Borehole Acoustic Imaging Research Articles

Field application of near-borehole fracture acoustic imaging using a three-dimensional scanning inversion method

Borehole acoustic remote imaging can help identify and evaluate abnormal geological bodies within tens of meters of a well, which has a very important application prospect in fracture imaging. Compared to the borehole mode wave, the echo amplitude from a near-borehole fracture is considerably small, and the currently used wave field separation method cannot completely filter the borehole mode wave. Due to the influence of the large amplitude of the remaining borehole mode wave, several artifacts are often produced near the well when using traditional migration imaging. Thus, imaging and characterizing near-borehole fractures via acoustic imaging becomes difficult. To overcome this, we propose a three-dimensional scanning inversion method that attenuates the effects of remaining refracted waves on borehole acoustic imaging. We then perform data processing and imaging of field-logging data based on this method, and multimode echoes are used to identify and evaluate fractures. Finally, a comparison of the inversion results of the borehole acoustic and borehole electrical imaging showed that the three-dimensional scanning inversion method can effectively attenuate the remaining refracted wave and better realize the clear imaging of near-borehole fractures. Different modes of echoes illuminate different areas of fractures near wells. Therefore, based on scattered S-waves, the proposed imaging method can effectively detect fractures that cross-cut a well. Moreover, based on multimode waves, the method can suppress artifacts and highlight the azimuth characteristics of near-borehole fractures. Using the proposed method, high-precision positioning and fine characterization of formation fractures in near and far wells can be achieved simultaneously. Further, near-borehole fracture parameters—such as tendency, dip, and depth—can be inverted, and the results are consistent with the inversion results obtained through borehole electrical imaging. Thus, the proposed method enables the quantitative evaluation of near-borehole fractures.

Elastic wave radiation and reception of a borehole dipole source in a vertically transverse isotropic formation

Single-well acoustic imaging using a dipole tool has been applied in anisotropic formations. For this application, the effect of anisotropy on the radiation and reception of acoustic waves in the borehole needs to be analyzed and a proper analysis method needs to be developed. We use the far-field asymptotic solution of the wavefield to calculate the radiation pattern of the dipole source in a borehole penetrating vertically transverse isotropic (VTI) formation. We model the spherical wave propagation characteristics of the radiated and reflected waves in the VTI formation and the receiving of the waves in the borehole for the on- and off-axis positions. Based on the developed analytical modeling methods, we study systematically the effects of the formation anisotropy on the radiation pattern and borehole response characteristics. Our modeling result demonstrates that due to the coupling between the longitudinal and the transverse particle motions, quasi-P-(qP-) and quasi-SV-(qSV-) wave radiation patterns contain two components. The result also indicates that the anisotropy causes the arrival time of the SH-reflection wave to differ from that of the qSV wave, and the misposition of the imaged targets would occur if the velocity measured along the borehole is used for migration. The azimuth sensitivity of the off-axis data can be used to help eliminate the 180° ambiguity for the dipole acoustic imaging in the VTI formation. Understanding the effects of anisotropy on borehole acoustic imaging can help improve the accuracy of the imaging result in anisotropic formations.

Borehole azimuthal acoustic imaging using 3D spatial scanning: Application in acoustic detection of nearby wells

At present, it is difficult to image small-scale abnormal geo-bodies near wells using borehole acoustic imaging, as information from scattered waves is neglected and that from reflected waves is mainly used, thus resulting in low imaging resolution. Imaging methods based on scattered waves can help obtain higher-resolution measurements. Herein, a 3D scattered wave spatial scanning imaging method is proposed to make full use of multi-mode scattered waves for imaging small-scale abnormal geo-bodies near the well. The 3D finite-difference time-domain method was used to simulate the acoustic fields for borehole azimuthal acoustic imaging with a fluid-filled target borehole near a measurement well, and the imaging method was applied for the acoustic detection of nearby wells based on forward modeling and field measurement waveform data. The results show that for the fluid-filled target borehole that is approximately parallel to and distant from the measurement well, the amplitude of the scattered PP-wave is the largest followed by the mode converted PS- and SP-wave; and the amplitude of the scattered SS-wave is the smallest, which is related to the radiation, scattering, and reception of the monopole acoustic field. In this case, the scattered wave from the target borehole can be regarded as a plane-wave. The spatial scanning imaging method based on single-mode scattered acoustic waves and the 3D plane-wave imaging method provide similar imaging results. The comprehensive use of multi-mode scattered acoustic waves can further improve the imaging signal-to-noise ratio and resolution. A field example was used to verify the correctness of the forward analysis and the effectiveness of the imaging method. This study will help improve the imaging resolution for the acoustic detection of nearby wells and the evaluation of downhole hydraulic fracturing.

Numerical simulation of borehole 3D scanning acoustic imaging using scattered waves

Oil and gas exploration increasingly requires high-resolution imaging of small, irregularly shaped, and highly heterogeneous well-side complex and abnormal geo-bodies. Conventional borehole acoustic imaging is often unable to accurately obtain the position and azimuth of small-scale abnormal geo-bodies. This study presents an inversion method that uses scattered waves for borehole 3D acoustic imaging and an implementation scheme that combines plane and spherical scanning imaging. The finite-difference time-domain method was used to simulate the acoustic fields for borehole azimuthal acoustic imaging of one and two caves next to a well. The proposed inversion method of 3D spatial scanning based on multi-mode wave information was validated through numerical simulations investigating the effect of different parameters on the imaging results. The simulation results show that the cave-scattered waves include the PP-, PS-, SP-, and SS-waves. When plane scanning imaging is performed based on a single wave mode, the other wave modes become interference factors. After the weighted processing of the PP-, PS-, SP-, and SS-waves, plane scanning imaging based on multi-mode scattered acoustic waves is shown to weaken pseudo-solutions, enhance the signal-to-noise ratio, and improve the radial and axial positioning accuracy of scatterers. When the scatterer is close to the borehole axis, the echo received by the receiver is not a real plane wave. In contrast with the 3D slowness time coherence (STC) and beamforming methods, spherical scanning imaging based on single-mode scattered acoustic waves completely considers this fact, which improves its azimuth positioning accuracy. Furthermore, spherical scanning imaging based on multi-mode scattered acoustic waves accurately estimates the azimuth of caves beside a well with a high imaging resolution. Finally, numerical simulation results were validated using the field measurement data of a well, and the actual imaging effect of the new method was tested. Therefore, rather than using single-mode reflected waves with limited information, the proposed method of scanning imaging using scattered acoustic waves can substantially improve the imaging resolution and positioning accuracy of small-scale abnormal geo-bodies beside a well and enhance the detection range.

Investigation of through-casing borehole acoustic reflection imaging with a dipole source: Modeling and field application

In view of most wells are completed by casing pipe in oil & gas exploration and development process, it is imperative to extend the application of borehole acoustic reflection imaging (BARI) from open to cased boreholes. To understand the characteristics of signals in borehole acoustic imaging, we study the radiated and reflected elastic waves in an open and cased borehole through numerical simulations. The results show that, for a broadband dipole source, the radiated SH-waves have the highest amplitudes and broadest radiation angles in the cased borehole, which is similar to the observation in an open borehole. It is believed that the cement and casing pipe would prevent to a certain extent the energy excited by the inner borehole sources from propagating into the outside formation. However, our modeling results clearly show the existence of the cement and casing will not influence the amplitude of the radiated SH-waves significantly. In fact, the decrease of the borehole radius, which is caused by cased borehole completion here, will increase the amplitude of the radiated SH-waves. As a result, the signal-to-noise ratio (SNR) of reflected waves relative to direct waves in the cased borehole is higher than that in the open borehole. Meanwhile, the cement bond quality does not heavily affect the SNR of the reflected waves and the SNR increases with the decrease of source frequency. Interestingly, as the cutoff frequency of the dipole flexural-wave mode in the cased borehole increases rapidly with the decrease of the formation S-wave velocity, the amplitude of the direct wave also decreases significantly. This results in an increase of SNR of the reflected waves, which is beneficial for extracting the reflected waves. The field data example certifies our theoretical analysis. Our results provide a theoretical foundation for the through-casing BARI with a dipole source.

Borehole Acoustic Imaging Using 3D STC and Ray Tracing to Determine Far-Field Reflector Dip and Azimuth

Borehole Acoustic Imaging Using 3D STC and Ray Tracing to Determine Far-Field Reflector Dip and Azimuth

Towards the multi-scale characterization of braided fluvial geobodies from outcrop, core, ground-penetrating radar and well log data

Abstract The integrated application of advanced visualization techniques – validated against outcrop, core and gamma ray log data – was found to be crucial in characterizing the spatial distribution of fluvial facies and their inherent permeability baffles to a centimetre-scale vertical resolution. An outcrop/behind outcrop workflow was used, combining the sedimentological analysis of a perennial deep braided outcrop with ground-penetrating radar profiles, behind outcrop optical and acoustic borehole imaging, and the analyses of dip tadpoles, core and gamma ray logs. Data from both the surface and subsurface allowed the recognition of two main architectural elements – channels and compound bars – and within the latter to distinguish between the bar head and tail and the cross-bar channel. On the basis of a well-constrained sedimentological framework, a detailed characterization of the gamma ray log pattern in the compound bar allowed several differences between the architectural elements to be identified, despite a general cylindrical trend. A high-resolution tadpole analysis showed that a random pattern prevailed in the channel, whereas in the bar head and tail the tadpoles displayed characteristic patterns that allowed differentiation. The ground-penetrating radar profiles aided the 3D reconstruction of each architectural element. Thus the application of this outcrop/behind outcrop workflow provided a solid database for the characterization of reservoir rock properties from outcrop analogues.

Acoustic reflection log in transversely isotropic formations

We have calculated the waveforms of sonic reflection logging for a fluid-filled borehole located in a transversely isotropic rock. Calculations have been performed for an acoustic impulse source with the characteristic frequency of tens of kilohertz that is considerably less than the frequencies of acoustic borehole imaging tools.It is assumed that the borehole axis coincides with the axis of symmetry of the transversely isotropic rock. It was shown that the reflected wave was excited most efficiently at resonant frequencies. These frequencies are close to the frequencies of oscillations of a fluid column located in an absolutely rigid hollow cylinder. We have shown that the acoustic reverberation is controlled by the acoustic impedance of the rock Z=Vphρs for fixed parameters of the borehole fluid, where Vph is the velocity of horizontally propagating P-wave; ρs is the rock density. The methods of waveform processing to determine the parameters characterizing the reflected wave have been discussed.

Wavefield simulation of 3D borehole dipole radiation

Reflections in acoustic borehole-logging data can be used to image near-borehole geologic structures, using either monopole or dipole measurements, the latter of which resolve azimuthal ambiguities. Dipole methods in particular can characterize small and subtle fractures in reservoir environments. Accurate tools for wavefield simulation are required to account for wave modes in and out of the borehole, and for the receiver responses to these modes. We have evaluated a 3D elastic staggered-grid finite-difference method for isotropic and anisotropic media in the context of the borehole acoustic imaging problem. Based on the convolutional perfectly matched layer and the multiaxial perfectly matched layer schemes, a hybrid perfectly matched layer method was developed and implemented to reduce artificial boundary reflections. The superiority of this hybrid perfectly matched layer beyond the convolutional perfectly matched layer and the multiaxial perfectly matched layer was proven based on numerical benchmarks in isotropic and anisotropic media. Numerical simulation of radiation, reflection, and multipole reception of elastic waves, excited by a dipole source, was carried out. To optimize azimuthal detection, the relationships between S-wave polarization as well as source-receiver offset and source-reflector angle was analyzed. Results indicated that the S-S reflection is most sensitive to the angle between the incident ray and the normal to the reflector. Its maximum amplitude occurs as the incident angle reaches its critical value, a fact that can be used to calculate the total propagation distance of the S-S wave. The critical angle as well as the SH-wave velocities of geologic structures outside the borehole can thus be determined.

