Dual Service Compressor System For Conditioning Hydrocarbon Gas

Abstract

A system for compressing gas from a wellbore that uses a single reciprocating compressor unit to boost pressure of the gas to an intermediate stage, and from the intermediate stage to a final stage. The final stage is at a destination pressure for distribution. Between the intermediate and final stages the gas is treated to remove water and higher molecular weight hydrocarbons so that the gas pressurized to the final stage is compressed natural gas. The reciprocating compressor is made up of a series of throw assemblies that are all driven by a single shaft. Each throw assembly includes a cylinder with a piston that reciprocates within the cylinder to compress and pressurize the fluid therein. The reciprocating compressor can be a non-lube design thereby eliminating lube oil contamination of downstream compressed natural gas or higher molecular weight hydrocarbons.

Description

BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The present disclosure relates in general to a system and method for compressing gas from a hydrocarbon producing well, where the gas is compressed to an intermediate pressure and to a final discharge pressure within a single unit.

[0003] Description of Prior Art

[0004] Systems for forming compressed natural gas (CNG) typically include a booster compressor that compresses the feed gas to an intermediate stage pressure. While at the intermediate stage pressure, the gas is treated to remove natural gas liquids, which typically include constituents having two or more carbon atoms. The remaining gas, the majority of which generally is made up of methane, is then compressed with a second compressor commonly referred to as a CNG compressor. The booster compressor and CNG compressor can often each have a weight in excess of 75,000 pounds and occupy a significant amount of space. CNG compressors use electric motors; when disposed in remote locations the motors require onsite generators for their power.

SUMMARY OF THE INVENTION

[0005] Disclosed herein is an example of a method of producing natural gas that includes providing a reciprocating compressor having a booster cylinder and a compressed natural gas (CNG) cylinder, directing an amount of gas from a wellbore to the compressor, compressing the amount of gas in the booster cylinder to an intermediate stage pressure to define an amount of intermediate stage gas, directing the intermediate stage gas to the CNG cylinder, and compressing the intermediate stage gas in the CNG cylinder to a destination pressure to form compressed natural gas. The method may further include treating the intermediate stage gas prior to directing the intermediate stage gas to the second one of the cylinders. In this example, treating the intermediate stage gas involves separating higher molecular weight hydrocarbons from the intermediate stage gas. Further in this example, treating the intermediate stage gas removes moisture from the intermediate stage gas. Removing moisture from the intermediate stage gas can take place by adding a hygroscopic agent to the intermediate stage gas. In an embodiment, the booster cylinder is made up of a first booster cylinder and a second booster cylinder, and wherein a discharge of the first booster cylinder connects to a suction in the second booster cylinder. In an example, the CNG cylinder is a first CNG cylinder and a second CNG cylinder, and wherein a discharge of the first CNG cylinder connects to a suction in the second CNG cylinder. The reciprocating compressor may include a body, a shaft extending axially through the body, pistons in the booster and CNG cylinders coupled to the shaft, and a motor/engine connected to the shaft, the method further including activating the motor/engine to rotate the shaft and to reciprocate the pistons in the cylinders. The reciprocating compressor may further have a control panel on the body, the method further involving manipulating the control panel to operate the motor/engine. Moisture may be removed from the gas from the wellbore before directing the gas from the wellbore to the compressor.

[0006] Another method of producing compressed natural gas disclosed herein includes providing a reciprocating compressor having a body, a shaft in the body, a series of cylinders that extend radially outward from the body, and pistons in the cylinders, supplying fluid from a wellbore to a one of the cylinders that is designated as a booster cylinder, creating intermediate stage fluid by pressurizing the fluid in the booster cylinder, removing moisture from the intermediate stage fluid to form intermediate stage gas, and forming an amount of compressed natural gas by pressurizing the intermediate stage gas in another one of the cylinders. Higher molecular weight hydrocarbons can be removed from the intermediate stage fluid. The series of cylinders can be a multiplicity of booster cylinders. Optionally, the another one of the cylinders is a compression cylinder, and wherein the series of cylinders are a multiplicity of compression cylinders.

