Wells – Processes – Producing the well
Reexamination Certificate
2001-01-24
2004-07-06
Bagnell, David (Department: 3672)
Wells
Processes
Producing the well
C166S250030, C166S250150
Reexamination Certificate
active
06758277
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system and method for optimizing fluid flow in a pipe and in particular, fluid flow in a gas-lift well.
2. Description of Related Art
Gas-lift wells have been in use since the 1800's and have proven particularly useful in increasing efficient rates of oil production where the reservoir natural lift is insufficient (see Brown, Connolizo and Robertson,
West Texas Oil Lifting Short Course
and H. W. Winkler,
Misunderstood or Overlooked Gas
-
lift Design and Equipment Considerations
, SPE, p. 351 (1994)). Typically, in a gas-lift oil well, natural gas produced in the oil field is compressed and injected in an annular space between the casing and tubing and is directed from the casing into the tubing to provide a “lift” to the tubing fluid column for production of oil out of the tubing. Although the tubing can be used for the injection of the lift-gas and the annular space used to produce the oil, this is rare in practice. Initially, the gas-lift wells simply I*njected the gas at the bottom of the tubing, but with deep wells this requires excessively high kick-off pressures. Later, methods were devised to inject the gas into the tubing at various depths in the wells to avoid some of the problems associated with high kick-off pressures (see U.S. Pat. No. 5,267,469).
The most common type of gas-lift well uses mechanical, bellows-type gas-lift valves attached to the tubing to regulate the flow of gas from the annular space into the tubing string (see U.S. Pat. Nos. 5,782,261 and 5,425,425). In a typical bellows-type gas-lift valve, the bellows is preset or pre-charged to a certain pressure such that the valve permits communication of gas out of the annular space and into the tubing at the pre-charged pressure. The pressure charge of each valve is selected by a well engineer depending upon the position of the valve in the well, the pressure head, the physical conditions of the well downhole, and a variety of other factors, some some of which are assumed or unknown, or will change over the production life of the well.
The typical bellows-type gas-lift valve has a pre-charge cylinder for regulating the gas flow between the annular space and the interior of the tubing string. The pre-charge forces a ball against a valve seat to keep the valve closed at operating pressures below the pre-charge pressure. Several problems are common with bellows-type gas-lift valves. First, the bellows often loses its pre-charge, causing the valve to fail in the closed position or operate at other than the design goal, and exposure to overpressure causes similar problems. Another common failure is erosion around the valve seat and deterioration of the ball stem in the valve. This leads to partial failure of the valve or at least inefficient production. Because the gas flow through a gas-lift valve is often not continuous at a steady state, but rather exhibits a certain amount of hammer and chatter as the ball rapidly opens and closes, ball and valve seat degradation are common, and lead to gas leakage. Failure or inefficient operation of bellows-type valves leads to corresponding inefficiencies in operation of a typical gas-lift well. In fact, it is estimated that well production is at least 5-15% less than optimum because of valve failure or operational inefficiencies. Fundamentally these difficulties are caused by the present inability to monitor, control, or prevent instabilities, since the valve characteristics are set at design time, and even without failure they cannot be easily changed after the valve is installed in the well.
It would, therefore, be a significant advantage if a system and method were devised which overcame the inefficiency of conventional bellows-type gas-lift valves. Several methods have been devised to place controllable valves downhole on the tubing string but all such known devices typically use an electrical cable or hydraulic pipe disposed along the tubing string to power and communicate with the gas-lift valves. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral with the tubing string or spaced in the annulus between the tubing string and the casing because of the number of failure mechanisms present in such a system. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhole communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes a communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil or diesel oil in the annulus between the casing and the tubing. The invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication because it is expensive, has problems with brine leakage into the casing, and is difficult to use. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,574,374; 5,576,703; and 5,883,516. Methods and uses of downhole permanent sensors and control systems are described in U.S. Pat. Nos. 4,972,704; 5,001,675; 5,134,285; 5,278,758; 5,662,165; 5,730,219; 5,934,371; 5,941,307.
It is generally known that in a gas-lift well, an increase of compressed gas injected downhole (i.e. lift-gas) does not linearly correspond to the amount of oil produced. More specifically, for any particular well under a particular set of operating conditions, the amount of gas injected can be optimized to produce the maximum oil. Unfortunately, using conventional bellows type valves, the opening pressure of the gas-lift bellows type valves is preset and the primary control of the well is through the amount of gas injected at the surface. Feedback to determine optimum production of the well can take many hours and even days.
It is also generally known that in two-phase flow regimes, such as in a gas-lift well, several flow regimes exist with varying efficiencies (see A. van der Spek and A. Thomas,
Neural Net Identification of Flow Regime Using Band Spectra of Flow Generated Sound
, SPE 50640, October 1998). However, while operating in a particular flow regime is known to be desirable, it has largely been considered impossible to practically implement.
It would, therefore, be a significant advance in the operation of gas-lift wells if an alternative to the conventional bellows-type valve were provided, in particular, if sensors for determining flow characteristics in the well could work with controllable gas-lift valves and surface controls to optimize fluid flow in a gas-lift well. Generally, it would be a significant advance to be able to detect the flow regime in a two-phase flow conduit and to control the operation to remain in a desirable phase.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.
SUMMARY OF THE INVENTION
The problems outlined above are largely solved by the system and method in accordance with the present invention for determining a flow regime and controlling
Burnett Robert Rex
Carl, Jr. Frederick Gordon
Savage William Mountjoy
Vinegar Harold J.
Bagnell David
Shell Oil Company
Stephenson Daniel P
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