the mud in the tanks is saved for use in drilling the next well. After the final well is drilled, all mud is generally barged ashore for recovery of chemical components or storage. Occasionally, when transport to shore storage facilities is not feasible, the unweighted components (whole mud less barite) are discharged overboard. The high cost of barite and other chemical additives makes their recovery economically desirable, however, and tends to keep overboard disposal at a minimum. Special Cases Occasionally, abnormal formation pressures, exceptionally tight formations, or other problems require the use of oil-based or highly treated drilling muds. Drill cuttings are then separated and cleared of entrained oil before being discharged overboard, and the drilling muds are retained and shipped to shore and stored in tanks for Wells usually are produced through tubing placed inside the final or production string of casing. During tubing installation, the blowout preventers remain in use to ensure control of the well. A system of in-tubing safety valves, plus other casing and tubing valves at the surface or seafloor, is installed at the platform cellar deck level and subsurface safety valves are installed at depths varying from a few hundred to several thousand feet in the tubing string. Of major concern in the operation and control of every production platform are the downhole control devices. Production tubing is fitted with one or more safety valves that are installed and located at least 100 feet below the mud line or seafloor. In the past, velocity choke valves designed to shut off production when the flow rate exceeds predetermined limits have been used. Such valves should close if surface equipment failure results in an excessive flow through the tubing. These chokes are particularly susceptible to failure from internal erosion in areas where sand is produced along with the oil and gas. Newer types of valves do not depend on the velocity of well fluids for actuation, but are held open by hydraulic or other fluid Release of this pressure by a pressure applied from the surface. control signal, or by an accident, causes them to close immediately. Their use increases costs significantly, but the need for more reliable valves has been shown by recent incidents in the Gulf of Mexico and elsewhere. The Environmental Protection Agency has noted, and we agree that reliability of velocity actuated subsurface safety devices has been very low in recent disasters (11 out of 22 wells failed in the Bay Marchand Fire). It is hoped that the newer pressure-release-closure valves will offer an increased degree of safety but they have not been in OCS waters long enough to allow a confident prediction of reliability. (2) Produced Formation Water The waters associated with oil and gas pools which are frequently produced along with the oil and gas are called formation waters. The lower edge or boundary of most oil and gas pools is marked by an oil-water or gas-water contact. In some pools, water is produced with the oil in early stages of production, whereas in others, appreciable water never comes up with the oil. Most formation waters produced in the Gulf of Mexico are brines, characterized by an abundance of chlorides, mostly as sodium chloride, and have concentrations of dissolved solids several times greater than that of seawater. The total amount of mineral matter commonly found dissolved in oil-field waters range from a few parts per million (ppm), nearly fresh water, to approximately 300,000 ppm, a heavy brine. One of the highest brine concentrations recorded was 624,798 ppm; or 64.3% from a field in Michigan (Case, 1945, as cited by Levorsen, 1958). 1/ 1/ Case, L. C., 1945; Exceptional silurian brine near Bay City, Michigan. Bull. Amer. Assoc. Petrol. Geol. 29:567-570. It is highly unlikely that any of the produced formation water resulting from this sale would ever be piped ashore. Both economic and environmental considerations weigh heavily towards choosing to treat and release the formation water into the ocean at the platform site or reinject it into subsurface formations. In nearly all cases, reinjection is utilized as a secondary recovery technique by pumping the formation water, under pressure, back into the lower reaches of the petroleum-producing zone and thus maintaining good reservoir pressure. Formation water which is to be discharged into the ocean is first passed through a water-polishing facility that removes all but traces (less than 50 ppm) of entrained oil. However, the water is still void of dissolved oxygen and contains large quantities of dissolved minerals. 1/ d. Workover Operations Since petroleum production involves the handling Nowhere is of flammable fluids under pressure, the safety systems control is 1/ Composition of formation water is discussed in detail in Vol. 1, Sec. II.B.1.b. |