Abstract:
A method and apparatus for heating at least one fluid tank that receives production fluid comprising produced oil and a produced liquid from a hydrocarbon producing well. A layer of liquid is provided in the fluid tank that has a higher specific density and a higher thermal conductivity than the produced oil. The layer of liquid at least partially covers a heat trace positioned in the fluid tank. The heat trace transfers heat to the layer of liquid.

Description:
FIELD 
       [0001]    This relates to a method and an apparatus for heating a liquid storage tank, such as a sales tank on a well site. 
       BACKGROUND 
       [0002]    Most producing oil wells within the Provinces of Alberta and Saskatchewan are set up in a similar fashion. A drive head is positioned on a well head. A production flow line extends from the well head to a liquid storage tank, which is spaced a distance from the well head. A propane burner shoots flame into a fire tube in the liquid storage tank. An engine positioned in a wooden shack adjacent to the well head, provides motive force to the drive head. Propane tanks positioned adjacent to the liquid storage tank, provide a fuel source for the propane burner and the engine respectively. 
         [0003]    Referring to  FIG. 5 , there is illustrated a prior art well site, generally indicated by reference numeral  100 . Well site  100  includes a liquid storage tank  12  having an interior  14 , a peripheral sidewall  16  and a heat tube  17 . A primary propane tank  20  provides fuel to a burner  21 . A secondary propane tank  23  provides fuel to an engine  24  that is housed separate and apart from liquid storage tank  12 . Hydraulic flow lines  26  driven by hydraulic pump  48  extend from engine  24  to drive head  28 . A production line  30  extends from a well head  32  to liquid storage tank  12 . 
         [0004]    U.S. Pat. No. 7,726,298 (St. Denis) entitled “Method and apparatus for heating a liquid storage tank” describes an apparatus for heating a liquid storage tank. Referring to  FIG. 1 , the apparatus, generally indicated by reference numeral  10 , includes a liquid storage tank  12  having an interior  14 , a peripheral sidewall  16 , and an engine compartment  18  appended to liquid storage tank  12 . Referring to  FIG. 2 , primary propane tank  20  provides fuel through fuel line  22  to engine compartment  18 . Referring to  FIGS. 1 and 3 , an engine  24  is disposed within engine compartment  18 . Referring to  FIG. 2 , hydraulic flow lines  26  extend from engine compartment  18  to drive head  28 . A production line  30  extends from well head  32  to liquid storage tank  12 . Hydraulic flow lines  26  are in a substantially parallel orientation and in close proximity to production line  30 . Referring to  FIG. 3 , engine  24  has exhaust manifold  34  which is connected to an exhaust conduit  36  which extends from peripheral sidewall  16  into interior  14  of storage tank  12 . Exhaust conduit  36  is depicted as a substantially horizontal loop that, upon exiting peripheral sidewall  16 , is adapted with a muffler  38 . Exhaust conduit  36  is further adapted with interior baffles  40 . An engine coolant conduit  42  extends from engine  24 . A thermostatically controlled valve  44  is positioned along engine exhaust conduit  36 . From thermostatically controlled valve  44 , engine coolant conduit  42  extends below exhaust conduit  36 . Referring to  FIGS. 3 and 4 , engine coolant conduit  42  is depicted as being positioned below the exhaust conduit  36  and proceeds horizontally below the exhaust conduit  36  and returns to engine  24 . Referring to  FIG. 3 , in addition, engine coolant conduit  42  may extend to a booster pump  46  that also returns engine coolant to engine  24 . Depending upon the orientation of thermostatically controlled valve  44 , the flow of heated engine exhaust may proceed by either the coil or by pass route. Engine  24  is further adapted with hydraulic pump  48  that provides hydraulic pressure to hydraulic flow lines  26 . 
         [0005]    Referring to  FIG. 3 , engine  24  is operated within engine compartment  18 . Referring to  FIG. 2 , fuel for the operation of engine  24  is provided by propane tank or casing gas.  20 . No other source of fuel is needed. The noise of engine  24  is dampened by muffler  38  and absorption of sound by liquid storage tank  12  through peripheral sidewall  16 . Heat  50 , given off from engine  24  during operation, heats engine compartment  18  and such heat is transferred through peripheral sidewall  16  to interior  14  of liquid storage tank  12 . Hot exhaust gases  52  from engine  24  pass through exhaust conduit  36  and heat  50  given off heats interior  14  of liquid storage tank  12 . Interior baffles  40  disrupt the linear flow of hot exhaust gases  52  to more evenly distribute and transfer heat from hot exhaust gases  52  through exhaust conduit  36  to interior  14  of liquid storage tank  12 . Referring to  FIG. 4 , in the illustrated embodiment, engine coolant conduit  42  is disposed below the exhaust conduit  36 . Heated engine coolant passes through engine coolant conduit  42  such that heat  50  is added to further transmit heat  50  to interior  14  of liquid storage tank  12 . Referring to  FIG. 3 , for operation in warmer conditions, thermostatically controlled valve  44  diverts heated engine exhaust, to the bypass route. Allowing 50% of the engine heat out of the  14  tank interior, the heated engine coolant is cooled and, in turn, moderates the temperature of interior  14  of liquid storage tank  12 . Referring to  FIGS. 1 and 2 , apparatus  10 , by configuring hydraulic lines  26  in a substantially parallel orientation and in close proximity to production line  30 , production fluid within production line  30  is also heated. 
