Controlled-source Electromagnetic Survey Research Articles

Guided and Direct Wave Evaluation of Controlled Source Electromagnetic Survey Using Finite Element Method

Deep target hydrocarbon detection is still challenging and expensive. Direct hydrocarbon indicators (DHIs) in seismic data do not correspond to economical hydrocarbon exploration. Due to unreliability in seismic data for the detection of DHIs, new methods have been investigated. Marine controlled source electromagnet (MCSEM) or Sea bed logging (SBL) is new method for the detection of deep target hydrocarbon reservoir. Sea bed logging has also the potential to reduce the risks of DHIs in deep sea environment. Modelling of real sea environment helps to reduce the further risks before drilling the oil wells. 3D electromagnetic (EM) modelling of seabed logging requires more accurate methods for the detection of hydrocarbon reservoir. Finite element method (FEM) is chosen for the modelling of seabed logging to get more precise EM response from hydrocarbon reservoir below 4000 m from seabed. FEM allows to investigate the total electric and magnetic fields instead of scattered electric and magnetic fields, which shows accurate and precise resistivity contrast below the seabed. From the modelling results, It was investigated that Hz field shows higher magni- tude with 342% than the Ex field. It was observed that 0.125 Hz frequency can be able to show better resistivity contrast of Hz field (31.30%) and Ex field (16.49%) at target depth of 1000 m below seafloor for our proposed model. Hz and Ex field delineation was found to decrease as target depth increased from 1000 m to 4000 m. At the target depth of 4000 m, no field delineation response was seen from the current electromagnetic (EM) antenna used by the industry. New EM antenna has been used to see the EM response for deep target hydrocarbon detection. It was investigated that novel EM antenna shows better delineation at 4000 m target depth for Ex and Hz field up to 10.3% and 15.1% respectively. Novel EM antenna also shows better Hz phase response (128.4%) than the Ex phase response (38.3%) at the target depth of 4000 m below the seafloor.

Application of DCIM on Marine Controlled-Source Electromagnetic Survey

The discrete complex image method (DCIM) is used to calculate the mixed-potential electric field integral equation. The DCIM has been used broadly in high-frequency domain, and now is successfully introduced to the extremely low-frequency situation of marine controlled-source electromagnetic (MCSEM) survey. The integral path in high-frequency situation does not hold valid for extremely low-frequency situation. The reason can be ascribed to the change of wave number formulas. A new integral path suitable for the extremely low-frequency situation is proposed. The appropriate ranges of the parameters used in DCIM are given. The computation is stable at these considerably large ranges. The results of a five-layer model calculated with DCIM are compared with those with the filter method and the direct integral method. The computation time with DCIM is comparable to the time with filter method. The effects of water depth, upper-sediment thickness, and oil-layer thickness are studied with DCIM. The results show that electric field varies with the change of thickness. The effects accord to the physical meanings of MCSEM survey. It is safe to conclude that the DCIM with the new path is valid in MCSEM survey.

Sensitivity of controlled-source electromagnetic fields in planarly layered media

The study of electromagnetic (EM) field sensitivities is useful for assessing the feasibility of controlled-source electromagnetic (CSEM) surveys. Sensitivity calculations are also a principal building block of EM inversion schemes. Sensitivities are formally given by the derivatives of the EM field components with respect to conductivity. For horizontally layered media, these derivatives can be evaluated analytically, offering advantages in computational efficiency and accuracy over numerical evaluation. We present a complete set of explicit analytic expressions for the EM field sensitivities in 1-D VTI-anisotropic media for horizontal and vertical electric and magnetic dipole sources, and also for finite horizontal electric sources. Since our derivations are based on a formulation for EM fields that is quite general in allowing for sources and receivers at any depth, our sensitivity expressions exhibit the same generality. We verify our expressions by comparison to numerical solutions, and finally present application examples that demonstrate the utility and versatility of these expressions for CSEM feasibility studies.

