Development and Field Test Results of an Efficient Downhole Centrifugal Gas Separator
Authors: Kobylinski L.S. , Taylor F.T. , Brienan J.W. 
Published: December 2007

Performance degradation of a centrifugal pump caused by ingested Performance degradation of a centrifugal pump caused by ingested free gas is well understood. High-GOR wells typically have a very high free-gas/liquid ratio at the pump intake, making the efficiency of the downhole gas separator crucially important in preventing free gas from entering the pump and hence producing the preventing free gas from entering the pump and hence producing the well successfully.

This paper describes the design, development, and laboratory testing of a new rotary gas separator that is currently being installed in high-GOR wells. Laboratory tests were conducted using air in water at low intake pressures. Free-gas and liquid separation theory is reviewed briefly to show that test conditions in many respects represent the worst case of a downhole condition from which valid predictions of downhole performance can be made. Actual oil and production rate data from several oil wells showing before and after performance of the rotary design and conventional reverse flow designs are presented also. An average improvement in production of 95% was observed. Free-gas separation efficiencies at pump intake conditions of pressure, temperature, and viscosity are compared also.

Introduction

The practical results of excessive free-gas ingestion in a downhole electrical submersible pump (ESP) are excessive on/off cycling, rough pump operation as evidenced by a "gassy" amperage chart, high dynamic fluid level, and reduced equipment operating life. The result is reduced oil and gas production and higher operating costs. Equally important is that an operator may select gas lift, hydraulic, or sucker rod pumps to produce a high-GOR field when ESP's would be the best economic choice if free gas could be separated from the oil more efficiently at the intake of the downhole pump.

This paper describes the development and field testing of an efficient centrifugal gas separator. Extensive laboratory tests were conducted to evaluate different design concepts and are reported briefly. Because of the uncertainty of downhole conditions such as fluid velocity, viscosity, water cut, and temperature, laboratory test results cannot predict downhole performance accurately. For this reason the primary emphasis in performance accurately. For this reason the primary emphasis in this paper is placed on actual field results. Approximately 1,500 centrifugal separators of the type we describe have been installed worldwide, and the reported results consistently show reduced dynamic or significant reduction of cyclic operation. The field results we present are from a number of Sun E and P Co. wells offshore California that were particularly well instrumented and recorded.

Theory

Many gas separators have been devised that use various physical mechanisms to obtain separation. Among these are gravity, agitation, and centrifugal force.

The oldest and simplest bottomhole gas separator is the natural gas anchor. The natural gas anchor consists of simply setting the pump intake below the casing perforations. As the oil and entrained gas bubbles flow through the casing and experience a pressure drop, the bubbles grow in size and begin to rise. As they pressure drop, the bubbles grow in size and begin to rise. As they join other bubbles, they grow even larger. The rate of bubble rise increases with bubble size, with larger bubbles rising at about 0.5 ft/sec [0.15 m/s]. Gravity separation occurs when the fluid downhole velocity is lower than the bubble rise velocity. This is accomplished by maximizing the downflow area. This area is limited by casing size, equipment diameter, and fluid viscosity. The reverse flow gas separator, shown in Fig. 1, is a modified gas anchor that permits placing the pump intake above the casing perforations by locating the housing intake ports above inner intake passages. This causes the flow to make a 180 turn, which causes free gas separation to occur. A low downward flow velocity also may permit some additional natural separation. This type of separator has been used in centrifugal pumping applications for 50 years, but its efficiency is limited in applications where moderate to high flow rates are encountered.

Screen-type gas/liquid separators using a fine screen have proved to be effective if the two-phase fluid is clean. proved to be effective if the two-phase fluid is clean. Experiments have shown, however, that the screen may clog quickly when used in an oilwell environment.

For a gas separator to operate efficiently, it must ingest the two-phase mixture with minimal pressure drop. This is necessary to prevent additional gas breakout inside the separator. The goal is to induce inflow and then to boost the pressure by means of an inducer, to separate the phases, and then to vent the free gas back into the wellbore while the liquid-phase fluid enters the first stage of the centrifugal pump.

JPT

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