Pyrolysis and Combustion Enhance Recovery of Gas for Two China

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Pyrolysis and Combustion Enhance Recovery of Gas for Two China Shale Rocks Wei Chen, Yafeng Lei, Yilin Chen, and Jiafeng Sun Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02274 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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Energy & Fuels

Pyrolysis and Combustion Enhance Recovery of Gas for Two China Shale Rocks Wei Chena,*, Yafeng Leib,Yilin Chen c, Jiafeng Sund a School of Energy, Soochow University, China b General Electrical Company, Houston, TX 77041, USA c School of Resource and Geosciences, China University of Mining and Technology, Xuzhou 211116, China d Department of Mechanical Engineering, Texas A&M University, College Station, TX77843, USA

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Corresponding Author:

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Professor: Wei Chen

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School of Energy,

17 18 19 20

Soochow University, China Suzhou 215006, China Email: [email protected]

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ACS Paragon Plus Environment

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Abstract

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Due to the low matrix permeability of shale formation in China, the extraction of shale gases

31

from the rocks become difficult. Recently, combustion and other thermal treatment methods

32

were employed to remove organic matters from the shale to increase rock's permeability. In

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this study, geothermal properties of two shale samples were tested and analyzed. It was found

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that shale 1# has a higher methane adsorption capacity, reflectance, and porosity than that of

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shale 2#. Since the shale has low permeability, the two samples were heated up in a TGA

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instrument in inert (N2 and helium) and un-inert environments (air and CO2) to increase shale

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permeability by removing the organic matters. From TGA weight loss curves, it was found

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that the weight loss rates were high in helium environment compared to CO2 and air since

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helium has higher thermal conductivity which can promote organic matter cracking. However,

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organic matters were not oxidized with air and some of them were still remained inside the

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samples at the end of experiments. As a result, pure air may not be a suitable combustion agent

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to burn the organic matters. Furthermore, the experiment reveal that carrying gases with high

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thermal conductivity was beneficial for the organic matter cracking.

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Key Words: Shale, Permeability, Thermal conductivity, Combustion, Organic matter

45

Acronyms

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BET

Brunauer–Emmett–Teller

47

FC

Fixed carbon

48

Ro

The vitrinite-like macerals reflectance

49

TC

Thermal conductivity

50

TGA

Thermogravimetric analysis

51

TOC

Total organic carbon


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VM

Volatile matter

53 54

1. Introduction

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With the increase of energy demand, shale gas has attracted increasing attentions. Shale gas/oil

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was considered to be a cleaner and efficient energy to replace other conventional fuels such as

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coal for power generation. As an unconventional fuel, shale gas was produced from

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organic-rich shale. With the development of horizontal drilling and hydraulic fracturing

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technology, shale gas has become more economical to produce around the world, especially in

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USA[1]. According to Perry et al, the total geological reveres of shale gas reservoirs are

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estimated to be approximately 456.24 x1012 m3 globally[2]. China has an abundant of different

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types of shales. These shales include not only organic-rich Paleozoic marine shale but also

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Mesozoic and Cenozoic continental shales. These shales contain large amount of gas reserve

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and it was estimated that the total geological reserves of shale gas reservoirs of china is around

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134 X1012 m3[3]. However, large scale shale gas exploration is the primary challenge due to

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the low porosity and low permeability of shale formation.

67

Normally, a shale gas reservoir is a source rock which retains hydrocarbons over geologic

68

times. It comprises total organic carbon (TOC) stored inside the matrix[4]. Gas storage in the

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shale is generally categorized into three groups[5]: (1) a free gas state in pores and fractures;

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(2) an adsorbed state on the surface or inside organic matter and minerals; and (3) a dissolved

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state in organic matter and water. For group (1), gas is adsorbed in micropores because of

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large specific surface area and great adsorption potential in narrow pores[6]. Pore structure is

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the key factor influencing the occurrence of shale gas and gas storage capacity[7]. However,

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due to extremely low matrix permeability, clusters of mineral-filled “natural” fractures, and an

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organic-rich deposition, extraction of the shale gas becomes difficult [8]. Heat treatment

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would be one of the methods to improve permeability. Jamaluddin et al found that thermal


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treatment can increase permeability in tight gas reservoirs by vaporizing the capillarily

