MXPA98001681A - Method to determine the density and the photoelectric factor of a training through an instrument with several ga ray detectors - Google Patents

Abstract

The present invention is an advanced method of determining the density of a formation using a density measuring instrument with several detectors. The use of three or more detectors produces a more accurate and accurate measurement of the density of the formation in the presence of a considerable separation between the instrument and the formation. Through a new algorithm of a simple detector a more accurate photoelectric factor is obtained. The use of the information on the photoelectric effect and the density of the three detectors makes it possible to measure a compensated photoelectric effect for the separation and the photoelectric factor of the mud scale. The use of the density responses of the different detectors allows a verification of the consistency and, therefore, a better control of the density measurement quality.

Description

METHOD FOR DETERMINING THE DENSITY AND THE PHOTOELECTRIC FACTOR OF A TRAINING THROUGH AN INSTRUMENT WITH VARIOUS GAMMA RAY DETECTORS FIELD OF THE INVENTION This invention is related to the determination of the characteristics of a geological formation. In particular, it is related to the determination of the density of the formation even when the separation between the apparatus and the formation is considerable. The invention also measures the photoelectric factor of a formation and provides better quality control when measuring density.

DATA OF THE INVENTION Nuclear instruments have been used for decades to determine the density of the rocky geological formations that surround a borehole. The measurements of the nuclear density depend on the Compton scattering of the gamma rays in the formation to perform the density measurements. A standard density measurement instrument consists of a gamma ray source (or x-ray), at least one gamma-ray detector and shielding between the detector and the source, in order to detect only the scattered gamma rays. During the density graph, the gamma rays from the instruncient source move through the borehole into the geological formation. The gamma rays will be scattered by the electrons in the formation or pozo and some of them will be scattered back to the detector in the diagnostic probe. Depending on the space between the source and the detector, the rate of scouting of the detected gamma rays will increase with the increase of the density of the formation (dominant dispersion term) or - decrease with the increase of the density of the formation (effect prevailing attenuating). In intermediate spaces, both the terms of attenuation and dispersion affect the response. In an ideal situation, the well of deo would have a uniform and straight shape. This uniform sounding well would allow a density measuring instrument equipped with a detector to be in close proximity to the formation surrounding the well and the separation of the instrument would be minimal. Under these conditions, a detector would be sufficient to perform the density measurement. However, since the boreholes usually do not have a uniform shape and are not straight, an important problem when performing density mapping is the contact of the borehole with the borehole wall of - probe. Density density probes can be designed as cushion or mandrel type instruments. In a mandril, the. Source and detectors are located in the body - of the straight cylindrical instrument. The rigid length of such an arrangement makes it difficult for the instrument to keep in close contact with the wall of an irregular borehole. In the cushion-type instruments, the detectors and, in many cases, also the source of the graph are mounted on a short and articulated cushion that can be moved relative to the body of the instrument. A strong decentralizing arm pushes the cushion against the wall of the borehole allowing better contact thanks to the shorter length of the device. All density density probes will find a mud crust, accumulated in the wall of the formation, which prevents good contact. The density measurement needs to be compensated also for this type of separation. Due to the imperfections of mandrel-type instruments, they are used only if a cushion-type instrument can not be designed due to size or cost constraints. Most modern density measuring instruments use an articulated cushion that contains the detectors and the gamma ray source. A backup arm pushes the cushion against the formation. The small length of the cushion and the great decentralization force exerted by the backrest arm ensure excellent contact of the cushion with - the training in most cases. However, for tools with a smaller diameter, the use of a cushion-like construction becomes difficult or impossible. In these cases, the detectors are placed inside the housing of the instrument (mandrel-type instrument). Decentralization is provided by an arc spring and / or a calibration device with a backup arm. However, the greater length and rigidity of the instrument produce an application of lower quality of the. Instrument to the wall of the well of -sondeo and requires a separation superior to the normal one. The basic design of a two-detector instrument is shown in Fig. 1. Instrument 1 consists of a gamma-ray source 2, a short-distance detector (DC) 3 and a long-distance detector (DL) 4 The instrument is in a borehole 5 which is substantially uniform. The gamma rays emitted by the source 2 are introduced into the borehole and geological formation 6, where they are scattered and some are then detected by the detectors. The DC 3 detector is more sensitive to the region near the instrument 7. The detector DL 4 detects the gamma rays 8 of the formation 6 at a greater depth than the DC detector and is less sensitive to the effects of the separation of the detector. . The apparent density derived from the measurement of the DL detector can be corrected for the separation of the instrument by comparing the readings of the apparent density of the instruments.

