CA2193184C - Recirculation of a lower alkyldisulphide in a well - Google Patents

Abstract

A method and system are provided for recirculation of lower alkyldisulphide, such as dimethyldisulphide (DMDS), in a well in which a hydrogen sulphide and sulphur containing fluid is,produced. The method and system are based on the insight that a lower alkyldisulphide reacts with the produced hydrogen sulphide and thereby decomposes into a lower alkanethiol and that the lower alkyldisulphide can be regenerated by selectively separating the hydrogen sulphide from the thus produced lower alkanethiol.

Description

RECIRCULATION OF A LOWER ALKYLDISULPHIDE IN A WELL
The invention relates to a method and system for recirculating a lower alkyldisulphide in a well in which a hydrogen sulphide and sulphur containing fluid is produced.
In sour gas production wells in which the produced fluid has a high sulphur content, such as those at Bearberry or Panther River in Alberta, Canada, it is required to mitigate sulphur deposition by injection of sulphur solvent into the well production tubing.
Dimethyl disulphide (DMDS) is an excellent sulphur solvent, dissolving over three times its weight of sulphur at 50 °C in the presence of about 5.5 MPa of hydrogen sulphide. The solvent functions by reacting chemically with elemental sulphur, forming dimethyl polysulphide (DMPS). An alkaline catalyst is normally added to dimethyl disulphide to facilitate this reaction.
US patent specification 4,290,900 discloses the use of dimethyldisulphide as a sulphur solvent and US patent specification 3,314,999 discloses a method for preparing organic disulphides, such as dimethyl or other alkyl, aryl or arylalkyldisulphides, by sulphidization of the corresponding mercaptans by means of elemental sulphur.
The paper entitled "Sulphur deposition in sour gas production facilities" presented by James B. Hyne of the University of Calgary, Alberta, Canada at the 1986 Gas Conditioning conference discloses that the use of dimethyldisulphide in a continuous circulation mode in super-sour gas production will require regeneration of the sulphur-loaded solvent. The paper further discloses that various techniques, such as distillation, cryogenic and chemical wash techniques, may be used to decompose the separated dimethylpolysulphides for the regeneration of dimethyl-disulphide.
Field experience has learned, however, that in sour gas wells dimethyldisulphide does not only form dimethyl-polysulphide, but also at least partly decomposes into methanethiol.
This decomposition cannot be adequately resolved by the known regeneration techniques and has thus far precluded the use of dimethyldisulphide in sour gas wells in which the produced fluid has a high hydrogen sulphide content.
The present invention aims to provide a solution to this problem and to provide a method for recirculating dimethyldisulphide and other lower alkyldisulphides in wells in which a hydrogen sulphide.containing fluid is produced.
The method and system according to the invention are based on the insight that when hydrogen sulphide containing fluids are produced the decomposition of a lower alkyldisulphide to a lower alkanethiol can be reversed by controlling the hydrogen sulphide partial pressure. In the event the lower alkyldisulphide is dimethyl disulphide the aforesaid decomposition is a result of the following chemical reaction:
CH3SSCH3 + H2S ---> 2CH3SH + 1/8 Sg.
The invention is furthermore based on the insight that if in a wellhead separator the still remaining hydrogen sulphide is selectively separated from the produced methanethiol containing fluid mixture the above chemical reaction is reversed and dimethyl-disulphide is regenerated.
Accordingly, in a first aspect of the invention, there is provided a method for recirculating a lower alkyldisulphide in a well in which a hydrogen sulphide and sulphur containing fluid is produced, the method comprising:

