Experimental Investigation on Hydrothermal Reduction of Sulfates to H2S and Organic Sulfides by Ethene

The kinetic characteristics of alkenes involved in thermochemical sulfate reduction (TSR) have been never reported in geological literature. In this study, TSR by ethene under hydrothermal conditions was performed in the constrained simulation experiments. Typical TSR products consisted of H2S, CO2, mercaptans, sulfides, thiophenes derivatives and benzothiophene. The apparent activation energy E and apparent frequency factor A for TSR by ethene were determined as 76.370 kJ/mol and 4.579 s-1, respectively. The lower activation energy for ethene involved in TSR relative to ethane suggested that the reactivity of ethene is much higher than that of ethane, in accordance with the thermodynamic analysis. Rate constants were determined experimentally using first-order kinetics extrapolate to MgSO4 half-lives of 67.329 years - 3.053 years in deep burial diagenetic settings (120°C - 180°C). These values demonstrate that the reaction rate for TSR by ethene is extraordinarily fast in high-temperature gas reservoirs (120°C - 180°C). Consequently, the newly formed ethene from thermal cracking and TSR alteration of natural gas and/or petroleum could not survive after TSR process and were rarely detected in natural TSR reservoirs.


Introduction
High sour gas with >10% H 2 S (hydrogen sulfide) has been reported in reservoirs from the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation in the NE Sichuan Basin [1]- [10]. The elevated H 2 S (up to 62%) is attributed to thermochemical sulfate reduction (TSR) in deep buried Triassic and Permian reservoirs with vertical depths over 4.8 km [8]. TSR is an organic-inorganic interaction between sulfates and hydrocarbons in high temperature diagenetic settings, in which sulfates are reduced to H 2 S, organic sulfides and elemental sulfur (S 0 ) while hydrocarbons are oxidized to carbon dioxide (CO 2 ) and water [11]. TSR-derived H 2 S not only critically affects the economic proportion of hydrocarbon gas, but is highly toxic to human beings and corrosive for petroleum processing facilities. H 2 S gas risk has become one of the major concerns for exploration in deeply buried carbonates [4] [5] [6]. Some laboratory experiments with gaseous saturated hydrocarbons were carried out to investigate the formation mechanism of elevated H 2 S in sour gas from TSR zones [12]- [17].
However, up to now, few TSR simulation studies utilized alkenes as reducing agents for sulfates [18]. Although alkenes are absent or only trace components in most natural gas and petroleum, they are produced in appreciable amounts during thermal degradation of kerogen [19] as well as in many previous TSR experiments [12] [18] [20] [21] [22] [23]. Notably, it was reported that alkenes were the most reactive hydrocarbon type during TSR [18], and the rapid increase in the yield of ethanethiol and propanethiol was closely related to the generation of ethylene and propylene from hexadecane thermal cracking at high temperatures [20]. Nevertheless, the kinetics of TSR by alkenes needs further investigation due to no reports related. To fill this gap, a series of simulation experiments on the reactions between magnesium sulfate (MgSO 4 ) and ethene (C 2 H 4 ) were conducted under controlled hydrothermal conditions. C 2 H 4 was selected as a representative of intermediate hydrocarbon species during TSR. Anhydrous MgSO 4 was utilized as oxidants for hydrocarbons due to its effective oxidant of hydrocarbons without the addition of H 2 S [24]. The objectives of the present study are to: 1) Investigate the thermodynamic characteristics of TSR by ethene; 2) Determine the kinetic parameters for sulfate reduction by ethene; 3) Discuss the reason that alkenes were not detected in natural TSR zones.