DL and DC detectors, Correction for the separation caused by the accumulation of the mud scale or separation of the instrument can be achieved by using two detectors with different depths of investigation. In this case, the first detector - (.DC) has little depth of investigation and is more sensitive to the wellbore fluid or to the mud scale between the instrument and the formation. A second detector (DL) a-a greater distance from the source is less sensitive to the environment of the well and more sensitive to formation. The difference between the readings of the two. detectors, can be transformed into a correction for the separation and the mud scale. However, due to larger separations due to the irregular shape of the borehole 9, the compensation of the two detectors is often insufficient or ambiguous. The inaccuracies of the measurement are two (.2) detectors reside in the fact that the measurement of two detectors is used to determine three unknowns: the density of the formation, the separation (distance between the instrument and the wall of the sounding well) and the density of the fluid and / or mud-scale between the instrument and the formation. With small separations, the last two unknowns can be combined in an effective thickness (separation of the density of the -love). With greater separations, this approach fails and the correction becomes ambiguous. Also, the depth of Short-distance debris investigation may become smaller than separation. This will prevent proper compensation. As shown in Fig. 1, due to the irregu- lar shape of the borehole wall 9 the instrument is separated from the wall by a considerable distance. The depth of investigation of the short-distance detector 3 is less-that the separation and achieving an effective compensation of the response of the density of the long-distance detector 4 is more difficult and, sometimes, impossible. The use of an additional detector located between the traditional DL and DC detectors can help to deal with the ambiguity of the correction before a considerable separation of the instrument and some of the limitations of the detection of two detectors can be overcome. The measurement of three detectors provides the ability to distinguish the effect produced by the thickness of the mud and / or mud scale from the effect produced by the density of the mud and / or mud scale between the instrument and the formation. Also, the best statistical accuracy provided by the average measurement will improve the speed of the probe's logging. The operation of the three-detector instrument is shown in FIG. 2. The detection of three detectors 11 has the ability to measure -three different depths of investigation in the formation. The instrument has a source 12, and detractors of sorta dis- tance (DC) 13, medium distance (DM) 14 and long distance - CDL) 15. Because of the shape of the wall of the well 9 there is a considerable separation 23 between the instrument 11 and the wall of the well 9. In order to compensate The effect of this separation, at least two detectors, must have greater depths of in-vestigation than the separation of the instrument. The detectors 14 and 15 have depths of investigation, 25 and 26 respectively, which extend into the interior of the -formation 6 and allow to measure the formation and the material in the region 23 between the instrument and the wall of the borehole. The idea of using three detectors to differentiate -different depths of research was described in the US patent. UU 4,129,777 (ahl). In Wahl, the main idea is to measure the density of the material at three depths - different from the instrument. This can be used to determine the density of the formation through the coating pipe, to determine the thickness of the cement behind the coating pipe or to determine the density and thickness of the mud scale between the instrument and the formation. In all three cases the measurement is also used to determine the density of the formation and the thickness and density of the layer of material between the instrument and the formation. In wahl, gamma radiation is emitted from the instrument into the surrounding medium, and the measurements correspond to the amount of radiation that returns to the As a result of the interassión of the radiation emitted, the first, second, and third sap, respec- tively, from the siren environment, one of them somenzando in the well of sounding and extending to the erect radial depths. These measurements are taken by three detectors which are calibrated at different distances from the radiation source - gamma in order to have three different depths of investigation. A representation of the thickness of the solid material is then obtained from the three measurements of gamma radiation. Specifically, the method proposed by Wahl is useful for determining the thickness of the material bound between the borehole lining pipe and the adjoining formation. In this case, the three measurements of the gamma radiation (superficial, intermediate and deep) are corrected for the mitigating effect of the coating. Thus, three densities are measured from the radiosion measurements super fi sial, intermediate and deep respec- tively. Another patent that insorporates the sonsept of three de-testers is US Pat. UU 5,525,797. Moa e. In this patent, as in the Wahl, the gamma ray source is stopped axially from the first, second and third detector. The first detector is axially separated from the gamma ray source by a distance defined as a first separation. The first separation and collimation for the first detes They are designed in such a way