Gas Lift Module, Petroleum Production Performance series - Part 2

Now, before discussing the design of Gas Lift systems, Let's understand the Gas Lift valves. A Gas Lift system requires a source of gas and sufficient pressure to inject it at the proper place in the system. The injected gas may come from production operations or an outside source of supply, often sufficient supply and pressure is available from the high-pressure separator. If you're available operating pressure is not high enough, then a compressor will be needed. Prior to injections against typically passes through a flow control choke which controls the injection rate. Gas Lift valves located in the tubing are sized and spaced according to the overall design. The method of operation and the type of installation depend largely on the type of valves used. As you might expect there are different types of Gas Lift valves.

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     Simply, we should discuss three types: - The Casing pressure operated valve, - The Fluid operated valve, and - The Throttling valve. Later we should discuss a 4th type called the Pilot operated valve. They're distinguished by their sensitivity to the casing pressures including pressures needed to open and close them. The Gas Lift in history categorizes Gas Lift valves according to which pressure has the greater effect on the opening of the valve. The sensitivity is determined by the mechanical design of the valve because it is the pressure exposed to the largest area in the valve that controls the valves operation. "Remember that P=F/A .So, F=P*A. a schematic of a typical Gas Lift valve installed in a tubing string is shown here. Nitrogen is normally injected into the dome and charged to a specified pressure. The bellows serve as a flexible or responsive element. Movement of the bellows causes the valve stem to rise and fall, and the ball to open and close over the port. When the port is open, the annulus and tubing are in communication. Because the area of the bellows A (b) is much larger than the area of the port in A (p), and since the bellows is exposed to casing pressure, it is casing pressure that controls the operation of the valve. This type of valve ten is referred to as a casing pressure operated valve or, more simply, a pressure operated valve. It requires a build-up in casing pressure to open and a reduction in casing pressure to close."

      the various parts of the pressure operating valve and are technically discussed in the video in details. Click Here to watch this part (2:54 to 12:27).

     Now let's look at a graph of Flow Rate (q) verses Tubing Pressure (P). It will help us understand the performance characteristics of the Throttling valves. The vertical axis is flow rate and the horizontal axis is tubing pressure. At very low tubing pressure to the left of point 1, the valve is closed. As the tubing pressure reaches point 1, the valve begins to open and gas close from the casing to the tubing. The flow rate increases as the port continues to open. Throttling occurs from point 2 point 3 at which point the point is fully opened and throttling ends. The maximum flow rate occurs at point 4 as the giving pressure increases from point 4 to point 5 the tubing and casing pressures become balanced and the flow rate drops to zero. During the reverse cycle as the tubing pressure decreases, the valve opens at point 5, throttling takes place between points 3 and 2 and the valve throttle close between points 2 and 1. There other valves referred to as Combination valves which are also available for Gas Lift operations. Information on these and other special-purpose valves are available for manufacturers. The type of valve to be used for a given installation depends on whether the well to be placed on intermittent or continuous lift. But it is not certain which type of Gas Lift operation will take place as in cases where wells performance is borderline then valves maybe selected which are suitable for both continuous and intermittent lift. Values used for continuous flow must be sensitive to tubing pressure when in the open position. As the tubing pressure decreases, the valve should begin the throttle closed so as to decrease gas throughput.as the tubing pressure increases, the valve should open so as to increase gas throughput. This proportional response to increase and decrease in tubing pressure, maintains the established flow in tubing pressure and intends to keep a constant pressure inside the tubing. The ideal valve for continuous flow Gas Lift then is the Throttling valve.

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Gas Lift Module, Petroleum Production Performance series - Part 2

12:06:00 PM Gas Lift, Petroleum, Videos

Gas Lift Module, Petroleum Production Performance series - Part 3

Transcript: The types of valves to be used for intermittent lift depend upon whether we are going to install a single point for multipoint injection system. In single point intermittent gas lift operation, all of the gas necessary to move the liquid slug to the surface is injected through the operating valves, generally the bottom valve in the string. Let's look at a single point injection on the gas lift simulator. For this type of installation, the valve should expand to a large port size as soon as it is open and remain in the fully open position until closing. Depending upon the completion configuration, the port side will range in diameter from ⅜ to ⅘ of an inch. For multipoint intermittent gas lift operation, each valve intern should allow sufficient gas to pass so as to move the slug to the next higher valve. The pressure under the slug opens the valve that has just passed and supplements against being injected through the lower valves.

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As the slug moves to the surface, the valve normally remains opened until the slug is produced at the surface. Because the opening and closing of the various valves in our gas lift systems are so important to its operation so we should understand how and when a valve will open, when it will close and what a difference in these two pressures. Here we have a casing pressure operated valve; it is a single element valve for which we would like to calculate the opening and closing pressures. To do this,

Click here to see the example (1:44 – 4:28)

For given bellows and tubing pressures, we may reduce the spread by reducing the size of the poor opening. This is particularly important in intermittent gas lift installations because it controls the volume of gas used in each cycle. As the pressure reduction or spread required to close the operating valve increases, remaining gas injected during the cycle also increases. A small port size though increases horsepower requirements and therefore a balance must be done between gas conservation and horsepower requirements. The pilot valve was developed in response to the need for a larger ports size while maintaining cost control over spread characteristics. It has a small port which is used for split control and a larger port which is used for more efficient gas passage. The pilot valve then answers this twofold need and as often used for intermittent gas lift operations.

