Thermodynamics and kinetics of reactions between C1-C3 hydrocarbons and calcium sulfate in deep carbonate reservoirs

Abstract

Organic-inorganic reactions during thermochemical sulfate reduction (TSR) are the main cause for the destruction of hydrocarbons in deep carbonate sediments. In this paper, thermal simulation experiments on C 1 -C 3 hydrocarbons and solid calcium sulfate were carried out using an autoclave operated at high temperature and high pressure. The gas and solid products were characterized by advanced analytical methods including microcoulometry, gas chromatography and FT-IR. The kinetics and thermodynamics were investigated on the basis of the experimental data, and the results showed that the reactions of C 1 -C 3 hydrocarbons and solid calcium sulfate can proceed spontaneously in the laboratory to produce H 2 S, H 2 O, and CaCO 3 as the main products. From the kinetic calculation, it is found that these three reactions are zero order reactions with activation energies of 152.9 kJ/mol, 131.0 kJ/mol and 120.6 kJ/mol, respectively. When extrapolated to geological temperatures, the time needed to reach 50% conversion at the temperature of 200°C for the reaction between CH 4 and CaSO 4 is 1.44 million years.

Simulation experiments on the reaction system of CH4-MgSO4-H2O

“…Tang et al [38] reported that MgSO 4 is an effective oxidant of hydrocarbons without the addition of initial ARTICLES GEOCHEMISTRY hydrogen sulfide which has been used as an initiator in other experiments [22,23] . The apparent activation energy of the reaction between methane and magnesium sulphate is 101.894 kJ/mol, which is lower than most previous studies (121-208 kJ/mol) [17,18,20,24,25] . This value is close to that calculated by Goldhaber and Orr [23] in the presence of high H 2 S (96 kJ/mol), which to some extent justifies Tang's viewpoint.…”

“…Properties of the solid phase were analyzed by FT-IR and X-ray diffraction. MgO had not been found in the blank tests, which showed that there was no hydrolysis of According to the former simulation experiments on C 1 -C 3 hydrocarbons and solid calcium sulfate [25] , long-chain hydrocarbons more easily participate in the reactions than short-chain hydrocarbons. Ethane should be more active than methane in the process of TSR.…”

“…(14), which leads to the formation of MgCO 3 . In our previous study of the reaction of methane and solid calcium sulfate, calcium sulfate is reduced by methane to produce calcium carbonate [24,25] . Thus, it is justifiably deduced that MgCO 3 might ever exist in CH 4 -MgSO 4 -H 2 O system.…”

“…However, experimental works on the reaction between methane and sulfates have been rarely reported. It should be noted that Yue et al [24,25] had carried out simulation experiments on C 1 -C 3 hydrocarbons and solid calcium sulfate, and found methane could react with calcium sulfate with no presence of water. But the results seemed to have aroused some researchers' suspicions [27] .…”

“…In comparison with field measurements, some simulation experiments in the laboratory have been conducted to investigate chemical characteristics of TSR [1,13,[17][18][19][20][21][22][23][24][25][26] . However, experimental works on the reaction between methane and sulfates have been rarely reported.…”