High-precision while-drilling natural gamma-ray spectrum imager

Abstract

According to the high-precision while-drilling natural gamma-ray spectrum imager, a high-precision gamma-ray spectrum detector is selected as a sensor module, a high-energy-resolution energy spectrum diagram is obtained through energy correction processing, a stable window method is selected for energy spectrum drift correction of instrument temperature change, and the energy spectrum drift correction step is simplified; the energy spectrum drift channel number and the energy window moving range are determined by searching a K peak, a U peak and a Th peak so as to ensure the accuracy of energy window division, the high-precision potassium, uranium and thorium element content measured in real time in each sector in the stratum is obtained after spectrum unfolding, data processing, interpolation imaging and other operations are carried out according to the measurement result of each sector, and the accuracy of energy window division is improved. The real-time azimuth imaging graph of each sector is displayed, parameters such as the organic carbon content and the shale type can be accurately judged according to the imaging graphs, more accurate and more comprehensive information is provided for geosteering, the drilling success rate is greatly increased, and the operation risk is reduced.

Description

Technical Field

[0001] The invention relates to a high-precision while-drilling natural gamma spectral imager, belonging to the technical field of while-drilling geological guidance. Background Art

[0002] The natural gamma logging system is a logging instrument that reflects the total gamma intensity emitted in the formation. The logging instrument transmits the logging information along the wellbore by means of cable transmission. The total natural gamma cable logging is the first logging instrument used for gamma ray measurement in drilling geophysical exploration. Since most shales are relatively rich in naturally occurring radioactive elements, these logs are mainly used to distinguish shales from other formations or to measure the shale content of the formation. This cable logging system is only used after drilling.

[0003] Measuring the total gamma intensity while drilling is an advantageous logging method, where data is transmitted via mud pulses. This type of system is often referred to as gamma logging while drilling.

[0004] Another type of natural gamma ray logging system is a logging instrument that measures the natural gamma spectrum in the formation. This type of logging system is usually called a gamma spectrum logging system. Natural gamma spectrum logging instruments are usually pulled up along the wellbore by cables for measurement. Low gamma count rate and poor detector stability are the main problems in any type of natural gamma spectrum logging system.

[0005] Most of the natural gamma rays found in formations are emitted by elements in the potassium (K), uranium (U), and thorium (Th) decay chains. The energies of natural gamma rays that can be measured in a borehole environment typically span a range of about 0.1 to less than 3 million electron volts (MeV). The elements K, U, and Th emit gamma rays at different characteristic energies. Therefore, by identifying these characteristic energies using a natural gamma spectroscopy logging system, the radiation components from K, U, and Th can be obtained, thereby obtaining the total measured gamma intensity. Through system calibration and modeling, these components can then be correlated to the corresponding elemental content of these elements in the formation. In addition to the shale content obtained from the total natural gamma ray logging, the elemental content of K, U, and Th can also be used to determine parameters. These additional parameters include, but are not limited to, clay typing, lithology identification, fracture detection, and radioactive tracer placement.

[0006] With the improvement of oil and gas field exploration and development and production needs, the target of oil and gas field exploration has gradually shifted to unconventional oil and gas reservoirs with more complex geological structures. The exploration and development of these complex oil and gas reservoirs require more accurate while-drilling geosteering technology as support and guarantee.

[0007] At present, the domestic azimuthal gamma logging instruments have inherent limitations. On the one hand, the data transmission speed using mud pulses is slow. On the other hand, in order to enhance the azimuthal characteristics, a shielding tube is added to the back of the detector, which makes the measurement count rate of the gamma detector low, resulting in a series of problems such as poor formation resolution and low energy resolution. Summary of the invention

[0008] The present invention is a high-precision natural gamma spectrum imaging instrument while drilling. The instrument uses a high-precision gamma spectrum detector, which can obtain the potassium, uranium and thorium content in the formation in real time with high accuracy. It can not only provide real-time azimuth imaging of total gamma rays and element content in each sector, but also clearly and visually display reservoir information through the chromaticity change of the three-dimensional imaging map, which is used to accurately determine the type of mud, estimate the organic carbon content, and provide more accurate and comprehensive information for geological guidance. Through the accurate description of the azimuth gamma image, three-dimensional image and organic carbon content of the drilled reservoir, the drilling trajectory is adjusted in time to guide the drill bit to smoothly drill through thin oil and gas layers and complex geological structure reservoirs. According to these accurate and reliable logging information, the success rate of drilling can be greatly improved, the risk of operation can be reduced, and the efficiency of drilling can be improved.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: a structure of a high-precision while-drilling natural gamma spectroscopy imager, including a high-precision gamma spectroscopy detector, an azimuth sensor, a shielding body, a data storage circuit, a data acquisition circuit, a high-temperature power supply, and a drill collar.

