Gamma - logging

Bore-hole surveying nuclear methods

Nuclear researches of holes are subdivided into methods of analysis of natural radioactivity (gamma methods) and syntheticly called radioactivity, termed is kernel-physical or is kernel-geophysical (gamma gamma and neutron methods).

Methods of analysis of natural radioactivity of rocks in holes

On analysis of natural radioactivity of rocks the gamma-ray logging or a gamma method (H'M) is grounded. It is analogue of radiometry.

Operations make by means of borehole radiometers of different brands. The electrical cues proportional to intensity of a gamma radiation, are transmitted from them on a cable in a usual well-logging instrument truck where their automatic logging is carried out.

As a result of a gamma-ray logging the continuum curve, or the chart, intensity of a gamma radiation enters the name. Magnitude is metered in pulses for a minute or in microroentgens at an o'clock (gammas). As dissolving of kerns is casual process intensity of a gamma radiation fluctuates about an average level, testing statistical fluctuations. Recurring records are applied to their account with smaller velocity of holding of observations. As gamma-rays almost are completely immersed by a course of muck thickness 1 - 2 m, and to 30 % of a nuclear energy is not skipped by case pipes the borehole radiometer can fix a gamma radiation of the mucks which have been had in radius, not exceeding 0,5 m from a borehole axis. The accretion of a borehole diameter and water or drill fluid presence in it reduce diagnostic study radius even more.

On gamma-ray logs beds with different extent of radioactivity come to light. Maximums mucks and the ores containing uranium, radium, thorium, potassium-40 and other radioelements, and also granite, clay allocate; minimums - sandy and carbonate rocks.

The spectrometry of a natural gamma radiation, i.e. definition of energy of gamma-rays, serves for an escape in sections of holes of mucks and the ores containing certain devices, for example, potassium, thorium, uranium, phosphorus, etc.

Natural radioactivity of rocks

Among other radioactive methods of bore-hole surveying the most abundant is the method of natural radioactivity of rocks or as it term is more often, gamma - a method. In its ground analysis of regularity of alteration of natural radioactivity of the rocks caused by presence mainly of uranium and thorium with decay products, and also radioisotope of potassium К⁴⁰ lays. Other radioelements (Rb⁸⁷, Zr⁹⁶, La¹³⁸, Sm¹⁴⁷ etc.) have so major half-lifes, that at existing abundance in earth crust of the appreciable contribution to the cumulative radioactivity cannot deposit.

Radioactivity of the basic minerals which are a part of sedimentary rocks, fluctuates in rather wide breaking points - from the 100-th fractions to several thousand Ra/gm. All these minerals on radioactivity can be battered into four bunches.

Relationship of the contribution of radioelements in a blanket gamma-ray activity of mucks variously. The basic contribution to a gamma-ray activity of buhrs and specially dolomites produce Ra (accordingly 64 % and 75 %), contribution Ra, Th, K in radioactivity of sandstones is approximately equal (Ra 23-26 %, Th 40 %, K 35 %). In this connection the spectrum of a natural gamma radiation terrigenous and carbonate rocks is various.

The first bunch characterised by low radioactivity, includes basic constituents of sedimentary rocks minerals:

• a quartz
• dolomite
• anhydrite
• gypsum
• a calcite
• iron earth
• a rock-salt

The second bunch of minerals with medial radioactivity is introduced by separate mineral differences of a type:

• a limonite
• a magnetic iron ore
• a tourmaline
• emery
• baryta
• oligoclase
• amphibole, etc.

To the third bunch of minerals refer to:

• clay
• micas
• felspars
• the potassium salts characterised by enhanced radioactivity, and some other minerals

The fourth bunch includes accessory minerals, which one radioactivity more than in 1000 times exceeds radioactivity of minerals of the first bunch.

In gamma - a research technique of holes about magnitude of natural radioactivity of rocks judge on intensity I_γ their natural γ-radiances, filed by the radiometer, moving on a wellbore.

