Impact of Water Pressure on the Organic Matter Evolution from

Article Cite This: Energy Fuels 2019, 33, 6283−6293

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Impact of Water Pressure on the Organic Matter Evolution from Hydrous Pyrolysis Lina Sun,† Jincai Tuo,*,‡ Mingfeng Zhang,‡ Chenjun Wu,‡ and Shunqi Chai§ †

Hubei Cooperative Innovation Center of Unconventional Oil and Gas, Yangtze University, Wuhan, Hubei 430100, China Key Laboratory of Petroleum Resources, Gansu Province, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, PR China § School of GeoSciences, Yangtze University, Wuhan 430100, China Downloaded via UNIV OF CAMBRIDGE on August 20, 2019 at 07:28:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Deep oil and gas, as an important part to make up the energy-deficient. A series of breakthroughs had been made in their exploration and development in the past few years. However, the effects of deep fluids during hydrocarbon formation and evolution are still an ambiguous problem. Therefore, to investigate the effect of water pressure (PW) on hydrocarbon generation and thermal evolution of type-I, type-II, and type-III kerogens in a deeper stratum, three series of pyrolysis experiments were conducted on three samples with different kerogen types (types II1, I, and III in TC, YMS, and XJ samples, respectively) in a high-temperature, high-pressure simulator. The type-I kerogen was pyrolyzed with 5, 20, 35, 50, 65, and 80 MPa water pressure at 375 °C for 48 h, whereas the type-II and type-III kerogens were pyrolyzed with 10, 20, 30, 40, 50, and 60 MPa water pressure at 350 °C for 48 h. The results showed that there was a threshold pressure PW affecting liquid hydrocarbons. In addition, before the threshold pressure, PW played a role in promoting, but then it was the inhibiting. For gaseous hydrocarbons, while the pressure effects and results were the same as with liquid hydrocarbons in TC samples under the near-critical or critical state, they had no obvious effects in YMS and XJ samples before the critical state, which proved that the organic matter (OM) evolution was associated with the state of fluid in the reaction system and the effects were stronger when the fluid was under the near-critical or critical state than when it is not under this state. The reasons could be concluded as follows: (1) the different properties of water, as the ionization product constant of water (K(w)) was higher in TC samples and may provide more H+ to participate in the OM reactions compared to YMS and XJ samples. Moreover, the particular characteristics of intersolubility with organic solvent occurred under near-critical or critical states, resulting in the more watersoluble hydrocarbons being expelled in TC samples. (2) Chemical mechanism: based on the first-order reaction equation for oil−gas generation and essential characters of samples and experiments, it can be concluded that the influence degree of PW on OM evolution was related to the type of OM, the thermal maturity as well as the nature of water. (3) Physical mechanism: the vapor in free space and generated hydrocarbons in pores were in a dynamic balance state, which also resulted in the existence of threshold pressure PW affecting OM evolution. Therefore, understanding the effects of PW on OM evolution would help us to study the oil−gas generation accurately in actual geology and help preferably in oil−gas exploration and exploitation.

1. INTRODUCTION Pyrolysis, which was based on the time−temperature equivalence principle demonstrated by Connan (1974),1 is an important technique to study the evolutional features of organic matter (OM). Experimental conditions, such as temperature, time, pressure, water, minerals, catalysts, and pore structure, were considered in pyrolysis.2−6 While considering the important factors of temperature and time, pressure should also not be ignored in the OM evolution.7−15 In addition, research studies about the effects of pressure on OM evolution by predecessors were controversial.16−20 For example, by carrying out hydrous pyrolysis in gold tubes, Monthioux et al. (1985)16 found that the yields of pyrolysis products were almost unchanged and hence pressure had no obvious effect on OM evolution. Xie et al. (1996)17 used a mudstone of type I to pyrolyze in a semiclosed simulation experimental system which resulted in higher oil yields by higher pressure. Therefore, it has been concluded that pressure promotes OM evolution. Jiang et al. (1998)18,19 conducted a series of simulation experiments on a mudstone in a closed system and found that pressure suppressed OM evolution. By comparing the differences among them, we © 2019 American Chemical Society

may consider that the contradictory conclusions were resulted from the different experimental methods, experimental conditions, pressure methods, and so on. At the same time, few have studied the effect of water pressure (PW) on OM evolution via pyrolysis experiments. Furthermore, the effects of PW under high temperature on OM evolution were still inconclusive. Therefore, the main focus of this article was on the effects of PW on pyrolysis products during high-temperature hydrous pyrolysis. Under different temperatures and pressures, the states of water are variable. The critical temperature (TC) and pressure (PC) of pure water were 374.5 °C and 22.1 MPa, respectively. When the temperature (T) < TC, the state of water could be called steam and liquid, and steam could be changed into liquid by compressing. When T > TC and P < PC, the state of water was defined as gas, which could not be changed into liquid only by compression. When T > TC and P > PC, it was difficult to distinguish the status as gas or liquid, and so, it was defined as the Received: April 13, 2019 Revised: May 29, 2019 Published: May 30, 2019 6283

DOI: 10.1021/acs.energyfuels.9b01125 Energy Fuels 2019, 33, 6283−6293

Article

Energy & Fuels Table 1. Geochemistry Information in Source Rock Samples before Pyrolysis sample number

lithology

S1 (mg/g)

S2 (mg/g)

S3 (mg/g)

TOC %

T-max °C

HI mg/g TOC

OI mg/g TOC

PI mg/g TOC

OM type

TC YMS XJ

shale shale carbon mudstone

2.78 1 0.25

66.48 138.58 5.5

0.13 2.29 4.3

13.75 19.96 23.44

451 443 419

483.49 694 23

0.95 11 18

1.00 0.98 0.57

I1 I III

Figure 1. Schematic diagram of the pyrolysis experimental procedure.