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blocked water, dehydrating the clay-bound water, destroying the clay lattice and creating

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thermally induced microfractures[9]. Different technologies such as electrical heater, high

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temperature steam, and microwave heating were employed in oil and gas industry to extract

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oil and gas[10]. When shale rocks get hot, the absorbed gases such as methane obtain energy

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from the injected steam or hot water and leave the rocks. Thus, a large amount of gases can be

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produced and collected from the shale. According to Fourier’s law, the amount of heat flow to

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the shale could be determined using the product of thermal conductivity and the temperature

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gradient. Different types of rocks, sandstones, mudstones, shales, marls, and lime stones, were

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analyzed to define the relationships between thermal conductivity and other physical

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properties of rocks[11] . The sandstones, with a large effective porosity typically ranging

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between 16% and 30%, have bulk thermal conductivities ranging between 2.1 and 3.9

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W/m/K[12]. Rock thermal conductivity is one of the most important parameters to affect shale

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gas extraction and its value strongly depends on mineral composition. It increases with

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augmenting content of minerals of high thermal conductivity mineral such as quartz, and

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decreases with the increase of low conductivity minerals such as clay minerals[11].

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In addition to mineral composition, pressures, temperature, porosity and water saturation

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would affect the rock thermal conductivity. Li et al found that thermal conductivity of the

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rocks increases with increase of pressure ,but decreases with rising of temperature[13].

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Gornov et al conducted “optical scanning” of core samples using “λ-profile” method to

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determine the rock thermal conductivity and found that the thermal conductivity of the

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regional rocks depends on mineral composition, structure, and texture[14]. Roy et al

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investigated the effect of water saturation on rock thermal conductivity measurements and

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found that thermal conductivity of porous rocks under water-saturated state is greater than that

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under dry state [15]. Popov et al used experimental methods to find the relationship between


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permeability, electrical and thermal conductivity[16].

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In addition to thermal conductivity, the permeability of gas shale is also affected by the

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organic rich deposition (kerogen) [17, 18].

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hydraulic fracturing , research aiming at alternative permeability enhancement methods such

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as combustion were considered to be one possible method to remove the organic matters such

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as kerogen, portion of naturally occurring organic matter [4] . However, few studies have been

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conducted using different carrying gases to remove organic matters during combustion and

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pyrolysis processes to investigate the effect of thermal treatment methods on the shale

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property changes .

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The main objective of this study is to study thermal and geochemical properties of two

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Chinese shales and explore the mechanics of shale combustion and pyrolysis behaviors under

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different gas environments. The thermal conductivity, pore size, and methane adsorption

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capacity of the two samples were investigated. Moreover, helium, CO2 and air were employed

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as carrying gas to remove the organic matters of shale rocks to enhance the permeability.

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Furthermore, the weight loss process and weight loss rate characteristics obtained from TGA

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experiment were studied to investigate the effect of heating rates and purging gases properties

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on organic matter liberation behaviors.

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2. Properties of Shale Samples

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2.1 Geochemical properties of shale samples

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These shale samples were collected from two different drilling holes at ShangGu basin,

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Shanxi province, China. The samples obtained from this area have a cylinder shape and gray

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color (Figure 1).

Due to the potential environmental impact of


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Figure 1. Illustration of shale 1# and 2#

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Shale 1# and 2# were buried at depth of 609 and 35 meters, respectively. Before the TGA tests,

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the raw samples were crushed and screened to small particles with different size. The samples

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of particles were sent to Jiangsu Coal Research Laboratory for different physical and chemical

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analyses. In order to explore the shale properties such as thermal properties, proximate and

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ultimate analyses were conducted as shown in Table 1. Similar to other fossil fuels, proximate

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analysis provides the composition of the shale in terms of moisture, volatile matter (VM),

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fixed carbon (FC), mineral matter and ash [17]. It also provided organic matter mass

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percentage of the shale samples. Table 1 lists the proximate and ultimate analysis results of the

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two shale samples.

