Ultrasonic Borehole Televiewer Research Articles

Wellbore breakouts of the main borehole of Chinese Continental Scientific Drilling (CCSD) and determination of the present tectonic stress state

The Sulu–Dabie high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt as the product of subduction-collision between the northern China plate and Yangtze plate underwent a process of formation and evolution from deep subduction→exhumation→extension→slow uplift. The study of its modern tectonic stress field has great significance for a complete understanding of the process of formation and evolution of the HP–UHP metamorphic belt, especially the exhumation and uplift of the belt. Wellbore breakouts are the most visual tectonic phenomenon which can characterize the modern stress action in the main borehole of Chinese Continental Scientific Drilling (CCSD). Ultrasonic borehole televiewer reflection wave data show that wellbore breakouts began to occur at 1216 m depth of the main borehole. A total of 143 borehole televiewer images were collected from 1216 to 5118 m depth (hole completion depth). After data processing and statistics, the average azimuth of the long dimension of the wellbore breakout obtained was 319.5° ± 3.5°, indicating that the average azimuth of the maximum horizontal principal stress causing wellbore breakout initiation was 49.5° ± 3.5°. The maximum and minimum horizontal principal stress values at 52 depths in the interval of 1269 to 5047 m were estimated using the elements of wellbore shapes (wellbore depth and width), combined with the cohesive strength and internal frictional angle of the rock obtained by rock mechanical tests on samples, and the static load stresses at corresponding depths were calculated according to the rock density logging data. The results indicate that: the maximum and minimum horizontal principal stresses are 41.4 and 25.3 MPa at 1269 m depth respectively and 164.7 and 122 MPa at 5047 m depth respectively; the maximum vertical stress is 141.3 MPa at 5047 m depth with a density of 2.8 g/cm 3; the in-situ stresses increase nearly linearly with depth. The magnitudes and directions of the three principal stresses reflect that the regional stress field around the CCSD main borehole is mainly in a strike-slip state, which is consistent with the basic features of the regional stress field determined using other methods.

Analysis of borehole televiewer measurements in the Vorotilov drillhole, Russia — first results

In the Eurasian part of the World Stress Map almost the whole region east of the Tornquist-Teisseyre line is terra incognita. The closure of this information gap is of fundamental importance to the understanding of the geodynamics of the Eurasian continent. A detailed analysis of stress-induced wellbore breakouts has been performed over a 4.1-km-long depth interval in the Vorotilov drillhole (VGS). The borehole is located in the central part of the Russian platform, right in the center of the Vorotilov meteorite impact crater 60 km to the NNE of the city of Nizni Novgorod. An ultrasonic borehole televiewer (BHTV) was used to obtain high-resolution acoustical images from the borehole wall. With an interactive system for analyzing BHTV data the azimuth and shape of borehole breakouts occurring in the depth range of 1.3–4.8 km were analyzed. A statistical analysis of the resulting orientation profile of the breakout azimuths yields an overall direction of the maximum horizontal principal stress SH of N 137°E ± 15°. Variations of breakout orientation with depth ranging from a few degrees up to more than 90° are seen on various depth scales.The observed stress direction of N 137°E agrees very well with the average SH orientation of N 145°E in Central Europe. If this measurement is taken as representative for the Russian platform, the stress field in Russia is only slightly rotated in comparison to Central Europe. This can possibly be interpreted as indicative for the stress field to be governed by broad scale tectonic forces, such as a strong contribution from the forces exerted by the collision zone in the Alpine-Himalayan belt and by the Mid-Atlantic ridge.

Evaluation of permeable fractures by multi-offset hydrophone vertical seismic profiling (VSP): T. Kiguchi, H. Ito, Y. Kuwahara, S. Nakao & D. T. Ohminato, Geothermal Science & Technology, 5(1–2), 1995, pp 21–29

We carried out a high frequency multi-offset hydrophone VSP experiment in the shallow wells which NEDO (New Energy and Industrial Technology Development Organization) drilled at the Fracture Evaluation Test site in Iidate, Fukushima prefecture, Japan in order to detect permeable fractures intersecting borehole and to characterize them. Although the optical borehole television (BTV) and ultrasonic borehole televiewer (BHTV) mapped numerous fractures at the wellbore, it is still difficult to discriminate permeable fractures from impermeable ones. Tube waves are observed at several depths. We calculated fracture dip, strike and permeability from the amplitude ratio of tube wave to P-wave. Our analysis is based on the model that tube wave is generated when the incident P-wave compress the fractures intersecting wellbore. Calculated permeability is of the order of 100 md. With tube wave analysis, we are able to distinguish permeable fractures from impermeable fractures observed by borehole imaging tools. For example, one tube wave is generated at the depth where high and low dip angle fractures are observed with well logging. We conclude that the low dip angle fracture is permeable on the basis of the tube wave analysis. This is consistent with the flow properties of the fractures estimated from the circulation test.