A marine electromagnetic survey to detect gas hydrate at Hydrate Ridge, Oregon

Gas hydrates are a potential energy resource and hazard for drilling and infrastructure, yet estimates of global volume vary by over three orders of magnitude. Hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations. A marine electromagnetic survey was carried out at Hydrate Ridge, Oregon, USA, with the aim of testing the use of controlled source electromagnetic (CSEM) and magnetotelluric (MT) methods to map gas hydrate and free gas below the gas hydrate stability zone. A 2-D CSEM inversion supports the scenario deduced from previous seismic and drilling results, which indicate two mechanisms of hydrate emplacement: a transport-dominated and reaction-dominated regime. A prominent resistive region of 2.5–4 ?m at a depth of about 130 mbsf, near the seismic bottom simulating reflector (BSR), suggests that 27 to 46 per cent of the bulk volume is filled with hydrate, depending on whether Archie's Law or the Hashin-Strikman bounds are used. This is representative of a reaction-dominated regime for hydrate emplacement, and where a significant low velocity zone exists based on a seismic tomography inversion, suggests large quantities of free gas below the BSR. Electrical resistivity logging while drilling (LWD) data show general agreement with the CSEM inversion model except for a CSEM-derived resistive region at seismic horizon A, known to transport free gas into the gas hydrate stability zone. Inversion of MT data collected simultaneously during the CSEM survey provides a complimentary low-resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model.

Joint elastic-electrical properties of reservoir sandstones and their relationships with petrophysical parameters

We measured in the laboratory ultrasonic compressional and shear-wave velocity and attenuation (0.7–1.0 MHz) and low-frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elastic-electrical properties of reservoir sandstones. P- and S-wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi-logarithmic scale for both clean and clay-rich sandstones; P- and S-wave attenuations showed a bell-shaped correlation (partial for S-waves) with apparent electrical formation factor. The joint elastic-electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co-located seismic and controlled source electromagnetic surveys.

Pressure effects on the joint elastic-electrical properties of reservoir sandstones

We conducted a laboratory study of the joint elastic-electrical properties of sixty-three brine-saturated sandstone samples to assess the likely impact of differential pressure (confining minus pore fluid pressures) in the range 8–60 MPa on the joint interpretation of marine seismic and controlled-source electromagnetic survey data. The samples showed a wide range of petrophysical properties representative of most sandstone reservoirs. We found that a regression equation comprising both a constant and an exponential part gave a good fit to the pressure dependence of all five measured geophysical parameters (ultrasonic P- and S-wave velocity, attenuation and electrical resistivity). Electrical resistivity was more pressure-sensitive in clay-rich sandstones with higher concentrations of low aspect ratio pores and micropores than in clean sandstones. Attenuation was more pressure-sensitive in clean sandstones with large open pores (macropores) than in clay-rich sandstones. Pore shape did not show any influence on the pressure sensitivity of elastic velocity. As differential pressure increases, the effect of the low aspect ratio pores and micropores on electrical resistivity becomes stronger than the effect of the macropores on attenuation. Further analysis of correlations among the five parameters as a function of pressure revealed potentially diagnostic relationships for geopressure prediction in reservoir sandstones.

Insight into the marine controlled-source electromagnetic signal propagation

In marine controlled-source electromagnetic (CSEM) surveys the subsurface is explored by emitting low-frequency signals from an electric dipole source close to the sea-bed. The main goal is often to detect and describe possible thin resistive layers beneath the sea-bed. To gain insight into how CSEM signals propagate, it is informative to study a stratified model. The electromagnetic field is then given in terms of integrals over TE- and TM-polarized plane-wave constituents. An asymptotic evaluation of the field integrals for large propagation distances results in explicit spatial expressions for the field components and the derived expressions can be used to analyse how the CSEM signals propagate. There are two major signal pathways in a standard CSEM model. One of these pathways is via the thin resistive layer and the resulting response is accounted for by a pole in the reflection response for the TM mode. The signal is propagating nearly vertically down to the resistor from the source, then guided while attenuated along the reservoir, before propagating nearly vertically up to the receiver. The response is slightly altered by the sea-bed interface and further modified in shallow water due to multiple reflections between the sea-surface and sea-bed at both the source and receiver sides. The other major signal pathway is via the resistive air half-space, the so-called airwave. The airwave is generated by the TE mode and interacts with the subsurface via vertically propagating signals reflected between the sea-surface and subsurface at both the source and receiver sides.

Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 1. Electromagnetic properties

The marked decrease in bulk electrical conductivity of sediments in the presence of gas hydrates has been used to interpret borehole electrical resistivity logs and, to a lesser extent, the results of controlled source electromagnetic surveys to constrain the spatial distribution and predicted concentration of gas hydrate in natural settings. Until now, an exhaustive laboratory data set that could be used to assess the impact of gas hydrate on the electromagnetic properties of different soils (sand, silt, and clay) at different effective stress and with different saturations of hydrate has been lacking. The laboratory results reported here are obtained using a standard geotechnical cell and the hydrate‐formed tetrahydrofuran (THF), a liquid that is fully miscible in water and able to produce closely controlled saturations of hydrate from dissolved phase. Both permittivity and electrical conductivity are good indicators of the volume fraction of free water in the sediment, which is in turn dependent on hydrate saturation. Permittivity in the microwave frequency range is particularly predictive of free water content since it is barely affected by ionic concentration, pore structure, and surface conduction. Electrical conductivity (or resistivity) is less reliable for constraining water content or hydrate saturation: In addition to fluid‐filled porosity, other factors, such as the ionic concentration of the pore fluid and possibly other conduction effects (e.g., surface conduction in high specific surface soils having low conductivity pore fluid), also influence electrical conductivity.

