P-wave Velocity Logs Research Articles

P-Wave Velocity Log Simulation for Petroelastic Inversion Using Gardner’s Equation, Neural Networks And Ensemble Methods Based on Trees: A Case Study of the Santos Basin, Brazil

One of the main tools for reservoir characterization is analyzing well-log data. The importance of such methods stems from petrophysical properties estimation, such as porosity, which is very important to the oil and gas industry. In scenarios where data is hard to collect, data loss and technical failures during the acquisition impose an extra challenge. Thus mathematical and petrophysical models are good candidates to fill information gaps in the well-log dataset. In such a way, the rock’s petroelastic and petrophysical properties can be successfully estimated. Several studies correlate the velocity of compressional waves (VP) to other basic well data. In this study, we used the Gardner equation and Machine Learning methods such as Neural Networks, Random Forest and Gradient Boosting regressions to generate VPlogs. We used real-world data acquired from twenty wells of the pre-salt formation from Santos Basin in Brazil to train and test the Machine Learning methods and evaluated the data estimated by those models using statistical metrics. We calculated the acoustic impedance from the estimated logs and used it to create a prior model for a petroelastic inversion, which allowed us to estimate the natural logarithm of the acoustic impedance for a seismic volume. The Machine Learning methods presented lesser errors between estimated and measured velocities when compared to Gardner’s equation.

Evaluating uncertainty in empirically derived unconfined compressive strength (UCS) estimates and implications for drilling applications; a case study from the Cooper Basin

Accurate estimates of Unconfined Compressive Strength (UCS) are essential for a range of subsurface applications, including drilling wells for subsurface fluid extraction or injection. However, measuring UCS of subsurface rock samples through laboratory-based uniaxial and triaxial testing is time consuming, expensive, and potentially subject to individual sample variance. P-wave velocity logs are routinely obtained in petroleum basins and are commonly used to estimate UCS using empirically-derived correlations. In this study we analysed P-wave velocity data from 43 wells in the Cooper Basin, Australia and created mean ranges of expected UCS through the Tirrawarra, Toolachee, Patchawarra, Murteree Shale and Epsilon formations, using literature-derived P-wave velocity-UCS correlations, and also estimated UCS based on selected available laboratory-determined P-wave velocity measurements. These two suites of P-wave velocity-derived estimates of UCS are then compared to available results from laboratory uniaxial testing of core samples. Our analysis indicates that P-wave velocities, and subsequent derived estimates of UCS from P-wave wireline logs, are typically lower than those from laboratory-analysed samples. This may reflect a sampling bias towards the selection of strong rocks for laboratory UCS testing or lower estimates of UCS due to wellbore damage and pore space content (air or liquids) from P-wave log derived velocities. However, the laboratory-derived P-wave velocities generally agree with laboratory-derived UCS data from the Patchawarra, Murteree Shale and Epsilon formations when using published empirical velocity-strength correlations. A retrospective case study presents the impacts of different UCS estimates on mud weight required to produce observed borehole breakouts in the Epsilon Formation. Breakouts were observed 90 degrees to the orientation of SHmax (117° N) with a mean width of 51°, the breakouts were produced during drilling using a 9.7 ppg mud weight. The back-calculated estimate of UCS from the observed breakout widths at 9.7 ppg is 154 MPa. This sits between the UCS estimate of 171 MPa from uniaxial testing and 145 MPa from the mean of laboratory sample-derived P-wave velocities. Estimates of UCS from empirical correlations based solely on p-wave log-derived are far lower and vary between 87.3 and 114.4 MPa.

Active and Passive acoustic logging applied to the detection of preferential flow in a sedimentary aquifer

Two boreholes of an experimental site located in the Cher region (France) were investigated via Full Waveform Acoustic Logging (FWAL). The acoustic tool used for the FWAL experiments is a flexible monopole tool holding two pairs of piezoelectric receivers and a magnetostrictive transducer. The tool was modified to perform both active and passive FWAL. To our knowledge, this change is a novelty. For passive acoustic logging, several runs were recorded to obtain a set of acoustic noise sections from which noise Root Mean Squared (RMS) amplitude logs and spectral amplitude logs in different frequency bandwidths were computed. The acoustic logs resulting from passive acoustic monitoring were compared with P-wave acoustic velocity, core data, and a flowmeter log. It is shown that: (1) the distribution of noise frequencies in the 0–5 kHz is strongly correlated with the variations of the flowmeter, (2) the distribution of noise frequencies and noise RMS amplitude is correlated with the lithology (core description), and the P-wave velocity log. As the noise is simultaneously recorded by two receivers of the tool, an interference noise section was elaborated by correlating and summing pairs of acoustic traces at each depth. This procedure, which can be interpreted as an interferometry analysis, points out the presence of low-frequency waves identified as Stoneley waves. It is shown that the Stoneley wave velocity obtained in passive mode can be used to estimate the shear velocity of the formation.