that the gamma rays detected in the first detector are the gamma rays scattered mainly by the coating. The second detector or average detector is axially spaced a greater distance from the gamma source than the first detector. The second . detector is separated from the source of gamma rays by a defined distance as second separation. The second separations and solimasion for the second test tube have been designed in such a way that the gamma rays detested in the second detector will be those dispersed mainly by the coating and the cement. Finally, the tester terser or. The remote debris is separated - axially at a greater distance from the gamma ray source than the first and second detectors, by a distance defined as a third separation. The third separasión and solimasión defined by the terser detestor have been designed in such a way that the gamma rays detested in the terser detestor -they are dispersed prinsipalmente from the coating, semento and formaión. It is this tester terser that allows the instrument to measure the density of the formations while the first and second detectors allow prinsipalmente-that the instrument compensates for the coating and the cement. However, the second detestor can be used to measure the density of the formation in the absence of cement. Preferably, the detectors are protected by a high-density material, between the source and the detector, which prevents the detection of gamma rays that simply move through the instrument. A step or vacuum is provided in the shield in the form of a collimating channel that extends from the tester, through the instrument, and terminates on the outside of the super fi cial surface of the instrument. The solosion channels are designed specifically for the detection scheme of the debris sada. Specifically, the first detector or proximal detector will have a soliasion directed at a small angle to the coating so that the first detestor detests the scattered gamma rays-primarily by the coating. The second detestor or -testor medium will be a solimasión directed to a more insular or perpendicular angle are respect to the coating because the second detestor has the function of detecting gamma rays dispersed through all the cement, in addition to the coating (greater depth of investigation). Finally, the third detector or far detector will be a large sanal of suction that is directed substantially perpendicular to the coating due to the distance to which the third detector of the source is located. Since the gamma rays detected in the far detector must pass through the coating, cement, formation, before passing back through the cement and coating, the statistical probability of this event occurring is lower than for the first. and second deterstores and, consequently, a wider collimation channel is required for the third detector. The density of the three detectors presented by -Wahl dessribe the general idea of using three detectors to tell the density in the presence of a material of considerable thickness and / or density between the instrument and the formation. The distinction between the different depth of investigation is achieved by the different axial separation of the detectors. The invention presented by Moake uses to a large extent - the same detector separations as the Wahl invention. The collision of the detestor is optimized for a medicion through the casing. The DC Oprimer detectors) and DL (third) use a collimation very similar to that used in the traditional density measurement instruments of two detectors. The collision of the DM (medium) detestor is very just and almost perpendicular to the borehole wall to obtain a deeper density lesion in media through the lining. The pronounced co-limation angle of the DM detector reduces its counting speed and statistical precision. In an openhole measurement, the depth of investigation of the DM and DL detectors will be very similar and the sensitivity to the mud scale, which has a lower density than the steel coating, will be reduced. This still requires a solution to determine a compensation of the separation in the diagnostic probes that can overcome these limits. A possible approximation for an algorithm of 3 deterstores is disclosed in US Pat. 5,390,115 (Case and Ellis). The astual invention provides a new altorithm - of several detectors optimized for situations where a density measurement instrument finds a substantial separation of the formation. The method of this invention may be implemented in conjunction with the multi-detector instrument disclosed in a signed appendix, filed on February 19, 1997, aplision number, which is incorporated by reference. In addition to measuring the density of the formation, this -invent can also measure the photoelectric factor (FFE) of the formation. This measurement depends on the absorption of low-energy gamma rays through the photo-esthesis in the formations. Since the photoelectric efesto largely depends on the atomic number of the elements of the formation, it will provide an indication of the lithology of the formation. Because the photoelectric absorption preferably eliminates low energy gamma rays, the instrument housing must allow low energy gamma rays to pass through the detectors inside the housing. This objective: L can be achieved using a window composed of a material is a low (Z) low number in the housing or using a - Z material low in the housing, such as titanium. The typical materials of the window are beryllium and titanium. The housing materials can be titanium or, for low pressure requirements, graphite or highly resistant sarbono compounds.

