Oil Production – Oil and Gas Facilities

Introduction

This post gives some basic information for oil production facilities in Oil and Gas industry.

When an oil-drilling rig strikes oil, the well is said to have “come in”. The drilling rig is removed, and the well is fitted with special equipment to allow the oil to be collected and processed in a controlled way. This collection process is called production. There are three main components of oil production equipment: the wellhead, the pipeline, and the gathering centre.

Wellhead

Oil does not come up out of the ground in a pure condition, but is mixed with water and contains gases such as propane. The gases are dissolved in the oil under great pressure, like the carbon dioxide in a bottled soda.

At the top of every oil well there is a well control head that is connected to the subsurface tubing and casing pipes that line the well. At the top of this well head are a number of valves, pipe fittings and chokes for the separate producing zones of casing and tubing. This assembly is called “The Well head Christmas Tree”. The valves are used to open and shut the well zones as required, while the bean chokes are used to control the flow of the oil/gas/water cut at predetermined levels to prevent over-producing and damaging the reservoir.

Well Pads

The Well Pads typically have 25 to 50 wells on each pad. One operator for every 3 or 4 Well Pads usually handles the Well Pads. The Well Pads are elevated above the tundra by gravel (thus gravel pads) that is about 1/4 mile long by 100 yards wide. Each Well Pad has one or more manifold buildings.

Lufkin Pumps

Low pressure wells sometimes are fitted with special pumps to draw the oil to the surface.

Distribution Centers

Manifold

The oil from the wells flows into and out of the manifold buildings. So oil flows out of a well house, out a flow line and into a manifold building. There it is choked and diverted to appropriate pressures and flow lines and directed to High Pressure (HP) flow lines or Low Pressure (LP) flow lines. The manifold buildings also have a test separator that tests each well monthly to determine the amount of oil, gas, and water that each well is producing.

There are several types of wells on the Well Pads. There is LP oil production wells that flow at about 180 to 200psi. Also, HP or high-pressure wells, that flows at 500 to 2,700 psi. Additionally there are water injection wells that flow or inject at 2,000 psi back down into the formation. Finally, there are MI or gas injection wells that inject gas at 3,000psi back into the formation.

Wells that are routed to the Large Diameter Flow lines (LDFs) go to the GC’s and are routed to one of three separation vessels called Slug Catchers. For general separation these serve the same purpose. However, there is no 1st STG. and the Slug Catcher is basically the equivalent of the 2nd STG. vessel (Serves the same purpose) and operates at about 150# pressure. The gas coming from these Slug Catchers is compressed by compressors (BIG GUYS!) to 650# and flowed with the other HP Gas to the Central Gas (formerly ARCO). Water taken from the vessels is cleaned and injected back.

Gathering Centers

After the oil is manifolded to either a High Pressure or Low Pressure pipeline at the Well Pad, the oil, gas, and water entrained with the oil from the underground formation go to a Gathering Center (GC) (similar to the picture below) associated with the Well Pad for separation of gas and water from the oil stream. Each GC separates the oil from the gas and water and sends the “dry” oil off to a Pump Station. The gas and water are sent back to the Well Pads to be reinjected via injection wells, forcing the gas and water back into the underground formation it came from.

Gathering Centre is a facility, which collects oil from a number of wells, for the purpose of separating out the associated gases and water. Gathering Centres are spread over oil fields. Each Gathering Centre can serve 40-50 wells or more.

The separation process is a series of vessels in which the pressure is dropped in each of 3 “stages” (STG) of separation. (1st STG. 650 pounds (#), 2nd STG. 150#, 3rd STG. 3#) Each stage “flashes” gas off the oil and the gas is compressed further (except the 650# gas) and dried via a TEG Reboiler System and sent to the Central Compressor Plant where it is further compressed to 2,000# or 4,000# and re-injected into the formation at the Well Pads.

Water “falls out” of the oil in the Separators by gravity as well as the pressure drops between stages. Each Gathering Center then removes as much retained oil from this “produced water” as possible before increasing the pressure to about 2,500# and injecting the water back into the formation at the Well Pads.

Gathering Centre

Gas Oil Separators

Oil and associated gas flows from wells to gas separators in the Gathering Centre where gas is separated in consecutive stages by HP (high pressure), and LP (low pressure) separators, and delivered via booster stations to the Liquefied Petroleum Gas Plant (LPG) for processing and fractionation. Some quantities of gas also accumulate at the top of tanks after oil stored in those tanks has settled. These gases, called “tank vapours”, are compressed and condensed and subsequently pumped to the LPG Plant for further processing.

Gas Oil Separators                           Gas Oil Separation Plants

The Pipelines

Crude oil flows out of the wells to a Gathering Centre through multi-phase pipelines laid out in organized corridors. After the three phases (oil, gas and water) are separated, the oil is pumped out to the Tank Farms via a complex of transit lines and mixing manifolds. From the Tank Farms, gravity lines transport the oil to the local refineries and export facilities.

Booster Stations

Likewise, the separated gases from the Gathering Centres are trans-shipped through separate pipeline networks and delivered to Booster Stations for onward delivery to the LPG plants. The Booster Station pressurizes the gas so it will flow properly through the gas lines. The crude oil, which is now free of gas, is pumped from the Gathering Centres to the Tank Farms for storage and shipment.

Field Dehydarators

Properties of crude

1. The viscosity of crude oils decreases as the temperature increases.
2. The flash point temperature increases as the value of API decreases.
3. Morddud crude oil, which is of lower API, obviously shows higher pour point.
4. Morddud crude oil has higher acidity number.
5. Safaniya (Saudi) crude oil contains higher percentage volumes of light distillates, while Bahrain crude oil yields higher percentage volumes of residual distillates.