135 136 137 138 139 140


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Energy & Fuels

Table 1. Proximate and ultimate analysis shale 1# 0.4 88.08 9.66 2.22 4.63 6.19 0.67 0.13 0.26 1755.50

Moisture Ash VM FC Carbon Oxygen Hydrogen Nitrogen Sulfur HHV (kJ/kg)

shale 2# 0.7 89.09 8.11 2.71 6.24 3.09 0.72 0.16 0.63 2764.08

Dry, Ash Free Moisture Ash VM FC Carbon Oxygen Hydrogen Nitrogen Sulfur HHV (kJ/kg)

0.00 0.00 81.31 18.69 39.00 52.08 5.61 1.10 2.21 14757.13

0.00 0.00 74.77 25.23 57.57 28.50 6.64 1.50 5.79 25498.21

Chemical formula

CH1.7105O1.0024N0.0242S0.0212

CH1.3715O0.3716N0.0223S0.0377

142 143

In addition to proximate and ultimate analysis, the thermal maturity was an important

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parameter to evaluate the gas self-generation and storage capacity, i.e., gas sorption capacity

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[19].Thus, understanding of the thermal maturity of the shale samples would be the first step

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to explore the gas adsorption mechanisms. Table 2 provides the geochemical characteristics

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analysis of two samples including exinite, vitrine, and inertinite weight percentage as well as

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the vitrinite-like macerals reflectance (Ro).

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Table 2. Geochemical characteristics of the shale samples Sample No.

Exinite(%) Vitrinite (%)

shale 1# shale 2#

54 68

18 14

Inertinite(%)

Ro (%)

28 18

1.92 1.85

150


Organic Matter type Ⅱ Ⅱ

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2.2 Shale Pore Size Analysis

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Pore system of the shale samples was analyzed by nitrogen adsorption. Measurements were

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conducted in a Surface Area and Pore Size Analyzer (V-Sorb 2800TP) according to Rock

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surface area and pore size distribution measurement method: static nitrogen adsorption

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capacity (SY/T 6154-1995). Pore Size Analyzer used the standard static volumetric method to

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measure the amount of adsorbed gas. In order to removal of bound water and capillary water

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adsorbed in the clays, about 200-300 mg of the small sized samples were outgassed at 383 K

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for 5 hours at a vacuum environment (10 µm Hg). Nitrogen with purity of 99.999% was used

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as an adsorbent at 77 K, and adsorption–desorption isotherms were obtained at relative

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pressures (p/p0) between 0.01 and 1. Figure 2 gives the adsorption and desorption process of

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the two samples under liquid nitrogen environments. Under the similar condition, shale 1# has

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larger adsorption capacity than that of shale 2#.

Absorption-shale 1#

Desorption-shale 1#

Absorption-shale 2#

Desorption shale 2#

8

Absorption （ml/g，STP）

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7 6 5 4 3 2 1 0 0

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0.2

0.4

0.6

0.8

1

Relative pressure (P/P0)

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Figure 2. Liquid nitrogen adsorption and desorption curves of shale samples

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Figure 3 gives the pore size distribution of the shale 1# and Shale 2#. It was found that shale


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1# has more small size pores than shale 2# (< 9 nm) while shale 2# has more large size pores (>

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10 nm). According to the test, the porosity of the shale 1# and shale 2# is 3.12% and 2.56%,

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respectively.

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Pore size distribution shale 1# shale 2#

1.20E-03

Pore Volume (ml/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.00E-03 8.00E-04 6.00E-04 4.00E-04 2.00E-04 0.00E+00 193.4

98.4

41

24.7

12.6

8.1

5.8

4.5

3.5

2.8

2.3

Pore size（nm）

170 171

Figure 3. Pore size distribution of the shale samples

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2.3 High-pressure Methane Sorption Experiments

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The sorption of hydrocarbon gases (mostly methane) in shales not only provides gas-storage

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capacity but also the “free-gas” capacity in the pore system[20]. In order to investigate the

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sorption and desorption behavior of shale samples, high-pressure sorption experiments of

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methane were performed by the volumetric method. The methane sorption capacity of the

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shale samples was determined by a high-pressure volumetric sorption apparatus (IS-300). All

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methane isothermal adsorptions were measured at 25 ± 0.1 °C. The adsorption data were then

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fitted to the Langmuir equation as shown in Equation (1)[21].

