T.A. Ma, E.L. Bigelow
Atlas Wireline Services Houston
Borehole imaging data can provide high quality geological and petrophysical information to improve fracture identification, dip computations, and lithology determinations in a well bore. The ability to visually quantify the area of a borehole wall occupied by fractures and vugs enhances reservoir characterization and well completion operations.
The circumferential borehole imaging log (CBIL) instrument is an acoustic logging device designed to produce a map of the entire borehole wall. In recent years, imaging devices have become more accepted because of the direct observation of structural diagnostics and the detailed information pertaining to internal sedimentary features. Visual observations of the well bore have been previously restricted to examinations of cores.
The visual images can confirm computed dips and the geological features related to dip. Borehole geometry, including breakout, are accurately described by complete circumferential caliper measurements, which is important information for drilling and completion engineers. In many reservoirs, the images can identify porosity type, bedding characteristics, and petrophysical parameters.
Image acquisition has also found successful applications finding fractures in horizontal and extended-reach well bores. The CBIL log is also effective in cased hole wells for pinpointing pits, scale, splits, and perforations. The tool can be used in well bores that contain any type of drilling fluid.
The images identified by the CBIL log can be used in well-to-well comparisons for predictions of lateral and vertical facies changes, identification of specific contacts, confirmation of the sedimentary environment, and reservoir characterization.
DATA ACQUISITION
The CBIL instrument is a new generation of the borehole televiewer (BHTV), a system developed by Mobil Research Corp. in the 1960s.1 The improvements in microelectronics, digital telemetry systems, and acoustic transducer technology have contributed to the development of the circumferential borehole imaging log.
As the tool is pulled up the borehole, a rotating transducer operating in pulse echo mode scans the entire circumference of the well. The transducer sends a sound pulse to the borehole wall and receives the reflected acoustic pulse from that portion of the formation. For each sound pulse, two parameters are measured and recorded: the amplitude of the reflected acoustic peak wave and the travel time of the reflected acoustic signal.
The transducer rotates six times per second, acquiring 250 amplitude and travel time samples per revolution. The CBIL data are recorded in the scanning mode for full coverage of the borehole wall. The logging speed is typically about 10 ft/min (3 m/min).
Modifications to transducer focusing and lower frequency measurements have increased logging speed and allowed running the tool in heavy drilling fluids. Additionally, high sampling rates increase the horizontal and vertical resolution of the images.2
Solid-state navigational equipment is used to establish tool and image orientation with respect to magnetic north and true vertical position. Accelerometer data are also used to correct depth disparities caused by excessive sticking and pulling during logging operations.
Well site plots are printed in black, white, and gray-tone images. An advanced postprocessing computer program can generate computer-enhanced color displays of the data.3 This program uses a multivariable file system capable of handling seismic, core, or other log data. Each variable can have its own starting and ending depths, level spacing, and units. The system is capable of accessing up to 10 files simultaneously for multiple well analyses.
The reflectance amplitude images show the contrast in acoustic impedance around the borehole. These variations in the acoustic properties of the formation affect the amplitude of the reflected acoustic wave. High reflectance generally corresponds to high impedance, which is typically found in dense, nonporous rock. White represents a highly reflective rock surface, whereas low reflectance is shown in black, and reflectance values between the two extremes have a corresponding gray tone (Fig. 1).
The amplitude images are well-suited for identifying various sedimentary features, such as planar or convoluted bedding. Low reflectance features are of particular interest because they help identify open or healed fractures, vugs, partially mineralized fractures, and microfractures. The images represent a cylinder that has been cut down the north axis and unrolled to form a two-dimensional flat plot with magnetic north at the left and right edges (Fig. 2).
BOREHOLE GEOMETRY
The travel time display (also referred to as flight time) is a sensitive resolution of the borehole shape and size. The image shading is a function of the distance from the transducer to the borehole wall; black or dark gray represents longer travel times and borehole enlargement (Fig. 3). Very fine marks or grooves caused by the bit, stabilizer, or other drilling tools can be identified. Travel time images are combined with a measurement of the mud velocity and used to produce plots of the borehole radius, shape, and elliptical extent (Fig. 4).
A gamma ray tool and six-arm dipmeter tool can be run simultaneously with the CBIL, and the recorded resistivity pad traces can be used to construct a resistivity image for correlation.
Dipping geological features appear as sinusoids on the unrolled images (Fig. 2). The amplitude traces can be extracted from different circumferential segments of the image to create synthetic traces for dip correlation and computation. An interactive dip processing program makes optical correlations and automatic computation of dip. Formation dips can then be compared to those computed from conventional dipmeter data.
Advanced postprocessing software permits the generation of a rolled up version of the reflectance amplitude or travel time image to produce a synthetic core to display the external features that would be seen on a full core from similar depth intervals. If a full core is available from the interval, the log data can be used to orient the recovered core and to make any necessary depth corrections. The location and nature of features observed on the core and from the CBIL images are corroborated, and where applicable, the dip, dip direction, or orientation can also be calculated from the synthetic core.
THIN BEDS
In thin, laminated sand/shale environments, an accurate sand count is often critical. Acoustic amplitude images are often superior to most resistivity measurements because of the very fine vertical resolution.4
The CBIL measurements are made at the interface between the mud column and the borehole wall; hence, there is virtually no depth of investigation. The differences in acoustic impedance properties of the rock are therefore readily identifiable (Fig. 5).
FRACTURE DETECTION
Horizontal wells can be logged with the CBIL and gamma ray spectral instruments using conventional conveyance systems. A combination of centralizers and knuckle joints were used to ensure tool centralization for a 1,500-ft horizontal section logged with the tool (Fig. 6).
The images were produced immediately following the logging operation and clearly showed fractured intervals. The operator was then able to select the exact positions for placing external packers and sliding sleeves before the pipe was run. The fractures are near vertical (greater than 70 in dip angle) and are producing oil.
The spectroscopic data showed an increase in uranium count rates (a phenomenon associated with fluid movement in fractures) in the same intervals where fractures appeared on the CBIL images.
The orientation data of the reservoir's fracture trend enabled more accurate planning for future horizontal wells in the field.
POROSITY
Carbonate rocks exhibit numerous types and amounts of pore space, including vugular porosity. The size and number of vugs in a reservoir rock directly affect petrophysical parameters used in conventional log analysis, particularly the m exponent (Fig. 7).
The relationship of resistivity to porosity, referred to as formation resistivity factor (F), is fundamental to log analysis. Log analysts commonly use a relationship of F = 1/0m, where m = 2 in carbonate log analysis, although a relatively large number of small vugs or a moderate number of large vugs affects the exponent. Vuggy porosity tends to affect m to a greater degree at higher porosity values (10%), where m values as high as 2.8 are not uncommon. Formation resistivity factor increases as m increases, and saturation calculations are subsequently increased.
Fractures also affect the formation factor relationship. The affect is more severe at low values of matrix porosity (
Therefore, recognition of pore type is important to accurate log analysis in carbonate reservoirs. The complete wall coverage provides a view similar to the external periphery of a full core.
ACKNOWLEDGMENT
The authors wish to thank Atlas Wireline Services and Western Atlas International Inc. for permission to publish this article.
REFERENCES
1. Zemanek, J., Caldwell, R.L., Glen, E.E. Jr., Holcomb, S.V., Norton, L.J., and Straus, A.J.D., "The Borehole Televiewer-A New Logging Concept for Fracture Location and Other Types of Borehole Inspection," Journal of Petroleum Technology, June 1969, pp. 762-774.
2. Zemanek, J., Strozeski, B., and Wang, Z., "The Operational Characteristics of a 250 kHz Focused Borehole Imaging Device," transactions from the SPWLA 31st Annual Logging Symposium, Lafayette, La., 1980.
3. Faraguna, K., Chase, D. M., and Schmidt, M. G., "An Improved Borehole Televiewer System: Image Acquisition, Analysis, and Integration," transactions from the SPWLA 30th Annual Logging Symposium, Denver, 1989.
4. Hackbarth, C.J., and Tepper, B.J., "Examination of BHTV, FMS, and SHDT Images in Very Thinly Bedded Sands and Shales," paper presented at the SPE 63rd Annual Technical Conference and Exhibition, Houston, Oct. 25, 1988.
5. Verdur, H., Stinco, L., and Naides, C, "Sedimentological Analysis Utilizing the Circumferential Borehole Acoustic Image," transactions from the SPWLA 32nd Annual Logging Symposium, Midland, Tex., 1991.
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