(1) In well logging, an aqueous drilling mud which is more saline than the formation water.

(2) Salts are quite often used in drilling fluids in order to reduce formation damage. These salts usually are sodium chloride or potassium chloride. In which case, to prevent formation damage, the salinity of the salt base mud may equal or may exceed formation water salinity.

A dome that is formed by the intrusion of salt into overlying sediments. A piercement salt dome is one that has pushed up so that it penetrates the overlying sediments, leaving them truncated. Formations above the salt plug are usually arched so that they dip in all directions away from the center of the dome. See also diapir.

• Collection centers (in which salt water from several wells is gathered).
• Central treating plant (in which salt water is conditioned to remove scale or corrosion forming substances).
• Disposal wells (in which treated salt waste is injected into a suitable formation).

(1) The total effective thickness of a permeable section excluding shale streaks or other impermeable zones. Often determined from electrical logs (SP curves and microlog).

(2) The number of separate permeable sands separated by impermeable zones.

(1) A wire line on a drilling rig often used to run or recover tools inside the drill pipe.

(2) A line that can be drafted through the maximum deflections on the SP curve for thick clean sands in a section where the formation water has constant salinity.

(1) Containing as much as it can contain under given conditions of temperature and pressure, as in:

1. solid dissolved in liquid.
2. gas dissolved in liquid.
3. liquid dissolved in gas

(2) Filled to capacity, as fluid (liquid or gas) in formation.

(3) Reached the limit of its measuring capacity, as in electrical instruments. The capability of an instrument to detect variations is decreased as the measuring instrument nears saturation.

(1) The fraction or percentage of the pore volume occupied by a specific fluid (e.g., oil, gas, water, etc.). The fluids in the pore spaces may be wetting or nonwetting. In most reservoirs, water is the wetting phase, but a few reservoirs are known to be oil wet. The wetting phase exists as an adhesive film on the solid surfaces. At irreducible saturation of the wetting phase, the nonwetting phase is usually continuous and is producible under a pressure gradient to the well bore.

(2) The occupation of fluids in a pore may take different forms:

1. Funicular saturation. A form of saturation in which the nonwetting phase exists as a continuous web throughout the interstices. The nonwetting phase is mobile under the influence of a hydrodynamic pressure gradient. The wetting phase might or might not be at irreducible saturation. In the illustration, the oil in the "A" figure is funicular
2. Pendular saturation. The wetting phase exists in a pendular form of saturation. An adhesive fluid film of the wetting phase coats solid surfaces, grain-to-grain contacts, and bridges fine interstices or pore throats. The wetting phase might or might not be at irreducible saturation. In the illustration, water in the "A" and "B" figures is pendular.
3. Insular saturation. A form of saturation in which the nonwetting phase exists as isolated insular globules within the continuous wetting phase. A drop in pressure might or might not cause the insular globules to collect into a continuous phase. In the illustration oil in the "B" and "C" figures is insular.

When a very permeable salt-water sand containing shaly streaks is invaded by fresh mud filtrate, the filtrate and saline interstitial solutions tend to separate because of the difference between their specific gravities. As a result, the filtrate tends to accumulate just below the shaly streaks setting up an electrochemical cell which causes the SP to develop a sawtooth appearance.

(1) Side-bed resistivity or shoulder-bed resistivity.

(2) Often refers to the deconvolver setting on the surface panel of some induction logging gear. Setting usually corresponds to the shoulder-bed resistivity.

(1) Depth scale. Depth scales vary with locale and requirements. Most single logs are recorded on two different films at once, and the two films may optionally be on different depth scales. One film may be for correlation and the other tor detailed interpretation, or one may be for normal use and the other for quick-look interpretation by the overlay technique.

In North America the depth scales for correlation use are: 1 or 2 in. of film per 100 ft. of hole (i.e., on a scale of 1:1200 or 1:600 in terms of feet of film per feet of hole). The usual scale for detail use is: 5 in. of film per 100 ft. of hole (i.e., 1:240). An expanded scale of 25 in. per l00 ft. (l:48) is available for close scrutiny of short sections, and a special 60 in. per 100 ft. scale (1:20) is used for dipmeter logs.

