Abstract:
A method of recovering liquid petroleum gases (LPGs) from a wellbore includes: performing a well treatment operation by injecting the LPGs into the wellbore to increase the wellbore pressure; flowing a fluid stream from the wellhead into a separation vessel, the fluid stream comprising the LPGs; reducing the pressure of the fluid stream from the wellbore pressure to a separation vessel pressure, the fluid stream in the separation vessel comprising the LPGs in liquid form and in vapour form; separating the vapour form from the liquid form; transferring the liquid form of the LPGs to a pressure vessel; and passing the vapour form through a condenser to condense the vapour form, and depositing the condensed vapour form into the pressure vessel.

Description:
FIELD 
     This relates to a method and apparatus for capturing LPGs using a separator vessel. 
     BACKGROUND 
     Hydrocarbon producing wells are often stimulated in order to enhance production. A fracing operation is a common method of stimulating a well. Propane is among the many types of frac fluids that may be used in the fracing operation. Propane has the advantage of being pumped into the well in liquid or gel form, and exiting the well as a gas. GasFrac Energy Services Inc. of Calgary, Alberta uses a proprietary liquefied petroleum gas in fracing operations. Other frac fluids may include propane, butane, or mixtures thereof. 
     SUMMARY 
     There is provided a method of recovering liquid petroleum gases (LPGs) from a wellbore, the wellbore having a wellbore pressure, the method comprising the steps of: performing a well treatment operation by injecting the LPGs into the wellbore to increase the wellbore pressure; flowing a fluid stream from the wellhead into a separation vessel, the fluid stream comprising the LPGs; reducing the pressure of the fluid stream from the wellbore pressure to a separation vessel pressure, the fluid stream in the separation vessel comprising the LPGs in liquid form and in vapour form; separating the vapour form from the liquid form; transferring the liquid form of the LPGs to a pressure vessel; and passing the vapour form through a condenser to condense the vapour form, and depositing the condensed vapour form into the pressure vessel. 
     According to another aspect, the liquid petroleum gases may comprise at least 80% propane by weight, or at least 90% propane by weight, or at least 80% butane by weight, or at least 90% butane by weight. 
     According to another aspect, the separation vessel pressure may be less than 500 psi, or less than 200 psi, or less than 100 psi, or less than 75 psi. 
     According to another aspect, the well treatment operation may be a fracturing operation. 
     According to another aspect, passing the vapour form through a condenser may comprise filtering the condensed liquid petroleum gases to remove entrained water and solid particles prior to condensing the vapour form. 
     According to another aspect, the method may further comprise the step of analyzing the composition of the fluid stream, and activating a heater to heat the fluid stream between the wellbore and the separation vessel in order to vapourize the LPGs once the LPG component of the fluid stream drops below a predetermined threshold. 
     According to another aspect, the method of claim  10 , wherein analysing the composition of the fluid stream comprises analysing the vapour phase, the liquid phase, or both exiting the separation vessel. 
     According to another aspect, the method may further comprise the step of, once the heater is activated, transferring hydrocarbon liquids from the separation vessel to a production fluid facility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 block diagram of an arrangement for recapturing LPGs (liquid petroleum gases) pumped into a wellbore 
         FIG. 2  is a block diagram of an arrangement for recapturing LPGs with heaters 
         FIG. 3  is a detailed block diagram of an arrangement for recapturing LPGs. 
         FIG. 4  is a side elevation view of a separation tank. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown equipment that may be used in a method to capture LPGs used in a well treatment operation. During a wellbore treatment procedure, treatment fluid is pumped through a wellhead  12  into a wellbore  14 . In the examples discussed below, the treatment fluid is one or more type of LPGs (liquid petroleum gases). The LPGs are pumped into the wellbore  14  to cause the wellbore pressure to reach the desired treatment pressure. Once the well treatment operation has been concluded, it is desireable to recapture as much of the LPGs as possible such that they may be reused. 