EXTRACT OF THE INVENTION The aim of this invention is to provide an optimized medium for performing a high-density density measurement in the presence of a considerable separation of the instrument. Another objective of this invention is to provide an improved, more robust measurement of the photoelectric factor of a geological formation. Another objective of this invention is to provide a better means to control the quality of the density measurement. This invention is an improved method for determining the density of the array using a series of gamma-ray detectors. This includes an improved coringion of the separation, better prescision and significantly improved measurement for the photoelitetric effect and a more reliable way - to ensure the pression of the density response. The detectors have different depths of research tion within the training. In small separations the DC detector mainly investigates the mud and the mud crust and the superficial layer of the formalin. As the -separation increases, the DC detector signal is no longer sensory to the formation or to the mud or mud crust that is in close proximity to the formation. The DM detector has a greater depth of investigation and is sensitive - to the well of sounding and formation even when larger separations of the instrument. - The long distance detestor (DL) is sensitive mainly to the density of the formaión. The density of DL is corrected using the information of the DM- and DC detectors to provide a more accurate density reading. This invention is also an improved method for de-terminating the photoelectric factor (FFE) of the array. The use of a set of three detectors in a titanium housing provides an FFE response of high salience, more accurate and accurate than that of traditional instruments of two detectors.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a two-detector logging probe in the case of a large separation caused by the irregular shape of the borehole.

