Well logging

Reservoir rock data obtained by well logging are of vital importance to the petroleum industry. With these data, the production potential of a well can be determined and many problems involving the structure, environment of deposition, and correlation of rock strata can be solved.

Logging Procedure

A well log is a continuous record of any rock characteristic that is measured in a well borehole. The log itself is a long, folded paper strip that contains one or more curves, each of which is the record of some rock property. Since the first “electric log” was run in a well in France for the Pechelbronn Oil Company in 1927, well logs have been the standard method by which well data have been displayed and stored. Well logs and the information they record can be classified in two ways: by type (radioactive, sonic, electric, or temperature) and by purpose (lithology, porosity, or fluid saturation determination).

After a well has been drilled, it is standard procedure to log it. Logging has been compared to taking a picture of the rock formations penetrated by the borehole. The technique consists of lowering the logging tool, or “sonde,” to the bottom of the borehole on the end of an electric cable that is attached to a truck-mounted winch at the surface. The truck also contains the instruments for recording the logged data. The sonde, which is 4.5 to 6 meters long and has a diameter of 7.5 to 13 centimeters, is then pulled up the borehole at a constant rate, measuring and recording the data of interest. Measurements are recorded coming “uphole” rather than going “downhole” because it is easier to maintain a constant sonde velocity by pulling it up. On the downward course, the sonde has a tendency to “hang up” on numerous irregular surfaces in the borehole. It is essential to run logs to evaluate petroleum potential before the borehole is lined with steel pipe, because the well completion process is expensive and will be done only if economically justified. Data obtained from the well by logging are used to determine such rock parameters as lithology, porosity, and fluid saturation.

Radioactive and Sonic Logs

Of the logging curves that can be run for lithology identification and correlation, the most useful are the gamma-ray, spontaneous potential, and caliper. The gamma-ray log records the intensity of natural gamma radiation emitted by minerals in the rock formations during radioactive decay. One advance in gamma-ray technology has been the development of the gamma-ray spectrometry tool, a device that measures the energies of the gamma rays and makes possible the identification of individual minerals. The spontaneous potential (SP) log measures small natural potentials (voltages) caused by the movement of fluids within the formations. These currents largely arise as a result of salinity differences between the pore waters of the formations and the mud in the borehole. Although the borehole is drilled with a bit of a particular size, its diameter is never constant from top to bottom because the rotating drill pipe wears away the rocks along the hole. The caliper log provides a continuous measurement of borehole diameter by means of spring-activated arms on the sonde that are pressed against the wall of the borehole. Softer rocks have larger-diameter boreholes, and harder rocks have smaller diameters because of their difference in resistance to wear.

Porosity is determined by using, singly or in combination, the sonic, neutron, and density logs. The standard sonic tool has an arrangement of two transmitters, each with its own signal receiver. The transmitters send out sound waves, which are detected back at the receivers after passing through the rock. The neutron log tool bombards the formation with fast neutrons. These neutrons are slowed by collisions with ions in minerals and fluids. Because a hydrogen ion has approximately the same mass as does a neutron, collisions with hydrogen ions are most effective in slowing the neutron for the same reason that a billiard ball is slowed more by a collision with another billiard ball than it is by a collision with the rail of the table. The slowed neutrons are deflected back to the tool to be counted and recorded. Like the neutron log, the density log is a nuclear log. The density sonde bombards the formations with medium-energy gamma rays. The gamma rays collide with electrons in the formation, causing the gamma-ray beam to be scattered and its intensity reduced before it returns to the detector on the sonde.

Electric and Temperature Logs

Most logs used to determine water and oil saturations in the formations employ some method of measuring the passage of an electric current through the rock. The electric logs can be subdivided into induction logs and electrode logs. The induction log measures the conductivity of the formation and is the most commonly used device. The induction-logging sonde generates a magnetic field that induces a current deep in the formation. The passage of this current is measured by the logging tool. In the electrode-log system, electrodes on the sonde put current directly into the borehole fluid or the formation. The resistance to the flow of the electric current through the formation is measured as the formation resistivity. The short normal log, microlog, and microlaterolog measure resistivity immediately adjacent to the borehole, while the laterolog and guard log measure resistivity deep in the formation. Deep readings are made by narrowly focusing the electric current beam and directing it straight into the formation rather than letting it diffuse through the mud and into the adjacent formations. The laterolog and microlaterolog are most commonly used when the borehole mud has a base of saltwater rather than freshwater.

The temperature log, a nonelectric log, continuously records borehole temperature and can also be used for fluid identification. The dipmeter log is a resistivity device run with three or four electrodes arranged around the perimeter of the sonde. If the rock layers are inclined at any angle to the horizontal, this inclination, or dip, can be detected, because the electrodes will encounter bed boundaries at slightly different times on different sides of the borehole. An online computer converts these differences to angle of dip. The cement-bond log is a sonic device that measures the degree to which cement has filled the space between the steel pipe, or casing, that lines the inside of the borehole and the formations behind it (complete filling is desired).