Building a continuous velocity model from acoustic data and drilling measurements

On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. During the drilling, some parameters such as rate of penetration (ROP) and Torque have been continuously recorded. Full Waveform Acoustic logging (FWAL) and seismic experiments, such as refracted tomography, were conducted. A linear relationship between Torque-to-ROP ratio and acoustic velocity has been computed, in a root mean square sense, to obtain an estimated P-wave velocity log from drilling parameters. A specific procedure based on zoning process applied on acoustic data is used to force the torque to respect the trends of variation of the P-wave velocity. After calibration with acoustic velocity in the 30 – 192 m depth interval, and validation with tomographic velocity in the 0 – 12 m depth interval, drilling parameters allow a prediction of P-wave velocity from the surface up to the terminal depth of the borehole, with a 10% relative uncertainty. The acoustic velocity log from FWAL is by that way extended over the total heigh of the borehole.

Heat flow and gas hydrate saturation estimates from Andaman Sea, India

Gas hydrate was recovered in the Andaman Sea along the eastern coast of the Andaman Islands during the India National Gas Hydrate Program (NGHP) Expedition-01 at Site NGHP-01-17. Coring confirmed gas hydrate occurs predominantly in discrete volcanic ash layers. Pore water chemistry, electrical resistivity and P-wave velocity logs are used to estimate gas hydrate saturations at Site NGHP-01-17. Gas hydrate saturation estimated from chloride concentrations shows values up to ∼85% of the pore space for distinct ash layers from ∼270 m below seafloor to the base of gas hydrate stability zone (BGHSZ). Gas hydrate saturations estimated from the electrical resistivity and acoustic velocity logs using standard empirical relations and modeling approaches are comparable to each other, but saturations are only ∼20% of the pore space on average. This much lower gas hydrate saturation estimate from the log data is a result of overall reduced resolution of the logging tools relative to the typically 20–30 cm thick hydrate-bearing ash layers. Available 2D multi-channel seismic data were also analyzed and a bottom-simulating reflector (BSR) was imaged along several seismic profiles. The depth of the BSR is more than 600 m along the seismic line crossing Site NGHP-01-17, which makes this one of the deepest BSRs observed worldwide. To understand the unusual depth of the BSR, we mapped its depth and estimated heat flow from the BSR depth using a simple conductive model. BSR-derived heat flow values range from ∼12 to ∼41.5 mW/m2 from the study area and follow the bathymetry trend of dominant North–South ridges and can be explained with the east-ward trending increase in heat-flow toward the current seafloor spreading center. We also modeled the BGHSZ to analyze the linkage between gas hydrate occurrences in the Andaman Sea and its relation to the tectonic activity. Our analysis suggests an extensively variable BGHSZ in the Andaman Sea controlled mainly by overall low geothermal gradients. Consistent local minor variations were observed with lower heat flow values over prominent topographic highs and higher values in valleys/troughs due to focusing and defocusing effects of the topography.

Characteristics and interpretation of fracture-filled gas hydrate – An example from the Ulleung Basin, East Sea of Korea

