Rama V. N. Rao

Spatial orientation and distribution of reservoir fractures from scattered seismic energy

Wepresentthedetailsofanewmethodfordeterminingthereflection and scattering characteristics of seismic energy from subsurface fractured formations. The method is based upon observations we have made from 3D finite-difference modeling of the reflected and scattered seismic energy over discrete systems of vertical fractures. Regularly spaced, discrete vertical fracture corridors impart a coda signature, which is a ringing tail of scatteredenergy,toanyseismicwaveswhicharetransmittedthrough or reflected off of them. This signature varies in amplitude and coherence as a function of several parameters including: 1 the difference in angle between the orientation of the fractures and the acquisition direction, 2 the fracture spacing, 3 the wavelength of the illuminating seismic energy, and 4 the compliance, or stiffness, of the fractures. This coda energy is most coherent when the acquisition direction is parallel to the strike of thefractures.Ithasthelargestamplitudewhentheseismicwavelengths are tuned to the fracture spacing, and when the fractures have low stiffness. Our method uses surface seismic reflection tracestoderiveatransferfunctionthatquantifiesthechangeinan apparent source wavelet before and after propagating through a fracturedinterval.Thetransferfunctionforanintervalwithnoor low amounts of scattering will be more spikelike and temporally compact. The transfer function for an interval with high scattering will ring and be less temporally compact. When a 3D survey is acquired with a full range of azimuths, the variation in the derived transfer functions allows us to identify subsurface areas with high fracturing and to determine the strike of those fractures.Wecalibratedthemethodwithmodeldataandthenapplied ittotheEmiliofieldwithafracturedreservoir.Themethodyielded results which agree with known field measurements and previously published fracture orientations derived from PS anisotropy.

Experimental studies of monopole, dipole, and quadrupole acoustic logging while drilling (LWD) with scaled borehole models

We develop a 1:12 scale model logging-while-drilling (LWD) acoustic tool for laboratory measurements in borehole models to investigate the effects of tool wave modes on our ability to determine formation velocities in acoustic LWD. The scaled tool is comprised of three sections: (1) the source section, consisting of four transducers mounted on the tool body that can generate monopole, dipole, and quadrupole waves; (2) the receiver section, consisting of six sets of receivers, each containing two transducers mounted on opposite sides of the tool center line; and (3) the connector section, a threaded steel cylinder that connects the source and receiver sections tightly to simulate an LWD tool. We use four borehole models to simulate fast and slow isotropic and anisotropic formations. The slow-formation models are constructed of synthetic material ( Lucite® for the isotropic case and Phenolite® for the anisotropic case). The fast-formation models are made from natural rock samples (sandstone for the isotropi...

Higher Order Modes in Acoustic Logging While Drilling

Summary In multipole acoustic logging while drilling (LWD), the fundamental modes dominate recorded waveforms. Higher order modes may also appear and complicate the processing of LWD data. In dipole LWD measurements, the dipole tool mode is often not well separated from the flexural mode. This makes the shear wave measurement more difficult. We conducted theoretical and numerical analysis on dipole LWD logging responses. We found that hexapole mode may be present in the dipole waveforms. Laboratory dipole data show the presence of hexapole mode, which asymptotes to the formation shear wave velocity. This observation supports our conclusion. We may make use of these higher order modes for accurate determination of formation shear wave velocity.

Fracture Spacing And Orientation Estimation From Spectral Analyses of Azimuth Stacks

Discrete, vertically aligned fracture systems impart one or more notches in the spectral ratios of stacked reflected seismic traces. This apparent attenuation is due to the azimuth dependant scattering introduced by the fractures. The most prominent notch is located at the frequency where the P wavelength is about twice the fracture spacing. The frequency location of the notches can be used to determine the fracture spacings. Azimuth stacks with an orientation parallel to the fractures tend not show these spectral notches – allowing for another way to detect the fracture orientation. An analysis of the vertical component of the 3D, ocean bottom cable seismic survey data from the Emilio field, offshore Italy, shows a prominent set of fractures with a spacing of about 30 to 40 meters with orientations that agree with previous studies.

An Experimental Study of Seismoelectric Signals In Logging While Drilling And Filtering Out of Tool Waves

Acoustic logging while drilling (LWD) may be complicated because of contamination of waves propagating along the drill collar (the tool waves). In this paper we propose a new method for separating tool waves from the true formation acoustic arrivals in borehole acoustic LWD. The method utilizes the seismoelectric signal induced by the acoustic wave at the fluid-formation boundary. The basis for seismoelectric conversion is the electric double layer (EDL) that exists in most rock-water systems. EDL does not exist at the tool (conductor) water interface. Therefore, there should be no seismoelectric signals due to tool modes. In this paper, borehole monopole and dipole LWD acoustic and seismoelectric phenomena are investigated with laboratory measurements. The main thrust of the paper is the utilization of the acoustic and seismoelectric signals to filter out the tool waves and enhance the formation acoustic signals in LWD.