|BRITAIN'S OFFSHORE OIL & GAS
Getting Every Last Drop Out
Crude oil can contain acidic fluids including hydrogen sulphide and carbon dioxide which corrodes casing. If necessary high grade steel production tubing is inserted into the well to collect oil and gas and protect the casing. Access to the reservoir is achieved either by perforating holes through the casing installed across the reservoir using small explosive charges, or by running casing with pre-drilled holes or slots. Many sandstone reservoirs are liable to collapse and produce sand along with petroleum - in these wells "sand screens", which filter out the sand particles downhole, are run. Flow from the well is controlled by valves on the "Christmas tree" (F97) at the wellhead.
For smaller fields, rather than being drilled from a large central platform, the wells are drilled from subsea clusters. For these types of wells, the wellhead and Christmas tree is installed directly on the seabed, with production from several wells co-mingled at a subsea manifold. Subsea manifolds are often linked by pipelines and umbilical control lines back to a nearby platform, where engineers can control and monitor the oil and gas production. Alternatively, the production can be piped to a Floating, Production, Storage and Offloading vessel (FPSO) for processing and export (F88). Floating production facilities are generally less expensive to install than fixed platforms but their operating costs are higher. The disadvantage of floating production systems is the weather may prevent the docking of the offloading shuttle tankers for several days during the winter period.
In oil reservoirs, to achieve as high a recovery factor as possible reservoir pressures must not be allowed to fall too low as oil and associated gas are removed. It is desirable to maintain pressures above the point where dissolved gas in the oil comes out of solution to form free gas. Seawater is pumped into the water-soaked rocks beneath the oil zone in volumes equal to the sub-surface volume of the liquids produced. Water injection wells are usually located around the periphery of an oilfield. Gas separated from oil on the platform may also be compressed and injected into the reservoir rocks to maintain pressure. Water and gas injection can improve recovery of oil from less than 15 percent to more than 50 percent. Very deep fields, such as Brae, with high pressures and temperatures may yield condensate, a valuable light oil which exists as dissolved in gas in the reservoir. Dry gas will be injected into the reservoir to maintain pressure, thus avoiding condensate drop out, and to sweep the gas condensate to the wells. Downhole pumps have been used offshore when reservoir pressures are insufficient to send the oil to the surface, as in the Beatrice Field. A more common technique is gas lift in which gas from the same nearby field is mixed with oil in the tubing to lessen the weight of the liquid column (F98).
Flow from every oil and gas well is tested and monitored throughout the life of the well. Replacement of worn equipment such as tubing and valves helps prolong the life of the well. In less productive wells, well stimulation may be tried. High-pressure fluids are pumped down the well to create deep fractures in the reservoir rock through which oil and gas can flow. These fractures are held open by sand grains which are forced into the fracture with the fluid. Acid stimulation helps remove clogging mineral scale such as calcium carbonate which may have accumulated during years of production.
In extended reach and horizontal wells, coiled tubing is often used to carry production equipment to the bottom of the well. Coiled tubing is more flexible and much quicker to use than the conventional drillstring.