Seismoelectric Laboratory Measurements In A Borehole

The seismoelectric logging method is based on measuring the electric field generated by seismic waves in a fluid-filled borehole. Two kinds of electromagnetic (EM) fields can be generated within the formation and at the interface of formations. One is a stationary or local EM wave and the other is a radiating EM wave. In this paper, we make various fractured borehole models with artificial materials or natural rocks and measure the electric field generated by a seismic source in a water-filled borehole. The experimental results show that the Stoneley wave generates both a stationary EM wave at the borehole wall and a radiating EM wave on the fracture, which propagates with light speed in the borehole. When the aperture of the fracture increases, the amplitude of the seismoelectric wave decreases due to the low ion concentration in the fracture. In a layered borehole model, a thin, permeable glued-sand zone is sandwiched between two nonpermeable or low-permeable layers, and the Stoneley wave generates two kinds of seismoelectric signals at the permeable zone. Compared with the acoustic waveforms in the same borehole, the seismoelectric waveforms are more effective in determining and characterizing a fracture or a fractured zone filled with a permeable layer.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation ConsortiumUnited States. Dept. of Energy (Grant DE-FG0293ERl4322

Seismoelectric And Seismomagnetic Measurements In Fractured Borehole Models

Seismoelectric and seismomagnetic fields generated by acoustic waves in fluid-saturated fractured borehole models are experimentally investigated with an electrode and a Halleffect sensor. In a borehole with a horizontal fracture, the Stoneley and flexural waves induce seismoelectric and seismomagnetic fields on the borehole wall and an electromagnetic wave propagating with light speed at the horizontal fracture. In a borehole with a vertical fracture, the acoustic field generated by a monopole or dipole source is similar to that in a borehole without a vertical fracture. However, the acoustic wave propagating along the vertical fracture induces seismoelectric and seismomagnetic fields, whose apparent velocities are equal to that. of a Stoneley wave. Experimental results show that two different kinds of electric and magnetic fields are generated by acoustic waves in borehole models with horizontal and/or vertical fractures. One is an electromagnetic wave propagating with light speed. The second is a stationary or localized seismoelectric and seismomagnetic field. Seismoelectric and seismomagnetic measurements might be a new logging technique for exploring fractures in a borehole.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumMassachusetts Institute of Technology. Earth Resources Laboratory. Reservoir Delineation Consortiu

Inversion of Shear Wave Anisotropic Parameters in Strongly Anisotropic Formations

Deepwater reservoirs use highly deviated wells to reduce cost and enhance hydrocarbon recovery. Due to the strong anisotropic nature of many of the marine sediments, anisotropic seismic imaging and interpretation can improve reservoir characterization. Sonic logs acquired in these wells are strongly dependent on well deviations. Cross-dipole sonic logging provides apparent shear wave anisotropy in deviated wells, which can be far from the truth. Although anisotropic parameters have been successfully obtained using data from wells of several deviations or using single well data based on weak anisotropy approximation, estimation of strong shear wave anisotropy from single well data remains a challenge. Using sensitivity analysis, we find Stoneley wave velocity has good sensitivity to qSV and SH wave velocities in deviated wells. We create a linear inversion scheme to estimate shear wave anisotropy using SH, SV, and Stoneley wave velocities logged in one well. We first apply the method to laboratory measurements from boreholes of various deviations relative to the symmetry axis of an anisotropic material. We then apply the method to a field data set acquired in a deviated well. We also compute the vertical and horizontal shear wave velocity logs in this well using the inverted elastic shear wave constants.Massachusetts Institute of Technology. Earth Resources Laborator

Sonic Logging in Deviated Boreholes in an Anisotropic Formation: Laboratory Study

