Qiang Ma†
ab,
Zhengda Yang†a,
Liqiang Zhanga,
Riyi Lin*a and
Xinwei Wanga
aCollege of New Energy, China University of Petroleum (East China), Qingdao 266580, P. R. China. E-mail: linry@upc.edu.cn; Tel: +86-0532-8698-1767
bChina Petroleum Pipeline Engineering Co., Ltd, Langfang, Hebei 100044, China
First published on 23rd October 2019
During the thermal enhanced oil recovery (EOR) process, the hazardous hydrogen sulfide (H2S) gas among the produced gases causes significant difficulty in the exploration and development of petroleum. In this study, the effects of superheat degree on the H2S generation by heavy oil aquathermolysis were explored through simulated experiments. The crude and residual oils before and after the reaction were separated into saturate, aromatic, resin and asphaltene fractions (SARA). The oil samples were analyzed from various perspectives by various characterization methods including Fourier transform infrared (FTIR) spectroscopy, elemental analysis, and X-ray photoelectron spectroscopy (XPS). The results showed that H2S generation was favored by larger superheat degree at the same temperature, and it increased from 0.178 to 0.345 mL g−1 oil with an increase in the superheat degree from 62.19 to 89.42 °C. The contents of the sulfur-containing substances, which were supposed to be the main sources of H2S generation, in the saturate and aromatic fractions decreased significantly with an increase in the superheat degree; the increase in the superheat degree led to a slight reduction in the contents of the methylene, methyl and carboxyl/carbonyl groups. Moreover, the analysis of the main sulfur existing forms before and after the reaction suggests that sulfur in the forms of sulfides, sulfones and sulfates is more likely to generate H2S under superheated steam conditions. This study provides an understanding of the mechanism of H2S generation during the process of injecting superheated steam for heavy oil recovery.
Currently, it has been pointed out that H2S is generated through aquathermolysis reactions.9 However, aquathermolysis is a very complex reaction, and the production mechanism of H2S is not fully understood.7 Hyne et al. first carried out the aquathermolysis test using two heavy oil samples at 200–300 °C.10 During the steam injection process, the condensation, ring-opening, hydrodesulphurization, and water gas shift reactions occurred between steam and heavy oil; the composition of the gas obtained from the heavy oil reaction was the same as that of the gas obtained from sulfur-containing model compounds such as thiophene and thiolane. The aquathermolysis reaction is very complicated such that a number of aquathermolysis experiments have been conducted to investigate the influencing factors for H2S generation via the aquathermolysis of heavy oil. Na et al. carried out aquathermolysis tests with bitumen samples in an H2S corrosion-resistant autoclave, and the gas composition after each reaction was analyzed, the results of which showed that the amounts of H2S and CO2 increased with an increase in the reaction time and temperature.11 Montgomery et al. separated the heavy oil into saturate, aromatic, resin and asphaltene fractions (SARA) and found that the H2S generation could be minimized using specific temperature and pressure.12,13 Moreover, some researchers have revealed that H2S generation can be promoted by the catalytic effect of minerals. The H2S concentration was higher when minerals were present in the aquathermolysis reaction mixture,14,15 and the reaction temperature of aquathermolysis could be reduced due to this catalytic effect.
The reaction temperature can influence the water status, and some researchers have explored the role of water status,16–18 water dielectric constant19 and solubility20,21 in H2S generation. The results showed that water participated in the aquathermolysis reaction simultaneously as a catalyst, reactant and solvent. The dissociation constant of water increased by three orders of magnitude at 200 °C, and consequently, water showed the catalytic effect of acid, alkali or acid–alkali double catalysts in the reaction process. The generation of H2S increased with a decrease in the pH value when the heavy oil reacted with water.22
Recently, the utilization of superheated steam for heavy oil recovery has gained extensive attention because superheated steam carries more energy as compared to saturated steam. However, the heat transfer characteristics of the two-phase flow in micro-channels can be significantly different.23 Sun et al. investigated the flow and heat transfer characteristics of superheated steam in steam injection wells.24,25 The distributions of pressure and temperature of superheated steam in multi-point injection horizontal wells could be predicted using a numerical model, and it was found that the wellbore heat efficiency increased with the increasing superheat degree.26 Compared with heavy oil recovery by conventional steam, the oil recovery was proven to be significantly improved by superheated steam.23 However, the mechanism of the aquathermolysis reaction and corresponding H2S generation under superheated steam conditions has not been well understood to date.
