The following figure gives a simplified overview of the typical oil and gas production process Figure 1 Oil and Gas production overview Figure 2 Oil and Gas production facilities Subsea production systems are wells located on the sea floor, as opposed to at the surface. Like in a floating production system, the petroleum is extracted at the seafloor, and then can be 'tied-back' to an already existing production platform or even an onshore facility, limited by horizontal distance or “offset”. The well is drilled by a moveable rig and the extracted oil and natural gas is transported by undersea pipeline and riser to a processing facility. This allows one strategically placed production platform to service many wells over a reasonably large area. Subsea systems are typically in use at depths of 7,000 feet or more, and do not hav the ability to drill, only to extract and transport. Drilling and completeion is performed from a surface rig. Horizontal offsets up to 250 kilometers, 150 miles ai currently possible. Photo:Norsk Hydro ASA miniature floating tension leg platforms, much like the semi submersible type, with tensioned tendons. areas, diluents and heating may be needed to reduce viscosity and allow flow. Typically the metering installation consists of a number of meter runs so that one meter will not have to handle the full capacity range, and associated prover loops so that the meter accuracy can be tested and calibrated at regular intervals. While the ethane, propane, butane, and pentanes must be removed from natural gas, this does not mean that they are all 'waste products. In fact, associated hydrocarbons, known as 'natural gas liquids' (NGL) can be very valuable by-products of natural gas processing. NGL include ethane, propane, butane, iso-butane, and natural gasoline. These NGLs are sold separately and have a variety of different uses; raw materials for oil refineries or petrochemical plants, as sources of energy, and for enhancing oil recovery in oil wells,. Condensates are also useful as diluent for heavy crude, see below. This process goes on continuously, even today. However, an oil reservoir matures : the sense that a too young formation may not yet have allowed the hydrocarbons to form and collect. A young reservoir (e.g. 60 million years) often has heavy crude, less than 20 API. In some areas, strong uplift and erosion and cracking of rock abo have allowed the hydrocarbons to leak out, leaving heavy oil reservoirs or tar pool: Some of the world’s largest oil deposits are tar sands where the volatile compound: have evaporated from shallow sandy formations leaving huge volumes of bitumen soaked sands. These are often exposed at the surface, and could be strip mined, but must be separated from the sand with hot water, steam and diluents and further processed with cracking and reforming in a refinery) to improve its fuel yield. The Drill String is assembled from pipe segments about 30 meters (100 feet) long normally with conical inside threads at one end and outside at the other. As each 30 meter segment is drilled, the drive is disconnected and a new pipe segment inserted in the string. A cone bit is used to dig into the rock. Different cones are used for different types of rock and at different stages of the well. The picture shows roller cones with inserts (on the left); other bits are PDC (polycrystalline diamond compact, on the right) and Diamond Impregnated. Photo: Kingdream PLC 3.5.1 Subsea wells The variable flow choke valve. The variable flow choke valve is typically a large needle valve. Its calibrated opening is adjustable in 1/64 inch increments (called beans). High-quality steel is used in order to withstand the high-speed flow of abrasive materials that pass through the choke, usually for many years, with little damage except to the dart or seat. If a variable choke is not required, a less expensive positive choke is normally installed on smaller wells. This has a built in restriction that limits flow when the wing valve is fully open. Wells are also divided into production and injection wells. The former is for production of oil and gas, injection wells is drilled to inject gas or water into the reservoir. The purpose of injection is to maintain overall and hydrostatic reservoir pressure and force the oil toward the production wells. When injected water reaches the production well, this is called injected water break through. Special logging instruments, often based on radioactive isotopes added to injection water, are used tc detect breakthrough. 4.2.2 Production separators 4.2.4 Third stage separator An oil heater could be located between the first and second stage separator to reheat the oil/water/gas mixture. This will make it easier to separate out water when initial water cut is high and temperature is low. The heat exchanger is normally a tube/shell type where oil passes though tubes in a cooling medium placed inside an outer shell. 4.3 Gas treatment and Compression For the compressor operate in an efficient way, the temperature of the gas should | low. The lower the temperature is the less energy will be used to compress the gas for a given final pressure and temperature. However both gas from separators and compressed gas are relatively hot. When gas is compressed, it must remain in thermodynamic balance, which means that the gas pressure times volume over temperature (PV/T ) must remain constant. (PV = nkT). This ends up as a temperature increase. The main operating parameters for a compressor is the flow and pressure differential. The product defines the total loading, so there is a ceiling set by the maximum design power. Further, there is a maximum differential pressure (Max P,) and choke flow (Max Q), the maximum flow that can be achieved. At lower flow, there is a minimum pressure differential and flow before the compressor will “surge”: there is not enough gas to operate. If variations in flow are expected or difference between common shaft compressors will occur, the situation will be handled with recirculation: A high flow, high pressure differential surge control valve will open to let gas from the discharge side back into the suction side. Since this gas is heated it will also pass through the heat exchanger and scrubber not to become overheated by circulation. point, means a transfer of responsibility or title from the producer to a customer, shuttle tanker operator or pipeline operator. Special tank gauging systems such as Level radars, Pressure or Float are used to measure the level in storage tanks, cells and caverns. The level measurement is converted to volume via tank strapping tables (dependent on tank geometry) and compensated for temperature to provide standard volume. Float gauges can also calculate density, and so mass can be provided. The main function of the control system is to make sure the production, processing and utility systems operate efficiently within design constraints and alarm limits. The control is typically specified in programs s a combination of logic and control function blocks such as AND, ADD, PID. For a particular system, a library of standard solutions such as Level Control Loop, Motor Control is defined. This means that the system can be specified with combinations of typical loops, consisting of one or more input devices, function blocks and output devices, rather than formal programming. same functions as the control system, and the difference mainly comes down to data architecture and use of communications. 5.1.7 Production Information Management Systems (PIMS) 5.3 Flare and Atmospheric Ventilation