Deepwater field development requires drilling of deviated or horizontal wells. Most formations encountered can be highly anisotropic and P- and S-wave velocities vary with propagation directions. Sonic logs acquired in these wells need to be corrected before they can be applied in formation evaluation and seismic applications. In this study, we make use of a laboratory model made of an approximate transversely isotropic Phenolite to study acoustic logging in deviated wells. We drill holes at various deviations relative to the symmetry axis in the Phenolite block. Then we perform monopole and dipole sonic measurements in these holes and extract the qP, qSV, SH, and Stoneley wave velocities using the slowness-time domain semblance method. The velocities measured using monopole and dipole loggings vary with borehole deviations. We also measure the qP, qSV, and SH wave velocities using body waves at the same angles as the well deviations. We then compute the theoretical qP, qSV, SH, and Stoneley wave velocities based on an equivalent transverse isotropic model of the Phenolite. We find the qP, qSV , and SH wave velocities obtained using the body wave measurement and acoustic logging method agree with the theoretical predictions. The Stoneley wave velocities predicted by the theory also agree reasonably well with the logging measurements.Massachusetts Institute of Technology. Earth Resources LaboratoryMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Multipole seismoelectric logging while drilling (LWD) for acoustic velocity measurements

In seismoelectric well logging, an acoustic wave propagates along a borehole and induces electrical signals along the borehole wall. The apparent velocities of these seismoelectric signals are equal to the formation velocities. Laboratory scale-model multipole acoustic and seismoelectric LWD tools are built to conduct measurements in a borehole drilled into a sandstone formation. The tools include either an acoustic receiver array of an electrode receiver array along with four acoustic sources to allow the generation of monopole, dipole, and quadrupole modes. Results show that the standard acoustic measurement of formation velocities are impacted by strong tool wave contamination in most situations. However, because the propagating tool waves do not induce any electrical signals, the seismoelectric measurements can provide a more robust velocity measurement. The multipole seismoelectric logging-while-drilling (LWD) could be used as a new logging method to measure the acoustic velocities of the borehole formations.Massachusetts Institute of Technology. Earth Resources Laboratory (Founding Member Consortium

Experimental and Theoretical Studies of Seismoelectric Effects in Boreholes

In a fluid-saturated porous formation, an impinging seismic wave induces fluid motion. The motion of fluid relative to the rock frame generates an electric streaming current. This current produces electric and magnetic fields, which are called seismoelectric and seismomagnetic fields, respectively. When there is a fracture or a discontinuity, a radiating electromagnetic wave is also generated, in addition to local fields. Seismoelectric and seismomagnetic fields depend on the amplitude, frequency, and mode of the seismic wave, as well as the formation porosity, permeability, pore size, and fluid conductivity. In this paper, we describe laboratory results of seismoelectric and seismomagnetic fields induced by an acoustic source in borehole models. We use a piezoelectric source for acoustic waves and a point electrode and a high-sensitivity Hall-effect transducer for measuring the localized seismoelectric and seismomagnetic fields in fluid-saturated rocks. The dependence of seismoelectric conversions on porosity, permeability and fluid conductivity are investigated. Three components of the seismomagnetic field are measured by the Hall-effect transducer. At a horizontal fracture, the acoustic wave induces a radiating electromagnetic wave.Massachusetts Institute of Technology. Earth Resources LaboratoryMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu

Polarization Of Flexural Waves In An Anisotropic Borehole Model

Two modes of flexural waves can be generated by a dipole source in an anisotropic borehole. Their velocities are related to those of the fast and slow shear waves in the formation. The particle motions and the polarization diagrams of the fast and slow flexural waves are measured in borehole models made of phenolite materials with transverse isotropy or orthorhombic anisotropy. The experimental results show that the particle motion of the fast flexural wave is linear and in the same direction as that of the fast shear wave in the formation. The polarization direction of the fast flexural wave coincides with that of the fast shear wave and is independent ofthe direction of the dipole source. The particle motion of the slow flexural wave is nonlinear and elliptic. Its polarization direction and variation are dependent on the anisotropic material and the source direction. This means that the slow flexural wave is a more complicated wave mode rather than the simple mode where the particle motion generated by a dipole source is in the direction of the slow shear wave. The polarization characteristics of the fast flexural wave can be applied to determine the principal axis of an anisotropic formation by in-line and cross-line logging data.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Contract DE-FG02-86ER13636