Figure 2 is a three-sensor depressor probe in the case of a large separation separated by the irregular shape of the borehole. Figure 3 is a typical gamma ray thickness observed in a density measurement instrument. Figure 4 is a flowchart of the preferred physical representation of the density algorithm method. Figures 5a and 5b show the basis of the density compensation algorithm and the need to limit the stress of the lithology. Figures 6a and 6b show the density compensation data at an intermediate distance. Figures 7a and 7b show a comparison between the tradisional algorithm and the improved algorithm for determining the FFE. Figure 8 is the flow diagram of the algorithm - to determine the FFE. Figure 9 shows the data that supports the something rhythm to control the quality of the density. Figure 10 is the flow chart for determining the density quality factor.

DETAILED DESCRIPTION OF THE INVENTION The density algorithm of 3 detestores is based on the tradisional approach of measuring the density of "column and shoulder". However, in order to fully utilize the 3-detector instrument, the "solum-na" algorithm was modified and the "rib" algorithm was adapted to the use of three detectors. The traditional "column" algorithm uses the following formulas to determine the apparent density of the measurement of a simple detector. The formula (1) is used for the detesters furthest from the source. Formula (2) is used for a debris near the source.

RHOaparent.e = A + B. In (Wd, ura) + C.In (.W, bl, and,) (1) Wcal, Wcal, hard soft RHO. = A + B. In (W, x A-) + C. In (W,,,) apparent lasts 1 soft, "» hard soft Figure 3 shows a typical spectrum of gamma rays observed in a density measurement instrument. The spectrum is divided into two windows: soft and hard. "Blan da" refers to the low energy part of the spectrum in the energy range of about 30 to 120 keV. "Dura" refers to -the high-energy part of the spectrum of about 200 to 500 keV- if a 137Cs source is used to perform the log. RHOaparent.e is the density measured p for the sensor simp? -le and _ is not sorregida for separating the instrument. "A" is - a constant (. Typically the density of a calibration medium), '"B" is the same for the sensitivity of the density of the sonous velosity in the high-energy window. The high-energy window in a typical density-measuring instrument using a 137Cs source is between 200 and 500 keV, it is desir, in the region in which the interassión of the prinsipal gamma rays oscillates through the Compton scattering. instead of photoelés-trisa absorption. W, represents the velosity of sound in the high-energy sale. Wsal is the equivalent soundness in a calibration measurement. The normalization - to the salibrasion eliminates the small differences of instrument to instrument. "C" is the correction coefficient of the lithology. This corression is necessary to eliminate the small deviations caused by different lithologies, it is to say, by different effective atomic numbers. The sensitivity to this efesto is higher at low gamma ray energies, hence the correction is based on the low energy window W,, which is normalized by the salibration sonication rate of the window. The formula (.2) also uses the term A-, which represents the sulfur A of a dementor located at a greater distance, and the density ap- RHO, determined by the farthest detector. The coefficients A, B and C may vary depending on the size of the well. probe. The manifold can be dessribir-se in form of values disretos before determined sizes -of the well of sounding for which the coefficients have been determined through measurements or models, where the values for other sizes of wells of sounding are obtained by means of interpolation . It is also possible to describe the A, B and C values as analytical functions of the size of the borehole. Since the three Co most detectors in the density measurement instrumentation in a sounding train have different depths of investigation, the actual density can be determined by comparing the apparent densities measured by the three (or more) detectors. . The preferred approach for determining the density of the geological formation is shown schematically in the flow chart of Figure 4. In the first step 40 the counting speeds of the soft and hard window are determined for the three detectors (DL, DM , DC). The velosities of sounding 41 are then continuously suppressed for the dead time of the thunderstorm and the secondary beam is subtracted from the source of the ganglion instability. This secondary espestro has been determined previously, before the source of the inscription was inserted in the instrumentation. The resulting "net" counting velocities 42 are standardized 43 by the counting speeds of the calibra-sidn of the instrument to obtain the normalized or calibrated counting velosities of the window -44. For each store a bulk density is determined using formula (1 or 2). Also, if necessary, the densities 46 are - corrected 47 for the effect of temperature on the net count rates. Since the density of the weight of the mud affects the transport of the gamma rays, the apparent densities have to be corrected 47 for the density of the mud (weight of the mud). Temperature correction 47 can be carried out as shown in the formula - (3), where the correction is a linear function of the difference between the temperature of the instrument T., -. 0 to Y A temperature of reference tref The last one is t. The latter is typically the temperature at which the instrument is calibrated. is the coefficient of temperature of the density measurement.

RHOcorr = RHOno corr. í (.l . (Iinst, rument.o - Tref_) "J (3) These corrections produce a corrected density 48. In - the formula (4) shows a possible correction of the weight of the mud, where fi. - is the density of the mud in the borehole, Bhs is the diameter of the borehole, day is the diameter of the instrument and M,, M "and M_ are coefficients of finished by experimentation and / or models.

RHO sorr = RHO no sorr. (1 - M 1p ('P l,? Od, o - 1). (Bhs-day) M2. EMe * RHOno sorr (4) This step is followed by the determination of a deferral term deltaRHDL. .,. What is the difference between the long-distance density and the sorta distansia 50? This difference is an indi- cation of the effect of the separation of the instrument 51. A correction based on this difference is then applied deltaRHODL. "To obtain the compensated densities DL 52 and DM 53. The correction is a monotonic function of deltaRHODL ^. QQj.ii -.- iQ And the corrected separation density is determined as shown in formula (5). The compensated densities DL 52 and DM 53 are then averaged 54 to obtain the apparent bulk density RHOB 55.

RHODL - corr = RHODL - no corr + f (deltaRHODLd, i.sponi., Bl, e) (5) f (deltaRHODL ,. -hle ^ can be an analytic function or can be described by a sequencing of segments in the subtracted line ("segmented rib".) Figure 5a shows the correlation between the stress of the RHOnecesari.a and the available sorbidity. deltaRHODL.,,. The points of the data in the They represent many measurements taken at densities between 1.7 and 3.1 g / cc, with simulated mud crusts of varying thickness and density. Most points follow - a trend line but some points deviate signifi cantly. This is due to the photoelectric effect of the mud or fluid sample from the borehole, which is overridden in the Cl span. The "soft" gamma rays are not very penetrating and are strongly afflicted by the elements' presensia - they are a high numb number Z in their path. The formula (1) solves the effect of the photoelitetric factor of the formations. However, if the gamma rays are displaced through a material from the well borehole with high Z Clodo heavy and heavy mud substratum), the corression will be too large and the response of the density will not be assured. Accordingly, this invention limits the "soft" corressidn to obtain an insensitive-insensitive response to heavy mud. The result is shown in formula (6).

