Abstract:
A vacuum system is disclosed for reducing heat loss within the annular chamber of a steam injection well. The apparatus includes elements for drawing vapor within the annular chamber outside the well, elements for condensing the vapor and elements for discharging the fluid. In addition, the apparatus incorporates elements to isolate and bypass the vacuum and condensing means in the event of high-steam leakage that may damage any components thereof.

Description:
INTRODUCTION 
     The present invention relates to a vacuum system for reducing heat loss from the well annulus of a steam injection well wherein a system is utilized to draw vapor from within the annular chamber formed in a well between the well casing and a steam injection tubing above a thermal packer, condense the vapor and remove it as a liquid. 
     BACKGROUND OF THE INVENTION 
     In a typical steam injection well, steam is injected through a relatively small diameter uninsulated tubing string. This steam injection tubing string is run inside the well casing and forms an annular chamber therewith. Near the bottom of the steam injection tubing string is a thermal packer that seals the space between the tubing string and the well casing. This seal insures that steam is injected into the producing formation. The annular chamber is vented to the air to relieve any pressure build-up from steam leakage passed the packer or from any other sources of leakage. 
     Heat losses from steam injection wells result in significant added fuel costs. Typical heat losses from steam injection tubing are 400 to 600 BTU/hr. per foot of tubing. Commercially available insulated tubing can be used to reduce the heat losses to approximately 75 to 150 BTU/hr. per foot of tubing, however, insulated tubing is relatively expensive. 
     Filling the annular chamber with insulation can provide insulating value near that of commercially available insulated tubings, but a major cost reduction. To be effective, insulation must be kept dry. When insulation becomes wet, it loses much of its value as an insulator and may also lose its structural integrity. It is extremely difficult to maintain a dry state within a steam injection well because of steam leakage passed the thermal packer; condensation of the steam on surfaces kept cool by insulation wets the insulation. Also, ground water penetration into the well casing can and often does, occur. There is need, therefore, for a practical and efficient method for maintaining the structural integrity and insulating value of insulation used in a steam injection well. 
     SUMMARY OF THE INVENTION 
     The present invention provides a practical method of establishing and maintaining a vacuum to preserve or dry insulation. If a partial vacuum is maintained such that condensation of water cannot occur on the coldest surfaces of the annular chamber, the water can migrate out of the well as a vapor and be condensed and disposed of outside the well. 
     The present invention is directed to a vacuum system for reducing heat loss. It is utilized to draw the vapor from the annular chamber and condense and dispose of the vapor outside the well. The apparatus includes elements for drawing the vapor outside the annular chamber, elements for condensing the vapor into a fluid and elements for discharging the fluid. Elements are incorporated to protect the vacuum and condensing components by isolating and bypassing them in the event of excessive steam leakage. 
     PRINCIPAL OBJECT OF THE INVENTION 
     The particular object of the present invention is to remove and discharge vapor from the annular chamber of a steam injection well. This would serve to effectively reduce heat loss from the steam injection well by allowing the introduction of insulation into the annular chamber and maintaining the insulation in a dry state. Also, insulation that becomes wet may be dried by utilizing the vacuum system to remove and discharge fluid within the annular chamber. 
     Additional objects and advantages of the present invention will become apparent from a detailed reading of the specification and drawings which are incorporated herein and made a part of this invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an elevation view, partially in section, illustrating the preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an elevation view, partially in section, of a well 1 penetrating an oil-bearing formation 2. A well casing 3 extends from the earth&#39;s surface 4 to the oil-bearing formation 2. A portion of the well casing 3 adjacent the oil-bearing formation 2 may contain producting liner 5, including perforations or slots, to permit oil from the oil-bearing formation 2 to flow into the well 1. A wellhead 6 or other suitable sealing means forms an air-tight seal at the upper portion of the well casing 3. 
     A steam injection tubing 7, sealed in air-tight relationship at the wellhead 6, extends through the wellhead 6 and laterally along the well casing 3, terminating adjacent the oil-bearing formation 2. At the earth&#39;s surface 3 a steam generator and injector control 8 connected through a pair of valves 9 and 10 to the steam injection tubing 7 generate and control the injection of steam through the steam injection tubing 7. Thermal packer 11, located above the oil-bearing formation 2, closes off the space between the steam injection tubing 7 and well casing 3, thereby forming a substantially annular chamber 12 between the wellhead 6 and said thermal packer 11. The substantially annular chamber may be partially or totally filled with insulating material 13. 
