Electromagnetic Logging Research Articles

An approximate 3D inversion method for inversion of single-well induction-logging responses

Electromagnetic logging is a technique used to probe differences in electric conductivity around a measurement device. Electromagnetic logging while drilling can contribute to proactive geosteering to improve well placement in reservoirs because such measurements can serve as an indication for the structure of the surrounding geology. We have developed a novel way of predicting the 3D conductivity distribution around a drilling tool, based on the contrast-source inversion method, in which we have replaced the full integral-equation approach by the single spherical scatterer (SSS) approximation. The approximation took into account the dominant features of the diffusive electromagnetic field. This allowed for a substantial gain in computational speed and storage of the inversion method for reconstruction of the conductivity distribution. In view of the limited range of the electromagnetic probing, the overall reconstruction can be segmented in several local windows. This reduced the computational speed requirements and the storage requirements dramatically, while safeguarding the overall 3D character of reconstruction. We have synthesized 3D electromagnetic logging data using synthetic models and conductivity maps from a hydrocarbon North Sea reservoir model. Reconstructions were made for multiple source frequencies, and the results were compared with the results obtained from the Born approximation. We have observed that reconstruction based on the SSS approximation was superior to the one based on the Born approximation. Our algorithm helped us determine the feasibility of producing reconstructions of reservoir sections in a short time frame, which allows for real-time decision making during drilling operations.

Sources of high-chloride water and managed aquifer recharge in an alluvial aquifer in California, USA

As a result of pumping in excess of recharge, water levels in alluvial aquifers within the Eastern San Joaquin Groundwater Subbasin, 130 km east of San Francisco (California, USA), declined below sea level in the early 1950s and have remained so to the present. Chloride concentrations in some wells increased during that time and exceeded the US Environmental Protection Agency’s secondary maximum contaminant level of 250 mg/L, resulting in removal of some wells from service. Sources of high-chloride water include irrigation return in 16 % of sampled wells and water from delta sediments and deeper groundwater in 50 % of sampled wells. Chloride concentrations resulting from irrigation return commonly did not exceed 100 mg/L, although nitrate concentrations were as high as 25 mg/L as nitrogen. Chloride concentrations ranged from less than 100–2,050 mg/L in wells affected by water from delta sediments and deeper groundwater. Sequential electromagnetic logs show movement of high-chloride water from delta sediments to pumping wells through permeable interconnected aquifer layers. δD and δ18O data show most groundwater originated as recharge along the front of the Sierra Nevada, but tritium and carbon-14 data suggest recharge rates in this area are low and have decreased over recent geologic time. Managed aquifer recharge at two sites show differences in water-level responses to recharge and in the physical movement of recharged water with depth related to subsurface geology. Well-bore flow logs also show rapid movement of water from recharge sites through permeable interconnected aquifer layers to pumping wells.

A Broadband Electromagnetic Propagation Logging With Linear Frequency Modulation Signal

In this letter, a novel broadband electromagnetic (EM) propagation logging method with linear frequency modulation (LFM) continuous waves (LFMCWs) as well-logging signals has been proposed for logging while drilling (LWD). The logging response of the layered media to the LFMCWs under the noise environment has been numerically modeled using numerical mode matching (NMM) and inverse fast Fourier transform (IFFT). Based on the frequency spectrum of LFM signals, the NMM method is used first to calculate both the amplitude and the phase of the signals in receiving coils for every frequency point, and then, IFFT is applied to convert them to the logging response in the time domain. After that, the matched filter method has been imposed to realize pulse compression of the received signals and to improve the peak SNR (PSNR) significantly. As the logging responses of the EM propagation logging tool, the amplitude ratio and the phase difference between the compressed signals from the two separated coils at the peak point are obtained, respectively. Then, the apparent conductivity based on the amplitude ratio and the phase difference is obtained to have the logging curves. It has been shown from the simulation results that using LFMCWs as wideband logging signals can improve the SNR and the logging resolution remarkably, particularly under the condition with correlated noise that restrained the SNR and logging resolution strongly in traditional logging with a single-frequency continuous wave.

Processing high-frequency electromagnetic logs from conducting formations: linearized 2D forward and inverse solutions with regard to eddy currents

High-performance algorithms and software have been designed for fast 2D forward modeling and inversion of high-frequency electromagnetic logs to recover the patterns of electrical conductivity and relative permittivity (dielectric constant) of rocks. Forward modeling implies linear representation, with regard to eddy current, of attenuations and phase shifts of electromagnetic fields propagating in 2D conducting formations. In order to improve the modeling quality, the background model is chosen proceeding from the eddy current pattern in a heterogeneous subsurface. The inverse solution is obtained by pseudoinversion of the sensitivity matrix of relative amplitudes and phases. Joint 2D inversion of phase shifts and attenuations improves the reliability of the inferred spatial conductivity distribution and allows estimating permittivity. The reported numerical experiments on inversion of synthetic and field EM logs prove the possibility of estimating the permittivity of complex terrigenous and carbonate reservoirs with water- and oil-filled porosity.