     LPGs are a useful treatment fluid as they tend to avoid damage to the formation, and tend to mix well with the formation fluids. In this document, LPGs are hydrocarbons or hydrocarbon mixtures whose characteristics are selected that, at standard ambient temperature and pressure, they exist as a gas, and they exist as a liquid under conditions used for the well treatment operation. For example, a hydrocarbon or hydrocarbon mixture will be considered an LPG for the purposes of this patent document if it is a gas at standard ambient temperature and pressure (SATP) but is a liquid under when used in downhole operations. Examples of LPGs include propane, propane mixtures that include 80% propane or more, or 90% propane or more by weight, mixtures of propane and butane, butane, butane mixtures that include 80% butane or more, or 90% butane or more by weight, etc. Other LPGs may include ethane, isobutene, pentaines, etc., depending on the circumstances. The most common type of well treatment operation is a hydraulic fracturing operation, where fluid is pumped under pressure to apply a pressure that is greater than the formation fracture pressure in order to stimulate the well. The formation fracture pressure is the point at which a formation will crack. The actual pressure that needs to be applied will depend on many factors, such as the depth of the zone, the water depth, the air gap, the formation pore pressure, etc. Proppant, such as sand, is pumped down with the treatment fluid in order to keep the fractures open once the pressure is released. 
     Once the treatment operation is complete, the pressure must be relieved by flowing back the treatment fluid. It is important that the fluid not be flowed back too quickly, otherwise the formation fractures may become clogged due to sand that flows out with the treatment fluid. Referring to  FIG. 1 , when flowing back a well, a fluid stream is allowed to exit wellhead  12  through line  16 . As shown, the fluid stream exits wellhead  12  and passes through a pressure control device, such as a choke  18  and an emergency shut down valve (ESD)  20 . Choke  18  controls the pressure and flow rate of the fluid stream. For example, choke  18  may limit the pressure to between 50 psi and 200 psi. The actual pressure may be outside this range, and will depending on the characteristics of the well (i.e. to prevent the fractures from being filled with sand) and will also depend on the pressure ratings of equipment downstream of wellhead  12 . A preferred flowback pressure is between 65 psi and 100 psi, although it may be as high as allowed by the pressure vessels and the preferences of the user. ESD  20  is a safety device that will stop the fluid stream if there is a problem. As the LPGs are volatile fluids and are at high pressures, ESD  20  is an important safety feature. The system also includes a number of check valves  21  for safety reasons. 
     It has been found that the fluid stream is made up primarily of the LPGs used to treat the wellbore when the well is first flowed back. This allows the user to capture LPGs as they flow from wellhead  12  in liquid form as shown in  FIGS. 1 and 3 , and will be described below. As more fluids are flowed out of wellhead  12 , more contaminants will rise with the LPGs, making it necessary to separate the LPGs from these contaminants. While water and sand will not dissolve in the LPGs, crude oil and other hydrocarbons will dissolve in the LPGs, making it necessary to flash off the LPG&#39;s once the amount of undesireable hydrocarbons exceeds a certain threshold. 
     Referring to  FIG. 3 , line  16  is connected to a separation vessel  22 . Referring to  FIG. 4 , separation vessel  22  is a pressure vessel capable of withstanding up to the desired flowback pressure, i.e. the flowback pressure and the separation vessel pressure will be equivalent. In many circumstances, a pressure rating of up to 285 psi would be sufficient, although the pressure rating may be up to 500 psi, 200 psi, 100 psi or 65 psi. As can be seen, separation vessel  22  includes a first zone  24  with a first outlet  25 , a second zone  26  with a second outlet  27 , a third zone  28  with a third outlet  34 , and a fourth zone  30  (for vapour) with a fourth outlet  36 . Separation vessel also has an inlet  32  that depicts the fluid stream into first zone  24 . 
