Abstract:
A method and apparatus for compressing rich natural gas is described. The apparatus has a compression stage that has a gas inlet for receiving rich natural gas. The compression stage includes one or more compressors and one or more cooling elements. A pressure vessel is also included for receiving the compressed gas from the compression stage and a liquid outlet is connected between the compression stage and a vessel having a pressure that is less than the pressure in the compression stage. The vessel has a gas outlet connected to the gas inlet. The method includes the steps of subjecting a stream of rich natural gas to a compression cycle to form a compressed gas and a condensate, separating the condensate from the compressed gas, flashing at least a portion of the condensate to a gas and recycling the flashed condensate into the compression cycle.

Description:
FIELD 
       [0001]    This relates to a method and apparatus for compressing rich natural gas. 
       BACKGROUND 
       [0002]    A hydrocarbon well being produced for its crude oil will often produce natural gas and water with the crude oil. This “solution” or “associated” natural gas is usually referred to as “rich,” meaning it is composed of methane along with heavier hydrocarbons. The composition of the natural gas and the amount of gas relative to the liquid varies between wells. If the well is not connected to a gathering system, the natural gas is often simply vented or flared. 
       SUMMARY 
       [0003]    There is provided a method of compressing rich natural gas. A stream of rich natural gas is subjected to a compression cycle to form a compressed gas and a condensate. The compression cycle may include more than one compression stage and more than one cooling stage and the rich natural gas may be compressed and cooled to a pressure and temperature that is outside the phase envelope of the compressed gas. The condensate is separated from the compressed gas and at least a portion of the condensate is flashed back to a gas. The flashed condensate is then recycled back into the compression cycle. 
         [0004]    There is further provided a method of compressing rich natural gas, the phase of the rich natural gas being defined by a phase diagram having a phase envelope. The method comprises the steps of: defining a path on the phase diagram from an uncompressed state to a compressed state, at least a portion of the path being within the phase envelope of the phase diagram; manipulating the rich natural gas along the path to form a compressed gas and a condensate; removing the condensate from the rich natural gas and flashing at least a portion of the condensate to a gas; and reintroducing the flashed condensate into the path. 
         [0005]    There is further provided an apparatus for compressing rich natural gas that includes a compression stage that has a gas inlet for receiving natural gas. The compression stage has one or more compressors and one or more cooling elements. A pressure vessel is included for receiving the compressed gas from the compression stage. A liquid outlet is connected between the compression stage and a vessel having a pressure that is less than the pressure in the compression stage. The vessel has a gas outlet connected to the gas inlet of the compression stage. The vessel is used to recycle gas back into the compression stage to capture additional rich natural gas. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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: 
           [0007]      FIG. 1  is a schematic view of a well site. 
           [0008]      FIG. 2  is a schematic view of a loading and unloading process. 
           [0009]      FIG. 3  is a schematic view of a multi-element gas container. 
           [0010]      FIG. 4  is a graph showing a phase diagram of a typical rich gas. 
           [0011]      FIG. 5  is a graph showing a compression path of a rich gas. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    The various components of molecules are often referred to by the number of carbon atoms they contain. For example, rich natural gas may be composed of methane (C1), ethane (C2), propane (C3), butane (C4), and heavier hydrocarbons (generally referred to as C5+ herein). 
         [0013]    When natural gas is produced from an oil well that is not connected to a gathering system, the gas must be disposed of in some way. Often, this is done by flaring or venting. It would be preferable to capture the rich natural gas in order to store and process the rich natural gas, such as by transporting the gas to a central processing location. In order to store the gas in a container in an economically viable manner, it must be stored at a higher density, which is generally achieved by compression and cooling. The state diagram of a rich natural gas is shown in  FIG. 4 . The rich natural gas is in a densified state in a particular temperature and pressure range. The presence of heavier hydrocarbons (or other additives) allows a denser phase to be reached than would be possible with, for example, methane alone. U.S. Pat. No. 7,137,260 (Perry) entitled “Method and Substance for Refrigerated Natural Gas Transport” describes the effect of additives, temperature and pressure on the density of natural gas mixtures. The difficulty is that the different components tend to separate in different phases during compression and cooling, as each component has a different transition point. 
         [0014]    The present discussion relates to a method and apparatus that permits the rich natural gas to be captured and stored in a single densified or compressed state, such that it may then be stored or transported, as the case may be. 
         [0015]    Referring now to  FIG. 1 , there is shown an example of a well site  10  that may be the source of rich natural gas. There are shown three wellheads  12  that each produce water, liquid hydrocarbons, and rich natural gas. The well site also has a three-phase separator  14  and a treater  16 . Three-phase separator  14  and treater  16  are used to help separate the produced fluids into the various components, with water being stored in water tank  18 , oil being stored in oil tank  20 , and natural gas being diverted to the flare stack  22 . The stored fluids are then periodically removed from tanks  18  and  20  through truck loading points  21 . It will be appreciated that other well sites may vary from what is depicted. For example, some well sites may produce from fewer or more wellheads  12 . In addition, the separation equipment may be different, for example, there may only be a single production tank, with the phase separation occurring within. As these processes are well known and the actual well site design will depend on the preferences of the operator and the characteristics of the well. What is important to note is that the discussion related to capturing the rich natural gas applies to each of these well sites, where rich natural gas is produced through the wellhead. 
