Abstract:
A system and method for producing a low specific gravity liquid from underground storage. The system includes a subterranean cavern and a sump in fluid communication with a floor of the cavern. A bottom intake ESP system is deployed to draw liquid from the sump to fully drain the cavern without vaporizing the liquid at the pump intake.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the transfer of liquids having a low specific gravity and a low vapor pressure, such as liquid natural gas, and particularly to an electric submergible pumping system that may be utilized in pumping such liquids from subterranean storage caverns without vaporizing the liquid. 
     BACKGROUND OF THE INVENTION 
     Currently, certain substances having a low specific gravity and a low vapor pressure are stored in underground, i.e. subterranean caverns, in liquid form. Exemplary substances are liquid natural gas (LNG), isobutane and propane. Liquid natural gas, for example, may be maintained in a liquid state at approximately 60° F. under a pressure of approximately 30 psi. Thus, placing such substances in large, subterranean caverns provides a way to maintain large quantities of the substance in liquid form during storage. 
     An exemplary subterranean, storage cavern may be formed by washing away the salt from a natural salt mine to create a cavern. The substance, such as liquid natural gas, is then pumped into the storage cavern and stored in this cool environment under sufficient pressure to maintain the substance in liquid form. A current method for producing the liquid from the storage cavern is to use brine to force the product from the cavern. This technique requires a separate pond, typically at the earth&#39;s surface, for storage of the brine when the cavern is full of the desired substance. A pump is utilized in moving the brine from the pond and into the cavern to force the liquid product from the cavern. The surface brine ponds are extremely expensive to build and maintain. 
     It would be advantageous to eliminate the use of brine introduced into the storage cavern for moving the low specific gravity liquid to a location at or above the surface of the earth. 
     SUMMARY OF THE INVENTION 
     The present invention features a method for producing a liquid that has a low specific gravity and is stored in a subterranean cavern. The method includes forming a sump adjacent a floor of a subterranean cavern in which a liquid having a low specific gravity is stored. The method further includes deploying an electric submergible pumping system to the sump, and locating a pump intake of the electric submergible pumping system in the sump. The intake is located at a sufficient distance beneath the floor such that the liquid in the sump has a head sufficient to prevent substantial vaporization of the liquid when exposed to a net positive suction head at the pump intake during operation of the electric submergible pumping system. 
     According to another aspect of the invention, a method is provided for producing a liquid from a subterranean environment. The liquid is of the type subject to vaporization at normal pressures and temperatures on the surface of the earth. Specifically, the method includes forming a subterranean storage cavern with a sump region extending downwardly from a lower floor. The method further includes storing a liquid in the subterranean storage cavern, and deploying an electric submergible pumping system such that it is in communication with the sump region. The electric submergible pumping system is used to draw fluid from the sump region through a pump intake. Additionally, the method includes locating the pump intake beneath the lower floor a sufficient distance to substantially eliminate vaporization of the liquid when subjected to a lowered pressure at the pump intake during operation of the electric submergible pumping system. 
     According to another aspect of the present invention, a system is provided for storing and producing a substance having a low specific gravity and a low vapor pressure. Low vapor pressure refers to a characteristic of the substance that tends to cause vaporization of the substance at normal temperatures and pressures along the surface of the earth. The system includes a storage cavern disposed in a subterranean environment. The storage cavern is designed to store the substance in a liquid form. The system further includes a sump region disposed in fluid communication with the storage cavern at a position generally beneath the storage cavern. Additionally, a pump is disposed in fluid communication with a pump intake. The pump intake is located in the sump region at a sufficient depth to substantially eliminate vaporization of the substance when exposed to a net positive suction head of the pump. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
     FIG. 1 is a front elevational view of an exemplary storage and production system, according to one embodiment of the present invention; 
     FIG. 2 is a front elevational view similar to FIG. 1 but showing an alternate embodiment; and 
     FIG. 3 is a cross-sectional view taken generally along the axis of the electric submergible pumping system illustrated in FIG. 2 to show various internal components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring generally to FIG. 1, a substance handling system  10  is illustrated according to a preferred embodiment of the present invention. System  10  provides for the storage of a substance in liquid form in a subterranean environment. Additionally, system  10  facilitates the production of such substance from the subterranean environment to a location at or above a surface  12  of a planet, e.g., the earth. 