[0010] The high-precision gamma spectrum detector comprises a dual-output crystal, a first photomultiplier tube, a second photomultiplier tube, a first light guide, and a second light guide.

[0011] The dual-output crystal is a scintillation crystal, which is generally cylindrical in shape.

[0012] The first light guide is located between the upper end of the cylindrical crystal and the first photomultiplier tube.

[0013] The second light guide is located between the lower end of the cylindrical crystal and the second photomultiplier tube, and its material and performance are completely the same as those of the first light guide.

[0014] The first photomultiplier tube and the second photomultiplier tube are both photomultiplier tubes composed of a photocathode, an electron optical input system, an electron multiplication system and an anode, and have completely the same material properties. The pulse output by the first photomultiplier tube is a first pulse, and the pulse output by the second photomultiplier tube is a second pulse. The incident position of the gamma ray can be calculated based on the first pulse and the second pulse, and the K, U and Th contents can be calculated with higher accuracy through energy correction.

[0015] The first pulse and the second pulse are both electrical pulses. Calculating the incident position of the gamma ray and / or correcting the energy of the gamma ray based on the first pulse and the second pulse includes: determining the amplitude values ​​corresponding to the first pulse and the second pulse; determining the difference between the amplitude value of the first pulse and the amplitude value of the second pulse; and calculating the incident position of the gamma ray and / or correcting the energy of the gamma ray based on the attenuation coefficient of the scintillator to the scintillation light.

[0016] The first pulse and the second pulse are both electrical pulses. Correcting the energy of gamma rays according to the amplitude values ​​of the first pulse and the second pulse includes: determining the amplitude values ​​corresponding to the first pulse and the second pulse; and determining the arithmetic mean or weighted mean of the amplitude values ​​of the first pulse and the second pulse as the energy measurement value of the gamma rays.

[0017] The high-precision gamma spectrum detector is located in the non-magnetic drill collar. The 'back' of the detector is made of shielding material tungsten to make a shielding tube to shield gamma rays outside the detection direction. The tungsten shielding tube adopts a concentric circle structure design with a window at a certain angle. The gamma rays in the formation can only enter the crystal from the window and be detected by the high-precision gamma spectrum detector. The azimuthal gamma spectrum downhole measurement system divides the annular section measured by the high-precision gamma spectrum detector into multiple sectors. When the drill bit rotates, the high-precision gamma spectrum detector rotates with the drill bit. During the rotation process, the high-precision gamma The energy spectrum detector collects and measures gamma rays from each sector, thereby obtaining the gamma ray intensity values ​​corresponding to each sector on the cross section and the potassium, uranium and thorium contents corresponding to each sector. The ground information processing system interpolates the gamma ray intensity data and potassium, uranium and thorium content data of each sector, and defines the imaging color spectrum according to dynamic or static color. The data of each sector is assigned different color according to certain scale rules and displayed, thereby generating two-dimensional images and three-dimensional stereo images of the gamma ray intensity data and uranium, thorium and potassium content data while drilling.

[0018] The window opening angle of the tungsten shielding tube directly affects the magnitude of the gamma counting rate. The larger the window angle, the larger the measured gamma counting rate.

[0019] The measurement accuracy of the high-precision gamma spectrum detector is highly correlated with the peak position of the spectrum during actual well logging. Temperature changes will cause the spectrum to drift, resulting in large errors in the spectrum interpretation results. Therefore, a temperature drift correction algorithm is added. The temperature drift correction method in the present invention is different from the conventional spectrum stabilization method. The conventional spectrum stabilization method requires the addition of additional spectrum stabilization sources such as Am sources, while the present invention uses a window stabilization method that does not require additional spectrum stabilization sources. It uses the characteristic peaks of K, U, and Th in the natural gamma spectrum to determine the energy window range of the five-window spectrum interpretation based on the channel addresses of these characteristic peaks. Therefore, the channel address changes caused by temperature drift will not affect the accuracy of its spectrum interpretation.

[0020] The technical effects and advantages of the embodiments of the present invention are:

[0021] 1. The embodiment of the present invention adopts a high-precision gamma spectrum detector. By processing the amplitude values ​​of the first pulse and the second pulse, the stratigraphic resolution can be improved by determining the photon incident position, and the calculation accuracy of the K, U, and Th contents can be improved by energy attenuation correction, so as to accurately judge the mud type and estimate the organic carbon content, providing more accurate and comprehensive information for geological guidance.