         [0006]    Referring to  FIGS. 6 through 8 , in some circumstances, the fluids held in liquid storage tank  12  contain suspended solids, or solids are transported with the fluids as they are deposited in liquid storage tank  12 , such as sand. As the liquids are stored, the solids settle out and come to rest on the bottom of tank  12 . When this is the case, the fluids stored in storage tank  12  can be more effectively heated by raising engine compartment  18  relative to the bottom of liquid storage tank  12 , such that at least a portion of the engine compartment is inset within the periphery of the storage tank  12  at a higher position than would otherwise be the case. As it is raised, it may be necessary to include a floor  54  with rails  56 , a support  58 , and stairs  60  shown in  FIG. 7 . Referring to  FIG. 8 , compartment  18  may also have a vent  62 , and an engine guard  65  to prevent individuals from entering the compartment during operation. 
       SUMMARY 
       [0007]    According to one aspect there is provided an apparatus for heating at least one fluid tank that receives production fluid comprising produced oil and a liquid from a hydrocarbon producing well. A layer of liquid that has a higher specific density and a higher thermal conductivity than the produced oil is positioned in the at least one fluid tank. A heat trace is positioned within the fluid tank at least partially in the layer of liquid and transfers heat to the layer of liquid to heat the tank. 
         [0008]    According to another aspect, a method of heating at least one fluid tank that receives production fluid comprising produced oil and produced liquid from a production tank on a hydrocarbon producing well includes the steps of: providing a heat trace in the fluid tank and providing a layer of produced liquid in the at least one fluid tank which submerges at least a portion of the heat trace and which has a higher specific density and a higher thermal conductivity than the produced oil. 
         [0009]    The at least one fluid tank may be at least one sales tank that receives fluid from a production tank. The heating of the sales tank including the steps of transferring oil from the production tank to the at least one sales tank and causing the heat trace to heat the layer of liquid in the sales tank. 
         [0010]    The heat trace utilized in the heating apparatus and method may include a coolant heated by an internal combustion engine being circulated through tubing in the layer of liquid. The layer of liquid may comprise primarily of water. The internal combustion engine may also be used to drive a wellhead pump. The production tank may also be heated by the same internal combustion engine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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 in any way to be limiting to the particular embodiment or embodiments shown, wherein: 
           [0012]      FIG. 1 , labelled as PRIOR ART, is a side elevation view of a well site utilizing an apparatus for heating a liquid storage tank. 
           [0013]      FIG. 2 , labelled as PRIOR ART, is a top plan view of the well site utilizing the apparatus illustrated in  FIG. 1 . 
           [0014]      FIG. 3 , labelled as PRIOR ART, is a top plan view of the liquid storage tank illustrated in  FIG. 1 . 
           [0015]      FIG. 4 , labelled as PRIOR ART, is a front elevation view of the liquid storage tank illustrated in  FIG. 1 . 
           [0016]      FIG. 5 , labelled as PRIOR ART, is a top plan view of a prior art well site. 
           [0017]      FIG. 6 , labelled as PRIOR ART, is a side elevation view of a well site utilizing a variation of the apparatus illustrated in  FIG. 1 . 
           [0018]      FIG. 7 , labelled as PRIOR ART, is a top plan view of the well site using the variation illustrated in  FIG. 6 . 
           [0019]      FIG. 8 , labelled as PRIOR ART, is a front elevation view of the variation illustrated in  FIG. 6 . 
           [0020]      FIG. 9  is a top plan view of the liquid storage tank illustrated in  FIG. 1  in fluid communication with a second tank. 
           [0021]      FIG. 10  is a side elevation view of the liquid storage tank and second tank in fluid communication shown in  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    The apparatus described below is an improvement on the apparatus for heating a sales tank described above with reference to  FIGS. 1 through 8 . The preferred embodiment, a variation in which heat is more effectively transferred, and in which a sales tank may also be heated, is described below with reference to  FIGS. 9 and 10 . 
         [0023]    Referring to  FIGS. 9 and 10  engine  24  may be used to heat a sales tank. In the depicted drawing, sales tank is identified using reference numeral  60  and reference numeral  12  is used to identify a production tank. Generally, the first tank to receive the production fluid is referred to as the “production tank.” If a second tank is connected to receive the overflow from the production tank, this is generally referred to as the “sales tank.” This terminology is used as the fluid is generally cleaner in the sales tank, having had more time to separate the multiple phases, and when a fluid load is taken from the well site, it is commonly taken from this tank because it is cleaner. However, while only one sales tank  60  is shown, it is not uncommon to have more than one sales tank on a well site, and it will be understood that the techniques described below may be used to heat more than one sales tanks  60 , if more than one sales tank is present, and if there is sufficient heat to do so. This will depend on the preferences of the user and the heating requirements compared to the available heat. 
