13.6. 1 electrical imaging logging
13.6. 1. 1 micro-resistivity scanning imaging logging
Formation micro-resistivity scanning imaging logging (FMS) and full borehole formation micro-resistivity imaging logging (FMI) are rapidly developed on the basis of high-resolution formation dip logging (SHDT), and are the most widely used methods in electrical imaging logging at present. It records hundreds of micro-resistivity (or conductivity) curves by using the button electrode array attached to the electrode plate on the well wall. These curves reflect the relative variation characteristics of the conductivity of the part of the borehole wall swept by the electrode plate, and have extremely high longitudinal resolution (about 0.5 cm).
By special processing and imaging the recorded data, these conductivity curves can be converted into images scaled by pixel color or gray scale, that is, micro-resistivity scanning images (FMI for short). In this calibration, the stratigraphic features larger than the instrument resolution (the ability of micro-conductivity to draw stratigraphic features) are usually expressed as several resolution unit pixels, while the stratigraphic features smaller than the instrument resolution are expressed as one resolution unit. The resolution of the instrument is related to the geometric structure of electrode buttons, such as the size of buttons, the spacing between buttons, the spacing between rows, the size of array, etc., which determines the clarity of scanned images. On the micro-resistivity scanning image, different colors or grayscales represent the resistivity of the formation near the borehole wall. The darker the color, the smaller the resistivity, and vice versa. Therefore, the micro-resistivity scanning image can clearly describe the subtle changes of borehole wall strata, such as various stratigraphic characteristics, sedimentological characteristics, as well as holes, fractures and their occurrence and orientation, as well as observe the core image.
At present, there are many kinds of imaging logging tools on the market, such as Schlumberger's formation micro-resistivity scanning imaging logging tool (FMS) and full-well formation micro-resistivity imaging logging tool (FMI), Atlas's micro-conductivity imaging logging tool (1022XA), Halliburton's electron microscopic imaging logging tool (EMI) and so on. Its main technical indicators are shown in table 1022xA.
Table 13-2 main technical characteristics of several micro-resistivity scanning imaging logging tools
13.6. 1.2 array induction imaging logging
Array induction imaging logging adopts an array induction logging tool composed of multiple receiving coils. By signal processing the measurement results of different detection depths, array induction curves with different longitudinal resolutions and different radial detection depths can be generated, and two-dimensional images of formation resistivity or oil saturation can be further generated by using these curves.
At present, the mature array induction imaging logging tool (AIT) consists of a transmitter coil, eight sets of receiver coil pairs and corresponding electronic circuits, as shown in figure 13-24. The transmitting coil works at the frequencies of 20kHz and 40kHz, eight groups of coils use the same frequency, and six groups of coils also use another higher frequency. In this way, eight groups of coils actually have a coil spacing of 14 detection depths, and each group of coils measures an in-phase signal R and a 90-degree phase shift signal X, and * * * measures 28 original signals. After borehole correction and "software focusing" processing, these original signals can obtain three longitudinal resolutions of 1ft(30.5cm), 2ft(6 1cm) and 4ft( 122cm), each with 10in(25.4cm). ..
Figure 13-24 Array Induction Imaging Logging Tool
Among the abundant logging information provided by array induction logging, the high-resolution logging curve is far superior to the conventional method in thin layer interpretation, which can distinguish the thin layer with a thickness of 0.3m The logging curves of five detection depths can be inversed by four parameter models, and reliable formation true resistivity Rt, transition zone (flushed zone) resistivity Rxo, transition zone inner diameter (flushed zone radius) r 1 and outer diameter r2 can be obtained. In addition, imaging array induction logging curves can obtain two-dimensional (axial Z and radial R) visual images of formation resistivity, apparent formation water resistivity and oil-gas saturation.
13.6. 1.3 azimuth lateral imaging logging
Azimuth resistivity imaging logging is a new lateral logging method developed on the basis of conventional bilateral logging. It adds an azimuth electrode array consisting of 12 electrodes with an angle of 30 to each other in the middle of the A2 shielding electrodes on both sides, and measures the directional resistivity values of 12 directions around the well.
12 electrodes cover the stratum around the well within 360 azimuth, and the calculated resistivity of each electrode is equivalent to the resistivity of the medium on the path through which the power supply current passes within the control range of 30 azimuth. Therefore, it is a real three-dimensional logging method.
Adding the supply currents of 12 azimuth electrodes can also provide high-resolution lateral logging (LLHR). At this time, the electrode of 12 azimuth lateral logging can be equivalent to a cylindrical electrode with a certain height, and the measured resistivity is equivalent to the average resistivity of the medium around the well. The longitudinal resolution of LLHR is 8in(20.3cm), which is obviously higher than that of shallow lateral logging.
The azimuth lateral imaging logging also retains deep and shallow lateral measurements, and can give three lateral logging curves of LLD, LLS and LLHR at the same time. In addition, by imaging 12 azimuthal resistivity curve, ARI images with conductivity as scale can be obtained, which is of great significance for analyzing the heterogeneity and fractures of the formation around the borehole.
13.6.2 acoustic imaging logging
13.6.2. 1 borehole acoustic imaging logging
Acoustic imaging logging (CBIL) or ultrasonic imaging logging (UBI) around a well uses a sensor to transmit and receive. The transducer emits 2MHz ultrasonic pulses perpendicular to the borehole wall at a certain emission frequency (2000 ~ 4200/s) and rotates at a certain speed to scan around the borehole.
During the pulse transmission interval, the reflected wave reflected by the borehole wall is recorded. The energy of the reflected wave depends on the acoustic impedance difference between the fluid in the well and the borehole wall medium (rock). Because the acoustic impedance of fluid in the same well can be regarded as a constant, the recorded reflected wave energy can reflect the change of acoustic impedance of borehole media. Obviously, the medium with high acoustic impedance has high interfacial reflection coefficient and strong reflected wave energy, while the reflected wave energy is weak.
The amplitude of the recorded reflected wave is displayed in the 360 orientation of the borehole. Through the high-resolution imaging of the whole borehole wall, the unfolded map reflecting the physical condition of borehole wall medium can be obtained. This is beneficial to detect cracks, analyze the occurrence of cracks and understand the heterogeneity of rocks.
It should be pointed out that in the logging process, the probe will rotate with the lifting of the instrument, so that the scanning trajectory of the acoustic pulse signal is threaded. In order to determine the orientation of the borehole wall diagram, the scanning diagram obtained in this way can be truncated at the magnetic north pole and expanded into an acoustic image of the borehole wall. In addition, the resolution of acoustic image is affected by well diameter, mud in the well and surface structure of target layer, while the vertical resolution of image is restricted by scanning speed and logging speed. Using focused transducers, low-frequency or large-size transducers and increasing vertical and horizontal sampling rates can reduce these effects to some extent.
13.6.2.2 dipole shear wave imaging logging
The transducers used in conventional acoustic logging are all radially expanded and vibrate uniformly, which is called monopole sound source. When using this sound source, when the velocity of shear wave in the formation is lower than the sound velocity of fluid in the well (such as soft formation or mudstone formation with low velocity), because there is no sliding shear wave on the borehole wall, it is impossible to record shear wave. In order to overcome this defect of acoustic logging, dipole shear wave imaging logging technology came into being.
The sound source of dipole shear wave logging consists of two point sound sources with similar distance, the same intensity but opposite phases. The receiver part consists of eight receiving stations, the distance between each receiving station is 6 inches (15.2 cm), and each receiving station consists of four receivers with an angle of 90, as shown in figure 13-25. When a dipole sound source vibrates in a well, one side of the borehole wall is compressed and the other side is decompressed, thus causing a slight deflection of the borehole wall. In this way, on the one hand, P wave and S wave are excited in the formation, on the other hand, this bending wave propagates in the borehole fluid along the borehole axis, which makes the borehole fluid form pressure deflection. Dipole receiver calculates formation shear wave by measuring bending wave.
At present, dipole shear wave imaging logging is a flexible combination of monopole and dipole transmitters and eight monopole and dipole receivers to measure, and finally output formation P-wave, shear wave and stoneley wave velocity or time difference, continuous Poisson's ratio curve and full wave train records. Using these P-wave and S-wave velocities or time differences with high longitudinal resolution, we can better determine formation porosity, calculate rock elastic mechanical parameters and estimate formation permeability. Using the attenuation change of acoustic energy and imaging processing, fractures can be identified, fracture orientation and formation anisotropy can be judged.
Figure 13-25 Overview of Dipole Shear Wave Imaging Logging Tool
13.6.3 nuclear imaging logging
Array neutron porosity-lithology imaging logging (APS) is a mature method in nuclear imaging logging technology. It uses a pulsed neutron generator to emit fast neutrons of 1.4 MeV, and an array detector consisting of five helium counting tubes records epithermal neutrons and thermal neutrons. All five detectors are shielded by boron-containing cemented carbide, among which three detectors record near-source thermal neutrons, one records far-source thermal neutrons, and the other records far-source thermal neutrons (as shown in figure 13-26). The longitudinal resolution of the instrument can reach 16.5cm (near source distance) and 23cm (far source distance) respectively.
In actual logging, the neutron porosity of formation can be calculated by counting ratio method, just like compensated neutron logging, by using short source distance and long source distance overheated neutron detectors. Using double short-distance epithermal neutron detectors can be used for high-resolution epithermal neutron logging, and the time distribution of epithermal neutron counting rate in neutron pulse interval can also be measured. Its decay constant is a measure of fast neutron deceleration time, which is related to the hydrogen index of the formation. Using long-distance thermal neutron detector, the time distribution of thermal neutron counting rate can be recorded, and the macroscopic capture cross section σ and thermal neutron lifetime τ related to lithology can be obtained.