CA2908977C

CA2908977C - Method of transporting hydrocarbon fluids containing sour gas

Info

Publication number: CA2908977C
Application number: CA2908977A
Authority: CA
Inventors: Gilles F. Martin
Original assignee: 1787939 Alberta Ltd
Current assignee: 1787939 Alberta Ltd
Priority date: 2015-10-19
Filing date: 2015-10-19
Publication date: 2019-01-29
Anticipated expiration: 2035-10-19
Also published as: CA2908977A1

Abstract

A method of transporting hydrocarbon fluids with sour gas includes using a displacement fluid that has sour gas and is oxygen free. When unloading the hydrocarbon fluids from a transport tank, the displacement fluid fills the transport tank to relieve the pressure and prevent oxygen from entering the tank. When loading the hydrocarbon fluids into the transport tank, the displacement fluid is transferred to a production tank.

Description

METHOD OF TRANSPORTING HYDROCARBON FLUIDS CONTAINING SOUR GAS
TECHNICAL FIELD
[0001] This relates to a method of transporting hydrocarbon fluids containing sour gas.
BACKGROUND

[0002] In many hydrocarbon producing wells, the produced fluid contains some natural gas. In crude oil, this is referred to as associated gas or solution gas. When agitated or pressure is reduced, this gas may be released from the production fluids. In wells where the natural gas that is released contains hydrogen sulphide (H2S), it is generally referred to as sour gas.

[0003] Hydrogen sulphide is highly toxic to humans and animals, and care must be taken when transporting or otherwise handling sour gas. Low levels of exposure to H2S can result in symptoms such as eye irritation, sore throat, nausea, and vomiting. At high levels of exposure, workers not wearing the appropriate protective equipment will pass out. Too much of the gas can kill a worker in seconds. In addition, the mixture of sour gas and oxygen may create a risk fire or explosion. As such, tank trucks are generally "sweetened" after unloading fluids that contain H2S. The various approaches to sweetening a sour tank are well known in industry and will not be described further. When loading a tank truck with a fluid containing sour gas, the air that is vented from the tank truck must be passed through a scrubber in order to ensure that H2S is not released to the atmosphere.

[0004] An example of a production process is shown in FIG. 5 and 6, where a well 10 produces fluid, which is then captured in one or more service or production tanks 16.
Referring to FIG. 5, a well site may have a flare stack 11, and a scrubber for sour gas 18, as required. Referring to FIG. 6, a transport tank 12, hauled by a truck 14, is loaded from production tank 16. When the production fluid enters transport tank 12, any sour gas that is carried by the production fluid will be released because of the change in pressure because and because of the agitation experienced by the fluid. As such, as transport tank 12 is filled, the displaced gas that is vented from tank 12 is generally passed through the H2S
scrubber unit 18 to make it safe to be released to atmosphere.

[0005] Referring to FIG. 7, when transport tank 12 is being unloaded at a facility that has a receiving tank 20 (FIG. 5), atmospheric air is allowed to enter tank 12 via a vent line 22 to relieve the vacuum. While tank designs may vary, a typical transport tank 12 is shown with two compartments, with vent line 22 used to equalize the pressure between the compartments.
After being unloaded, tank 12 must be "sweetened" in order to receive a load that does not contain H2S, which can be done using known techniques.
SUMMARY

[0006] There is provided a method of transporting hydrocarbon fluids in a transport tank that is designed to store and transport fluids, the method comprising the steps of: loading a hydrocarbon fluid into the transport tank, the hydrocarbon fluid carrying hydrogen sulphide;
transporting the hydrocarbon fluid to an offload location; and offloading the hydrocarbon fluid from the transport tank while displacing the hydrocarbon fluid with a displacement gas, wherein the displacement gas is substantially free of oxygen and at least a portion of the displacement gas comprises hydrogen sulphide.