Through the use of 2-D and 3-D seismic data, a total of thirteen sites were selected and drilled in the East Sea of Korea in 2010. A suite of logging-while-drilling (LWD) logs was acquired at each site. LWD logs from the UBGH2-3A well indicate significant gas hydrate in clay-bearing sediments including several zones with massive gas hydrate with a bulk density less than 1.0 g/m3 for depths between 5 and 103 m below the sea floor. The UBGH2-3A well was drilled on a seismically identified chimney structure with a mound feature at the sea floor. Average gas hydrate saturations estimated from the isotropic analysis of ring resistivity and P-wave velocity logs are 80 ± 13% and 47 ± 16%, respectively, whereas they are 46 ± 17% and 45 ± 16%, respectively from the anisotropic analysis. Modeling indicates that the upper part of chimney (between 5 and 45 m below sea floor [mbsf]) is characterized by gas hydrate filling near horizontal fractures (7° dip) and the lower part of chimney (between 45 and 103 mbsf) is characterized by gas hydrate filling high angle fractures on the basis of ring resistivity and P-wave velocity. The anisotropic analysis using P40H resistivity (phase shift resistivity at 32 mHz with 40 inch spacing) and the P-wave velocity yields a gas hydrate saturation of 46 ± 15% and 46 ± 15% respectively, similar to those estimated using ring resistivity and P-wave velocity, but with quite different fracture dip angles. Differences in vertical resolution, depth of investigation, and a finite fracture dimension relative to the tool separation appear to contribute to this discrepancy. Forward modeling of anisotropic resistivity and velocity are essential to identify gas hydrate in fractures and to estimate accurate gas hydrate amounts.

Pore- and fracture-filling gas hydrate reservoirs in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Green Canyon 955 H well

High-quality logging-while-drilling (LWD) downhole logs were acquired in seven wells drilled during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II in the spring of 2009. Well logs obtained in one of the wells, the Green Canyon Block 955 H well (GC955-H), indicate that a 27.4-m thick zone at the depth of 428 m below sea floor (mbsf; 1404 feet below sea floor (fbsf)) contains gas hydrate within sand with average gas hydrate saturations estimated at 60% from the compressional-wave (P-wave) velocity and 65% (locally more than 80%) from resistivity logs if the gas hydrate is assumed to be uniformly distributed in this mostly sand-rich section. Similar analysis, however, of log data from a shallow clay-rich interval between 183 and 366 mbsf (600 and 1200 fbsf) yielded average gas hydrate saturations of about 20% from the resistivity log (locally 50−60%) and negligible amounts of gas hydrate from the P-wave velocity logs. Differences in saturations estimated between resistivity and P-wave velocities within the upper clay-rich interval are caused by the nature of the gas hydrate occurrences. In the case of the shallow clay-rich interval, gas hydrate fills vertical (or high angle) fractures in rather than filling pore space in sands. In this study, isotropic and anisotropic resistivity and velocity models are used to analyze the occurrence of gas hydrate within both the clay-rich and sand dominated gas-hydrate-bearing reservoirs in the GC955-H well.

Characterization of geological formations by physical parameters obtained through full waveform acoustic logging

In this paper, we will show through a field example that full wave form acoustic logging allows a quantitative evaluation of geological formations. For that purpose, conventional logs and their associated standard deviation (Std) must be computed (formation velocities, amplitudes, frequencies, etc.) since the Std is used to estimate the uncertainties associated with the log and to edit other logs. The missing values are then reconstructed by geostatistical interpolation (ordinary kriging and co-kriging). The shear velocity and density of the formation are also estimated in order to obtain mechanical parameters such as Poisson’s ratio or shear modulus. Since the converted refracted shear waves can be recorded in fast formations, a joint method based on the local measurement of the shear velocity by picking the arrival times of the refracted S wave and interpolation by co-kriging with P-wave velocity log has been used to compute a continuous shear velocity log. The Analysis of the dispersive properties of the Stoneley modes has then been used to estimate density variations and build iteratively a density log from an a priori density model. Furthermore, we will show that a dimensionless shape index can be used as a qualitative acoustic attribute to detect the presence of interfering waves, anomalic zones and to obtain a measurement of the attenuation. We will also show that P-wave attenuation , P-wave frequency and acoustic porosity logs can be fruitfully used to compute an acoustic permeability log.

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient ( a), cementation factor ( m) and saturation exponent ( n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 ( S h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 ( S h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km 2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.

Exploration for gas hydrates in the deepwater, northern Gulf of Mexico: Part II. Model validation by drilling

This study examines the accuracy of the predictions of gas hydrate saturations made based on five-step analysis of 3D seismic data prior to 2005 drilling, logging, conventional coring, and pressure core sampling through the gas hydrate stability zone at two focus sites in the northern Gulf of Mexico. These predictions are detailed in Part I (2008). Here we conduct a detailed analysis of the gas hydrate saturation using both resistivity and P-wave velocity log data and analyze the pre-drilling predictions, which were made almost exclusively on the basis of seismic data, with no local logging control. Well log measurements, core data analysis, and pressure core-degas experiments all indicated general agreement with the pre-cruise analysis regarding the location and approximate concentration of gas hydrates in the sediments. We find that seismic predictions are generally consistent with log-based estimates after upscaling to seismic frequencies. We recalibrated the pre-drill model based on the new field data so that a refined version of the model could be used for future work.

