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Coal reservoir heterogeneity in multi-coal seams of the Panguan syncline, Western Guizhou, China: Implication for the development of superposed CBM-bearing systems. Shida Chen, Dazhen Tang, Shu Tao, Zhenlong Chen, Hao Xu, and Song Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01617 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018
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Coal reservoir heterogeneity in multi-coal seams of the Panguan syncline, Western Guizhou, China: Implication for the development of superposed
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CBM-bearing systems.
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Shida Chen a, b, Dazhen Tang a, b, Shu Tao a, b, *, Zhenlong Chen c, Hao Xu a, b, Song Li a, b
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a
School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, PR China;
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b
Coal Reservoir Laboratory of National Engineering Research Center of CBM Development &
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Utilization, Beijing 100083, PR China;
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c
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Nanjing 210011, China
Petroleum Exploration & Production Research Institute, Sinopec East China Oil & Gas Company,
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* Corresponding Author-Email:
[email protected].
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Abstract: The reservoir heterogeneity of multiple-coal seams in the Panguan area was
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systematically analyzed based on coalmine geological exploration data, field-test data and
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laboratory tests. The results show that the physical properties (e.g., pore size distribution) of
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adjacent coal seams in the same well are basically the same due to the similar coal rank and
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non-existence of a coalification jump, whereas the ash yield and sulfur content related to the
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sedimentary environment exhibit a “high-low-high” trend with increasing depth. After the third
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coalification jump, coal becomes much more compact with a reduction in the seepage space and
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increase in the specific surface area. Additionally, the NMR T2 spectrum decreases from a
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bi-modal to a unimodal curve. The reservoir temperature (15~50 ℃) increases linearly with depth,
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but the influence of temperature on the CH4 adsorption capacity is insignificant in this area.
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Additionally, the well test parameters reveal that the pressure systems are vertically superposed
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due to the well water resistance of the key stratigraphic units. Specifically, the reservoir pressure
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tends to increase with depth in the same pressure system, and a corresponding increase in gas
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content is observed. An abrupt point of the pressure coefficient can be regarded as a boundary of
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different gas bearing systems. At the end of a gas-bearing system, coal seams are characterized
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by supersaturated reservoirs with a gas saturation greater than 100%. Furthermore, the change
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rule of coal permeability is more complex in multiple-coal seams due to the existence of
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superposed pressure systems. In the same stress field, a higher coal permeability is observed due
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to the higher reservoir pressure. Generally, the reservoir pressure and in-situ stress distribution
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are the two main determinants of the CBM enrichment and development in multi-coal seams,
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which should be paid more attention in the selection of favorable target layers.
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Keywords: Reservoir heterogeneity; Multi-coal seams; CBM-bearing system; Western Guizhou;
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China.
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1. Introduction
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Coalbed methane (CBM) development is a national energy security need and is also a way
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to reduce gas disasters and protect the natural environment.1,2 China is the largest consumer and
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producer of coal in the world and has abundant CBM resources, and thus, the development and
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utilization of CBM has a bright prospect in China.3,4,5 In the last few decades, CBM theory and
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technology have developed rapidly in China, and large-scale commercial exploitation has been
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carried out in the southern Qinshui and eastern Ordos basins. 6-8 Recently, low-rank CBM in the
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Junggar Basin and multi-seam CBM in western Guizhou have aroused general concern.9-12
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However, large scale development has not been realized in those areas due to the lack of research
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on CBM geologic conditions.
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Unlike other coal bearing basins, the CBM geologic conditions of multi-coal seams have
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unique complexity and particularity. Currently, the CBM development in western Guizhou
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province has revealed that the coal reservoirs in this area are characterized by “multi-seams with
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thin thickness, high in-situ stress, weak water bearing capacity, complex coal texture distribution,
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and varied coal rank”.13-18 Additionally, Qin et al. (2008) 19 and Shen et al. (2016)11 have found
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that the coalbed gas content variation is fluctuant in multi-coal seams and put forward the
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academic viewpoint of “Independent superposed CBM-bearing systems”, which means that the
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pressure systems are vertically superposed due to the water-resistance of the key stratigraphic
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units. Due to its special geological conditions, multilayer commingled production is considered
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as an efficient way for enhancing the CBM recovery in multi-seam area. However, the existence
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of superposed CBM-bearing systems may lead to the interlayer interference in practical
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production process.20-24 For example, the fluid pressure difference can cause high-pressure
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production layers to prevent the output of low pressure production layers through the wellbore.