Most of the rest of the world uses decimal scaling (1:1000, 1:500, 1:200, 1:40, 1:20) based either on the foot or the meter. Metric logs are easily identified by the fact that depth numbers appear at each 50-m. level, while logs scaled in feet have depth numbers at each 100-ft. level.

(2) Grid scale type. Usualy linear, logarithmic, or split. Observe illustration. The hybrid scale, which is not shown, was used before the logarithmic scale for use with laterolog type surveys.

(3) Grid scale sensitivity. The scale on which the magnitude of the measured signal is recorded. It corresponds to the value given to the full-scale deflection of the galvanometer or pen, or the width of one track (100 small divisions) on linear scales; center or midscale (one-half width of one track = 50 small divisions) on hybrid scales

(4) Chemical scale. A deposit that accumulates on solid surfaces (e.g., casing. tubing, rock. etc.) or in perforations and other apertures. It reduces fluid production from the formation.

(1) A view screen on the camera or recorder. Part of the light trace beamed from each galvanometer is reflected onto the screen on a simulated grid. The curve responses of each log can be observed on the screen during the survey operations.

(2) Filter screen.

Recovery of petroleum oil from underground reservoirs by using secondary sources of energy, such as injected natural gas or water to increase producing rates and to improve ultimate oil recovery. Water injection, commonly known as water flooding, usually affords higher recovery than gas injection. Gas injection is generally limited to those reservoirs which have a gas cap and in which gas cap expansion is expected to be an efficient natural drive mechanism.

Although the terms "primary" and "secondary" imply a sequence of use, both mechanisms might work concurrently once secondary recovery is implemented. See also primary recovery and tertiary recovery.

(1) Solid matter which has been transported from its source by wind or water and then deposited.

(2) Solid matter which has been precipitated from solutions or formed from skeletal remains or secretions of organisms.

Descriptive term for rock formed of sediment, especially:

1. clastic rocks such as conglomerate, sandstone, and shales formed of fragments of other rock transported from their sources and deposited in water;
2. rocks formed by precipitation from solution, such as rock salt and gypsum, or from secretions of organisms, such as most limestone.

(1) The difference observed between two different well-logging measurements of the same or related parameters recorded or plotted on the same log (e.g., porosities, formation factor curves, etc.)

(2) The differences observed between two similar well logging measurements, made at different depths of investigation, recorded or plotted on the same log (e.g., resistivities, movable oil plot, etc.).

(1) A line drawn through the deflections characteristic of shale on an SP curve, which is used as the reference in making measurements to determine the characteristics of permeable rocks and their formation waters.

(2) The characteristic of thick shale on the gamma-ray log or other well logs.

(1) Short in dimension.

(2) Electrical short, short circuit.

A percussion-type device (i.e., gun) which can be attached to the well-logging cable, provided with a means of accurately depth-positioning the tool (SP or gamma-ray curve), which is used to obtain formation samples. Hollow. cylindrical core barrels (i.e., bullets) can be shot in sequence, from the gun into the formation. After each core barrel has been fired into the formation wall, it is pulled free and retrieved by wires connecting the barrel to the gun. Core barrels are available for penetrating formations of different hardness. The type of barrel and size of charge are varied to optimize recovery in different formations.

(1) A meaningful response to a well-logging instrument which can be detected or measured.

(2) A measurement by a well-logging instrument which conveys the desired information as opposed to unwanted noise.

(1) Usually, the projecting portion of the body of a sonde, containing detection and measuring devices, which is pressed firmly against the face of the formation with enough force to cut into mud cake. This minimizes the effects of the borehole when using shallow investigative svstems (i.e., density, sidewall neutron techniques, etc.)

(2) Sometimes referred to as pad.

As originally defined for the case of a metallic wire carrying an alternating current, the skin depth is the distance into the wire at which the current density is reduced to 1/e (or 37%) of its value at the surface of the wire.

The term is retained in induction logging as a qualitative indicator of how deeply the magnetic field penetrates into the formation. However, because of the different geometry, the magnetic field penetrates much deeper into the formation in terms of skin depths than for the wire geometry. For the frequency used by some induction logging sondes, one skin depth in a formation of 1 ohm-m resistivity is 140 inches. Skin depth varies in direct proportion to the square root of the resistivity. See skin effect part (1).

