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4 Fracture Detection Methods
Pages 167-242

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From page 167...
... Fracture detection methods rely on the fact that fractures are thin compared to their lengths and heights; that is, they are essentially two-dimensional anoma 167
From page 168...
... 168 Cal JO o ._ 4 C)
From page 169...
... 169 ~ ~ ~Cal ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~so so~ so , - ~ ~ ~ ~ , - ~ ~ ~ ~ ~ ~ ~ C)
From page 170...
... 170 .`, A Cal so o .~ C)
From page 172...
... Typically, the reduced data from each detection method (e.g., seismic travel times) must be inverted to yield estimates of local rock properties (e.g., seismic velocities)
From page 173...
... Obliquely traveling P waves are affected by vertical fractures, both in velocity and attenuation, if their rays do not lie in the plane of the cracks (see Chapter 3~. Hence, when data from a set of such oblique paths are processed into a conventional seismic reflection section, the zones of intense fracturing may appear as velocity or amplitude anomalies, most commonly as "dim spots." Kuich (1989)
From page 174...
... An example is shown in Figure 4.1, where reflections obtained from a subhorizontal fracture zone or fault are compared to acoustic televiewer (BHTV; see "Borehole Imaging WN 380 t 390 400 410 180 170 TRANSIT-TIME IN URETER WN - 4 ACOUSTIC LOG BHTV SEISMIC SECTION \ N WN BOREHOLES 1 o 20 ~30 0.61 0.1 ACOUSTIC LOGBH1V "~A , 500 O 050 | ZONE A OF 38t 20 __~ 2 400 410 420 MA ZONE B ~ 740: TRANSIT.TIMC IN URETER ..~ _ 7~ _ FIGURE 4.1 Comparison of a conventional P-wave reflection seismogram with acoustic televiewer image logs. The image logs show the distribution of fractures where two boreholes penetrate the reflector at locations close to the center of the seismic line.
From page 175...
... The scale differences between the remote seismic sounding and the local borehole measurements are apparent in the figure. The P-wave reflections indicate a single, horizontally continuous fracture zone, whereas the borehole images indicate a great deal of variability in the local distribution of fractures in the fracture zone.
From page 176...
... Shear waves polarized obliquely to the fractures resolve themselves vectorially into these two particular directions, determined by the rock mass. These two modes travel at different speeds; the faster mode is polarized in the plane of the fractures.
From page 177...
... Meadows and Winterstein (1994) report the detection and characterization of an artificial hydraulic fracture via analysis of the reflection of split shear waves back to the surface.
From page 178...
... , provides a good cross-section of recent advances. Electrical and Electromagnetic Methods Detection of water-filled fractures by electrical and electromagnetic methods is possible because water-filled fractures generally have higher electrical conductivities than intact rock.
From page 179...
... Electrical soundings are made to investigate the earth beneath the sounding point, most commonly in horizontally stratified environments where the method is used to find the depth, thickness, and electrical properties of different strata. In this context, "Schlumberger sounding" is a standard technique in the search for horizontal aquifers in such environments (Ward, 1990~.
From page 180...
... Electromagnetic Methods Electromagnetic methods utilize electric and magnetic fields in the earth that satisfy the diffusion equation, which ignores the "displacement currents" that are necessary for true wave propagation. This implies that anomalies in the data, attributed to anomalous conductors in the ground, in most cases are caused by induced currents.
From page 181...
... ,_ dHz/dt, (dHX/dt) Sounding profiling FIGURE 4.4 Common electromagnetic methods in geophysical sounding.
From page 182...
... Thus, these methods can be used to detect semihorizontal fracture zones. Resolution deteriorates with depth owing to the diffusive nature of the electromagnetic field.
From page 183...
... In ~ plane Two-wa travel time surface Position along profile FIGURE 4.5 Schematic illustration of ground-penetrating radar used in profiling mode. T is the transmitter; R is the receiver.
From page 184...
... FIGURE 4.6 Semihorizontal fracture zones observed by ground-penetrating radar along a profile measured on granitic outcrops at the Underground Research Laboratory, Manitoba, Canada, showing distribution of fractures along borehole WB2. Reflectors S-4 and S-5, seen at depths of 40 to 50 m and 65 m, respectively, are verified by increased fracture frequency observed in the slanted borehole NB2.
From page 185...
... One specific application of tiltmeter measurements to hydraulic fracture studies is the identification of fracture orientation changes around boreholes. These changes occur in response to changes in the direction of principal stress in rock around the borehole.
From page 186...
... The main advantages of performing measurements underground is that they provide subsurface confirmation of the surface measurements. They also allow surface measurements (e.g., seismic reflection data)
From page 187...
... For a 100-m-deep borehole, the volume of investigation in a typical VSP survey would range from 100 to about 1,000 m in diameter. Other types of P-wave VSP surveys have been used successfully to characterize fracture zones in crystalline rock.
From page 188...
... One particular form of VSP survey, the VSP hydrophore tube-wave analysis, has been found especially useful for detection of fractures intersecting boreholes. "Tube waves" are confined to the interior and neighborhood of the borehole, and travel somewhat slower than shear waves in rock.
From page 189...
... Consequently, two types of tomograms can be constructed: one based on travel times that resolves the distribution of velocity, and the other that resolves the distribution of attenuation. The latter type has only recently been applied, but a number of good examples exist.
From page 190...
... . Arrows and letters give the positions of fracture zones inferred from these and other data.
From page 191...
... This approximates the physical problem as a matrix equation, which must be solved numerically. If raypaths are straight, there is a linear relationship between travel time and slowness or inverse velocity (Eq.
From page 192...
... One difficulty with anisotropy correction is that the variation in travel times caused by fracture zones that are aligned with the fabric of the rock may be removed by the correction. The difference between heterogeneity and anisotropy is blurred in this case.
From page 193...
... An example from a detailed characterization of a fracture zone at the Grimsel test site in the granitic Alps of Switzerland, using sources and receivers on all four sides of the investigated area, is shown in Figure 4.9. Radar Tomography Borehole radar normally implies the application of a pulsed source.
From page 194...
... The low-velocity and highattenuation anomalies correspond to fracture zones, with widths ranging from 1 to 8 m, intersecting a granite intrusion. The agreement between seismic and radar results is remarkable considering that they represent different physical properties of the rock mass.
From page 195...
... This results in a vertical electric field component between the conductive layers, with the magnetic field component horizontally polarized in the seam. Data from multiple sources and receivers can be inverted by tomographic techniques to yield maps of electric properties in the resistive bed.
From page 196...
... This makes it possible to uniquely determine the orientation of a fracture zone from measurements in a single borehole (Sandberg et al., 1991~. Figure 4.11 shows the main components of the short-pulse directional borehole radar system developed in the Stripa Project.
From page 197...
... In: ~ ~ ::::. : ~ ::::: ::::::::::: lo: :: FIGURE 4.11 Main components of the short-pulse directional borehole radar system From Olsson (19921.
From page 198...
... . Hence, borehole radar will detect fracture zones, clusters of fractures, and tectonized zones rather than single fractures.
From page 199...
... These procedures are commonly referred to as seismic reflection tomography and are applied to surface measurements. An integrated scheme for wave equation and tomographic inversion of borehole data also has been suggested by Pratt and Goulty (l991~.
From page 200...
... SINGLE-HOLE METHODS Boreholes and tunnels provide the only direct means of access for measurements of fracture properties at depth. Most geophysical logging methods were designed to meet the need for measurements performed in situ, adjacent to a borehole, and in sample volumes small enough to significantly improve spatial resolution over that obtained from surface or surface-to-borehole methods.
From page 201...
... Another measure of fracture density is the number of fractures intersecting the core per unit length. However, core is commonly lost or the core barrel is filled with rubble in many intensely fractured intervals, strongly biasing such fracture frequency determinations.
From page 202...
... Even small core samples are likely to have been damaged during drilling, and core recovery may be poor in the intensely fractured and weathered parts of the rocks that provide most of the hydraulic conductivity. Hence, core studies may not be reliable for determining characteristics of dominant fractures or fracture zones.
From page 203...
... , multiple geophysical logs can be used to identify methods for uncoupling these dependencies. These examples illustrate the advantages of well logging as a general invests gallon technique giving fine-scale (sample volumes ranging from 10 cm to 10 m in diameter)
From page 204...
... If fracture zones are present or if concentrations of subparallel fractures are associated with faults and geological contacts, the thickness may be great enough that the average property of the zone will be indicated by the geophysical measurement. Even then, the properties of fractures will be averaged with those of the intervening rock and borehole fluids.
From page 205...
... Hostile borehole conditions also degrade the wall rock, making it more difficult to characterize fractures using geophysical logging methods. The general approach used in log analysis for geothermal applications is similar to that outlined previously for more general fracture studies.
From page 206...
... These cameras are used extensively in shallow water wells and other near-surface applications for a variety of borehole and casing inspection tasks. Fracture detection applications are sometimes limited because cameras require clear borehole fluids, and they commonly image the borehole at oblique angles using local illumination sources that produce shadows, which interfere with fracture detection and interpretation (Paillet et al., 1990~.
From page 207...
... l, · L · ~ 15 ~6 S W ~E S C' ~ I ' ' I ' ' I ' ' 1 FIGURE 4.13 A color visual band digital scanner borehole log of fractured granite using the CORE borehole scanner (shown here in black and white)
From page 208...
... The BHTV log indicates a single, large, near-vertical fracture, with faint indications of near-horizontal fractures close to the bottom of the larger fracture.
From page 209...
... The subhorizontal fractures were nearly closed under confining pressure in situ as indicated by the BHTV log, but opened during core recovery. Although fractures shown in BHTV
From page 210...
... BHTV has been a reliable and tested means of borehole imaging in fracture studies for nearly two decades. In recent years the conventional approach in BHTV logging of photographing and displaying an image of the intensity of ultrasonic reflections with an oscilloscope has been modified in a number of useful ways (Paillet et al., 1990; Maki et al., 19911.
From page 211...
... The latest applications of FMS use independent measurements of fluid conductivity to relate the measured electrical anomaly associated with the fracture to fracture aperture (Hornby et al., 19921. Probably one of the most important limitations of this system in fracture studies is the incomplete azimuthal coverage of the wellbore provided by the FMS image.
From page 212...
... . Until recently, acoustic and electric borehole imaging devices have been extensively used in fracture studies despite the much greater potential spatial resolution of optical imaging devices.
From page 213...
... axis from a conventional transducer. Early attempts to develop this method applied established seismic data processing techniques in a scaled-down version of conventional acoustic refraction and reflection methods (Christensen, 1964~.
From page 214...
... . 60 cm at: transmitter -Centralizing pads FIGURE 4.18 Schematic illustration of acoustic waveform logging as a scaled-down refraction study of fractures and adjacent rock.
From page 215...
... nearly horizontal fracture zone and (b) nearly vertical fracture.
From page 216...
... In this sense the two methods complement each other: the high-frequency amplitude methods can be used for fine-scale fractures, and the lower-frequency reflection method is appropriate for larger fracture zones. The discrepancy between theory and experiment has only recently been resolved (Paillet et al., 1989a)
From page 217...
... In the past two years, such "crossed-dipole" tools have been marketed worldwide by several service providers. High-Resolution Flowmeter Methods The availability of borehole imaging methods for fracture identification and other geophysical logs for fracture characterization provides effective methods for describing fractures that intersect exploratory boreholes.
From page 218...
... These methods seem especially useful in bridging the gap between small-scale measurements of conventional well logging and intermediate-scale measurement techniques such as straddle packer tests and borehole-to-borehole sounding. Fluid replacement logging is a recently developed technique for the identification of permeable fractures that intersect the borehole (Tsang et al., 1990~.
From page 219...
... Although fluid replacement logging is somewhat more time consuming and cumbersome than flowmeter logging, both techniques are effective in identifying the fracture zones associated with inflow to this borehole. FLUID FLOW MONITORING USING GEOPHYSICAL METHODS The best way to detect and characterize the actual flow of fluid in fractures is through differential measurements that is, measurements before, dunng, and after the application of a known perturbation of the fluid system.
From page 220...
... and correlated these with changes in the seismic measurements. The dependence of bulk electric properties on pore fluids allows monitoring of fluid flow by electrical or electromagnetic methods.
From page 221...
... There are a number of examples where repeated borehole radar tomography has been used to map the transport of a brine tracer in fractured granite. Figure 4.23 shows the difference tomogram obtained after continuous injection of saline tracer into a fracture zone.
From page 222...
... The following methods are particularly useful at various detection distances: for detection at great distances, seismic methods based on split shear waves; · for detection at moderate distances, the directional borehole methods; · for detection in the borehole, the televiewer, EMS, and scanner methods; and · for characterization of the actual flow associated with fracture systems, the high-resolution flowmeter methods and radar difference tomography. The objectives of most studies are to both detect and characterize fractures.
From page 223...
... Given the breadth of methodologies evident in Table 4.1, it might seem unnecessary to look for new methods of acquisition, processing, and inversion. However, major new methods have recently been developed (e.g., split shear waves that do not rely on oversimplified theories.
From page 224...
... Add PI ane Points _ Hide O Checksum O Dipole Azimuth ~ Magnetic O _ Gravity ~ Licensee 1991: Conterra AB Uppsala, Sweden l ~iMuth 60 Para~eters |Sa~pI . Frec fluency ~ Direct Pulse at |I Depth Increment |I Depth of Trace 1~ Nominal Ueloci to |121 ~R Separat i on | ~ 90 Sta us In 0 FIGURE 4.A1 Screen display of a commercially available software product for interactive interpretation of directional radar data.
From page 225...
... HI Zone ~ Zone B __ ~ | __ ~ I'm Zone M/' _ ~ FIGURE 4.A2 Distribution in space of radar reflections derived from single-borehole directional radar data at the Site Characterization and Verification (SCV) site at the Stripa Project's mine.
From page 226...
... ~ · ~ ~ _ ~ _ 2~ ~ 160 2.5 3~0 1 ·0 1.010.0 12 13 14 -02 0.0 02 H 1 _ . A ~ ~_ ~r C ~DOWNF: Or ~ UPFLOW l FIGURE 4.B 1 Comparison of fracture permeability interpreted from borehole televiewer image logs with results of high-resolution flowmeter measurements of flow under ambient hydraulic head conditions and permeability measured by packer isolation-hydraulic injection tests for two boreholes in fractured dolomite at a site near Rockford, Illinois.
From page 227...
... These spikes represent the effects of mechanical enlargement of fracture mouths during drilling. Experience demonstrates that caliper logs indicate the depths where almost all significant fractures and fracture zones intersect boreholes.
From page 228...
... `, 20 30 _ 40 _ 50 ~ ~ Natural . J ~ ( Gamma Neutron Singl+Point Resistance ~C ~ Increasing ~ -<, Resistance 3 B Caliper ,_ D: ~ jE MA jA f A FIGURE 4.B2 Example of conventional caliper, resistance, neutron, and gamma logs for a borehole in granitic crystalline rocks in New Hampshire.
From page 229...
... Note the extent to which the geophysical logs respond to the most prominent fractures on the televiewer image and fail to indicate many of the smaller and possibly closed fractures.
From page 230...
... (a) Distribution of fractures indicated by caliper and BHTV logs; (b)
From page 231...
... . Caliper and BHTV logs before and after stimulation indicate that the borehole is intersected by a number of fractures and that the stimulation process produced no measurable change in these fractures in the immediate vicinity of the borehole.
From page 232...
... Left, distribution of fractures indicated by cal~per and BHTV logs; right, distribution of vertical flow under about 7 m of hydraulic head during injection before and after stimulation. From Paillet et al.
From page 233...
... S''. N 46° W FIGURE 4.D1 Schematic illustration of shear-wave propagation in a homogeneous medium with aligned fractures.
From page 234...
... . After leaving the fractured interval, both shear waves propagate with the same velocity but have different polarizations and travel times, which were established by the fractured interval.
From page 235...
... and perpendicular (right) shear-wave vertical seismic profiles from the Silo field in southeastern Wyoming.
From page 236...
... Pp. WW1-WW25 in the 30th Annual Logging Symposium Transactions.
From page 237...
... Pp. II1-II21 in 26th Annual Logging Symposium Transactions.
From page 238...
... Pp.1-26 in the 20th Annual Logging Symposium Transactions. Houston, Tex.: Society of Professional Well Log Analysts.
From page 239...
... Pp. HH1-HH24 in 32d Annual Logging Symposium Transactions.
From page 240...
... 1992. Borehole radar applied to the characterization of hydraulically conductive fracture zones in crystalline rock.
From page 241...
... 1991. Combined interpretation of fracture zones in crystalline rock using single-hole, cross-hole tomography and directional borehole-radar data.
From page 242...
... Pp.470-481 in Society of Exploration Geophysicists (SEG) Convention Expanded Abstracts.


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