Figure 2. Schematic diagram of the pyrolysis experimental procedure.

supercritical fluid. While high-temperature water, were the generic terms of liquid water above 200 °C and supercritical water (SCW).21 Tao et al. (2010)12 conducted a series of anhydrous pyrolysis experiments on coal in confined gold tubes at 380 °C with pressures of 50−250 MPa. The results showed that pressure could promote the synthesis process between carbon dioxide and hydrogen as well as affect the fractionation of carbon and hydrogen isotopes in methane and ethane. Uguna et al. (2012)13 investigated the effect of PW (17.5, 50, and 90 MPa) on hydrocarbon generation of coal in a 25 mL Hastelloy pressure vessel at 350 °C. The relatively lower-pressure (17.5 MPa) hydrous conditions promoted the generation of hydrocarbons, but an opposite result occurred at higher pressures (50 and 90 MPa). Uguna et al. (2015)15 conducted another series of experiments at 380 and 420 °C with pressures of 23 (30), 50, 70, and 90 MPa, which indicated that gas yields were not affected by pressure before 70 MPa but reduced at 90 MPa. At 380 °C, the pressure retarded the generation of oil by delaying the conversion of bitumen to oil. At 420 °C, the process of oil cracking to gas was retarded at higher pressures (70 and 90 MPa). The confined gold tube pyrolysis system was used by Mi

et al. (2014)14 to investigate the effects of pressures (10, 25, 50, 75, and 100 MPa) on the generation of gas from coal. They believed that the pressures mainly affected the secondary gas generation. The highest retardation occurred at 50 MPa but diminished after a pressure of 75 MPa. In conclusion, there were different beliefs regarding the effect of pressure on pyrolysis products. However, this may be caused by the different experimental samples, methods, and conditions. To obtain an understanding closer to the geology of how PW affected OM evolution in different basins or deepness, the conditions of temperature, time, lithostatic pressure, and original source rock were considered in our pyrolysis experiments. Thus, we could find the possible influence and mechanisms of PW on the generation of oil and gas, which could help us to understand the theory of hydrocarbon generation in a deep source rock or overpressured formation. Therefore, this research will provide theoretical foundation and technology support for oil−gas exploration and exploitation. 6284


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the sample cell from the autoclave, which was prepared to the following analyzing on residues. And then we could prepare for a new experiment. In this article, these hydrous pyrolysis experiments were all carried out with the same time (48 h) and lithostatic pressure (100 MPa, to ensure the impermeability of the experimental system) (the experimental conditions are listed in Table 2). For TC samples, the

2. EXPERIMENTAL SECTION 2.1. Pyrolysis. Three samples (big block samples from outcrops) for this study were obtained from two different basins in western China. These samples were collected from Yc and Tri in Ordos Basin (sample number: TC) and Lcg, Per and Bdw, and Jur in Junggar Basin (sample number: YMS and XJ), respectively. In addition, their lithology was shale, shale, and lime mudstone, respectively. The minerals in TC samples mainly consisted of quartz (21.7%), feldspar (9.5%), clay mineral (57.6%), and pyrite (11.2%). The minerals in YMS samples mainly consisted of quartz (46.0%), feldspar (12.6%), clay mineral (27.4%), and dolomite (14.0%). In addition, the minerals in YMS samples mainly consisted of quartz (0.7%), calcite (54.5%), siderite (11.2%), and dolomite (33.6%). These samples were all in the stage of early maturation. The geochemistry information is presented in Table 1. These pyrolysis experiments were conducted on a WYMN-3 hightemperature, high-pressure (HTHP) simulation instrument (Figure 1), which belonged to a controllable and confined system of hydrocarbon generation and expulsion. The original sample pores were retained as far as possible, and similar geological conditions of hydrodynamic pressure and overlying lithostatic pressure were taken into consideration. In general, this equipment comprised a software control system (computer and control panel), reaction (autoclave), heating, hydraulic control (lithostatic pressure), fluid supplement (added deionized water and then controlled PW), and collection (stored pyrolysis products) systems, which was reported earlier.22 Its working principle is as follows: with the aid of two axial hydraulic control systems and a high pressure water supplement system, the instrument could be used to simulate the in situ lithostatic and hydrostatic pressure in the source rock. An associated episodic hydrocarbon expulsion occurred via the controlling of two-position three-way solenoid valves A and B (Figure 1). The main technical features of the instrument are as follows: (1) exerted lithostatic pressure, surrounding rock pressure, and higher water pressure, (2) the original core samples were maintained and the original pores were retained as much as possible, (3) simulation experiments were conducted in both closed and semiopen systems with a certain hydrodynamic pressure (PH), (4) it is a hydrous experiment system, and (5) conditions of different samples, temperatures, pressures, fluid media, inorganic minerals, and so on were considered for experiments to develop the study of hydrocarbon generation and kinetics. The experimental procedure is summed up as follows (Figure 2): (1) sample preparation is as follows: before pyrolysis, the three samples were drilled to many core columns with a diameter of 2.5 cm (centimeter) and a height of approximately 5−6 cm, which was suitable for the sample cell in the autoclave. (2) Experimental samples were installed into the autoclave: the drilled columned samples were placed in the middle of the sample cell (length: 13.92 cm), and other free spaces at both ends were filled by foraminiferos heel blocks. Then, the samples were placed together into the autoclave with the upper part of the sample cell being connected to the hydrocarbon expulsion pipeline and the bottom part being linked to the intake pipe. After that, force was exerted on the autoclave to fix the sealing of the reaction system. (3) The vacuumized and leakage test was conducted: before heating, deionized water was added to the sample cell to check whether the reaction system has a leak; if not, we continued to check the collection system for leaks by a vacuum pump. The valves among these connected systems (in order: autoclave, sample cell, hydrocarbon expulsion pipelines, gas and liquid collector, gas collecting pipe, and vacuum pump) were opened for vacuum. (4) Procedure settings on computer are as follows: the experimental procedure (such as variable temperature, pressure programming, pressure threshold, etc.) was carried out using the software control system. (5) After running both software and the hardware instrument simultaneously, the experiment was begun. (6) At the end of the experiment, the procedure was stopped and experimental products were collected. (7) The gas collecting pipe was washed first with saturated salt solution to collect generated gases. (8) The liquid products were collected in a gas and liquid collector and the pipe was washed. (9) Took out the residues: took the sample out of