W81 w0 * 1 w81 hard-soft hard ^ is the soefisiente that determines the minimum value that W, must be in comparison with the high energy window W,. The effect of this sorption is shown in Figs. 5a and 5b, Fig. 5a shows the need for density density (deltaRHO nesesary) as a function of the -diferensia between the apparent densities of DL and DC- (delta RHODIg. * ie ^? ^ "p the -Limitation in the density correction.Figure 5b shows the same data include the limitation in the lithology sorption.The points of the data are marsados according to the factor fo-toeléstriso of the mud and / or mud substrate The density of the average detestor can be corrected in the same way that they are the density of long distansia.- The preferred physical representation uses deltaRHODL, ... that is determined from the difference between the densities of long and short-distance, as shown in Formula (7).

RHODM-corr = RHODM-no sorr + g (deltaRHODL ^. Ni £) le) (7) The function for the DM correction differs from that of the DL detector. The reason for preferably using deltaRHODL,. * ble > it is shown in Figures 6a and 6b. The diagram -from Figure 6a deltaRHODL ,. 'ble' versus the correction - nosesaria deltaRHODMnesesap.o shows a disphoxion musho-minor, Figure 6a, that if deltaRHODM is used ,. - The fact that the points of the data in the diagram of Figure 6a come in different lines as a function of the separation of the Instrument can be used to make additional corrections to separations sonsiderablés. In the preferred physical representation the response of the final density is obtained by the simple average of the DL and DM densities sompensed, as shown in Formula (8a).

RH0final = < RH0DL corrected + RH0DM corrected > 2 • < 8a) = (cl * RHODL corregi.d, o + c2 * RHODMcorregi.d, o) (8b) A weighted average can also be used as shown in equation (8b), where cl + c2 = 1. And it is possible to further correct the density based on the difference between the DM and DL denned offsets as shown in Formula (.9): RHO ^. , = (RH0DL_., + RHCDM.,) /2.+h (RHODL., Corrected corrected final corrected - RHODM.,) C9) corrected v Other applications of the algorithm are possible. In particular, the sorption for the size of the borehole and the weight of the mud can also be carried out in the final response of the RHOB density ..., instead of the apparent densities of the simple detector.

The photoestrial efesto repersute prinsipalmente -in low-energy gamma rays Cblandos), while -the high-energy window Cduros) is almost exaslumente sly afflicted by the density of the formanidn. For this reason it is tempting to use the soft / hard gamma ray relay as a measure of the photoelectric fastor of the array. The traditional FEE algorithm is based on the formula shown in the equation. (10) Use the esho that low-energy gamma rays are afflicted more by the absorption of gamma rays through the efesto fotoeléstriso than high-energy gamma rays. The ratio between the number of lights in a low power window C < 120 keV) and a high energy window C > 200 keV) is an indication of the photo-eléstriso effect of the formaión. Wblanda CIO) This is a good result of instruments that use beryllium windows to allow low-energy gamma rays to deviate from the form to the debris, which is a minimal dispersion or absorption. In this case a window of very low energy can be used Ces desir, 30 to 70 keV). The velosity of sound in this window is dominated -by the influence of the photoelectric effect. If an alo is used titanium, the very low energy C30 gamma rays at 60 keV) are absorbed energetically in the housing material. This requires that an energy window that includes higher energy gamma rays be used to obtain a response suf fi ciently pres- sure. However, the velosity of sound in this window is more afflicted by the density of the formaión. This is evident in sasos where the FFE is high at a low density. The density of the density can be corrected in a simple and elegant way by slightly changing equation (10). The resulting equation CU) is shown as follows: Soft W + B (11) FFE + C (W,) cL hard The sambio you were in taking a potency ° < . of the - sound of sound in the window of the density before forcing the soft-hard relasion. If the potency < * • is less than 1.0 the density of the density is reduced. Figures 7a and -7b show a real example of both approaches. In Figure 7a there is a point 60 that has a low density and a high FFE that does not follow the evident tendensia suando the two sides of the esuasión (10) are represented one against the other. In Figure 7b, the span (11) is used with ot = 0.94. Point 61 also with a low density and high FFE matches better with the general tendensia. The FFE can be derived, therefore, from the sounding velocities shown in the reinforcement (12).