[0007] Also disclosed herein is a compression system for generating compressed natural gas that has a body, cylinders mounted on the body and pistons in the cylinders that comprise a booster compressor and a compressed natural gas compressor, a feed line containing fluid from a wellbore and having an end connected to a suction side of the booster compressor, a suction side on the compressed natural gas compressor that is in fluid communication with a discharge side on the booster compressor via an intermediate circuit, and a discharge line containing compressed natural gas and connected to a discharge side of the compressed natural gas compressor. The compression system may also have a dehumidification system disposed in the intermediate circuit. Optionally, a tank can be disposed in the intermediate circuit for removing higher molecular weight hydrocarbons. A crankshaft may be included with the compression system that is coupled with each of the pistons, and a motor/engine can be included that is coupled with the crankshaft. Further included with this example is a control system mounted on the body and in signal communication with the motor/engine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 is a schematic view of an example of a system for processing fluid from a wellbore.

[0010] FIG. 2 is a schematic example of a dual service compressor for use with the system of FIG. 1.

[0011] While the invention will be described in connection with the embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The method and system of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The method and system of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout. In an embodiment, usage of the term "about" includes +/-5% of the cited magnitude. In an embodiment, usage of the term "substantially" includes +/-5% of the cited magnitude.

[0013] It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

[0014] An example of a compressed natural gas (CNG) system 10 is schematically illustrated in FIG. 1. The CNG system 10 is downstream of a wellhead assembly 12 shown mounted over a wellbore 14 that intersects a formation 16. Hydrocarbons, both liquid and gas, from the wellbore 14 are produced through the wellhead assembly 12 and transmitted from wellhead assembly 12 via a connected production line 18. Production line 18 terminates in a header 20. The header 20 may optionally be the destination for other production lines 22, 24, 26 that also transmit production fluid from other wellhead assemblies (not shown). A feed line 28 provides a communication means between the header 20 and CNG system 10. The end of feed line 28 distal from header 20 terminates in a knockout drum 30 and which optionally provides a way of separating water and other liquids from the feedline 28. A drain line 32 connects to a bottom of knockout drum 30 and directs liquids separated out from the fluid flow in feed line 28. The gas portion of the fluid in feed line 28 directed into knockout drum 30 exits knockout drum 30 through an overhead line 34 shown extending from an upper end of knockout drum 30. The end of overhead line 34 distal from knockout drum 30 connects to a suction line of a compressor 36. In the example of FIG. 1, compressor 36 includes a booster compressor 38 and a CNG compressor 40. In this example, overhead line 34 terminates at a suction end of booster compressor 38 so that the gas in line 34 can be pressurized to an interstage pressure.

[0015] The interstage gas discharged from booster compressor 38 is treated in an interstage conditioning system 42. More specifically, a discharge line 46 provides communication between a discharge side of booster compressor 38 to a dehydration unit 48. In one alternative, an injection line 50 for injecting hygroscopic agent into the intermediate stage gas flow stream is shown connected to dehydration unit 48. In one example the hygroscopic agent includes triethylene glycol (TEG), and extracts moisture contained within the interstage gas. A discharge line 52 is shown connected to dehydration unit 48, and provides a means for moisture removal from the intermediate stage gas. Overhead line 54 is shown connected to an upper end of unit 48 and which is directed to a heat exchanger 56. Within heat exchanger 56, fluid from within overhead line is in thermal communication with fluid flowing through a bottoms line 58; where bottoms line 58 connects to a lower end of natural gas liquid (NGL) tank 60. Downstream of heat exchanger 56, overhead line 54 connects to a heat exchanger 62. Flowing through another side of heat exchanger 62 is fluid from an overhead line 64, where as shown overhead line 64 attaches to an upper end of NGL tank 60. An optional chiller 66 is shown downstream of heat exchanger 62 in line with overhead line 54. Further in the example of: FIG. 1 is a control valve 68 illustrated in overhead line 54 and just upstream of where line 54 intersects with NGL tank 60. Liquid within line 58 is transmitted to offsite 70, and is controlled to offsite 70 via a valve 72 also shown set within line 58. Valve 72 can be manually or motor operated.

