G. Bernasconi

AVO and AVA inversion for fractured reservoir characterization

Seismic wave reflection amplitudes are used to detect fluids and fracture properties in reservoirs. This paper studies the characterization of a vertically fractured fluid‐filled reservoir by analyzing the reflection amplitudes of P‐waves with varying incident and azimuthal angles. The reservoir is modeled as a horizontal transversely isotropic medium embedded in an isotropic background, and the linearized P‐waves reflection coefficient are considered. The conditioning of the inverse problem is analyzed, and fracture density is found to be the best conditioned parameter. Using diffraction tomography under the Born approximation, an inversion procedure is proposed in the transformed k–ω domain to detect fracture density variations within the reservoir. Seismic data are rearranged in pairs of incident and reflected plane waves, enlightening only one spectral component of the fracture density field at a time. Only the observable spectral components are inverted. Moreover, working in the transformed domain, p...

Efficient data compression for seismic-while-drilling applications

Seismic-while-drilling services efficiently support drilling decisions. They use the vibrations produced by the drill-bit during perforation as a downhole seismic source. The seismic signal is recorded by sensors on the surface, and it is processed in order to obtain/update an image of the subsurface around the borehole. To improve the characterization of the source, some sensors have been experimentally installed also downhole, on the drill pipes in close proximity to the bit: data logged downhole have been able to give better quality information. Currently, the main drawback of downhole equipment is the absence of a high-bit-rate telemetry system to enable real-time activities. This problem may be solved by employing either an offline solution, with limited memory capacity up to few hundreds of megabytes, or an online solution with telemetry at a very low bit-rate (few bits per second). However, following the offline approach with standard acquisition parameters, the internal storage memory would be filled up in just a few hours at high acquisition rates. On the contrary, with the online solution, only a small portion of the acquired signals (or only alarm information about potentially dangerous events) can be transmitted in real time to the surface by using conventional mud-pulse telemetry. We present a lossy data compression algorithm based on a new representation of downhole data in angle domain, which is suitable for downhole implementation and may be successfully applied to both online and offline solutions. Numerical tests based on real field data achieve compression ratios up to 112:1 without major loss of information. This allows a significant increase in downhole time acquisition and in real-time information that can be transmitted through mud-pulse telemetry.

Advanced Pipeline Vibroacoustic Monitoring

Remote real-time monitoring of pipelines reliability is becoming a key factor for the environmental sustainability of oil&gas industry. Multipoint Acoustic Sensing (MAS) technology makes use of multi sensors placed at discrete distances to detect Third Party Interference (TPI) and fluid leaks along the pipeline. In fact, any interaction with the pipe generates pressure waves that are guided within the fluid (gas or oil) for long distances, carrying information on the source event. Pressure propagation is mainly governed by the absorption coefficient and the sound speed. These parameters are in turn complicated functions of the frequency, the geometrical and elastic parameters of the pipe shell, the elastic parameters of the surrounding medium, and the acoustic and thermodynamic properties of the transported fluid. We have analyzed these aspects while processing acoustic data collected on crude oil and gas transportation pipelines, in different operational and flow conditions. This study describes the acquisition campaigns and the data analysis steps used for the experimental derivation of fluid properties and pipe anomalies. The results are also used for the validation of mathematical models of pressure waves propagation in fluid filled pipes.Copyright

Acoustic Response of a Sinusoidally Perturbed Hard-Walled Duct

Acoustic wave propagation in hard-walled ducts is of interest in many fields including vehicle design, musical instruments acoustics, and architectural and environmental noise-control. For the case of small sinusoidal perturbation of the cross-section, it is possible to derive simple though approximate analytical formulas of its plane wave acoustic reflection and transmission spectral response that resembles the optical situation of uniform Bragg gratings. The proof is given here, starting from the “horn equation” and then exploiting the coupled-modes theory. Examples of the results obtained with these analytical formulas are shown for some sinusoidally perturbed ducts and compared to results obtained through a numerical method, revealing a very good agreement.

