Gas-Lift Surveillance Page 16
Gas-Lift Surveillance
In Search of Hidden Production and Efficiency
By
Dan Dees, AppSmiths
Cleon Dunham, Oilfield Automation Consulting
Burney Waring, Shell EP Technology
Introduction
Surveillance is a process. It's a process of planning and designing for optimum operation; then continually looking for, finding, and addressing opportunities to overcome inefficiencies, weaknesses, and faults in that operation so that improvements can be made and maintained.
Few production processes in the oil field are more in need of this surveillance process than gas-lift. Many people say that gas-lift is simple. Inject gas in a well and oil will come flowing out. This may be true in many cases, but there is a significant difference between achieving "some" oil production and achieving optimum production with optimum investment in gas-lift equipment and gas. Murphy said it. There are many things that can go wrong with this process, and the chances are that several of them will go wrong at the same time, or at the time least expected.
Gas-lift surveillance, real surveillance, is a comprehensive process that empowers trained oil-field operators, analysts, technicians, and engineers to optimize the artificial lifting of oil wells using gas-lift. This is a profound statement. There can be differences of hundreds of barrels of oil per day, and therefore thousands of dollars of income, between poor performing gas-lift wells and optimized ones.
The goal of gas-lift surveillance is to keep an entire gas-lift system - the producing oil wells, gathering systems, treating systems, compression systems, distribution systems, and injection systems - all operating optimally, all of the time. This goal cannot be achieved by traditional, manual or partially automated means. It requires state-of-the-art tools and skilled personnel using these tools, on a continuous basis.
Is this feasible? Is it cost effective? Yes, it is feasible, as this paper will demonstrate. And yes, it is highly cost effective. In fact, to not pursue this process on a continuous basis can be economic folly.
Types of Gas-Lift
While there are many possible combinations and permutations, there are two basic forms of gas-lift - continuous and intermittent.
Continuous Gas-Lift. The three goals of continuous gas-lift are to inject the optimum (more on this later) rate of gas into the well; to inject on a continuous (stable) basis; and to inject as deep as possible. These goals are undisputed. Anything short of the optimum injection rate (either too little or too much gas) will result either in under production or excessive injection, both of which reduce profitability. Anything other than stable injection, that is any fluctuations in the injection rate or pressure, will lead to production inefficiency and may cause other problems, both with the well and with associated systems. Anything other than injection from the deepest feasible point will result in severe inefficiency.
Intermittent Gas-Lift. The goal of intermittent gas-lift is to inject an optimum volume of gas beneath a column of liquid to lift the liquid "slug" from the well. For this process to be optimum, the timing and size of the gas-lift injection volume must be just right to obtain maximum oil production per gas injection cycle.
Gas-Lift Surveillance. Many factors can affect the successful, optimum operation of a gas-lift system and its wells. The goal of gas-lift surveillance is to design gas-lift systems and wells to operate optimally and then to be continuously on the watch to detect any inefficiencies or negative or troublesome factors, determine the most cost effective steps to overcome these problems, implement the "best" solution(s), and validate that they have been successful.
Gas-lift surveillance is difficult but very rewarding work. And, there is some very good news. There are tools, techniques, and training available today to permit effective gas-lift surveillance to be practiced in almost every gas-lift operation, any where in the world. These will be discussed in this paper.
And there is more good news. Gas-lift surveillance can be highly profitable work. It has been repeatedly shown that production from truly optimized gas-lift wells can be routinely increased by 10% (and often much more) relative to wells that are essentially ignored and allowed to "limp along" on their own. And, there can be other economic rewards. This increased production can often be obtained with much less gas than might be used in wells that are not optimized. And in some cases, this can mean less capital investment in gas-lift handling, compression, and dehydration equipment.
Let's place this in perspective. Suppose a gas-lift field has 100 wells. Suppose each well is potentially capable of producing 600 BOPD (about 100 M3/Day). Based on much experience, it is likely that the daily oil production from this field, where the potential is 60,000 BOPD, is only about 55,000 BOPD, or perhaps 50,000, or maybe even less. With effective gas-lift surveillance, the daily production can be increased by 5,000 BOPD, 10,000, or even more. At today's oil prices, this is an increase in income of $100,000 per day, or more, or over $35 MM per year. And, this is with no new drilling; no new completions; no new facilities; just good gas-lift surveillance. To paraphrase a seasoned Shell field manager, "good surveillance is the least expensive way there is to produce more oil."
Continuous Gas-Lift --- What Surveillance is Looking For
As stated above, optimum continuous gas-lift is about three things - injecting gas at the optimum rate, in a stable manner, from the deepest feasible point. Anything that interferes with any of these three goals must be found and overcome by the gas-lift surveillance process.
These objectives are not all equal. By far the most important is injecting at the deepest feasible point. This gives the maximum “lifting" effect. The second most important is stability. If a well is unstable, it may be multi-pointing or cycling between valves with a significant loss in efficiency. The third is injecting at the optimum rate. This may be thought of as "fine tuning."
Depth. Ideally, the optimum depth for gas-lift injection is just above the producing formation. Since most gas-lift wells have a packer, or perhaps more than one packer if the well is dually completed, the maximum depth of lift is normally limited to just above the shallowest packer. However, other things can limit the depth as well. The most important of these is the available pressure of the gas-lift injection system.
This paper is not intended to provide a treatise on gas-lift design. For this, we recommend API Recommended Practice 11V61. However, the design process is critical. The gas-lift mandrels and valves must be spaced and designed correctly to allow the well to "work down" (unload the well's annular completion fluid) to the optimum injection depth and then stay there.