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V=

VL P PL + P

(1)


Energy & Fuels

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where V is the volume of adsorbed gas per unit volume of the shale sample at pressure P, VL

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is the Langmuir volume, P is the gas pressure, and PL is the Langmuir pressure, or the pressure

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at which half the Langmuir volume of gas is adsorbed. It was found from Figure 4 that with

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the increase of pressure P, the adsorption capacity of both samples increased. Furthermore,

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sample 1 has higher methane adsorption capacity than that of sample 2. shale 1#

shale 2#

3.5 3 2.5 Absorptiopn (CM 3/g)

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2 1.5 1 0.5 0 0

2

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4

6 Pressure (Mpa)

8

10

12

188 189

Figure 4. CH4 adsorption of shale 1# and 2#

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2.4 Thermal Conductivity Test

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The thermal conductivity of shale rocks is an important property for determining the amount

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of heat to extract the methane gas from rock. Typical techniques for the measurement of rock

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thermal conductivity include the divided-bar steady-state technique, the needle-probe transient

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method and the optical scanning method[12]. In this study, the hot-wire method (instrument

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XIATECH TC3000) was employed to experimentally determine the thermal conductivity (TC)

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of shale samples. The shale samples were embedded in a hot wire and heated with a constant


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electric heating power. The TC of the samples could be determined through the

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temperature-increment rate with the temperature change obtained over time (Figure 5). The

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experiments were repeated five times and average values were obtained for each sample.

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Figure 5. Hot wire method of temperature change rate with time of shale 1#

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2.5 TGA Test of Shale Sample

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In order to study the effect of thermal treatment methods such as combustion and pyrolysis on

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the shale rock permeability, thermal gravimetric and differential thermal analyses on shale

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samples were conducted using a TGA-DTA (TA SDT Q600). In each experiment, about 10-12

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mg samples were heated in a ceramic pan at a rate of 20 oC/min from ambient temperature to

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1000 oC in different gas environments. The mass loss of the samples (m) as a function of

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temperature (T) was recorded for the trace. Figure 6a gives the shale 1 # sample weight loss

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processes under different purge gas environments, and Figure 6b gives the sample weight loss

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processes of Shale 2 # under different heating rate conditions using helium as the purging gas.

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Both shales have low moisture (less than 1%) and volatile matter (8% for shale 1# and 9% for

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shale 2#). Moreover, both shale rocks share the same weight loss trend. The main weight loss

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process was occurred between the temperature 300 to 600 oC, which corresponding to organic


Energy & Fuels

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matter decomposition and cracking of heavy hydrocarbons. With the temperature inccreasing

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further (above 600 oC), the weight of the shale decreased slightly which may be due to the

216

dissociation of clay materials and the carbonate minerals components of the shale samples[22].

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Moreover, it was observed that it was easier for samples to lose weight when helium was used

218

as purge gas, followed by air and then CO2. This is because helium has significantly higher

219

thermal conductivity (0.16 w/m2) while the thermal conductivities of air and CO2 are around

220

0.02 w/m2. Sample lost more weight under helium condition since more heat flux could

221

penetrate into the shale samples which resulted in quickly breakdown of the bonds among the

222

hydrocarbons of the shale samples. It was interesting that samples were not oxidized by air

223

since similar weight percentage remained at the end of the experiment.

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According to the TGA manufacturer, the uncertainty of the temperature is 0.1% of the

225

measured value. The uncertainty of lignite weight is 0.1 mg.

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He

Air

CO2

100 99 98 97

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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96 95 94 93 92 91 0

227 228 229

100

200

300

400

Temperature

500

600

700

800

(oC)

Figure 6a. Weight loss process of shale1# under different purge gas conditions at heat rate 10 o C/min


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5 oC/min

100

10 oC/min

20 oC/min

99 98

Weight %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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97 96 95 94 93 92 91 0

200

400

600

800

1000

Temperature (oC)

230 231

Figure 6b. Weight loss process of shale2# under helium conditions at different heat rates

232

3 Results and Discussion

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3.1 Fuel Properties

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It can be found from Table 1 that both shale samples have very high ash content (>88%) and

235

low moisture content (600 oC), weight loss rate did not significantly increase

308

for air. More weight was supposed to lose if organic matters were oxidized by oxygen under

309

high temperature. However, the percentages of weight were similar at the end of experiments

310

under air and CO2 condition (Figure 6a), which means some organic matter such as fixed

311

carbon did not oxidize and remained inside the samples. This may be due to the fact that

312

organic matters were covered by the minerals and muds or the small pores were blocked