Analysis of the Thermal and Hydraulic Stimulation Program at Raft River, Idaho

The Raft River geothermal field, located in southern Idaho, roughly 100 miles northwest of Salt Lake City, is the site of a Department of Energy Enhanced Geothermal System project designed to develop new techniques for enhancing the permeability of geothermal wells. RRG-9 ST1, the target stimulation well, was drilled to a measured depth of 5962 ft. and cased to 5551 ft. The open-hole section of the well penetrates Precambrian quartzite and quartz monzonite. The well encountered a temperature of 282 °F at its base. Thermal and hydraulic stimulation was initiated in June 2013. Several injection strategies have been employed. These strategies have included the continuous injection of water at temperatures ranging from 53 to 115 °F at wellhead pressures of approximately 275 psi and three short-term hydraulic stimulations at pressures up to approximately 1150 psi. Flow rates, wellhead and line pressures and fluid temperatures are measured continuously. These data are being utilized to assess the effectiveness of the stimulation program. As of August 2014, nearly 90 million gallons have been injected. A modified Hall plot has been used to characterize the relationships between the bottom-hole flowing pressure and the cumulative injection fluid volume. The data indicate that the skin factor is decreased, and/or the permeability around the wellbore has increased since the stimulation program was initiated. The injectivity index also indicates a positive improvement with values ranging from 0.15 gal/min psi in July 2013 to 1.73 gal/min psi in February 2015. Absolute flow rates have increased from approximately 20 to 475 gpm by February 2 2015. Geologic, downhole temperature and seismic data suggest the injected fluid enters a fracture zone at 5650 ft and then travels upward to a permeable horizon at the contact between the Precambrian rocks and the overlying Tertiary sedimentary and volcanic deposits. The reservoir simulation program FALCON developed at the Idaho National Laboratory is being used to simulate and visualize the effects of the injection. The simulation model uses a discrete fracture network generated for RRG-9 using acoustic borehole imaging and analysis of microseismic activity. By adjusting the permeability of the fractures, a pressure history match for the first part of the stimulation program was obtained. The results of this model indicate that hydraulic fracturing is the dominant mechanism for permeability improvement for this part of the stimulation program.

Heterogeneous bedrock investigation for a closed-loop geothermal system: A case study

This paper investigates bedrock heterogeneity by applying three different geophysical approaches, in order to study the long-term behaviour and the interaction between closed-loop geothermal systems. The investigated site consists of four boreholes equipped with geothermal pipes on the campus of University of Liege, Belgium. The first approach includes acoustic borehole imaging, gamma-ray logging and cuttings observation and results to a detailed fracture characterisation, rock identification and layer dip angle determination. The second approach consists of measuring the thermal conductivity of cuttings at the laboratory. Study of cuttings thermal conductivity measurements can contribute to bedrock heterogeneity knowledge concerning the transition of one formation to another and the layer dipping. The third approach is based on high-resolution temperature profiles, measured during the hardening of the grouting material and the recovery phase of a Distributed Thermal Response Test. Through this approach a correlation of the temperature profiles to the geological characteristics of the surrounding bedrock is identified. The analysis of this correlation can provide information on fractured zones, alternation of different rock types and layering dipping. This latter approach can be easily applied on closed-loop geothermal systems to characterise the bedrock and investigate its heterogeneity as well as contribute to the their long-term behaviour prediction and to the optimisation of their efficiency.

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.

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.

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.

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.

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.

Development of a new acoustic borehole imaging tool : Sci Drilling V2, N4, 1991, P203–214

Development of a new acoustic borehole imaging tool : Sci Drilling V2, N4, 1991, P203–214

New Developments in Acoustic Borehole Imaging: ABSTRACT

New Developments in Acoustic Borehole Imaging: ABSTRACT