Shihong Chi

Theoretical and Numerical Studies of Seismoelectric Conversions in Boreholes

We present theoretical and experimental studies on the effects of formation properties on seismoelectric conversions in fluid-filled boreholes. First, we derive the theoretical formulations for seismoelectric responses for an acoustic source in a bore- hole. Then, we compute the electric fields in boreholes penetrating formations with different permeability and porosity, and then we analyze the sensitivity of the con- verted electric fields to formation permeability and porosity. We also describe the lab- oratory results of the seismoelectric and seismomagnetic fields induced by an acous- tic source in borehole models to confirm our theoretical and numerical developments qualitatively. We use a piezoelectric transducer to generate acoustic waves and a point electrode to receive the localized seismoelectric fields in layered boreholes and the electric component of electromagnetic waves in a fractured borehole model. Numeri- cal results show that the magnitude ratio of the converted electric wave to the acoustic pressure increases with the porosity and permeability increases in both fast and slow formations. Furthermore, the converted electric signal is sensitive to the formation permeability for the same source frequency and formation porosity. Our experiments validate our theoretical results qualitatively. An acoustic wave at a fracture intersect- ing a borehole induces a radiating electromagnetic wave. AMS subject classifications: 65Y10, 76Q05, 76S05, 78A25, 86A25

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.

Comparison of Discrete Fracture and Effective Media Representation of Fractures on Azimuthal AVO

In fractured reservoir development, azimuthal AVO (AVOaz) properties of reflected PP waves from reservoir tops are often used to infer fracture properties. The fracture parameter inversion is based on either an effective media model (EMM) or a discrete fracture model (DFM). We address the differences in fracture properties that may be inferred by AVOaz based on the two models. For the DFM we focus on fractures whose length and spacing are comparable to the seismic wavelength. First, we compute the elastic parameters describing the fractured reservoir for each type of model. Then we synthesize seismic data using a finite-difference program for both sets of elastic parameters. By performing AVOaz analysis, we find that EMM and DFM predict different offsets for maximum AVOaz magnitudes. The DFM results show larger AVOaz magnitude with farther offsets, and phase changes at offsets larger than 35 degrees may indicate compliant fractures in a reservoir. For compliant fractures, the fracture strike determined using AVOaz effect based on the EMM is opposite to that from the DFM. This difference could cause incorrect estimation of fracture orientation if the EMM is used to interpret data from a reservoir with discrete fracture zones. DFM may be better suited for modeling wavelength-scale fractures.

F-K Domain Characteristics of the Seismic Response of a Set of Parallel Discrete Fractures

We model seismic wave propagation in a reservoir with discrete fracture zones using a finite difference scheme, which implements the Coates-Schoenberg formulation for fractured media. We study the variation of scattered energy in the direction perpendicular and parallel to the fracture strike. In the modeled data, we observe variations in the coherence of seismic energy and interference between backward and forward scattered energy. We then sorted data from the Emilio field in Italy in azimuthal gathers. These panels show a striking qualitative resemblance to the modeled data. We conclude that, in this case, a discrete representation of the fractures in the reservoir predicts the observation in the field data well. This supports the idea that fractures can cluster into fracture zones that scatter seismic energy. We then analyze the seismic energy on a profile in the direction perpendicular to the fracture strike. First we use estimated scattered energy in a window around the target zone to estimate the spacing between large fracture zones. The scattered energy in a later time window is shown to consist of mainly P to S scattered energy. For the estimation of smaller spacings, we rely on the smaller wavelength of these converted waves to illuminate finer structure. The result of spacing estimation is not very sensitive to the time window from which we estimate the scattered energy, because, in f-k domain, the wavenumber values of the dominant backscattered energy within successive time windows are almost the same, but frequency content drops gradually. Finally we apply this analysis to field data from Emilio Oil Field, and estimate a fracture spacing of about 40 m.

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.

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.

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

Deepwater field development requires drilling of deviated or horizontal wells. Most formations encountered can be highly anisotropic and Pand 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.