This study focused on the effects of superheat degree on the H2S generation during a simulated thermal enhanced oil recovery process. The superheat degree of steam was well-controlled by changing the reaction temperature and pressure. The mechanism of H2S generation was revealed by analyzing the gas composition, elemental contents, structure and functional groups of the heavy oil before and after the reactions. This study provides an insight into the mechanism of H2S generation and may help to analyze the possible H2S generation using superheated steam during the thermal enhanced oil recovery process in the future.
Contents (%) | Element contents (%) | |||||
---|---|---|---|---|---|---|
H | C | N | S | Others | ||
Crude oil | 100 | 11.04 | 85.18 | 0.97 | 0.45 | 2.36 |
Saturate | 25.43 | 12.01 | 85.08 | 0.00 | 0.12 | 2.79 |
Aromatic | 21.08 | 10.68 | 84.11 | 0.51 | 0.78 | 3.92 |
Resin | 44.28 | 11.15 | 86.26 | 1.62 | 0.48 | 0.49 |
Asphaltene | 9.21 | 8.68 | 82.74 | 1.60 | 0.43 | 6.55 |
The gas products were dried by a drying cylinder prior to detection. The H2S concentration was measured by an on-line H2S detector (AKRT-H2S-W, AKRT, USA). The other gases were obtained by a gas sampling bag at ambient temperature and then analyzed by gas chromatography (7890A, Agilent, USA). The residual oil and the separated SARA were analyzed by an elemental analysis, Fourier transform infrared spectroscopy (FTIR 870, NEXUS, USA) and X-ray photoelectron spectroscopy (XPS, ESCALab-250Xi, ThermoFisher, USA). The FTIR spectroscopy was utilized to characterize the functional groups containing heteroatoms in heavy oil. The oil samples were prepared for the FTIR tests by homogenizing 1 mg of oil with 100 mg of potassium bromide. XPS was utilized to characterize the existing form of heteroatoms in heavy oil. The oil samples were spread on a slide of 5 mm × 5 mm and dried in a drying oven for 2 hours for the XPS tests. The elemental peaks were fitted using the software XPSPEAK41.
No. | t (h) | T (°C) | P (MPa) | Ts (°C) | ΔT (°C) |
---|---|---|---|---|---|
Case 1 | 24 | 250 | 2.62 | 226.46 | 23.54 |
Case 2 | 24 | 250 | 2.07 | 214.12 | 35.88 |
Case 3 | 24 | 250 | 1.71 | 204.59 | 45.41 |
Case 4 | 24 | 250 | 1.57 | 200.46 | 49.54 |
Products | Superheat degrees (°C) | |||
---|---|---|---|---|
23.45 | 35.88 | 45.41 | 49.54 | |
CH4 | 2.250 | 4.017 | 9.055 | 14.784 |
C2H6 | 2.133 | 4.510 | 8.176 | 15.916 |
C2H4 | 11.309 | 14.399 | 11.357 | 18.043 |
C3H8 | 0.000 | 0.000 | 0.000 | 0.000 |
C3H6 | 0.000 | 0.000 | 0.000 | 0.000 |
i-C4H10 | 24.044 | 14.417 | 12.480 | 22.433 |
n-C4H10 | 3.076 | 19.557 | 2.662 | 0.660 |
n-C4H8 | 10.837 | 7.630 | 9.455 | 1.879 |
i-C5H12 | 0.000 | 0.000 | 0.000 | 0.202 |
n-C5H12 | 0.000 | 3.004 | 15.595 | 1.277 |
C6+ | 2.161 | 2.981 | 7.598 | 13.924 |
H2 | 0.236 | 0.185 | 0.185 | 0.147 |
CO | 0.000 | 0.000 | 0.000 | 0.000 |
CO2 | 0.392 | 0.438 | 0.368 | 0.362 |
H2S | 0.0016 | 0.031 | 0.053 | 0.078 |
Olefins | 22.146 | 22.030 | 20.812 | 19.922 |
Alkane | 31.503 | 45.505 | 47.969 | 55.273 |
The generation of H2 during the aquathermolysis reaction might be due to the water gas reforming reaction.17,27,28 Hydrogen has the ability to rapidly capture hydrocarbon radicals, which can effectively inhibit the polycondensation reaction during aquathermolysis and prevent heavy oil from charring.29 The viscosity of heavy oil can be greatly reduced even by an order of magnitude or more when acid gases, such as CO2 and H2S, are dissolved in the heavy oil,30–32 and the role of CO2 in the aquathermolysis reaction has been further discussed by combined characterization methods. Particularly, the C–S bond energy is lowest in the heavy oil, and it is believed that the C–S bond cleavage of the sulfur-containing organic compounds in heavy oil is the main cause of H2S generation.33 On this basis, H2S generation was used to represent the extent of the aquathermolysis reaction.