Table 2. Experimental Conditions in This Pyrolysis Experiment hydrodynamic pressure (MPa) (floating range) sample number TC

YMS and XJ

temperature (°C)

simulated depth (m)

fiducial value

minimum value

maximum value

375 375 375 375 375 375 350

500 2000 3500 5000 6500 8000 1000

5 20 35 50 65 80 10

4.5 18 31.5 45 58.5 72 9

6 24 42 60 78 96 12

350 350 350 350 350

2000 3000 4000 5000 6000

20 30 40 50 60

18 27 36 45 54

24 36 48 60 72

target temperature was 375 °C, and its corresponding PW values were 5, 20, 35, 50, 65, and 80 MPa. However, for YMS and XJ samples, the target temperature was 350 °C, and the corresponding PW values were 10, 20, 30, 40, 50, and 60 MPa. However, the PW was in a dynamic balance state between decreasing (achieved by the expelling of oil, gas, and water vapor from the reaction system) and increasing (achieved by adding deionized water from the high-pressure pump to the sample cell). In addition, the opening degree of the pyrolysis system could be controlled by a two-position three-way solenoid valve and also achieved by the relationship between the hydrocarbon expulsion pressure limit and the pressure threshold. In these experiments, the hydrodynamic pressure, hydrocarbon expulsion pressure limit, and pressure threshold set on the computer were P MPa (P represents the target simulation pressure), (P + 5) MPa, and 5 MPa, respectively. When the hydrodynamic pressure in the sample cell reached the pressure limit (P + 5) MPa, the solenoid valves opened automatically to discharge hydrocarbons into the gas and liquid collector. When the degree of pressure exceeded the pressure threshold, that is, when the hydrodynamic pressure was reduced to (P − 5) MPa, the solenoid valves automatically closed. Thus, this pyrolysis system was in a dynamic evolution process when transformed between an open and closed system. However, there were three stages in one heating procedure in every experiment: a rapid heating from room temperature to 375 or 350 °C in 2 h, maintaining the temperature constant at 375 or 350 °C for 48 h, and cooling from 375 to 350 °C to room temperature (Table 2). Once the experiment was completed, the pyrolysis products of the gas were collected. Then, after unloading the autoclave, the hydrocarbon expulsion pipeline was carefully cleaned with dichloromethane, and the gas and liquid collector contained oil, dichloromethane, and water (the gas was already collected). This separated portion of the liquid oil is expelled oil. The separated part of the liquid oil from the sample cell washed with dichloromethane is the washout oil, while the portion extracted from the solid residues is the residual oil. The summation of expelled, washout, and residual oils is the total oil. 2.2. Gas Analysis. 2.2.1. Gas Components. The gas collected from pyrolysis was analyzed on a DANI GC 1000 gas chromatograph equipped with a hydrogen flame ionization detector and a GDX column (4 m × 3 mm i.d.). Hydrogen was used as a carrier gas at a flow rate of 30 mL/min. The detectors include a flame ionization detector, a thermal conductivity detector, a flame photometric detector, and an electron 6285


5 20 35 50 65 80 10 20 30 40 50 60 10 20 30 40 50 60

TC

XJ

YMS

pressure (MPa)

sample number

52.626 58.483 181.079 182.196 72.500 12.689 26.137 51.959 60.502 40.802 29.553 11.676 1.684 2.582 4.524 6.432 3.768 2.571

expelled oil (mg/g TOC) 10.260 0.739 1.492 3.480 0.073 0.538 5.508 9.436 4.033 11.335 3.260 3.156 0.005 0.005 0.005 0.004 0.004 0.002

washout oil (mg/g TOC) 178.021 35.744 9.073 25.339 3.600 18.340 299.952 417.167 536.778 430.500 367.021 252.109 0.217 0.251 0.197 0.214 0.161 0.328

residual oil (mg/g TOC) 240.907 94.965 191.643 211.015 76.172 31.567 331.597 478.562 601.312 482.636 399.834 266.941 1.907 2.838 4.726 6.650 3.933 2.900

total oil (mg/g TOC) 10.175 7.485 162.906 120.186 136.929 147.249 33.563 22.184 22.509 16.020 19.048 46.899 56.636 47.720 74.723 92.754 28.771 92.558

total gases

Table 3. Liquid and Gaseous Hydrocarbon Products from Pyrolysis Experiments

0.0014 0.060 32.983 38.472 5.934 12.573 3.729 3.346 3.625 3.682 2.208 4.845 3.299 1.732 6.450 5.723 1.673 4.341

C1 (mL/g TOC) 0.0003 0.016 12.664 16.263 2.135 2.309 1.021 0.979 1.061 0.990 0.890 1.288 0.920 0.346 1.137 0.841 0.418 0.724

C2 (mL/g TOC) 0.0001 0.004 6.274 7.849 1.082 0.886 0.533 0.506 0.548 0.504 0.554 0.748 0.427 0.136 0.527 0.362 0.201 0.359

C3 (mL/g TOC) 0.0001 0.001 1.818 2.281 0.347 0.229 0.265 0.246 0.266 0.233 0.309 0.457 0.149 0.044 0.173 0.129 0.066 0.129

C4 (mL/g TOC) 0.0003 0.000 0.229 0.352 0.087 0.064 0.136 0.098 0.106 0.088 0.147 0.242 0.067 0.020 0.070 0.058 0.024 0.070

