantitative patterns thus computed to the maturity of shale.

The Influence of Analytical Particle Size on the Pore System Measured by CO2, N2, and Ar Adsorption Experiments for Shales

“…The conventional analytical methods applied to sandstone/carbonate reservoirs are not suitable for characterizing nanometer-scale pore networks in shale reservoirs. , Until present, a couple of advanced analytical techniques have been applied to characterize pore networks in shale reservoirs, including low-pressure gas adsorption (LPGA), mercury injection capillary pressure (MICP), field-emission scanning electron microscopy (FE-SEM), ,, transmission electron microscopy (TEM), atomic force microscopy (AFM), computed tomography (CT) scanning, nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS/USANS) techniques. − However, these methods have their own advantages and limitations in characterizing pores with different sizes. FE-SEM and TEM can only qualitatively characterize mesopores and macropores, failing to capture micropores because of resolution limitation .…”

Influence of particle size on gas-adsorption experiments of shales: An example from a Longmaxi Shale sample from the Sichuan Basin, China

“…Due to its potential impact on gas storage and production, shale reservoir properties have been extensively studied. Scanning electric microscopy (SEM) [3][4][5][6], atomic-force microscopy (AFM) [7], mercury intrusion porosimetry (MIP) [8], computed tomography (CT) scanning ( [9,10], nuclear magnetic resonance (NMR) [11], low-pressure gas adsorption (LPGA) [12], and small angle X-ray and neutron scattering are all used to evaluate the reservoir potential of shales [13,14]. Compared with other methods, LPGA experiments are cost-effective and can provide quantitative information on the pore system in shale.…”

Pore-scale micro-computed-tomography imaging: Nonwetting-phase cluster-size distribution during drainage and imbibition

“…Over the past decade, micro-computed-tomography (μCT; synchrotron-based and bench-top instruments) has become a frequently used technique to study porous media and geological samples [8][9][10][11]. Together with nuclear resonance (MRI) and acoustic techniques, X-ray-based tomography is one of the few nondestructive techniques that give detailed insight into the structure of porous media over length scales ranging down to a few hundred nanometers [12,13]. The resolution of MRI depends primarily on the strength of the applied magnetic field gradient and can theoretically be increased to the wavelength of the scattering vector.…”