[0016] Overhead line 64 is shown connected to a suction end of CNG compressor 40 and where the gas within overhead line 64 is compressed to a CNG pressure. A discharge line 74 connects to a discharge side of CNG compressor 40 and provides a conveyance means for directing the compressed natural gas from CNG compressor 40 to a tube trailer 76. Optionally, a valve 78 is provided in discharge line 74 and for regulating flow through discharge line 74; and to selectively fill tube trailer 76. Alternatively, each booster compressor 38 may include a first stage 80 and second stage 82. In this example, discharge from first stage 80 flows through suction of second stage 82 for additional pressurization. Similarly, CNG compressor 40 contains a first stage 84 and second stage 86, wherein gas within first stage 84 is transmitted to a suction side of second stage 86 for additional compression. Examples exist wherein the booster compressor 38 and CNG compressor 40 are reciprocating compressors and wherein each have a number of throws, wherein some of these throws may be what is commonly referred to as tandem throws.

[0017] In one example of operation, a multiphase fluid from well 14 flows through lines 18, 20, 28 and is directed to knockout drum 30. Embodiments exist where the fluid flowing through these lines contains at least an amount of flare gas, which might commonly be sent to a flare and combusted onsite. An advantage of the present disclosure is the ability to economically and efficiently produce an amount of compressed natural gas that may be captured and ultimately marketed for sale. Liquid within the fluid in line 28 out flows to a bottom portion of knockout drum 30 and is separated from gas within the fluid. From within drum 30, the gas is directed into overhead line 34. Line 34 delivers the gas to the suction of booster compressor 38, where in one example the gas is pressurized from an expected pressure between 50 to 100 psig to a pressure of 400 psig, and which forms the interstage gas. Gas, which may include hydrocarbons, is directed through line 46 into drum 48. For the purposes of discussion herein, lower molecular weight hydrocarbons are referred to those having up to two carbon atoms, wherein higher molecular weight hydrocarbons include those having three or more carbon atoms. To remove moisture from within the interstage gas in line 46, hygroscopic agent is directed from injection line 50 into dehydration unit 48 and allowed to contact the gas within dehydration unit 48. Alternatively, a molecular sieve 88 may be provided within dehydration unit 48. Hygroscopic agent, or sieve 88, can then absorb moisture within the interstage gas. Sieve 88 may be regenerated after a period of time to remove the moisture captured within spatial interstices in the sieve 88. Regeneration can be by pressure swing adsorption or temperature swing adsorption.

[0018] To remove higher molecular weight hydrocarbons from the interstage gaseous mixture in line 54, the fluid making up the mixture is cooled within exchangers 56 and 62 and flashed across valve 68. Cooling the fluid stream, and then lowering the pressure across valve 68, is an example of a Joule-Thompson method of separation and can condense higher molecular weight hydrocarbons out of solution and into tank 60. The resulting condensate can be gravity fed from within tank 60 and to offsite 70. An optional flare 90 is schematically illustrated in communication with fluid from the wellbore 14 via an end of header 20. Fluid in header 20 can be routed to flare 90 when system 10 is being maintained or otherwise out of service.

[0019] In alternatives employing the optional chiller 66, the higher molecular weight hydrocarbons are separated from the fluid stream by a mechanical refrigeration unit instead of the Joule-Thompson method of gas conditioning. In examples where the Joule-Thompson method is employed, the discharge from the booster compressor 38 can be at about 1,000 psig. In examples using the mechanical refrigeration method, the discharge from the booster compressor 38 can be at a pressure of around 400 psig. An advantage of treating the gas at the interstage pressure is the ability to remove additional moisture from the gas as well as to optimize the separation of the higher molecular weight hydrocarbons. As such, a higher quality of compressed natural gas can be obtained and delivered via line 74 into the tube trailer 76. Moreover, a higher quality of NGL can be delivered to offsite 70. In currently known processes, methanol is sometimes added to the gas mixture to prevent the formation of hydrates during the gas treatment process. However, the addition of methanol is not only costly, but also reduces the quality and marketability of the end products.

