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Radiation Surveying for Oil and Gas
One of the first methods of "direct" or "semi-direct" exploration for oil was the chemical analysis of soil for hydrocarbons. According to observations these soil hydrocarbons concentrated in a band or "halo" directly above the margins of the producing area. These light hydrocarbons tended to decrease in hot, dry weather, but experiments have determined that water-soluble non-organic compounds in the soil formed the same pattern. One explanation for this is the theory that gasses passing through a moist zone tend to stimulate water evaporation, resulting in a concentration of these compounds in the "halo". The further possibility developed that some of the water-soluble inorganic substances are radioactive and that the gamma-ray emanations could be measured.
First attempts to develop a radiation pattern was by the collection of "soil-air" samples and the use of an electroscope in analyzing them. It was impossible to standardize results in this procedure, which made this radon method impractical.
The next step was based on the measurement of gamma radiation. The Geiger Counter was not sensitive enough to handle the low grade radiation of ordinary soils. Other more sensitive instruments were too heavy or bulky for field work. However, a light ionization chamber type of instrument was developed which could be carried by two men. Following this the search for uranium brought about the development of the scintillometer, and eventually the application of that instrument to oil and gas exploration came about.
In the start of the direct method of exploration the successes of chemical and radiation surveys were emphasized while the weaknesses remained unknown. It has now taken ten years of experience in the use of radiation equipment to bring out all the difficulties to be taken into account and to learn how to overcome them.
Either scintillometers or ionization chambers may be used if they meet certain conditions. They must (1) be sensitive enough to record accurately the low grade variations of soil radiations, (2) have control of temperature drift, (3) be constructed so that values may be integrated to a minimum of variation and (4) be light enough for carrying but rugged enough for field use.
In sampling, a compass and chain survey is sufficiently accurate for the location of stations. The sample pattern is chosen to cover all possibilities of the extent of an anomaly.
Contamination of the reading may come about from the use of a luminous wrist watch. A fall out from nuclear explosions may keep instruments off scale for days. Showers may bring down radioactive particles and approaching cold fronts may affect readings for a short period. Cuttings, drilling mud and well brines usually are excessively radioactive; for this reason sample stations should be uphill from dry holes, drilling wells or producers.
Natural causes which bring about false readings are:
1. Diurnal variations in cosmic readings. These may be controlled by check stations.
2. Thin soil above bed rock, which yields lower readings than thick soil, must be taken care of by adjustment.
3. Rock fragments in soil, which also require adjustment.
4. Unweathered shales, which are frequently highly radioactive.
5. Granite wash, which in soil greatly increases radiation values. It must be avoided.
6. Marshy soil, which may be washed free of radioactive material.
7. Dry lake basins, which have concentrations of mineral salts.
8. Irrigation, which causes transportation of different materials.
9. Soil interference patterns. This is the one problem which has been almost insurmountable and which has caused abandonment of much radiation surveying. To meet the problem a soil classification method has been developed in which soil samples are tested by a mechanical device for what is called a soil radiation self-potential; by comparison this establishes a correction factor to be applied to readings. In areas in which this correction factor has not been employed about 50 percent of surveys are undependable, about 25 percent are half correct and the remaining 25 percent requires no change.
In practically all cases the use of the correction factor results in stronger contrasts between high and low values. Frequently soil changes are so rapid that a correction of 50 percent in the radiation range is required in short distances.
The interpretation still remains difficult, however. The only time that a simple pattern may be developed is when the productive zone of a pool has uniform porosity and is perfectly continuous. If there are several productive horizons each will yield its own pattern unless the margins of each horizon lie vertically above one another. A scrambled pattern therefore usually results from multiple zone production, and the best to be expected from it is an outline of the entire area segrated from the non-productive background.
The method is not necessarily related to structure, but a thorough knowledge of the structure and stratigraphy is necessary for the best interpretation.
It is believed that the radiation methods with the use of the corrections is highly useful. It goes beyond the accepted methods of exploration, especially in the search for stratigraphic traps. However, there is still much to learn about it from future experience.
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