P-wave Logs Research Articles

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.

Neural Network Prediction of P-wave Log for Reservoir Characterization in the Niger Delta Basin

This study present a new technique that integrates several logs for P-wave prediction to minimize some errors and uncertainties associated with most estimation methods. The adopted method involves application of an artificial neural network technique that integrates density, resistivity and gamma ray logs for data training and the prediction of P-wave log. The results obtained gave correlation coefficient of 0.77, 0.24 and 0.42 between the acquired P-wave log and the acquired density, resistivity and gamma ray logs respectively, to demonstrate the relationship between P-wave log and the selected logs for the prediction process. The correlation coefficient of the estimated P-wave from Gardner and Faust methods with the acquired P-wave log are 0.64 and 0.59 respectively, while that of the neural network derived P-wave gave a better correlation coefficient of 0.81. Cross plot validation of P-wave derive Acoustic Impedance against density for both lithology and fluid discrimination revealed clusters for neural network derived P-wave parameter similar to the acquired P-wave derived parameters. Results of the presented neural network technique have been demonstrated to be more effective than results of the two conventional techniques.
 Keywords: Sonic log, Gardner’s method, Faust method, Neural network, Cross plot.

The statistical nature of the upper continental crystalline crust derived from in situ seismic measurements

SUMMARY In situ measurements in the main borehole of the German continental deep drilling project (KTB) point to the statistical nature of the continental crystalline crust between 286 and 6000 m depth. P-wave, S-wave and density logs display deviations from Gaussian distribution functions that may be partly due to the approximately bimodal nature of the lithology, because the crust consists mainly of metabasites and paragneisses. The autocovariance functions and power spectra of the logs indicate that the velocity and density perturbations have a complex statistical nature, but the autocovariance functions can be approximated by a superposition of two exponential functions with correlation lengths of 1 m and 20 m. The associated relative standard deviations for each exponential function are about 3 per cent of the mean for the sonic logs, and nearly 6 per cent of the mean for the density. A vertical seismic profile measured between 3000 and 6000 m depth shows small traveltime fluctuations, which can be explained by a generalization of a recently published scattering theory for transverse wavefield fluctuations to include the case of longitudinal fluctuations. The theory is valid for harmonic scalar plane-wave propagation in random media with isotropic statistics, and scattering mainly in the forward direction. One important theoretical result is that, although longitudinal and transverse autocovariance functions of amplitudes and phases differ, the expectations of their respective autovariances are the same. The observed traveltime fluctuations agree with theoretical predictions based on normally distributed velocity perturbations that are described by the average of the autocovariance functions of the two P-wave logs measured from 286 to 3000 m and between 3000 and 6000 m depth. Owing to the lack of traveltime information that can be averaged, and because the traveltime fluctuations predicted by the theory do not differ sufficiently for 2-D and 3-D random perturbations, no statement can be made on how anisotropic the observed random velocity perturbations are. The observed heterogeneities are, however, consistent with the fact that reflection seismic profiles reveal a nearly transparent upper crystalline crust.