         [0016]    As depicted, the gas phase from treater  16  is traditionally flared by a flare stack  22 . However, in the preferred embodiment, the gas phase is captured through line  24 , and injected into a compressor  26 , as shown in  FIG. 2 . Compressor  26  works in conjunction with a dryer  28  to remove moisture in the rich natural gas. A suitable dryer may be obtained from Xebec of Montreal, Canada. Compressor  26  is used to compress and cool rich natural gas captured from well site  10  with the goal of obtaining a single densified state that is enhanced by the presence of heavier hydrocarbons. A possible path across through the phase diagram is shown in  FIG. 5 . Gas at about 20 PSI and 60° F. is input into compressor  26 . The gas is then compressed to about 150 PSI. As the compression will cause the temperature to rise to about 210° F., the gas is then cooled to about 100° F. The cooling element in this example uses ambient air to cool the gas. This allows the compressed gas to be cooled to no more than ambient temperature. The present example assumes that the compressed gas temperature will be cooled to around 10 to 20° F. of ambient summer temperatures, estimated at 80 to 90° F. During other seasons, it may be possible to cool the gas further, or it may be necessary to use a refrigeration unit and a coolant to achieve the necessary temperatures. During the second and third stages of compression, the gas may exit the phase envelope, but is cooled into the phase envelope  25 . The path is defined by the behavior of a modeled gas that is first compressed, then cooled. The actual path may have more or fewer cycles, and may have different intermediate pressures and temperatures, depending on the composition of the gas, the requirements of the equipment and the preferences of the user. As is apparent, heavier hydrocarbons will fall out during the compression and cooling cycle, the majority of which will occur during the cooling process. Along the depicted path, these condensates will typically be composed of C3 and heavier hydrocarbons, although there may be some C2 as well. As a single dense phase is desired, the condensates are removed from compressor  26 . However, heavier hydrocarbons help achieve a single dense phase, and also have economic value. Accordingly, some or all of the condensates are recycled to a point before the compressor  26 . As depicted on  FIG. 1 , the condensates are injected at a point indicated by arrow  30 , upstream of treater  16 . Because treater  16  is at a lower pressure, at least a portion of the condensates are flashed. Some of the heavier hydrocarbons, primarily C5+, may not be flashed simply by reducing the pressure. These heavier hydrocarbons may be encouraged to flash, such as by heating, or exposing them to even lower pressure. They may also be deposited into oil tank  20  for delivery with the other liquid hydrocarbons. As these heavier hydrocarbons are valuable, it may be desirable to encourage the C5+ condensates not to flash. On many well sites, a treater  16  will not be available. In these circumstances, a different low pressure tank may be used, such as the production tank itself, where any liquids that do not flash are immediately deposited with the other liquid hydrocarbons. 
         [0017]    Referring again to  FIG. 5 , compressor  26 , illustrated in  FIG. 2 , may achieve the desired final pressure and temperature using a three-stage compression and cooling approach. The final temperature and pressure is preferably outside the phase envelope  25 , which ensures that the hydrocarbon mixture will remain as a gas as during storage or transportation. Once the gas is stored in a pressure vessel, only the temperature may be changed by external conditions. Preferably, the location of the gas on the phase diagram is such that any increase or decrease in temperature will not cause the gas to re-enter the phase envelope  25 , which could cause condensates to form. The box  29  on the phase diagram represents a preferred operating range, based on the composition of the gas. The operating range is preferably outside the phase envelope, and preferably maximizes the densifying effect, but still have reasonable pressures and temperatures. Operating outside the envelope may be necessary in order to ensure liquid does not fall out of the gas during transport, as transporting two phases is against regulations in some jurisdictions. The optimal range will be determined in each situation, such as for each well. The gas composition is preferably monitored to make any necessary adjustments. 
         [0018]    In another example, the target temperature may be well below ambient temperature, which results in a lower pressure and higher densification. For example, the target temperature may be as low as 0 to −40° F. The final temperature and pressure will be selected based on the composition of the gas being compressed and its phase envelope, and will be selected based on the cost of additional cooling and compression and the benefit of additional compression and densification. For example, richer gases (gas with a larger component of heavier hydrocarbons) reach a densified state at a higher temperature than leaner gases. Accordingly, it may be sufficient to use a cooler that uses ambient air as described in the example above, which is less expensive to run than a refrigeration unit, which would be necessary to achieve colder temperatures. In addition, it is less economically viable to cool smaller volumes. 
