Borehole Sonic Logs Research Articles

Low Vp/Vs Values as an Indicator for Fractures in the Critical Zone

AbstractPoisson's ratio for earth materials is usually assumed to be positive (Vp/Vs > 1.4). However, this assumption may not be valid in the critical zone because near Earth's surface effective pressures are low (<1 MPa), porosity has a wide range (0%–60%), there are significant texture changes (e.g., unconsolidated vs. fractured media), and saturation ranges from 0% to 100%. We present P‐wave (Vp) and S‐wave (Vs) velocities from seismic refraction profiles collected in weathered crystalline environments in South Carolina and Wyoming. Our data show that ∼20% of the subsurface has negative Poisson's ratios (Vp/Vs values < 1.4), a conclusion supported by borehole sonic logs. The low Vp/Vs values are confined to the fractured bedrock and saprolite. Our data support the hypothesis that weathering‐generated microcracks can produce a negative Poisson's ratio and that Vp/Vs values can thus provide insight into important critical zone weathering processes.

A comprehensive approach to karst identification and groutability evaluation – A case study of the Dehou reservoir, SW China

Yunnan Province in SW China has one of the most developed karst terrains in China and has been historically affected by sinkholes and other instability phenomena caused by unpredictable cavity formation and well-developed underground hydraulic conduits. During construction of a karst seepage control reservoir project (grouting of a 4.6-km long anti-seepage curtain) in Yunnan province, some typical detection methods (electromagnetic and water injection tests) failed to map the complex geo-structural karst zones clearly, resulting in considerable grout loss and excessive leakage during the grouting phase of this project. To deal with this limitation, a comprehensive approach, including a new drilling process monitoring system, borehole electrical resistivity tomography, borehole sonic logging, and water injection tests, is employed to identify the general characterization, geometry, and spatial distribution of karst zones along the anti-seepage curtain. Additionally, a deep belief network (DBN) model replaced the empirical relationships typically used to predict the grouting quantity for groutability evaluation in karstified areas. The DBN algorithm is trained by data collected using this comprehensive approach (rate of penetration, rock permeability, resistivity, and P-wave velocities) and two previous grouting quantities as input data. The lessons learned in this case study permit us to define an effective comprehensive approach for delineating the general characterization of karst zones between two boreholes and locations of potential conduits for grout flow through the borehole. The proposed DBN model can predict the grouting quantity for karst regions, and the accuracy of prediction is greatly affected by previous grouting quantity rather than the collected parameters.

Elastic waves in porous media saturated with non-wetting fluid

Elastic waves have widely been used as a non-destructive probing method in oilfield exploration and development, and the most well-known applications are in seismic exploration and borehole sonic logging. For waves in porous media, it is popular to use the Biot theory, which incorporates the wave-induced global flow, accounting for the frictional attenuation. The Biot theory assumes that the fluid is wetting to the solid matrix. However, the fluid is not always wetting the rock in real reservoirs. It was previously revealed that a non-wetting fluid parcel tends to slip on the solid wall pore boundary where the intermolecular potential between the fluid and solid wall is weaker than in wetting fluid conditions. This particular slippage feature means that the coupling relationship between the fluid and solid frame and frictional dissipation is likely to be very different between non-wetting and wetting fluid situations. We characterise this wave-induced slippage using an apparent viscosity for the non-wetting fluid within the thin viscous boundary layer. This apparent viscosity is smaller than the viscosity of the bulk fluid. We demonstrate that the slip correction affects the dynamic permeability and dynamic tortuosity and results in slippage/wettability dependent phase velocities and attenuation of the fully fluid-saturated rock.

Interpretation of borehole sonic measurements acquired in vertical transversely isotropic formations penetrated by vertical wells