(1) In well logging. Sometimes called propagation effect. As applied to induction logging, a reduction in the conductivity reading (increase in the resistivity reading) observed in very conductive media. A thick-bed correction for the effect is usually provided in the instrument. Residual corrections. when needed in thin beds, may be made by means of charts.

Simply stated, skin effect results from the interactions between adjacent ground loops when, because of their high conductivity. they carry induced currents of appreciable magnitudes. The magnetic fields of the ground-loop currents induce additional eddy currents in adjacent ground loops which are superimposed on those induced by the transmitter coil field. The net result is to nullify somewhat the magnetic field of the transmitter coil, and the resultant field may be shifted in phase. The conductivity signal seen at the receiver coils and the depth of investigation are thereby decreased.

The term "skin effect" originated from the tendency in metallic conductors for high-frequency alternating-current flow to be concentrated near the surface or "in the skin" of the conductor.

(2) In pressure transient testing and analysis. Skin effect is the additional resistance to fluid flow which causes an additional pressure drop at the well bore and/or in the formation near the well bore. The additional pressure drop is the result of some form of damage occurring during the drilling, completion. and/or production operations. Some factors which can cause this alteration are: invasion by mud filtrate or whole mud; cement; partially plugged perforations; movable fines; introduced contaminants such as rust, pipe dope, etc. The zone where permeability has been reduced by these factors creates an additional pressure drop during production or injection and is referred to as skin. The resulting effect is skin effect. The skin factor reflects the effects of the additional pressure drop. A positive value of the skin factor denotes formation damage. or damage at the well bore. A negative value of skin effect can result from man-made fractures or any form of stimulation which increases the ease with which fluid can flow in the region immediately surrounding the well bore.

A tradename for a method and system developed for obtaining a number of well logs in highly deviated and bad boreholes. Drill pipe is lowered to a predetermined well depth. The survey cable is then threaded into a side-entry sub and attached to the cable head. which in turn is attached to the subsurface instruments that are to be lowered into the well through the drill pipe. The side-entry sub is attached to the drill pipe.

Once all connections are made, the logging instruments, drill pipe, and sub can be lowered to a predetermined depth by ad ding nmore drill pipe above the sub. After the drill pipe has been lowered to the desired depth, the logging instruments are lowered or pumped out of the drill pipe to perfornm logging operations.

The survey cable is now outside the drill pipe and extending from the side-entry sub to the derrick floor. Mud circulation through the drill pipe can be accomplished at any time during operations.

Once the openhole logging is completed. the subsurface in struments reenter the drill pipe and retrieval can be started. Presently, the Induction Electrolog, compensated neutron log, Compensated Densilog/caliper, BHC Acoustilog, gamma-ray log, and Neutron Lifetime Log can be run on this system. Slant-hole Express is a tradename of Dresser Atlas. See illustration at high-angle borehole logging system.

(1) A quantity of radioactive material injected into the borehole during radioactive tracer surveys.

(2) A solid mass of metal debris which results from the collapse of the metallic liner during detonation of a shaped charge. The slug follows the jet at about one-tenth the velocity of the jet. Sometimes sufficient debris exists to partially plug perforations.

The sonic log is used in combination with other logs (e.g., density and neutron) for porosity, shaliness, and lithology interpretation. Integrated transit time is helpful in interpreting seismic records. See acoustic log.

Spontaneous potential, self potential. The difference of potential (DC voltage) between a movable electrode in the borehole and a distant reference electrode usually at the surface. The SP results from the IR drop measurable in the borehole produced by the flow of SP currents in the hole. These currents are generated by the electrochemical and electrokinetic potentials.

In impermeable shales, the SP tends to follow a fairly constant shale base line. In permeable formations, the deflection depends on the contrast between the ion content of the formation water and that of the following: drilling mud filtrate, the clay content, the bed thickness and resistivity, hole size, invasion, and bed boundary effects, etc. In thick, permeable, clean, nonshale formations, the SP value approaches the fairly constant static SP value which will change if the formation water salinity changes. In dirty reservoir rocks, the SP will not reach the same value, and a pseudo-static SP value will be recorded.