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Permeability differences will cause different fluid supplies between different production layers.
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Gas saturation or the critical desorption pressure determines whether the productive layer can
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concentrate and produce gas continuously. Therefore, characterizing the reservoir heterogeneity
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(e.g., pore-structure, gas content, adsorption/desorption capacity, coal permeability and reservoir
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pressure) of multi-coal seams is significant for the enrichment of CBM and the selection of
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favorable development layers.
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However, few studies have investigated the reservoir heterogeneity of different coal seams
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or different CBM-bearing systems vertically due to the lack of measured field data. Recently,
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several parameter wells were drilled in western Guizhou province, providing data for analysis. In
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this work, two parameter wells in the Panguan area were taken as an example to investigate the
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reservoir heterogeneity of different coal seams and coalfields according to coalmine geological
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exploration data, field-test data and laboratory test results from coal cores, including the material
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composition, pore-fracture structure, adsorption capacity, gas content, gas saturation, and well
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test parameters. The main controlling factors of reservoir heterogeneity in the vertical and
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horizontal were determined with the goal of providing theoretical support and engineering
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guidance for future CBM development strategies.
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2. Geological setting
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The Panguan area is situated in the Liupanshui mining district in southwest Guizhou
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Province (104°18′~104°52′ E, 25°34′~25°04′ N), which covers a coal-bearing area of 1350 km2
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(Fig. 1). The study area is 58 km from E to W and 60 km from S to N. The exploration and
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development of coal-bearing strata in this area mainly lies in the Upper Permian Longtan
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Formation, which is a marine and continental alternating deposition that averages 180 m in
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thickness (Fig. 1c). For the upper and lower parts, the Longtan Formation is mainly classified to
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lagoon and tidal flat deposits, whereas the middle part is dominated by lower delta plain
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deposits.11 The quantity of coal seams in this formation is approximately 40 layers from 30 m to
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42 m (averaging 34 m) in thickness. The 12#, 17#, 18# and 24# coal seams are developed
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steadily in the whole region. Although the study area is rich in CBM resources, the degree of
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geological exploration and development is low. By the end of 2017, 14 CBM wells (including 4
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parameter wells) were completed, and the gas rate of 9 multilayer commingled production wells
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was approximately 600-2200 m3/d.
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According to the simulation results using the Basin-Mod Basin simulation software, the
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Late Permian coal-bearing strata in the Panguan area is characterized by two periods of
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subsidence and burial, two periods of uplift and erosion, and three periods of coalification. 25 The
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Middle Yanshanian tectonic movement was a key period for CBM accumulation in which the
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modern occurrence of the coal rank was created by telemagmatic metamorphism superimposed
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on plutonic metamorphism (Fig. 2). During the Middle Yanshanian tectonic movement, although
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the coal seams were shallow due to tectonic uplift, the maximum geothermal gradient reached
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5.5°C / 100 m as a result of a tectonic heat event (magmatic intrusion). At this time, the
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paleo-temperature increased rapidly and reached 140°C in the west wing, corresponding to the
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metabituminous stage (Ro, 1~1.2%); in the east wing, T reached 200 °C and the coal transformed
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into meagre coal (Ro>1.9%). Therefore, the coal rank in this area varies widely with the
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vitrinite reflectance from 0.8% to 2.5%.