C5 (mL/g TOC) 0.0026 0.001 0.032 0.051 1.559 0.200 0.045 0.019 0.021 0.018 0.045 0.085 0.029 0.014 0.036 0.035 0.012 0.042

C6+ (mL/g TOC)

0.005 0.082 54.000 65.268 11.144 16.261 5.728 5.193 5.627 5.514 4.153 7.665 4.890 2.291 8.393 7.148 2.394 5.664

total hydrocarbon gases

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Figure 3. Yields of expelled, washout, residual oil, and total oil with the increasing hydrodynamic pressures from hydrous pyrolysis. capture detector, which were maintained at 250 °C. The column program was as follows: an initial temperature of 29 °C which is maintained for 4 min, a first ramp to 90 °C at 15 °C/min which is maintained for 32 min, and a second ramp to 190 °C at 50 °C/min which is maintained for 15 min. 2.2.2. Stable Carbon Isotopes. Carbon isotope analysis was conducted on an Agilent 6890 gas chromatography-combustionthermo isotope ratio mass spectrometer (GC-C-IRMS) via a combustion interface (GC combustion III).23 For the chromatographic separation, a Varian Capillary Column was used (CP-Carbon BOND, 25 m × 0.53 mm i.d., 10 μm film thickness). Helium was used as a carrier gas at 15 mL/min. The applied temperature program was as follows: 50 °C for 3 min, followed by a ramp of 20 °C/min to 200 °C which is maintained for 10 min. Individual compounds were oxidized in a microvolume ceramic tube with NiO, CuO, and Pt wires at 950 °C. For the calculation of δ13C values, three pulses of standard pure CO2 gas, precalibrated against an interlaboratory-recognized reference CO2, were injected into the IRMS via the GC-C III interface. The analytical precision of the carbon isotopic composition was better than 0.3‰. The carbon isotope values were reported as δ-values, which was expressed as δsample = (Rsample/Rstandard − 1) × 1000 (‰) (where R denoted the ratio of 13C/12C). The δ13C values were normalized to Pee Dee Belemnite.

total oil at 35 MPa were essentially the same as those at 50 MPa, which also showed that the generated liquid hydrocarbons had no large change or had a maximum from 35 to 50 MPa. Moreover, as the trend line of residual oil showed an increase in PW (>65 MPa), that is, from 35 to 50 MPa, the pressure promoted the OM evolution, which resulted in the increase of expelled oil and the decrease of residual oil. The opposite conditions occurred after 65 MPa; the TOC of residual oil increased by 14.740−18.340 mg/g at 80 MPa. This provided evidence to the inference that PW was in favor of OM evolution before 50 MPa and then an opposite effect occurred after 50 MPa. In addition, as the evolutionary trend of washout oil with pressure was consistent with residual oil, this may also imply that PW went against the thermal cracking of kerogen or bitumen after a certain pressure range. 3.1.2. Generated Liquid Hydrocarbons from YMS Samples. In YMS samples, the tendency of total oils with increasing pressures was the same as that of TC samples (in the pressure range of 15−60 MPa), whose pressure also first increased and then decreased (Figure 3-b-YMS). However, the difference was that the main contributor to total oil was residual oil, though the tendency of residual oil was the same as that of expelled oil with increasing pressure. However, the residual oil yields were obviously higher than the expelled oils, and both of them were also higher than those in TC samples. The differences between these oils were the type of kerogen, mineral compositions, and the simulation temperature. The type-I kerogen in YMS should have a higher yield of expelled oil than type-II1 kerogen in the TC sample, which was opposite to the results in this study. Therefore, this phenomenon may be associated with the different OM degradation process and liquid properties at different simulated temperatures. On the one hand, in other temperature-controlled experiments on TC and YMS samples, the simulated temperature from 350 to 375 °C corresponded to the thermo-cracking oil−gas-generated stage.22,24 Moreover, the yields of YMS samples at 350 and 375 °C were all higher than those in TC samples under the same temperature-controlled experimental conditions. However, as the results of pressurecontrolled experiments in this article showed, the yields of TC samples at 375 °C were higher than those in YMS samples at 350 °C, which further showed that temperature was the main factor affecting OM evolution compared to the effect of pressure. In addition, previous research studies suggested that the inorganic minerals in source rocks had participated in and affected the isotopic compositions of hydrocarbon gases under the same simulated temperature.25 Therefore, the effects of minerals could be weak with increasing Pw, as shown in this article. On the other hand, the simulated temperature in YMS samples was 350 °C, in which situation, the water was not in the critical state. However, in TC samples, the simulated temperature was 375

3. RESULTS 3.1. Generation of Liquid Hydrocarbons. The liquid hydrocarbons generated in hydrous pyrolysis were composed of expelled, washout, and residual oils, which are presented in Table 3 and Figure 3. 3.1.1. Generated Liquid Hydrocarbons from TC Samples. In TC samples, the expelled oil yields increased at 5 MPa and reached the maximum at 50 MPa and then decreased to a minimum of 12.689 mg/g TOC at 80 MPa (Figure 3-a-TC). The residual oil decreased from 178.021 mg/g TOC at 5 MPa to 18.340 mg/g TOC at 80 MPa. Washout oil is a part of the liquid hydrocarbon generated and migrated from the source rock, but it did not leave the reaction system to the collecting system, which could be seen as a mixture of expelled oil and residual oil. Although its yield was lower compared to the expelled and residual ones, it was helpful for us to have a more accurate mass balance of the generated hydrocarbons. Therefore, we would not make a detailed analysis on washout oil with increasing pressures because of its lower yield. We would only discuss the relationship between expelled, residual, and total oil and pressure. In general, the pressure of total oil increased first and then decreased; this tendency was almost the same as expelled oil. Therefore, there were two conclusions that could be made. The first was that PW promoted the generation of liquid hydrocarbons in a certain pressure range (50 MPa in this sample), a suppression or retardation effect occurred on the generation of liquid hydrocarbons. The yields of 6287


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Figure 4. Yields of generated gases with the increasing hydrodynamic pressures from hydrous pyrolysis.