Detecting vertical transversely isotropic (VTI) formations and quantifying the magnitude of anisotropy are fundamental for describing organic mudrocks. Methods used to estimate stiffness coefficients of VTI formations often provide discontinuous or spatially averaged results over depth intervals where formation layers are thinner than the receiver aperture of acoustic tools. We have developed an inversion-based method to estimate stiffness coefficients of VTI formations that are continuous over the examined depth interval and that are mitigated for spatial averaging effects. To estimate the coefficients, we use logs of frequency-dependent compressional, Stoneley, and quadrupole/flexural modes measured with wireline or logging-while-drilling (LWD) instruments in vertical wells penetrating horizontal layers. First, we calculate the axial sensitivity functions of borehole sonic modes to stiffness coefficients; next, we use the sensitivity functions to estimate the stiffness coefficients of VTI layers sequentially from frequency-dependent borehole sonic logs. Because sonic logs exhibit spatial averaging effects, we deaverage the logs by calculating layer-by-layer slownesses of formations prior to estimating stiffness coefficients. The method is verified with synthetic models of homogeneous and thinly bedded formations constructed from field examples of organic mudrocks. Results consist of layer-by-layer estimates of [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]. We observe three sources of error in the estimated coefficients: (1) bias error originating from deaveraging the sonic logs prior to the sequential inversion, (2) error propagated during the sequential inversion, and (3) error associated with noisy slowness logs. We found that the relative bias and uncertainty of the estimated coefficients are largest for [Formula: see text] and [Formula: see text] because borehole modes exhibit low sensitivity to these two coefficients. The main advantage of our method is that it mitigates spatial averaging effects of sonic logs, while at the same time it detects the presence of anisotropic layers and yields continuous estimations of stiffness coefficients along the depth interval of interest.

Corrigendum to “Fracture detection in oil-based drilling mud using a combination of borehole image and sonic logs” [JMPG: 84 (June 2017); pages 195- 214

Corrigendum to “Fracture detection in oil-based drilling mud using a combination of borehole image and sonic logs” [JMPG: 84 (June 2017); pages 195- 214

Inversion-based method to mitigate noise in borehole sonic logs

A major challenge in the interpretation of seismic measurements and sonic logs is the presence of deleterious noise that impacts the quality and reliability of the estimated seismic wavelets and seismic inversion products. Spatial averaging effects and borehole drilling damage can also bias the estimation of in situ stress and elastic properties from sonic logs. We have developed an inversion-based method to mitigate processing errors, spatial averaging effects, and borehole environmental noise on sonic logs, which does not require arbitrary numerical filters, effective-medium theory models, or time-consuming waveform reprocessing. The inversion-based method estimates layer-by-layer slownesses via joint inversion of shear and compressional logs measured in a vertical well, and it uses the estimated slownesses of the assumed horizontal layers to model noise-mitigated sonic logs. By making use of geometric and physical constraints for noise reduction implicit in the inversion-based method, we obtain sonic logs that more accurately reflect the physical properties of rock formations penetrated by wells. Sonic logs are efficiently modeled by invoking axial sensitivity functions. First, we test the inversion-based method with synthetic sonic logs contaminated with noise. Estimated layer-by-layer slownesses agree with those of the original model within a standard deviation of [Formula: see text], while effectively reducing the numerical noise included in the input measurements. When bed-boundary locations are unknown, we perform the inversion-based method by assuming bed boundaries uniformly spaced at the same sampling interval of sonic logs; in this case, although the accuracy of the estimated layer slownesses decreases, the noise on sonic logs decreases. Then, we apply the method to sonic logs acquired in the North Sea and estimate angle reflectivity from the noise-mitigated logs. Results verify the reliability of the inversion-based method to reduce biases in the calculated angle reflectivity within a few minutes of central processing unit time.

Effective Media for Transversely Isotropic Models Based on Homogenization Theory: With Applications to Borehole Sonic Logs

Sonic log records, including measurements of wave speeds in boreholes, provide critical input to the geological, geophysical, and petrophysical studies of a region under exploration. 1D background models are routinely built based on sonic log records for applications such as seismic imaging of hydrocarbon reservoirs and microseismic source inversions. Smoothing or ‘upscaling’ techniques are required to produce models in coarser scales than the very fine layers in the raw log data. In this paper, we follow the recently popular homogenization theory, derive its application to the special case of 1D TI models for both P-SV and SH waves, and show that it is consistent with the Backus averaging technique commonly used to upscale 1D fine-layered models. We examine a study case of sonic log data from a well in the Horn River Basin in northeastern British Columbia, a region known for its tight shale-gas deposit. We demonstrate the computational accuracy and efficiency gained by proper upscaling procedures for spectral-element simulations of seismic wave propagation, and discuss the effect of control parameters on wavefield recovery.

