Pumping Oil Wells. In areas with low bottomhole pressure, wells must be put on the pump. Production cannot reach the surface or if it does, production is small. Often large quantities of water must be handled. Downhole pumps can be run on wireline, rods or on the bottom of the tubing. These pumps may be mechanically, electrically, or hydraulically operated. The most common and visual pumping unit is the mechanical surface pumping unit (SUCKER ROD PUMPING SRP), or more commonly called the pump jack.
This consists of a power source on the surface, normally an electric motor or hydrocarbon fueled engine, a gear unit, and a beam type pumping unit. Occasionally a band wheel is used to drive the pumping units on many wells from one central station. Pumping jacks on several wells can be run from this station, or may be powered by cable from a single pumping unit.
On a surface pumping unit, there is a stuffing box at the surface that packs off the rods to prevent loss of fluid while pumping. The beam unit has a fold back horses head that allows it to be moved back from over the hole while rods and tubing are being pulled. The downhole pump may be made to seat in the tubing and be pulled with the rods.
Other types of downhole pumps are run as part of the tubing string, and require that the rods and tubing both be pulled. Often a tubing anchor is run at the bottom of the tubing string to prevent movement during normal operation. When the pump is pulled, it is normally sent to a pump shop for repair and a reconditioned or new pump replaces it. Pulling rods from a pumping well requires different pulling capacities. The derrick should be strung accordingly. Special rod elevators are attached to the pickle to grasp the rod boxes. A special set of rod wrenches is used to make or break rod boxes when rods are pulled from inside tubing. The rod elevators are alternated when pulling or running rods. One is used going into/out of the hole. The other elevator acts as slips below the tool joint to be broken. Elevators are equipped with a safety latch to be sure the rod does not slip out of the elevator. Always check this before breaking a connection. Rods are normally hung in the derrick from a basket or fingers in the derrick.
Rods are hung in stands of two, three, or four joints depending on the height of the rig. Rod guides to prevent excessive box and tubing wear are used. When a large surface unit is not present it does not necessarily mean that the well is not pumping. Electric driven submerged pumps and hydraulic pumps are used.
The electric submersible pump uses electric current controlled from the surface to power an electric motor downhole. As the electric motor downhole operates, it turns impellers which force fluid from bottom to surface. The cable providing the electric current is run and strapped outside of the tubing string to prevent contact with corrosive wellbore fluids. In a hydraulic pump, the well’s fluids are separated into produced (flow line fluid) and power fluid. The power fluid operates the downhole hydraulic pump.
Power fluid at high pressure (low velocity) is converted to a low pressure (high velocity) jet by the pump’s nozzle. The pressure at the nozzle exit becomes lower than the pressure in suction passageway, and fluid is drawn in from the wellbore.
The suction fluid becomes entrained with the high velocity jet and the pumping action begins. After mixing in the throat of the pump, the power and wellbore fluid is slowed by the diffuser. Because the velocity is lowered, the pressure is raised. This pressure increase should be sufficient to pump the fluid to the surface. Nozzle and throat areas must be sized to the job to insure proper pumping action.