     Referring to  FIG. 1 , as mentioned above, choke  18  controls the pressure to less than 200 psi, and preferably around 100 psi or less. This represents a significant pressure drop from the pressure in wellbore  14 , where the pressure will be at the wellbore treatment pressure to begin with. The pressure drop across choke  18  will cause a drop in temperature for any vapours exiting wellhead  12 , such that the primary component of the fluid stream will be liquids. Referring to  FIG. 3 , these liquids enter separation vessel  22  via inlet  32 . As depicted, separation vessel  22  is in a configuration to capture LPGs in liquid form in the fluid stream. As the fluid stream is primarily LPGs with little contaminants at the beginning, this may be done by drawing liquids out of first zone  24  through first outlet  25  and into a holding tank  37  via line  35 . As there will be some LPGs in vapour state, these vapours are captured in fourth zone  30  and removed through line  33  via fourth outlet  36 . These vapours pass through a condenser stage  38 , through a flash tank  40  and into holding tank  37 . Condenser stage  38  may include a chiller  42 , a coolant holding tank  44 , and coolant pumps  46  that pump coolant through a heat exchanger  48  that is used to condense the LPG vapour. There may also be some sample catchers  50  that are used to analyse the components of the gas. Referring to  FIG. 1 , there may also be some filters, such as aerolecent and coalescent filters  52  and  54  that remove water and dust, sand or other solid particles from the LPG vapour stream. In addition, each of these tanks is preferably connected to a flare stack  56  as a safety measure and also to flare of any vapours that are not condensed at the end of the process. It will be noted that, in  FIG. 1 , the liquid LPGs are placed in a different holding tank, labelled  58 , than holding tank  37  as shown in  FIG. 3 . It will be understood that the ultimate destination of the recaptured LPGs is at the user&#39;s discretion, and may also include a pipeline or other capture/processing facility. 
     As noted above with reference to  FIG. 3 , there may be one or more sample collectors  50  that are used to analyze the composition of the LPG vapour stream through line  33 . This is used to determine whether the LPG stream is sufficiently pure to continue capturing liquid LPG directly from separation vessel  22 . Other analyzers may also be used, and may also be positioned in different locations, such as on line  16 , line  35 , or any other convenient location that will permit meaningful results. Once a certain threshold of hydrocarbon contaminants in the fluid stream is reached, the configuration of the equipment shown in  FIGS. 1 and 3  is changed to heat the fluid stream in order to flash off the LPGs. The threshold will depend on the desired quality of LPGs. For example, the threshold may be at 98% LPGs by weight, 95% LPGs by weight, 90% LPGs by weight, 85% LPGs by weight, or 80% LPGs by weight, or any percentage therebetween. The desired purity will depend on how the LPGs will be processed afterward and what they are intended to be used for. A higher purity may be achieved by activating the heaters earlier. 
     An example of a heating strategy is shown in  FIG. 2 , where line heaters  60  are included. As shown, line heaters  60  are used to heat the fluid stream as it flows out of wellhead  12  in order to cause the LPGs to flash off. The heated fluid stream enters separation vessel  22 . As shown in  FIG. 2 , line heaters  60  may be powered by LPGs that are collected from the fluid stream, and may also heat a coolant fluid that is pumped via pump  62  through a loop  64  in separation vessel  22  and also a loop  66  in flash tank  40 . The capacity of line heaters  60  will depend on the heating requirements. There should be sufficient heat to cause the LPGs to flash off from the fluid stream and to prevent freezing of any components. The amount of heating may be calculated by a person of ordinary skill based on the phase change characteristics of the particular LPG being recovered, the temperature of the fluid stream before heaters  60 , the pressure of the fluid stream, etc. 
     Once heaters  60  are in use, the operation of separation vessel  22  may change. For example, referring to  FIG. 4 , while liquid LPGs were previously withdrawn from first zone  24 , crude oil or other liquid hydrocarbons that have not flashed may be collected in this zone  24 . Alternatively, if there is a significant sand component and water component, zone  24  may be used to collect the sand, zone  26  may be used to collect the water, and zone  28  may be used to collect the crude oil, such that outlet  25  withdraws sand and other solids, second outlet  27  withdraws water and third outlet  34  withdraws oil. If there is no sand component, or no water component, zone  26  may also be used to collect the crude oil. The crude oil that is collected may be transferred to production tanks  68  as shown in  FIG. 2 . Throughout the process, vapour zone  30  is used to collect the LPG vapours, which are condensed using similar techniques to those discussed previously with respect to the vapour present when the liquid LPGs were recovered directly. 
     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. 
     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. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.