Formation micro-resistivity imaging logging is developed on the basis of high-resolution formation inclinometer, which was first represented by formation micro-scanner (FMS) introduced by Schlumberger Company in 1980s. FMS can provide images reflecting the formation resistivity around the borehole wall, which has gained great advantages in formation evaluation and geological application and promoted the rapid development of this technology. Schlumberger Company made three major improvements to FMS in less than three years, and introduced FMI (Full Borehole Microscope Scanner). Atlas and Halliburton followed closely, and introduced STAR Imager and EMI (ElectricalMicro Imaging). The following mainly introduces the whole borehole micro-resistivity scanning imaging logging tool FMI of Schlumberger Company.
6.2. 1. 1 FMI instrument structure and measuring principle
The FMI instrument mainly consists of five parts: telemetry, control, insulation nipple, acquisition nipple and inclinometer, polar plate and probe, as shown in Figure 6. 2. 1 (a)。
1) telemetry part. It is used to transmit data. The formation information and various auxiliary and control measurements collected by button electrode scanning are sent to the ground through logging cable, and the data transmission rate is 200 KB/s. ..
Figure 6. 2. 1 FMI structure and measurement schematic diagram
2) Control part. The automatic control loop in the control joint can amplify the signal describing the rock characteristics, expand the dynamic range of the instrument, regularly check the working state of each branch and feed it back to the logging engineer, so as to realize the optimal control of the downhole instrument, enhance the flexibility of the use of the instrument, provide convenience for the operation of the instrument, and enable the three logging methods to collect the required data in the shortest time.
3) Insulation joint. It can insulate the probe from the electronic circuit shell, and make the current flow from the polar plate into the stratum and return to the electronic circuit shell. There is a certain potential difference between them. One advantage of this arrangement is that FMI can be used as the low-end loop electrode of ARI during combined logging.
4) Collect joints and inclinometers. The acquisition connector has the following functions: filtering DC components, such as spontaneous potential, from micro-conductivity data; Digitize the signal to improve the anti-interference of the signal; Filter the digital signal to improve the signal-to-noise ratio; The digital signal is processed to determine the in-phase amplitude of formation micro-conductivity data.
Inclinometer can measure the inclination of instruments and drilling holes, and can also measure the inclination of drilling holes. The measurement accuracy of azimuth is 2, and the measurement accuracy of inclination is 0. 2. It can also measure the acceleration of the instrument, which is used to correct the speed in the process of image processing and inclination calculation.
5) polar plate and probe. The polar plate part consists of button electrode array and high-precision electronic circuit. Electronic circuit is used to sample, detect and amplify the button electrode signal, which ensures the resolution and clarity of the image. The design of polar plate can make the instrument have reliable response in highly deviated wells or horizontal wells.
The instrument has four push arms perpendicular to each other, and each push arm is equipped with two polar plates, the upper part is the main polar plate and the lower part is the hinge polar plate, as shown in Figure 6. 2. 1 (b)。 After the folding polar plate is opened, it can automatically adapt to the shape of the borehole and make it close to the borehole wall, which can ensure that the polar plate can still be in close contact with the borehole wall when the instrument body is not parallel to the borehole axis. Two rows of button electrodes are installed in the center of each polar plate, with 12 electrodes in each row, and 192 electrodes are installed on eight polar plates. The diameter of the button electrode is 0. 16 inch (4. 1mm), and the diameter of the outer edge of the surrounding insulating ring is 0. 24 inches (6 inches. 1mm)。 The distance between the two rows of electrodes is 0.3 inch (7.62 mm), and the upper and lower rows of electrodes are staggered. The lateral distance between the upper and lower electrodes is 0.08in(2.05mm) of the electrode radius, that is, half of the electrodes overlap between the two electrodes [Figure 6.2. 1 (b)], so that all the shaft walls are within the control range of the electrode array during the measurement. The resolution of this instrument is 0.2 inch (5. 1 mm).
The measuring principle of FMI is shown in Figure 6.2. 1(a). The current loop is the upper electrode-stratum-lower electrode. The upper electrode is the shell of the instrument electronic circuit, and the lower electrode is the polar plate. During the measurement, all eight polar plates are close to the borehole wall. The imaging logging ground system controls the emission of current to the formation, records the current and applied voltage of each electrode, and reflects the change of formation micro-resistivity around the borehole wall. FMI can log in in in three modes.
1) Full hole mode. Use 192 button electrode for measurement. In 6 1/4in borehole, the coverage rate of borehole wall is 93%; In 8 1/2in borehole, the coverage rate of borehole wall is 80%; In the 12 1/4ing borehole, the coverage rate of the borehole wall is 50%.
2) Quadrupole mode. Only four main plates are used. This mode is similar to FMS logging, and is suitable for areas with familiar strata, which can save costs and improve logging speed.
