Gas lift for unusual environments

Gas lift is especially suited for application in unusual environments. This page discusses a few of these environments and how gas lift is used in the particular application.

Offshore platforms

Gas lift is a widely used artificial-lift system in offshore installations and has performed exceptionally well. Most downhole gas lift equipment has few if any moving parts and requires little maintenance. What little maintenance that is required can normally be done with wireline equipment. For this reason, downhole gas lift equipment is much less costly to replace than other forms of downhole artificial-lift equipment. The required space, or "footprint," and weight of gas lift surface controls are minimal. Because produced gas from the offshore wells generally must be conserved, it can be compressed and provide a ready source of high-pressure gas for gas lift.

The process of gas lift from offshore platforms is slightly different from gas lift onshore. In many cases, the surface installations on offshore platforms are better designed than onshore installations. If designed properly, there will be very little pressure loss caused by restrictions or flowlines. The compressors for gas lift are located nearby, so the distribution of gas should be no problem. Because of the importance to the overall installation, gas compression and dehydration equipment is normally operated and maintained by people who are provided for this specific purpose.

Safety devices

There are many more safety devices on offshore platforms than on onshore installations. High-/low-pressure shutoff devices are installed at the wellhead on both the injection-gas line and the flowline to automatically close in the well at the surface, should there be a radical change in either line’s operating pressure.

Surface or downhole safety devices are a necessary part of any offshore well. With so much property and human life at stake on the platforms, it is an absolute necessity to prevent downhole or surface catastrophic failure. Safety valves are included on the production string of a gas lift well and may cause some restriction to flow. In the North Sea, governmental agencies also require a safety valve on the gas-injection side of gas lift wells. The reason is the possibility of the check valve on the gas lift valve failing, which would allow well fluids to flow into the casing. Restrictions from subsurface safety equipment may enter into the design of a gas lift installation.

Gas lift installations drilled from offshore platforms

Wells drilled from offshore platforms have varying degrees of deviation that must be accounted for in predicting vertical multiphase flow. Most gas lift design programs can take this varying deviation into account. Wireline operators report little problems in setting valves in mandrels at deviations up to 60°. Motors have been developed to move tools downhole at high deviations and have been used in some instances. The gas lift retrievable-valve mandrels with orientation sleeves are designed to insure that the valve enters the pocket regardless of pocket orientation relative to the vertical.

Subsea gas lift installations

Subsea installations increased dramatically in the last few years with the addition of producing areas in greater and greater water depths. Today, it is not unusual to find production from water depths over 6,000 ft with gas lift being the preferred production method in most of these completions. With extreme water depths, some form of artificial lift is usually required just to kick off the well and move the production from the seafloor to the surface.

Devices and equipment for carrying gas lift equipment into subsea wells were perfected at the onset of subsea technology. These include devices for carrying the valves and engines for driving the valve downhole through flowlines and other systems. Although such tools are available, these operational techniques are not widely used. Most subsea gas lift today is done with high-pressure gas and a single orifice placed as deep as the pressure will permit. By using this approach, there is no gas lift equipment in the well that could fail or need replacing. In many deepwater installations, where the water depth itself adds considerable head that must be overcome to produce the well, gas injection is often through a single point at the wellhead or near the mudline.

Heavy-oil production

Gas lift has been used to produce high-viscosity oil in many parts of the world. Heavy oil is being produced by gas lift in Venezuela. Diluent injection has been found to be very beneficial in producing all types of artificial-lift wells in eastern Venezuela. Diesel fuel is usually used as the diluent, and the injection of approximately 10% by volume lowers the specific gravity and increases the API gravity of the oil.

Water can be used as a diluent as well as diesel fuel and has been proven effective to reduce backpressure in large pipelines. In most cases, water acts to reduce backpressure by adding a water ring around the viscous crude that reduces friction between the crude and the pipe wall. When this occurs, water becomes the wetting phase and the resulting friction is similar to that of water. However, unlike the diesel fluid which increases the value of the produced oil by lowering its viscosity, water adds another phase that must eventually be removed.

Gas lift with air, nitrogen, and carbon dioxide

Typically, gas lift designs are based on natural gas as the injection gas. However, gas lift with natural gas did not begin in a big way until the 1920s. Early gas lift operations were conducted using air as the injection gas. Many of the early great oil fields such as Spindletop and Goose Creek were produced with air lift. Air has its disadvantages when used for gas lift because oxygen in the air causes serious corrosion, scale, and the possibility of combustion when it is mixed with well fluids. Air is still used in gas lift installations today but on a very limited scale.

Nitrogen and carbon dioxide offer good alternatives to natural gas for gas lift. Nitrogen can be used for gas lift because it is inert, relatively inexpensive, and noncorrosive. Quite often, nitrogen is available at high pressure near the producing facilities, where it is being used for various enhanced-oil-recovery and pressure-maintenance projects.

Carbon dioxide is also readily available from miscible displacement projects and can be used for gas lift purposes. Natural gas containing 75% carbon dioxide was used for gas lift in two large fields in Argentina without noticeable problems. In heavy-oil reservoirs, carbon dioxide is also very useful in decreasing the viscosity of the oil. Both nitrogen and carbon dioxide can be purchased at a price competitive with natural gas in many locations. The use of small nitrogen plants for gas lift of remote offshore and onshore locations has proved feasible under certain conditions.

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See also

Gas lift

PEH:Gas_Lift