The critical factors, from a gas-lift surveillance perspective, are:
· The gas-lift mandrels must be spaced to allow the well to "work down," given the available injection pressure, the reservoir pressure, the gravity (weight) of the produced fluid, and many other factors. Since several of these factors can change over the life of a well, the mandrel spacing must be able to accommodate changes or it must be possible to change the spacing from time to time. The prudent approach is to "over design" by installing more mandrels - that is, placing the mandrels closer together - than initially thought necessary. It is easy and inexpensive to skip mandrels that are not needed. It can be very expensive to work a well over to add or re-space mandrels.
· The gas-lift valves must be designed to be open to support the unloading process, and then remain closed to permit continuous gas-lift from the desired operating depth. In general, gas-lift valves do not have a wide, effective operating range. Therefore, it is essential that the gas-lift valve design be kept current to the needs of the well. And the good news is that with an effective gas-lift surveillance system, it is relatively easy to keep the valve design current. And in most cases, it is relatively inexpensive to change gas-lift valves using wireline techniques. The key word here is diligence. If the current gas-lift valve design is not sufficient to allow the well to work down and stay on bottom, change it. Do not "live" with an incorrect design that can cause severe inefficiencies by forcing the well to lift from an upper valve or valves, or be unstable.
· Unfortunately, even if the gas-lift mandrel and valve designs are optimum, the well may still not be able to lift from the desired depth. There may be holes, leaks, blockages, etc. Any such impediments must be quickly found and corrected if gas-lift is to be optimum.
So, the role of the gas-lift surveillance system is to help assure that each well can "work down" to the desired depth of injection through proper design, and then to continuously monitor the well to assure that it stays there. If any problem is preventing the well from lifting from the deepest feasible point, it must be identified and corrected as quickly as possible to optimize the over-life economic performance of the well.
Stability. Continuous gas-lift is more effective and efficient when the injection rate and pressure are stable. Many bad things can happen if the injection is allowed to be unstable or heading. If the well is unstable but still lifting only from the desired operating depth, it is, by definition, over injecting part of the time and under injecting the rest of the time. If the well is unstable and cycling between valves, this is even worse. When it is lifting from an upper valve, the gas-lift effect is much less.
Instability is easily detected. It can be seen by monitoring the casing and tubing pressures on a continuous basis, with a two or three-pen recorder, or with a modern surveillance system. The trick is in determining how to correct instability. The old operator's method of injecting more gas to stabilize a well often does more harm than good. It can exacerbate the problem of multi-pointing, and it may very well waste gas.
Instability is most often caused by a mismatch between the injection rate and the capacity of the operating gas-lift valve or orifice to handle this rate. If the injection rate and/or pressure are too low, the pressure may not be high enough to keep the valve open, or open far enough, and it may intermittently close and open. (This is often referred to as "self intermitting" and is normally highly inefficient.) Also, if the valve is not fully open, it may throttle. This is also an inherently unstable state. Or, if the well is lifting from an orifice, the differential pressure from casing to tubing may be too low to sustain stable flow.
If the rate or pressure is too high, the valve or orifice may enter "critical" flow. The valve or orifice may not be able to handle all of the "extra" gas, so the pressure will rise. It may very well rise to the point where the next higher valve opens. This will cause multi-pointing.
Instability is not always caused by a mismatch between the injection rate/pressure and the valves. It can also be caused by a leaking mandrel/valve, or by a hole in the tubing. Clearly, a problem such as this will cause continual inefficiency until it is corrected.
Optimum Rate. If a gas-lift well is properly designed and operated, it may be possible to inject gas on a continuous, stable basis, from the deepest feasible point, over a limited range of rates. At any given point in time, there is one injection rate that is optimum; one rate that optimizes the economic return based on unit value of production per unit cost of injection. A goal of gas-lift surveillance is to always know this optimum rate, which can change over time as well conditions change, and inject at this rate, if possible.
However, several factors must be considered in a comprehensive gas-lift surveillance system.
First, the rate that is "optimum" from a well production point of view may not be optimum from a gas-lift equipment or design point of view. For example, the economic "optimum" injection rate for a well might be 1,000 MSCF/Day (about 30,000 M3/Day). However, if the maximum capacity of the "operating" gas-lift valve or choke is only 800 MSCF/Day (24,000 M3/Day), the well may go unstable at the higher rate and both production and efficiency may suffer greatly. Therefore, for effective gas-lift operation, the surveillance system or process must find the true optimum injection rate, with all factors taken into consideration. Now, if the well would actually produce significantly more oil at the higher injection rate, the logical step would be to change the operating gas-lift valve or orifice so that the higher rate could be injected in a continuous, stable way.
Second, it will normally be the case that the amount of gas available for injection into all of the wells in a gas-lift system is not equal to the sum of the optimum injection rates of the individual wells. So, a balancing act is required. If there is less total gas available, it must be distributed in a way to optimize the overall performance of the field, even though this will mean that some wells will be producing less than optimal, and some may actually be closed in. If there is more gas available than needed for optimum production of all wells, the excess should be sold or used for other purposes. It should not be injected as over injection can cause just as severe inefficiencies as under injection.
There is one very important point to remember. For optimum operation, a gas-lift well must be designed to produce at its optimum rate. This means that the entire gas-lift system as it affects the well - gas-lift injection rate, gas-lift pressure, mandrel spacing, gas-lift valves, tubular sizes, surface facilities, etc. - must be designed (sized) to handle the optimum injection and production. To accomplish this, the surveillance system must allow the gas-lift staff to determine the optimum production rate to be designed for.