     System  10  comprises a subterranean storage cavern  14 , a sump region  16  and an electric submergible pumping system  18 . Sump region  16  is in fluid communication with storage cavern  14 . Similarly, electric submergible pumping system  18  is in fluid communication with sump region  16 , such that liquid may be drawn from sump region  16 . 
     Subterranean storage cavern  14  preferably is disposed within a geological formation  20 . Storage cavern  14  may be formed in a variety of ways including drilling, mining, blasting, utilization of existing caverns, or by washing away material, such as naturally occurring salt. Generally, cavern  14  includes a lower surface or floor  22 , an upper surface or ceiling  24  and a side wall  26  extending between floor  22  and ceiling  24 . A substance  28 , having a low specific gravity and a low vapor pressure, is stored in cavern  14 . Exemplary substances are liquid natural gas (LNG), isobutane and propane. Typically, a wellbore  30  extends between storage cavern  14  and surface  12 . Wellbore  30  is sealed by a cap  32  to prevent the escape of vaporized substance  28 . 
     Substance  28  is pumped into storage cavern  14  according to conventional methods utilized in moving such substances to underground storage caverns. When cavern  14  is filled, substance  28  is primarily a liquid  34 . However, the substance tends to vaporize and form a vapor  36  above liquid  34 . As liquid  34  is removed, a greater portion of cavern  14  is filled with vapor  36 . As explained above, vapor  36  typically is not allowed to escape. 
     Sump region  16  is in fluid communication with storage cavern  14 , and extends downwardly from floor  22 . Sump  16  may be formed by, for example, drilling, and it generally is axially aligned with wellbore  30 . Thus, when electric submergible pumping system  18  is deployed, it can be lowered into sump region  16 , as illustrated in FIG.  1 . An exemplary sump region is large enough to permit fluid to flow from storage cavern  14  into sump region  16  about the portion of electric submergible pumping system  18  that extends into sump region  16 . An exemplary sump region  16  is approximately 8 feet to approximately 32 feet deep. In other words, the sump extends approximately 8 feet to approximately 32 feet below floor  22 . 
     An exemplary electric submergible pumping system  18  for use in substance handling system  10  comprises a bottom intake electric submergible pumping system. Pumping system  18  includes a submergible pump  38  in fluid communication with a pump intake  40 . In the illustrated embodiment, submergible pump  38  is connected directly to pump intake  40 . Submergible pumping system  18  further includes a submergible motor  42  coupled to submergible pump  38  to provide power thereto. A motor protector  44  is disposed between submergible motor  42  and submergible pump  38 . Additionally, an expansion chamber  46  may be coupled to submergible motor  42 . It should be noted that the electric submergible pumping system components are listed to represent an exemplary electric submergible pumping system, and that a variety of other or additional components can be utilized in such submergible pumping systems. 
     Pumping system  18  is deployed in wellbore  30  by a deployment system  48  that may have a variety of forms and configurations. For example, deployment system  48  may comprise tubing, such as production tubing  50  or coil tubing, connected with submergible pumping system  18  at a connector  52 . Power is provided to submergible motor  42  via a power cable  54 . 
     In the illustrated pumping system, submergible pump  38  includes a liquid discharge  56 , and production tubing  50  includes a liquid inlet  58  Inlet  58  may be in the form of perforations through the outer wall of production tubing  50 . A shroud  60  extends from submergible pump  38  at a position beneath liquid discharge  56  to production tubing  50  at a point above liquid inlet  58 . Shroud  60  is sealed to pump  38  and production tubing  50  at its lower and upper ends. Additionally, shroud  60  preferably is concentric with the submergible pumping system components and sized to provide flow space between the components of electric submergible pumping system  18  and an interior surface  62  of shroud  60 . 