[0020] Referring now to FIG. 2 shown is a schematic side sectional example of the compressor 36, where the compressor 36 includes a body 90. Throw assemblies 92, 94, 96, 98 are shown coupled to the body 90 and each along a path generally transverse to an axis of the body 90. Cylinders 100, 102, 104, 106 are shown respectively in each of the throw assemblies 92, 94, 96, 98. Shown in each of the cylinders 100, 102, 104, 106 are pistons 108, 110, 112, 114, which reciprocate in the cylinders 100, 102, 104, 106 to compress gas within the cylinders 100, 102, 104, 106. Piston rods 116, 118, 120, 122 respectively connect pistons 108, 110, 112, 114 to a crankshaft 124 shown projecting axially through the body 90. The crankshaft 124 is driven by a motor 126 shown optionally mounted to the body 90. Operating the motor 126 causes rotation of the crankshaft 124, which in turn reciprocates pistons 108, 110, 112, 114 within their respective cylinders 100, 102, 104, 106. In one example, the motor 126 includes an internal combustion engine that can be powered by gasoline, gas from the wellbore 14, another combustible material, or combinations thereof. In another alternative, the motor 126 can be electrically powered.

[0021] Further shown in the example of FIG. 2 is that throw assemblies 92, 94, are included in the booster compressor 38 portion of compressor 36. In this example overhead line 34 terminates in throw assembly 92, so that gas exiting overhead line 34 can be compressed by reciprocation of piston 108 within cylinder 100. Gas being compressed in cylinder 100 by piston 108 is transmitted to throw assembly 94 via line 128. Gas exiting line 128 into cylinder 102 can be compressed by reciprocating piston 110. Gas compressed within cylinder 102 exits into discharge line 46, where it is transmitted to interstage conditioning system 42.

[0022] Throw assemblies 96, 98 are shown in CNG compressor 40 portion of compressor 36. As shown, overhead line 64 terminates at throw assembly 96 so that interstage gas from interstage conditioning system 42 is transmitted into cylinder 104. Reciprocation of piston 112 in cylinder 104 compresses gas exiting overhead line 64. Gas compressed in the cylinder 104 is transmitted to throw assembly 98 via line 130 shown having an upstream end connected to cylinder 104 and a downstream end connected to cylinder 106. Piston 114 compresses the gas exiting line 130 into cylinder 106, which is then discharged into discharge line 74. A control panel 132 for sending controls to the compressor 36, and/or motor 126 is shown adjacent body 90 and connects to body 90 via bus 134. In and embodiment, bus 134 provides connection for transmitting signals and/or power to body 90 and motor 126 from control panel 132. Further shown is a power line 136 connected to motor 126, which can convey fuel to motor 126 in embodiments when motor 126 is an internal combustion engine. Alternatively, power line 136 can provide electricity to motor 126 when motor 126 is powered by electricity.

[0023] The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While embodiments of the invention have been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. In one example the compressor is a non-lube design, an advantage of which is the reduction of oil and associated equipment requirements, e.g. day tank, strainer, and/or heavy weight oil. A non-lube design can prevent oil carry over to downstream equipment like NGL storage tank, tube trailer, molecular sieves, etc., which eliminates the need of filtration equipment for critical processes and alleviates any operational issues such as contamination, catalyst degradation and the like. Moreover, oil cost savings that results in direct operating expenditures saving for end users. An additional advantage is that a non-lube design eliminates the need for forced feed lubrication system (pumps, PSV, internal gearing, labor etc.) to all cylinders, and packing. It also eliminates the auxiliary components/instrumentation such as tubing, check valves, poppet valves, distribution blocks, no-flow switch etc. This would in turn reduce the overall compressor price to customer. The non-lube cylinder design can implement non-metallic wear resistant materials for internal moving components and by the use of appropriate clearances to maximize heat dissipation in the absence of lube oil. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.