         [0019]    It will be understood that the phase envelope  25  of the compressed rich natural gas will not necessarily be the same as the phase envelope  25  defined by the original rich natural gas. As heavier hydrocarbons fall out, particularly if they are not all recycled back to compressor  26 , the phase envelope  25  will shift toward that represented by dotted line  27 . Dotted line  27  is merely an example, as the actual phase envelope  25  will depend on the composition of the gas. 
         [0020]    It has been found that this recycling process does not create a build-up of C3 and C4 hydrocarbons at the inlet of compressor  26 . It is suspected that one reason for this is that the path chosen does not cause all C3+ hydrocarbons to fall out, based on the limited range in which the compression and cooling stages occurs. 
         [0021]    Referring again to  FIG. 2 , once suitably compressed by compressor  26 , the gas is loaded into a pressure vessel  32 , such as may be carried by a transport truck, via a loading station  34 . Pressure vessel  32  may be, for example, a multi-element gas container, also called a “tube trailer.” These types of vessels generally have a capacity of between 150 L to 3000 L (water volume), but other volumes may be designed for if desired. While the description above refers to the gas being compressed to its final pressure and temperature, this is not done from the beginning. Instead, the output of compressor  26  is substantially the same as the pressure of pressure vessel  32 . By “substantially the same,” it will be understood that the pressures are in the same range in order to avoid significant pressure drop. However, some pressure differential is necessary in order to have the pressurized gas flow into pressure vessel  32 , and to account for the increase in pressure that will occur as the gas is loaded. Referring to  FIG. 3 , pressure vessel  32  includes a dip tube  33  that extends to the bottom of vessel  32 . By opening a valve connected to dip tube  33 , the pressure in pressure vessel  32  will force any liquid out of dip tube  33 . Accordingly, it is preferred to have pressure vessel  32  at a slight angle, such that the liquids accumulate at dip tube  33 . 
         [0022]    Referring to  FIG. 2 , once pressure vessel  32  is filled, it is transported to an unloading station  35  and offloaded. Unloading station  35  may be located at a processing plant, an access point to a natural gas pipeline or at the end user directly. A pressure reducing station  36  may be used to obtain a low pressure output as is known in the art. 
         [0023]    In one example, the pipeline at unloading station  35  may have a pressure of 250-300 PSI, resulting in an “empty” pressure of 350-400 PSI for pressure vessel  32 . The pressure in pressure vessel  32  after unloading will vary depending on the pressure at unloading station  35 . Some compressors  26  may be limited in their minimum output to, for example, 600 PSI. Accordingly, the compressed gas will experience a pressure drop as it is loaded into pressure vessel  32 , which may result in condensates forming. This may be avoided by only emptying pressure vessel  32  to match the minimum pressure that can be achieved by compressor  26 , or using a compressor with a lower minimum output. However, another solution is to remove any condensates via dip tube  33 , illustrated in  FIG. 3 , until the pressure in pressure vessel  32  is matched to the output of compressor  26 . The condensates may be recycled as described above. Once the minimum pressure output of compressor  26  is substantially the same as pressure vessel  32 , the output pressure follows the pressure increase in pressure vessel  32  as progresses toward the target temperature and pressure. Referring to the phase diagram in  FIG. 5 , this means that compressor  26 , illustrated in  FIG. 2 , will only pass through the first two stages of the path initially and only partially through the third stage until toward the end of the filling process. Referring to  FIG. 2 , it is preferred that any cooling occur in compressor  26  as this is more likely to result in condensates forming, and liquids are not desirable, and in some cases not permitted by regulation, in pressure vessel  32 . However, as described above, it is possible to remove condensates from pressure vessel  32  if necessary during loading. While the path followed by compressor  26  involves changes in both pressure and temperature, the path in pressure vessel  32  will preferably minimize any temperature changes to also minimize the formation of condensates, which generally occur as the result of a temperature or pressure drop. 
         [0024]    Referring again to  FIG. 3 , pressure vessel  32  is preferably a tube trailer made up of multiple tubes  38  that can be loaded and unloaded through valves  40  at either or both ends. This is particularly useful in unloading, as it can accelerate the unloading process. During unloading, a pressure drop is occurring, such that condensates are likely to fall out of the compressed gas. In order to prevent a slug of liquid, it is preferred that the liquid be removed continuously from pressure vessel  32 , which involves removing liquid through dip tube  33 . Generally speaking, the amount of liquid falling out of the gas will be the limiting factor in the ratio of gas removed from either end of tubes  38  in order to ensure the liquid is continuously removed. 
         [0025]    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 element is present, unless the context clearly requires that there be one and only one of the elements. 
         [0026]    The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. Those skilled in the art will appreciate that various adaptations and modifications of the described embodiments can be configured without departing from the scope of the claims. The illustrated embodiments have been set forth only as examples and should not be taken as limiting the invention. It is to be understood that, within the scope of the following claims, the invention may be practiced other than as specifically illustrated and described.