Experiment
MgSO 4 is analytical pure, purchased from Aladdin Industrial Corporation (Shanghai, China). C 2 H 4 (99.999%) was supplied by Wuhan Newradar Special Gas Co., Ltd.. Simulation experiments on the reduction of MgSO 4 by C 2 H 4 were operated at elevated temperatures using an autoclave. The autoclave was made of Hastelloy C-276 alloy with a volume of 200 mL. The sample basket is a quartz glass cylinder with a height of 40 mm and diameter of 20 mm. The quartz glass cylinder has a bottom to hold MgSO 4 powder and its other side is left open. The Open Journal of Yangtze Gas and Oil basket was placed on the bottom of the reactor. Each time, the basket with about 2.000 g MgSO 4 powder and 1.0 mL distilled water was put into the reactor, and then the reactor was vacuumed. Ethene was charged to a given pressure. In the reaction system of C 2 H 4 -MgSO 4 -H 2 O, the initial pressure was about 2.5 MPa at room temperature and the final pressure of the reaction system ranged from 4.5 MPa -5.6 MPa. The reactor was first heated to 250˚C directly and then to the final temperatures by a program control. The final temperatures were 280˚C, 300˚C, 320˚C, 340˚C and 360˚C, and the heating times were 240 h, 400 h, 560 h, 720 h and 880 h, respectively. Constant heating rate β is set as 0.125˚C/h. After simulation experiments, the basket with reaction products was taken out of the autoclave, firstly put into an oven to be heated at 120℃ for about 6 h to remove water, and then heated in a muffle at 500˚C. The weight of solid products was measured by an electron balance. The calculation of TSR conversions was based on gravimetry. The products were characterized by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and X-ray diffractometer (XRD) methods, respectively.
After thermal experiments, headspace gaseous products of the tube reactor were removed through the valve, and characterized using an Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA). Subsequently, 2 mL of dichloromethane (DCM) was then injected into the reaction vessel through the valve to extract organic products, and then organic sulfur products in the DCM extract were analyzed by gas chromatography-mass spectrometry (GC-MS). The analysis of the extracted organic sulfur products was carried out at Beijing Risun Chemical Technology Institute Co., Ltd., China (courtesy of Shijuan Du). Characteristics of the crystalline structure of the solid products at the termination of each experiment were confirmed by a PANalytical X'Pert PRO X-ray diffractometer (PANalytical Co., Almelo, Netherlands). The analysis of the crystalline structure of the solid products was carried out at Beijing Risun Chemical Technology Institute Co., Ltd., China (courtesy of Xinpo Feng). Detailed operation conditions for GC, GC-MS and XRD are available in Ding et al.

TSR Products
According to the GC analysis, the composition of gaseous products from the reaction between C 2 H 4 and MgSO 4 was shown in  [20]. Figure   2 indicated the contents of ethene and organic sulfides vs. temperature. Figure 2 indicated the contents of ethene and organic sulfides vs. temperature. With the decrease of C 2 H 4 , contents of LSC vs. temperature exhibited a positive relationship in Figure 2. The diminishing C 2 H 4 resulted from TSR effect as well as thermal chemical alteration (Figure 1 and Figure 2).   Table 2. Besides H 2 S and LSC, thiophene derivatives and benzothiophene were also detected in the DCM extract (  Table 2 should be attributed to thermal alteration of LSC [22]. The positive correlation between contents of SOSCs and temperature in Figure 3 indicated that the conversion of inorganic sulfur of MgSO 4 to thiophene sulfur in TSR was enhanced by elevated temperatures. Tentative formation pathways for LSC and SOSCs in this study were given in Equations (2)

H S C H C H SH
Some black material was observed on the surface of solid products at 360˚C.
After calcinations at 500˚C, the solid products turned white. Therefore, the black deposit corresponded to be coke [22]. X-ray powder diffraction patterns of solid reaction products at 360˚C were shown in Figure 4. From the X-ray patterns, two distinct crystallographic phases, MgO and MgSO 4 were found to coexist in the products. These results suggested that MgO was produced during TSR by

Thermodynamic Analysis
Our present experimental results indicated that TSR by ethene could be written as the general Equation (4). Previous study of Ding et al. (2011) [15] showed that thermochemical reduction of MgSO 4 by ethane (C 2 H 6 ) conformed to Equation (5). Based on Equations (4) and (5), the standard Gibbs free energy of molar Open Journal of Yangtze Gas and Oil reaction for TSR by ethene or ethane at various temperatures was calculated by use of thermochemical modeling program HSC 6.0 (Outokumpu Research Oy, Pori, Finland) and shown in Figure 5. It is evident that the reactions (4) and (5) proceed spontaneously at 0 -600˚C according to the negative values of Gibbs free energy ( Figure 5). The absolute value of Gibbs free energy of the two reactions increased with increasing temperature, which implied that elevated temperature is favored. Comparing the absolute value of Gibbs free energy of the two reactions (4) and (5) at the same temperature, it can be concluded that ethene thermodynamically more easily participates in TSR than ethane.

Kinetic Study
In the present study, the molar conversion for the reaction between C 2 H 4 and MgSO 4 was calculated from the weight change of the solid products and reactants, and the results were given in Figure 6. From Figure 6, it can be seen that The reaction of C 2 H 4 and MgSO 4 is assumed as first order reaction, and its kinetic equation could be written as [28]: where constant heating rate d d According to the slope and intercept of regressed line in Figure 7, the apparent activation energy E and apparent frequency factor A for TSR by ethene were determined as 76.370 kJ/mol and 4.579 s −1 , respectively. Notably, Xia et al.
(2014) [29] reported that the activation energy for reduction of MgSO 4 by ethane is 264.5 kJ/mol. The lower activation energy for ethene involved in TSR relative to ethane suggested that the reactivity of ethene is much higher than that of ethane, in accordance with the above thermodynamic analysis. High reactivity of alkenes is possibly due to their susceptibility to electrophilic attack [6]. The reaction rate constant k and half-live t 1/2 for TSR by ethene at geological temperatures was determined using Equation (6). The results were listed in Table 3.