Article pubs.acs.org/EF
Characterization of Coalbed Methane Reservoirs at Multiple Length Scales: A Cross-Section from Southeastern Ordos Basin, China Yong Li,*,†,‡ Dazhen Tang,† Derek Elsworth,‡ and Hao Xu† †
Coal Reservoir Laboratory of National CBM Engineering Center, School of Energy Resources, China University of Geosciences, Beijing, Beijing 100083, People’s Republic of China ‡ Department of Energy and Mineral Engineering, G3 Center and Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Coalbed methane (CBM) reservoirs are notoriously difficult to characterize for the existence of heterogeneity at several length scales. These length scales affect processes of desorption at the grain scale (1000 nm) and meso (100−1000 nm) pores shows an apparent decrease with an increase in coal burial depth, which is also confirmed by transverse relaxation time (T2) spectra from NMR analyses. The water saturating the macropores is generally removable, while the water in the mesopores is only partially removable, and the abnormal increase in centrifuged T2 amplitudes reflects poor connectivity between pores. Three kinds of N2 adsorption/ desorption (BET) curves are recovered and interpreted to be slit-like/plate-like pores, narrow slit-like pores, and ink-bottle (narrow throat and wide body) pores (∼10 nm). Results, using the same source samples throughout, show strong heterogeneity at the microscopic scale for the pore distribution characteristics, even for a single coal seam, and emphasize the utility of using multiple methods of characterization to infer heterogeneity and the textures and connectivity of pore structures.
1. INTRODUCTION Coalbed methane (CBM) reservoirs behave very differently from conventional reservoirs as a result of the heterogeneous structure and geochemical composition of the coal. From a genetic point of view, the mineral matter in coal, similar to the organic matter, is a product of the processes associated with peat accumulation and rank advance as well as in changes in subsurface fluids and other aspects of sediment diagnosis.1 The ash content of coal, which can be calculated from proximate analysis, correlates inversely with methane sorption capacity, which implies that the organic matter is a key component in the ability to store methane.2,3 Coal-type variations are typically expressed in terms of many physical and chemical properties (e.g., proximate analysis, maceral group, and mineral matter composition). Pores within coal are an order of magnitude smaller (nanometer scale) than pores within conventional carbonate and sandstone reservoirs (micrometer scale).4,5 Pore classification systems already exist for materials that contain nanometerscaled porosity, which categorize pore sizes based on physical adsorption properties and capillary condensation theory.6−10 In this study, we adopt the pore classification system by Hodot,6 which divides coal pores into micropores (1000 nm). The lower size limit of micropores © XXXX American Chemical Society
depends upon the kinetic diameter of the probing gas molecule (i.e., 0.35 nm for carbon dioxide).10 Macro- and mesopores allow for either strong laminar/turbulent gas flow/infiltration or slow gas laminar flow infiltration, respectively. Thus, they are part of the main gas flow path during CBM production and/or CO2 injection. However, transition pores and micropores control adsorption, where primarily gas capillary cohesion may occur, and represent the main pore systems for physical adsorption and diffusion.11−13 Quantitative analyses by mercury intrusion porosimetry (MIP) and low-pressure gas adsorption were used to determine the pore structure distribution of samples from the coal section and identify relationships between the pore size distribution and the fabric, texture, composition, and geochemistry of the coal.14,15 MIP uses the gradual injection of liquid mercury into an evacuated pore system under the action of external pressures; smaller pores become accessible only at successively elevated pressures. These measurements are run up to a pressure of 35 MPa, accessing pore throats as small as 0.018 μm.16 However, the high-pressure mercury may damage the pore Received: February 24, 2014 Revised: August 20, 2014
A
dx.doi.org/10.1021/ef500449s | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 1. Site locations and stratigraphic column of the Permo-Carboniferous coal-bearing sequence in southeastern Ordos Basin, with the lithotype distribution of the number 3, 5, and 11 coal seams.
2. MATERIALS AND METHODS
structure and generate false results, especially for the micro- and mesopores, whereas gas adsorption can avoid these limitations and spurious outcomes and can provide the pore size information from a few nanometers to hundreds of nanometers, including the size range of micro- and mesopores.17,18 The distribution of coal pores defined from gas adsorption analyses is generally based on the Brunauer−Emmett−Teller (BET) model using nitrogen as a probe gas.19 Nuclear magnetic resonance (NMR) imaging has been widely used in coal pore analysis because it is both non-destructive and convenient.20−22 Pore types are typically identified by using three transverse relaxation (T2) spectrum peaks;21 pore size distribution is determined by centrifuging; and permeability is estimated by adopting fitting. Coal is composed of organic components and inorganic minerals with the void space, including pores and fractures. Because the three basic components are of different density, their X-ray computed topography (CT) numbers are also different, allowing for successful discrimination of these phases in images. This investigation focuses on the variation of the coal reservoir characteristic of a single coal seam as well as between different coal seams. The primary objectives are 3-fold: (i) to determine the maceral and chemical composition of three main coal seams in the southeastern Ordos Basin, China (Figure 1), as they relate closely to the CBM development, (ii) to document the pore structure and distribution characteristics of continuous sublayers within and between the coal seams, and (iii) to provide an awareness of the nature and variability in coal composition and especially pore structures over an entire crosssection of a coalfield. To understand the pore system of these coal samples, the total porosity, pore size distribution, surface area, and organic geochemistry were analyzed, including the use of image analysis.