Electromagnetic fields generated by finite-length wire sources: comparison with point dipole solutions

In present-day land and marine controlled-source electromagnetic (CSEM) surveys, electromagnetic fields are commonly generated using wires that are hundreds of metres long. Nevertheless, simulations of CSEM data often approximate these sources as point dipoles. Although this is justified for sufficiently large source-receiver distances, many real surveys include frequencies and distances at which the dipole approximation is inaccurate. For 1D layered media, electromagnetic (EM) fields for point dipole sources can be computed using well-known quasi-analytical solutions and fields for sources of finite length can be synthesized by superposing point dipole fields. However, the calculation of numerous point dipole fields is computationally expensive, requiring a large number of numerical integral evaluations. We combine a more efficient representation of finite-length sources in terms of components related to the wire and its end points with very general expressions for EM fields in 1D layered media. We thus obtain a formulation that requires fewer numerical integrations than the superposition of dipole fields, permits source and receiver placement at any depth within the layer stack and can also easily be integrated into 3D modelling algorithms. Complex source geometries, such as wires bent due to surface obstructions, can be simulated by segmenting the wire and computing the responses for each segment separately. We first describe our finite-length wire expressions and then present 1D and 3D examples of EM fields due to finite-length sources for typical land and marine survey geometries and discuss differences to point dipole fields.

Role of fine-scale layering and grain alignment in the electrical anisotropy of marine sediments

Electrical resistivities of seafloor sediments determined by controlled source electromagnetic (CSEM) surveys have been found to be significantly greater than those measured by electrical well logging, in some instances by a ratio of as much as 5:1. Because borehole logging techniques invariably measure electrical resistivity using currents circulating in horizontal planes and CSEM surveys are sensitive to the currents circulating in vertical planes, a possible cause of this discrepancy is strong electrical anisotropy of the sediments. We have examined electrical log data from vertical exploration and appraisal wells and deviated production wells in the North Sea. By correlating the same sedimentary units between wells, we are able to compare resistivities measured at different borehole inclination angles. As the inclination angle changes, the amount of vertical and horizontal resistivity contributing to the resistivity changes. Hence we can estimate the electrical anisotropy in the sediments. Results indicate that anisotropy ratios of 1:5 and greater are present within the shale-dominated units. We show that fine-scale horizontal layering can make only a relatively small contribution to this anisotropy, but that a model of horizontal alignment of highly oblate spheroidal grains can account for most or all of it.

Electric field responses of different gas hydrate models excited by a horizontal electric dipole source with changing arrangements

Abstract The exploration of marine controlled source electromagnetic survey for gas hydrate generally consists of a dynamic horizontal electric dipole source towed in the sea water and a static receiver array allocated on the seafloor. In this paper, two orthogonal horizontal components of electric field excited by a horizontal electric dipole source upon one-dimensional horizontal-layered seafloor are calculated to study the effect of different arrangements and gas hydrate models on the response of electric field. There are several dominant factors determining the variation of induced field, including transmitting frequency of the source, distance between the source and the seafloor, water depth, thickness of the overburden layer, as well as physical and chemical structure of the gas hydrate model. Modeling results suggest that the amplitude of received electric fields can reach up to 10-16–10-10 V /(A·m 2 ) by adjusting transmitting frequency; keeping the source being towed several tens of meters above the seafloor can improve the survey efficiency but not decrease the quality of collected data; deep water environment and shallow thin overburden layer are favorable for the survey because of their attribution to signal-to-noise ratio; the porosity and saturation of gas hydrate have significant influence on medium resistivity-the higher resistivity the medium has, the bigger the amplitude of electric field response will be. Forward modeling can provide valuable information for the development of instruments and the design of survey to detect gas hydrate.

Unlocking the value of CSEM

Lucy MacGregor1 and Richard Cooper suggest that for marine controlled source electromagnetic survey methods to achieve more general adoption in the E&P industry, the applications need to be refocused with more emphasis on prospect appraisal and reservoir monitoring rather than exploration.