°C, and the water was already in the critical state. Then, at the critical state, the solubility of the liquid to organic solvent (pyrolysis products) would result in the higher liquid hydrocarbons being expelled from the hydrocarbon generation system. Thus, the expelled oil yields of TC samples were obviously higher than those of YMS samples at 350 °C, which was also consistent with previous research results of the intersolubility between critical liquid and organic solvents.26,27 Combining the yields of liquid hydrocarbons in different states with the increasing PW indicated that PW was in favor of liquid hydrocarbon generation in a certain range (30 MPa). 3.1.3. Generated Liquid Hydrocarbons from XJ Samples. In XJ samples, the tendency of total oil yields was also the same as those of TC and YMS samples, which also increased first and then decreased with pressures (Figure 3-c-XJ). Compared to TC and YMS samples, the contributor to total oil was expelled oil and the expelled oil yields were obviously lower in XJ samples, which may be related to the properties of themselves. The types of kerogen in XJ, TC, and YMS samples were type III, II1, and I, and the corresponding generating potentiality is preferred to gas, oil−gas, and oil generation, respectively.28 In addition, we could also find that the effect of PW on XJ samples was the same as those of TC and YMS samples, which was in a certain range (40 MPa in this samples). 3.2. Generation of Gaseous Products. The yields of gaseous products generated in hydrous pyrolysis are presented in Table 3 and Figure 4. They were composed of hydrocarbon (C1, C2, C3, C4, C5, and C6+) and nonhydrocarbon gases. 3.2.1. Generated Gaseous Products from TC Samples. The total gas yields (mL/g of initial rock TOC) increased to the peak value (162.906 mL/g TOC at 35 MPa) and then increased by small-scale fluctuations (Table 3 and Figure 4-a-TC). In general, the yields of nonhydrocarbon gases presented an increasing tendency, and the peak value was 130.811 mL/g TOC at 80 MPa. Therefore, we may infer that PW generally improved the generation of nonhydrocarbon gases under the temperature of 375 °C in a II1-type sample. In gaseous hydrocarbons, the tendency was increased first and then decreased with increasing pressures. The yields of total hydrocarbon gases (C1−C6+) increased slightly from 5 to 20 MPa but then increased dramatically to a maximum at 35 MPa (54.000 mL/g TOC) and 50 MPa (65.268 mL/g TOC) before decreasing to 65 MPa (11.144 mL/g TOC) and 80 MPa (16.261 mL/g TOC). In detail, the gas yields of C1 (CH4), C2 (C2H4 and C2H6), C3 (C3H8), C4 (n-C4 and i-C4), and C5 (n-C5 and i-C5) with PW were similar to those of the total hydrocarbon gases. Therefore,

the gases were mainly original gases that were associated with the generation of oil from the primary cracking of kerogen or bitumen.29 However, their differences mainly lay in the point of 65−80 MPa, which represented that the yield of C1 had the same increasing tendency as total hydrocarbon gases, but C2 had almost no change and C3, C4, and C5 all presented a slightly decreasing trend. Furthermore, the C6+ (heavy hydrocarbon) yield increased from 5 MPa (0.003 mL/g TOC) to 65 MPa (1.560 mL/g TOC) and then decreased by 87% to 0.200 mL/g TOC at 80 MPa. Thus, by the tendency of gaseous hydrocarbons, inhibition occurred on the thermal cracking of kerogen and bitumen from 65 to 80 MPa. Furthermore, as the tendency of total gaseous hydrocarbons with increasing PW was almost the same as that of liquid hydrocarbons, the pressures promoted the generation of hydrocarbon gases before 50 MPa but retarded after this pressure point. 3.2.2. Generated Gaseous Products from YMS Samples. The total gas yields decreased first and then increased to the peak value (46.899 mL/g TOC at 60 MPa) with the increasing pressures (Table 3 and Figure 4-b-YMS), and the same situation occurred on the nonhydrocarbon gases. Thus, we may find that PW suppressed the generation of nonhydrocarbon gases. However, compared to nonhydrocarbon gases, the hydrocarbon gases generated in YMS samples almost remained and their TOC ranged from 4.153 to 7.665 mL/g with PW. Therefore, no obvious effects of PW occurred on the generation of hydrocarbon gases before the critical state. The same situation also occurred on the yield of C1 to C2+, which also had little effects on PW. Comparing the results of YMS samples to those of TC samples, we could find that the difference between them was the different simulation temperatures and kerogen types, which was 350 °C (before critical state), type I, and 375 °C (critical state), type II1. The type I kerogen in the YMS sample had the tendency to produce oil and type II1 kerogen in the TC sample had the tendency to produce oil and gas. Therefore, in contrast to the case of liquid hydrocarbons, the gaseous hydrocarbon yield in the TC sample was significantly higher than that of the YMS sample. However, for the different trends of gaseous hydrocarbons with increasing PW, the different OM evolution processes should be related to the different simulated temperatures and fluid states. 3.2.3. Generated Gaseous Products from XJ Samples. In this sample, the total gases were changing in a wave mode with the increasing pressures, and the peak values were 92.754 mL/g TOC at 40 MPa and 92.558 mL/g TOC at 60 MPa (Table 3 and Figure 4-c-XJ). Nonhydrocarbon gases were the main contributors to the total gases. As a sample of type III kerogen, its gas potential was significantly higher than that of oil. The hydrocarbon gases were changing in smaller fluctuations with 6288