The gamma - radiance plugs as well a so-called background radiation (phon). The Background radiation is called by pollution by radioactive substances of stuffs of which one the deep-seated instrument is fabricated, and a cosmic radiation. Cosmic radiation agency sharply drops with depth and depth of several tens metres on observed datas does not affect any more.

Gamma - logging

Gauging of intensity I_γ natural γ-radiances of mucks along a wellbore is termed gamma - logging.

Conditionally consider, that an effective radius of action of array gamma - logging (the sphere radius from which one 90 % of the radiances accepted by the detecting instrument) radiate matches approximately 30 sm; radiance from more remote sections of muck is immersed by a circumambient, not having attained the detecting instrument. The accretion d_с because of a hole wall and caving scour (it is routine in argillaceous gangues) is tracked by abatement of indications gamma - logging. The cement ring in most cases also influences magnitude filed γ-radiances, moderating it. For definition γ-activity of a bed at the quantitative interpretation data gamma - logging result in to reference conditions.

Intensity of radioactive radiance of mucks in the hole metre by means of the detecting instrument γ-radiance which have been had in the deep-seated instrument. Recording is carried out in the course of interacting gamma - radiances with atoms and moleculas of the material charging the detecting instrument. In the capacity of the detecting instrument use Geiger-Muller counters or more effective better decomposing a section scintillation counters.

Geiger-Muller counter

In this counter one of welding rods (anode) alive 800 - 1000 In is seated in the cabinet spacefilled by the ionising gas under low pressure (≈ 0.01 atm.). The part gamma - quanta, driving through the cabinet, does not interreact on the path with gas moleculas that reduces efficiency of the counter. Others gamma - quanta call ionisation of several moleculas of gas.

Everyone filed by the counter gamma - the quantum calls a current pulse in a mains circuit of the counter.

Scintillation counter

The detecting instrument the gamma - radiances is the transparent crystal which one moleculas possess property of a scintillation - emittings of quantums of light at affecting gamma - quanta. Quantums are noted by the photomultiplier and call an electron stream to the anode (current).

Major advantage of a scintillator is high performance of the account (it is filed to 50 - 60 % gamma - the quanta driving through a crystal) in comparison with other types of counters which one efficiency 1 - 5 %. It allows to diminish length of counters C 90 to 10 sm, to enrich vertical dismembering and to provide small static fluctuation.

Statistical fluctuations

Radioactive disintegration is changeable in a time, therefore for reception of stable values of radioactivity value of indications undertakes long enough period. As this season cannot be rather major the metered radioactivity is not a stationary value even in the event that the deep-seated instrument is in the hole without excursion. Observable alterations of radioactivity in this case are termed as its statistical fluctuations.

Statistical fluctuation on the chart should not exceed some centimetres, otherwise because of chart distortion cannot be correlated. Adjustment of voltage of fluctuation is carried out by selection of time constant of an integrating mesh.

Time constant of an integrating mesh

Adjustable devices of an integrating mesh allow to variate its time constant from 1 to 6 seconds Sampling of this or that value of time constant from which one a downhole survey will be made, comes from two discordant rules: major duration of time constant moderates statistical fluctuations, but calls backward creep in record of filed magnitude and demands decrease of velocity of indication for abatement of distortion of a curve.

Curves gamma - logging

The curve gained as a result of indication characterising intensity γ-radiance of benches along a wellbore, is termed gamma - a logging trace.

The configuration of a receivable curve of alteration of magnitude I_γ depends on variety of the factors bundled to features of the investigated section, a well design and procedure of effecting of gaugings (radioactivity of the rocks driven by the hole, radioactivities of drill fluid, a borehole diameter and casing presence).

Exact analytical consideration of agency on magnitude I_γ all plurality of these factors represents rather the challenge which till now completely has been not solved. However agency of each of these factors is separately learnt in detail enough.

Thanks to statistical fluctuations the radioactivity curve has the deflections which have been not bundled to alteration of physical properties of benches (a lapse of gaugings). The lapse bundled to fluctuation, that more than is less than the pulses irradiated in unit of time (counting rate). Generally intensity γ-radiances of the benches prized by the hole, is low fidelity proportional γ-activity of mucks. However at equal g-activity of muck with great gravity the gamma - logging because of livelier sorbtion γ-beams is noted by smaller indications. Indications gamma - logging are function not only radioactivities and gravities of mucks, but also requirements of down-the-holes (a borehole diameter, gravity of flushing fluid, etc.).