FFE = C Wblanda (.12) A + B or dura Substrates A, B, and C can be funsions of the size of the borehole. In particular, in the preferred physical representation, the "A" subject can be written -as A = A * (1-const * (size of the borehole - diameter of the instrument) (13) where sonst is a small number. This makes it possible to sorrow the effet of the borehole fluid between the instrument and the formations, insluso if the instrument is entered into proper contact are the formaión. If there is a significant difference between the survivorship of the instrument and the well-sounding, the only gamma rays that will not find fluid-from the sounding well in its path will be those that enter the line where the instrument touches the formations. The average sanctity of the wellbore fluid penetrated by the ra- yos gamma increases with increasing unconformity of the curvature. If it is not corrected, this causes a deviation in the response. If two detesters are used to determine the FFE of the shape, and the two detectors hold significantly different azimuth angles, the fact that the average length of gamma ray travel through the borehole fluid varies can be used to correct-the effect of the EBF on the borehole fluid. Figure 8 shows the different steps necessary to determine the photoelectric factor of gamma ray threshers. In the first step 70, which is similar for detesting sada, the thickener of the gamma-ray sonar velosity measured by each detector is obtained and divided between two windows ("hard" and "soft") at least. In the second step 71, the counting speeds of the window are corrected - to compensate for the losses of soundness due to sonating - to dead time elestrdniso, and the secondary sonication velosities of the window of the source of stabilization of the tector are subtracted . In the terser step 73, the net window vortices are normalized (salibrated) by the respectable net sonorous velosities of the window of the instrument's salibration. This provides the calibrated counting rates of window 74, which are corrected for the temperature of instrument 75. In the fourth - step, the photo-esthesis of the simple debris is determined according to the equation (12), where some or all of the particles (A, B, C) may be the size func- tions-of the borehole 77 and / or the weight of the mud. . If more than one sensor measures the FFE, the FFE resulting from the simple debris 78 can be combined in steps 79 and 80 to obtain an FFE 82 that is compensated for the photoelitetry effect of the mud and the separation of the instrument. To follow this, it may be necessary to add to the calculation additional information such as -the delta RHO d, i.sponi.b.l.e 81. The FFE can be determined from all the detectors of the set. This allows two things: to compensate the density and the photoelectric effect of the mud crust and the separation, and to perform salinity controls by separating the photo-thermostats determined by two or more detectors. The sompensation for the mud scale and the separation is different from the compensation for the density. The difference of FFE between two detestores depends on the solitude of the detestores, the separation and the density and photo-eléstriso of the sludge or sludge of mud. Therefore, the -FFE sorriginated is not only a function of the difference of FFE between two detestores, but also depends on the measured density and the difference dRHO between the densities of the single detestor.

FFE_sorr = FFE + g (FFE, dRHO, FFE and RHO) (14) The FFE is the density of the simple detestor salsulated from one of the detestores, dRHO is the difference between the densities DL and DC or DM and DC of the simple detestor and dFFE is the difference between the FFE of two detestores. The RHO can be the sorrected density or one of the densities of the simple detestor. When carrying out the density measurement, it is possible that situations arise where the response is not precise or is incorrupt. It is important to have quality controls that indicate when the response of the instrument is no longer reliable. The use of a set of three or more detectors allows the use of the consistency between the responses of the detesters to indicate locations where the instrument is not able to offer a reliable response or indicate a failure -in the instrument. Traditional density measurement instruments (two detectors) depend on the quality verifications made by the simple detector Salinity Mathematics) and the size of the sornession aplisted in the "column and rib" algorithm to infer the validity of the it turns two. However, the deltaRHOd, i.sponi., Bl..e is not an unequivocal quality indicator, although it can equitably indicate that the data are sorrestos, insluso if the instrument he experiences excessive separation. A more accurate and adequate method for detecting and unavailability of a definite data output is a combination of responses from the three detesters. It is based on the following sanctities, which are derived from the apparent densities measured by the three detestores. dDM = (RHODM-RHODCalto) / RHODL (15) dDL = (RHODL-RH0DCmed) / RHODL (16) If the above quantities Cl) and C2) are plotted against each other, we obtain the diagram shown in Figure 9. RHODM is the average apparent distance density (corrected for the effect of the size of the well-depth and weight of the mud), RHODL is the density of apparent longitude. RHODC, denotes the density of distance - apparent cut of a window of energy of gamma rays that is in the center of the spectrum (around 300 keV for an instrument using a 137Cs source). RHODC, refers to a window of higher energy (approximately -400 keV). The position of the energy windows must be determined through experiments and models for any-density measurement instrument. The output speed is determined as shown in Figure 10. The first steps 90 to 97 are - the same as in the density algorithm and serve to determine the softer or more apparent densities of the simple detestor 96 that are sorrected for the weight of the sludge 97 -producing sorrected densities 98. In step six 99 the reassessments of the scaffolds are determined 15 and 16. In step seven 100 it is determined whether the set of two relasions -sae within a predetermined region in the transverse diagram. The quality factor 101 is then determined as the distance of the data points from the limit of the predetermined region. The distancies that are taught-within the limit are called arbitrarily positive and those that are taught outside the limit. A fastor of negative sálidad index, therefore, defisent or doubtful data. In somparasidn are the prosesamiento of the density, two apparent densities are determined from the velosidades of sonteo of the salibrada window of the detesting of short distàsia. This is an indisasion of hesho that different energies of scattered gamma rays respond to different depths of investigation. The selection of appropriate energy windows, which may differ from those used for the density algorithm, must be carried out through experiments and models. The methods of this invention provide advantages-significant in comparison are the astute designs. East invention has been dessrito in connection are the preferred physical representations. However, it is not limited to them. Changes, variations and modifications to the basic design can be made without departing from the inventive concepts in this invention. Also, these sambios, variations and modifications will be evident to the design experts who will have the advantage of the descriptions -previous.