Random Heterogeneous Model with Bimodal Velocity Distribution for Methane Hydrate Exploration

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing zones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data.This model is applied to two simulations of elastic wave propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering.We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing zones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.

Random heterogeneous model with bimodal velocity distribution for Methane Hydrate exploration

We have developed a random heterogeneous velocity model with bimodal distribution in methane hydrate-bearing zones. The P-wave well-log data have a von Karman type autocorrelation function and non-Gaussian distribution. The velocity histogram has two peaks separated by several hundred metres per second. A random heterogeneous medium with bimodal distribution is generated by mapping of a medium with a Gaussian probability distribution, yielded by the normal spectral-based generation method. By using an ellipsoidal autocorrelation function, the random medium also incorporates anisotropy of autocorrelation lengths. A simulated P-wave velocity log reproduces well the features of the field data.This model is applied to two simulations of elastic wave propagation. Synthetic reflection sections with source signals in two different frequency bands imply that the velocity fluctuation of the random model with bimodal distribution causes the frequency dependence of the Bottom Simulating Reflector (BSR) by affecting wave field scattering. A synthetic cross-well section suggests that the strong attenuation observed in field data might be caused by the extrinsic attenuation in scattering.We conclude that random heterogeneity with bimodal distribution is a key issue in modelling hydrate-bearing zones, and that it can explain the frequency dependence and scattering observed in seismic sections in such areas.

Mapping brittle fracture zones in three dimensions: high resolution traveltime seismic tomography in a granitic pluton

SUMMARY Fractured and altered zones within a granitic pluton are mapped in three dimensions by means of high resolution seismic traveltime tomography. The input traveltimes were picked from offset and azimuth variable vertical seismic profiles (OVSP) acquired in three boreholes and from seismic shot gathers of four CDP high resolution seismic reflection profiles recorded on the surface. For the OVSP data a hydrophone streamer placed in the boreholes recorded the acoustic energy generated (a signal with a frequency content between 15 to 150 Hz) by a Vibroseis truck at source points distributed every 30 m in a rectangular grid of 620 m by 150 m. The combination of borehole and surface seismic data resulted in an increase in the ray density of the shallow subsurface. The tomographic algorithm uses a variable model grid, with a finer grid spacing close to the surface were ray density is highest and the velocity variations are strongest. Therefore the resulting velocity models feature more detail at shallow levels. A simple and smooth starting velocity model was derived from P-wave velocity logs. Careful surface geological mapping, and borehole geophysical data, P- and S-wave velocity logs and Poisson’s ratio depth functions, provided key constraints for a physically reasonable 3-D interpretation of the tomograms. The low velocity anomalies constrained by the tomographic images were interpreted as unconsolidated rock, fractures and altered zones which correlate with structures mapped at the surface or velocity anomalies identified in the logs. Subsequent resolution analysis revealed that the derived velocity model is well constrained to depths of 60 m.

