PWT

Produced Water Treatment with Deoiling Hydrocyclones - Misconceptions & Corrections (B-PWT015)

Putting it all Together

Deoiler hydrocyclones are part of a properly integrated process package for optimum Produced Water Treatment results
Process order is important
Desanders first if there are any significant solids present
Polishing afterwards with Dissolved Gas Flotation units best, especially when pressure is available – Mother Nature’s superior Induced Gas Flotation method
Further filtration downstream may be required to meet more stringent specifications, but adds order of magnitude to size, weight & cost.

Thank you.

Produced Water Treatment with Deoiling Hydrocyclones - Misconceptions & Corrections (B-PWT014)

Misconception 6 : Deoilers need pumps to work properly / Deoilers don’t work with pumps

Correction 6 : Deoilers require pumps only when the available operating pressure is below a specified minimum, and operate effectively and efficiently when strict design guidelines are followed.

We saw previously that Deoilers have minimum pressure drop requirement to operate efficiently, and in some cases a maximum pressure drop limit. The minimum pressure drop requirement depends on the Deoiler liner size, but with the latest generation Deoiler this figure is around 40 psi. Below this figure a Deoiler system requires pumps.

At this point one should ask the question, “For the produced water process conditions provided, which include a low operating pressure, should a Deoiler system be chosen as the most appropriate technology?” In some instances the answer is no, so evaluate the equipment choices available carefully (seen earlier in this series), before a final decision on what’s appropriate.

The issue in using a pump with any produced water equipment, let alone a Deoiler, is the fact that many pumps will interact with what might be a quite benign produced water stream, and create a tough to separate emulsion of small oil droplets.

Now there are various low shear pumps, typically progressive cavity pumps, which do a good job in not creating this emulsion. Multistage centrifugal pumps may be applicable as well. However, in both cases the equipment can be quite large and certainly quite expensive.

For the majority of service work in the upstream oil & gas industry, single stage centrifugal pumps are the preferred choice.

Single-stage centrifugal pumps can be used if the shearing effect is minimised by following these strict design requirements;

Single stage centrifugal pump, closed impeller type
Pump speed at less than 1800 rpm
Maximum suction to discharge pressure differential of 100-120 psi
Pump operating at a constant flowrate and at 75% or greater pump curve efficiency
Operating at a constant flowrate using a system recycle design, which allows for any turndown requirement

By following these criteria, a pumped Deoiler system can provide an effective, efficient, produced water treatment system.

Bibliography:

Flanigan D.A., et al., Conoco Speciality Products Ltd., “Use of Low-Shear Pumps and Hydrocyclones for Improved Performance in the Clean-up of Low-Pressure Water”, SPE 19743, ATCE San Antonio, October 8-11, 1989.

Produced Water Treatment with Deoiling Hydrocyclones - Misconceptions & Corrections (B-PWT013)

Misconception 5 : Deoilers are not flexible in dealing with high turndown requirements

Correction 5 : Deoiler systems can handle any online turndown requirements.

This misconception first came about when large numbers of second generation Deoiler liners were first packaged and bundled into single vessel arrangements to handle large flowrates. The turndown was a function of the pressure drop available, and the lower the pressure drop the lower the turndown.

Now this situation was initially dealt with by the provision of blanks which allowed for the slow build-up to the required maximum flowrate, but this also required the unit to be shut down to install more liners. Alternatively, multiple vessels, with multi Deoiler Liners systems could be deployed for higher online turndown, however this required more space, and was more costly.

Then in the mid 1990’s the advent of Packaged Active Cyclone Systems (PACS^TM) was introduced to allow a high turndown capability online within a single vessel. This was achieved by segmenting or isolating banks of Deoilers Liners within the pressure vessel, and turning these banks of liners on or off online, as required, with external vessel piping and valve arrangements.

Packaged Active Cyclone System (PACS^TM)

Notwithstanding this capability, it should be noted that most of the time the design turndown requirements of a PWT system requested are not warranted. This is due to the fact that most of the time water production rates do not need to match the requested water treatment rates. Often forgotten when specifying water treatment rates is that between the producing wells, and the PWT system, is a production separator – typically one of substantial size.

When located and installed properly on the water outlet leg of the production separator upstream of the LCV, a Deoiler system treats the volume of water between the low and high levels of a production separator, up to a maximum capacity.

If there’s little or no water production, the Deoiler system happily shuts-in, until the required water volume builds up. There’s no problem with this batch operation, as the Deoiler with a 2-3 sec residence starts to work instantaneously when required, and shuts down just as quick.

Produced Water Treatment with Deoiling Hydrocyclones - Misconceptions & Corrections (B-PWT012)

Misconception 4 : Deoilers Do Not Work in High Pressure Gas Condensate Systems

Correction 4 : Deoilers work every bit as well, if not better, in high pressure gas condensate systems. The main issue to address here is the pressure profile limits required to ensure the Deoiler is running within its operating envelope.

The misconception here has come about due to the assumption that gas condensate systems contain very small drop sizes and that these prevent the Deoiler from operating efficiently.

Although it’s true that condensate has a higher propensity to shear into small drop sizes given a sizable pressure drop source, this isn’t the problem here.

Produced water in gas condensate systems almost always exists at the higher end of design and operating pressure in the production facility. Operating pressures in excess of 600-800 psig are common. This high pressure source produced water always contains a substantial amount of dissolved gas, and any slight reduction in pressure results in the breakout of this gas.

The Deoiler liner is an oil water separator, and is not designed to deal with gas as well. Any free gas has a detrimental effect on performance, although some gas can be handled in the vessel ‘package’. Now we saw that the Deoiler is a simple, but effective, separator with a 2-3 second residence time and no moving parts.

However the pressure drop required through the Deoiler always results in some gas breakout. The amount depends on the operating pressure, the volume of dissolved gas, and other process conditions. Due to the short residence time in the Deoiler, this gas breakout occurs after it exits the Deoiler, without effecting the Deoiler performance – up to a point.

At increasing flowrates, experienced in gas condensate systems due to the high operating pressure available, this gas breakout can occur within the Deoiler liner. The free gas displaces the oil core to exit through the overflow reject stream. The oil is then choked away from the reject port and exits with the water, resulting in poor performance.

The answer here is to restrict the pressure drop through the Deoiler to a nominal maximum, to ensure the gas breakout does not occur within the Deoiler liner. This nominal maximum pressure drop is around 400 psi, from inlet to reject outlet. The actual figure should be evaluated from field testing beforehand or during commissioning.

Produced Water Treatment with Deoiling Hydrocyclones - Misconceptions & Corrections (B-PWT011)

Misconception 3 : Deoilers cannot deal with Emulsions

Correction 3 : Deoilers can deal and treat emulsions, just not all emulsions efficiently without chemicals.

This misconception comes about due to the expectation that the Deoiler is only a primary separation device. Although this is correct for the first generations of Deoilers, the performance of the latest generation of Deoilers reflect those of a secondary separation device.

Furthermore we saw in the first chapter of this series that when a stable emulsion is present, any mechanical solution alone will not facilitate the separation process, and other mechanisms, specifically the use of chemicals, need to be addressed and used in order to solve the separation problem.

Where the emulsion is unstable this latest generation of Deoilers can effectively treat the fluid with reasonably high efficiency.

Notwithstanding the above, in applications where an unstable emulsion is present, it is highly recommended to undertake a Deoiler hydrocyclone field test to adequately evaluate the performance of the technology under actual operating conditions.

The compact nature of the Deoiler lends itself to an easy field test arrangement where the equipment required is small, compact, and easily mob / demobilised.