Erik H. Saenger

Laboratory‐based seismic attenuation in Fontainebleau sandstone: Evidence of squirt flow

At seismic frequencies (1–100 Hz), we studied attenuation in the laboratory using the forced oscillation method. We adopted the longitudinal mode of oscillation, which yields the Youngs modulus and the corresponding attenuation, here defined as the inverse quality factor. A Fontainebleau sandstone with a porosity of 8% and a permeability of 12 mD was saturated with different fluids and investigated at the confining pressures of 5, 10, and 15 MPa. At all the measured confining pressures, while attenuation was zero for the dry sample, for partial and full water saturation, it gradually increased from nearly zero to ~0.02 with increasing frequency. The sample was then fully saturated with glycerin-water mixtures of varying viscosities, up to that of glycerin (8, 92, 485, and 1414 cP). At the confining pressure of 5 MPa, a bell-shaped attenuation curve peaking at ~6 Hz with a magnitude of ~0.11 was observed when the sample was fully saturated with glycerin (1414 cP). A decrease in viscosity of the saturating fluid shifted the attenuation curve to higher frequencies, and an increase in confining pressure caused a decrease in the overall magnitude of attenuation. The data obtained for glycerin were compared to a simple squirt flow model with sufficient agreement, implying that squirt flow is the dominant mechanism responsible for the observed attenuation.

Analysis of high-resolution X-ray computed tomography images of Bentheim sandstone under elevated confining pressures

A sample of Bentheim sandstone was characterized using high-resolution threedimensional X-ray microscopy at two different confining pressures of 1 MPa and 20 MPa. The two recordings can be directly compared with each other because the same sample volume was imaged in either case. After image processing, a porosity reduction from 21.92% to 21.76% can be deduced from the segmented data. With voxel-based numerical simulation techniques, we determined apparent hydraulic transport properties and effective elastic properties. These results were compared with laboratory measurements using reference samples. Laboratory and computed volumes, as well as hydraulic transport properties, agree fairly well. To achieve a reasonable agreement for the effective elastic properties, we define pressure-dependent grain contact zones in addition to mineral phases in the digital rock images. From that, we derive a specific digital rock physics template resulting in a very good agreement between laboratory data and simulations. The digital rock physics template aims to contribute to a more standardized approach of X-ray computed tomography data analysis as a tool to determine and predict elastic rock properties.

Analysis of high resolution X-ray CT images of Bentheim Sandstone under elevated confining pressures

A sample of Bentheim sandstone was characterized using high-resolution threedimensional X-ray microscopy at two different confining pressures of 1 MPa and 20 MPa. The two recordings can be directly compared with each other because the same sample volume was imaged in either case. After image processing, a porosity reduction from 21.92% to 21.76% can be deduced from the segmented data. With voxel-based numerical simulation techniques, we determined apparent hydraulic transport properties and effective elastic properties. These results were compared with laboratory measurements using reference samples. Laboratory and computed volumes, as well as hydraulic transport properties, agree fairly well. To achieve a reasonable agreement for the effective elastic properties, we define pressure-dependent grain contact zones in addition to mineral phases in the digital rock images. From that, we derive a specific digital rock physics template resulting in a very good agreement between laboratory data and simulations. The digital rock physics template aims to contribute to a more standardized approach of X-ray computed tomography data analysis as a tool to determine and predict elastic rock properties.

Application of alternative digital rock physics methods in a real case study: a challenge between clean and cemented samples: Applying alternative DRP methods in a real case study