     In operation, submergible motor  42  drives submergible pump  38  which draws liquid from sump region  16  through pump intake  40 . This liquid is discharged through liquid discharge  56  and flows upwardly along the submergible pumping system components within shroud  60 . The liquid is forced into production tubing  50  through liquid inlet  58  and delivered upwardly through the production tubing to a location at or above surface  12 . The submergible pump  38  is able to maintain the liquid at sufficient pressure to prevent vaporization during the transfer, for instance, to the earth&#39;s surface. For example, the output of the pump and the outflow of liquid can be controlled to maintain the produced liquid at 150 psi above atmospheric pressure, 300 psi above atmospheric pressure, or greater depending on the type of substance being pumped. The higher pressures potentially allow the liquid to be pumped directly into a pipeline without the use of a transfer station. 
     In the preferred embodiment, pump intake  40  is located at a position in the sump region at a sufficient depth within the sump region to substantially eliminate vaporization of the substance  28  when exposed to a net positive suction head at pump intake  40 , even when the liquid is drained to cavern floor  22 . The net positive suction head is created by electric submergible pumping system  18 , and particularly submergible pump  38 , which draws liquid into pump intake  40  by reducing the internal pressure within submergible pump  38 . Thus, pump intake  40  must be located at a sufficient distance (labeled distance  64 ) beneath floor  22  to avoid vaporization of the low specific gravity liquid when liquid  34  is drawn down to floor  22 . In other words, the pressure applied on the liquid  34  as it enters intake  40  by the liquid head established in sump region  16  must be sufficient to prevent vaporization, i.e. flashing, of the liquid when exposed to the net positive suction head created by submergible pump  38 . 
     Without sump region  16 , submergible pumping system  18  cannot be used efficiently in the complete removal of liquid from storage cavern  14 . In the latter situation, as liquid  34  is drawn downwardly towards the lowest level at floor  22 , substance  28  tends to vaporize as its liquid level lowers to a point where the pressure applied by the liquid head is less than the net positive suction head created by pump  38  at pump intake  40 . Once the liquid vaporizes, the electric submergible pumping system  18  no longer functions, and components, such as submergible pump  38 , can be damaged. Consequently, a large volume of liquid cannot be pumped from the lower portion of cavern  14 . Sump region  16 , on the other hand, permits the liquid to be drained to floor  22 , leaving only a small volume of liquid in sump region  16 . 
     Preferably, pump intake  40  is placed at a location proximate the bottom of sump region  16 . In a typical example, pump intake  40  is placed approximately 1 foot to 3 feet above the bottom of sump region  16 . 
     As illustrated in FIG. 2, the net positive suction head created by electric submergible pumping system  18  can be decreased through the addition of an inducer  66 . An inducer  66  is coupled to or combined with submergible pump  38  and effectively lowers the net positive suction head created at pump intake  40 . Thus, for a given bottom intake electric submergible pumping system  18 , the net positive suction head can be lowered, and the depth of sump region  16  decreased. For example, inducer  66  may be designed to decrease the net positive suction head by approximately fifty percent. The depth of sump region  16  then also can be decreased by approximately fifty percent. 
     Liquids having low specific gravity, such as LNG, isobutane and propane, tend to have low lubricity. Accordingly, it is preferred to substitute standard electric submergible pumping system bearings with self-lubricating bearings that are made of, for example, graphite or carbon impregnated bronze. As illustrated in FIG. 3, a plurality of self-lubricating bearings  70  are disposed throughout submergible motor  42 , motor protector  44  and submergible pump  38 . Self-lubricating bearings  70  promote the longevity of electric submergible pumping system  18  when utilized in substance handling system  10 . 
     It will be understood, however, that the foregoing description is of preferred embodiments of this invention, and that the invention is not limited to the specific forms shown. For example, a wide variety of subterranean caverns may be utilized; a variety of submergible pumping systems and pumping system components may be used; and various substances may be stored. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims.