Ejector Wells

[Sherman Desrosiers]

Like the deepwell systems, the ejector or eductor system is based on wells that are drilled to lower the groundwater level to provide stable working conditions. Ejectors use air within the wells to produce a vacuum to draw water out of the soil.

The system works by circulating high pressure water which is fed from a tank and supply pumps at ground level down the well into the ejector nozzle and venturi located at the foot of the well. The flow of water through the nozzle generates a vacuum in the well and draws in the groundwater. It is then piped back to ground level and back to the supply pump for recirculation.

Ejectors use water as the pumping mechanism which creates a vacuum by recirculating water at a high pressure through a nozzle and venturi within the ejector body. An ejector system is typically used in soils such as Silts, silty Sands or laminated Clays which have a low permeability. They are well suited for deep excavations were the depth is beyond the scope of a vacuum wellpoint system and where well yields are below the effective use of a deep well pumping system.

Ejectors wells are installed at relatively close spacing (typically 3m-6m centres) and drilled using a rotary or cable percussive drilling rig (typically 200mm bore) into which a well liner is placed (typically 100mm diameter) with a surrounding filter annulus. To assist in generating flow inducing vacuum within the well a bentonite or grout seal is installed. For most projects vertical wells are installed, however where access is an issue, wells can be installed at an inclined angle.

After airlift development of the wells to increase efficiency and reduce the risk of pumping fines. A twin pipe ejector is installed on supply (25/32mm) and return (32/40mm) riser pipes that in turn are connected to two parallel header mains. One header main is a high-pressure supply line and the other header is low pressure return line. Both lines run to a recirculating central surface pump station which supplies pressurised water (typically 5-7 Bar) to the ejector units, which then returns water to a tank, to feed the pump and were any excess (or induced groundwater) is discharged.

Typically, a single pump can operate 100 lin.m system (30no-15no ejectors) although this varies considerably on the volumes of water to be pumped, the lift or supply flow and pressure required. The volumes of water which can be pumped by each system are generally low, typically less than 5 m per hour.

Ejectors are predominantly used for temporary construction dewatering. However, they can be used for permanent dewatering schemes and as a means of abstraction for irrigation or private water supply systems.

Ejector wells work on the same principle as a well point system but allow water to be drawn from deeper in the ground. An ejector dewatering system consists of an array of wells pumped by jet pumps installed at the base of each well up to depths of 150 metres.

Certain types of ground condition require alternative techniques such as ground where the permeability is very low or where the depth and nature of the excavation precludes the use of wellpoints and centrifugal pumps. One of the techniques commonly used then is the high pressure ejector system. This system works on the principle of forcing water at high pressure down the well and through a nozzle to create a venture effect, which in turn creates a vacuum of sufficiently high level to draw up the water surrounding the bottom of the well and return it to the surface.

Where groundwater must be lowered more than six metres below ground in lower permeability soils, such as silt or fine sand, ejectors often offer the best dewatering solution. At depths greater than 45-50m, ejector systems can become inefficient, often making a vacuum deepwell system more appropriate.

Ejectors occupy a niche in the dewatering industry, where pumping levels are too deep for wellpoints, but well yields are too low to allow the use of electric submersible pumps alone (as with classic deepwells).

Ejector systems require both a high-pressure supply and low pressure return header main following the line of wells. Project Dewatering offers clients the choice of either a double pipe or concentric pipe ejector system to suit the site conditions.

Ejectors require a greater level of experience to successfully operate than traditional wellpoint and deepwell systems, and can often require more regular maintenance, due to the loss of performance and efficiency caused by naturally occurring iron-related bio-fouling. Project Dewatering has a wealth of experience in the installation, monitoring, control and maintenance of these systems, ensuring drawdown requirements are met and sustained throughout the course of the dewatering process.

An ejector is used in upstream processing to compress or boost the pressure of an entrained fluid. It is an alternative to a vapour recovery unit (small compressor recovering gas) in some applications. It can be less capital cost, lower operational costs, and less maintenance intensive.