Determination of Lithology

Lithology refers to the mineralogical composition of the rock unit, or formation. Oil and natural gas occur almost exclusively in the sedimentary rocks sandstone, limestone, and dolomite; the latter two are known as carbonate rocks. Shale, the most abundant of all sedimentary rocks, is never a reservoir rock for hydrocarbons because it is impermeable. An important purpose of well logs is to determine the lithology of the rock formations and thus, to identify those that possess suitable permeability to serve as reservoir rocks. The principal radioisotopes (thorium, uranium, and potassium), from which most natural gamma radiation emanates, are usually found in minerals in clays and shales. Therefore, the gamma-ray log is used to differentiate shales from sandstones and limestones and to calculate the amount of clay that might be present in some sandstones. Since most fluid movement is in or out of porous and permeable formations, the SP curve may be used to identify such rocks. These rocks are usually sandstones—hence, the identification of lithology. While permeable zones can be located, it is not possible to calculate actual permeability values. Next to permeable formations, the diameter of the borehole is reduced by the buildup of mud cake on the borehole wall. Borehole diameter also changes dramatically through shales, because shale is weak and crumbles, or “caves,” enlarging the borehole. The caliper log can therefore be used as a lithology log to identify permeable sandstones and “caving” shale.

In addition to assessing rock units for their petroleum content and environmental information, well logs are used extensively for correlation—that is, the matching and tracing of rock units from one locality to another. Since it is lithology rather than porosity and fluid saturation that is geologically the most significant factor in rock identification, the lithology logging curves are most commonly used for this purpose. Correlation may be accomplished either by matching log curves “by eye” or by statistical and computer analysis.

Determination of Porosity

Porosity is a measure of the total open space in a rock unit that is available for the storage of hydrocarbons. Such space is normally expressed as a percentage of the total rock volume. Knowledge of formation porosity is necessary to determine the total petroleum reserves in the formation or oil field. The sonic log has historically been the most widely used porosity tool. The time, in microseconds, required for a sound wave to travel through one meter of the rock is continuously plotted on the log. This travel time is the reciprocal of velocity, so a wave that has a high velocity has a short travel time. The use of a dual transmitter-receiver system for modern sonic logs eliminates the effects of changing borehole diameter and deviations of the borehole from the vertical. Formation travel times are functions of lithology and porosity. If the formation lithology is known, the porosity can be calculated. Because a sonic wave travels faster through a solid than through a liquid or gas, increasing porosity causes greater travel times. The presence of shale in the rock formations, however, can also cause unusually high travel times and erroneously high porosity calculations. The neutron tool principally senses the hydrogen ions present in the formation fluids, which, in turn, are found in the pore spaces. This log is affected by lithology, because clays in shales have water within their crystal structure, and the tool senses this water as if it were pore water. The density log measures electron density, which is directly related to the overall, or bulk, formation density. The greater the bulk density, the lower the porosity, because mineral matter is denser than fluid-filled pore space. A related log is the variable-density log, which is used to locate rock zones that are highly fractured and thus, potential reservoir rocks. The fractures in the rock have the effect of lowering the bulk density.

Because accurate interpretation of the data is dependent on a knowledge of lithology, it is common practice in the petroleum industry to run porosity tools in combination, particularly the neutron and density logs. Cross-plotting the readings from the two logs provides both lithology and porosity information. In addition, neutron and density logs respond oppositely to the presence of natural gas in a formation. Density porosity readings increase, whereas neutron porosity readings decrease. Therefore, this log combination will detect the presence of gas-bearing zones by the separation of the two curves.

Determination of Fluid Saturation

Fluid saturations—water and oil—are the most important quantities to be determined from well-log analysis. Because the water and oil saturations together must equal 100 percent, knowing one necessarily determines the other. The significance of these values is clear: If the rock unit of interest is not oil-bearing, or if it contains hydrocarbons in quantities that are not economically feasible to produce, the well will be abandoned rather than completed.

To understand the quantitative assessment of formation-fluid saturation, one must first understand what occurs within the borehole and in the formations that are penetrated by the well. Because high temperatures are generated by friction as the drill bit grinds its way through solid rock, specially formulated “drilling mud” is continuously circulated down the borehole to cool the bit. In addition, the mud clears the borehole by bringing to the surface the pulverized rock material, or cuttings. The drilling mud is usually a water-based fluid with various mineral additives. Within the borehole, there is a tendency for the fluid portion of the mud to separate from the mineral fraction. The fluid, or mud filtrate, seeps into the permeable rock formations, completely flushing out and replacing the natural formation fluids adjacent to the borehole. This area is the “flushed zone.” Some of the filtrate moves deeper into the formation, where it continues to displace the natural fluids, creating a partially flushed area, or “invaded zone.” The solid portion of the mud that has separated from the filtrate forms a “mud cake,” lining the inside of the borehole on the surfaces of the permeable formations.