[0022] 2. The embodiment of the present invention adopts a high-precision gamma spectrum detector and installs an azimuth sensor on the instrument, which can simultaneously obtain gamma spectrum information and azimuth gamma imaging information in one measurement, and can accurately determine the interface position of different strata and calculate the stratum inclination.

[0023] 3. The embodiment of the present invention adopts the method of logging while drilling, which can perform azimuthal natural gamma spectroscopy logging while drilling, so that the potassium, uranium and thorium content in the formation can be measured in real time with high accuracy.

[0024] 4. The embodiment of the present invention adopts a stabilization window energy spectrum drift correction method, which can perform temperature drift correction on the energy spectrum without adding an additional stabilization spectrum source, and the correction method is simpler and more convenient. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0026] Figure 1 is an overall structural diagram of an embodiment of the present invention;

[0027] Figure 2 is a structural diagram of a high-precision gamma spectrum detector according to an embodiment of the present invention;

[0028] Figure 3 This is the working principle diagram of the high-precision gamma spectrum detector;

[0029] Figure 4 It is the flow chart of energy spectrum drift correction;

[0030] Figure 5 It is the installation profile and sector division of high-precision gamma spectrum detector in drill collar;

[0031] Figure 6 is the imaging method of this embodiment;

[0032] Figure 7 2 is an imaging effect diagram of this embodiment. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The specific embodiments described here are only used to explain the implementation methods of the embodiments of the present invention, but are not limited to the embodiments.

[0034] See also Figure 1 , showing the overall structure of an embodiment of the present invention. The gamma spectroscopy imaging system while drilling is mainly composed of a sensor module 11, a data acquisition module 12, a downhole telemetry module 13, and a ground receiving imaging device 14. The sensor module 11 measures the formation azimuth gamma spectrum, bottom hole temperature, well inclination angle, drill bit speed and other data, and sends them to the central control instrument through the downhole bus. The central control instrument then processes the data and sends it to the downhole telemetry module 13, and uploads it to the ground receiving imaging device with the help of drilling fluid pulses, so that the geological guidance software can image and analyze the drilled formation.

[0035] See also Figure 2 , provides a structural diagram of the high-precision gamma spectrum detector in this embodiment. The detector includes a housing 21, a cylindrical dual-output crystal 22, a light guide 23 located at the upper end of the crystal 22, a light guide 24 located at the lower end of the crystal 22, a photomultiplier tube 25 located at the upper end of the light guide 23, and a photomultiplier tube 26 located at the lower end of the light guide 24. Gamma rays in the formation pass through 21 and enter the crystal 22, and lose energy in the crystal 22, causing ionization and excitation of scintillator atoms, generating scintillation photons, which pass through the light guide 23 / 24 through reflection, transmission and other light transmissions, hit the photocathode of the photomultiplier tube 25 / 26, and are converted into photoelectrons, and then fly to the dynode system of the photomultiplier tube for multiplication and output and then transmitted to the data acquisition module.

[0036] See also Figure 3 , showing the working principle of the high-precision gamma spectrum detector in this embodiment. The high-precision gamma spectrum detector uses a dual-output crystal, and can simultaneously obtain the amplitude values ​​UAMP and DAMP of the photomultiplier tube 25 at the upper end of the crystal and the photomultiplier tube 26 at the lower end of the crystal in one measurement. According to the attenuation ability of the crystal to gamma rays of different energies, the amplitude data UAMP and DAMP are energy-corrected. The corrected amplitude CAMP is related to UAMP, DAMP and the energy attenuation coefficient. According to the amplitude value after energy correction, the natural gamma spectrum can be statistically analyzed and drawn. It has a higher energy resolution than the energy spectrum obtained by an ordinary single-end detector. Therefore, by decomposing the spectrum, more accurate gamma spectrum data, that is, the uranium, thorium, and potassium element content values, can be obtained, which effectively helps to determine the mud type and calculate the organic carbon content. In addition, the incident position of the gamma photon on the crystal can be determined by using UAMP and DAMP and the attenuation coefficient, so that the formation resolution of the gamma logging curve can be improved, which is convenient for better judgment of the formation interface.

[0037] After energy correction, the corrected natural gamma spectrum is obtained, the horizontal axis is the gamma ray energy in units of mega electron volts (MeV), and the vertical axis is the count rate on each energy channel. Among them, the characteristic peaks of K, U and Th are displayed at energies of 1.46MeV, 1.76MeV and 2.61MeV respectively. To obtain the content of these three elements, energy spectrum analysis must be performed. The energy spectrum is first pre-processed, including the removal of abnormal values, smoothing of energy spectrum filtering, and correction of energy spectrum drift, and finally the spectrum is decomposed. Commonly used spectrum decomposition methods include inverse matrix, least squares, and weighted least squares. In this embodiment, the weighted least squares method with higher accuracy is used for spectrum decomposition. The weighted least squares spectrum decomposition method is: AT*W*A*X=AT*W*C, where A is the response coefficient of the instrument, X is the content matrix of the elements uranium, thorium, and potassium to be determined, C is the five-energy window count rate matrix, and W is the diagonal weight matrix. According to this formula, the element content X can be calculated.

