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Combustion Characteristics of Tight Sandstone Yu Zhou, Wei Chen, and Yafeng Lei Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00417 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 3, 2018
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Energy & Fuels
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Combustion Characteristics of Tight Sandstone
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Yu Zhou a, Wei Chen a,*, Yafeng, Lei b, a b
School of Energy, Soochow University, Suzhou, China General Electrical Company, Houston, TX 77041, USA
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Corresponding Author: Wei Chen
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School of Energy
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Soochow University, Suzhou 215006, China
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E-mail:
[email protected] 14 15
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Abstract
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As a promising unconventional energy source, tight sandstone gas has attracted
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increasing attention. Because of the extremely low porosity/permeability of tight
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sandstone, large scale exploration and development are still challenging. Recently,
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high-temperature combustion or pyrolysis is employed to increase sandstone porosity
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/permeability. In this study, the sandstone samples obtained from Xinjiang province
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China were heated for 30 minutes in air atmosphere at temperatures of 350 oC, 500 oC,
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700 oC, 800 oC and 900 oC respectively. The combustion characteristics and property
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changes of sandstone were investigated. In the beginning, thermal decomposition
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process of sandstone and gas emission were tested using thermogravimetry combined
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with fourier-transform infrared spectroscopy (TG-FTIR). Then, the mineral
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composition and pore structure of different combusted sandstones were tested and
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analyzed. Moreover, the effects of combustion temperature on particle size of
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sandstone was also investigated. Finally, scanning electron microscopy (SEM)
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technology was performed to study the surface appearance change of sandstone.
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According to the experiment results, organic matters started releasing at 350-500 oC
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and most of the minerals such as carbonate decomposed at 600-870 oC. During the
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combustion process, carbon dioxide and water vapor were the main product gases.
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With the increase of combustion temperature, the mean pore diameter increased from
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8.0 nm to 22.6 nm while the particles size almost kept constant. In addition to the
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increase of pore size, it can be found from SEM photos that the compact surface of
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sandstone became smooth and some new pores and small cracks appeared on the
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surface of sandstone after high-temperature combustion, especially at 900 oC. As a
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result, high-temperature combustion is one of the feasible methods to improve the
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porosity/permeability of sandstone.
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Key Words: sandstone, combustion characteristics, mineral, organic matter, pore size
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1. Introduction
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With the development of natural gas extraction techniques, large scale extraction of
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tight sandstone gas, shale gas , and coalbed methane has aroused increasing interest in
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petroleum industry [1]. Countries such as China, Canada, and United States are
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developing strong programs to study and explore large scale extraction of sandstone
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gas. Permeability/porosity is the critical parameter in evaluating the capacity for
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hydrocarbon flow transportation in the rocks [2, 3]. Tight gas sandstone reservoirs
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are commonly characterized by low permeability (less than 0.1 mD), low porosity
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(less than 10%), complicated pore structure and strong heterogeneities [1, 4]. Thus,
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the extraction of large reserves of sandstone becomes challenging. Tight sandstone
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has poor connectivity, which contains pores ranging from nano-scale to micro-scale
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and the natural fractures were at millimeter scale [5, 6]. According to Liu et al., the
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pore space of tight sandstone was mainly made up of intergranular pores, dissolved
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pores and micro pores, and the micro pores in clay minerals dominated pore space and
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permeability [6].
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For most tight sandstones, main mineral compositions are quartz and feldspar, with a
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small quantity of other minerals [7]. The high quartz content results in high
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mechanical stability, but the variety of accessory minerals (feldspars, clays, iron
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oxides and carbonate minerals) might lower its chemical and mechanical stability [8].
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Due to the presence of various minerals, the pore connectivity became poor, which
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resulted in the reduction of storage and gas transport capacity [9]. In order to increase
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permeability of reservoirs, acids have already been used to react with minerals in
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sandstones [10]. However, chemical acids have potential negative effects on the
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ecology system. For example, the inject of the common mud acid (a combination of
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hydrofluoric acid and hydrochloric acid ( HF: HCl )) may not only cause the secondary
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and tertiary precipitation damage, but also collapse the sandstone formation because of
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unconsolidated formation caused by excessive corrosion of clay minerals [11].
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Moreover, organic acids such as acetic and formic acids were also used to acidize
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sandstone which could result in underwater contamination [12]
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Recently, hydraulic fracturing and CO2 sequestration are regarded as the main
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promising enhanced gas recovery techniques for tight gas reservoirs [13]. These
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techniques have greatly increased gas production, but it can also threaten water
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resource [14]. For example, hydraulic fracturing requires a large amount of water,
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which is not applicable to some areas that lack water resources, especially in Xinjiang
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province, China which is covered by large deserts. In addition, water-based fracturing
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fluid increases water saturation and decrease gas permeability, then causes the
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so-called permeability jail [15, 16]. Moreover, using CO2 for gas recovery needs to
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consider the problems of pipe corrosion, pipe leaking and compressor investment [16].
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Therefore, many researchers are searching for other exploitation technologies for gas
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and oil extraction. For example, Wang et al. applied microwave heating to enhance
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shale gas recovery [17]. Nicolini et al. applied nanofiltration to improve oil recovery
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in sandstone reservoirs by changing the injection water salinity and ionic components
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[18]. Recently, high-temperature combustion and pyrolysis have been used to enhance
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shale rock permeability [19, 20]. With the increase of pore size and storage space of
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sandstone, both porosity and permeability increased [21].