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by a reversed trend toward less negative δ13C values beyond the pressure point of 65 MPa. This suggested that there may be two distinct generation mechanisms of CH4 and C2H6 before and after 65 MPa.33,34 Therefore, before 65 MPa, the PW promoted the generation of hydrocarbon gases, followed by the increasing breaking of 12C−12C or 12C−13C. However, after 65 MPa, the heavier carbon isotope indicated that the PW retarded the gaseous hydrocarbons generated as well as the further breaking of 12C−12C or 12C−13C. However, compared to TC samples, the δ13C values of CH4, C2H6, and C3H8 almost had no change with increasing pressures in YMS and XJ samples, which indicated that PW almost had no significant effects on the generation of gaseous hydrocarbons in YMS and XJ samples. However, in three samples, all the values of δ13C1 < δ13C2 < δ13C3 occurred, which present the organic origin of natural gas.35,36 3.3. Analysis of Solid Residues. The TOC of solid residues is presented in Figure 6.

the increasing pressures, which ranged from 2.291 to 8.393 mL/ g TOC. They had no obvious change and further indicated the smaller effect of PW on generated hydrocarbon gases. Comparing the results of TC and YMS samples, we also concluded that PW had little effect on the generation of hydrocarbon gases. 3.2.4. Carbon Isotopes of C1 to C3 in Three Samples. In Figure 5, the δ13C values of CH4, C2H6, and C3H8 generated

Figure 6. Change of TOC (solid residues) with increasing hydrodynamic pressure.

In TC samples, from 5 to 80 MPa, the TOC of residues were 11.50, 7.08, 6.71, 4.72, 7.51, and 6.95%, which were in sequence decreased by 16.36, 48.51, 51.20, 65.67, 45.38, and 49.45%, respectively, relative to the original TOC. Obviously, with an increase in PW, TOC presented a trend of decreasing first and then increasing. In addition, the main turning point was at about 50 MPa, which could be observed as the demarcation of the PW point between the effect of suppression and promotion of OM evolution. In YMS samples, the original TOC was 19.96%, and from 10 to 60 MPa, the residual TOC almost had not changed. In XJ samples, the situation was the same as that of the YMS sample. Therefore, based on the results from the three samples, we may also infer that the OM evolution was mainly affected by temperature compared to pressure. In addition, under the same temperature, OM evolution was also associated with the state of fluid in the reaction system. The effects were stronger when the fluid was under the critical state than when it was not under this state.

Figure 5. Carbon isotopes of C1 to C3 with the increasing hydrodynamic pressures.

from pyrolysis at different PW values were presented. According to previous research studies, in the whole stage of thermal evolution, the carbon isotope of hydrocarbon gases had a tendency to become lighter first and then heavier. This was because the bond energy was weak to strong in the order of 12 C−12C < 12C−13C < 13C−13C, and the order of carbon− carbon bond cleavage was 12C−12C < 12C−13C < 13C−13C with increasing thermal interaction.30 Therefore, the lighter carbon− carbon bond preferentially cleaved and then resulted in the preferential enrichment of 12C and then heavier 13C in the product system, that is, the tendency of carbon isotopes is to become lighter and then heavier. In addition, because the simulation temperature was the same in one series of samples and the temperature was the main influencing factor on thermal maturity,13,31,32 the carbon isotope of hydrocarbon gases should be almost the same in one sample. However, as there was a difference in PW in each series of experiments, the carbon isotope of hydrocarbon gases was affected by PW in this study. In TC samples, as the pressure increased, a similar trend occurred among the δ13C values of CH4, C2H6, and C3H8, which was an initial trend toward more negative δ13C values that was followed

4. DISCUSSIONS 4.1. Effect of Water Pressure on Hydrocarbon Generation. In these three samples, the variation tendency of total oils with increasing PW all presented an increasing trend and then a decreasing trend. To their gaseous hydrocarbons, they increased first and then decreased with increasing pressures in TC samples but were almost invariant in the YMS and XJ samples. The same tendency also occurred on their δ13C values of CH4, C2H6, and C3H8 in the three samples. In addition, the 6289


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Energy & Fuels corresponding TOC of residual solids was decreased first and then increased in TC samples but almost had no change in YMS and XJ samples. Thus, before the pressure limit point, the degree of OM evolution increased continuously, and the consumed OM increased. However, after that point, the suppression effects occurred, and the consumed OM decreased. Therefore, combing the liquid, gaseous hydrocarbons, and the TOC in residual solids together, we could infer that the effects of PW were associated with the different simulation temperatures, types of kerogen, as well as the nature of water. Compared to PW, the temperature remained as the major influential factor to OM evolution.13 In addition, at different temperatures, the effect of PW on liquid hydrocarbons was the same, which played a promoting role in a certain pressure range, but suppression effects occurred after that pressure range. Price and Wenger (1992)37 proved by experiments that the pressure effect was not a developmental process but had a threshold pressure. Only when the pressure reached the threshold value could an obvious effect be produced. Hao et al. (1995)38 noticed that not every overpressured system suppressed the OM evolution but put forward the question of threshold pressure. In addition, they also speculated that the higher the maturity, the greater was the threshold pressure. Therefore, combining previous research studies with our study, we also could consider “a certain pressure range” for the threshold pressure, which may be associated with the types of kerogen and the maturity in samples. In Figure 3, the threshold pressure that promoted the generation of liquid hydrocarbons in three samples was about 30 MPa (type I kerogen in the YMS sample and simulated at 350 °C), 43 MPa (type II1 kerogen in the TC sample and simulated at 375 °C), and 40 MPa (type III kerogen in the XJ sample and simulated at 350 °C). The hydrodynamic pressure (PH, had a limit value) in the reaction system was the sum of PW and PP (pressure produced by pyrolysis products), and PW and PP were in a dynamic balance after PH reached the limit value. Therefore, in the same reaction system, the pyrolysis products gradually increased with PW and further resulted in the increase of PP. When PW + PP = PH (max), as PW continued to increase, the PP was gradually reduced, which would result in the reduction of corresponding pyrolysis products. Therefore, threshold pressure PW affected the OM evolution. To gaseous hydrocarbons, the role of PW was different at different temperatures. At 375 °C, the effect of pressure was the same as that of total oil. However, at 350 °C, the pressure almost had no effect on gaseous hydrocarbons in YMS and XJ samples. This may be associated with different temperatures, kerogen types, and the corresponding state of liquid in the reaction system. Also, the existence of threshold pressure to generated gaseous hydrocarbons occurred at 375 °C, but it was not responding to the origin of hydrocarbon gases from bitumen. In addition, the changes in liquid and gaseous hydrocarbons in TC samples and changes only in liquid but no obvious changes in gaseous hydrocarbons in YMS and XJ samples could also prove that PW was more condutive to the first migration of hydrocarbons in the form of the water-soluble homogeneous phase at 375 °C than that at 350 °C. 4.2. Influencing Mechanisms of Water Pressure on Pyrolysis Products. 4.2.1. Chemical Mechanism. As PW was achieved by the process of adding deionized water to the sample cell in the autoclave, water was always present in the reaction system. Michels (1995)9 pointed that the extent of the pressure effects depended on the nature of the pressurizing medium, and PW (hydrous pyrolysis) induced a strong suppression effect on

