US2906882A - Radiation survey method - Google Patents

Description

Sept. 29, 1959 //0 Ill 0 0 0 P onucme WELLS DRY HOLES J. w. MERRITT RADIATION SURVEY METHOD Filed Oct. 25. 1956 a: 62 6/ Q O O 214 235254 Q9 97 96 o o o 234 2% 282 4 Still another object of the invention is to provide an im- The readings from which the Fig. 1 map was prepared proved gamma ray survey technique in which the abnorare found in column A of Table 1.

mal high and low readings in gamma ray activity caused by changes in theretentivity characteristics of the soil Gamma Son 9860 Correction True value are corrected and 1n which the final plot of the readings Station A B O D E reflects the variation due primarily to presence or absence 93-13 AH) of concentrations of radio-active mineral substances in the hydrocarbon band or halo. g3? g9 3g A further object of the invention is to provide a gamma 272 53 42 314 ray survey technique of the character described in which g3 2g 2? in one step of the method the retentivity characteristics 9 e4 34 41 330 of the soil from point to point are established through g? 2; measurement of the electrical conductivity or resistance 275 53 54 329 of the soil. A feature of the invention in this respect g2 32 3g resides in the provision of a procedure for eliminating 295 so 18 22 317 variations or anomalies in resistivity or conductivity 53g 3g 3 measurements which occur primarily from the presence 300 98 0 0 300 of excessive concentrations of water-soluble mineral con- 38g 3g 8 8 stituents which have accumulated in the soil as the result 295 98 o 0 295 of phenomena other than the capillary accumulation of 33; g: g g such constituents resulting from gas movement through 31s 98 0 0 31s the SOilv 2i 1? 2g Another object of the invention is to provide a survey 320 98 0 0 s20 technique which is simple in operation, can be carried Z23 2; i3 out quickly and yet which is extremely accurate in result. 254 58 40 48 312 Other and further objects together with the features of 38 3; 8 8 333 novelty appurtenant thereto will appear in the course of 72 5 0 8 9 the following description. 2% In the accompanying drawings, which are included to 8 76 22 6 310 illustrate a manner of practicing the invention and the Z3; Z3 f3 3% advantages resulting therefrom, 296 80 18 22 318 Fig. 1 represents a map of an oil producing field show- 32 Z3 Z2 mg the locations of producing wells and dry holes on 269 43 52 321 which have been plotted the gamma ray contour lines re- Si 3; 22 sulting from a conventional gamma ray survey. The con- 263 66 32 39 302 tour lines connect points of equal gamma ray activity Z; g and are hereinafter referred to as isogams; and 3 $2 2 29 275 Fig. 2 represents a map of the area illustrated in Fig. 233 71 27 32 1, but shows the isogam pattern obtained by employing g2 g8 the method described and claimed herein. 321 93 Q 0 Referring initially to Fig. 1, this map represents a ,23 $2 $3 typical example of the pattern obtained by known gamma 272 87 11 13 285 ray survey techniques. The surface gamma ray readings gi g2 at spaced points along parallel traverse lines are shown 268 5a 40 48 315 by the numbers below the small circles, each circle repre- 53g 8g 8 3 senting the exact location of the station where the reading 294 78 20 24 31s was taken. The stations are identified in numerical order 332 3g 3 8 ggg the station identifying number appearing above the circle. 268 84 14 17 285 Equipment for obtaining readings of ground gamma 3g 8 8 ray activity is well known and in general use. Perhaps 255 98 0 0 205 the most accurate type and the type I prefer is the porta- 33g 2% 29 g? ble ionization chamber which fundamentally consists of 204 44 54 55 329 two electrodes immersed in an inert gas contained under g8 g3 g2 pressure in a steel chamber. The electrodes are con- 239 82 16 19 30s nected externally to a circuit containing batteries. When fi gamma rays (which have sufiicient penetrability to pass 275 73 2., 30 305 through the steel walls) enter the chamber and ionize the 32% if 3g gas, a current flows in the circuit which is directly pro- 259 45 52 62 331 portional to the ionization or intensity of radiation. Ordi- 23 2; g 3%,? narily the current is stepped up through amplified cir- 292 98 0 292 cuits and passed through a meter where the values can 222 Si i2 be read directly or may be automatically recorded on a 271 98 0 0 31 chart. It will be understood, of course, that other equip- 335 33 1 2% 518; ment for measuring gamma ray radiation emanating from 293 79 19 23 321 the ground may be used and my invention is not limited 8' 1 33 to the use of any particular type of equipment. 274 64 34 41 315 In obtaining a map as exemplified by Fig. 1, the port- 33: iii) i; able gamma ray measuring unit is moved from point to 299 80 18 22 330 point along the traverse lines, a reading being taken at 333 ii; iii each succeeding station. As a satisfactory spacing for 236 81 17 29 356 the stations, I prefer proceeding along parallel traverses 2 6 32 3? 2i 2% 660'feet apart, readings being taken at 330 foot intervals 272 88 10 12 along the traverses. The sides of the field can be closed gi 38: by taking intermediate 330 foot readings between the 269 69 38 45 315 traverses at each end, as indicated. Z3 42 2i 2%:

Gamma, Soil 98-Soil Correction True Value Station A B O D 98-B 1.228)) A+D Once plotted on the map, isogam lines are drawn to define the readable pattern of activity. It will be observed from Fig. 1 that the areas in which readings below 280 gamma units are found are represented by the portions of the map free from cross hatching. The areas between isogam lines representing an increase of from 280 to 290 are distinguished by diagonal cross hatching. Between isogams of 290 and 300 the area is distinguished by horizontal cross hatching and above 300 by vertical cross hatching.

As is believed at once evident, the pattern obtained in Fig. 1 shows no correlation with the actual production characteristics of the region mapped. As has been hereinbefore noted, the pattern indicating possible producing zones of oil is a band or series of high values surrounding the location of the oil deposit and appearing substantially vertically above the outer limits of the deposit.

Fig. 1, however, shows no conclusive or readable pattern of this character. No halo or parallelband effect is present, and such a pattern would not justify a recommendation of the area as a favorable prospect. However, as shown by the symbols on the map, there is definitely a large producing field running through the central portion of the area. a

I have discovered that the primary reason for the failure of the known gamma ray techniques to adequately reflect the anomalies which indicate the presence or absence of oil deposits isthe rather extreme variation in localized areas in soil characteristics with particular reference to the ability of the soil to retain the water-soluble mineral concentrations resulting fromgas movements. It is generally agreedthat surface gamma ray readings ordinarily. measure the radiation only from the radio-active mineralslocated to a depth of from ten to'twelve inches below the surface. The intensity of'radiatio'n from-concentrations below that depth-is suppressed by the shielding effect of the surface layer. The ability of the surface layerto retain the mineral concentrations resultingfrom subsurface hydrocarbon activity thushas ad irecteifecton the gamma activity at any point: 111 loosesandy soils it oft en happens thatflven-though' the mineral concentratiens were once present in 'thesurfaee 'layer; the

continual leaching caused by rainfall, flooding and the like carries some or all of the minerals downwardl'y'into lower strata to a point well below the limits at which their radiation can be measured by surface equipment. On the other hand, in tight clay loams surface moisture has little or no eifect and the anomalies in gamma ray measurements accurately reflect the degree to which radioactiveminerals have been concentrated. In almost every field the character of the soil varies from point to point between these extremes, and this variation has a marked effect on the results of the gamma ray surveys.

The map appearing as Fig. 2 is aplot of the same area shown in Fig. 1 but prepared in accordance with the method embodying nay-invention. That this map is in complete accord with thelocation of the production area is believed evident. The fifteen. producing wells are surrounded by a closedring or halo of high values which clearly delineate the area in which production can be ex pected. It will be noted that the abandoned wells lie outside the halo and further drilling in this area would appear to bepointless.

To obtain the Fig. 2 map, the following procedure is carried out. Gamma ray readings are taken at each station as explainedin connection with Fig. 1. However, in addition to the gamma ray reading there is taken at each station according to my method a sample of the soil. Preferably it istaken at a six-inch depth with any one of the several conventional devices employed for such purposes, for example, a. sampling pick or soil auger. The samples are placed in individual bags or other suitable containers, each bag marked with the identifying data for the particular station. The samples are accumulated as the survey proceeds and are preserved for later use, as hereinafter described.

Following thecompletion of the field survey which includes the obtaining of the. gamma readings and the soil samples at each station, the next step is the obtaining of a measurement of retentivity characteristics of the soil at the stations. For this, the samples collected during the survey are employed.

A number of procedures for measuring the retentivity characteristics.- of the samples may be used. In one preferred method. the. retentivity. is obtained by. measuring the conductivity of the sample after it has been exposed toandallowed to adsorb moisture. Highly porous soils such as loose sands lose by leaching action whatever minerals that may collect. Therefore they do not adsorb and retain moisture readily and'will have a high ,resist' ance (orlow. conductivity) while tight and heavier soils are not so easily leached and thus will adsorb and retain more moisture and will consequently have a lower resist" ance (or. higher. conductivity).

In another method, a Weighed quantity of soil is placed in. the bottom of a centrifuge tube and is covered with water. Following centrifuging the unabsorbed water is poured off and the soil is againwcighed. The difference in weight ofcourse provides a value which can be compared with values obtained from other samples to determine their relative water absorbing and retaining proper ties. In a third method, a measured volume of water is placed in the centrifuge with aweighed quantity of soil, and the discharge water is caught in the process of spinning the sample. The figure obtained by subtracting the measuredvolumeof discharged moisture from the orig inal volume of water placedwith the centrifuge soil sample provides a direct measure of the retentivity of the soil.

' In'obtainingthe Fig. 2 map, the electrical conductivity method was relied upon. Each soil sample is first dried at below 212 R, preferably at 180 F. .to remove all free moisture and then is ground sufficiently fine to break up clods and lumps but without destroying its natural texture. The-sample is then thoroughly dried again and placed ina humidifier maintained at a constant temperafare and humidity. By. way of example, suitable values '7 for this are 80 F. and a relative humidity of 80%. Each sample is kept in a humidifier for the same period of time (say six hours), after which it is removed and immediately tested.

Any suitable testing equipment adapted to be used in measuring electrical conductivity of fine granular materials may be employed. I prefer a test cell comprising a simple tube of conductive material having in its in* terior a centrally disposed coaxial rod, also formed of material which is'a good conductor. The rod and tube are insulated from one another and the opposite leads from any suitable source of electrical energy of known potential are connected respectively therewith. The sample is placed within the annular space between the two electrodes (the tube and rod) and its conductivity is measured by connecting a voltmeter in series between the source of potential and the tube.

The figures in column B of Table I represent the voltage readings for the samples when tested according to an electrical method. It will be noted that the values obtained vary over a wide range. These readings point up very clearly the changing character of the soil from station to station across the field, particularly in connection with the ability of the soil to retain the mineral solution concentrations deposited by gas movement. The analysis or testing of the soil provides the basic data necessary for correcting the gamma ray survey to counteract so far as possible the influence of the soil conditions on the radiation measurements.

In a preferred embodiment of my method, the necessary correction of the original radiation measurements is accomplished through the use of a correction factor reflecting the ratio of the range between maximum and minimum gamma ray readings (taken from Table I) to the range of soil retentivity measurements. In the instant example, the ratio is obtained by dividing the difference between the maximum and minimum gamma readings (329 minus 259) by the range of voltage readings (98 minus 41) which gives a factor of 1.228.

The gamma correction to be applied to the original reading is obtained by converting the variations in retentivity of the soil to gamma units through the use of the correction factor. As stated earlier, the ability of a dry soil sample to absorb moisture from the air is a direct indication of its ability to retain the water soluble minerals which may have been deposited therein by gas movement. Soils which adsorb the least moisture are most likely to lose the water-soluble minerals by the leaching action of rain or flood waters, while those that are fine, tight and relatively highly absorbent exhibit much better retention qualities. In the electrical testing method the most retentive soils are represented by the highest voltage readings in Table I while the least retentive and highly porous soils have the lowest values. Therefore, in order to convert the readings into values which tend to indicate the degree of correction in the original gamma reading which must be made, the highest value is accepted as the standard for the area and all values are subtracted therefrom; the differences in each case are listed in column C of Table I. To convert these differences to gamma ray corrections, the figures in column C are multiplied in each case by the correction factor. The resultant (column D) is then added to the original reading (column A) and the sum of the two is the corrected radiation value.

The final step in my method consists of the plotting of the corrected readings (column E) on the area map and the interpolation of the map of the isogam lines in order to delineate the pattern of gamma ray activity. In the specific example herein disclosed, the final corrected readings are taken from column E of Table I and were spotted on the map next to the stations to which they correspond. Isogam lines are then drawn'to connect the points of equal radiation intensity with the result observed in'Fig. 2. The band of highs encompassing within it the producing wells reflects the outline of the subsurface deposit, and it will be seen that this band correlates very closely with the results indicated by the producing wells and abandoned dry holes.

Essentially the same procedure is carried out when the retentivity characteristics are determined by the use of the non-electrical procedures hereinbefore set forth. In the first of these, that is, when weighed samples of dried and ground soil are placed in a centrifuge tube, covered with Water, and after centrifuging and after the free water has been poured off, weighed again, the gains in weight represent the retentivity characteristics of the particular samples. The correction factor is obtained by dividing the difference between the greatest and smallest gain into the radiation range, and the correction value is reached by multiplying each gain value by the correction factor. As in the electrical method, the correction is added to the radiation reading to arrive at the figure to be charted on the map for the particular station. In the volumetric method, the measured water thrown off during centrifuging is used as a basis. However, to properly correlate the measurements with the degree of retentivity, the amount thrown off from each sample is subtracted from the volume originally placed in the centrifuge. Highly retentive soils will retain more water and thus the difference will be greater than in the case of soils with low retentivity. Again, the range of retentivity measurements is divided into the radiation range to correlate the two as a correction factor, and the factor is utilized in the same fashion as in other methods.

Turning now to a modified form of the invention in which the electrical conductivity or resistance of the samples provides the basis for determining retentivity characteristics, I have found that in some isolated fields there occur instances of extreme concentration of watersoluble minerals which can serve to cloud the true radiation picture even after correction as contemplated in the preferred embodiment. In low spots there may be a soil of highly alkaline or saline characteristics which, due to the excessive concentration of the mineral constituents, will not under normal atmospheric conditions leach out to the same degree as the more uniform soil surrounding the area. In this case if conductivity measurements in accordance with the preferred embodiment are employed, samples taken from the areas of excessive concentration Will produce abnormally high conductivity (low resistance) measurements which do not accurately reflect the relative retentivity characteristics of the soil in the area of concentration as compared with the surrounding soil in which the concentrations are not present. This is due to the excessive quantity of electrolytic material in the soil at the areas of concentration. Where this condition is suspected, either from visually observable phenomena or from a spot chemical analysis, then the following steps may be employed.

The radiation measurements of the field are taken precisely as described in the preceding embodiment and at the same time a soil sample is obtained at each station. The samples are, of course, tagged to identify them with a particular station.

When ready for the determination of the water soluble retentivity characteristics of the soil at the various stations, the samples are first thoroughly dried and then screened or sieved to segregate pebbles therefrom. Following this, a fixed quantity of each sample is weighed out and placed individually in a suitable mixer along with a fixed volume of a water whose electrolyte content is known. In most areas city tap water will serve the purpose, care being taken to periodically check the electro-.

lyte content to see that the Water is maintained at a standardized value. After a thorough mixing of the soil sample and water, in which the water leaches the watersoluble minerals from the sample, and after the solids have been permitted to settle, the-electrolyte content of the resulting solution is measured and compared with the known value for the water. If the electrolyte content of the solution is substantially above that for the Water prior to mixing, then the solution is decanted, more raw water is added, and the process repeated. The leaching of the soil samples should be continued until the electrolyte content of the solution is substantially equal to that of the raw water.

In measuring the electrolyte content of the raw 'water and the solution, any convenient method or apparatus may be employed. I prefer the use of a conductivity measuring instrument of the conventional type such as the conductivity bridge and dip cell employed in the type RC instrument manufactured and sold by Industrial Instruments, Inc., of Jersey City, New Jersey. This ins'trument is of the Wheatstone bridge type employing electrodes which are immersed in the liquid to be measured. The conductivity is measured in terms of resistance of the electrolyte and the range is approximately from .2 ohm to 25 megohms.

When the electrolyte content of the leach solution for each sample has been reduced to substantially a standardized value for all samples (which in this example is the electrolyte content of the raw water) the solution is decanted or otherwise removed and the sample thoroughly dried. As in the earlier embodiments the sample should be dried at a temperature below 212 F., preferably 180 F. or under, to prevent the breakdown of crystals formed by Water crystallization. If the dried sample is in cake form, it should be ground to the extent necessary to reproduce as closely as possible the original texture and grain size.

After the soil samples have been individually leached to a standardized condition and dried as set forth above, their comparative retentivity characteristics are determined in exactly the same fashion as described earlier herein in connection with the preferred embodiment of my invention by which the Fig. 2 map was prepared. That is, each sample is placed in a humidifier maintained at a constant temperature and humidity and kept there for the same period of time. When removed from the humidifier each sample is immediately tested for conductance as earlier described and the values obtained are utilized exactly as described in connection with the preparation of Table I to obtain a corrected radiation value for each station. The corrected values, as in the illustrative example, are charted on a map of the area to reveal the corrected radiation pattern.

From the foregoing it is believed clear that the modi fied form of the electrical conductivity method of determining water soluble retentivity characteristics differs from the preferred form in that the soil samples are leached before conductivity measurements are taken to remove excessive minerals which have been concentrated in the area by conditions not arising from the gas movements through the soil, but from other either natural or artificial phenomena. The modified form of the method as herein described is particularly valuable in obtaining a correct pattern in areas Where surface alkali concentrations are apparent or where they have been established or are known to be present from other tests and investigations.

From the foregoing, it will be seen that I have provided a method of radiation surveying in which the retentivity characteristics of the soil from point to point are correlated with the radiation measurements to produce a pattern of activity reflecting the true character of the field. My invention is thus one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the method.

It will be understood that certain features and subcombinations are of utility and may be employed Without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

v Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described my invention, I claim:

1. A method of exploring for and locating subsurface hydrocarbon deposits comprising the steps of measuring and recording the surface radiation and intensity of the radioactive constituents of the earth at a plurality of stations in a selected area to obtain for each of said stations a radiation value, obtaining a representative sample of the soil at each of said stations, leaching said samples individually with a water of known mineral content until the leached solution of each sample is at substantially the same level of electrolyte content, drying said samples, exposing said samples to water vapor for a fixed period, passing an electric current through each of said samples to determine the relative water soluble mineral retentivity characteristics of said samples and recording same, applying to the original radiation values a correction based on the variation from station to station in said retentivity characteristics thereby to obtain a corrected radiation Value at each station, and charting the corrected values to visually reveal the corrected radiation pattern.

2. A method of exploring for and locating subsurface hydrocarbon deposits comprising the steps of measuring and recording the surface radiation intensity of the radioactive constituents of the earth at a plurality of stations in a selected area to obtain for each of said stations a radiation value, obtaining a representative sample of the soil at each of said stations, mixing a fixed volume of water of known electrolyte content with a fixed volume of each sample, measuring the electrolyte content of the solution, decanting the solution and repeating said mixing step until the solution for all samples measures substantially the same electrolyte content, drying said samples, exposing said samples to air of standardized humidity for a fixed period of time and at a fixed temperature,

passing an electric current through each of said samples to determine the relative water-soluble mineral retentivity characteristics of said samples and recording same, applying to the original radiation values a correction based on the variation from station to station in said retentivity characteristics thereby to obtain a corrected radiation value at each station, and charting the corrected values to visually reveal the corrected radiation pattern.

References Cited in the file of this patent UNITED STATES PATENTS 2,165,440 Bays July 11, 1939 2,269,889 Blau Jan. 13, 1942 2,390,931 Fearon Dec. 11, 1945 2,725,281 Bond Nov. 29, 1955

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