An ejector is not as common as a compressor in upstream processing (especially offshore applications) because it needs a high-pressure motive fluid (gas). This is not available if the site mainly recovers oil. Ejectors are also more complicated to design and use in production fields where new wells are regularly being drilled and wells are continuously changing output over time.

Gas ejectors use high-pressure gas to compress surplus or low-pressure gas, flare gas, or vent gas. They are useful when the gas (and motive gas) can be compressed high enough to get to its destination. If the gas cannot be compressed high enough, it will need recompression and it may be more efficient to instead use a single compressor for all the compression.

Figure 1 shows the basic components of an ejector designed for use with gas. It has three connection points: one for the high-pressure gas (motive fluid), one for the low-pressure gas, and one for the discharge. The ejector uses a converging nozzle to increase the fluid velocity, which converts the pressure energy of the highpressure fluid into kinetic energy.

This conversion of static pressure to velocity pressure results in a low-pressure zone at the vena contracta that provides the motive force to entrain a side fluid (or gas). The mixed fluid then flows through a diffuser section comprising a diverging nozzle. This reduces the velocity and increases the pressure, thereby recompressing the mixed fluid. This enables the ejector to discharge at a pressure that is greater than that of the low suction branch.

System designs in which the flare gas is compressed into the fuel gas system are common. An example of an ejector in a flare gas recovery system is shown in Figure 2. The ejector system should be designed to avoid creating a vacuum in the flare gas line to avoid air ingress (flammable mixture) and ensure safe operation.

This equipment can be used to restart production of existing low-pressure wells which have been shut in for years due to high backpressure. An example of this is shown in Figure 3. If a suitable high-pressure well is available nearby, the pressure energy that is normally wasted across a choke could be used to drive an ejector to entrain the gas from the low-pressure well.

This can then bring the well back into production, and the need for additional external gas compression is reduced.

In some cases, an increase in gas production is not possible without adding another compressor. Yet, by using an ejector upstream of a compressor, the manifold pressure of the wells is reduced and thus gas production is boosted. The increase in production can reach up to 15% as a function of well performance.

Ejectors can be used to recover gas that currently is flared due to working losses from storage tanks (which occur when tank crude level changes and when crude is agitated) and standing losses (which result from the thermal expansion and contraction of the tank and vapour mixture from the daily heating cycle).

An example of this is shown in Figure 5 (and described more in Reference 1). The ejector system should be designed to avoid creating a vacuum in the storage tank vent line which could cause air ingress and a flammable mixture hazard.

Ejectors can be used offshore on vessels that deaerate seawater. The ejector pulls a vacuum on the vessel containing seawater, which pulls more air out of the water. In this case the motive fluid is typically compressed air which did take energy to compress but is likely already in service at the site.

The most common application of an ejector in downstream processing is on a vacuum distillation column in the crude unit. There are usually several ejectors in series that pull a vacuum, and the motive fluid is steam (not an existing high-pressure motive fluid). The vacuum allows the crude oil to vaporize at a lower temperature. This minimizes the cracking of molecules and reduces furnace firing.

Alternatively, a high-pressure fluid can be generated to be used with an ejector. An example would be the use of a closedloop water system where water is pumped so it can be used as a motive fluid for an ejector. The water is then recovered in a downstream separator and recycled back to the pump.

Onshore facilities might be better applications for ejectors because people can more easily make adjustments to the ejectors as motive fluid flows and pressures change. Limitations in people resources are not present to the extent that they are in offshore applications.

Flow rate variability of the low-pressure gas is common. If this variation is not controlled, the suction pressure created by the gas ejector will also vary. To maintain the desired pressure on the low-pressure side of the gas ejector, some standard control techniques are available, including the following:

Another operational risk is erosion fouling from solids in the gas. Also, at sonic velocities through the venturi throat, there will be wear and tear of the nozzle. Wear and tear increases the motive fluid demand for the same performance. This often goes unnoticed or, if noticed, unactioned. This causes the ejector to operate at ever-increasing motive fluid demand / steam demand and associated increased operating cost.

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