Glossary

Occupational Safety and Health Administration	OSHA. A U.S. government agency that establishes and enforces safety standards for industry employees.

OCS	Outer Continental Shelf.

offset well	(1) A well drilled on the next location to the original well. The distance from the first well to the offset well depends upon spacing regulations and whether the original well produces oil or gas. (2) A well drilled on one tract of land to prevent the drainage of oil or gas to an adjoining tract where a well is being drilled or is already producing.

offshore drilling	Drilling for oil in an ocean or large lake. A drilling unit for offshore operations may be a mobile floating vessel with a ship or barge hull, a semisubmersible or submersible base, a self-propelled or towed structure with jacking legs (jack-up drilling rig), or a permanent structure used as a production platform when drilling is completed. In general, wildcat wells are drilled from mobile floating vessels (as semisubmersible rigs and drill ships) or from jack-ups. while development wells are drilled from platforms.

ohm	A unit of electrical resistance. The resistance of a conductive material in which a potential difference of one volt produces a current of one ampere.

ohmic	A term which describes a material or substance which has electrical resistance.

ohm-meter	A unit of electrical resistivity, also written ohm meter²/meter The resistivity of a cubic meter of material which offers a resistance of one ohm to the flow of electrical current between two opposite faces. The reciprocal is mho per meter.

Ohm's law	E = IR, where E is the potential in volts produced by the flow of current (I) in amperes through a length of material exhibiting resistance (R) in ohms.

oil-base mud	An emulsified drilling mud in which the continuous phase is oil and the discontinuous aqueous phase occupies less than ten percent of the volume. Electrically nonconductive. Compare invert oil emulsion, oil emulsion, and water-base mud.

oil-cut	Containing less than measurable amounts of oil (describes a liquid; as oil-cut mud recovered in a drill-stem test).

oil emulsion	Refers to a fluid mixture, usually drilling mud, in which the continuous phase (external phase) is water and the discontinuous phase (internal phase) is oil. Electrically conductive. Compare oil-base mud and invert oil emulsion.

oil field	The surface area overlying an oil reservoir or reservoirs. Commonly, the term includes not only the surface area but may include the reservoir, the wells, and production equipment as well.

oil in place	The amount of crude oil that is estimated to exist in a reservoir and which has not been produced.

oil mud	A drilling mud in which oil is the continuous phase.

oil patch	A term referring broadly to the oil field and the activities of oil and gas exploration and production.

oil shale	The term applied to several kinds of organic and bituminous shales, most of which consist of varying mixtures of organic matter with marlstones, shale, and clay. The organic matter is chiefly in the form of a mineraloid, called kerogen. Oil shales are widely distributed throughout the world and become of economic interest because of the large amounts of oil which can be obtained from them. See kerogen.

oil stain	Visible oil seen on surfaces of grains or fragments of rock samples.

oil-water contact	The highest depth (shallowest depth in a well) opposite a formation at which virtually l00% water can be produced. This depth is at the bottom of the oil-water transition zone.

oil-water interface	See oil-water contact.

oil wet	Oleophilic. A condition in which oil wets the rock surfaces. Often described by the angle of contact of an oil droplet on the solid surface. The lower the angle (measured inside the oil phase) the greater the adhesion and the greater the degree of wettability for oil. If the nonwetting aqueous phase should be forced to move, it would advance over the adhesive layer of the oil.

oleophilic	Having a strong affinity for oils. Preferentially oil wet.

open hole	Uncased hole, or uncased portion of the hole.

openhole completion	A method of preparing a well for production in which no production casing or liner is set opposite the producing formation. Reservoir fluids flow unrestricted into the open well bore.

operator	The person or company, either proprietor or lessee, actually operating an oil well or lease. Compare unit operator.

OSHA	Abbreviation of the Occupational Safety and Health Administration.

osmosis	The spontaneous flow of molecules of the solvent of a more dilute solution into a more concentrated solution when separated from one another by a suitable semipermeable membrane. Compare diffusion.

outcrop	(1) The exposed portion of a buried layer of rock. (2) To appear on the earth's surface (as a rock).

overburden	Geostatic load. The aggregate of overlying rock column including the fluids contained within the pores of the rock.

overlay	(1) To place one recorded curve over another. See also normalize. (2) A well log in which one curve has been drafted or recorded over another and the relationship between the curves can be observed. The position of one curve with respect to the other and the amount of separation between the two curves provides specific information with regard to rock properties, lithology, mineralogy, and fluid saturations.

overpressure	Is an expression which has been used commonly to refer to high pressure found in some formations; super-normal pressure or surpressure. Technically, it should be said that overpressure is that amount of pore pressure which is in excess of normal pore pressure in overpressured formations. Any pore pressure greater than normal pore pressure can result from a number of conditions, some of which are listed below: (1) Abnormally high pore pressure related to geostatic load. As geostatic load increases. porous clay rock compresses with the resulting expulsion of associated water. Fractures and porous and permeable reservoir beds serve as conduits to carry off the expelled water. If the water in the pore spaces of the reservoir rock cannot be expelled, the water will be trapped. Under these conditions, as overburden is increased, the clay rock is prevented from further compaction and the compressive stress is transmitted to the interstitial water. Pore pressure in isolated reservoir beds will increase along with the pore pressures in the overlying clay rock (shale). (2) Abnormally high pore pressure related to the density contrast between reservoir fluid (if oil or gas) and interstitial water. Some reservoirs (which when filled with water, exhibit normal pore pressures) exhibit abnormal pressure at the crest of the structure when containing a column of oil or gas. For example. in a gas-filled reservoir, the normal pore pressure at the level of the gas-water contact may be transmitted through the gas column to the crest of the structure with only a small reduction (since the pressure exerted by the weight of a substantial column of gas is low compared to that of the same height of interstitial water). This results in higher than normal pore pressure at the crest of the structure. (3) Abnormally high pore pressure related to causes other than those found in (1) or (2). For example, high pressure may result from the charging of one bed in communication with another at higher pressure.

overshot	Cable-guided overshot used in wireline fishing operations. A device mounted on an appropriate adapter which can be attached to the bottom end of the drill pipe or tubing. The overshot is open-ended, hollow, and contains a spiral grapple. When used in fishing for stuck logging tools, for example, it is guided into position by the unbroken cable. The grapple engages the fishing bell housing when rotated slowly to the right. See also cut-and-thread fishing technique.

oxidation-reduction potential	Redox potential. Eh. A quantitative measure of the energy of oxidation. Oxidation is equivalent to a net loss of electrons by the substance being oxidized, and reduction is equivalent to a net gain of electrons by the substance being reduced. The oxidation-reduction reaction involves a transfer of electrons. The oxidation-reduction potential may be expressed as the ability to give or receive electrons and is expressed in terms of millivolts.