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3. Methodology
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In this work, 13 coal cores were collected from two parameter wells (Well 1: 5#, 9#, 12#,
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15#, 24#, 27#, 29#; Well 2: 6#, 9#, 10#, 17#, 18#, 26#) in two typical coalfields (including the
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Yueliangtian and Songhe coalfields) for laboratory experiments. Additionally, the well test
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parameters of those two wells (Well 1:1+3#, 9#, 16#, 27#; Well 2: 3#, 10#, 13#, 22#, 26#) were
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obtained and the gas content of different coal seams was measured (Well 1: 4#, 5#, 9#, 10#, 12#,
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15#, 16#, 22#, 27#, 29#; Well 2：1#, 3#, 7#, 10#, 17#, 18-1#, 18-2#, 24-1#, 24-2, 26#, 29#). In
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addition, coalmine geological exploration data from several coalfields in the Panguan area were
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collected from the Guizhou Bureau of Coal Geological Exploration. The specific experimental
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steps were as follows:
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(1) Material composition: Samples were first analyzed to determine the vitrinite reflectance
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and maceral content (500 points) following ISO 7404.3-1994 (1994) 26 and ISO 7404.5-1994
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(1994) 27, respectively. Proximate analysis (including ash yield, moisture content and volatile
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matter) measurements were performed according to the Chinese national standard GB/T
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212-2008 28.
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(2) Seepage space: Saturated columns (approximately 2.5 cm in diameter and 2 - 5 cm in
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length) were measured with the low field nuclear magnetic resonance (NMR) by using a
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MiniMR60 instrument, and the test procedures were the same as those presented by Yao et al
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(2010) 29. Mercury injection (MIP) was carried out using a Micromeritics Auto Pore IV 9500,
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following the national standard SY/T 5346-2005 30.
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(3) Adsorption space: Currently, N2 and CO2 adsorption are always used to determine the
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pore size distribution of microporous medium. Because of kinetic restrictions at cryogenic
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temperatures (87 K), N2 adsorption is of limited value for the characterization of very narrow
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micropores ( Well 2, but there is no obvious change rule in different layers.
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The differences of the proximate analysis results of different coal seams in different coalfields
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are shown in Fig. 4. The volatile content decreases gradually from top to bottom, completely
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consistent with the coal rank. However, there are no significant trend changes in the moisture
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content, which may be related to the non-existent coalification jump in the vertical. The coal ash
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yield and sulfur content exhibit a trend of high-low-high with increasing depth, which is closely
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related to the water depth and oxidation of the coal sedimentary environment.11 Here, the ash and
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sulfur contents of coals from the upper and lower parts of coal-bearing strata are greatly
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influenced by the chemical action of seawater due to the Lagoon and Tidal flat deposits. By
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contrast, the ash and sulfur contents of coals form the middle part are relatively low due to fewer
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effects of seawater, reflecting a lower delta plain. Generally, coal seams No. 9 and No. 18 can be
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roughly regarded as the dividing line of the sedimentary facies.
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4.2 Pore structure characterization with MIP, N2 and CO2 adsorption
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The pore structure of coal reservoirs, including the pore volume, pore size distribution
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(PSD), specific surface area (SSA), and pore morphology, directly influences the occurrence and
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migration of coalbed methane (CBM).
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influence the adsorption capacity of coal due to their enormous internal SSA, namely, adsorption
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pores, while pores with a diameter >100 nm are the main channel for gas and water flow
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(Seepage space). 29,37,38 Here, MIP, N2 and CO2 adsorption were selected to determine the PSD,
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pore volume and SSA across multi-scales. Fig. 5 presents the test results of MIP. The total pore
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volume (TPV, >7 nm) of 7 coal samples in well 1 range from 23 to 37 ×10-3 cm3/g (averaging
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26.23 ×10-3 cm3/g), whereas in well 2, they range between 19.4 and 29.6×10-3 cm3/g (average of
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23×10-3 cm3/g). The porosity in well 1 is 4.3-5.07% (averaging 4.62%), whereas in Well 2, it is
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only 2.95-3.9% (averaging 3.38%). The average pore diameter (APD) of coal in well 1 (18.33
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nm) is also greater than that in well 2 (15.4 nm). The anomalously high values of the 24# coal
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seam in well 1 and 17# coal seam in well 2 are because of damage to the initial coal structure
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during sample preparation, which increased the fracture density. It seems that, for most of the
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coal seams, the physical properties (porosity, TPV, PSD, seepage space) of the adjacent coal
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seams in the same well are basically the same according to the pore-fracture conditions for
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multiple-zone production.
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Generally, pores with a diameter
