Wells – Processes – Separating material entering well
Reexamination Certificate
2001-01-24
2003-04-29
Bagnell, David (Department: 3672)
Wells
Processes
Separating material entering well
C166S105500
Reexamination Certificate
active
06554066
ABSTRACT:
SCOPE OF THE INVENTION
The present invention relates generally to the field of petroleum production.
The invention particularly includes equipment for effecting a process for the gravitational separation of immiscible fluids of different density.
More particularly, it relates to equipment capable of efficiently separating the gaseous phase from a mixture of liquid and gas, the equipment being provided internally with an automatic level control system. Preferably, the equipment is used at the bottom of oil wells which bring oil to the surface through the use of lifting pumps.
A separator of this kind may also be used at the surface.
It may find application in the petrochemical, chemical or other industries.
BASIS OF THE INVENTION
In nature, petroleum is generally found mixed with water and gas.
In wells where there is no natural lift one of the alternatives available for lifting the petroleum from the well bottom to the surface is to use lifting pumps. Pumping may be of the sucker rod pumping (SRP) type, progressive cavity type (PCP), electrical submersible pumping (ESP) type or, where seabottom Christmas trees are used, of the subsea electrical submersible pumping (ESP in wet Christmas tree) type. Whatever type of pumping is selected, the presence of free gas above a certain percentage in the liquid mixture being pumped will cause a significant loss of efficiency in the pumping process.
To increase pumping efficiency it is common for separators for separating gas from the liquid mixture to be installed at the bottom of an oil well. There are various types of separators for this purpose available at the present time but, nevertheless, conventional separators, which largely use bubbling separation, generally have a lower separating efficiency than is desirable.
Separators based on other effects, such as e.g. the cascade flow type, stratified type, Jukovski effect type, etc. generally offer greater efficiency. However, these separators depend on the level of liquid within them being maintained within a specified range. This requires the use of an external manual or automatic control system based on sensors, valves and links between them, which are vulnerable points and add complexity to the system.
STATE OF THE ART
The fact that free gas reduces the efficiency of oil-well pumping systems is widely known. The first patent for a separator, to reduce the quantity of free gas at the inlet to a well-bottom pump, was published in 1881. Since then, many other separators have been proposed but, depending on operational conditions, the use of known separators has not always resulted in good pumping efficiency.
The process most commonly adopted in conventional well-bottom gas separators currently in use consists of adding a two-phase mixture to a medium in which the continuous phase present is liquid. Under these conditions gas is compelled to bubble in the direction of the dynamic level of the well and separation is subject to the rate of rise of bubbles through the liquid which, according to Stokes Law, is inversely proportional to the viscosity of the liquid.
The present applicant has lodged a patent application in Brazil (Application No. PI 9905912.6) entitled “Well Bottom Gas Separator” relating to a new design of separator based on flow of the cascade type, which might or might not be followed by flow of the segregated or stratified type. In this separator the two-phase mixture enters a settling vessel through openings located in the upper part of its side surface, above the level of the liquid which has accumulated in the separator. Thus, unlike the previous conventional separators in use at the time, the mixture is injected in a region where gas is the continuous phase present and separation takes place very much more rapidly than in a medium where the continuous phase is liquid. In order to optimize separation conditions, converting chaotic vertical cascade type flow into an inclined and segregated flow, a component of helicoidal shape was located within the settling vessel occupying a space above the level of liquid in the separator. To avoid turbulence and flooding, among other reasons, the pitch of the helicoidal surface was made to be variable so as to control the speed and thickness of the liquid layer over the helicoidal surface.
In general wells begin production with a high static level, and even in the case of separators of the cascade type separation initially takes place by bubbling. In order to ensure a changeover from this type of separation to cascade type separation the level of liquid within the settling vessel has to be lowered. This can be achieved by fitting a choke valve in the gas line for example, which has to be kept closed until the level of liquid in the settling vessel lies within a selected range. Thus, when the well is started, this choke valve must be kept closed as long as the level of the mixture is above the specified level and must be kept open when the level of liquid in the separator varies within the specified range.
Maximum pumping of liquid is achieved when the level of the mixture is stabilized in the settling vessel within the selected range, with the choke valve fully open. If the level only becomes stabilized when the choke valve is partly closed, output falls because the well casing, which is pressurized with gas, applies a back-pressure against the producing rock. Also, if the valve is open to eliminate the back-pressure from the gas, output can be even less still because the back-pressure of the gas is replaced by an even greater back-pressure from the liquid and under these conditions the level of liquid in the separator will rise well above the region of the openings and the cascade effect will cease to occur. As the efficiency of separation by bubbling is less than that of cascade separation, there is an adverse effect on the efficiency of pumping.
As manual control of the level of liquid within the cascade or cascade/segregated separator is burdensome, it is recommended that automatic level control should be used. This can be achieved, e.g., using a control valve in the gas line and level sensors within the well.
The volumetric flow from the pump downstream of the separator is normally constant. Nevertheless, depending on the type of two-phase flow, the flow of liquid upstream of the separator can vary. Slug flow can result in very great changes in the level within the separator, mainly when the area of the annulus in the separator is small. A long separator, which permits considerable variations in level, apart from being difficult to construct and install, has the disadvantage that it reduces production from the well by causing back-pressure against the producing rock as a result of an excessive hydrostatic column extending from the perforated casing to the top of the separator.
A level control valve located in the gas outlet at the wellhead prevents excessive variation in the level of liquid in the separator. Nevertheless level sensors are required at the bottom of the well, resulting in an expensive installation of limited reliability.
U.S. Pat. No. 3,451,477 discloses a system for direct control of the level in a well using a valve with two stages; a principal and a secondary stage, which has the advantage of being easily operated when there is a blockage due to differential pressure. The design of this valve apparently avoids gas entering the lifting pump. In reality this system does not prevent a considerable quantity of gas from entering the pump because the valve, when closed, maintains the volume of liquid in the separator but does not maintain the flow of liquid in the pump constant, and due to this the flow of gas in the pump increases to compensate for the lack of liquid. When the flow of gas in the separator falls, there is increased gas flow in the pump because the gas expands as it passes through the valve, due to the fact that the pressure on the suction side of the pump is less than the pressure in the separator. It is therefore concluded that the control system used in this separator cannot function adequately.
Li
Lopes Divonsir
Soares Rogerio Floriao
Souza Robson de Oliveira
Bagnell David
Dougherty Jennifer R.
Nixon & Vanderhye PC
Petroleo Brasileiro S.A.--Petrobras
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