Fracture Statistics Derived From Digital Ultrasonic Televiewer Logging

Abstract A 300 m section of core from a highly fractured igneous formation is directly compared to digital ultrasonic televiewer images of the borehole wall. The two different images consist of a mapping of the amplitudes travel-times of ultrasonic pulses reflected from the wall rock with respect to depth and magnetic north. These data were acquired during logging using a specially designed high-speed digitization system. The number of planar features seen in the amplitude and travel-lime images were 867 and 322, respectively, whereas 567 fractures and veins were observed in the core. The depth distributions of planar features seen in the core and in the travel time image appear to correlate. This correlation suggests that the travel-time log features in this paper are indicative of those in situ fractures which are open and permeable as determined by other means. Further, the travel-time planar features dip in the same direction as that expected for the greatest compressive stress. Conversely, this observation implies that the travel-time feature orientations possibly reveal in situ stress directions. Core examination indicates that the observed fractures are predominantly shear and, for a given state of stress, the orientations of these type of fractures differ from the orientations of extensional joints which open perpendicular to the direction of the least compressive stress. Extensional joints have been reported in televiewer logs by other workers. Hence, to delineate in situ stress directions from fractures observed in televiewer images, it must be distinguished whether a fracture results from either shear or extensional failure. Introduction Both natural and artificial fractures enhance fluid flow in the crust. In dense limestone, tight gas sand, coal and most igneous and metamorphic rock formations where the intrinsic permeability is low, economic extraction of hydrocarbon and geothermal resources can only originate from natural or artificial joints and fractures. Further, natural joints also impede and complicate underground construction. Hence, some knowledge of the density and orientation of in situ fractures is desirable for the efficient completion of many different engineering endeavours and both fracture detection and evaluation is important. A short listing of fracture detection methods includes borehole and surface seismics(1, 2, 3), radar(4), temperature gradient perturbations(5), sonic logging(6), resistivity scanning(7), and ultrasonic borehole televiewer analysis(5, 8, 9, 10). Each of these methods provides a piece of relevant information on fracture density, aperture, orientation, permeability, or location. Differentiating which fractures are open and permeable from those which arc closed and impermeable is especially crucial. Because the resolution of the fracture sensing techniques varies from a few metres for seismic measurements to centimetres for some wireline logging procedures, accurate characterization of in situ joints and fractures requires application or a number of methods. Of the above techniques mentioned, high resolution ultrasonic borehole televiewer logging(8) has been commercially exploited for nearly two decades. In conventional analog usage the televiewer provides a two-dimensional linear grey scale image of the borehole wall by mapping me strength, i.e. amplitude, of ultrasonic pulses reflected from rock from the interface between the wellbore fluid and the wall rock.

Comparison between natural fractures and fracture parameters derived from VSP : Geophys J V107, N3, Dec 1991, P475–483

SUMMARY The feasibility of resolving a subsurface fracture system using P-wave and cross-polarized shear-wave vertical seismic profiling (VSP) was examined. In situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer (BHTV) and by measurements of core sample velocities. The BHTV images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part. Two sets of dominant parallel fractures are observed by BHTV in the fracture-rich depth interval of the shallower part; the orientations of fracture strikes in the two sets are almost the same directing to NW, whereas the average dip angles of the fractures in the two sets are 73 and 46, respectively. The fracture orientations in the fracturepoor depth interval of the deeper part are found to be random. The degree of anisotropy of the core sample velocity is so small that the present VSP measurement cannot resolve the expected anisotropy from the core sample measurement. P- and two shear-wave velocities (Vp, V,, and V,) are determined from the downgoing phases of three-component VSP with a zero-offset P-wave source and with zero-offset shear-wave sources polarized in two orthogonal directions, respectively. The VSP results are as follows. (1) The shear-wave polarization anisotropy is not detected within the accuracy of 4 per cent. (2) V, and V, (average of V,, and V,) for the fracture-rich interval are 94 per cent and 86 per cent, respectively, of those for the fracture-poor interval. The VSP results are consistent with expected velocity changes due to an isotropic distribution of cracks. However, a biplanar crack model cannot satisfy the VSP results, whereas the BHTV images show two such sets of parallel fractures. It is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present VSP experiment.

Applications of real time digital acquisition of ultrasonic borehole televiewer data on a personal computer

A personal computer based hardware has been developed for the real time, rapid digitization of echo amplitudes and times-of-flight of ultrasonic pulses produced by existing analog ultrasonic borehole imaging systems. Both amplitude and travel-time B-scan images are mapped circumferentially around the wellbore wall with reference to magnetic north. 12-bit analog to digital conversion of pulse amplitudes provides a resolution of approximately 1 mV out of a peak amplitude voltage of 1 V; travel times are determined to within 100 ns to a maximum time of 512 μs after which it is assumed that no signal will be detected. This information provides high-resolution digital images of the borehole wall and cross section which are useful for delineation of open fractures, discrimination of complex cross sections, and calculation of wellbore volumes.

Comparison between natural fractures and fracture parameters derived from VSP

SUMMARY The feasibility of resolving a subsurface fracture system using P-wave and cross-polarized shear-wave vertical seismic profiling (VSP) was examined. In situ distributions of fractures and microcracks in a test borehole are characterized by an ultrasonic borehole televiewer (BHTV) and by measurements of core sample velocities. The BHTV images show that the fracture frequencies in the shallower part of the borehole are much higher than those in the deeper part. Two sets of dominant parallel fractures are observed by BHTV in the fracture-rich depth interval of the shallower part; the orientations of fracture strikes in the two sets are almost the same directing to NW, whereas the average dip angles of the fractures in the two sets are 73 and 46, respectively. The fracture orientations in the fracturepoor depth interval of the deeper part are found to be random. The degree of anisotropy of the core sample velocity is so small that the present VSP measurement cannot resolve the expected anisotropy from the core sample measurement. P- and two shear-wave velocities (Vp, V,, and V,) are determined from the downgoing phases of three-component VSP with a zero-offset P-wave source and with zero-offset shear-wave sources polarized in two orthogonal directions, respectively. The VSP results are as follows. (1) The shear-wave polarization anisotropy is not detected within the accuracy of 4 per cent. (2) V, and V, (average of V,, and V,) for the fracture-rich interval are 94 per cent and 86 per cent, respectively, of those for the fracture-poor interval. The VSP results are consistent with expected velocity changes due to an isotropic distribution of cracks. However, a biplanar crack model cannot satisfy the VSP results, whereas the BHTV images show two such sets of parallel fractures. It is suggested that this discrepancy is due to the lack of depth resolution in the velocity determination in the present VSP experiment.

Analysis of fractures from borehole televiewer logs in a 500m deep hole at Xiaguan, Yunnan province, Southwest China

Fractures from a 500 m deep hole in the Red River fault zone were analyzed using an ultrasonic borehole televiewer. Four hundred and eighty individual fractures were identified between 19 m and 465 m depth. Fracture frequency had no apparent relation to the major stratigraphic units and did not change systematically with depth. Fracture orientation, however, did change with stratigraphic position. The borehole intersected 14 m of Cenozoic deposits, 363 m of lower Ordovician clastic sediments, and 106 m of older ultramafic intrusions. The clastic sequence was encountered again at a depth of 484 m, suggesting a large fault displacement. Fractures in the top 162 m of the sedimentary section appear randomly distributed. Below that depth, they are steeply dipping with northerly and north-westerly strikes, parallel to the major active faults in the region. Fractures in the ultramafic section strike roughly eastwest and are steeply dipping. These orientations are confined to the ultramafic section and are parallel to an older, inactive regional fault set.

Well bore breakouts and in situ stress

The detailed cross‐sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data. We show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations. Both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression. This work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ.

Natural resonance caused by fracture-induced venting: An analysis of multichannel acoustic logging in a deep borehole

A four-channel sonic log of a 1-km section of oceanic crust at site 504B, Eastern Equatorial Pacific Ocean has been spectrally analyzed. The cross-spectrum of seismic waveform pairs was converted into integrated energy and peak-frequency at each shot point for both compressional and shear coda. These plus an ultrasonic borehole televiewer fracture count were combined into a “fracture spectrum log” over the bottom 500 m of the borehole. Cross correlation of P and S frequency and energy variations downhole revealed a natural resonance at wavelengths of 30–40 cm in the oceanic crust. This resonance appears as a low-pass filter of P energy and a high-pass filter of S energy. Because the energy attenuation is phase coherent with the fracture density, we interpret this natural resonance to be caused by venting of acoustic energy through intersecting fractures of an aspect ratio appropriate to the resonance wavelength. This is the first time such uniform fracturing has ever been observed in the oceanic crust. [Work supported by ONR.]

The distribution of natural fractures and joints at depth in crystalline rock

This paper presents the results of studies of the natural fracture distribution encountered in 10 test wells drilled in three areas of the United States. Seven of the wells were drilled to depths of 200–250 m, while three were drilled to depths of about 1 km. Using an ultrasonic borehole televiewer, fracture depths, strikes, and dips were determined. Steeply dipping fractures were found throughout each of the wells, and in general, few horizontal fractures were observed. Statistically significant fracture pole concentrations were found for each well which were basically invariant with depth, although some variation of fracture orientation with depth was found in two wells. The significant fracture orientations were not found to be the same in wells only several kilometers apart in a given region. In none of the wells did the number of observable fractures decrease markedly with increasing depth. No simple relationship of fracture orientation or fracture density with major structural features such as the San Andreas fault were observed, and no simple relation between the significant fracture orientations and either past or present regional stress fields could be determined.

Ultrasonic borehole televiewer investigation of oceanic crustal layer 2A, Costa Rica Rift

In situ lithostratigraphy and the distribution of fracture and void zones in oceanic layer 2A were examined with an ultrasonic borehole televiewer at IPOD sites 501 and 504B on the south flank of the Costa Rica Rift. These records indicate a decrease in the size of basalt pillows with depth and a corresponding increase in fracture and void density. The observed increase in P-wave velocity with depth in this hole, as layer 2A merges into 2B, is best explained by an increase in the degree to which clays and zeolites fill fractures and voids. This also explains the order-of-magnitude drop in permeability observed in hole 504B. An increase in alteration-filling with age could explain the transition from convective to conductive heat flow and the correlative disappearance of layer 2A on the flanks of oceanic spreading centres.