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
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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