Critical investigation of calculation methods for the elastic velocities in anisotropic ice polycrystals

Abstract. Crystallographic texture (or fabric) evolution with depth along ice cores can be evaluated using borehole sonic logging measurements. These measurements provide the velocities of elastic waves that depend on the ice polycrystal anisotropy, and they can further be related to the ice texture. To do so, elastic velocities need to be inverted from a modeling approach that relate elastic velocities to ice texture. So far, two different approaches can be found. A classical model is based on the effective medium theory; the velocities are derived from elastic wave propagation in a homogeneous medium characterized by an average elasticity tensor. Alternatively, a velocity averaging approach was used in the glaciology community that averages the velocities from a given population of single crystals with different orientations. In this paper, we show that the velocity averaging method is erroneous in the present context. This is demonstrated for the case of waves propagating along the clustering direction of a highly textured polycrystal, characterized by crystallographic c axes oriented along a single maximum (cluster). In this case, two different shear wave velocities are obtained while a unique velocity is theoretically expected. While making use of this velocity averaging method, reference work by Bennett (1968) does not end with such an unphysical result. We show that this is due to the use of erroneous expressions for the shear wave velocities in a single crystal, as the starting point of the averaging process. Because of the weak elastic anisotropy of ice single crystal, the inversion of the measured velocities requires accurate modeling approaches. We demonstrate here that the inversion method based on the effective medium theory provides physically based results and should therefore be favored.

Advanced methods for the evaluation of a hybrid-type unconventional play: The Bakken petroleum system

Hybrid-type plays such as the Bakken petroleum system (BPS) can be particularly challenging from an interpretation, completion, or production perspective due to the mix of conventional and unconventional elements coexisting within a relatively short depth interval. In the BPS, conventional aspects include the presence of separate reservoir intervals, which, depending on your location within the basin, may include the Scallion, Middle Bakken, Sanish, and Three Forks. Unconventional aspects include the Lower Bakken and Upper Bakken shales, which are organic-rich shales comprising source rock and reservoir. Developing an accurate petrophysical evaluation of these formations requires a priori knowledge of the mineralogy, fluids, and geomechanical properties such that appropriate logging measurements, core analysis methods, and interpretation techniques can be obtained and used. During the development phase of an oil field, the log and core measurements being acquired and the petrophysical evaluation being performed may vary significantly from well to well across the field. Some wells may have triple-combo wireline or logging-while-drilling measurements consisting of bulk density, neutron porosity, and induction or laterolog resistivity, supplemented with a total gamma ray measurement. Borehole sonic logs may also have been acquired in some wells primarily for seismic calibration, geomechanical modeling, and hydraulic stimulation design. If the “standard” suite of measurements and petrophysical evaluation being provided fail to accurately represent the true complexity of the formations being evaluated, the asset valuation will, in most cases, be negatively impacted. Our formation evaluation of the BPS led to the identification of unique petrophysical challenges for each of the formations comprising the BPS. Traditional formation evaluation methods were applied to the BPS based on triple-combo measurements, a traditional petrophysical analysis, and the evaluation of net feet of pay. Advanced evaluation methods and techniques were then applied to address the petrophysical complexities identified with core evaluation, advanced log measurements, and discrepancies between the two. New petrophysical models were developed and fine-tuned to address the shortcomings of the simple models, and the net feet of pay were reevaluated using these new models. The detailed formation evaluation program used to characterize the BPS consisted of standard triple-combo logs supplemented with advanced downhole measurements including: (1) triaxial resistivity for thin-bed analysis, (2) nuclear magnetic resonance for porosity, free-fluid, and kerogen identification, (3) dielectric dispersion for water saturation, (4) geochemical spectroscopy for mineralogy and total organic carbon, and (5) dipole sonic for dynamic rock properties. Petrophysical models were developed using deterministic and probabilistic methods to integrate the measurements acquired for the most accurate analysis of porosity, saturation, and mineralogy and to best describe the hydrocarbon production potential of the BPS.