kerogen thermal breakdown while effluent pressure (confined pyrolysis) had only a limited influence. However, as the results investigated by previous investigators on the role of water in pyrolysis showed, the products generated during hydrous pyrolysis were higher than those generated in anhydrous pyrolysis.5,6,39−41 Therefore, these studies proved the hypothesis that water participated in the pyrolysis reaction of OM. Thus, we should also consider the role of water itself on pyrolysis products with different states. There were different states of water under different pressures and temperatures. The properties of water varied from ambient conditions to critical conditions and above critical conditions over a remarkable wide range (Figure 7). The critical

Figure 7. State diagram of liquid.27

temperature (Tc) and pressure (Pc) points of pure water were 374.2 °C and 22.1 MPa, respectively. The SCW occurred when the temperature and pressure in the reaction system exceeded the critical points. Liquid and gas occurred under the critical state.27 From ambient to supercritical temperature, water changed its character from a solvent for ionic species to a solvent for nonionic species, and electrochemical properties varied substantially. For example, the dipole moment decreased from the high value at ambient conditions to a value common for normal solvents at supercritical conditions. However, even in the critical region, water was still as polar as acetone. The pH value of liquid water decreased by three units with temperature increasing to about T = 250 °C, thus providing more H+-ions for acid-catalyzed reactions. Just below the critical temperature, the ionization product constant (K(w) = C(H+)·C(OH−)) of water changed tremendously and further rendered near-critical state water and SCW a much less ionized compound than that under ambient conditions. Therefore, the reactivity of water increased in the neighborhood of the critical point with or without a catalyst. On the basis of the water properties, we could divide the experimental conditions into two sections. TC samples, which were simulated under 375 °C and 5−80 MPa, almost existed in the near-critical state and critical state. YMS and XJ samples, which were simulated under 350 °C and 10−60 MPa, were almost listed in the before-critical state. In addition, according to the research about another experimental series by the same PW and different temperatures, they considered that the corresponding stage was thermal cracking of kerogen to oil and gas at 350 and 375 °C.22,24 Besides, pyrolysis could be taken as a threestage process: (1) the primary degradation of the kerogen: some bitumen and little oil−gas were generated, (2) the further degradation of bitumen: a mass of oil−gas was obtained, and (3) the secondary cracking of oil to gas, which resulted in the large 6290


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Figure 8. Schematic diagram of the physical mechanism of water pressure on pyrolysis products.

amount of gases generated.42,43 According to the classification of chemical reaction, pyrolysis could be broadly separated into two categories: decomposition and condensation reaction.43−46 The process of decomposition mainly participated in the rupture of the covalent bond and belonged to increased entropy and adsorption reactions, which actually obtained the depolymerization of kerogen, the removal of oxygenated functional group, dehydrogenation, and so on. The process of condensation reaction was almost exothermic reactions with reduced entropy, which mainly included coincidence, addition, cyclization, aromatization, condensation, and so on. As in the early OM evolution stage at 350 and 375 °C, the liquid and gaseous hydrocarbons were almost derived from the primary cracking of kerogen and bitumen, which could be supposed as the following two reactions in a series:

the solubility property of near-critical water could be miscibility with generated hydrocarbons. Therefore, when the valve (connect the reaction and collection system) was open for expelling pyrolysis products, some of the hydrocarbons would be expelled to the collection system in a form of miscibility with water in TC samples but did not obviously occur in YMS and XJ samples. Therefore, the less generated gaseous hydrocarbons were expelled more in TC samples than in the other two samples, which also proved that PW had not affected gaseous hydrocarbons obviously when water is listed before the critical state. Then, as the limited reaction space, the number of “functional” H+ may also have a certain limitation, which may promote the primary cracking of kerogen or bitumen and provide source H+ to liquid hydrocarbons generation first before its limitation, but opposite effects occurred after its limitation. Therefore, to liquid hydrocarbons, they had the same tendency with PW in the three samples. Also, Uguna et al. (2015)15 reported that the higher PW slowed the conversion rate of bitumen to oil and resulted in the higher residual oils after a certain pressure range. However, to the gaseous hydrocarbons, they decreased after the threshold pressure in TC samples and did not change obviously in the YMS and XJ samples. Therefore, there was a threshold pressure during the process of PW affecting the OM evolution, and the influence degree was related to the type of OM, the thermal maturity, as well as the nature of water. 4.2.2. Physical Mechanism. In a confined space, we could use the equation of state of a hypothetical ideal gas to analyze the changes of reactants and products. It was a good approximation of the behavior of many gases under many conditions, although it had several limitations. It was first stated by Clapeyron (1834)48 as a combination of the empirical Boyle’s law, Charles’s law, and Avogadro’s law. The ideal gas law was often written as PV = nRT, where P is pressure, V is volume, T is temperature, n is the number of moles of gas, and R is the ideal gas constant. As the reaction space (including the space occupied by the sample and the residual free space), the samples and the simulation temperature were the same, so the number of vapor was higher at higher PW. Moreover, temperature was the main influence factor to OM evolution, so the corresponding OM evolution stage was almost the same in three samples. In each experimental series, we supposed that the generated hydrocarbons existed in pores of samples and the vapors were in free space for the original reaction state (Figure 8). As T, n, R, and V were constant in the reaction system at different pressure points, when PW was lower, the concentration of generated products was relatively higher which would then be expelled from pores to free space for further expelling (Figure 8lower PW). However, the concentration of generated products in pores and vapors in free space reached the maximum dynamic balance states, it