         [0024]    Produced oil moves from production tank  12  to sales tank  60  by transfer pipe  61 , and is removed from sales tank  60  by a riser  63 . In the preferred example depicted in  FIGS. 9 and 10 , the heat produced by engine  24  is used to heat both production tank  12  and sales tank  60 . In this example, production tank  12  is equipped with engine compartment  18  and is heated as described previously, and sales tank  60  is heated using excess heat not required in production tank  12 , as described below. However, it will be understood that variations may also be used. For example, production tank  12  may not have an engine compartment  18 . Alternatively, only production tank  12  or sales tank  60  may be heated. 
         [0025]    Referring to  FIGS. 9 and 10 , sales tank  60  is connected to receive fluids from production tank  12 . Generally speaking, a rough separation of phases occurs in production tank  12 , with sand  68  falling to the bottom, and oil  70  floating to the top of the water portion  64 . As the oil reaches the height of transfer pipe  61  of production tank  12 , oil flows into sales tank  60 , which helps improve the quality of fluids that are transported from the well site. As with fluids in the production tank  12 , the fluids in sales tank  60  are also generally kept warm. However, the heating requirements in sales tank  60  are not as high, as the fluids were pre-warmed in production tank  12 , production and sales tanks  12  and  60  are often insulated, and the temperature does not have to be maintained at as high a temperature as in production tank  12 . 
         [0026]    Referring to  FIG. 10 , as shown, a fluid line  67  is positioned within sales tank  60 . This fluid line  67  is connected to engine  24 , such that heated engine coolant, such as glycol, is circulated through fluid line  67 . Fluid line  67  has isolation valves  72  positioned at points along fluid line  67  to alter the course of heated engine coolant by either allowing it to flow through fluid line  67  or forcing heated engine coolant through a bypass  74  to prevent it from being used to heat sales tank  60 . It will be apparent that other methods of heating the fluid in fluid line  67  may also be used. For example, fluid line  67  may circulate a heat transfer fluid in a closed circuit that is heated in a heat exchanger connected to engine  24 . In addition, there may be separate fluid circuits for tanks  12  and  60 , where one or both is heated using a heat exchanger. 
         [0027]    To assist in the transfer of heat to the fluid in either production tank  12  or sales tank  60 , a layer of liquid  64  is positioned below the oil layer  70 , such that at least a portion of fluid line  67 , and preferably all of fluid line  67 , is submerged within layer of liquid  64 . Layer of liquid  64  is maintained at the height required to ensure a sufficient portion of fluid line  67  is submerged. In sales tank  60 , engine compartment  18  is preferably also submerged within layer of liquid  64 . Layer of liquid  64  has a higher density than the oil expected to be produced from the well, and has a higher thermal conductivity than the oil as well. Layer of liquid  64  is maintained at a level to ensure that a sufficient portion of fluid line  67  is kept submerged. In one example, a depth of five feet at the bottom of sales tank  60  is found to be sufficient. Other depths may be used depending on the preferences and requirements of the user, and the tank specifications. The appropriate level is maintained by providing a riser pipe  66  used to draw off water that extends to the desired height, which prevents drawing layer of liquid  64  down below the desired height accidentally. Another riser pipe  63  extends above riser pipe  66  and is used to withdraw oil from oil layer  70 . The valves  71  for riser pipes  63  and  66  are preferably located conveniently at a bottom of the tank, such as in an ENVIROVAULT™. While not shown, similar standing pipes and valves are preferably provided on sales tank  12  as well. Tank  60  is also shown with a sand removal port  76  that is used to remove sand  68  from the bottom of tank  60 . 
         [0028]    Fluid line  67  is preferably made from coil tubing with a 1″ to 2″ diameter. Preferably a minimum of 50 feet of tubing is maintained within layer of liquid  64 . The minimum length may also be 100 feet, 150 feet, 200 feet, or more. The minimum length of coil tubing that is kept submerged will depend on the size of the tubing and the heating requirements in the tank. In one example that provided adequate results, 200 feet of 1″ tubing was kept in layer of liquid  64 . 
         [0029]    In the event that the heat from engine  24  is insufficient to maintain the desired temperature, other sources of heat may be provided. For example, an additional heat source may be provided within the tank. Referring to  FIG. 10 , an electric coil  78  may be positioned within oil layer  70  as shown, or within liquid layer  64 . Electric coil  78  is preferably equipped with a thermostat  80  that activates electric coil  78  in the event that heated engine coolant flowing through fluid line  67  is not sufficient to maintain an appropriate temperature of oil  70 . Electric coil  78  may be powered by an on-site power source, such as a generator, or by an alternator connected to engine  24 . Alternatively, an external heat source (not shown) may be provided that heats the fluid in liquid line  62  before it passes into the relevant tank. 
         [0030]    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. 
         [0031]    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.