Seismic properties and effects of hydrothermal alteration on Volcanogenic Massive Sulfide (VMS) deposits at the Lalor Lake in Manitoba, Canada

Borehole sonic and density logs are essential for mineral exploration at depth, but its limited availability to link rock properties of different ore forming geologic structure is a hindrance to seismic data interpretations. In situ density and velocity logs provide first order control on the reflectivity of various lithologic units. We analyzed borehole logs from 12 drill holes over and around the Lalor VMS deposits geographically located in the northern Manitoba, Canada, in an attempt to characterize lithologic units based on its seismic properties. The Lalor Lake deposit is part of the Paleoproterozoic Flin Flon Belt, and associated with an extensive hydrothermal alteration system. Volcanogenic Massive Sulfide (VMS) zones are distributed in several ore lenses with relatively shallower facies comprise solid to solid sulfides, tend to be disseminated or Stringer sulfides, while deeper lenses are gold and silver enriched and occurred in the highly altered footwall region. Our analysis suggests that massive sulfide and diorite have higher acoustic impedance than other rock units, and can produce useful reflection signatures in seismic data. Bivariate distributions of P-wave velocity, density, acoustic impedance and Poisson's ratio in end-member mineral cones were used for qualitative assessment of the extent of alteration of various lithologic units. It can be inferred that hydrothermal alteration has considerably increased P-wave velocity and density of altered argillite and felsic volcanic rocks in comparison to their corresponding unaltered facies. Amphibole, garnet, kyanite, pyrite, sphalerite and staurolite are the dominant end-member alteration minerals affecting seismic rock properties at the VMS site.

Robust Detection and Estimation for Logging While Drilling Monopole Acoustic Data

In this paper, we present methods and strategies for robust detection and slowness estimation of weak compressional (P) and shear (S) waves in borehole sonic logging in noisy environments such as while logging while drilling, by proposing methods for enhancing the semblance used in the slowness time coherence method used in the logging. In this direction our contributions are two fold. First, we propose a novel class of shrinkage estimators in the discrete Radon transform domain derived from data semblance, for waveform de-noising to combat random additive noise and reflections. In this context we also identify necessary and sufficient conditions for the optimality of the proposed estimator. Our second contribution lies in developing a novel method to cancel energy propagating at the slowness of borehole modes, chiefly Stoneley, and to a lesser extent, shear, that significantly lowers the semblance of the weaker head wave arrivals. This approach is based on representing the data using time-frequency compact space-time propagators. Finally, we present an algorithm that combines the space time shrinkage in the discrete Radon transform domain and the proposed interference cancelation strategies for enhanced compressional and shear detection via semblance processing. The algorithm is validated on synthetic data and demonstrated to enhance performance on real logging while drilling field data sets.

Introduction to this special section: Borehole geophysics and sonic logging

Sonic logs are most frequently used to tie surface and borehole seismic amplitude measurements to P- and S-wave velocities encountered along wellbore trajectories. Other common uses of sonic logs include the estimation of dynamic elastic properties for geomechanical analysis, such as wellbore stability studies and the design and planning of hydrofracturing operations. Modern sonic logs are acquired with multiple transmitters and an array of closely spaced receivers (anywhere between eight and 13 receivers) in the form of time waveforms. Sonic transmitters can come in the form of monopole, dipole, and quadrupole actuators. By design, sonic transmitters are immersed in a fluid — the borehole mud — whereby formation shear waves can be detected and quantified only by the elastic coupling that exists between wave motion taking place in the borehole fluid and in the surrounding rock formations. Such an elastic coupling gives rise to markedly frequency-dispersive behavior of the detected waves that presents some technical challenges when one interprets their speeds.

New constraints on an existing mineral resource through 3D seismic

SUMMARY The Kevitsa nickel-copper deposit in northern Finland is a large, low-grade, mafic-hosted accumulation of disseminated sulphides with rare, spatially restricted occurrences of net-textured to semi-massive sulphides. The mineable limits of the resource grow or shrink with commodity prices, but it has also been recognised that subtle mafic layering in the intrusion controls sub-horizontal layering of sulphides. The net-textured and semi-massive mineralisation styles occur near the base of the intrusion. Data from a 3D seismic survey demonstrate the unpredicted ability to image the sub-horizontal mafic layering, as well as the expected reflections at the base of the intrusion in contact with interlayered volcanic and sedimentary country rocks. The ability to trace the lateral extents of the mafic layering, backed up by analysis of borehole sonic and density logging, offers the possibility to predict the ultimate envelope of the resource. The interpretation of the base of the intrusion provides a horizon along which to target the net-textured to semi-massive contact mineralisation.

The crystal fabric of ice from full‐waveform borehole sonic logging

In an ice sheet, a preferred crystal orientation fabric affects deformation rates because ice crystals are strongly anisotropic: shear along the basal plane is significantly easier than shear perpendicular to the basal plane. The effect of fabric can be as important as temperature in defining deformation rates. Fabric is typically measured using analysis of thin sections under the microscope with co‐polarized light. Due to the time‐consuming and destructive nature of these measurements, however, it is difficult to capture the spatial variation in fabric necessary for evincing ice sheet flow patterns. Because an ice crystal is similarly elastically anisotropic, the speed of elastic waves through ice can be used as a proxy for quantify anisotropy. We use borehole sonic logging measurements and thin section data from Dome C, East Antarctica to define the relations between apparent fabric and borehole measured elastic speeds (compressionalVP and vertically polarized shear VSV). These relations, valid for single maximum fabrics, allow in‐situ, depth‐continuous fabric estimates of unimodal fabric strength from borehole sonic logging. We describe the single maximum fabric usinga1: the largest eigenvalue of the second‐order orientation tensor. For ice at −16°C anda1in the 0.7‐1 range the relations areVP = 248 a13.7 + 3755 m s−1 and VSV = −210a17.3 + 1968 m s−1.

Lateral continuity of basement seismic reflections in 15 Ma ultrafast-spreading crust at ODP Site 1256

The Ocean Drilling Program (ODP) initiated drilling at Site 1256D in the Guatemala Basin, about 1,000 km off the East Pacific Rise to penetrate plutonic rocks, anticipated to be relatively shallow in this region, formed at an ultra-fast spreading rate. IODP Expedition E312 successfully drilled into gabbros at ~1,150 m in basement. Multi-channel seismic traces show weak laterally coherent sub-basement reflections at borehole depths. Synthetic reflectivity seismograms were computed using a Ricker wavelet and impedance profiles from borehole sonic logs. These seismograms show significant sub-basement amplitude peaks. A zero-offset vertical seismic profile, shot on E312, was processed to investigate the authenticity of these reflections and their relationship to borehole geology. A dual scheme of the median filtering and F–K dip filtering was used. Tests with synthetic seismograms indicate the approach is effective at reasonable SNR levels. Downgoing energy is clearly identified but negligible upgoing energy is visible over random noise. These results indicate that lava flows and igneous contacts in upper ocean crust have significant topography on lateral scales less than the Fresnel Zone (~300 m) due to igneous and tectonic processes.

Sub-basalt seismic imaging using optical-to-acoustic model building and wave equation datuming processing

The presence of highly heterogeneous basalt layers has a detrimental effect on seismic imaging of underlying structures. Due to the prevalence of basalt, the challenge of imaging below basalts impinges on a broad spectrum of geological problems, from hydrocarbon exploration to volcano seismology. Both internal basalt characteristics and rough interfaces can degrade the sub-basalt image quality. In this paper we investigate the effects of both internal and surface heterogeneity on the wavefield. Using highly heterogeneous models of basalts derived from optical imagery and borehole sonic logs, combined with wave simulations to generate synthetic data, we explore the causes of poor sub-basalt imagery. We establish the significant role that surface roughness plays in the imagery problem and demonstrate, through a novel application of wave equation datuming, how to suppress its effect. The methodology demonstrated herein may be applicable in any environment where heterogeneous surface structures/interfaces severely degrade seismic image quality.

A comparative study of borehole size and tool effect on dispersion curves

Sonic wave dispersion characteristics are one of the most important targets of study, particularly in estimating shear wave velocity from borehole sonic logging. We have tested dispersion characteristics using monopole and dipole sources. Theoretical dispersion curves were computed for tool-absent and tool-included models having the same physical properties but different diameters (including Φ520 mm, Φ150 mm, and Φ76 mm). Comparisons were made between boreholes of different sizes and between tool-absent and tool-included models. Between the tool-included and the tool-absent boreholes, a close similarity in dispersion curve shape was revealed for the monopole source, and a significant difference was shown for the dipole source. However, for the cut-off frequency, particularly in the engineering boreholes (Φ76 mm and Φ50 mm), a significant difference was observed for signals from the monopole source, but approximately the same cut-off frequencies were found with the dipole source. This indicates the need of careful choice of source frequency in monopole-source sonic logging, particularly in an engineering borehole.The results of numerical experiments show that cut-off frequency is exponentially proportional to the inverse of borehole radius, irrespective of the mode type and the presence of a tool, and that the cut-off frequencies for each borehole environment could be expressed as an exponential function, rather than the inversely proportional relationship between the cut-off frequency and the borehole radius that was previously generally recognised. From the direct comparison of dispersion curves, the effects on the dispersion characteristics of borehole size and the presence of the tool can be revealed more clearly than in previous studies, which presented the dispersion curve and/or characteristics for each borehole environment separately.

