Radioactive Well Logging

 “Well Logging” is a technique for obtaining geological information about subsurface rocks by obtaining rock samples from the well and/or by using remote sensing devices.   Cores and well “cuttings” are actual samples of the subsurface rock materials.   Devices have been developed that can test the rocks of the well hole for their electric properties, acoustic properties, radioactive properties, hydrocarbon contents, etc.  For example, well tests known as, spontaneous potential tests are conducted on the rocks of a well hole by detecting the small amount of natural electrical current that exists in the rocks.  Cations and Anions, especially Sodium and Chlorine in salt water can set up a very weak electrical current that flows through pore fluids.   Petroleum is a very good insulator in respect to current flow.  Salt water vs. petroleum as a pore fluid in a sedimentary rocks can be distinguished using the present or absence of spontaneous electrical current.

Logs are made as a record of the well hole.   A device is lowered the length of the well hole and the characteristics are recorded on a continuous graph known as the log sheet.

GAMMA RAY LOGS:

Gamma ray logs reflect naturally occurring radiation in rocks penetrated by the borehole.  Although several types of rays are emitted, only gamma rays have enough penetration to be of practical use in logging the natural radioactivity of rocks.  All natural rocks contain some radioactive material. However, compared to that of uranium or radium ore, even of low grade, the radioactivity of most rocks is very small. The radioactivity of a rock is usually expressed in terms of equivalent amount of radium per gram of rock required to produce the same gamma ray intensity. Although there is no fixed rule regarding the amount of radioactivity a given rock may have, because of potassium decay, shales, clays and marls are generally several times more radioactive than clean sands, sandstones, limestones and dolomites.

Because shales, clays and marls have radioactivities that are of the same order, the term "shale" is used in this article to generally denote any of these three formations. Similarly, for the sake of generalization, the term "sand" is used to denote either sands, sandstones, limestones, and dolomites, since these four rocks have radioactivities of the same order.

The radioactivity of clean sands, i.e., sands free of shaly materials (shale, clay, marl) is generally very low. Sands that contain some shaly material have a somewhat higher radioactivity, and the increase is proportional to the amount of shale contained. Therefore, shaly sands and sandy shales generally have a radioactivity that is between that of clean sands and that of shale. In a given area, the radioactivity of shales does not generally vary too much, so that a gamma ray log is an approximate measurement of the quantity of shale contained in a formation.  Marine sandstones with uranium, however, have high radioactivity.

The radioactivity of shale varies from area to area. In the tertiary and more recent formations, i.e., those usually found in the Gulf Coast and in California, the radioactivity of the sediments, as a whole, is generally several times weaker than in the older rocks found in other petroleum provinces.

Some organic marine shales have a much higher radioactivity than the ordinary shales in the same area. However, they are generally relatively thin and are not found too frequently. When present, marine shales make excellent geologic markers.

Gamma ray logs have been used to distinguish some Uranium-producing horizons in sedimentary rocks from the western United States.  In more recent years gamma ray logging has been used to distinguish different fresh-water aquifers separated by clay/shale in alluvial aquifers in sedimentary bedrock confined/artesian type aquifers.   The drop in gamma radiation shows the transition from clays to sands and gravels

Effect of Casing

Most of the gamma rays emitted by the formation can penetrate casing, so that a gamma ray curve can be obtained in cased holes, although the amplitudes of the curve are somewhat reduced. For example, a 5/16 inch thickness of steel reduces the gamma ray intensity about one fourth.

Effect of Mud

The mud has two effects on the gamma ray curve:

1.      It absorbs a small percent of the radiation and therefore reduces the log amplitude; unless the hole diameter is very large (more than 24") this effect is very small and can be ignored.

2.      The shale or clay contained in the mud increases the radioactivity background, so that even clean sands show a slight radioactivity on the log. If the mud is uniform, this small increase is constant from top to bottom. However, if the shale has settled at the bottom of the hole there will be an increase in the radioactivity measured in this interval that has to be considered when interpreting the log. The effect of the mud on the gamma ray log is the same whether the mud is fresh or salty. Because this effect is usually very small, a gamma ray log is very useful in wells containing salty mud since, in this case, the electric log is generally poor.

Effect of Hole Size

The larger the hole, the smaller the gamma ray intensity reaching the probe. However, this effect is small and can generally be neglected.

Interpretation of Gamma Ray Logs

The interpretation of gamma ray logs can be summarized as follows:

1.      In a given area, only the relative radioactivity of the various rocks is of significance.

2.      Rocks of low radioactivity include primarily clean sands, sandstones, limestones, and dolomites. Anhydrite, salt, lignite and coal have also a low radioactivity. Their radioactivity increases when they are shaly.

3.      Ordinary shales have a much higher radioactivity than the rocks listed above. The radioactivity of sandy shales is less than that of shales. Shales are sufficiently high in radioactivity and can generally be easily distinguished from the other rocks on a gamma ray log.

Application of Gamma Ray Logs

Gamma ray logs are used in the following instances.

1.      To log cased holes (no electric log can be obtained in cased holes).

2.      To log dry holes (no electric log can be obtained in holes that do not contain water or mud).

3.      To log holes containing salt water or salty mud (the electric logs obtained in such holes are generally poor).

4.      To supplement the information given by the electric log (identification of formations, estimating the amount of clay in sands, etc.)

5.      To locate radioactive ores, uranium in particular.

6.      To help locate lignite and coal beds.

7.      To help locate clay and fresh water sands.

NEUTRON LOGS:

Neutron logs record the response of formations to neutron bombardment induced artificially.   A neutron source is lowered into the well hole.   Porous units absorb more neutrons and return or reflect fewer neutrons.  Denser rock units reflect more neutrons and yield higher neutron radiation values.  Porous sandstones usually have a low neutron values.

BULK DENSITY LOGS:

Bulk density logs (sometimes called a density log or "densilog") record induced radiation  from the well hole.   A skid-mounted device lowered into the well hole emits mdedium-energy electrons and measures returning gamma ray values.  Gamma rays are generated by electron collisions with atoms within the rock formations.  The gamma ray return values are related to rock density, and most bulk density values on the log graphs are recorded as g/cm3.  The average density for rocks of the crust is approximately 2.5g/cm3. Well Consolidated shales such as most Paleozoic shales have a bulk density of around 2.3 g/cm3.  Well  consolidated sands have bulk densities are usually higher than average crustal density if well cemented and lower than average crustal bulk density if  less well cemented and more porous.  More consolidated Paleozoic limestones have higher bulk densities than anverage crust; so it may be possible to distinguish sandstones, shales, and limestones from well holes, especially when well cuttings accompany well logs.

Usually Bulk Density logs are run in conjunction with Gamma Ray logs and Caliper logs1.  These three may be the only logs run on a well.  Wide well bores with high natural gamma radiation but moderate density (from moderate induced gamma radiation ) suggest the presence of shales.  Narrower well bores with low natural gamma radiation and low density suggest harder units with higher porosity such as sandstones.



1 A caliper log is created by an instrument that measures the width of the well hole.   Caliper logs are especially helpful in discerning boundaries between soft and hard strata such as shales and sandstones.  Shales may continue to cave and “slough” off into the well, mixing cuttings of lower sandstone horizons.   A caliper log can log the exact boundary where a sandstones begins by the narrowing of the hole at the horizon of the harder denser rock.

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