Abstract:
A system and method captures and processes flare gas so that the gas is usable as compressed natural gas (“CNG”). The flare gas is pressurized by a combination of a booster compressor and a CNG compressor. While interstage and between the booster compressor and the CNG compressor, the gas is treated to remove moisture and to separate out higher molecular weight hydrocarbons. The moisture is removed by contacting the interstage gas with a hygroscopic agent within a dehydration unit. The moisture free hydrocarbon fluid is expanded, and/or externally cooled and directed to a knock out drum. Higher molecular weight hydrocarbons are separated from the fluid in the knock out drum. Gas from the knock out drum is compressed in the CNG compressor.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The present disclosure relates in general to a system and method for compressing gas from a hydrocarbon producing well, where the gas is compressed in multiple stages, and conditioned between stages. 
       Description of the Prior Art 
       [0002]    Hydrocarbons produced from subterranean formations are often multiphase fluid mixtures of gases and liquids. The liquids from these multiphase mixtures are usually collected and transported to processing facilities for further refinement. However, as it is not always economical to store or transmit the produced gases, they are sometimes sent directly to flare instead of being captured. When the gases are captured they are often processed to remove moisture and other undesirable compounds. Hydrate inhibitors, such as methanol, are occasionally used to prevent hydrate formation within the gas. However, the hydrate inhibitors can be difficult to separate from the gas and thus introduce added complexities when trying to obtain a marketable gas product. 
       SUMMARY OF THE INVENTION 
       [0003]    Described herein is an example method of producing compressed natural gas which includes obtaining fluid from a wellbore; where the fluid contains liquid and gas, and also includes a mixture of higher molecular weight hydrocarbons and lower molecular weight hydrocarbons. The gas from the wellbore is pressurized to an interstage pressure, and moisture is removed from the gas while the gas is at the interstage pressure to form a dry gas. Higher molecular weight hydrocarbons are removed from the gas while the gas is at the interstage pressure to isolate natural gas, and the processed natural gas is pressurized to form compressed natural gas. Removing moisture from the gas can involve contacting the gas with a hygroscopic agent that couples with the moisture, and separating the moisture and hygroscopic agent from the gas. The step of separating the higher molecular weight hydrocarbons from the gas can include cooling the gas, flashing the gas across a flow restriction so that the higher molecular weight hydrocarbons condense to from a liquid, and separating the liquid from the gas. In this example, during the step of cooling heat from the liquid is transferred to the gas. Alternatively, the step of cooling includes directing the gas through a chiller. The liquid can be transferred to an offsite location that is remote from the wellbore. The step of removing moisture from the gas can include contacting the gas with a molecular sieve. The compressed natural gas can be transferred to a container, where the container is transported to a location remote from the wellbore. The steps of pressurizing the gas can take place proximate the wellbore. Moisture can be removed from the gas prior to the step of pressurizing the gas to the interstage pressure. 
         [0004]    Another example method of producing compressed natural gas involves receiving an amount of gas directly from a wellbore, pressurizing the gas to an interstage pressure, dehumidifying the gas at the interstage pressure to form a dry gas, and compressing the dry gas to form compressed natural gas. The dry gas can include a mixture of higher molecular weight hydrocarbons and lower molecular weight hydrocarbons, the method may further involve separating the higher molecular weight hydrocarbons from the dry gas at the interstage pressure. In this example, the step of separating the higher molecular weight hydrocarbons includes cooling the dry gas with a lower temperature fluid selected from the group consisting of liquid comprising the higher molecular weight hydrocarbons, a chilled fluid, and combinations thereof. The step of dehumidifying the gas at the interstage pressure may include contacting the gas with a hygroscopic agent. 
         [0005]    Also disclosed herein is an example of a system for producing compressed natural gas which is made up an interstage conditioning system with a dehumidifying system for removing moisture from gas from a wellbore, a booster compressor having a suction line in communication with the gas from the wellbore and a discharge line in communication with the interstage conditioning system, and a compressor having a suction line in communication with an exit of the dehumidifying system and a discharge line having compressed natural gas. The system can also have a separation tank in the interstage conditioning system for separating higher molecular weight hydrocarbons from the gas. The dehumidifying system optionally has a tank with an injection system for a hygroscopic agent. The dehumidifying system optionally includes a tank having a molecular sieve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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: 
           [0007]      FIG. 1  is a schematic view of an example of a system for processing fluid from a wellbore. 
       
    
    
       [0008]    While the invention will be described in connection with embodiments, it will be understood that it is not intended to limit the invention to the embodiments. 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 
       [0009]    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. 
         [0010]    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. 
         [0011]    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  34  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. 
         [0012]    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 motor or manually operated. 
         [0013]    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. 
         [0014]    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 dehydration unit  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 . 
         [0015]    Hygroscopic agent, or sieve  88 , can then absorb moisture within the interstage gas. Sieve  88  may be regenerated after a period of time (by pressure swing adsorption or temperature swing adsorption) to remove the moisture captured within spatial interstices in the sieve  88 . 
         [0016]    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. 
         [0017]    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. 
         [0018]    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. 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.