High-Rate Artificial Lift

Abstract

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. This paper summarizes the major considerations in the selection, design, installation, or repair of high-rate artificial-lift systems. The majortypes of artificial-lift-sucker-rod pumps, gas-lift systems, electricalsubmersible pumps, hydraulic pumps and jets, and hydraulic turbine-drivenpumps--will be discussed. An extensive bibliography of artificial-lift papersis included. Introduction The need for better design and more efficient operation of high-rateartificial lift continues to be one of the more important artificial-liftproblems. Many of the older fields that are now flowing will require high-rateartificial lift. Also, waterflooding and water injection for pressuremaintenance have increased the need for higher lift rates. Selection Selection of the type of artificial lift is very important. In such aselection, the three major economic factors are income, operating cost, andcapital cost, generally in that order. The type of artificial-lift systemchosen should produce the reserves in a timely fashion with minimum operatingcosts. An efficient system may well be worth some additional capital costs. Making a bad selection will have adverse effects on the net income and mayresult in a change to a different type of artificial lift. One of the initialsteps in any design selection is to set priorities. Such things as location, lift depth, and casing size have a large influence on the type and size ofequipment. Important issues in an offshore field can be significantly differentfrom a dry-land location. The importance of cash flow, total present valueprofit, and reserves must be weighed. High-Rate Rod Pumping Rod pumping is not normally considered to be particularly applicable forhigh-rate artificial-lift systems. However, use of relatively long strokeunits, large plungers, and high pumping speeds should be reviewed if for noother reason than to establish a norm. A good general rule is to install rodpumping if the desired production can be obtained without encountering severeor unusual lift problems. Possibilities range from lifting 4,000 BFPD from1,500 ft [635 m3/d fluid from 460 m] to 1,000 BFPD from 4,000 ft [160 m3/dfluid from 1220 m]. The problems of lifting high rates from shallow wells arequite different from lifting from deep wells. In high-rate, shallow wells, themajor problems are normally rod fall and high peak torques, not rod stress, TheAPI design method (API RP 11L) appears to be inaccurate for design of shallowwells where the conditions of low nondimensional pump speed (N/No) and lownondimensional fluid loads (Fo/Skr) exist. In design of high-rate, shallow rodsystems, we must rely on other design approaches (i.e., use of thewave-equation programs or field experience). In general, for high-rate, shallowlift, relatively large pumps should be installed and the units run as fast asthe rods will fall without excessively overloading the equipment. Larger pumpsizes running at relatively slower speeds are theoretically more efficient thansmaller pumps running at higher speeds. Deep, high-rate rod-pumped wells arelimited primarily by the high rod loads. The API design method usually gives"ballpark" load answers and predicts reasonably well the change in loads fordifferent operating conditions. To deal with high rod loads, the users areforced to go with higher-strength rods, such as the API Type D or specialhigh-strength rods. High cyclic loads and rod abuse will cause repeated rodfailures, which in turn will result in extremely high operating costs andexcessive downtime. Efforts to improve rod life are almost always worthwhile. Good surveillance is essential in rod-pumped fields when lifting high rates. Also, it is very important to provide effective corrosion inhibition as soon aswater becomes the wetting phase. Pumping-unit care and maintenance also deserveto have high priority because the units are the most expensive piece ofequipment in the rod-pumping system. Both the gearbox loads and the structureload must be considered. The API design assumes that the unit is in perfectbalance, which seldom occurs in practice. As water cuts and lift depthsincrease. there is a tendency to overload the gearbox. Loads exceeding thegearbox rating may significantly shorten life. Keeping the structure of theunit from being overloaded is also important. Beam bending or failure of thebase seldom occurs. Occasionally, a base weld may crack but can be easilyrepaired. The primary problem with overloading is bearing failures. JPT P. 277^