Zhenya Zhu

Experimental studies of electrokinetic conversions in fluid-saturated borehole models

Experimental and theoretical studies show that there are electromagnetic (EM) fields generated by seismic waves with two kinds of conversion mechanisms in a fluid‐saturated, porous medium. Within a homogeneous formation, the seismic wave generates a seismoelectric field that exists only in the area disturbed by the seismic wave and whose apparent velocity is that of the seismic wave. At an interface between differing formation properties, the generated seismoelectric wave is a propagating EM wave that can be detected everywhere. An electrode, used as a receiver on the ground surface, can detect the propagating EM wave generated at an interface, but cannot detect the seismoelectric field generated in a homogeneous formation. When the electrode is in a borehole and close to a porous formation, it can detect both the EM waves and the seismoelectric field. In this paper, electrokinetic measurements are performed with borehole models made of natural rocks or artificial materials. Experimental results show that...

Seismoelectric and seismomagnetic measurements in fractured borehole models

Seismoelectric and seismomagnetic fields generated by seismic waves in fluid-saturated fractured borehole models are experimentally investigated with an electrode and a Hall-effect 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 at 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.

Experimental studies of seismoelectric conversions in fluid‐saturated porous media

When seismic waves generate a relative fluid-solid motion in a fluid-saturated porous medium, the moving charges (streaming current) in the electric double layer induce an electromagnetic (EM) field. This paper first experimentally confirms that the coupling between the seismic wave and the electromagnetic field in the kilohertz range is electrokinetic in nature. Seismoelectric signals are measured in homogeneous cylindrical porous rock samples and multilayered models. The seismoelectric signals in homogeneous rock are electric fields that move along with the acoustic wave. The mechanism of the seismoelectric conversion is completely different from the piezoelectric effect of quartz grains. The seismoelectric sensitivity with respect to salinity of the saturant has been experimentally determined. The amplitude of seismoelectric signals increases as the saturant conductivity decreases. The seismoelectric effects are generated by two different mechanisms. Both the EM radiation and the electric potential generated at an interface and within a porous medium, respectively, were measured as the P wave, at ultrasonic frequencies, passes through the layered models. Our experimental results demonstrate that seismoelectric effects exist and are measurable in the kilohertz range. The paper concludes with a comparison of experimental data and modeled data in a three-layer porous model. Seismoelectric measurements could be an effective means of obtaining transport coefficients such as hydraulic permeability and other porous rock properties.

Electroseismic and seismoelectric measurements of rock samples in a water tank

An electromagnetic (EM) or seismic wave can induce seismic or EM waves because of the electrokinetic conversion based on the electric double layer in a fluid-saturated porous medium. We tested a new method for observing electroseismic and seismoelectric conversions in rock samples. Our method is designed to overcome the shortcoming of previous attempts to separate signals generated by a continuous electric or seismic source in a small container. We first observed acoustic fields around electrodes excited by an electric pulse in a water tank or in a water-saturated porous sample. In our approach, we immersed rock samples in a water tank and measure the seismic or electric responses using electric or acoustic pulses conveyed through the immersed electrodes or hydrophone. We measured electroseismic- and seismoelectric-frequency responses in Berea Sandstone and Westerly Granite samples at a frequency range of 15–150 kHz . The measurements clearly separate the effects of the electric source, background noises,...

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...

Sonic logging in deviated boreholes penetrating an anisotropic formation : Laboratory study

Development of deepwater fields requires drilling deviated or horizontal wells. Many formations are highly anisotropic, that is, the P- and S-wave velocities vary with propagation direction. Sonic logs acquired in these wells need to be corrected for anisotropy effects before the logs can be used in formation evaluation and seismic applications. In this study, we use a laboratory model made of an orthorhombic Phenolite block to study acoustic logging in deviated wells. We first measure the qP-, qSV-, and SH-wave group velocities by using body waves at angles of 0°, 15°, 30°, 45°, 60°, 75°, and 90° relative to the slowest P-wave principal axis of the Phenolite block. We then drill holes at the same angles in the block. We record monopole and dipole sonic waveforms in these holes and extract the qP-, qSV-, SH-, and Stoneley-wave velocities by using the slowness-time semblance method. The velocities measured through the use of monopole logging and dipole logging vary with borehole deviations. We find that an equivalent transversely isotropic (TI) model can fit the measured qP-, qSV-, and Stoneley-wave velocities very well. The S-wave velocities at low to medium borehole deviations can be used to differentiate an orthorhombic material from a TI one.

Seismoelectric experimental data and modeling in porous layer models at ultrasonic frequencies

When seismic waves propagate through a fluid saturated sedimentary rock, the motion of the pore fluid relative to the solid matrix causes relative flow. A fluid electrolyte in contact with a solid forms an electric double layer. The diffuse distribution of mobile cations can move when the fluid moves. The seismic wave motion which generates the relative flow also induces a ‘streaming’ electrical current due to the cation motion. This induced streaming current acts as a current source in Maxwell’s equations. Therefore, when seismic waves travel through fluid saturated sedimentary materials, current systems are set up in the material inducing non-radiating fields. But when the seismic waves hit a contrast in electrical and or mechanical properties the current systems on both sides form a complex dynamic current system across the interface generating electromagnetic waves. In this paper we model this conversion behavior and compare the modeled results against laboratory measurements. Conversions generated by both S and P wave transducers are measured and compared against theoretical results.

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

Massachusetts Institute of Technology. Earth Resources Laboratory (Founding Member Consortium)

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.

Experimental Study of Flexural Waves In a Fractured Or Cased Borehole Model

The ultrasonic logging is performed with dipole transducers in aluminum and lucite borehole models to study the propagation of flexural waves in a fractured or cased borehole. The experimental results show that the flexural wave is much more sensitive to a horizontal fracture than to a vertical one. The propagation of flexural waves in a borehole with an inclined fracture is related to both the polarization of the flexural wave and the direction of the fracture. The experimental results show that a very strong low-frequency flexural wave can be generated by a dipole source in a cased borehole and that it propagates with the shear wave velocity of the formation. High-frequency waves generated by a dipole source propagate with the compressional wave and flexural wave velocities of the casing. Dipole acoustic well logging could be an effective means for determining horizontal and declined fractures and measuring the formation shear wave velocity in a cased borehole.