Numerical modeling and inversion of electromagnetic logs in the wells drilled with biopolymer and oil-based mud

Algorithms and software for numerical modeling and inversion of electromagnetic logs in the wells drilled with biopolymer and oil-based mud are developed. The algorithms are based on linearized solutions of the forward and inverse problems of electromagnetic logging and permit fast modeling of induction logs and efficient recovery of electric conductivity around the well. Mathematical modeling is based on numerical-analytical solution of the 2D forward problem taking into account high conductivity contrast between the well and the formation. Linear inversion is based on SVD-decomposition of information matrix. The results of numerical modeling and inversion of synthetic and field logs at the intervals of fluid-saturated terrigenous and carbonate formations drilled using biopolymer and oil-based mud are given.

A real-time borehole correction of electromagnetic wave resistivity logging while drilling

Abstract The response of electromagnetic wave logging while drilling is influenced greatly by borehole and drilling fluid resistivity when the size of borehole is relatively large and drilling fluid resistivity is low. Borehole radius and drilling fluid resistivity were introduced to obtain more accurate transformed resistivity on the basis of the commonly used resistivity transformation model. The influence of borehole was considered in the newly established three dimensional transformation model, and a new borehole correction method was proposed. The resistivity transformation database can be established by calculation according to a certain instrument, and the true resistivity is obtained by three dimensional interpolation search technology of real-time correction in practical use. The results of numerical simulation and modeling verification show that the transformed resistivity by real-time correction coincides with the resistivity corrected by charts. The method can eliminate the borehole influence, reduce calculation dimension, and improve the inversion efficiency of highly deviated and horizontal wells logging data.

A New Resistivity Logging Method in Production Well

This paper presents a new method for resistivity logging in production well. Based on electromagnetic theory, an electromagnetic response model is established for multi-cylindrical-layer, and the general expressions for through casing electromagnetic logging are deduced. We further calculate and simulate the effects of casing permeability on electromagnetic response and demonstrate the necessity of casing magnetization. Finally, the effectiveness of casing magnetization is validated by the magnetization experiment. The result shows signal penetration capability is significantly enhanced after casing magnetization. It demonstrates that casing magnetization is an effective method for resistivity logging in production well.

Fast solution of two-dimensional inverse problem of high-frequency electromagnetic logging in oil-gas boreholes

The paper is dedicated to development of physical-mathematical tools of interpretation of the data of electromagnetic logging in oil-gas boreholes. Program-algorithmic means of numerical inversion of relative gain-phase characteristics in axis-symmetric models have been worked out. Algorithm of solving fast inverse problem is based on linear inversion and includes analysis of a matrix of sensitivities of the measured signals to model parameters. Inversion is accompanied by determination of areas of ambiguity of parameters recovery. High productivity of two-dimensional direct problem made possible realization of alternative approach to inversion based on plotting and analysis of all the ensemble of quasi-solutions of two-dimensional inverse problem. Elaborated algorithms give possibility to determine geo-electric parameters of penetration zone and the layer and estimate their inaccuracies. Experiments on recovery of electro-conductivity in typical models of collectors on synthetic and experimental data have been performed.

Invasion zone modeling using water- and oil-based muds

The hydrodynamic processes of displacement of reservoir fluids in an oil-saturated reservoir in vertical well drilling are studied by numerical simulation accounting for the drilling conditions and the type of mud (water-clay or oil-based). The influence of the hydrodynamic characteristics of the invasion zone on the specific electrical resistance depending on the type of mud is studied. The results of numerical simulations for the interpretation of high-frequency electromagnetic logs were used to construct geoelectric and hydrodynamic models in the reservoirs with different fluid saturations.