Click here to know about the operation of a pilot valve. (5:31-8:13)

Let me summarize this section on valves. A gas lift valve is categorized according to the pressure which has the greater effect on its opening. The casing pressure or pressure operated valve is dominated by the casing pressure both to open enclosure. The fluid operated valve depends on the tubing pressure to open and close it. The throttling valve or alternatively the continuous flow valve depends on the casing pressure to open it and the tubing or casing pressure to close it. See the whole video for more…

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Gas Lift Module, Petroleum Production Performance series - Part 3

5:30:00 PM Gas Lift, Petroleum, Videos

Gas Lift Module, Petroleum Production Performance series - Part 1

Exploration geophysics: Gas Lift Module, Petroleum Production Performance series, The basic technical video library for the exploration and production specialist, Part 1. Welcome to the first module of a series on petroleum production engineering. On this module, we should learn how oil is produced from a well by Gas Lift, one of the important methods of artificial lift. Objective should be to discuss Gas Lift applications in general, point out the different types of Gas Lift installations, illustrate the various pieces of surface and subsurface equipment needed for Gas Lift system, and finally we should show you how to design Gas Lift installations. But first, let's visit briefly on California. "Believe it or not, a good example of a Gas Lift installation is located in that building behind us, here in downtown Los Angeles   

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     We shall be visiting it soon, but let's first see how Gas Lift fits into the life of a producing well." they've already considered the life of a flowing well. I'm going to calculate the pressure and flowing relationships that exist throughout the flowing well system. We found that, as the average reservoir pressure decreases so does the flowing production rate. We also found that at some point in the life of most wells, flow to the surface stops. at that point or even earlier, the well maybe place an artificial lift using either a Gas Lift ,the topic of this module, or one of several possible pumping systems. The purpose of any artificial lift system including Gas Lift is to reduce the bottomhole pressure in order to allow the well to flow under the existing formation pressure. With Gas Lift, this can be accomplished by forcing gas through a choker valve located at the surface down the annulus through valves in the tubing. Injected gases allowed to aerate the liquid column in the tubing.

     

     The aeration reduces the bottomhole pressure caused by the weight of the column of liquid in the tubing. With sufficient aeration, the bottomhole pressure may be reduced to a point where the well once again begins to flow. The continuous aeration of the fluid column in the tubing will cause more oil to flow from the formation into the wellbore and then to the surface. Overtime though, the more fluids are produced, the average reservoir pressure decreases requiring increasing amounts of aeration to maintain a constant production level. The lifting of fluids can be accomplished by either continuous or intermittent gas injection. in continuous flow Gas Lift, a continuous volume of high-pressure gases introduced into the annulus in the tubing at a fixed rate causing a continuous flow of fluids from a well. Thus, artificial with method is usually applied to high productivity index wells which have high bottomhole pressures relative to their depth. For normal tubing strings, it is possible to lift from 200 to 20,000 barrels per day. But we choose instead to inject gas down the tubing and produce the fluid off the annulus. It is possible to lift up to 80,000 barrels per day using continuous Gas Lift. When small Macaroni tubing strings are used, it is possible to obtain production rates as low as 25 barrels per day using a continuous lift.

      The range of continuous Gas Lift then is anywhere from 25 to 80,000 barrels per day. The other Gas Lift method involves intermittent rather than continuous injection of lift gas. With generally applied only when a limited amount of fluid is flowing from the reservoir into the wellbore. Under these conditions, it becomes necessary to wait until the fluid volume in the wellbore builds up to a level worth lifting. once the fluid builds up to a high enough level, a slug of gas is injected down the annulus through a Gas Lift valve into the tubing, there by pushing the column of fluid to the surface as a slug. Cycling is regulated to coincide with the buildup of the fluid level in the wellbore. Intermittent injection and therefore intermittent production is accomplished by the use of a time cycle controller and adjustable choke located at the surface on the gas injection line. Intermittent flow Gas Lift is ideally suited for the well which has a high productivity index but a low average reservoir pressure, or alternatively a well with a low productivity index but high reservoir pressure. The major advantage of Gas Lift in an artificial Lift mechanism is the fact that the specific gravity of gas is so much less that oil or salt water. The following example illustrates the statement. "Assume that we have three 6000ft wells each completed with tubing on a packer and each having a surface pressure of 100 psi. The first well is filled with salt water, the second with oil and the third with gaps.

      Our objective is to calculate the bottomhole pressure of each. Let's begin with the well filled with salt water. the specific gravity of salt water is 1.07 which is equivalent to a hydrostatic gradient of 0.465 psi per ft. the static bottomhole pressure for this well then will be 100+0.465(6000)=2890 psi. Now let's turn to the oil well. if the column is filled with 0.8 specific gravity oil with the pressure gradient of 0.346 psi/ ft, then the static bottomhole pressure will be 100+0.346(6000)= 2176psi. This is more than 700 lb less than that for the well filled with salt water. Now we turn to the gas field well. we are told that it has an average specific gravity relative to water of 0.16 which gives an equivalent pressure gradient of 0.069 psi/ft. this gives a static bottomhole pressure of 514 psi. This is much lower than those for oil and water. The pressure profiles for the conditions obtained in each well are shown graphically in the video. we see the very low bottomhole pressure that exists when a well is filled with gas." we conclude from this that if we have a well filled with oil or water and can saturate all or a portion of the liquid column with gas, the bottomhole pressure will be reduced significantly. With a reduced bottom pressure, fluid in-flow from the formation will be increased and perhaps become continuous. But, is the design engineers job to select the gas volumes, points of Injection, and frequency of injection? Yes, so as to optimize the production from the well. We shall see how this is done later on in this module.

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Gas Lift Module, Petroleum Production Performance series - Part 1

5:18:00 PM Gas Lift, Petroleum, Videos