The SP is most useful when the mud is fresher than the formation water, a good contrast exists between mud filtrate and formation water resistivities, and formation resistivity is low to moderate. In these cases, it indicates permeable beds by large negative deflections, permits easy sand-shale discrimination, is useful for correlations, and under favorable conditions, can be used for the estimation of formation water resistivity.

The curve still remains useful in some saline muds. If the formation water is less saline than the mud filtrate, the SP deflection will be positive. However, when the mud column becomes so conductive it will not support a demonstrable IR drop, the SP curve becomes featureless. See electrochemical potential, electrokinetic potential, SSP, activity, and differential SP.

(1) Of solids or liquids, it is the ratio of the density of the material at standard conditions of temperature and pressure to the density of pure water under standard conditions.

(2) Of a gas, it is the ratio of the density of the specific gas at standard conditions of temperature and pressure to the density of dry air standard conditions.

SFL. A log of formation resistivity measured by a tool developed for the limited investigation of the invaded zone.

Focusing is used to enforce an approximately spherical shape on the equipotential surfaces within the formation in spite of the presence of the borehole. Borehole effect is virtually eliminated for hole diameters up to 10 in., yet investigation of the tool is kept shallow enough that its response is, in the majority of cases, mostly from the invaded zone. SFL is a mark of Schlumberger.

(1) With a well-logging tool, to raise the tool a short distance and drop it repeatedly against minor obstructions in the hole in order to reach greater depths in the borehole. An operation which should be carried out only with care.

(2) With well-drilling equipment, to move the drill stem up and down in the hole over a short distance without rotation in order to clear ohstructions in the hole.

Static spontaneous potential. The maximum SP that would be recorded when the SP electrode passes from a position well inside a very thick, porous, permeable clean sand (or other reservoir rock) to a point well within a thick shale. The static spontaneous potential given by the sum of the components of the electrochemical potential is:

SSP = –K log₁₀(a_w / a_mf) ,

where

and a_w and a_mf are the activities of the formation water and mud filtrate, respectively. Because of the inverse relationship between activity and equivalent resistivity in dilute solutions, this equation is approximated by

SSP = –K log₁₀ (R_mfe / R_we)

where R_mfe and R_we are the equivalent resistivities of mud filtrate and formation water, respectively. For NaCI solutions which are not too saline, R_mfe = R_mf and R_we = R_w; for more concentrated solutions, an activity correction should be made. Since the static SP in a sandstone is equal to the potential causing current (I) to flow in a mud column of resistance (R_m), shale of resistance (R_sh), and a sandstone of resistance (R_ss), then

SSP = IR_m + IR_sh + IR_ss

where the measured SP is IR_m.

(1) The distance separating a sonde from the wall of the borehole.

(2) A device for producing the separation in (1). Compare centralizer.

(3) In perforating, the distance a jet or bullet must travel in the wellbore before encountering the wall of the formation.

(1) at rest, immobile.

(2) Noise sometimes appearing on well log curves.

(1) See interval transit-time stretch.

(2) Survey cable stretch.

The direction or bearing of a horizontal line drawn on the plane of a structural surface; e.g., inclined stratum, fault plane, etc. The strike is perpendicular to the direction of dip. The bearing can be referred to south or north; e.g., N 30° E or S 30° W.

(1) In the microscopic sense, it pertains to the framework of rock. It refers to that rock material (called matrix by some log analysts) which creates the load-bearing structure. The interstices and other voids in the structure constitute porosity.

(2) In the megascopic sense, it pertains to large visible features of rock.

Electrical conductance occurring at the surfaces of some solid crystalline materials when they are exposed to aqueous solutions.

Ion exchange provides a mechanism of electrical conduction in wet clay-like materials whereby the (usually positively charged) ions move under the influence of an impressed electrical field between "exchange sites" on the surfaces of the particles. See ion exchange.

(1) To take and record borehole geophysical measurements; i.e., the act or performance of a well-logging operation. To a well log

(2) The result of a well-logging operation, a well log.

(1) A hollow, rubber-faced cylinder mounted on a hollow mandrel with a pin joint on the upper end to connect to the swab line. A check valve that opens upward on the lower end provides a way to remove the fluid from the well when pressure is insufficient to support flow.