3D finite-difference frequency-domain modeling of controlled-source electromagnetic data: Direct solution and optimization for high accuracy

Three-dimensional modeling of marine controlled-source electromagnetic (CSEM) data is vital to improve the understanding of electromagnetic (EM) responses collected in increasingly complex geologic settings. A modeling tool for simulating 3D marine CSEM surveys, based on a finite-difference discretization of the Helmholtz equation for the electric fields, has been developed. Optimizations for CSEM simulations include the use of a frequency-domain technique, a staggering scheme that reduces inaccuracies especially for horizontal electric-dipole sources located near the seafloor, and a new interpolation technique that provides highly accurate EM field values for receivers located in the immediate vicinity of the seafloor. Source singularities are eliminated through a secondary-field approach, in which the primary fields are computed analytically for a homogeneous or a 1D layered background; the secondary fields are computed using the finite-difference technique. Exploiting recent advances in computer technology and algorithmic developments, the system of finite-difference equations is solved using the MUMPS direct-matrix solver. In combination with the other optimizations, this allows accurate EM field computations for moderately sized models on small computer clusters. The explicit availability of matrix factorizations is advantageous for multisource modeling and makes the algorithm well suited for future use within an inversion scheme. Comparisons of simulated data for (1) 1D models to data generated using a 1D reflectivity technique and (2) 3D models to published 3D data demonstrate the accuracy and benefits of the approach.

Investigating the exploration potential for 3D CSEM using a calibration survey over the Troll Field

We demonstrate the ability of the controlled source electromagnetic (CSEM) technique to reveal hydrocarbon reservoirs by reinvestigating the North Sea Troll field with a 3D acquisition grid. Unlike previous applications of 2D CSEM surveys over this field, which have shown a large electromagnetic response over the gas province, the focus in this study is on the thinner and smaller oil reservoir. A weak but consistent anomaly enables detection and delineation of the oil zone and reduces the ambiguity associated with traditional 2D lines. This holds true even for a sparse receiver grid where the target is located between the receiver and source lines. The 3D data show the extent of the oil zone and that it is matched to the top reservoir structural high as well as north-south aligned faults. Variation in magnitude of the anomaly is qualitatively in good correlation with resistivity logs at the well locations. This suggests that 3D CSEM data pave the way for exploration use because of its superiority over 2D data in terms of reduced sensitivity to survey layouts.

Fast numerical methods for marine controlled-source electromagnetic (EM) survey data based on multigrid quasi-linear approximation and iterative EM migration

In this paper we consider an application of the method of electromagnetic (EM) migration to the interpretation of a typical marine controlled-source (MCSEM) survey consisting of a set of sea-bottom receivers and a moving electrical bipole transmitter. Three-dimensional interpretation of MCSEM data is a very challenging problem because of the enormous number of computations required in the case of the multi-transmitter and multi-receiver data acquisition systems used in these surveys. At the same time, we demonstrate that the MCSEM surveys with their dense system of transmitters and receivers are extremely well suited for application of the migration method. In order to speed up the computation of the migration field, we apply a fast form of integral equation (IE) solution based on the multigrid quasi-linear (MGQL) approximation which we have developed. The principles of migration imaging formulated in this paper are tested on a typical model of a sea-bottom petroleum reservoir.

Removal of Sea Surface Related Wavefields from CSEM Data

For a controlled-source electromagnetic (CSEM) survey in a shallow water envi- ronment, the presence of the sea surface signiflcantly hinders the interpretation of the measured data. The electromagnetic (EM) waveflelds are disturbed by the sea surface. The removal of the sea-surface related wave phenomena from the data is an important step in order to robustly interpret the collected data. We propose a processing method by which the sea-surface related multiples would be removed, while a priori knowledge of the EM source wavelet becomes super∞u- ous. The governing equations are obtained from an appropriate application of the EM reciprocity theorem that relates on one side the EM flelds in the actual measurement conflguration including the sea surface and on the other side the EM flelds in a desired source conflguration and in the absence of the sea surface, where the water layer is extended to inflnity. In a controlled-source EM (CSEM) survey (1), it is necessary to interpret the measurements in such a way that a prediction of the presence of hydrocarbons in the sedimentary layers can be made. However, in a shallow water environment, the presence of the sea surface hinders this interpretation. Electromagnetic waveflelds are partly re∞ected and partly transmitted by the sea surface. This means that the source signal is contaminated by its so-called source ghost signal and that the received signals are contaminated by the so-called receiver ghost. Further the receiver ghost can be considered as secondary source signals that are transmitted in the earth. Hence, removal of all these sea surface related electromagnetic waveflelds from the data is a prerequisite step before actual interpretation of the data can take place. In this paper we show that an appropriate use of the electromagnetic reciprocity theorem leads to the mathematical equations for a consistent removal of sea-surface related waveflelds. The actual EM wavefleld is denoted as f ^ E(x); ^ H(x)g in the frequency domain (with frequency parameter s = j!). The position in space is denoted as x = fx1;x2;x3g. The coordinates x1 and x2 denote the horizontal directions, while x3 denotes the vertical direction pointing into the earth. The wavefleld is generated by an electromagnetic source in the sea at x S = fx1;x2;x S g. On the sea bottom (not necessarily a horizontal plane), we measure the EM wavefleld re∞ected by the earth geology at the sea bottom (see Figure 1, left). We assume that there exists a horizontal plane at x3 = x obs between the source level and the sea bottom. It is assumed that the sea water is homogeneous with complex permittivity ^ and real constant permeability . In this domain, outside the sources, the electromagnetic flelds satisfy Maxwell's equations r £ ^