2?93184 - injecting lower alkyldisulphide into the produced well fluid, thereby causing the lower alkyldisulphide to decompose at least partially into alkanethiol;
- feeding the produced fluid mixture to a separator which is operated at such a temperature and pressure that hydrogen sulphide is substantially separated from the lower alkanethiol containing fluid mixture, thereby inducing the lower alkanethiol to regenerate into lower alkyldisulphide; and - reinjecting at least part of the thus regenerated lower alkyldisulphide into the well.
Preferably the lower alkyldisulphide is dimethyldisulphide, which is a widely available sulphur solvent. However, alternatively it comprises ethyl-disulphide and/or any other lower alkyldisulphide since these will broadly have similar chemical and physical characteristics as dimethyldisulphide.
It has been found that because of the difference in boiling points, i.e. -60.7 °C for hydrogen sulphide and +6.2 °C for methanethiol at atmospheric pressure, these substances can be effectively separated in a separator e.g. a fractionating column or vessel which is operated at a relatively low temperature and elevated pressure such that methanethiol becomes liquid and hydrogen sulphide remains gaseous.
Since the produced fluid mixture may have a temperature of around 100 °C at the wellhead it is usually necessary to cool off the mixture such that in the separator the temperature is below 50 °C and it is preferred to maintain the temperature of the fluid mixture in the separator between 0 °C and 30 °C.
Since cooling off the produced fluid mixture is expensive it is preferred to reduce the amount of cooling and to maintain a relatively high pressure within the separator in order to keep methanethiol in a liquid and hydrogen sulphide in a gaseous phase.
Preferably the pressure of the fluid mixture within the separator is maintained above 0.6 MPa and it is particularly preferred to maintain the pressure above 1.4 MPa.
According to another aspect of the invention, there is provided a system for recirculating a lower alkyldisulphide in a well in which a hydrogen sulphide and sulphur containing fluid is produced, the system comprising:
means for injecting the lower alkyldisulphide into the produced well fluid;
- a separator for.processing of the produced fluid mixture, the separator being provided with temperature and/or pressure control means that are adjustable such that hydrogen sulphide and lower alkanethiol are separated; and - means for injecting regenerated lower alkyldisulphide obtained from the thus separated methanethiol into the well.
The separator may comprise a water separation vessel andjor a fractionating column, or a series of fractionating columns. It is preferred that the fractionating column or columns separate the more volatile fractions, such as hydrogen sulphide, carbon dioxide and methane from the more easily condensed fluids, such as dimethylpolysulphide and methanethiol.
The invention will be explained in more detail on the basis of the following examples and drawings, in which Fig. 1 shows a dimethyldisulphide recirculation system according to the invention.
Fig. 2 shows the plotted outcome of experiments used to show the effect of temperature on decomposition of dimethyldisulphide (hereinafter referred to as DMDS or CH3SSCH3) into methanethiol (hereinafter referred to as MT or CH3SH) using the gradual gas depletion technique (no distillation column);-2j93184 Fig. 3 shows a schematic vertical sectional view of an experimental apparatus used to simulate the effects of gas depressurized through a condenser column;
Fig. 4 shows the plotted outcome of experiments used to show the effect of the use of the condenser or reflux column shown in Fig. 3 on CH3SH removal using the gradual gas depletion technique;
Fig. 5 shows the plotted outcome of experiments used to show the effect of variation of the temperatures in the vessel and column sY~own in Fig. 3 on the gradual gas depletion technique;
in Fig. 2, 4 and 5 the vertical axis represents the measured percentage of DMDS decomposed to CH3SH and the horizontal axis the percentage of the volume of gas removed; and Fig. 6 shows the plotted outcome of experiments used to show the influence of pressure on the appearance of CH3SH
in the gas phase when the gradual gas depletion technique is used.
In Fig. 6 the vertical axis represents the percentage of CH3SH detected in the gas phase and the measured pressure in the vessel of Fig. 3 while the temperature in the vessel V and condensor C are maintained at various constant levels.
Referring now to Fig. 1 there is shown a sour gas production well in which DMDS is injected at a downhole location into the production tubing 1 via an injection tube 2 passing through the wellhead 3.
The injection of DMDS serves to prevent formation of solid elemental sulphur in the production tubing 1.
DMDS acts as a sulphur solvent by forming dimethyl polysulphide (DMPS), but, as demonstrated by the experiments described below, will also decompose within the production tubing into MT as a result of the following chemical reaction:

CH3SSCH3 + H2S ---> 2 CH3SH + 1/8 Sg At the wellhead, a dewatering vessel 4 is present in which water is removed from the produced fluid mixture.
This dewatering vessel 4 is optional and therefore shown in dotted lines. A fractionator in the form of a distillation column 5 is mounted behind the dewatering vessel 4. In use this column 5 is operated at such a pressure and temperature that volatile components such as hydrogen sulphide, C1-C3 and carbon dioxide are separated from the more condensable components such as MT, DMPS and DMDS. The volatile components are removed in a gaseous phase from the top of the distillation column 5 and further processed in a hydrocarbon stripper and Claus plant. The condensable components are removed in a liquid phase from the bottom of the column 5 and pumped into a dewatering vessel 6 in which any still remaining MT is permitted to be regenerated into DMDS. The resulting DMPS/DMDS fluid mixture is then pumped into an amine stripper 7 in which an amine solution is used to remove sulphur from the DMPS thereby regenerating DMDS which is then recirculated into the well as illustrated by arrow 8. Preferably the distillation column 5 is operated at a temperature which is about 20 °C lower than that within the dewatering vessel 4. It is furthermore preferred that the pressure of the fluid mixture in the distillation column 5 is maintained at an elevated level, e.g. between 0.6 and 1.4 MPa and that the temperature of that fluid mixture in the distillation column 5 is maintained below 50 °C, and preferably between 0 °C and 30 °C.
Several experiments have been conducted that show the extent of decomposition of DMDS when the method according to the invention is and is not practiced. As much as possible, the experiments attempted to simulate conditions expected at the Bearberry sour gas wells. The 2~93~84 _ 7 _ static bottom hole temperature and pressure are about 116 °C and 37.2 MPa. At the wellhead or inlet separator, these conditions will vary but will normally be around 50 °C and at 0-4.5 MPa.
An experiment was conducted to determine the extent of decomposition of DMDS in fluid production wells of the Bearberry field. A simulated Bearberry fluid was prepared by mixing 1.45 g of methane, 5.10 g of C02, 70.50 g of H2S, and 4.052 g of elemental sulphur in a pressure cell.
The mixture was held at a temperature of 120 °C and a pressure of 34.7 MPa. These conditions are similar to those at bottom-hole of the Bearberry wells. After 24 hours, 2.67 g of DMDS were injected into the mixture.
The mixture was allowed to stand for 15 minutes. The cell and its contents were then cooled to 50 °C. This was intended to simulate wellhead or inlet separator conditions. At 50 °C, the pressure declined to about 12 MPa because of thermal contraction. After 24 hours, the cell was depressurized in stages with no procedures being applied to selectively separate hydrogen sulphide and methanethiol, and samples taken of the effluent stream. Analysis showed that 24% of the DMDS had decomposed to methanethiol. No carbon disulphide was detected.
The base case experiment was repeated several times, including the cooldown to 50 °C, but with lower temperatures during discharge of the contents of the cell. In this way, different conditions in an above-ground separation vessel were simulated. Discharge of the final fractions of the 0 °C experiments was done at room temperature, to prevent retention of condensed methanethiol. (The normal boiling point of methanethiol is 6.2 °C.) During the discharge, the pressure continuously reduced to atmospheric pressure. Initial quantities of reagents are given in the table below:

_8_ Effect of Discharge Temperature - Initial Reagent Quantities Exper- CH4 C02 H2S S DMDS Discharge invent g g g g g , temp.C

SS-34 1.45 5.10 70.50 4.0520 2.6702 50 SS-35 1.45 5.10 70.25 4.0533 2.7107 30 SS-36 1.45 5.10. 70.50 4.0500 2.5952 23 SS-31 1.45 5.05 70.50 4.0510 2.6802 0 Because of the scale of the experiments and the difficulty of conducting this type of experiment, it was not possible to measure directly the final amount of DMDS. The extent of decomposition of DMDS was calculated by measuring the amount of methanethiol formed. The data are tabulated below.
Experiment Cell DMDS
Temp., °C Decomposition, Although some variability exists in the results, the experiments taken together show a significant reduction in decomposition at lower temperatures. Since all the experiments were held for 24 hours and lower decompositions were measured at lower temperatures, it is concluded that recombination of methanethiol with sulphur is taking place. The chemical equation is:

2193$4 2CH3SH + S ---> CH3SSCH3 + H2S.
A still lower discharge temperature, -10 °C, was also attempted. This experiment also showed low decomposition (14%), but did not achieve decomposition below that observed at 0 °C. It is possible that the rate of recombination would become limiting at these very low temperatures.
Data in the previous table give a clear indication that DMDS decomposition was limited by the recombination of methanethiol during recovery of gases from an autoclave. Not shown in those data is a secondary and perhaps more important distillation effect which results in partial separation of H2S and other gases from methanethiol such that methanethiol becomes concentrated in the last few liters of gas removed from the autoclave.
This effect is best illustrated in Figure 2 which plots methanethiol concentration against volume of gas removed from the autoclave. This separation effect is expected to be a consequence of the boiling points of H2S and methanethiol and could be maximized by use of a distillation system.
To test the fractional distillation hypothesis, a few experiments were conducted using the apparatus shown in Figure 3. The apparatus shown comprises an autoclave vessel V in which a mixture 11 of Bearberry fluid and DMDS + MT is present. The autoclave vessel V is located within a vessel 13 which is filled with mixed coolant 14 and which is mounted on a magnetic stirrer 15. A
thermocouple (not shown) is arranged within the autoclave vessel V and means are present to maintain the temperature of the coolant 14 and fluid mixture 11 at a predetermined level. A conduit 16 is connected to the top of the autoclave vessel V. The conduit 16 is provided with a valve 17, a pressure gauge 18 and thermocouple 19.

A reflux column C is connected to the top of the conduit 16 and comprises 0.125 L condenser filled with stainless steel shavings. The column C is surrounded by cooling coils 20 that are maintained at a constant temperature.
An exhaust conduit 21 is connected to the top of the column C. This exhaust conduit 21 comprises a valve 22, a thermocouple 23 and a gas meter 24 and leads to a gas scrubber (not shown).
The sour gas fluids and DMDS were subjected to the same simulated production conditions as described earlier and then were discharged through the reflux column C
under a variety of conditions. Figure 5 illustrates the advantage of a column operated at -10 °C in comparison to earlier experiments carried out with no column. As can be seen from Run 44 in Figure 4, very little methanethiol was detected until over 90% volume of the gas had been removed from the system.
Since it may be costly to run the column in the field at temperatures as low as -10 °C, a variety of column and discharge vessel temperatures were investigated as shown in Figure 5. It is evident from these experiments that the column could be run at temperatures ranging from -10 to 30 ° C before significant concentrations were observed prior to discharge of 90% volume of the gases.
In the experiments, as the gas was discharged, the pressure in the autoclave dropped. As with any similar distillation system, the separation behaviour is dependent on both temperature and pressure. Figure 6 shows the appearance of methanethiol as a function of vessel pressure. For the temperatures employed, practically no methanethiol was observed in the vapour product at pressures above 0.6 MPa. These data suggest that DMDS recovery will be maximized when a pressure of at least 0.6 MPa is maintained in the distillation system. These observations and the ones depicted in Figure 4 are remarkable when it is remembered that the column used in this work consisted of a steel cylinder packed with stainless steel shavings. Commercial columns or fractionators possess many more theoretical plates and would likely enable complete separation of methanethiol and DMDS solvent from the sour gases.
As will be seen in the following experiments, separation of H2S from methanethiol by distillation promotes the recombination process described earlier.
An experiment (SS-47) was conducted with a reflux column to improve the separation of H2S from the mixture of sour fluids containing methanethiol. Except for the use of the column, the experiment was done the same way as in the previous section. The cell temperature was 30 °C and the column temperature was 10 °C. Initial reagent concentrations are given in the Table below:
Use of Reflux Column - Initial Reagent Quantities Experiment CH4 C02 H2S S DMDS
g g g g g SS-47 1.45 5.05 71.50 4.0510 2.6024 Ss-44 1.45 5.05 70.00 4.0500 2.5272 Analysis of the discharged gases showed that only 5.10 of the DMDS had decomposed to methanethiol. (Compare with 18% for SS-35, which was discharged at 30 °C but without a reflux column.) A similar run (SS-44) used vessel and column temperatures of 0 °C and -10 °C. The DMDS decomposition was found to be 6.3%. (Compare 14 and 12% for SS-31 and SS-37, with the cell also operated at 0 °C.) The low total DMDS decomposition values for SS-47 and SS-44 (5.1 and 6.30) combined with information in Figures 4 and 5 give a strong indication that the ??9384 recombination effect is enhanced greatly by controlling the separation of H2S from methanethiol. This is a key point of this invention.
Thus, a commercial system might comprise a continuous gas/liquid separator and one or more attached fractionators operating at appropriate temperatures and pressures to separate H2S from methanethiol. However, the method according to the invention is not limited to the pressure and temperature conditions used in the examples.
At higher pressures,, for instance, it is expected that higher operating temperatures could be used for the separation of H2S and methanethiol.
It will be understood by those skilled in the art that the invention is not limited to regeneration of dimethyldisulphide. Other lower alkyldisulphides such as ethyldisulphide will also be suitable for use as sulphur solvents and will also decompose into lower alkanethiols.
Such decomposition can be reversed by selectively separating the still remaining hydrogen sulphide from the lower alkanethiol containing mixture. Accordingly the invention is not limited to regeneration of dimethyldisulphide only.