High-resolution three-dimensional images are used in digital rock physics to numerically compute rock physical properties such as permeability and elastic moduli. These images are not widely available, and their preparation is both expensive and time consuming. All of these issues highlight the importance of alternative digital rock physics methods that are based on two-dimensional images and use different approaches to compute effective properties of three-dimensional samples. In addition, the scale of study in both standard and alternative digital rock physics is very small, which applications of its results are questionable at wells or reservoir scale. The aim of this study is to use two-dimensional images and alternative digital rock physics techniques for computing seismic wave velocity and permeability, which are compared with well and laboratory data. For this purpose, data from one well in a reservoir located in the southwestern part of Iran are used. First, two clean (carbonate) and two cemented (limy sandstone) samples were collected from well cores at different depths. Then, two-dimensional images by scanning electron microscope and conventional microscope were captured. In the next step, two alternative digital rock physics methods, namely, empirical relations and conditional reconstruction, have been employed to compute P-wave velocity and permeability of a three-dimensional medium. Results showed that, in clean (mono-mineral) samples, velocity values were reasonably close to well data. However, permeability values are underestimated compared with laboratory data because laboratory data were obtained at ambient pressure, whereas alternative digital rock physics results are more representative of reservoir pressure conditions. Nevertheless, permeability–porosity trends are valid for both samples. In the case of cemented samples, a two-scale procedure, along with a method for two-scale computation and grain-cement segmentation, is presented and developed. Results showed that P-wave velocity is overestimated probably due to random sampling in this method. However, velocity–porosity trends are in agreement with well data. Moreover, permeability results obtained for cemented samples were also similar to those obtained for the clean samples.

Numerical simulations of passing seismic waves at the Larderello‐Travale Geothermal Field, Italy

Passing seismic waves released by large-magnitude earthquakes may affect geological systems located thousands of miles far from the epicenter. The M9.0 Tohoku earthquake struck on 11 March 2011 in Japan. We detected local seismic activity at the Larderello-Travale geothermal field, Italy, coinciding with the maximum amplitudes of the Rayleigh waves generated by the Tohoku earthquake. We suggest that the earthquakes were triggered by passing Rayleigh waves that induced locally a maximum vertical displacement of approximately 7.5 mm (for waves with period of 100 s). The estimated dynamic stress was about 8 kPa for a measured peak ground velocity of 0.8 mm/s. Previous similar observations pointed out local seismicity at the Larderello-Travale Geothermal Field triggered by the 2012 Mw5.9 Po Plain earthquake. We conducted forward numerical modeling to investigate the effects caused by passing P, S, Love, and Rayleigh waves through the known velocity structure of the geothermal field. Results indicate that maximum displacements focus differently when considering body or surface waves, with displacement values being higher within the first 2 km of depth. The focusing of the displacement below 3 km seems to be strongly controlled by the velocity structure of the Larderello-Travale geothermal field. We propose that seismic activity triggered by passing seismic waves may be related to a clock-advancing mechanism for local seismic events that may have occurred in any case. Furthermore, our analysis shows that local anisotropies in the velocity structure of the Larderello-Travale geothermal field (possibly linked to compartments of elevated pore pressures) strongly control the reactivation of regions of the geothermal field affected by passing seismic waves.

Defect mapping in pipes by ultrasonic wavefield cross-correlation: A synthetic verification

HighlightsWe introduce the wavefield cross‐correlation imaging condition to nondestructive testing of pipes.The full wave‐equation based imaging does not require event picking and mode separation of the measured signals.The imaging result is presented in terms of a 3D reflectivity map, and flaws of various sizes are simultaneously detected.Implementation details and various test cases are discussed. ABSTRACT This work presents a reverse‐time imaging technique by cross‐correlating the forward wavefield with the reverse wavefield for the detection, localization, and sizing of defects in pipelines. The presented technique allows to capture the wavefield reflectivity at the places of ultrasonic wave scattering and reflections. Thus, the method is suitable for detecting pipe defects of either point‐like or finite‐size types using data from a pulse‐echo setup. By using synthetic data generated by 3D spectral element pipe models, we show that the 3D wavefield cross‐correlation imaging is capable in the case of cylindrical guided ultrasonic waves. With a ring setup of transducers, we analyze the imaging results obtained from the synthetic single‐transducer and all‐transducer firings. The presented pipe flaw imaging method is straightforward to carry out using a suitable wave equation solver. Also, the method does not suffer from long iterative runs and numerical convergence issues commonly connected with imaging methods based on either deterministic optimization or statistical inference. The imaging procedure can be fully baseline‐free by performing data processing to remove direct arrivals from the ultrasound data.