3) Stratigraphic dip mode. With only eight measuring electrodes on four polar plates, we can get the same results as the high-resolution inclinometer.
6.2. 1.2 data processing
The mapping from FMI measurement information to borehole micro-resistivity image requires the following processing steps.
(1) pretreatment
1) automatic gain and current correction. The dynamic range of measured formation resistivity changes greatly, so it is necessary to realize the dynamic range change of measuring electrode current by automatic gain control and changing power supply current.
2) Detection and compensation of faulty electrodes. By analyzing the current distribution histogram of each electrode current in the selected processing window, the electrode information that the electrode current does not change with the stratum is removed, and the measured value of the invalid electrode is filled by interpolating the measured value of the corresponding measuring point of the effective adjacent electrode.
3) Speed correction and electrode orientation. The first step is to map the current time domain measurement information of the array electrode to the depth domain measurement information by using the measurement information of the three-component accelerometer, that is, to determine the depth of each measurement point. This correction method is completely equivalent to the velocity correction of dip logging. The second step is to determine the azimuth angle of each electrode with respect to the magnetic north pole by using the three-component magnetic flux measurement information and acceleration measurement information.
In addition, the information (or curve) measured by each electrode must be "depth aligned". Because the distance between the two rows of electrodes on the polar plate is 0.3 inches, the anomalies displayed by the two rows of electrodes have depth deviation and no depth alignment. The electrode on the wing plate (i.e. hinge plate) is 5.7 inches away from the electrode on the main board, and the abnormality displayed has a large depth deviation. When processing pixels, the measurement results of each electrode must be depth aligned first. Figure 6.2.2 shows the abnormal display of electrodes before and after depth alignment.
The above processing is also called imaging logging preprocessing, and the goal is to obtain the image information set with correct electrode spatial position. Reconstruct into borehole wall image.
Fig. 6.2.2 FMI resistivity curve before and after depth alignment
(2) converting into an intensity image
In order to convert the current of each button electrode into an image with variable intensity, the output image is displayed in 16 gray level, and the image can be displayed in 256 color levels on the interpretation workstation. Each "pixel" point in the image corresponds to a range of current levels. Generally, two schemes can be used to select gray scale and color scale, namely, so-called "static" normalization and "dynamic" normalization. Also known as equalization processing.
1) "Static" normalization. In a large depth profile (corresponding to a certain interval or a certain reservoir interval), the response of the instrument is normalized, that is, the resistivity expressed in a specific color at one depth, and if the color is the same at another depth, it means that the resistivity at that depth is the same. The advantage of this standardization is that the resistivity is compared by the gray scale and color in a long time interval. Its disadvantage is that it cannot distinguish the micro-resistivity change in a small range. Fig. 6.2.3(a) is a "static" normalized imaging image.
Fig. 6.2.3 FMI image
2) "Dynamic" normalization. That is to say, in a short time interval, the depth of gray and the depth of color are selected to represent the current level, so the change of local micro-resistivity can be reflected, and the changes of rock structure and cracks in the shaft wall can be studied in detail. Usually, the advantage of this method is that the longitudinal window length is 3ft, which can show the relative change of local micro-resistivity. Figure 6.2.3(b) is a "dynamic" standardized image of the same interval. Compared with Figure 6.2.3(a), it can divide the variation of borehole wall strata in more detail, especially at the top of the profile, and clearly show the variation of stratum bedding. , and in the figure,
There is no such indication in 6.2.3 (a).
3) Graphic display. When a plane is tangent to the wellbore cylinder vertically, the borehole wall is a straight line on the development diagram of 0 ~ 360. When a plane intersects the wellbore cylinder obliquely, the sidewall and the inclined plane cut an ellipse, which is sinusoidal on the development diagram of 0 ~ 360. The greater the angle between the plane and the shaft axis, the greater the amplitude of the sine curve, and the inclination and direction of the plane can be determined from the development diagram (Attached Figure 6.2.4). According to the imaging display, the bedding of the formation or the occurrence of cracks can be determined, so that the geological characteristics of the formation around the well can be studied by using borehole wall imaging.
Interpretation and application of data
If the resistivity between adjacent strata rocks is different, it will be reflected in FMI image; The greater the difference in resistivity, the more obvious the difference reflected in the image. In FMI image, high resistivity lithology corresponds to light color image, such as oil-bearing layer and dense layer. Lithology with low resistivity corresponds to dark images, such as mudstone and cracks filled with drilling fluid (water-based drilling fluid).
Interpretation of FMI images requires rich geological knowledge, because different geological phenomena may have the same or similar images on FMI images, for example, dissolved holes and clay particles with high conductivity or mineral nodules with high conductivity are all displayed as black spots on FMI images. Only by using geological laws and knowledge to calibrate FMI images and distinguish different geological phenomena can we get correct interpretation results.
FMI images can be used to identify fractures and dissolved pores in rocks, and also to explain pore characteristics, sedimentary facies, stratigraphic structure and lithologic correlation.
Figure 6.2.4 Characteristics of borehole wall imaging display
The main geological applications of FMI images include the following aspects: ① fracture identification and evaluation; ② geological structure interpretation; ③ Interpretation of stratigraphic sedimentary facies and sedimentary environment; ④ Reservoir evaluation; ⑤ Determination of in-situ stress direction; ⑥ Core depth homing and orientation; ⑦ High resolution TLC analysis and evaluation.
Usually, a representative parameter well is selected for coring in an area, and the whole borehole micro-resistivity scanning imaging logging is carried out. Through the detailed comparison with the core column, the display of relevant geological features in the borehole wall image can be studied, which can make full use of these features to solve geological problems. Here are some examples to illustrate its application.
In Figure 6.2.5, Figure (a) clearly shows the bedding and fractures of the formation, and Figure (b) clearly shows the low-angle fractures and high-angle fractures. Figure 6.2.6 shows holes, argillaceous bands, gravels and conglomerates.
Fig. 6.2.5 Formation bedding and fractures displayed by FMI images
Fig. 6.2.6 FMI image shows holes, muddy bands, glutenite and conglomerate (a); (b) argillaceous bands; Gravel; Giant joint venture
Formation micro-resistivity imaging logging has a broad application prospect in identifying thin layers, pore changes, fractures and sedimentary characteristics because of its high resolution. Therefore, it is necessary to select several representative parameter wells or key wells to scan and image the formation micro-resistivity in a certain area, and compare it with the core to find out the changing law of geological characteristics in this area, which can greatly reduce the number of coring wells and provide important and rich geological information for oilfield exploration and development.
6.2.2 borehole acoustic imaging logging
BHTV (Borehole TV) developed by Mobile Company in the late 1960s is the first downhole imaging equipment that can be used in typical oil wells. Downhole TV is equivalent to ultrasonic scanning of borehole wall, which can record borehole wall images continuously. Early imaging logging images show some interesting phenomena on the borehole wall, such as cracks, collapse, main lithologic interfaces, casing perforation and connection. Amoco, Shell and Arco successively improved this technology. Today, all oil companies provide ultrasonic borehole imaging measurement. Although some refraction experiments have been carried out, all borehole ultrasonic imaging measurements are carried out in reflection mode. These newer instruments still use most parts of the original downhole TV, but the word "TV" has been replaced by "ultrasonic imaging" or "scanning". At present, the representative ultrasonic imaging logging instruments are Schlumberger's ultrasonic imager USI and ultrasonic borehole imager UBI. CBIL of Atlas Company, CAST of Halliburton Company, downhole TV set of Huabei Oilfield in China, etc. These instruments can be used for open hole and cased hole logging filled with clear water, crude oil, conductive and non-conductive mud, but not for empty wells.
6.2.2. 1 measuring principle
The core component of the instrument is an ultrasonic transducer made of sheet piezoelectric ceramic material, which is used as both a transmitter and a receiver. It is driven by a motor and can rotate 360 degrees underground [Figure 6. 2.7 (a) and (b)]. Generally, the transducer is excited by an electric pulse of 1500Hz and emits ultrasonic waves. The acoustic wave propagates along the drilling fluid, is reflected at the borehole wall and returns to the transducer. The transducer converts the received acoustic signals into electrical signals and sends them to the ground system through electronic circuits. The working frequency of transducers in early instruments was about 1. 3MHz has been reduced to several hundred kHz in the instruments currently used. There is a triaxial accelerometer and magnetometer in the downhole tool, which is used to obtain the orientation of the instrument and can be used as a reference mark (instrument zero point) to obtain the orientation of the pulse emitted by the transmitter.
Geophysical logging course
Figure 6. 2.7 principle of borehole acoustic imaging logging | (1) schematic diagram of driving motor, transducer and magnetometer; (b) Schematic diagram of acoustic pulse scanning line of transducer on borehole wall; (c) The measured pulse echo signal instrument can measure two parameters: ① the amplitude of the echo signal received by the transducer; ② The travel time of sound wave from transducer to borehole wall and back to transducer is also called travel time or two-way travel time [Figure 6.2.7 (c)]. The change of rock acoustic impedance will cause the change of echo signal amplitude, and the change of well diameter will cause the change of propagation time. The measured reflected wave amplitude and propagation time are displayed as an image according to the azimuth angle of 360 degrees in the borehole, which can be a gray map or a color map. From some characteristic differences in the image, we can see the changes of underground lithology and geometric interface, such as erosion zone, cracks, holes and so on.
The main factors affecting the resolution of ultrasonic imaging logging tool mainly include the following aspects: ① the working frequency of transducer; ② Drilling fluid in the well; ③ Measure the distance; ④ the surface structure of the target layer; ⑤ Dip angle of target layer; ⑥ Wave impedance difference of rocks.
6.2.2.2 data processing
After receiving the acoustic signal, the ultrasonic transducer converts it into an electrical signal, which is an analog signal. In the early downhole TV imaging logging, the analog signals of downhole instruments cannot be corrected and processed after being transmitted to the ground. Digital imaging technology can use a variety of methods to process various signals, optimize image parameters, and get high-quality images. The processing of borehole acoustic imaging logging data includes image processing and image output.
(1) image processing
The main work of image processing includes: ① signal mediation, necessary correction and calibration of logging raw data to eliminate interference and improve data quality; (2) Image enhancement, that is, processing logging images to improve image clarity and visual effect; (3) Image analysis, geological interpretation of logging images and fracture statistics.
(2) Image output
Image output formats include: ① Plane development map of borehole wall, which is also the most commonly used map. There are two kinds of graphs, namely amplitude graph and propagation time graph. Usually, these two maps are displayed side by side for comparison and interpretation (Figure 6. 2.8); ② perspective view of borehole (fig. 6). 2.9); ③ sectional view; ④ Fracture trajectory diagram, including amplitude image, fracture trajectory and fracture parameters; ⑤ Fracture parameter curve, including amplitude image and four parameter curves of fracture density, fracture length, fracture width and fracture surface ratio; ⑥ Echo amplitude waveform diagram, there are two waveforms, one is vertical and the other is horizontal; ⑦ The waveform diagram of acoustic caliper can also be expressed vertically and horizontally; ⑧ Schmidt diagram of the crack, which is represented by an icon according to its appearance on a hemisphere, inclines outward from the center of the sphere and clockwise. In addition, there are fracture data tables and fracture grouping data tables.
Figure 6. 2.8 Borehole development amplitude diagram and propagation time diagram
Figure 6. 2.9 Borehole development amplitude diagram and perspective diagram
Image output colors are generally black and white and color (Table 6. 2. 1). Black and white images are actually gray-scale modulation. Generally speaking, black represents weak echo amplitude or long propagation time, and white represents strong echo amplitude or short propagation time. Color images are actually pseudo-colors. The intensity value of the modulated signal is divided into 256 (0, …, 255) levels, and different intensity values correspond to different colors. There are many different schemes, such as black-red-yellow-white scheme and red-white-green scheme.
Table 6. 2. 1 image color classification scheme
Interpretation and Application of 6.2.2.3 Data
On the propagation amplitude image of the borehole wall plane: ① Any structure intersecting the borehole, whether oblique or vertical, has mirror symmetry, but the scratches on the borehole surface caused by drilling tools, logging cables and fishing tools generally cannot produce this mirror symmetry (Figure 6. 2.8).(2) Natural cracks, holes, casing cracks, perforation holes, etc. Show black characteristic lines or areas in cased wells; The hard and smooth shaft wall without structure shows white areas because the reflected signal is very strong [Figure 6.2.9]. (3) The plane cracks (or layers) oblique to the borehole are black sine curves (Figure 6. 2.8); The plane horizontal fracture intersecting the borehole can be regarded as a special case of inclined fracture, which is shown as a horizontal line segment on the logging map. ④ The vertical structure intersecting the borehole is a vertical straight line; Any deviation from the vertical structure, such as the depression near the middle of the vertical crack in the figure, shows a curve. ⑤ The holes in the borehole wall are isolated and irregularly shaped points (Figure 6. 2.8).
On the propagation time diagram of the borehole wall plane: ① The open fracture intersecting with the borehole has a characteristic line similar to the amplitude diagram. (2) borehole collapse, borehole out of round, casing corrosion and damage, etc.
At present, borehole acoustic imaging logging plays a huge role in the oil field and can be used to solve the following related problems:
1) 360 spatial high-resolution borehole diameter measurement, and the borehole geometry is analyzed (Figure 6. 2.8. Figure 6. 2. 10), and calculate the geostress direction;
Figure 6. 2. 10 borehole perspective 1in≈2. 54 cm
2) determine the stratum thickness and dip angle;
3) Detect cracks, identify cracks and divide fracture zones (Figure 6. 2.8);
4) Analyze the formation morphology and structure;
5) Return to the borehole wall for coring (Figure 6. 2. 1 1);
6) Measure the change of casing inner diameter and thickness, and check the perforation quality and casing damage;
7) Cement cementation evaluation.
Figure 6. 2. 1 1 Core homing and BHTV images