Kerogen (K) → bitumen (B) + oil (O1) + gas (G1) + pyrobitumen (P1)

(1)

Bitumen (B) → oil (O2 ) + gas (G2) + pyrobitumen (P2) (2)

Considering the above two reactions as first order, the kinetic equations could be written as dK/dt = −k1 × K and dB/dt = −k2 × B (k was the reaction rate). In addition, based on the Arrhenius equation, there was the following relationship between the activation energy (Ea) and temperature (T) for any chemical reactions k = A exp(Ea /RT )

(3)

where k, A, Ea, R, and T were a reaction rate, pre-exponential frequency factor, activation energy, the universal gas constant (J/mol/K), and absolute temperature (K), respectively. In addition, as previous studies found that Ea under nearcritical water was lower than that under before critical water, Ea TC (Ea in TC samples) < Ea YMS and XJ (Ea in YMS and XJ samples). Therefore, comparatively speaking, the yields of liquid and gaseous hydrocarbons in the type-II1 kerogen of TC samples were higher than those in the type-I kerogen of YMS samples and type-III kerogen of XJ samples. However, in this pyrolysis stage, the chemical bonds in kerogen or bitumen were broken and mainly resulted in the generation of C15+, C6−C15, and C2− C5, C1. As Espitalié et al. (1988)47 considered when Ea was the same, the initial potentials were listed in the order C15+, C6−C15, C2−C5, and C1, which proved that the generation of oil was more than gas in this OM evolution stage. On the one hand, K(w) was higher at 375 °C than 350 °C, and the hydrogen ion (H+) was involved in the OM evolution and provided part of hydrogen sources to hydrocarbons.5,6,30,40 On the other hand, 6291


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would result in maximum expelling of products (Figure 8 dynamic balance states). In addition, PW increased sequentially, which would lead to the diffusion of vapors from free space to pores and result in the decrease of expelling products (Figure 8higher PW). Therefore, the explanation given above could also prove that PW promoted the OM evolution in a certain pressure and then the opposite effects occurred after that threshold pressure.

Lina Sun: 0000-0003-0103-4002 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was conducted during the period of doctoral study, and the scientific achievement belonged to the Key Laboratory of Petroleum Resources Research, Chinese Academy of Sciences. In addition, this research was supported by the National Natural Science Foundation of China (grant nos.: 41672127 and 41602151), the Key Laboratory Project of Gansu Province (grant no. SZDKFJJ20160504), Support Program Fund of Yangtze University for the Youth (no. 2016cqn18), Training Program of Innovation and Entrepreneurship for Undergraduates (no. 2017112), and Natural Science Foundation Project of Hubei province (no. 2018CFB389).

5. CONCLUSIONS The results obtained from the hydrous pyrolysis on three samples were respectively pyrolyzed at 375 °C and different PW from 5 to 80 MPa in TC samples with type II1 kerogen, at 350 °C and different PW from 10 to 60 MPa in YMS with type I kerogen, and XJ samples with type III kerogen. The two simulation temperatures occurred in the primary cracking stage of kerogen or bitumen to oil and gas, so the liquid hydrocarbons were the main products and gaseous hydrocarbon yields were lower. Thus, we compared and discussed the effects of PW on pyrolysis products among the three samples, which were listed as follows: (1) With the increase of PW, as the main pyrolysis products, liquid hydrocarbons generated in three samples all presented an increasing tendency and then a decreasing tendency; therefore, in a certain PW range (before threshold pressure), there was a promotion effect on the primary cracking of kerogen and bitumen, but an inhibiting effect occurred after that pressure. However, as the accompanying products of liquid hydrocarbons, gaseous hydrocarbons had the same tendency with liquid hydrocarbons in the TC sample, but there were no obvious changes in the YMS and XJ samples. Comparing their respective characteristics together, it can be concluded that PW was more conductive to the first migration of hydrocarbons in the form of water-soluble homogeneous phase under the near-critical or critical state than that before the critical state. (2) The influence degree of PW on OM evolution was associated with the type of OM, the thermal maturity, as well as the nature of water. Among them, the type of OM determined the main compositions of pyrolysis products, the thermal maturity stage determined the corresponding chemical reactions, while the nature of water at different temperatures affected K(w) of water as well as supplied quantities of H+ to the reaction system, intermiscibility with organic solvent, and so on. (3) On a physical level, the dynamic equilibrium between vapor in free space and generated hydrocarbons in pores also explained the existence of water threshold pressure in the process of OM evolution. (4) In geology, the appropriate PW values were conducive to the generation of pyrolyzate in the maturity stage, but the opposite occurred after the threshold pressure on a macro level. Thus, for further precisely understanding the hydrocarbon generation and preferably serving the oil− gas exploration, we should do more work on the role of PW during OM evolution.

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