Submersible chemical injection pump
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to a submersible centrifugal well pump assemblies, and in particular to an assembly that includes also a downhole secondary pump for injecting scale inhibiting chemicals. 
     2. Description of the Prior Art 
     In a conventional well having a centrifugal pump assembly, the well will be cased and will have a string of tubing extending downward to the downhole pump assembly. The pump assembly includes a centrifugal pump mounted above an electrical motor. A seal section mounted between the pump and the motor protects against the entry of well fluid into the motor. Electrical power is supplied by cables extending to the surface. The pump has an intake on its lower end and discharges into the tubing. 
     In certain fields, scale deposition on the downhole equipment is a serious problem. Mineral scale depositing on the submersible pump assembly can lead to extensive damage. One prior technique used to inhibit the deposition of scale on the equipment is to introduce chemicals into the annulus between the tubing and the casing at the surface. The chemicals will flow downwardly in the annulus into the intake of the pump and back up the tubing. This retards the deposition of scale on the equipment from the intake of the pump inwardly. However, it will not prevent scale deposition below the intake of the pump, and the motor and seal section are located below the intake of the pump. 
     SUMMARY OF THE INVENTION 
     A secondary pump is incorporated into the downhole pump assembly for injecting chemicals below the motor. The secondary pump is mounted between the seal section and the primary pump. The secondary pump has an intake connected to an intake tube that extends upwardly above the intake of the primary pump. Chemicals introduced into the annulus at the surface will flow down the annulus and into the intake tube. The secondary pump has a discharge port connected to a discharge tube that leads to a point below the motor. The chemicals drawn into the intake of the secondary pump from the annulus are pumped downwardly out the discharge tube. The chemicals flow upwardly around the motor and back into the intake of the primary pump. 
     The secondary pump preferably is a single stage centrifugal pump driven by the motor which also drives the primary pump. An impeller and diffuser is mounted inside the housing of the secondary pump in an inverted manner from normal operation. The impeller intake is on the upper side, and the diffuser outlet is on the lower side. The impeller and diffuser stage pump the annulus fluid downwardly to the discharge port. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG&#39;s. 1a, 1b, and 1c are simplified side views of a submersible pump well assembly having a chemical injection pump constructed in accordance with this invention. 
     FIG. 2 is a vertical sectional view, enlarged, of the chemical injection pump of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1A, well 11 includes a string of casing 13 that is cemented in the well. A string of tubing 15 extends downwardly from the surface in the well, defining an annulus 17 between the tubing 15 and the casing 13. Control equipment 19 is located at the surface for controlling the flow of production fluid up the tubing 15 to the surface. A chemical tank 21 is located at the surface, containing chemicals for retarding scale and corrosion in the well. The chemical tank 21 has a discharge line 23 for introducing liquid chemicals from the tank by gravity into the top of the annulus 17. The discharge line 23 would extend no more than a few feet into the top of the well. 
     A downhole submersible pump assembly 25 is connected to the lower end of tubing 15 for pumping fluid from the formation up the tubing 15. The downhole submersible pump assembly 25 includes a primary pump 27. Primary pump 27 is a centrifugal pump having a plurality of stages of impellers and diffusers (not shown) stacked together in a conventional manner. Primary pump 27 has an intake 29 at its lower end. A typical primary pump 27 would have about 100-200 stages of impellers and diffusers. 
     Referring to FIG. 1B, a secondary pump 31 has its upper end directly connected to the lower end of the primary pump 27. The secondary pump 31 has an intake port 33 on its upper end that is connected to an intake tube 35. Intake tube 35 is a small tube that extends upwardly in the annulus 17 between the casing 13 and primary pump 27. The intake tube 35 terminates about 6  to 10 feet above the intake 29 of the primary pump 27. The upper end of the intake tube 35 is open for drawing in fluids from the annulus 17, including chemicals introduced from the chemical tank 21. 
     The secondary pump 31 has a discharge port 37 on its lower end that is connected to a discharge tube 39. Discharge tube 39 is also a small diameter tube that extends in the annulus 17 offset from the axis of the well. The discharge tube 39 extends downwardly. 
     The lower end of the secondary pump 31 is connected directly to the top of a seal section 41. Seal section 41 is a conventional component in downhole submersible pump assembly 25. Seal section 41 has a shaft 43 that extends through it for driving the secondary pump 31 and the primary pump 27. There are a number of chambers 45 spaced along the length of the seal section 41. Each chamber has a partition (not shown) separating it from the other chambers 45. A face seal 47 is mounted around the shaft 43 at each chamber to pevent the leakage of well fluid into the seal section 41. Seal section 41 will be filled with a lubricating oil. U.S. Pat. No. 4,406,462, Witten, Sept. 27, 1983, provides more details concerning seal sections of this nature. 