To clarify the influence of superheated steam on aquathermolysis, H2S generation at different superheat degrees was explored, as shown in Fig. 2. The results agreed well with the previous conclusion that the H2S generation increased with the increasing reaction temperature.22 This indicates that H2S generation increased with an increase in the superheat degree even at the same reaction temperature. For example, when the reaction temperature was 300 °C, the H2S generation increased from 0.178 to 0.345 mL g−1 oil with an increase in the superheat degree from 62.19 to 89.42 °C. Furthermore, the change in H2S generation was more significant under higher temperature conditions even with a similar magnitude of change in the superheat degree. Higher superheat degree led to changes in the physicochemical properties of steam. The H2S generation was promoted by the increase in the dielectric constant, solubility, and acidity and alkalinity.34,35 In addition, the intermolecular forces and the hydrogen bonding forces were reduced at high superheat degrees. The conjugated large π bonds in the heavy oil structure, the side chain of the aromatic ring, and the C–S, C–O, and C–N bonds in the branches easily broke, leading to ring-opening, desulfurization, hydrogenation and de-chaining reactions.
Compared with the case of the crude oil, the saturate, aromatic and asphaltene fractions increased significantly, whereas the resin fraction decreased with the increasing superheat degree in the residual oil. Some free radicals were formed by molecular bond cleavage during the aquathermolysis reaction. These radicals could break macromolecular hydrocarbons to form small molecular saturated hydrocarbons, resulting in an increase in the saturate and aromatic fractions. With the increasing superheat degree, the intermolecular and hydrogen bonding forces weakened. Consequently, the aromatic ring structure and its branched C–S and C–O were more easily broken to form more free radicals.36 These radicals can induce polycondensation to form asphaltene. In addition, sulfidation is another reason for the simultaneous formation of asphalt and H2S.37
In addition to the overall analysis of SARA, the elemental content was detected to identify the primary source of H2S generation. The S and H element contents at different superheat degrees are displayed in Fig. 4a and b, respectively. As can be observed from Fig. 4a, the content of S in all the fractions decreases as the superheat degree increases, indicating that all organic sulfur species contribute to the generation of H2S. However, the specific contribution varied from one fraction to another. Particularly, the S element in saturate could not even be detected after all the reactions. The H element in water may participate in the reactions of H2S generation, and it can be observed from Fig. 4b that the H element in the saturate fraction increases significantly. In addition, the sulfur content was more highly concentrated in the aromatic fraction according to the elemental analysis of crude oil. Fig. 4a suggests that the content of the S element in the aromatic fraction presents largest reduction with the increasing superheat degree. Therefore, organic sulfur in the saturate and aromatic fractions may be the main source of H2S generation.
![]() | ||
Fig. 6 XPS spectra of C1s, O1s, N1s and S2p and corresponding fitting curves for the oil obtained after the reaction at 250 °C (B.E., binding energy). |
The peaks at 284.70 and 285.22 eV represented the aliphatic and aromatic carbons combined with oxygen atoms, respectively. The connection modes of the carbon and oxygen atoms can be further classified into three kinds. The peaks at 531.50, 532.36 and 533.26 eV correspond to the carbons combined with the oxygen atoms via ethers and hydroxyls, carbonyl, and carboxyl.44 The existing forms of nitrogen and sulfur in the heavy oil can also be detected by XPS. The N1s spectra mainly consisted of pyrrolic, pyridine, quaternary and chemisorbed NOx corresponding to the peaks at 398.78, 399.47, 400.00 and 400.60 eV.45 Similarly, the peaks at 163.70, 164.46, 165.50, 168.07, and 168.96 eV in the S2p spectra can be categorized into the peaks of sulfides, thiophenes, sulfoxides, sulfones, and acids/sulfates, respectively.
The XPS results of the organic C, O, N, and S forms in heavy oil before and after the reaction are summarized in Table 4. As can be seen, the heavy oil is mainly composed of aliphatic and aromatic compounds. Compared to the case of the oil before reaction, the number of the carbon–oxygen bonded atoms in the oil after the reaction was reduced; the number of the carbonyl and carboxyl groups was also relatively reduced, but that of the hydroxyl group increased. The carbonyl group was formed by the cleavage of the C–O bond in the carboxyl group. On this basis, gaseous CO2 was produced from the carbonyl group, and the escaping CO2 was detected in the gas products (Table 1).
Name | Group type | B.E. (eV) | FWHM (eV) | Relative peak area (%) | |
---|---|---|---|---|---|
Before reaction | After reaction | ||||
C1s | C (al and ar) | 284.70 | 1.00 | 62.22 | 62.11 |
C joins to O | 285.22 | 1.00 | 37.96 | 37.89 | |
O1s | C![]() |
531.50 | 1.13 | 31.03 | 27.75 |
C–O | 532.36 | 1.21 | 33.92 | 39.48 | |
O![]() |
533.26 | 1.47 | 35.05 | 32.77 | |
N1s | Pyrrolic | 398.78 | 0.77 | 26.32 | 25.11 |
Pyridinic | 399.47 | 0.65 | 18.34 | 25.32 | |
Quaternary | 400.00 | 0.75 | 27.80 | 30.67 | |
Chemisorbed NOx | 400.60 | 1.00 | 27.54 | 18.90 | |
S2p | Sulfidic | 163.70 | 1.00 | 19.71 | 15.25 |
Thiophenic | 164.46 | 1.81 | 19.80 | 30.48 | |
Sulfoxidic | 165.50 | 2.84 | 19.94 | 40.01 | |
Sulfones | 168.07 | 2.74 | 20.20 | 12.36 | |
Acidic/sulfates | 168.96 | 0.50 | 20.34 | 1.90 |
H2S generation was not only determined by the content of sulfur in heavy oil but also closely related to the chemical structure of sulfur-bearing molecules.17 As can be seen from Table 4, the main existing forms of sulfur in the heavy oil after reaction are sulfoxide and thiophene, which account for 70.49% of the total sulfur. In general, sulfoxide is more stable than other sulfides. It can only be decomposed to produce H2S when the reaction temperature exceeds 300 °C.46 Therefore, a large amount of sulfoxide remained in the residual after the reaction. By comparing the sulfur-containing substances before and after the reaction, it was found that H2S was mainly formed by sulfides, sulfones or sulfates under superheated steam conditions. Payzant et al.47 believed that the depolymerization of macromolecules, such as colloidal and asphaltenes, also produced thiophenes. By combining the abovementioned findings with those of the SARA analysis, it can be further concluded that the increase in the content of thiophene at high superheat degrees is mainly caused by resin degradation. Although the sulfone and sulfate contents were low before and after the reaction, they were still detected, and both of them decreased after the reaction due to the thermochemical reduction reaction.
Footnote |
† Equal contribution author. |
This journal is © The Royal Society of Chemistry 2019 |