Wells – Processes – Producing the well
Reexamination Certificate
2001-02-21
2003-05-27
Neuder, William (Department: 3672)
Wells
Processes
Producing the well
C166S169000, C166S242500
Reexamination Certificate
active
06568478
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATION
This application is based upon and claims priority from Brazilian Patent Application No. PI 0004685-0 filed Oct. 5, 2000, the contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and device to stabilize the production of oil wells. The device may be used with an oil production pipe and is intended to overcome the harmful effects caused to the well by the flow of unstable mixtures produced by certain wells. More particularly, the present invention is preferably related to a device which is used with a flow pipe of an oil well equipped to produce oil by means of gas lift, and to a method for its use.
2. Description of the Related Art
Oil is usually found in accumulations under pressure in the subsoil, in porous and permeable sandstone known as reservoir stones, or else hydrocarbon producing rock formations. Wells are drilled from the surface to drain off such reservoirs so as to communicate the reservoir with processing facilities in the surface, which are assembled to collect and to treat the produced fluids.
The wells are bores which traverse several rocking formations. Usually a steel pipe is inserted into such bores, and is called a casing. At least one pipe of smaller diameter is inserted into such casing, through which fluids from the reservoir flow.
Oil is a complex mixture of heavy and light hydrocarbons, comprising from dry gas (methane) to heavy oil. Depending on the features of the reservoir, some components may appear in higher concentration than other. Other substances may also accompany the produced oil, such as water, carbonic gas, hydrogen sulfide gas, salts and sand, only to mention some examples.
Depending on the conditions of pressure and temperature, the constituents of the oil may be in the gaseous phase or in the liquid phase. Thus, it is concluded that the fluids that usually flow into an oil well may be defined as a multi-phase multi-component mixture.
The flow of the fluids into an oil well, from the reservoir to the surface, can occur as a consequence of the accumulated energy in the reservoir, that is, without the presence of an external source of energy which provokes such production. In such a case it is said that the production of the well is normally flowing, or else it is said that the well is producing by surge. When an external source of energy is used, e.g. a down hole pump, there is then what is called an artificial lift.
Among the various known artificial lift methods the continuous gas lift can be noted. In a usual configuration for this method, natural gas at high pressure is injected into an annulus formed between the casing and the pipe through which the production of fluids from the reservoir flows, which is also named the production string or tubing.
Valves known as gas lift valves are located at certain points along the tubing, which control the flow of gas flowing from the annulus to the interior of the tubing. The expansion of such pressurized gas provides the necessary additional energy to allow fluids from the reservoir to flow at a certain flow rate.
In some oil wells the flow of fluids into the tubing occurs in an unstable way, that is, there are variations of pressure and flow rate with time, which can even be harmful to the integrity of the well and its associated equipment.
There are in the technical literature many citations of severe cases in which unstable flows in oil wells cause a halt in production. Such instabilities are also known as “heading”, as it is at the surface, at the well head, where they are more vigorously sensed, and such instability is able to occur in the tubing, in the annulus, or in both.
The phenomenon of the instability in the flow of multiphase mixtures is complex, and the causes for such instability are not totally understood. Generally, small disturbances give rise to great variations in the flow rates of the produced oil and the injected gas, as well as in the pressures. Many times such phenomenon is characterized by being cyclical.
In the article “These methods can eliminate or control annulus heading”, by A. W. Grupping, C. W. F. Luca e F. D. Vermeulen (Oil&Gas Journal, Jul. 30, 1984, p. 192), the authors show that the unstable behavior of the flow in wells producing by means of continuous gas lift may frequently be attributable to the pressure oscillations in the annulus formed between the tubing and the casing. According to the authors, keeping the pressure constant causes the flow in such wells to stabilize.
The control of the injection of gas in wells equipped to produce by means of continuous gas lift is usually made by a gas choke valve, located at the surface, and by another valve located at a certain point in the tubing, which is the gas lift valve.
According to Grupping, Luca and Vermeulen, and some others, the ideal situation is to remove the control from the surface, allowing it to be made only by means of the gas lift valve. The authors also recommend that the gas lift valve be provided with an internal passage comprising a single orifice. However, this is not enough to keep the flow rate constant.
The conventional gas lift valves used to control the flow rate of injected gas in wells equipped to produce by means of continuous gas lift are not really valves, although they are designated as valves by the experts and by the manufacturers. Actually they are flow regulators equipped with a small disc provided with a round orifice having a certain diameter. The edges of the orifice are usually sharp or smoothly rounded.
Such gas lift “valves” are also provided with a check valve, located downstream of the orifice, so as to preclude an undesirable flow of oil from the tubing to the annulus to occur.
When a gas flows throughout a constriction, such as an orifice, and the pressure upstream of the orifice is kept constant, the flow rate of the flowing gas increases as the pressure downstream of the orifice decreases, until, for a certain upstream pressure known as critical pressure, the sonic speed of the constriction is achieved. From then on a decrease in the pressure downstream of the constriction will not cause the injected gas flow rate to raise.
Thus, there are two dynamic behaviors, or rates of flow, for a valve provided with an orifice. The first can be defined as a sub-critical rate of flow, in which a reduction in the downstream pressure causes a raise in the gas flow rate, and the second can be defined as a critical rate of flow, in which the gas flow rate is constant, independently of the downstream pressure (considering a constant upstream gas pressure).
In use, the pressure upstream of the orifice is basically the pressure of the injection gas existing in the annulus at the position where the gas lift valve is installed, and the pressure downstream of the orifice is basically the pressure of the flow of fluids into the tubing at the position where the gas lift valve is installed.
Thus, according to the above technical literature, in a situation where the flow is critical the use of the gas lift valve contributes to stabilize the flow into the well, as in this situation the flow rate of injection gas is constant (assuming that the pressure in the annulus is constant).
However, due to the irreversible losses of energy in a gas flow passing through such orifices, deriving basically from the heat, the friction and the sound coming from the extremely turbulent flow of gas under pressure passing through the orifice, there is a necessity for the pressure into the tubing being essentially less than 55% of the existing pressure in the annulus so as a critical flow is achieved.
Such differential of pressure is not usually found in most of the actual cases, and consequently the orifice valve operates in a sub-critical rate of flow, the variation in the gas flow caused by the variation of pressure into the tubing contributing to the instability of the flow in the well.
The Brazilian patent application PI9300292-0, commonly owned by the applicant and the
Nixon & Vanderhye PC
Petroleo Brasileiro S.A. - Petrobras
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