Claims

1. A method of producing compressed natural gas, the method comprising: providing a reciprocating compressor having a booster cylinder and a compressed natural gas (CNG) cylinder; directing an amount of gas from a wellbore to the compressor; compressing the amount of gas in the booster cylinder to an intermediate stage pressure to define an amount of intermediate stage gas; directing the intermediate stage gas to the CNG cylinder; and compressing the intermediate stage gas in the CNG cylinder to a destination pressure to form compressed natural gas.

2. The method of claim 1, further comprising treating the intermediate stage gas prior to directing the intermediate stage gas to the second one of the cylinders.

3. The method of claim 2, wherein treating the intermediate stage gas comprises separating higher molecular weight hydrocarbons from the intermediate stage gas.

4. The method of claim 2, wherein treating the intermediate stage gas comprises removing moisture from the intermediate stage gas.

5. The method of claim 4, wherein removing moisture from the intermediate stage gas comprises adding a hygroscopic agent to the intermediate stage gas.

6. The method of claim 1, wherein the booster cylinder comprises a first booster cylinder and a second booster cylinder, and wherein a discharge of the first booster cylinder connects to a suction in the second booster cylinder.

7. The method of claim 1, wherein the CNG cylinder comprises a first CNG cylinder and a second CNG cylinder, and wherein a discharge of the first CNG cylinder connects to a suction in the second CNG cylinder.

8. The method of claim 1, wherein the reciprocating compressor comprises a body, a shaft extending axially through the body, pistons in the booster and CNG cylinders coupled to the shaft, and a motor connected to the shaft, the method further comprising activating the motor to rotate the shaft and to reciprocate the pistons in the cylinders.

9. The method of claim 8, wherein the reciprocating compressor further comprises a control panel on the body, the method further comprising manipulating the control panel to operate the motor.

10. The method of claim 1 further comprising removing moisture from the gas from the wellbore before directing the gas from the wellbore to the compressor.

11. The method of claim 1, wherein the reciprocating compressor is a non-lube design.

12. The method of claim 11, wherein the motor comprises an internal combustion engine powered by gas from the wellbore.

13. A method of producing compressed natural gas, the method comprising: providing a reciprocating compressor having a body, a shaft in the body, a series of cylinders that extend radially outward from the body, and pistons in the cylinders; supplying fluid from a wellbore to a one of the cylinders that is designated as a booster cylinder; creating intermediate stage fluid by pressurizing the fluid in the booster cylinder; removing moisture from the intermediate stage fluid to form intermediate stage gas; and forming an amount of compressed natural gas by pressurizing the intermediate stage gas in another one of the cylinders.

14. The method of claim 13, further comprising removing higher molecular weight hydrocarbons from the intermediate stage fluid.

15. The method of claim 13, wherein the series of cylinders comprises a multiplicity of booster cylinders.

16. The method of claim 13, wherein the another one of the cylinders comprises a compression cylinder, and wherein the series of cylinders comprises a multiplicity of compression cylinders.

17. A compression system for generating compressed natural gas comprising: a body; cylinders mounted on the body and pistons in the cylinders that comprise a booster compressor and a compressed natural gas compressor; a feed line containing fluid from a wellbore and having an end connected to a suction side of the booster compressor; a suction side on the compressed natural gas compressor that is in fluid communication with a discharge side on the booster compressor via an intermediate circuit; and a discharge line containing compressed natural gas and connected to a discharge side of the compressed natural gas compressor.

18. The compression system of claim 17, further comprising a dehumidification system disposed in the intermediate circuit.

19. The compression system of claim 17, further comprising a tank disposed in the intermediate circuit for removing higher molecular weight hydrocarbons.

20. The compression system of claim 17, further comprising a crankshaft coupled with each of the pistons, and a motor coupled with the crankshaft.

21. The compression system of claim 20, further comprising a control system mounted on the body and in signal communication with the motor.