[0007] According to an aspect, the transport tank may be purged of oxygen prior to being loaded with the hydrocarbon fluid. An internal volume of the transport tank is purged by loading the tank with a hydrocarbon fluid that is free of hydrogen sulphide.

[0008] According to an aspect, the hydrocarbon fluid may further carry natural gas, the natural gas and the hydrogen sulphide mixing to form sour gas.

[0009] According to an aspect, the hydrocarbon fluid may release at least a portion of the hydrogen sulphide during offloading. The hydrocarbon fluid may be agitated to promote the release of hydrogen sulphide from the hydrocarbon fluid.

[0010] According to an aspect, the method may further comprising the steps of:
transporting the transport tank to a well site after offloading the hydrocarbon fluid; and loading a further load of hydrocarbon fluid into the transport tank at the well site. The hydrocarbon fluid may be loaded from a production tank, and the method may further comprise the step of transferring the displacement gas to the production tank as the transport tank is loaded. The method may further comprise the step of redirecting the displacement gas to a hydrogen sulphide scrubber unit if oxygen is sensed in the displacement gas.

[0011] According to an aspect, there is provided a method of transporting hydrocarbon fluids in a transport tank, the method comprising the steps of: filling the transport tank with sour gas, the sour gas comprising hydrogen sulphide and natural gas and being substantially free of oxygen; at a well site, loading a hydrocarbon fluid into the transport tank, the hydrocarbon fluid comprising a liquid component and a sour gas component; as the hydrocarbon fluid is loaded into the transport tank, transferring the sour gas from the transport tank to a production tank as the hydrocarbon fluids are removed from the production tank.

[0012] According to an aspect, the method may further comprise the steps of:
transporting the transport tank containing the hydrocarbon fluid to an offload location; and offloading the hydrocarbon fluid from the transport tank while displacing the hydrocarbon fluid with a displacement gas, wherein the displacement gas is substantially free of oxygen and at least a portion of the displacement gas comprises hydrogen sulphide. At least a portion of the displacement gas may be released from the hydrocarbon fluid during offloading. The method may further comprise the step of agitating the hydrocarbon fluid to promote the release of displacement gas.

[0013] In other aspects, the features described above may be combined together in any reasonable combination as will be recognized by those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS

[0014] These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to be in any way limiting, wherein:
FIG. 1 is a schematic of fluids containing sour gas being transported.
HG. 2 is a schematic of a tank with fluids containing sour gas being loaded.
HG. 3 is a schematic of a tank with fluids containing sour gas being offloaded.
FIG. 4 is a schematic of a tank with fluids containing sour gas being offloaded in an alternative manner.
FIG. 5 is a schematic of a prior art method for producing and transporting fluid.

FIG. 6 is a schematic of a tank with fluids containing sour gas being loaded in the prior art.
FIG. 7 is a schematic of a tank with fluids containing sour gas being offloaded in the prior art.
DETAILED DESCRIPTION

[0015] The method described herein relates to a method of transporting hydrocarbons with sour gas. The method was developed primarily to assist in transporting production fluids from a well site where the fluids are produced to a destination, such as an upgrader, storage facility, etc. using a transport tank such as may be hauled by a truck.
However, it will be understood that the method may be adapted to other situations where it is necessary to transport sour gas, which may include transport tanks that are transported using other means.
Accordingly, while the embodiment described below will be discussed in terms of transporting production fluids from a production tank or other storage vessel at a well site to a treatment facility, it will be understood that the method may be applied to other circumstances based on modifications that are apparent to those skilled in the art once the principles discussed below are understood.

[0016] In the description below, the terms sour gas and hydrogen sulphide (H2S) may be used interchangeably. Generally speaking, the term sour gas refers to hydrogen sulphide mixed with natural gas, such as methane or other light hydrocarbons in vapour form. It will be understood by those skilled in the art that different forms of H2S and sour gas may be encountered as the composition of fluids being transported changes, and that the terms sour gas and H2S as used herein are intended to cover those situations or concentrations of H2S that require safety measures.