A Magnetic Gradiometric Method as an Adjunct for Marine CSEM

The marine Controlled-Source Electromagnetic (CSEM) method has proven itself an invaluable adjunct to the seismic method in petroleum exploration in the last few years. The problem is that it is not easily adapted to the Australian scene because most of Australia's petroleum deposits are in relatively shallow waters where CSEM does not work very well. Depending on the target, depths up to 500 m can be considered shallow water for CSEM. CSEM surveys in shallow water can have problems with the airwave dominating the received CSEM signal and also with electromagnetic noise induced by ocean waves. Modelling studies suggest that it is feasible to measure CSEM magnetic field gradients as a supplement to E and B field measurements and use this extra information to remove environmental noise (wave effects and magnetotelluric noise), airwave and direct wave components, and other unwanted contributions to the measured E and B fields. This can improve the signal/noise ratio of the CSEM measurements, thereby producing more reliable interpretations, and extend the useful range of the method, which can reduce survey costs. We have been developing a magnetic gradiometer that can be used for marine CSEM surveys. This poster will present the results from preliminary field trials conducted off Sydney's coast, in water depths up to 62 m, which show that the OceanMAG gradiometer was able to measure the magnetic field induced by ocean waves. Approaches for using magnetic gradient measurements as an adjunct to marine CSEM data will also be described.

Introduction to this special section: CSEM

The marine controlled-source electromagnetic (CSEM) method for finding hydrocarbons was introduced in the 1980s but only found commercial acceptance about five years ago. However, this brief time has been long enough for this method to earn the label of being possibly the most significant technology in oil exploration since 3D seismic. Indeed, the field has grown so rapidly that it will be the subject of simultaneously published special sections in both SEG journals—perhaps an unprecedented occurrence. The March-April issue of GEOPHYSICS will have 10 peer-reviewed papers on CSEM. TLE's special section presents several case studies that show the value of the technology in reducing risk for exploration and development projects. Most CSEM surveys are marine but CSEM is also done on land, mainly in Russia and China. Marine CSEM is mostly done in the frequency domain; land activities are split between frequency domain and time domain.

Rigorous 3D inversion of marine CSEM data based on the integral equation method

Marine controlled-source electromagnetic (MCSEM) surveys have become an important part of offshore petroleum exploration. However, due to enormous computational difficulties with full 3D inversion, practical interpretation of MCSEM data is still a very challenging problem. We present a new approach to 3D inversion of MCSEM data based on rigorous integral-equation (IE) forward modeling and a new IE representation of the sensitivity (Fréchet derivative matrix) of observed data to variations in sea-bottom conductivity. We develop a new form of the quasi-analytical approximation for models with variable background conductivity (QAVB) and apply this form for more efficient Fréchet derivative calculations. This approach requires just one forward modeling on every iteration of the regularized gradient-type inversion algorithm, which speeds up the computations significantly. We also use a regularized focusing inversion method, which provides a sharp boundary image of the petroleum reservoir. The methodology is tested on a 3D inversion of the synthetic EM data representing a typical MCSEM survey conducted for offshore petroleum exploration.

Evaluating reconnaissance CSEM survey designs using detection theory

Marine controlled-source electromagnetic (CSEM) surveys have recently found wide application for mapping resistive hydrocarbon reservoirs (Ellingsrud et al., 2002). Typically, these surveys have been target oriented and have been used to delineate potential reservoirs already mapped by other methods. However, CSEM surveys can also be used for reconnaissance in areas where no specific targets have been identified (Srnka et al., 2005). The objective of a reconnaissance CSEM survey is to detect the presence of resistive bodies that are large enough to be economic hydrocarbon reservoirs. These bodies would then be candidates for more detailed seismic or CSEM surveys.