High‐quality compression of MWD signals

Measurement-while-drilling (MWD) services efficiently support drilling decisions and are emerging as a powerful option on well sites. Presently, the main drawbacks of MWD equipment are the low data bandwidth of standard telemetry systems (up to 10 bits/s), and the high cost of storage memory in the downhole tool. Both these limitations can be reduced using data compression. We investigate the application of a lossy compression algorithm based on the wavelet transform to data recorded downhole while drilling. The user can select the maximum level of noise in the reconstructed signal; the compression ratio is adapted automatically. Numerical tests based on real field data achieve compression ratios up to 15:1 without noticeable signal degradation. This allows a significant increase in downhole acquisition time and in real-time information transmission rate through the telemetry system. We present signal-quality indicators to analyze the performance of the compression. The algorithm is simple enough to be embedded in current-generation downhole equipment without major modifications. This implementation can also be achieved using specialized hardware, lowering the computational requirements and synchronization issues of the downhole main controller.

3-D traveltimes and amplitudes by gridded rays

Seismic imaging in three dimensions requires the calculation of traveltimes and amplitudes of a wave propagating through an elastic medium. They can be computed efficiently and accurately by integrating the eikonal equation on an elemental grid using finite‐difference methods. Unfortunately, this approach to solving the eikonal equation is potentially unstable unless the grid sampling steps satisfy stability conditions or wavefront tracking algorithms are used. We propose a new method for computing traveltimes and amplitudes in 3-D media that is simple, fast, unconditionally stable, and robust. Defining the slowness vector as p=(px,py,pz) and assuming an isotropic medium, the ray velocity v is related to the slowness vector by the relation v=p/(p⋅p). Rays emerging from gridpoints on a horizontal plane are propagated downward a single vertical grid step to a new horizontal plane. The components of the slowness vector are then interpolated to gridpoints on this next horizontal plane. This is termed regriddi...

Localization Algorithms for Search and Rescue Applications

Detection of microseismic noise generated by survivors trapped by debris is a method already used by S&R teams. Present S&R equipment work exclusively on energy analysis while ignore information carried by propagation delays. We explore the potential of traveltime analysis compared to energy analysis for 2D and 3D localization. Results obtained on a couple of debris fields used for training S&R teams demonstrate that traveltimes are not less reliable than energy. A joint analysis of both the signal parameters can be an appropriate strategy to improve the 2D localization reliability. Besides, traveltimes can potentially extend the localization to the third dimension by returning an approximate estimate of the survivor depth. Main obstacles to achieve this goal are the inhomogeneity of the debris pile, the need of a real-time response, the limited extension of the sensor array. Despite of these difficulties, the preliminary results obtained in the fields with two different algorithms, one based on statistical traveltime inversion, the other based on data focusing, are encouraging and show accuracy in the limit of the seismic resolution. Both the algorithms are flexible and can be customized to face the peculiar constraints posed by this application.

Seismic and geometrical attributes from image gathers

We examine migrated angle domain gathers to extract seismic and geometrical attributes of reflectors. We illustrate the procedure on a real 2D dataset: the angle domains we refer to are the illumination dip angle and the aperture scattering angle. We use the illumination dip angle gather to track the main reflectors and to create a map of the dip values. This information and the scattering aperture angle gather enables to improve AVA analysis and to build maps of seismic attributes. The procedure is automatic.

Linearized target-oriented inversion - Application to real data

We apply our linearized target oriented inversion scheme (Bernasconi et al., 1994) to the Mobil data set (Keys and Foster, 1995). The seismic line consists of 1001 shot records. Each shot record was recorded on 120 channels for six seconds. Two wells (well A and well B) intersect the seismic line. Well log data are available. We have selected a target zone located at a depth of 2 km, centred on CDP 808 (well A) where an increase in oil content is evident.(Fig 1). Processing is limited to a corresponding portion of the whole data set, ranging from CDP 660 to CDP 1130. It covers an horizontal range of 5875 m.

An Integrated Geophysical Approach for Sub Basalt Exploration

We discuss an integrated work flow applied to multidisciplinary data acquired at two different scales. First, we performed an integrated geophysical experiment (on land) at relatively small scale. We performed a multidisciplinary survey along a line running on a lava flow outcrop. Our objective was to acquire a seismic and electromagnetic data set in correspondence of a basalt outcrop overlaying a well known sedimentary sequence. Using this data set we tested an integrated interpretation work flow based on modelling and inversion of complementary geophysical data. The objective was to verify if and how seismic and electromagnetic data can be combined for obtaining an improved imaging of the basalt and sub basalt sequences. Then we applied our integrated work flow to a multidisciplinary data set acquired in a much larger geological context in offshore Norway.