Upstream Oil and Gas Production Reservoir Fluids

Surface Handling of Reservoir Fluids

Our previous lessons on reservoir drive, artificial lift and enhanced oil recovery were focused on getting the reservoir fluids to the surface. Once the fluids are produced, several steps are necessary to get the hydrocarbons ready for sale. The hydrocarbons must be separated, treated, processed, gathered and stored. This lesson will discuss the processes and surface facilities used to perform these tasks.

Learn more about Oil and Gas Production Reservoir Fluids

Learn About Our Online oil and gas training with EKT Interactive

Separation

Reservoir fluids produced at the wellhead are a complex mixture of liquids, gas and solids. An oil well often produces small amounts of natural gas, along with salt water. And natural gas production often includes salt water and liquid hydrocarbons. The solids produced can include sand, scale and shale sediments. The solid matter must be removed before the oil and gas are separated into specific streams.

When the reservoir fluids flow out the wellhead, they enter an attached pipe called a flow line. The fluids pass through the flow line into a hollow steel vessel called a separator. There are two-phase and three-phase separators. The type of separator used depends on the characteristics of the water produced from the well.

A two-phase separator uses the force of gravity to divide the reservoir fluids into liquids and gas. In this case, minute droplets of produced water are uniformly distributed within the oil and closely bound to the oil. Oil and water in this state is called an emulsion. The emulsion, which is heavy, is driven out the bottom of the separator. Gas, which is lighter, goes out the top.

A three-phase separator is used when some of the produced water rapidly settles out of the oil. This type of water is referred to as free water. Here, free water goes out the bottom of the separator. Oil, or emulsion if it also contains water droplets, goes out the middle while gas still goes out the top.

There are good animations of the separation process on YouTube, and we have included links to some in this lesson’s relevant links section.

Oil and gas go through other treatments and processing in addition to separation.

Let’s first look at how oil, or emulsion, is treated.

Treating facilities for emulsion

After separation, the emulsion is piped into a special vessel called a heater-treater. Various types of equipment and processes are used to separate the water from the oil.

Chemicals called emulsion breakers or demulsifiers may be added to make the droplets of water merge. As the water droplets combine, they get bigger and heavy enough to be separated from the oil.

Electricity can also be used to break the emulsion. When the water droplets take on an electric charge, they move rapidly. The droplets can collide with each other, merge and get heavy so that they separate from the oil.

Finally, the emulsion may also be heated. This reduces the viscosity of the emulsion which allows the oil and water molecules to move rapidly. As with the electricity process, the water droplets can strike each other, merge and separate from the oil. Heat alone will not cause an emulsion to break down – it is used to support the other processes.

Storing crude oil

Oil and water produced by an oil well move from the wellhead, to the separator, through the treating facilities and finally into stock tanks.

A group of stock tanks is referred to as a tank farm or battery. The number of tanks in a battery, and their individual size, will vary depending on the daily production.

For onshore locations, oil production is often gathered by a tank truck from tank batteries that are widely dispersed. In this case, the oil is consolidated into one separation facility with a larger set of tanks where it undergoes treatment.

Now, let’s turn our attention to how natural gas is handled.

Handling natural gas

In addition to separation, gas requires additional treatment and processing for a variety of reasons.
First is the prevention of hydrate formation. A hydrate is a solid crystalline compound formed by hydrocarbons and water under reduced temperatures and pressure, as it leaves the wellhead.

Hydrates resemble snow and can partially or completely block production tubing, pumping equipment and flow lines. To combat hydrates, natural gas is often sent through a series of heaters. Chemical inhibitors can also be injected into the gas stream at various points to prohibit hydrates from forming.

Next, gas is sent through a dehydrator to remove water vapor. Almost all the water vapor must be removed before gas can be sold. Removal of water vapor prevents hydrates and corrosion once the gas is transported via pipeline.

Gas is also treated to remove unwanted contaminants such as hydrogen sulfide, carbon dioxide and nitrogen to get it acceptable for injection into a pipeline for sale.

Finally, gas is processed to remove natural gas liquids, or NGLs. In most instances, NGLs have a higher value as separate products, and it is thus economical to remove them from the gas stream. For more about NGLs look at this lesson’s relevant links.

Some of the needed gas processing can be done at or near the wellhead. This is called field processing. However, the complete processing of natural gas takes place at a processing plant, usually located in a natural gas producing region.

We’ve addressed how oil and gas are separated, treated and processed to get ready for sale. Let’s now discuss how the hydrocarbons are moved from the wellsite.

Gathering systems

Once field treatment and processing is completed, oil and gas moves into a gathering system of low pressure, small diameter pipelines. The gathering system transports and controls the flow from the wells to a main storage facility, processing plant or shipping point.

There are two types of gathering systems, radial and trunk line. The radial type brings all the flow lines to a central header. The trunk-line system uses several remote headers to collect oil and gas and is used most often in large fields that cover a large geography.

For natural gas, the gathering system transports the gas to a processing plant that produces pipeline-quality gas that can enter the pipeline distribution system.

Oil is transported by the gathering system either to storage, the pipeline distribution system or a marine vessel loading facility to be sold to a third party or taken to a refinery to be processed into refined products.

One final important topic related to handling production at the surface is disposal of the produced water.

Water disposal

We’ve seen throughout this lesson that the separation of salt water from other the other reservoir fluids is an integral part of oil and gas production operations. The produced water itself has to be treated and disposed of in an appropriate manner.

Produced water is often pumped into a well specially drilled for reinjection into the reservoir. In addition to disposing of the water, this helps maintain the reservoir pressure.

Produced water can also be reinjected as part of an enhanced oil recovery project or utilized in hydraulic fracturing projects.

If no other alternative is available, produced water may also be trucked or pipelined to a certified water-disposal facility. Disposal of produced salt water is regulated by state oil and gas commissions.