Claims (10)

1. A method for determining characteristics of a geological formation around a borehole and comprising the following steps: (a) providing a source for irradiating such a geological formation with gamma rays or X-rays; (b) provide some distance detectors -sorta, medium and long, locating each detector, respectively, at fixed distances, successively larger, from the source, so that each detector exhibits a non-negative response to the increase in density of the formation, and being said detestores sapases to generate indiscriminate signals of the energy of gamma radiasión detested by one of them sada; (s) dividing the thicket of detested gamma rays into a plurality of windows in the debris beam, with windows showing a gamma-ray sounding and representing different energies of the gamma rays; (d) sorregir the velosidades of -conteo of the gamma rays detected for the dead time; (e) determine a measure of density in the testicle; and - (f) calculating a corrected density from the determined density measurement in each detector.

2. The method of claim 1 wherein step (d) further comprises subtracting a measurement from the source of -stabilization of the counting speed.

3. The method of claim 1 or 2 where the - step (e) includes, in addition, the limitation of density measurements according to the following expression: RHDaParente = A + B- ^ dura ^ "111 (MaxCWblanda 'ß. Wdura.}. > where (Wcaldura) (Wsalblanda) Wsaldura,) RHO. is the density measured by the detector, A is a constant representing the density of a calibration medium, B is a density sensitivity coefficient for the counting speed in a high energy window, C is a -soefficient the lithology, W,, W _ß ¡_. represent, respec- tively, the counting speed in a high-energy window and the equivalent counting velocity in a sali- bration measure, W l da W lbla da represent, respec- tively, the counting speed in a low energy window and the velocity of equivalent count in a calibration measurement, and A is a coefficient determining a minimum value for soft.

4. The method of claim 3, wherein the step (e) additionally, the step of calculating the difference between the density measured by the detectors of long and long distance is also shown.

5. The method of claim 4, wherein step (f) also assumes the measurement of the density of the deformation of the long-distance detestor, adding to the measure of the long-distance density a difference in the difference between the density measured by the long-distance and long-distance detesters.

6. The method of claim 4, wherein step (f) also assumes the measurement of the density of the deformation of the mean distansia detestor, adding a frassion to the measurement of the average distansia density. of the difference between the density measured by the -detestores de larga and sorta distansia.

7. The method of claim 3, wherein step (e) also assumes the step of increasing the difference between the density measured by the medium and short distance detectors.

8. The method of claim 7, wherein step (f) also comprises correcting the density measurement of the formation of the average distance detector, adding to the measure of average distance density a fraction of the difference between the density measured by the middle and short distance deterrents.

9. The method of any of the presequent claims arising from the step of monitoring the measurement of the density of the test result for efestos of the borehole.

10. The method of any of the presumptive claims that arise, likewise, the step of absorbing the of the density of each detector for the weight of the mud and temperature of the detector.