[0017] In transporting production fluids using a transport tank, the tank is first filled with fluid, transported to the destination, and then offloaded. When the production fluids carry sour gas, care must be taken to ensure that no sour gas, or a negligible amount of sour gas, is released to the environment or exposed to the workers. In addition, care must be taken to ensure that sour gas and oxygen do not mix, particularly in an enclosed body such as a tank, as such a mixture has been known to pose a risk of fire or explosion.

[0018] Referring to FIG. 1, the method described herein allows fluids with sour gas, such as production fluids from a sour well, to be loaded from a service tank 16 or other source of 5 fluid, transported in tank 12, and offloaded at a destination facility 20 in a cycle, without releasing or treating any sour gas and while avoiding a mixture of oxygen and H2S. It will be apparent that beneficial results may be obtained when both the loading and offloading steps are used together. However, it will also be understood that the loading and offloading steps may be performed separately, although separating the steps may require other remedial steps to be taken, such as cleaning the transport tank, scrubbing the sour gas, etc.

[0019] Referring to FIG. 2, transport tank 12 is shown being loaded from a service vessel 16, such as a production tank or other storage tank that may be found on a well site.
Generally speaking, when transporting production fluids, tank 16 will be located at a well site.
Prior to being loaded, transport tank 12 is preferably flushed with an oxygen-free gas, such as nitrogen, natural gas, sour gas, etc. either as part of the unloading process described below, or in anticipation of being filled using the presently described method. If the gas in tank 12 contains oxygen, the prior art method of treating the displaced gas must be used. In the steps that follow, it will be assumed that tank 12 is free of oxygen.

[0020] As production fluids are loaded into tank 12, sour gas in the fluids will be released, such that the gas inside tank 12 will also be sour. This gas is displaced out of tank 12 through vent line 22. Preferably, the gas passes through an oxygen sensor 24 to detect any oxygen in the gas. If sensor 24 detects oxygen, the gas is instead disposed of as in the prior art, represented by scrubber unit 18. If no oxygen is detected, or the level of oxygen is sufficiently low that it is determined to be safe, it passes into a fluid line 26 that connects back to storage tank 16. Fluid line 26 permits gas to pass back into tank 16, thus equalizing the pressure between tanks 12 and 16. By allowing the gas to pass between tanks 12 and 16, it is unnecessary to release the sour gas to atmosphere, and the systems used to ensure the safety of transport tank 12 and storage tank 16 can be relied upon to avoid releasing sour gas to atmosphere. Once filled, transport tank 12 can then be hauled to an offloading location, such as a central storage facility, a pipeline inlet, an upgrader, etc. As the load within tank 12 is similar to what would otherwise be loaded into tank 12, it may be transported using common transportation methods.

[0021] Referring to FIG. 3, at the offload location, the production fluids in tank 12 are transferred out of tank 12 via fluid line 30 into a receiving tank 20. Rather than vent tank 12 to atmospheric air as it is emptied, tank 12 is instead filled with a displacement fluid that contains sour gas. This may be accomplished, for example, by using an agitator, represented by an impeller 34 within tank 12 as shown in FIG. 4, to encourage sour gas to be released from the production fluid. As shown, agitator is in one compartment of tank 12, however both compartments are connected by vent line 22, which allows the pressure to be equalized.
Other types of agitators may also be used. As tank 12 is not open to atmosphere, the pressure in tank 12 may be reduced as fluid is offloaded. However, the reduced pressure will result in more gas being released from the fluid.