Special-Purpose Logs

Within a reservoir, the rock matrix, freshwater, and hydrocarbons act as electrical insulators. Any electric current that passes through the rock is carried by dissolved ions in saltwater in the pore spaces of the formation. Therefore, where a current flows readily, the pore fluid is saltwater. Where electrical resistance is high, it is likely that hydrocarbons occupy the pore spaces. An induction log is used to assess electrical conductivity. The principal advantage of the induction log is that electrical currents largely bypass the high-resistance invaded zone where rocks have been penetrated by well fluids, and give a better picture of the true formation resistivity (the inverse of the conductivity) deep in the formation. Even so, the flushed zone and the invaded zone are still sampled to some extent, and corrections must be made to obtain the true resistivity of the uncontaminated formation. This can be accomplished by running logs that sample the formations only immediately adjacent to the borehole and, therefore, read either the flushed zone or the invaded zone resistivity.

In addition to corrections for flushed zone and invaded zone resistivities, other corrections must be applied to the log readings to allow for the effects of changing borehole diameter and bed thickness. When the true resistivity value and formation porosity are known, one can calculate the percentages of water and oil in the formation and make a quantitative determination of the total volume of hydrocarbons. Formations that contain natural gas rather than oil can be readily identified with the temperature log. As gas moves out of the formation and into the borehole, it is under less pressure and expands. As it expands, it cools, and the borehole temperature opposite the gas-bearing formation is significantly lowered.

Other special-purpose logs are available to the petroleum industry. One of these, the dipmeter log, provides information on the angle of dip of the formations encountered. This information can be used to determine environments of deposition, because rock layers in channel deposits, reefs, offshore bars, and other sedimentary features will have some unique pattern of dip. Increasingly, geologists have been using other well-log curves to refine their environmental interpretations. This is done by examining the curve patterns within the sedimentary rock units and noting whether the log parameters increase or decrease downward, for example, or change gradually or abruptly. The log, in effect, is measuring rock properties, such as particle size, that are controlled by the physical conditions within the site of deposition.

Importance to the Petroleum Industry

Well logging is a little-known but necessary part of the petroleum industry. It is the method whereby geologists and petroleum engineers obtain the information on petroleum reservoir rock characteristics that allow them to make decisions about the economic potential of an oil well—that is, whether it can be completed as a “producer” or must be plugged and abandoned as a “dry hole.” Such decisions involve millions of dollars and cannot be made without an examination of all the available relevant data. They must usually be correct, or the oil company will not survive financially.

The 1990s brought increased use of directional drilling, the downhole tool, and a nonrotating drill stem. A different type of well logging called “measure while drilling” (MWD) was well suited to this method of drilling. MWD uses a sonde as part of the drill stem near the tool. The sonde carries the usual logging instruments, along with magnetometers and accelerometers to help in establishing the drill stem’s position and the orientation of the hole. High-capacity batteries power the equipment. A transmitter assembly creates a series of pulses in the pressure of the drilling mud by opening and closing the valve. This allows small amounts of mud to pass directly from the drill stem into the borehole, thereby bypassing the drill tool. These pressure pulses are monitored by sensors at the top of the borehole. Computers in the sonde and in the logging truck communicate by means of these pulses and thereby provide logging information continuously while drilling. The logging information is therefore immediately available, and it is easier to interpret than with the older system because the sonde is in a relatively fresh borehole so the drilling mud has had little time to penetrate into the surrounding rock and modify its properties. Based on the logging data, decisions can be made regarding whether the well should be completed, the direction of the borehole, and which rock zones should be tested for their fluid content and producibility. As in many business endeavors, time saved in the decision-making process can be turned into money earned.

Principal Terms

conductivity: the opposite of resistivity, or the ease with which an electric current passes through a rock formation

correlation: the tracing and matching of rock units from one locality to another, usually on the basis of lithologic characteristics

gamma radiation: electromagnetic wave energy originating in the nucleus of an atom and given off during the spontaneous radioactive decay of the nucleus

hydrocarbons: organic compounds consisting predominantly of the elements hydrogen and carbon; mixtures of such compounds form petroleum

lithology: the mineralogical composition of a rock unit

neutron: an uncharged, or electrically neutral, particle found in the nucleus of an atom

permeability: measured in millidarcies, the capacity of a rock unit to allow the passage of a fluid; rocks are described as permeable or impermeable

petroleum: a natural mixture of hydrocarbon compounds existing in three states: solid (asphalt), liquid (crude oil), and gas (natural gas)

porosity: the volume of pore, or open, space present in a rock

reserves: the measured amount of petroleum present in a reservoir rock that can be profitably produced

reservoir: any subsurface rock unit that is capable of holding and transmitting oil or natural gas

resistivity: a measure of the resistance offered by a cubic meter of a rock formation to the passage of an electric current

sonde: the basic tool used in well logging; a long, slender instrument that is lowered into the borehole on an electrified cable and slowly withdrawn as it measures certain designated rock characteristics

Bibliography

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