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Many studies analyzed the mechanical properties of sandstone such as compressive
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strength, elastic properties and mechanical wave in different normal conditions
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[22-26]. Moreover, thermal conductivity of sandstone was also studied since it is
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related to permeability and porosity [27, 28]. However, limited researches focus on
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the high-temperature combustion characteristics of sandstone. Temperature could
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affect the dissolution and precipitation processes of minerals through its impact on
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mineral reaction rates and equilibrium constants of chemical reactions, etc [29]. For
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sandstones, high temperature could cause thermal damage and change pore structure
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[30-32]. Thus, it is necessary to investigate the geochemical prosperities and
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combustion characteristics of sandstone at different combustion temperatures.
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In this article, temperatures of 350 oC, 500 oC, 700 oC, 800 oC and 900 oC were applied
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to sandstone samples combustion for 30 minutes, respectively. The effects of
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combustion on the samples’ physical and geochemical properties such as pore size
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and mineral compositions was investigated. Moreover, the combustion characteristics
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and gas emission of sandstone samples were studied with TG-FTIR. The particle sizes
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of sandstone samples under different combustion temperatures were also investigated.
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2.
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The sandstone samples were obtained from a drilling hole at Donggou basin, Xinjiang
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province, China. It is located in northwest China and most of it is covered by deserts.
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As mentioned before, it is not suitable to exploit oil and gas reservoirs using hydraulic
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fracturing due to the lack of water resources. As a result, combustion and pyrolysis
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technologies are being employed in tight sandstone gas recovery.
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In the beginning of the experiments, a raw sandstone sample with gray color (Fig.1)
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was crushed and screened to small particles (Fig. 2a) with size of 100-180 mesh
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(0.088-0.150 mm). These raw small particles were heated at temperatures of 350 oC,
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500 oC, 700 oC, 800 oC and 900 oC respectively, for 30 minutes in a tube furnace in air
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environment. Afterwards, combusted and raw sandstone samples were collected for
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XRD tests, liquid nitrogen absorption and desorption tests, and particle size
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measurement.
Sample Preparation and Experiments
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Fig.1. Raw sandstone sample from Donggou basin
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As shown in Fig. 2, the color of combusted sandstone samples gradually became puce
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due to the decomposition of minerals and oxidization of organic matters with
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increasing combustion temperature.
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(a)
(b)
(d)
(e)
(c)
(f)
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Fig. 2. The raw and combusted sandstone samples. (a) Raw, (b) 350 oC, (c) 500 oC, (d)
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700 oC, (e) 800 oC, (f) 900 oC
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2.1 Proximate Analysis
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In the beginning of experiments, the raw sandstone samples were sent to a
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commercial lab for proximate analysis. The proximate analysis provided the weight
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percentages of moisture, ash, volatile matter (VM) and fixed carbon (FC) of sandstone
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[33]. Table 1 gives proximate analysis of the sandstone samples on air dry basis.
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Table 1. Proximate analysis of sandstone Air dry basis
Wt.%
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Moisture Ash VM FC
0.85 98.21 0.71 0.23
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2.2 TGA-FTIR Tests
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In order to study the thermal decomposition process of sandstone samples, about 20
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mg finely crushed raw samples were heated from ambient temperature to 900 oC with
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a heating rate of 20 oC/min in a TG-FTIR instrument (STA6000-Frontier). As
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temperature increased, mass loss and gas emission as a function of combustion
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temperature were both recorded.
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2.3 XRD Tests
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Since minerals will decompose during combustion and it affects the pore connection
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inside the sandstone samples, raw and combusted sandstone samples were used for
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XRD tests to study the effects of combustion temperature on mineral decompositions
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using an X-ray diffraction meter (D/MAX-2000PC). The scattering angle (2θ) was set
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from 10° to 80° and scanning rate was set at 5°/min with a step size of 0.02°. The
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relative content of raw sandstone samples was given in Table 2.
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Table 2. The XRD analysis of mineral composition and relative content of sandstone
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(Wt. %). Sample
Quartz Anorthose Orthoclase Calcite Illite Biotite
Donggou sandstone 5
3
12
70
9
1
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2.4 Liquid Nitrogen Adsorption and Desorption Tests
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Liquid nitrogen adsorption and desorption tests were performed to analyze the change
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of pore microstructure such as pore volume and pore size after high-temperature
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treating. Six groups of heated sandstone samples were measured using a Surface Area
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and Pore Size Analyzer (TriStar II 3020).
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2.5 Particle Size Measurement
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In order to investigate the effect of combustion temperature on particle size change, the
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six different types of sandstone samples (particle size 50-300 µm) were measured using
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a laser scattering particle analyzer (Malvern Instruments MS3000). The measurements
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were repeated for five times and average size distribution was obtained. Fig. 3 gives the
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size distribution curves of raw sandstone particles. It was found that the peak volume
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density (about 17 %) was at the size of 130 µm, which means the mean particle size of
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this sandstone group is range of 125-135 µm.
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Fig. 3. Size distribution of raw sandstone samples
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3. Results and Discussions
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3.1 Sandstone Sample Properties
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It can be found from Table 1 that the total organic carbon (TOC) of the sandstone
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samples including VM and FC is about 0.94 wt.%. TOC content of shale rock can be
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divided into 5 types: poor (＜0.5%), fair (0.5–1%), good (1–2%), very good (2–4%)
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and excellent (＞4%) [34]. Compared to shale, the TOC content of this sandstone is
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fair (