Claims (9)

1. ~A method for recirculating a lower alkyldisulphide in a well in which a hydrogen sulphide and sulphur containing fluid is produced, the method comprising:
- injecting the lower alkyldisulphide into the produced well fluid, thereby causing the lower alkyldisulphide to decompose at least partially into a lower alkanethiol;
- feeding the produced fluid mixture to a separator which is operated at such a temperature and pressure that hydrogen sulphide is substantially separated from the lower alkanethiol containing fluid mixture, thereby inducing the lower alkanethiol to regenerate into the lower alkyldisulphide; and - reinjecting at least part of the thus regenerated lower alkyldisulphide into the well.

2. ~The method of claim 1, wherein the lower alkyldisulphide is dimethyldisulphide and the lower alkanethiol is methanethiol.

3. ~The method of claim 1 or 2, wherein the temperature of the fluid mixture in the separator is maintained below 50 °C.

4. ~The method of claim 3, wherein the temperature of the fluid mixture in the separator is maintained between 0 °C and 30 °C.

5. ~The method of claim 1, 2, 3 or 4, wherein the pressure of the fluid mixture in the separator is maintained above 0.6 MPa.

6. ~The method of claim 5, wherein the pressure of the fluid mixture in the separator is maintained above 1.4 MPa.

7. ~A system for recirculating a lower alkyldisulphide in a well in which a hydrogen sulphide and sulphur containing fluid is produced, the system comprising:

- means for injecting the lower alkyldisulphide into the produced well fluid;

- a separator for processing of the produced fluid mixture, the separator being provided with temperature and/or pressure control means that are adjustable such that hydrogen sulphide and a lower alkanethiol are separated; and - means for injecting regenerated lower alkyldisulphide obtained from the thus separated lower alkanethiol into the well.

8. ~The system of claim 7, wherein the separator comprises a water separation vessel and fractionating column which is mounted downstream of the separation vessel.

9. ~The system of claim 8, wherein the fractionating column is provided with means to control the pressure and/or temperature of a dewatered fluid mixture obtained from the water separation vessel such that in use hydrogen sulphide flows in gaseous phase from a top of the fractionating column and the lower alkanethiol flows in liquid phase from a bottom of the fractionating column.