The hole effect on indications gamma - logging is manifested in intensity raise γ-radiances at the expense of natural radioactivity of columns, flushing fluid and cement and in an abatement g-radiances of rocks owing to sorbtion g-beams by a column, flushing fluid and cement. In connection with predominating value of the second process a hole effect affect mainly in sorbtion γ-beams of rocks. It result ins to that at a runout of the deep-seated borehole tool from fluid the accretion γ-radiances is observed. At its transition from a barefoot part of the hole in the cased decrease of intensity natural γ-radiances that calls shift of curves and abatement differentiate charts is noted. The same phenomenon is observed at transition of the deep-seated instrument from a single-casing part of the hole in the two-string.

A quantitative assessment of radioactivity of rocks

Ultimate goal of a geophysical data interpretation the gamma - a method is a contents quantitative assessment in rocks of radioelements.

Basically the appraisal on curves gamma - a contents method in investigated mucks of radioelements qп can be solved on the basis of use of one of two following relationships:

• q=S/K_γH
• q=I^∞_γ/K_γ

• Where

  • S - the abnormality square on a curve I_γ against an investigated bed
  • I^∞_γ - intensity of the γ-radiances, filed against an investigated bed under condition of it perpetually high power
  • H - a seam thickness
  • K_γ- so-called a γ-instrument constant, numerically equal to intensity γ-radiances which one is fixed by the used radiometer against an infinite thickness reservoir with the individual contents of radioelements.

Thus, in both cases the problem is reduced to stationary value definition K_γ the radiometer by which one the curve I_γ is gained, i.e. practically to a problem of a calibration of the radiometric instrumentation.

The solution of this problem rather difficult as magnitude K_γ depends on variety difficultly allowed and, that the most important thing, changeable factors. Routinely it is experimentally.

Range of application of a method

In a complex with yielded other methods of a petroleum geophysics effects gamma - a research technique of holes are used for lithologic dismembering of sections of holes, for their correlation and for an escape in them minerals. In sedimentary deposits they are the most reliable geophysical measure of extent of shaliness of rocks.

Secreting of minerals

Among minerals, was onevalued gated out by data gamma - a method, first of all it is necessary to term radioactive ores (uranium, radium and thorium), and also potassium salts.

In the holes bored for the purpose of looking up and survey of fields of radioactive ores, gamma - the method is the basic geophysical research technique, ground which one data the contents quantitative assessment in these ores of radioelements is carried out not only an escape in a section of ore beds and pays, but also. These data are widely used at count of fields of radioactive ores.

In many cases on curves gamma - a method in a section of holes accumulations of phosphorites, manganese, lead and other rare non-ferrous metals confidently allocate. On the pointed curves all these minerals are noted anomalously enhanced intensity I_γ

Dismembering

At the heart of lithologic dismembering by data gamma - a method of sections of holes regularity of alteration of radioactivity of rocks lays.

In holes of the oil, gaseous, coal and other fields dated for sedimentary deposits, curves gamma - a method mirror first of all extent of shaliness of rocks and presence in a section of low active mucks of a hydrochemical parentage. As a rule, the enhanced intensities I_γ on curves note the clayeyest differences of sedimentary rocks. The minimum intensities I_γ characterise chemical deposits (halites, gypsums, anhydrites) and pure not clayey differences of spigot products, sandstones, buhrs and dolomites. In a hemogenno-carbonaceous stratum of mucks it allows to gate out anhydrites and the rock-salts which are not differing from mucks a stratum on magnitude of electrical resistance and on neutron properties, and also highly active potassium salts and clayey differences among buhrs and dolomites. In sandy - a clayey part of a section of holes among the impermeable clayey deposits characterised by enhanced radioactivity, the under intensities I_γ on curves gamma - a method beds pure non-argillaceous sands and sandstones - possible oil traps confidently allocate. The role gamma - a method for secreting of collecting channels in a case when investigated holes are spacefilled by drill fluid which one specific electrical resistance is close to resistance of edge water specially increases. In these requirements method curves are feeblly differentiated also data gamma - a method become a basic stuff for secreting of penetrable differences - collecting channels. Besides, the gamma - a method gives the chance to decompose geologic sections of old cased boreholes, to tie up to depths muffs and the beds driven by the hole, and by that to enhance accuracy of perforations.