Obtaining reliable localizations with Time Reverse Imaging: limits to array design, velocity models and signal-to-noise ratios

Time reverse imaging (TRI) is evolving into a standard technique for locating and characterising seismic events. In recent years, TRI has been employed for a wide range of applications from the lab scale, to the field scale and up to the global scale. No identification of events or their onset times is necessary when locating events with TRI; therefore, it is especially suited for locating quasi-simultaneous events and events with a low signal-to-noise ratio. However, in contrast to more regularly applied localisation methods, the prerequisites for applying TRI are not sufficiently known. To investigate the significance of station distributions, complex velocity models and signal-to-noise ratios with respect to location accuracy, numerous simulations were performed using a finite difference code to propagate elastic waves through three-dimensional models. Synthetic seismograms were reversed in time and reinserted into the model. The time-reversed wave field back propagates through the model and, in theory, focuses at the source location. This focusing was visualised using imaging conditions. Additionally, artificial focusing spots were removed using an illumination map specific to the set-up. Successful locations were sorted into four categories depending on their reliability. Consequently, individual simulation set-ups could be evaluated by their ability to produce reliable source locations. Optimal inter-station distances, minimum apertures, relations between the array and source locations, heterogeneities of inter-station distances and the total number of stations were investigated for different source depths and source types. Additionally, the accuracy of the locations was analysed when using a complex velocity model or a low signalto-noise ratio. Finally, an array in southern California was investigated regarding its ability to locate seismic events at specific target depths while using the actual velocity model for that region. In addition, the success rate with recorded data was estimated. Knowledge about the prerequisites for using TRI enables the estimation of success rates for a given problem. Furthermore, it reduces the time needed to adjust stations to achieve more reliable locations and provides a foundation for designing arrays for applying TRI.

Analysis of high-resolution X-ray computed tomography images of Bentheim sandstone under elevated confining pressures: High-resolution X-ray CT images

A sample of Bentheim sandstone was characterized using high-resolution threedimensional X-ray microscopy at two different confining pressures of 1 MPa and 20 MPa. The two recordings can be directly compared with each other because the same sample volume was imaged in either case. After image processing, a porosity reduction from 21.92% to 21.76% can be deduced from the segmented data. With voxel-based numerical simulation techniques, we determined apparent hydraulic transport properties and effective elastic properties. These results were compared with laboratory measurements using reference samples. Laboratory and computed volumes, as well as hydraulic transport properties, agree fairly well. To achieve a reasonable agreement for the effective elastic properties, we define pressure-dependent grain contact zones in addition to mineral phases in the digital rock images. From that, we derive a specific digital rock physics template resulting in a very good agreement between laboratory data and simulations. The digital rock physics template aims to contribute to a more standardized approach of X-ray computed tomography data analysis as a tool to determine and predict elastic rock properties.

Seismic Attenuation in Fontainebleau Sandstone Saturated with High-viscosity Fluids

Attenuation, corresponding to the anelastic loss caused by viscous dissipation in a rock, has been measured in the laboratory over the seismic frequency range (1-100 Hz) in a Fontainebleau sandstone sample at confining pressures of 5, 10 and 15 MPa. The sample has been completely saturated with liquids of viscosities varying from 1 cP (water) to 1414 cP (glycerine), the intermediate viscosities being obtained from a water-glycerine mixture. By increasing fluid viscosity, it has been observed that the magnitude of attenuation increases and its peak is shifted to lower frequencies. Moreover, the overall magnitude of the different attenuation curves decreases with increasing confining pressure. These results suggest that attenuation is dominated by a fluid flow mechanism, possibly, squirt flow.

Numerical estimation of Carbonate properties using a digital rock physics workflow

Digital rock physics combines modern imaging with advanced numerical simulations to analyze the physical properties of rocks -- In this paper we suggest a special segmentation procedure which is applied to a carbonate rock from Switzerland -- Starting point is a CTscan of a specimen of Hauptmuschelkalk -- The first step applied to the raw image data is a nonlocal mean filter -- We then apply different thresholds to identify pores and solid phases -- Because we are aware of a nonneglectable amount of unresolved microporosity we also define intermediate phases -- Based on this segmentation determine porositydependent values for the pwave velocity and for the permeability -- The porosity measured in the laboratory is then used to compare our numerical data with experimental data -- We observe a good agreement -- Future work includes an analytic validation to the numerical results of the pwave velocity upper bound, employing different filters for the image segmentation and using data with higher resolution