[0022] Another option includes recirculating some fluid or any released gas back into tank 12 through vent line 22 as it is being offloaded, as shown in FIG. 4.
Circulation may also be considered a form of agitation and encourages sour gas to be released from the production fluid. While these represent simple and relatively safe approaches to generating gas from the fluids that can be used as a displacement gas, other options for encouraging sour gas to be released may also be used, as will be recognized by those skilled in the art. In addition, other options for displacing the offloaded fluid will be apparent to those skilled in the art, such as by connecting tank 12 to be vented by receiving tank 20, or by providing a separate source of gas. In some cases, depending on the amount of gas released from the production fluid and the specifications of transport tank 12, it may be necessary to leave some production fluid in the tank 12 to prevent creating too great a vacuum, or provide another oxygen-free source of gas to make up the difference.

[0023] Once the fluid in tank 12 has been offloaded and tank 12 is filled with an oxygen-free gas, preferably sour gas, tank 12 may then be transported back to a well site or other storage tank to be filled with a fluid containing sour gas, as described above. By filling tank 12 with an oxygen-free tank during offloading, any risks associated with mixing oxygen and H2S are avoided.

[0024] In this patent document, the word "comprising" is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article "a" does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements.

[0025] The scope of the following claims should not be limited by the preferred embodiments set forth in the examples above and in the drawings, but should be given the broadest interpretation consistent with the description as a whole.

Claims

What is Claimed is:

1. A method of transporting hydrocarbon fluids in a transport tank that is designed to store and transport fluids, the method comprising the steps of:
loading a hydrocarbon fluid into the transport tank, the hydrocarbon fluid carrying hydrogen sulphide;
transporting the hydrocarbon fluid to an offload location; and offloading the hydrocarbon fluid from the transport tank while displacing the hydrocarbon fluid with a displacement gas, wherein the displacement gas is substantially free of oxygen and at least a portion of the displacement gas comprises hydrogen sulphide.

2. The method of claim 1, wherein the transport tank is purged of oxygen prior to being loaded with the hydrocarbon fluid.

3. The method of claim 2, wherein an internal volume of the transport tank is purged by loading the tank with a hydrocarbon fluid that is free of hydrogen sulphide.

4. The method of claim 1, wherein the hydrocarbon fluid further carries natural gas, the natural gas and the hydrogen sulphide mixing to form sour gas.

5. The method of claim 1, wherein the hydrocarbon fluid releases at least a portion of the hydrogen sulphide during offloading.

6. The method of claim 5, further comprising the step of agitating the hydrocarbon fluid to promote the release of hydrogen sulphide from the hydrocarbon fluid.

7. The method of claim 1, further comprising the steps of:
transporting the transport tank to a well site after offloading the hydrocarbon fluid;
and loading a further load of hydrocarbon fluid into the transport tank at the well site.

8. The method of claim 7, wherein the hydrocarbon fluid is loaded from a production tank, and further comprising the step of transferring the displacement gas to the production tank as the transport tank is loaded.

9. The method of claim 8, further comprising the step of redirecting the displacement gas to a hydrogen sulphide scrubber unit if oxygen is sensed in the displacement gas.

10. A method of transporting hydrocarbon fluids in a transport tank, the method comprising the steps of:
filling the transport tank with sour gas, the sour gas comprising hydrogen sulphide and natural gas and being substantially free of oxygen;
at a well site, loading a hydrocarbon fluid into the transport tank, the hydrocarbon fluid comprising a liquid component and a sour gas component;
as the hydrocarbon fluid is loaded into the transport tank, transferring the sour gas from the transport tank to a production tank as the hydrocarbon fluids are removed from the production tank.

11. The method of claim 10, further comprising the steps of:
transporting the transport tank containing the hydrocarbon fluid to an offload location;
and offloading the hydrocarbon fluid from the transport tank while displacing the hydrocarbon fluid with a displacement gas, wherein the displacement gas is substantially free of oxygen and at least a portion of the displacement gas comprises hydrogen sulphide.

12. The method of claim 11, wherein at least a portion of the displacement gas is released from the hydrocarbon fluid during offloading.

13. The method of claim 12, further comprising the step of agitating the hydrocarbon fluid to promote the release of displacement gas.