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.