Scale and Angle Dependent Reflection Properties of Self-Similar Interfaces

Journal of Computational AcousticsVol. 09, No. 03, pp. 1015-1023 (2001) ICTCA'99 PapersNo AccessSCALE AND ANGLE DEPENDENT REFLECTION PROPERTIES OF SELF-SIMILAR INTERFACESJEROEN GOUDSWAARD and KEES WAPENAARJEROEN GOUDSWAARDDelft University of Technology, Centre for Technical Geoscience, P.O. Box 5028, 2600 GA Delft, The Netherlands Search for more papers by this author and KEES WAPENAARDelft University of Technology, Centre for Technical Geoscience, P.O. Box 5028, 2600 GA Delft, The Netherlands Search for more papers by this author https://doi.org/10.1142/S0218396X01000887Cited by:2 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail AbstractWe propose an alternative parameterization of seismic reflectors in the subsurface, in terms of self-similar singularities, which are generalizations of stepfunctions. This parameterization captures the multi-scale behavior of real sonic P-wave velocity logs, as can be derived by performing modulus maxima analysis on wavelet-transformed well-logs. Results on synthetic seismic reflection data, modeled in real well-logs, show that a singularity parameter can be retrieved, that is consistent with the parameter derived directly from the well-log.Presented at ICTCA'99, the 4th International Conference on Theoretical and Computational Acoustics, May 1999, Trieste, Italy. FiguresReferencesRelatedDetailsCited By 2Improved structural characterization of the Earth's crust at the German Continental Deep Drilling Site using advanced seismic imaging techniquesF. Hloušek, O. Hellwig and S. Buske8 October 2015 | Journal of Geophysical Research: Solid Earth, Vol. 120, No. 10The origin and nature of seismic reflections of sharp-based shoreface deposits (upper Jurassic Siliciclastics, northern France)H. Braaksma, G.G. Drijkoningen, N. Filippidou, J.A.M. Kenter and J.N. Proust1 Mar 2006 | Geophysical Prospecting, Vol. 54, No. 2 Recommended Vol. 09, No. 03 Metrics History Received 30 June 1999 Revised 6 March 2000 PDF download

SCALE AND ANGLE DEPENDENT REFLECTION PROPERTIES OF SELF-SIMILAR INTERFACES

We propose an alternative parameterization of seismic reflectors in the subsurface, in terms of self-similar singularities, which are generalizations of stepfunctions. This parameterization captures the multi-scale behavior of real sonic P-wave velocity logs, as can be derived by performing modulus maxima analysis on wavelet-transformed well-logs. Results on synthetic seismic reflection data, modeled in real well-logs, show that a singularity parameter can be retrieved, that is consistent with the parameter derived directly from the well-log.

Using P-wave velocity logs with petrofabric effects to map natural and blast-induced fractures in hard rocks

Abstract A challenging task in environmental geophysics is to locate fractures near a leaching stope in an underground mine. Existing methods for interpreting sonic logs do not incorporate petrofabric effects. The petrofabric effects are variations of P-wave velocities caused by textural variations in the lithology. This paper describes a new concept of using the petrofabric effects in the logs to determine anomalies of natural and blast-induced fractures in hard rocks. Full-waveform acoustic logs were acquired near an underground stope at the Colorado School of Mines Experimental Mine, Idaho Springs, Colorado. Data acquisition occurred once before the stope was blasted and twice after the blast event. Laboratory studies show that the petrofabric effects range from 4 to 15 percent. This variation depends on rock types. To interpret location of fractures, variation envelopes of petrofabric effects were placed in P-wave velocity logs. P-wave velocities that are lower than lower limits of the variation envelopes indicate natural and blast-induced fractures. Results show that the blasting broke the entire rock mass within 6 ft from the stope's perimeter. The use of petrofabric effect interpretation improves effectiveness of P-wave velocity logs in identifying fractures.

Seismic imaging of the site response using microearthquake recordings. Part II. Application to the Swabian Jura, southwest Germany, Seismic network

Abstract The subsurface site structure of the Swabian Jura, southwest Germany, seismic network has been modeled from the inversion of locally recorded SH wave microearthquake seismograms. The observed records have been deconvolved from the effects of apparent source pulse broadening due to the combined influence of frequency bandlimited source signals, absorption, peg leg multiple reflections, and the recording system following the procedure of Scherbaum and Stoll (1985). Corresponding pseudo-reflection seismograms were calculated based on the Kunetz-Claerbout relation for SH waves and inverted using a Levison recursion algorithm. The final structural models have been optimized in terms of numbers of layers and depth resolution using Ferber's (1985) noise stabilization technique. The resulting impedance models for the Swabian Jura, southwest Germany, seismic network show some strongly correlated reflectors in the uppermost layers, which are well understood in terms of the local stratigraphy. The comparison of the inverted impedance model HSN with the P-wave velocity log from a nearby exploration drilling (Trochtelfingen, BEB) shows a high degree of similarity of the dominating reflectors down to a depth of several hundred meters. As a consequence, the coda-generating process for the early part of the coda of the observed Swabian Jura microearthquake seismograms (Wood-Anderson magnitude MWA &amp;lt; 3.0) can be completely interpreted as an effect of the local subsurface structure.