Electrokinetics Of A Fluid-Saturated Rock Sample: Laboratory Experiments

The conversion between seismic and electromagnetic energies was discovered in a fluidfilled porous formation. When seismic waves propagate through a fluid-saturated porous formation, relative motion between the pore fluid and the solid matrix is generated and cation motion in the fluid is formed. The streaming electric current induces electromagnetic waves in the formation. There is an opposite phenomenon, i.e., the conversion of electric energy into acoustic energy in the porous formation. The electroseismics in porous sandstone samples are investigated by ultrasonic experiments in our laboratory. A compressional or a shear transducer excites an acoustic wave in the water-saturated sample and the electric signals generated on the surface are measured by an electrode. The relationship between the electric potential and acoustic wave or the conductivity of water-saturated rocks is studied. The electro-seismic conversion in rock samples is also investigated. Electroseismics could provide an effective means for estimating the fluid-saturated porous formation.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ERI4322

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

Experimental Studies Of Electrokinetic Conversions In A Fluid-Saturated Porous Medium

The electrokinetic effect in a fluid-saturated porous rock is defined as the coupling and conversion between seismic and electric energies. When seismic waves propagate through a fluid-saturated formation and cause a pore fluid-flow relative to the solid matrix, the motion of the cations in the fluid- flow forms a streaming electrical current and induces an electromagnetic wave at any discontinuous interface of the formation or stationary electric potential inside the homogeneous formation. Another conversion of energies opposite to the seismoelectric conversion is when an alternating electric field induces a relative fluid-flow in a fluid-saturated porous rock where fluid-flow can generate an electroseismic wave in the rock. In this paper we study the electrokinetics in porous sandstone and man-made porous models at high frequencies. A P-wave or S-wave transducer excites different acoustic wave modes in a cylinder, layer, or borehole model. Our experiments observe and record the radial or stationary seismoelectric signals induced at the interface or inside the formation. Some relative experiments have confirmed the reliability of the electrokinetic phenomenon observed in our experiments and the mechanism that is different from the piezoelectric effect. The results show that the seismoelectric signal induced by the extensional or flexural wave in the sandstone cylinder is a stationary local electric potential. The seismoelectric signal induced at the interface of the layer model is an electromagnetic wave which can be received within the fluid-filled porous medium. Experimental measurements performed in a borehole model by means of vertical seismic profiling (VSP) and single borehole logging show it is possible to conduct seismoelectric measurements in a deep borehole of petroleum formation. Measurements of electrokinetics can thus provide an effective means for estimating parameters in a fluid-saturated porous formation.Massachusetts Institute of Technology. Borehole Acoustics and Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-93ER14322

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

Experimental studies of streaming potential and high frequency seismoelectric conversion in porous samples

Streaming potential across a porous medium is induced by a fluid flow due to an electric double layer between a solid and a fluid. When an acoustic wave propagates through a porous medium, the wave pressure generates a relative movement between the solid and the fluid. The moving charge in the fluid induces an electric field due to the seismoelectric conversion. In order to investigate the streaming potential and the seismoelectric conversion in the same rock sample, we conduct quantitative measurements with cylindrical and plate samples of Berea sandstone 500 saturated by NaCl solutions with different conductivities. We measure the electric voltage (streaming potential) across a cylinder sample in solutions with different conductivities and under different pressures. In a solution container, we measure the seismoelectric signals induced by acoustic waves at different frequencies and solution conductivities. We calculate the quantitative coupling coefficients of the seismoelectric conversion at DC and high frequencies with samples saturated by solutions with different conductivities. According to the streaming potentials, we calculate the theoretical coupling coefficients at the DC and high frequency range. The experimental and theoretical results are compared quantitatively and their differences are discussed.Massachusetts Institute of Technology. Earth Resources Laborator

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

Electroseismic and Seismoelectric Measurements of Rock Samples in a Water Tank

An electromagnetic wave or a seismic wave can induce seismic or electric waves due to the electrokinetic conversion based on the electric double layer in a fluid-saturated porous medium. In this paper, we observe the acoustic fields generated around the electrodes excited by an electric pulse in a water tank. The electroseismic or seismoelectric waves are measured in the water tank system to confirm that the recorded seismic or electric signals are induced in porous samples due to the electrokinetic conversions. The electroseismic and seismoelectric frequency-responses in Berea sandstone and Westerly granite samples are measured at frequency range of 15 kHz to 150 kHz. The experimental measurements show that the effects of the electric source, background noises, and electroseismic or seismoelectric conversions are separated very clearly. We calculate the electroseismic and seismoelectric normalized coupling coefficients in the rock samples and compare them with the theoretical calculation. The variation trends of the normalized coupling coefficients are in agreement with the theoretical predictions. The measurement method in this paper could be used to investigate other electroseismic and seismoelectric properties for petroleum exploration applications.Massachusetts Institute of Technology. Earth Resources LaboratoryMassachusetts Institute of Technology. Borehole Acoustics and Logging Consortiu