313

which prevent the air to contact with organic matters. Thus, simply using air as oxidizer to

314

burn the organic matter of shale rocks may not be an effective method to improve the

315

permeability. More experiments including pure oxygen combustion will be conducted in the

316

near future to explore the reason behind this. Helium

Air

CO2

1 0.9 0.8 0.7

wt%/Min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.5 0.4 0.3 0.2 0.1 0 0

200

400

Temperature

317

600

800

(oC)

318

Figure 8. Weight loss rate process of shale 1# in different environment at heating rate 20

319

o

320

Figure 9 gives the weight loss rate curves of shale 1# under different heating rates with air

321

as carrying gas. It is not surprising that the maximum weight loss rate (the peak point)

C/min


Energy & Fuels

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increased with the increase of the heating rates. Moreover, the maximum weight loss rate (the

323

peak point) switched to higher temperature. For example, the maximum weight loss rate was

324

almost 1wt%/min at 520 ⁰C with heating rate 20 oC/min, while it was only 0.3 wt%/min at 420

325

o

326

helped the organic matters to liberate from the shale samples.

C with heating rate 5 oC/min. Thus, higher heating rate promoted the weight loss rate and

5 oC/min

10 oC/min

20 oC/min

1 0.8

wt%/min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.4 0.2 0 0

327

200

400

600

800

1000

Temperaure (oC)

328 329 330 331

Figure 9. Weight loss rate curves of shale 1# under different heating rates with air as carrying gas

332

rate 5 oC/min using helium as carrying gas. These two curves exhibit the similar weight loss

333

trend. The weight loss rate started increasing when temperature was above 300 oC and the

334

quick organic releasing zone was between temperature 330 oC-500 oC. Moreover, shale 2# has

335

a higher release rate when temperature was below 420 oC while shale 1# lost more organic

336

matters above temperature 420 oC since shale 1 # has a higher activation energy.

Figure 10 gives the comparison of the weight loss rate curves of shale 1# and 2# at heating

337


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Shale 1#

Shale 2#

0.25

wt%/min

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0.2

0.15

0.1

0.05

0 0

200

400

600

Temperature

338

800

1000

（oC）

339

Figure 10. Weight loss rate of shale 1# and 2# at 5 oC/min under helium environment

340

4. Conclusions

341

In the study, geothermal properties of two shale samples were tested. Combustion and

342

pyrolysis were employed to remove their organic matters to increase their permeability of the

343

shale samples. Following are the conclusions for this study:

344

(1) The total organic content of the shale samples is around 10% and hydrocarbon generating

345

potential of these two shale samples is high. Moreover, the Ro of shale 1# and 2# are 1.92

346

and 1.85 respectively and these two shales are thermally matured and located in the peak

347

gas window.

348 349

(2) The mean activation energy of shale 1 # and 2# are 235, 000 kJ/kmol and 205,600 kJ/kmol, respectively. It was easier for shale 2# to lose organic matters when it was heated up.

350

(3) The TC of shale 1# and shale 2# are 1.39 and 1.54 KJ/(m2·s·K) respectively and shale 1#

351

has higher porosity than that of shale 2#. Moreover, shale 1# (with higher Ro) has larger

352

specific surface area and greater adsorption potential in narrow pores, and higher methanol

353

adsorption capacity.

354

(4) When air, CO2 and helium were employed as purging gas to remove the organic matter


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355

and to increase their permeability of the two samples, both samples had the same weight

356

loss behaviors. It was found the weight loss rate peaks occurred at temperature between

357

330 oC and 500 oC due to the organic matter decomposition and cracking. It was easier for

358

the samples to lose weight when helium was used as purge gas since helium has higher

359

thermal conductivity, followed by air and then CO2.

360

(5) There is no significant departure between the weight loss rate curves of CO2 and air.

361

Simply using air as oxidizer to burn the organic matters of shale rocks may not be an

362

effective method to improve the permeability. Higher heating rate could promote the

363

weight loss rate.

364

Acknowledgements

365

The authors wish to acknowledge the financial support by “National Natural Science

366

Foundation of China-41602169”

367 368

References

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[1]

S. Gai, H. Liu, S. He, S. Mo, S. Chen, R. Liu, et al., "Shale reservoir characteristics

370

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Pyrolysis and Combustion Enhance Recovery of Gas for Two China

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