Electromagnetic‐Induction Logging to Monitor Changing Chloride Concentrations

Water from the San Joaquin Delta, having chloride concentrations up to 3590 mg/L, has intruded fresh water aquifers underlying Stockton, California. Changes in chloride concentrations at depth within these aquifers were evaluated using sequential electromagnetic (EM) induction logs collected during 2004 through 2007 at seven multiple-well sites as deep as 268 m. Sequential EM logging is useful for identifying changes in groundwater quality through polyvinyl chloride-cased wells in intervals not screened by wells. These unscreened intervals represent more than 90% of the aquifer at the sites studied. Sequential EM logging suggested degrading groundwater quality in numerous thin intervals, typically between 1 and 7 m in thickness, especially in the northern part of the study area. Some of these intervals were unscreened by wells, and would not have been identified by traditional groundwater sample collection. Sequential logging also identified intervals with improving water quality-possibly due to groundwater management practices that have limited pumping and promoted artificial recharge. EM resistivity was correlated with chloride concentrations in sampled wells and in water from core material. Natural gamma log data were used to account for the effect of aquifer lithology on EM resistivity. Results of this study show that a sequential EM logging is useful for identifying and monitoring the movement of high-chloride water, having lower salinities and chloride concentrations than sea water, in aquifer intervals not screened by wells, and that increases in chloride in water from wells in the area are consistent with high-chloride water originating from the San Joaquin Delta rather than from the underlying saline aquifer.

Application of Digital Phase Sensitive Detection for Electromagnetic Propagation Logging while Drilling

Intense electromagnetic interference while drilling prevents traditional analog phase sensitive detection (APSD) from correctly acquiring electromagnetic wave signal of logging while drilling (LWD). A digital phase sensitive detection (DPSD) utilizes high-performance floating-point DSP and FPGA to separately process real part and imaginary part of the acquired logging signal, changes traditional calculating method, improves processing speed and calculating precision. The programmable technique used can fully utilize logging information, simplify control logic, improve precision of timing control and measuring precision. Discussed are the principle, characteristics, implement of circuit and analysis of experimental data. Testing result shows that the DPSD may utilize logging information effectively and optimize the LWD system.

3D modeling of new-generation (1999–2010) resistivity logging tools

Electromagnetic (EM) logging is an im-portant method of formation evaluation because of its sensitivity to resistivity, which is a function of fluid saturation and fluid properties. Conventional induction and propagation resistivity logging tools excite the geological formation by axial magnetic dipole transmitter(s). Axial re-ceivers measure the formation response, reflecting the medium resistivity. Such axial measurement has no azimuthal sensitivity and is insensitive to the anisotropy and many other details of the formation.

On-Site Bias Noise Correction in Multi-Frequency Slingram-type Electromagnetic Induction Measurements

Slingram-type horizontal-loop electromagnetic induction systems are popular for environmental and engineering investigations of relatively shallow subsurfaces. The Slingram method is effective for the detection of anomalous zones by mapping and profiling large areas. However, analogous to airborne electromagnetic and induction borehole logging methods, zero level adjustment is crucial for further inverse analysis to estimate subsurface resistivity structures. To address this problem, we propose a simple and practical procedure for on-site measurement and correction of bias noise. To attenuate the response generated by electromagnetic induction in the ground, we raise the Slingram-type sensor vertically off the ground and measure background noise as the bias noise. By subtracting the bias noise from the original raw data, bias correction is implemented. Two applications, a high-salinity groundwater investigation and a levee assessment survey, demonstrate that this procedure is effective for quadrature data, but is inadequate for in-phase data. For a more precise correction, a more sophisticated method should be developed.

Dielectric spectroscopy of oil-bearing rocks at 0.05–16 GHz

We have measured the dielectric spectra of sand and bentonite mixtures with oil and saline water (NaCl solution) in the frequency range from 50 MHz to 16 GHz at 22 °C. The measured spectra were compared with predictions in terms of the generalized refractive mixing dielectric model (GRMDM), and the respective error was estimated as a check of the model applicability to describe oil-bearing formations. The results may be useful in developing new methods of electromagnetic logging.

Modeling and Analysis of the Response of Electromagnetic Logging Tool with Tilted-Coil Antennas in Cylindrically Stratified Anisotropic Formations

A new well-logging tool with tilted-coil can provide azimuthal information about formation and can improve the estimates of formation anisotropy. In this paper, an efficient algorithm is employed to simulate the response of electromagnetic logging tool with tilted-coil antennas in cylindrically stratified anisotropic formations. Firstly, tilted-coils are approximated into tilted magnetic dipoles. Then, we decompose it into three orthonormal components of magnetic dipoles, and the electromagnetic field generated by the three orthonormal components can be solved with the analytic method. Finally, we analyze the response of electromagnetic tool with tilted-coil antennas at different tilted angles and in different borehole environments, such as the responses in anisotropic formations, borehole effects and tool's eccentricity effect.

Method of ultrawideband radio-pulse logging of horizontal wells

Results of measuring the properties of oil-and-gas collector media using ultrawideband (UWB) radio pulses are presented. A new method of pulsed electromagnetic logging is suggested.