     Vacuum inlet tube 14 connected to the well casing 3 and communicating with the substantially annular chamber 12 extends through a condensing means 15 to a vacuum producing means 16, including a vacuum pump 30, vacuum pump tank 31 and vacuum outlet tube 32. A vacuum of twenty-eight inches mercury is preferred, the minimum and maximum values being twenty-five inches mercury and twenty-nine inches mercury respectively. The condensing means 15 contains a condensing drain outlet 18 for discharging the condensed vapor therethrough. A second vacuum producing means 24, including a drainage drop tube 17 connected to the condensing drain outlet 18 and extending to and submerged in a partially filled drain tank 33 below, procedures a second partial vacuum at the condensing drain outlet 18 having a pressure at most equal to the pressure of the first partial vacuum connected to the condensing drain outlet 18. 
     A shutoff means 34 closes off the vacuum inlet tube 14 upstream of the vacuum producing means 16 and condensing means 15 in the event pressure in the vacuum inlet tube 14 exceed a predetermined value. The predetermined value ranges from a minimum pressure of five psig to a maximum pressure of thirty psig; fifteen psig being optimum. The shutoff means 34 consists of an emergency automatic shutoff control valve 20 and a critical flow orifice 21, both in the vacuum inlet tube 14 and upstream of the condensing means 15 and the vacuum producing means 16. The critical flow orifice 21 is located upstream of the emergency automatic shutoff control valve 20. 
     Emergency pressure relief pipe 22 is connected to and communicates with the vacuum inlet tube 14 upstream of the emergency automatic control valve 20. Emergency pressure relief pipe 22 is sealed with a rupture disk 23 to maintain the partial vacuum within the substantially annular chamber 12 during normal operation. Ruptured disk 23 bursts when pressure in the emergency pressure relief pipe 22 exceeds a second predetermined value. The second predetermined value is equal to or slightly greater than the predetermined pressure value used to activate the shutoff means. 
     The vacuum system reduces heat loss from a steam injection well. Vapor drawn from within the annular chamber is condensed and discharged outside the well. Condensation within the annular chamber is virtually eliminated. This by itself will reduce heat loss, but the ability to introduce and maintain insulation within the annular chamber significantly increases the well&#39;s efficiency. 
     The following vacuum system for reducing heat loss from steam injection tubing has been implemented at well 7-2W, Section 27, Kern Front. Demonstrations have clearly shown the feasibility, economy and successful operation of the vacuum system. 
     In accordance with the present invention, a well was drilled into a subsurface formation and casing was placed into the well to prepare the formation for production. The casing may be perforated or provided with slotted liners in those areas of the subsurface formation where production is expected. The formations are treated in many and several different manners to prepare them for injection of hot fluids or steam. 
     A tubing string was passed down into the casing with its lower end aligned with the formation where the steam was to be injected. A thermal packer was placed above the formations to be treated to insure that the steam that was injected down the tubing string was retained in the area where the heavy gravity crude is located. The outside of the tubing string was anchored in the packer and the packer substantially prevented the injected hot fluids from flowing upwardly through the annular chamber between the thermal packer and wellhead. 
     The tubing string is usually centralized within the casing to prevent heat loss from the tubing string into the casing. The centralizers are usually constructed with a low heat conducting material to improve the efficiency of the system. 
     To prevent heat loss passed the thermal packer and from the steam injected tubing string, the annular chamber is filled with insulation. This insulation must be kept dry or it loses much of its value as an insulator. 
     By establishing a partial vacuum within the annular chamber such that condensation cannot occur on the coldest surfaces in the annular chamber, the water may pass as a vapor out of the well and be condensed and discharged outside the well. Vacuum drying of this type has been applied to food processing and packaging operations, but its application to well insulation has not yet been apparent. The primary components include a vacuum pump, vacuum pump tank and vacuum exhaust, a condensing means, a condensing drainage means and shutoff means to isolate the vacuum and condensing components in the event of high-steam leakage rates; as may occur with a steam injection tubular or thermal packer failure. The vacuum pump serves to establish and maintain a vacuum in the annular chamber. A condensing means located upstream of the vacuum pump minimizes pump on time by removing vapor as a liquid. The liquid may then be removed from the condensing means by creating a second partial vacuum at a condensing drain outlet at least equal to the vacuum passing through the condensing means. One method of creating this second partial vacuum includes a drainage drop tube connected to the condensing drain outlet and extending to and submerged in a partially full drain tank below. The drain tank must be a vertical distance below the condensing system drain outlet at least equal to that of the vacuum passing through the condensing means. Another method of achieving the required vacuum at the condensing system drain outlet would be to attach a second vacuum pump to the condensing system drain outlet or anywhere along the drainage drop tube. 
     The shutoff means protects the vacuum and condensing components by closing a control valve in response to excessive pressure upstream of a critical flow orifice within the vacuum inlet tube. A rupture disc prevents backflow into the vacuum inlet tube and annular chamber by bursting under excessive pressure and venting the system. 
     While a certain preferred embodiment has been specifically disclosed, it should be understood that the invention is not limited thereto as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.