November 1987
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149 Reads
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25 Citations
SPE Production Engineering
Consideration of the energy requirements in the design of a sucker-rod pumping system is very important. Examples are given that detail how the use of the largest possible pump with the lightest, strongest rod string and special-geometry units can provide a substantial energy reduction. Introduction A number of variables are important in the design of a sucker-rod pumping system. To get the best design for a specific application, each variable must be evaluated in terms of the particular requirements of that specific system. A system that might be ideal for the operating conditions of one area might be a very poor selection for another area with different operating conditions. The importance of the energy requirement is in proportion to the cost of energy. Twenty years ago, proportion to the cost of energy. Twenty years ago, electricity sold for about 0.002/MJ] and gas for about 0.0088/m3]. The energy bill was so, small that it was not even considered in the design of the system. Because most of the expertise in the design of sucker-rod systems was gained in this era of low energy costs, the impact of energy costs on the system design has been largely ignored. Today, the average cost of electricity is about 0.019/MJ], and the average lease value of natural gas is about 0.106/m3]. Now the energy bill is usually the largest operating cost item on the lease and has become a very important design consideration. Theoretical Energy Costs A discussion of energy usage and cost should start with hydraulic horsepower. This is the theoretical work that is required to lift the pounds of well fluid from the net depth. The equation for calculating hydraulic horsepower for a fluid weighing 8.34 lbm/gal [999 kg/m3] (specific gravity of 1.0) is Lw h = 33,000 h t q × 42 × 8.34 1 h = Lx ×, h 1,440 33,000 h = 0.00000736 qL, h where h = hydraulic horsepower, h L = net left, ft, w = weight, lbm t = time, minutes, and q = flow rate, B/D. In SI units, W = 0.0001135 qL, h The theoretical hydraulic horsepower to lift 500 B/D [79.49 m3/d] of fluid with a specific gravity of 1.0 from 6,000 ft [1828.8 m] would be 0.00000736 × 500 B/D × 6,000 ft = 22.08 hhp. In kilowatts, 22.08 hhp × 0.746 kW/hp = 16.47 kW. In SI units, 0.0001133 × 79.49 m3/d × 1828.8 m = 16.47 kW. The annual power bill, x, with 0.019/MJ] electricity would be x = 10,100/yr. This is the theoretical cost to do the work of lifting 500 B/D [79.49 m3/d] from 6,000 ft [1828.8 m]. Actual Energy Considerations Polished-rod horsepower starts with this theoretical work at the Polished-rod horsepower starts with this theoretical work at the pump and includes the other downhole requirements of the system: pump and includes the other downhole requirements of the system:the horsepower required to move the sucker-rod string dynamically,the frictional losses between the sucker rods and the tubing,hydraulic frictional losses of the fluid as it flows between rods and tubing, andbottomhole pump losses and inefficiencies. The dynamic horsepower requirements are related to the weight of the sucker-rod string and the pumping speed (strokes per minute times stroke length). The pumping speed for a given stroke length and producing rate is determined primarily by the size of the bottomhole pump. The frictional losses between the sucker rods and the tubing are related principally to fluid viscosity and tubing straightness. The hydraulic flow losses are a function of the producing rate and the annular area between rods and tubing. The bottomhole pump losses are related to the amount of slippage that passes the plunger and valves. The inefficiencies are related to the free gas in the pump from poor gas separation and the volume shrinkage resulting from the gas absorbed in the oil at pump intake pressure and then desorbed at atmospheric pressure.