The gamma - a method is applied also to secreting of mucks of the under radioactivity, for example bituminous coal.

In case of high stable values of radioactivity against clay and low indications of radioactivity in spigot products some authors give the quantitative interpretation of curves gamma - a method for definition of shaliness of collecting channels. For this purpose will run a line, corresponding to pure (not clayey) deposits, and a shale deflection line. The curve amount of deflection is accepted linearly bundled to shaliness X. Some explorers apply following dependence:

lg_X=AI_γ+B

Where A and B - the stationary values spotted on a core for each square.

Correlation

At the heart of use of data the gamma - a method for correlation of sections of holes lays good consistency of radioactivity of separate lithologic differences of mucks within the major squares and terrains. In comparison with other methods use of data gamma - a method for correlation are characterised by following advantages.

1. Independence of filed intensity I_γ from a salinity of edge water and drill fluid.
2. Independence of magnitude I_γ from an oil saturation of rocks.

It allows the gamma - a method to carry out by data a correlation of strata without technics of erection of overhead line of the hole and alteration on the square of a salinity of edge water, and also without a rule of observed holes under the water oil-bearing capacity attitude. Affects magnitude of filed intensity I_γ and alteration such changeable on the square of arguments of rocks, as their porosity and pore space frame in carbonate deposits a little. All is together taken result ins to that effects gamma - a method are the most reliable stuff for the intervulgar and a regional correlation.

Appraisal of shaliness

The basic worth gamma - a method at exploration of sedimentary rocks consists in possibility of quantitative determinations on its data of shaliness C_sh of rocks or contents in carbonate rocks of insolublis rest C_ro - arguments which one knowledge is necessary at an appraisal collector's properties of rocks, and also at the quantitative data interpretation of other methods of a petroleum geophysics.

At the heart of quantitative determinations the radioactivity correlation q_n rocks with the contents in them of clayey stuff C_sh and the insolublis rest C_ro, characterised by the enhanced radioactivity lays.

In all rocks at least in small amounts there are radioisotopes, which one contents at different mucks variously, therefore by means of recording of radioactive radiances in the hole it is possible to judge character of rocks.

The Gamma-ray logging is grounded on gauging natural gamma - activity of rocks. At gamma - logging are filed gamma - beams in the hole.

The gamma - radiance represents the high-frequency electromagnetic radiation resulting kernel processes, and is observed as a current of discrete corpuscles (gamma - quanta).

Operations make by means of borehole radiometers of different brands. The electrical cues proportional intensities of a gamma radiation, are transmitted from them on a cable in a usual well-logging instrument truck where their automatic logging is carried out.

As a result the gamma - logging enters the name the continuum curve, or the chart, intensities of a gamma radiation. Magnitude is metered in pulses for a minute or in microroentgens at an o'clock (gammas). As dissolving of kerns is casual process intensity of a gamma radiation fluctuates about an average level, testing statistical fluctuations. Recurring records are applied to their account with smaller velocity of holding of observations. As gamma - beams almost are completely immersed by a course of muck thickness 1 - 2 m, and to 30 % of a nuclear energy is not skipped by case pipes the borehole radiometer can fix a gamma radiation of the mucks which have been had in radius, not exceeding 0,5 m from a borehole axis. The accretion of a borehole diameter and water or drill fluid presence in it reduce diagnostic study radius even more.

On charts gamma - logging beds with different extent of radioactivity come to light. Maximums mucks and the ores containing uranium, radium, thorium, potassium-40 and other radioelements, and also granite, clay allocate; minimums - sandy and carbonate rocks.