A total of 18 fresh bench samples were obtained from three underground working faces in the Hancheng coalfield (see Figure 1 for sampling location). All of the samples were collected following the Chinese Standard Method GB/T 19222-2003 and were carefully packed and returned to the laboratory for experiments. Most of the samples collected were large blocks (approximately 15 × 15 × 15 cm3) augmented by numerous small samples for thin sections. Coalbeds were separated into bright and dull belts by hand picking lithotypes. Maceral composition was carried out on a total of 40 moderate-size blocks. Before the analyses, all of the blocks were prepared as 3 × 3 cm2 polished slabs. Maceral analyses (500 points) were performed on the polished slabs following method GB/T 69481998. For the measurement of frequency, network geometry, and connectivity of microfractures, each polished slab was divided into nine microblocks (1 × 1 cm2 for every microblock) to be accommodated in the visual field of a microscope. Proximate analysis (following GB/T 212-2001) was performed for all 18 samples to measure the ash, moisture, and volatile material contents of the coals. Samples were stage-ground in a ring mill to pass through a 60-mesh sieve and placed in an atmosphere over a saturated solution of potassium sulfate at 30 °C to obtain equilibrium moisture (ASTM D1412-04). Moisture was measured by oven-drying weight-loss calculations. Ash contents of samples were measured in accordance with ASTM D3174-04. To compare and contrast the petrographic composition with methane capacity, all samples are reported on a dry and ash-free basis. MIP analyses were performed on 14 selected samples (the other four samples are either quality- or quantity-limited) following Chinese Oil and Gas Industry Standard SY/T 5346-1994 and using a Micrometics 9310 porosimeter, which automatically registers pressure, pore diameter, intrusion volume, and surface area. Before the porosimetric analysis, all samples were dried at 75 °C for 48 h. The specific surface area of each sample was determined by Micromeritics ASAP 2020 using N2 gas adsorption/desorption at a low temperature B
dx.doi.org/10.1021/ef500449s | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Table 1. Proximate Analysis and Maceral Group Composition of Samples (Minerals Included)a proximate analysis sample number
coal seam
burial depth (m)
Ro,max (%)
target coal thickness (cm)
Md (%)
XS-3-1 XS-3-2 XS-3-3 XS-3-4 XS-5-1 XS-5-2 XS-5-3 XS-5-4 XS-5-5 XS-5-6 XS-5-7 XS-5-8 XS-5-9 XS-5-10 XS-11-2 XS-11-3 XS-11-4 XS-11-5
3 3 3 3 5 5 5 5 5 5 5 5 5 5 11 11 11 11
321.10 321.30 321.70 322.00 345.20 345.65 346.05 346.50 346.80 346.95 347.55 347.95 348.35 348.63 387.20 387.70 388.10 388.70
2.36
20 40 30 50 45 40 45 30 15 60 40 40 28 38 50 40 60 80
0.91 0.75 0.76 0.81 1.30 0.87 1.61 0.84 0.91 0.92 1.02 1.00 1.10 1.06 0.56 0.59 0.56 0.65
2.39 2.34
2.38
2.34
2.39
maceral group analysis (%)
Ad (%)
Vdaf (%)
type
vit
lipt
inert
min
48.54 15.32 12.47 17.64 14.29 9.90 82.67 8.80 14.02 9.81 8.10 24.68 33.12 7.820 19.87 51.71 17.87 19.13
19.04 14.10 12.09 13.73 14.04 12.70 53.14 12.55 13.32 11.74 10.77 14.43 16.18 11.16 12.20 21.62 14.68 12.16
dull semi-dull semi-bright semi-dull bright bright mudstone semi-bright semi-bright semi-dull semi-dull semi-bright semi-dull semi-bright semi-dull semi-dull semi-bright bright
58.07 72.06 73.83 57.71 80.53 80.35 24.73 78.57 73.38 78.58 76.18 77.59 74.99 78.70 73.78 72.83 75.84 80.92
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
10.33 14.24 19.27 13.89 14.77 12.75 6.77 14.63 18.92 16.32 17.52 14.01 14.71 16.00 15.22 11.37 15.76 13.28
31.50 13.20 6.60 28.30 4.60 6.70 68.40 6.70 7.60 5.00 6.20 8.30 10.20 5.20 10.80 15.50 8.20 5.70
a
Ro,max, maximum vitrinite reflectance ratio; Md, moisture content (%, air-dried basis); Ad, ash yield (%, air-dried basis); Vdaf, volatile matter (%, dry and ash-free basis); vit, vitrinite; lipt, liptinite; inert, inertinite; and min, mineral.
Figure 2. Ash, mineral, and parts of maceral composition content (mineral-matter-free basis) variation of the full coal column. and pressure (77 K and
