In upstream sector, the activities up to the producing wellhead (drilling, casing, completion, wellhead) are often called “pre-completion,” while the production facility is “post-completion.” For conventional fields, they tend to be roughly the same in initial capital expenditure. Once produced oil and gas from reservoir come up to the surface, they need to be stored and pretreated at field processing facility. Unlike refinery or chemical plant where complex chemical reactions are conducted to process hydrocarbons and create final products from them, the field processing facility is different in a number of ways. The process is simpler with its main tasks are to connect the wells and the transportation system (pipeline, truck, ship…); separate the liquid, gaseous, solids and remove unnecessary substances (H2S, CO2); change and measure temperature, pressure for further transportation.
Although there is a wide range of sizes and layouts, most production facilities have many of the same processing systems shown in this simplified overview
Wellhead
The wellhead sits on top of the actual oil or gas well leading down to the reservoir. The wellhead structure, often called a Christmas tree, must allow for a number of operations relating to production and well workover. It is an assembly of valves, spools, chokes, manifolds and fittings able to withstand high pressure from reservoir or from production activities.
Manifolds and gathering
The individual well streams are brought into the main production facilities over a network of gathering pipelines and manifold systems. The purpose of these pipelines is to control and distribute the well flow to further facilities, that also provides selecting production rate corresponding to potential of specific well. At this phase of production process, parameters of well flow such as pressure, temperature and flow rate are controlled and monitored closely by sensors, gauges and flow meters throughout the manifolds and gathering systems.
For gas gathering systems, it is common to meter the individual gathering lines into the manifold. For multiphase flows (combination of gas, oil and water), the high cost of multiphase flow meters often leads to the use of software flow rate estimators that use well test data to calculate actual flow.
Separators
A separator is a large vessel designed to separate production fluids into their constituent components of oil, gas and water. Based on the vessel configurations, the separators can be divided into horizontal, vertical, or spherical separators. Separators are also classified by function (two phases or three phase operation), by operating pressure (high, medium, low) and by application (metering, testing…). There are some different methods are used to separate each phase of production fluids, the method based on density difference (gravity separation) is the most common and used widely because of its simplicity. In gravity separation, the well flow is fed into a horizontal vessel. The retention period is typically five minutes, allowing gas to bubble out, water to settle at the bottom and oil to be taken out in the middle. The pressure is often reduced in several stages (high pressure separator, low pressure separator, etc.) to allow controlled separation of volatile components. A sudden pressure reduction might allow flash vaporization from oil leading to instability and safety hazards should be taken into consideration.
Metering and storage
In most cases, produced gas is not stored at the field site but is lead to immediate consumption or chemical plant for further processing through pipeline. On the other hand, oil is often stored before loading on a vessel such as shuttle tanker taking oil to a larger tanker terminal, or direct to a crude carrier. Offshore production facilities without a direct pipeline connection generally rely on crude storage in the base or hull, allowing a shuttle tanker to offload about once a week. A larger production complex generally has an associated tank farm terminal allowing the storage of different grades of crude to take up changes in demand, delays in transport, etc.
Metering stations allow operators to monitor and manage the natural gas and oil exported from the production installation. These employ specialized meters to measure the natural gas or oil as it flows through the pipeline, without impeding its movement. This metered volume represents a transfer of ownership from a producer to a customer (or another division within the company), and is called custody transfer metering. It forms the basis for invoicing the sold product and also for production taxes and revenue sharing between partners. Accuracy requirements are often set by governmental authorities.
Typically, a metering installation consists of a number of meter runs so that one meter will not have to handle the full capacity range, and associated with prover loops so that the meter accuracy can be tested and calibrated at regular intervals.
Utility systems
Utility systems are systems which do not handle the hydrocarbon process flow, but provide some service to the main process safety or residents. Depending on the location of the installation, many such functions may be available from nearby infrastructure, such as electricity. Many remote installations are fully self-sustaining and must generate their own power, water treatment, cooling system, etc. Utility systems are auxiliary pieces to overall solutions, but are a vital part of a successful oil and gas field.
Conclusion
From the wellhead to facilities where oil and gas can be stored, treated or transferred for further processing, is a seamless system with many components and their own task, but sole purpose of providing the flowline under control, efficiency for production and further treatment. As mentioned above the capital expenditure of production facilities is as much as pre-completion activities, therefore this is also a very important part of oil and gas production process, this area of production phase must be taken seriously, apply new technologies to optimize performance is essential as any other part of the upstream sector.