     Referring to FIG. 1C, an electrical motor 49 has its upper end connected to the lower end of the seal section 41 in a conventional manner. Electrical motor 49 is a large alternating current motor. Cables (not shown) lead from a power source at the surface to the electrical motor 49. 
     A tail pipe 51 is mounted to the lower end of the electrical motor 49. The discharge tube 39 extends in the annulus 17 alongside the seal section 41 and along side the motor 49. The discharge tube 39 extends downwardly into the tail pipe 51. Tail pipe 51 may have a plurality of aperatures 53, or an open lower end, or both, for the discharge of chemicals received from the discharge tube 39. 
     Referring to FIG. 2, secondary pump 31 has a tubular housing 57. An upper adapter 59 is screwed into the upper end of the housing 57. Bolts 61 enable the adapter 59 to be bolted to the lower end of the primary pump 27. A lower adapter 63 is screwed into the lower end of the housing 57. Bolts 65 enable the lower adapter 63 to be bolted to the upper end of the seal section 41. A shaft 67 extends concentrically through the housing 57, and is rotatably supported in a conventional manner by components shown but not specifically numerated. An upper coupling 69 rotatably couples shaft 67 to shaft 71, which extends through the primary pump 27 for driving the primary pump 27. A lower coupling 73 couples the lower end of the shaft 67 to shaft 43 of the seal section 41. 
     A diffuser 75 is stationarily mounted in the housing 57 about halfway between the upper and lower ends. Diffuser 75 has a plurality of curved passages 77 that extend from the periphery inwardly and downwardly. An impeller 79 is mounted inside the diffuser 75. The impeller 79 has a plurality of passages 81 that have an intake on the upper side and outlets at the periphery spaced below the intake of the passages 81. The outlets register with the diffuser passages 77. Impeller 79 and diffuser 75 are conventional, except that they are mounted in an inverted manner from the impellers and diffusers (not shown) of the primary pump 27. The intake to the impeller 79 and diffuser 75 assembly faces upwardly in housing 57 and the discharge of the assembly faces downwardly in the housing 57. 
     Referring to FIG&#39;s. 1A, 1B, and 1C, in operation, electrical power will be supplied to the motor 49, which will rotate shafts 43, 67 and 71 (FIG. 2). As indicated by arrows 83 in FIG. 1C, fluid from perforations 85 in the casing 13 will flow upwardly around tail pipe 51, motor 49, seal section 41, secondary pump 31 and into the intake 29 of the primary pump 27. The primary pump 27 will discharge the fluid into the tubing 15 to flow to the surface. Chemicals from chemical tank 21 will flow through the discharge line 23 into the top of the annulus 17. The suction at the intake 29 of primary pump 27 causes the chemicals and fluid in the annulus 17 to flow downwardly. As the fluid flows downwardly, as indicated by the arrows 87, some of the fluid will flow into the intake tube 35 and into the secondary pump 31. 
     Referring to FIG. 2, the fluid flows within the housing 57 into the intake of the impeller 79. The impeller 79 is spinning with the shaft 67, and forces the fluid outwardly through the passages 81 to the periphery of the impeller 79. The fluid then flows downwardly and inwardly through the passages 77 of the diffuser 75 at a higher pressure. The fluid flows out the discharge port 37 and into the discharge tube 39. From the discharge tube 39, the fluid flows into the tail pipe 51, FIG. 1c. As indicated by the arrows 89, the chemicals will discharge into the annulus 17 above the perforations 85 and below the motor 49. The chemicals will flow around the motor 49, seal section 41, chemical pump 31 and into the intake 29 of the primary pump 27. The chemicals will flow to the surface through the tubing 15 along with the produced well fluid. The capacity of the chemical pump 31 is about 31/2 to 41/2 gallons per minute, while a typical primary pump 27 might be pumping in a range of 100 gallons per minute, depending on well characteristics and pump size. 
     The invention has significant advantages. The chemical pump enables the chemicals introduced into the annulus at the surface to flow also around the components of the downhole pump assembly that are located below the intake of the primary pump. This reduces scale deposition on these components, lengthening the time between the need to pull the pump assembly for maintenance. Placing an impeller and diffuser in an inverted position, enables the discharge end of the chemical pump to be located below the intake of the pump. This avoids having to cross intake and discharge lines or to have complex passageways within the pump. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention.