Acoustic Borehole Research Articles

Characterisation of hydraulically-active fractures in a fractured granite aquifer

In this study, the practical usefulness and fundamental applicability of a self-potential (SP) method for identifying the hydraulically-active fractures were evaluated by a comparison of SP methods with other geophysical logging methods and hydraulic tests. Potentially permeable fractures intersecting a shallow, 10-m borehole were first identified using conventional geophysical methods such as acoustic borehole televiewer imaging and temperature, electrical conductivity and gamma-gamma logs. These results were then compared to results of analysis using the SP method. Constant pressure injection and recovery tests were conducted for verification of the hydraulic properties of the fractures identified by various logging methods. The acoustic borehole televiewer and gamma-gamma logs detected the openings or weathering zones associated with fractures intersecting the borehole, but they could not prove that groundwater was flowing through the detected fractures. The temperature and fluid-column electrical conductivity logs were unable to detect the fractured zones where groundwater in the borehole flows out to the surrounding rock aquifers. Comparison of results from the different methods tested showed that the most effective correlation between logs and the distribution of hydraulic conductivity was given by the SP signals, and that SP logging can accurately indicate the location of hydraulically-active permeable fractures. Based on these results, the SP method is recommended for determining the location of hydraulically-active fractures rather than other conventional geophysical logs. This self-potential method can be effectively applied in the initial stage of a site investigation to select the optimal site location or to evaluate the hydrogeological properties of fractures in underground exploration studies, such as those related geothermal reservoir evaluation and radioactive waste disposal.

A method for characterizing basement rocks from borehole images

Abstract Basement rocks tend to have negligible primary (rock matrix) porosity and, where present, almost all porosity is fracture-related. An insight into fracture type and distribution is therefore of importance when attempting to predict potential flow zones in hydrocarbon reservoirs. In the absence of core, borehole images provide a proven means of characterizing fractures within basement rocks. This paper will concentrate upon the classification and characterization of basement structural elements, including fractures, faults and breccias, versus lithological elements, such as foliations and intrusion boundaries from borehole images. In addition, suggestions of how to differentiate present-day in-situ stress indicators from natural fractures will be made as these features may provide important information regarding open fracture set orientations. Examples of microresistivity and acoustic borehole images acquired in fractured igneous basement rocks from oil wells in Yemen will be presented together with a suggested methodology for their interpretation. The discussion will be based on hydrocarbon reservoirs; however, the methodology outlined can potentially have wider applications in, for example, groundwater pollution prevention schemes or groundwater extraction, radioactive waste disposal, geothermal energy resources and deep-drilling research programmes.

Processing and analysis of high temperature geothermal acoustic borehole image logs in the Taupo Volcanic Zone, New Zealand

Acoustic borehole televiewer (BHTV) logs provide direct observations of lithology, structure and in situ stress in reservoirs, essential for successful well targeting and field management. Analyses of BHTV logs acquired in twenty-three high temperature (≤288°C) geothermal wells in the Taupo Volcanic Zone, New Zealand, resulted in the modification of BHTV processing techniques and the creation of a descriptive feature classification for hydrothermally altered, volcano-sedimentary-basement type reservoirs lacking other complementary information common in hydrocarbons or lower temperature geothermal systems. Lithological characteristics observed on these BHTV logs are presented, alongside an assessment of the reliability of the structural measurements and image interpretation.

Phase-based dispersion analysis for acoustic array borehole logging data.

A phase-based dispersion analysis method for velocity (slowness) extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented. The technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component. The approach is nonparametric and completely data-driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes. Results are free of the aliases and spurious modes which are characteristic of some prior approaches. Examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools; results are compared with those from the weighted spectral semblance (WSS) and amplitude and phase slowness estimation (APES) methods to demonstrate the effectiveness and utility of the proposed method.

In-situ stress measurements and regional stress field assessment of the Beishan area, China

Knowledge of the in-situ stress field in the rock mass at depth is important to provide stress boundary conditions for underground excavation design, stability evaluation, and the optimization of ground support systems. In the present study, in-situ stress measurement using hydraulic fracturing method is conducted in the Beishan area in Gansu Province, China, a preferred candidate site for China's high-level radioactive waste repository. Information on the stress magnitudes and orientations in three sub-areas is obtained from 136 measurement points in nine sub-vertical boreholes at depths ranging from 30 to 700m, and the distribution characteristics of the in-situ stress field is analyzed. The pressure–time records obtained from the test intervals with pre-existing fractures are discussed according to the acoustic borehole televiewer (BHTV) images. The measurement results indicate that the regional stress field is dominated by horizontal stress within most of the measurement depth ranges. A piecewise distribution of the principal stresses reveals approximately three stress domains with increasing depth. At shallow and medium depths, the stress fields are characterized by σH>σh>σv and σH>σv>σh, respectively, where σH, σh, and σv are maximum horizontal, minimum horizontal, and vertical stresses, respectively. With further increase of depth, the maximum principal stress changes from horizontal to vertical (i.e. σv>σH>σh). Fracture impression results reveal that the maximum horizontal stress is dominantly oriented in the NNE–NEE direction, which is in agreement with the orientation of regional tectonic stress field generated from movement of the Earth's tectonic plates. In addition, it is found that the orientations of σH are affected by the geological structures significantly.

The Results of Borehole Acoustic Imaging from a Granite in the Jihlava District, Czech Republic: Implications for Structural Geological Research

The paper presents utilisation of the acoustic borehole imaging method within the frame of structural geology research. Acoustic borehole imaging is a useful tool describing borehole walls. The method allows an identification of various inhomogenieties along the borehole including fractures. Despite the more or less subjective nature of interpretation of the acoustic borehole imaging similar to other geophysical method interpretations, is a careful comparison of the interpreted fractures and the fractures observed on a core allows us to orient both core and fractures. Furthermore it is possible to determine direction of present kinematic indicators on the oriented core (and fracture planes). Identified fault planes and kinematic indicators are valuable information for structural geology research, particularly for paleostress analysis. The procedure described here was applied in the PDV-1 borehole drilled in the immediate vicinity of the quarry near the Panské Dubenky village, Jihlava district, Czech Republic, Europe. The results are consistent with previous standard structural geology research conducted in the quarry.

Modified S transform for time-frequency analysis of borehole flexural waves dispersion

Guided wave propagation usually exhibits dispersive behaviour. The time-varying spectral components of dispersive waves have been characterized effectively using the time-frequency approach; the short-time Fourier transform (STFT) and the continuous wavelet transform (CWT) are commonly used for this purpose. However, the resulting energy distributions suffer from poor resolution related to the uncertainty principle, and this complicates the allocation of energy to individual propagation modes especially when the dispersion curves of these modes are close to each other in the time-frequency domain (in which case the separation becomes a challenge). Therefore there is a need for high resolution time-frequency techniques. To meet this challenge we introduce a modified version of the S transform, a method which combines the advantage of the STFT and CWT but with greater resolution. An adaptive algorithm is presented which identifies frequency regions of interest related to different energy modes, and employs the modified S transform to a dispersion curve model to extract the proportional energy distribution of a specific mode from a multimode dispersive wave signal. The effectiveness of this approach is demonstrated on dispersive flexural waves obtained from an acoustic borehole logging tool.

In Situ Stresses in Borehole Blanche-1/South Australia Derived from Breakouts, Core Discing and Hydraulic Fracturing to 2 km Depth

The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of Sv ≤ Sh < SH and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress Sh derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.

Method To Generate Full-Bore Images Using Borehole Images and Multipoint Statistics

Summary Borehole-image logs, which are produced by tools being lowered into a well, provide oriented electrical and acoustic maps of the rocks and fluids encountered in the borehole. Electrical borehole images in water-based (conducting) and oil-based (nonconducting) muds are generated from electrodes arranged in fixed patterns on pads that are pressed against the borehole wall. Depending on the borehole diameter, gaps nearly always occur between pads. Because of these gaps, it is common to have nonimaged parts of the borehole wall. Full-bore images are complete, 360° views of the borehole wall. They are generated by "filling in the gaps" between the pads in borehole-image logs. This method uses the Filtersim algorithm of multipoint statistics (MPS) to generate models, or realizations. Measured (incomplete) borehole images themselves are used as "training images." Recorded data are perfectly honored (i.e., the models are conditioned to the real data). Gaps are filled with patterns similar to those seen elsewhere in the log. Patterns in the gaps match the edges of the pads. The frequency distribution of continuously variable pixel colors in the gaps matches the distribution of pixel colors in the measured images. Full-bore images facilitate visualization and interpretation of borehole-image logs in any lithology, although case studies shown in this paper are developed in vuggy and fractured rocks. These images can be used to draw closed contours around electrically resistive or nonresistive patches in the borehole wall. Full-bore images can be used to repair logs with bad electrodes, low pad pressure, or poor acoustic reflections. Therefore, they can be used to enhance any commercially available electrical or acoustic borehole images.

Using borehole logging and electron backscatter diffraction to orient an ice core from Upper Fremont Glacier, Wyoming, USA

AbstractWhile glacier fabric reflects the accumulated strain, detailed azimuthal information is required to link the microstructure to the flow, and this is not easily gathered at depth. Borehole logging provides a way to obtain a log of azimuthal orientation of tilted stratigraphic features that can be used to orient the core with respect to glacier flow. We demonstrate this using acoustic borehole logs and the ice core from a 162 m borehole in Upper Fremont Glacier, Wind River Range, Wyoming, USA. We measured the dip of tilted dust and bubble layers in the actual ice core, identified them on the borehole log, then used their strike to orient the core sections containing them. We examined the crystal orientation fabric of our samples, using electron backscatter diffraction in a scanning electron microscope. When we compared the orientation of the tilted layers in some samples with the fabric, we found that the normal to the foliation and the c-axes maxima both pointed in the direction of maximum shear stress. This illustrates the usefulness of borehole logs for orienting ice cores after removal from the borehole, and for developing a better understanding of fabric development.

Damage assessment for concrete structure using image processing techniques on acoustic borehole imagery

The deterioration of concrete structures is a growing problem worldwide. Drilled cores are usually collected from large dams for testing the concrete and rock foundation underneath to assess their safety. However, the borehole itself can be used to assess cracks and other damage and collect additional information on the surrounding materials. This paper evaluates various edge-detection algorithms, as well as transform and statistical-based methods, for their effectiveness in assessing damage in a concrete dam from digital borehole imagery obtained using an acoustic televiewer. The statistical-based approach was found to be the most efficient technique for damage assessment from acoustic imagery. A clustering technique was used to quantify damage from the imagery, such as vertical cracks, horizontal cracks, voids, stains, and foundation damage. Results were verified using log data. Further damage analysis consisted of determining minimum, maximum and mean crack-width openings.

Single-borehole measuring method for broken rock zone in gently inclined thin layer weakness structure

According to the structural characteristics of gently inclined thin layer rock mass in which lots of weak interlayer existed, the concept of gently inclined thin layer weakness structure was proposed. If single-borehole measuring method of the acoustic along the conventional arrangement mode was used in measuring the broken rock zone in this structure, the change of the relationship curves (Vp-L) between acoustic p-wave velocity (Vp) and borehole depth (L) would present the irregular feature due to the mechanical characteristics of layered rock mass and harmful effects of weak interlayers, and the scope of broken rock zone couldn’t be defined quickly. Based on the analysis of the mechanical characteristics of layered rock mass, the propagation rule of acoustic and distributions characteristics of plastic zone and slip zone in layered rock mass, new arrangement mode of acoustic measuring boreholes for broken rock zone in gently inclined thin layer weakness structure was proposed. Namely, the measuring boreholes in two sides were parallel to the strata, the measuring boreholes in the roof and floor perpendicular to the strata. Besides the controlling depth of the measuring boreholes in the scope of the large plastic zones or the large slip zones should be increased. Engineering example showed that new acoustic measuring boreholes arrangement mode had the better applicability and could determine the scope of the broken rock zone in the gently inclined thin layer weakness structure quickly.

Engineering characterization of hydraulic properties in a pilot rock cavern for underground LNG storage

Feasibility of storing LNG in a lined rock cavern was evaluated using a pilot cryogenic rock cavern constructed in Daejeon, Korea. The pilot program included hydrogeological and engineering characterization of the rock mass around the cavern, design and construction of a drainage system, and pilot operation of the cryogenic cavern. An appropriate drainage system is most important to protect the containment system of LNG from thermal shocks due to ice lenses and hydrostatic pressure of groundwater. As a part of the pilot program, this study focused on the evaluation of hydraulic and engineering properties of the rock mass around the cavern. For this purpose, engineering logging of the rock cores, single and cross-hole hydraulic tests, and recharge/drainage tests were performed using seven drilled holes with different trends and plunges. Three main joint sets were found from the logging of the rock cores, acoustic borehole televiewer, and window mapping. The orientations of the three major joint sets were 60/209, 40/171, and 29/331, which can provide the main groundwater flow paths. Mean RQD values ranged from 56 to 88, which were classified as fair and good, although varying with depth along single boreholes. Hydraulic conductivity from the single and cross-hole hydraulic tests estimated in the order of 10 −6 or 10 − 7 m/s and corresponding transmissivity ranged between 10 − 5 and 10 − 6 m 2/s. Permeable intervals identified from the hydraulic tests were mostly located above the cavern roof. Below the roof, the permeable zone was difficult to observe. According to the hydraulic communication tests performed for some designated intervals, hydraulic connection between boreholes was highly varied with depth or location, which indicated a very different distribution of water conducting joint sets along the boreholes. When water was injected at R1 with constant or varying flow rates, monotonous and stable seepage was observed at observation boreholes. From this, some stable drainage was expected even in relatively heavy rainfalls. When designing the drainage system of the cavern, the drainage holes should be orientated to maximize frequency of encountering the major joint sets and the permeable intervals identified from this study.

Natural fracturing and petrophysical properties of the Palisades dolerite sill

Abstract This investigation of naturally occurring fractures in the mafic rocks of the Palisades dolerite sill characterizes the porosity of this crystalline rock sequence, and yields a method of determining the in situ porosity when complete down-hole information is not available. Two holes, 229 m and 305 m deep, were drilled 450 m apart through the sill and into the underlying Triassic sediments of the Newark Basin. Both holes were logged with geophysical tools, including the acoustic borehole televiewer (BHTV), to identify intervals of high porosity, fracturing, and potential zones of active fluid flow. Using the BHTV data, 96 and 203 fractures were digitally mapped within the sill in Well 2 and Well 3, respectively. Most fractures dip steeply (76–78°). There is a shift in fracture orientation between Well 2 and Well 3, although the lithology of the sill is continuous. The dolerite penetrated in both holes is fresh and unaltered, and intersects a 7-m thick olivine-rich layer about 15 m above the bottom of the sill. Several fractures identified in the sill have large apparent aperture (&gt;6 cm) that correspond to high-porosity zones (6–14%), measured from both resistivity and neutron logs in Well 2. We use a relationship between porosity and apparent fracture aperture in Well 2 to infer the porosity in Well 3. This correlative method for estimating porosity may be applicable between holes in other crystalline rock environments where down-hole log data are incomplete. Changes in the temperature gradient log also indicate active fluid flow, although flow appears to be most active in fractured and high-porosity zones in the sediments.

New developments in real-time processing of full waveform acoustic televiewer data

The new full wave Acoustic Borehole Imager tool (ABI) is a multi-echo (amplitude and traveltime) system that gives optimum performance under a wide range of borehole conditions and is an improvement over existing single echo acoustic televiewer tools. The principle of the multi-echo system is the digital recording of each reflected acoustic wave train. Then, real-time processing of the acoustic data is made by a downhole Digital Signal Processor (DSP) to extract all valuable information, which is further compressed before transmission to the surface such that no special requirements are imposed on data transmission rates or on logging speed. Also, recording of the full acoustic waveform enables major improvements to the dynamic range of the system. Information about echoes from the tool's acoustic window provides the possibility to predict tool generated coherent noise and allows detection of echoes from the borehole wall, which are much smaller than coherent noise signals. When used in a PVC cased borehole, the system can be automatically adapted to record both the reflection at the casing and at the borehole wall. When used inside steel casing, the tool can detect echoes from the inner wall and the outer wall of the steel casing and the resulting data can used to calculate amount inner corrosion, outer corrosion, and remaining casing thickness. If the borehole casing was grouted with cement, information can be gathered about the presence and absence of cement and the quality of cement bonding.

Stress orientation on the Norwegian continental shelf derived from borehole failures observed in high-resolution borehole imaging logs

Knowledge of stress orientation is crucial for the understanding of many processes in the earth's crust such as tectonic development, earthquake occurrence, and fluid transport along faults. In the case of the Norwegian continental shelf, which is a prolific area for hydrocarbon production, this knowledge also plays an important role for engineering decisions with respect to wellbore stability and reservoir management. Therefore, data on stress orientation have been collected extensively over the past few years in this region. However, nearly all data published for the Norwegian continental shelf for depths less than 6km are based on the evaluation of four-arm caliper data, a method with intrinsic limitations. For greater depths stress orientation is derived from earthquake focal plane mechanisms. This article presents stress orientation data derived from an extensive set of image logs throughout the Norwegian continental shelf. The analysis is based on the identification of compressive and tensile failures of the borehole wall in electrical and acoustic borehole imaging logs. This method for the determination of the stress orientation is found to be highly reliable and capable of delivering detailed and accurate results, and thus is far superior to the analysis of breakouts from four-arm caliper logs. The analysis of image logs shows that the stress orientation is consistent both regionally and with depth. Strong variations in stress orientation, which are often interpreted from earlier analyses of four-arm caliper data, are not observed. This may indicate that some of the earlier four-arm caliper analyses may have been biased by the misinterpretation of drilling-induced artefacts as stress-induced breakouts. Stress-induced failures of the borehole wall are observed in only about 40% of the wells analysed in this paper. This does not support the conclusion that the horizontal stresses are isotropic. It just indicates that the stress difference in combination with the borehole conditions (downhole pressure and temperature) was insufficient to induce these failures.

Development of a Doppler Acoustic Imaging System for Borehole Wall to Evaluate Fluid Flow Distribution around Subsurface Fractures

We are in the process of developing a doppler borehole televiewer (DBHTV) which quantitatively evaluates the permeability of independent subsurface fractures in a borehole. The system employs an ultrasonic pulsed Doppler method, and is compatible with the conventional acoustic borehole imaging system called borehole televiewer (BHTV). Using the DBHTV, back-scattered waves from fine particles in the borehole fluid such as drill mud or cuttings in a water filled borehole are detected along with the reflected waves from the borehole wall, and the Doppler shift of the back-scattered waves is used to estimate the fluid velocity. A method to estimate the distribution of Doppler shift was examined using a laboratory experimental model. An attempt to locate the sampling volume and a method to quantitatively estimate the flow velocity by scanning the transducer are examined. This study shows that the location of permeable fractures, the distribution of fluid velocity and the fluid volume can be visualized using the DBHTV.

Stress estimated using microseismic clusters and its relationship to the fracture system of the Hijiori hot dry rock reservoir

A method for estimating the in situ stress state by analyzing microseismic clusters induced by hydraulic injections is proposed. The method is based on focal mechanism analysis, supplemented with microseismic doublet analysis. The latter is used to discriminate between the fault plane and the auxiliary planes of the focal mechanism solution. The method was applied to estimate the stress field within the Hijiori hot dry rock geothermal reservoir and yielded results which were consistent with other estimates. The mechanics of permeability creation within the reservoir during major hydraulic injections was then examined by evaluating the interaction of the inferred stress field with the natural fracture system. The characteristics of the latter were determined by integrating information obtained from core samples, acoustic borehole televiewer images and pressure, temperature and spinner logs. The results indicated that the observed microseismicity and orientation-dependent permeability characteristics of the fracture population are well explained by shear failure inferred from Coulomb's friction theory. It is also shown that the growth direction of the reservoir is strongly controlled by the distribution of favorably oriented pre-existing fractures and their interaction with the stress field.

An Implementation of the Hough Transformation for the Identification and Labelling of Fixed Period Sinusoidal Curves

Hough transforms have long been used as computationally efficient methods for detecting particular shapes in images. This paper describes a practical implementation of a Hough transform for the characterisation of rock fractures in acoustic borehole images, where planar fractures ideally result in sinusoidal curves with a period equal to the perimeter of the borehole. The three remaining degrees of freedom of the sinusoid, namely its amplitude, baseline position, and phase, are successfully determined in ultrasound return images with considerable noise and clutter.

The appearance of potential sealing faults on borehole images

Abstract Electrical and acoustic borehole images provide data on the structure, width, spacing and orientation of potential sealing faults in the subsurface. These data bridge the ‘scale of observation gap’ between the resolution limits of more conventional data such as seismic data and core, and provide continuous records often up to several thousand feet long. Potential fault seals can be measured from vertical, deviated or horizontal wells and from either clastic or carbonate rocks. Five categories of potential fault seals can be described from images: juxtaposition fault seals place reservoir against seal lithologies and appear as sharp image contrasts; clay lined fault seals commonly appear as shale rich bands on electrical images, which may be either resistive or conductive depending on the resistivity of the host lithology, and low amplitude bands on acoustic images; grain size reduction fault seals appear as resistive or high amplitude features; cemented fault seals appear as highly resistive or very high amplitude features, and open/vuggy faults appear as conductive or very low amplitude features. An integrated approach to image interpretation incorporates not only borehole image data but also core, conventional wireline logs, pressure data and production information.