[0038] See also Figure 4 , showing a flow chart of energy spectrum drift correction. The problem of energy spectrum drift correction is involved in the process of data preprocessing. If the temperature changes during instrument logging, the energy spectrum will drift, resulting in a large error in the spectrum interpretation result. Therefore, it is necessary to add an algorithm for temperature drift correction. The temperature drift correction method in the present invention is different from the conventional spectrum stabilization method. The conventional spectrum stabilization method requires the additional addition of a spectrum stabilization source such as an Am source, while the present invention uses a window stabilization method that does not require an additional spectrum stabilization source. It uses the characteristic peaks of K, U, and Th in the natural gamma energy spectrum, and determines the energy window range of the five-window spectrum interpretation based on the channel addresses of these characteristic peaks. Therefore, the channel address changes caused by temperature drift will not affect the accuracy of its spectrum interpretation. The specific implementation is as follows:

[0039] During the gamma spectrum imaging logging while drilling, a real-time accumulated gamma spectrum can be obtained. According to the energy window range where the characteristic peaks of K, U, and Th in the standard spectrum are located, the peaks are searched to obtain the measured peak positions of K, U, and Th 41;

[0040] Compare the difference between the measured peak position of K, U, and Th and the standard peak position by 42 to determine whether the logging instrument has energy spectrum drift. If the measured peak position is completely consistent with the standard peak position, there is no energy spectrum drift and the spectrum can be directly analyzed and calculated. Otherwise, there is energy spectrum drift and the drift channel number n needs to be calculated based on the difference between the measured peak position and the standard peak position.

[0041] According to the energy spectrum drift channel number n calculated in 42, the standard energy window range is shifted 43. If the measured peak position is greater than the standard peak position, the energy window is shifted to the right as a whole. Otherwise, the energy window is shifted to the left as a whole to obtain a new energy window range. This new energy window range needs to be selected for calculation in subsequent spectrum interpretation.

[0042] See also Figure 5 , showing the installation profile of the high-precision gamma spectrum detector in the drill collar and the sector division in this embodiment. The side opening of the drill collar outer wall 54 is used to embed the high-precision gamma spectrum detector 51. The high-precision gamma spectrum detector 51 is installed in the drill collar 55. There is a tungsten shielding tube 57 with a side window on the back of the high-precision gamma spectrum detector 51. The middle of the drill collar 55 is a mud channel 53. The part between the outside of the drill collar outer wall 54 and the formation 56 is the annulus 52. The azimuthal gamma spectrum logging instrument can detect the gamma intensity and uranium, thorium and potassium content in a certain sector of the formation in a directional manner by using a side window. With the rotation scanning of the downhole drilling tool, the gamma intensity change map and uranium, thorium and potassium content change map of the formation in different sectors of the whole wellbore can also be obtained, that is, the azimuthal gamma spectrum imaging map.

[0043] See also Figure 6 , showing the imaging method of this embodiment. The high-precision gamma spectrum imaging method while drilling is a technology that measures the formation around the entire wellbore, obtains the gamma spectrum logging data of each sector in the wellbore, summarizes, pre-processes, edits, etc. these data, and finally obtains a visualized two-dimensional image or three-dimensional image. The high-precision gamma spectrum detector is installed in the drill collar. As the drill bit rotates, the detector will rotate accordingly. When it rotates to different azimuths around the well, it will measure the gamma spectrum measurement data of the azimuth, and then output it to the ground system. The ground system will perform comprehensive processing and interpolation on the gamma spectrum data of each sector, and define the imaging color spectrum according to dynamic or static chromaticity. The data of each sector is assigned different chromaticities according to certain scale rules and displayed, thereby generating a two-dimensional image and a three-dimensional stereo image of the gamma ray intensity data and uranium, thorium, and potassium content data while drilling.

[0044] The gamma spectroscopy imaging system while drilling can be used to measure the K, U and Th element contents at different borehole azimuths and different borehole depths. Azimuth measurement requires additional components to be set up in the data acquisition module so that the azimuth element contents K, U and Th can be determined. The signal from the high-precision gamma spectroscopy detector is transmitted to the processor, which again controls the gain through an adjustable high-voltage power supply, and finally outputs the gamma spectrum and element content through the processor.

[0045] When the instrument rotates 360 degrees, the gamma-ray energy spectrum is stored in the FLASH data block according to the time interval. The time interval is preferably about 50 milliseconds. In each time interval, the average reading of the azimuth sensor is determined to define the azimuth sector associated with each time interval, and the azimuth sector is thereby assigned to each box. Therefore, each box contains a gamma-ray energy spectrum measured in a known wellbore azimuth sector. The binning of the energy spectrum data, as well as the average value of the azimuth sensor readings in each time interval, are controlled by a processor, and the binned gamma energy spectrum data and the corresponding azimuth sectors are preferably stored in the processor. This process is repeated through multiple 360-degree rotations within a given depth interval to maximize the statistical accuracy of each natural gamma-ray energy spectrum stored in each box.

[0046] If the sensor module of the gamma spectrum imaging instrument while drilling only includes one gamma spectrum detector, the measurement count rate will be low and the statistical error will be large when the measurement time is short. If the sensor module includes multiple gamma spectrum detectors, a higher accuracy azimuthal gamma spectrum measurement value can be obtained. The amplitude of the azimuthal sector is determined by the number of detectors in the sensor module, and the circumferential imaging resolution of azimuthal gamma spectrum logging can be improved by increasing the number of detectors.

[0047] See also Figure 7 , showing the imaging effect diagram of this embodiment. The display of gamma spectrum imaging is related to the selection of color spectrum. Generally, light colors are used to represent low gamma values ​​or low potassium, uranium, and thorium contents, while dark colors are used to represent high gamma values ​​or high potassium, uranium, and thorium contents. The color in the gamma imaging diagram becomes darker as the gamma value or potassium, uranium, and thorium content increases, and becomes lighter as the gamma value or potassium, uranium, and thorium content decreases. The color corresponds to the gamma value or potassium, uranium, and thorium content. The two-dimensional and three-dimensional imaging diagrams of the gamma spectrum can clearly and accurately reflect the bottom characteristics. The mud type can also be accurately judged based on these data, and the organic carbon content can be estimated, providing more accurate and comprehensive information for geological guidance.

Claims

1. A high-precision while-drilling natural gamma ray spectrum imaging instrument, comprising: (a) a high-precision gamma-ray spectrum detector for measuring the gamma-ray energy spectrum; (b) The measured energy spectrum of the high-precision gamma spectrum detector is related to the downhole temperature during instrument measurement. Temperature changes will cause the energy spectrum to drift, and energy spectrum drift correction is required.

2. The high-precision gamma spectrum detector is a double-end window crystal gamma spectrum detector, which is installed in a non-magnetic drill collar. The back of the gamma spectrum detector is made of a shielding tube made of shielding material tungsten to shield gamma rays outside the detection direction. The tungsten shielding tube adopts a concentric circle structure design with a window at a certain angle. The gamma rays in the formation can only enter the crystal from the window and be detected by the gamma spectrum detector.

3. The method for correcting the energy spectrum drift phenomenon under temperature change according to claim 1, characterized in that: The instrument uses a stable window method to correct energy spectrum drift. The stable window method does not require an additional stable spectrum source. It first searches for the characteristic peak addresses of 1.46MeV of K, 1.76MeV of U, and 2.62MeV of Th in the natural gamma spectrum measured by the gamma spectrum detector. Then, the energy window is shifted according to the addresses of these characteristic peaks, thereby obtaining a new five-energy window range. Finally, the weighted least square method is still used for spectrum interpretation, but the energy window range is changed to the newly obtained five-energy window range. Using a stable window method to correct energy spectrum drift will not affect the accuracy of its spectrum interpretation.

4. The energy window range shifting method according to claim 3, characterized in that: After peak searching, the characteristic peak addresses of 1.46MeV of K, 1.76MeV of U, and 2.62MeV of Th are obtained. These characteristic peak addresses are compared with the standard characteristic peak addresses of the natural gamma energy spectrum without energy spectrum drift. If the peak positions after drift are greater than the corresponding peak positions of the standard spectrum, the five-energy window range is shifted to the right as a whole, and the number of channels moved is determined according to the difference between the measured channel address of the 1.46MeV characteristic peak of K after drift and the 1.46MeV characteristic peak address of the standard spectrum K. Conversely, when the peak positions after drift are less than the corresponding peak positions of the standard spectrum, the five-energy window range is shifted to the left as a whole, and the number of channels moved is also determined according to the difference between the measured channel address of the 1.46MeV characteristic peak of K after drift and the 1.46MeV characteristic peak address of the standard spectrum K.

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