The Quasi Total Reflected Lateral Shift of Inhomogeneous P‐Polarized Electromagnetic Wave on the Interface of Conductive Media

AbstractUsing the phase shift of reflection coefficient at the interface between two conductive media, we derive the lateral shift of quasi total reflection of inhomogeneous P‐polarized electromagnetic wave (whose electric field parallels to the incident plane), and draw the curves of lateral shift with respect to the incident angle. The results show that there are three discontinuous points at the critical angle of phase constant, the critical angle of attenuation constant and 90°. When the incident angle approaches to one of the three angles, the lateral shift becomes very large. When the incident angle equals to one of them, the lateral shift becomes infinite, i.e. the electromagnetic wave propagates along the interface. In other incident angle the lateral shift has the same order of magnitude as the wavelength. After accounting the lateral shift effect, we discuss the measuring error of electromagnetic wave logging caused by the lateral shift effect, and give a method of computing the measuring error

Three-dimensional mapping of salt stores in the Murray-Darling Basin, Australia: 2. Calculating landscape salt loads from airborne electromagnetic and laboratory data

Salt is widespread in the Australian landscape – in soil, regolith and groundwater – leading to concerns that land management practices may be putting much agricultural land and important water resources at risk of salinisation. Defining the location and nature of salt stores is an important first step in understanding the processes leading to salinity of soils, streams and groundwater resources, and predicting areas that may be at risk. Airborne geophysics can define subsurface salt stores and mobilisation pathways. Airborne electromagnetics (AEM) can map the three-dimensional conductivity structure of the landscape but does not, intrinsically, quantify the amount of salt. Salinity, moisture content, porosity and mineralogy all contribute to the electromagnetic signal, and each can vary significantly throughout the landscape.In the Lower Balonne catchment, Queensland, the relationship between AEM and the amount of salt in the landscape was quantified using laboratory analyses of pore fluids from core samples. A general statistical relationship was established between AEM conductivity and salt load (defined as the product of pore fluid salinity, porosity and moisture content)—with a significant positive correlation although data were generally widely dispersed. Comparison of calculated salt load with borehole electromagnetic logs gives insight into the factors contributing to dispersion in the AEM data.The relationship transforms bulk conductivity to salt load in 5m layers, allowing the generation of a three-dimensional map of the salt load. This is a powerful tool for identifying areas that may require monitoring and management interventions to reduce salinity risk. An example is given of salt loads beneath an established irrigation area in the Lower Balonne catchment, Queensland.

Study on the Two‐Dimension Full‐Parameter Inverse Method of the Electromagnetic Propagation Resistivity Logging

AbstractThe multiple propagation resistivity (MPR) tool of the Baker‐Hughes INTEQ is the widely used electromagnetic propagation resistivity logging tool. It has four phase difference curves and four amplitude ratio curves. With it as an example, the application of the two‐dimension inverse method to the electromagnetic propagation resistivity logging data in vertical well is presented. Because the sensitivity of logging response to layer's boundary is much higher than that to other formation parameters, so how to accurately pick up layer's boundary is important. With the method described in this paper, we begin by picking the layer's boundaries from the highest resolution curve with the characteristic recognition method, and perform vertical one‐dimension inversion for the shallow and deep detection phase difference curves. Utilizing this result to construct the initial model for two‐dimension inversion, we carried out the full‐parameter inversion of MPR data by using an improved damping regularized Gauss‐Newton optimization algorithm. The satisfying inversion result is obtained for the two typical 23‐layer models. The inversion result can significantly improve the quality of MPR interpretation. In addition, it is found that the inversion results from only five curves are nearly the same to the results from eight curves.

ISRM Suggested Methods for borehole geophysics in rock engineering

In recent years, many geophysical methods have been used in rock engineering. For effective application of geophysics, it is important to ensure that all aspects of a geophysical survey from the planning to the reporting are properly implemented. To assist this in the field of rock engineering, the suggested methods for geophysics have been drafted by the International Society for Rock Mechanics Commission on Application of Geophysics to Rock Engineering. Those for the land geophysics were already published in this journal last year. This is the second volume of the suggested methods which describe borehole geophysics. Considering the present status of their utilization in rock engineering, six types of geophysical methods used in boreholes, velocity measurement along a borehole, electric and electromagnetic logging, nuclear logging, vertical seismic profiling, seismic tomography and resistivity tomography are described. In addition, since tunnels can be considered as boreholes, Seismic Ahead of a Tunnel Face is also included in this volume because it has been often used in tunneling. For each of the methods, the following seven aspects of their deployment are described: general, applicability, planning, field operation, data processing, interpretation and output and report. Great emphasis is placed on aspects of planning and interpretation because they become more important in recent application of geophysics to rock engineering.