Applications of the Boundary Absorption Using A Perfectly Matched Layer for Elastic Wave Simulation in Poroelstic Media

AbstractIn elastic wave numerical modeling for a porous medium, artificial boundaries should be treated so that they have minimum effects on the modeling results. In this paper, a high‐order velocity‐stress staggered‐grid finite‐difference scheme, with a perfectly matched layer (PML) absorbing boundary condition was proposed for simulating wave propagation in poroelastic media. The construction of the perfectly matched layer was discussed in detail, and the implementation of the high‐order finite‐difference scheme of the PML boundary conditions was also studied. The numerical results were validated by using analytical solutions in a homogeneous porous model. The performance of the PML for body waves with various incidence angles and various absorbing thicknesses was demonstrated. Also, free surface Raleigh waves were investigated for their absorptions. The PML was compared with two kinds of absorption boundary conditions to confirm the efficiency of the PML method. Our numerical results show that the PML can greatly absorb or reduce outgoing waves, not only for body waves, but also for surface waves. The PML method is one of the best absorption boundary conditions for porous elastic wave modeling. This work will play an important role in investigating elastic wave response of inhomogeneous porous media, especially in studying borehole sonic logging response of heterogeneous porous media.

In-situ stress dependence of wave velocities in reservoir and overburden rocks

Seismic waves propagate through a stressed earth. Sonic waves recorded by a borehole sonic logging tool propagate in the near-well environment—which is also stressed, albeit under different stress conditions than the untouched formation far away. Cores excavated from within the earth undergo stress release, and are then reloaded in the laboratory—but hardly ever to the complete, fully anisotropic in-situ state of stress and pore pressure. Based on this, we easily recognize that stress sensitivity affects our ability to compare and correlate geophysical and petrophysical measurements of wave velocities. On the other hand, it also provides an avenue to extract information about rock stresses. As an example, the presence of abnormally low velocities in seismic or log data is evidence of high pore pressure.

Anisotropy correction for deviated‐well sonic logs: Application to seismic well tie

Borehole sonic logs acquired in deviated wells penetrating the HRZ and Colville shales in the Niakuk field in Alaska's North Slope are seen to be significantly faster than vertical well logs. These differences are attributed to shale anisotropy. An iterative inversion scheme was created to invert for shale anisotropy parameters using multiple wells penetrating shale sections at different angles. The inversion involves fitting the sonic log data at a range of borehole angles to the compressional wave group velocity surface. The result is an estimate of the anisotropy parameters (ε and δ) and the vertical P‐wave velocity. The results show that the shales are strongly anisotropic, with compressional‐wave anisotropy (Thomsen's parameter ε) on the order of 40% and the anisotropy parameter δ (relates vertical velocity to short‐offset NMO velocity) around 10%. This large anisotropy can affect seismic imaging, AVO, and time–depth calculations.A procedure was created to estimate the anisotropy‐corrected vertical sonic logs from sonic data recorded in a deviated well. The inputs are well deviation, P‐wave sonic log, volume of shale or gamma ray data, and anisotropy parameters for rock with 100% shale volume. With these inputs the compressional‐wave group velocity surface is computed and the equivalent vertical P‐wave sonic log is output. The equivalent vertical sonic log can then be used for standard seismic applications using isotropic velocity assumptions. The application was applied to a well deviated at approximately 67°. Shale anisotropy parameters were taken from the sonic log inversion, and an anisotropy‐corrected sonic log was produced. Seismic well ties were attempted using both the recorded logs and the anisotropy‐corrected logs, with the result that the well tie using the measured logs was poor while a tie using the anisotropy‐corrected logs was good.