The Role of Two-Speed Motors In Lifting Heavy Oil

Abstract

Blankenhorn, C.H., Member AIME, Creole Petroleum Corp., Tia Juana, Venezuela Abstract During the past few years, the use of two-speed electric motors has been applied to sucker-rod pumping for lifting high volumes of heavy, viscous crude in Venezuela's Lake Maracaibo area. The crude involved ranges from 12–15 API gravity, is very viscous and contains considerable entrained gas that is highly resistant to separation. Lifting maximum rates (1,500–3,000 BOPD) of this oil presents many and varied problems. The major problems encountered in pumping high volumes of the heavy oil are motor sizing, rod fall and low pumping efficiencies due to the en trained gas. Extensive field studies have shown that under single-speed motor operation, approximately twice the normal operating horsepower is required to provide the necessary start-up torque under dead oil, or start-up conditions. This leads to improper motor utilization, lower efficiencies and lower power factors throughout the entire electrical system. Pumping speeds are also limited, under single-speed operation, to approximately 75 per cent of normal desired speeds due to rod fall limitations encountered upon start-up under dead oil conditions. This, in turn, aggravates the low efficiency problem of pumping viscous oil containing entrained gas, resulting in an over-all decreased producing rate. Under two-speed motor operation, constant horsepower, single winding, squirrel cage induction motors have been utilized to overcome or greatly relieve these problems in the Lake Maracaibo operation. These motors have met with very satisfactory results by permitting start up at half speed, thus providing considerably in ore starting torque when most needed. After displacing the dead, thick oil fro in the well tubing and flow line, the pumping speed is automatically increased to twice the startup speed and normal operating speeds are achieved. The utilization of the two-speed pumping motor has resulted in approximately a 50 per cent reduction in motor sizes required, reduced cable sizes necessary for the higher hp motors previously required, and has increased the efficiency of the electrical system since the two-speed motor is essentially fully loaded (85- 95 per cent) at all times. These motors have also enabled pumping speeds to be increased approximately 30 per cent with corresponding increases in producing rates. Introduction In the Bolivar Coastal fields of Western Venezuela's Lake Maracaibo region, one of the largest producing reservoirs is the Bachaquero-2 heavy oil reservoir. The crude produced from the B-2 reservoir, as it is commonly called, is a low gravity (12–16 API) asphaltic oil with viscosities ranging from 5,000–15,000 SSU at 85 F. One of the big problems involved in producing this reservoir is that of maintaining high producing rates in areas of declining bottom-hole pressure, by artificial lift methods. Since the wells are located in Lake Maracaibo and the area is completely electrified, the most common method of artificial lift is the use of sucker-rod pumps with pumping units powered by electric motors. To date, rates of 1,300 3,000 BOPD/well have been achieved by this method, utilizing standard equipment. The present average production from 179 pumping wells in the field is 400 BOPD/well with 62 high volume pumping installations averaging 700 BOPD/well.Due to the physical characteristics of this viscous, low gravity crude, three major problems must be overcome to achieve highpumping rates from the wells:low pump efficiency,rod-fall limitations, andmotor utilization and sizing. Pumping Efficiency The oil produced from the B-2 reservoir, aside from its low gravity viscous nature, is best described as containing a homogeneous entrainment of gas which results in a low density mixture that is extremely resistant to separation. Extensive studies on pumping wells in the field have shown that due to the effect of the entrained gas, the maximum pump efficiency is 40–50 per cent for wells having a gas-oil ratio above 150 scf/bbl (Fig. l). This means that to achieve the desired pumping rates from the wells, the pumping installation has to be designed with a 40–50 per cent pump efficiency factor being considered, or approximately twice the size that would normally be required to pump an equivalent volume of fluid having a density similar to water. JPT P. 1195^