Surface Electromagnetic Methods Research Articles

A new borehole electromagnetic receiver developed for controlled-source electromagnetic methods

Abstract. Conventional surface electromagnetic methods have limitations of a shallow detection depth and low resolution. To increase the detection depth and resolution, borehole–surface electromagnetic methods for electromagnetic three-dimensional observations of the ground, tunnels, and boreholes have been developed. Current borehole receivers only measure a single parameter of the magnetic field component, which does not meet the special requirements of controlled-source electromagnetic (CSEM) methods. This study proposes a borehole electromagnetic receiver that realizes synchronous acquisition of the vertical electric field component in the borehole and the three-axis orthogonal magnetic field components. This receiver uses Ti electrodes and fluxgate magnetometers (fluxgates) as sensors to acquire electric and magnetic field components. Multi-component comprehensive observation methods that add the electric field component can effectively support the CSEM method, improve detection accuracy, and exhibit a strong potential for detecting deep ore bodies. We conducted laboratory and field experiments to verify the performance of our new borehole electromagnetic receiver. The receiver achieved a magnetic field noise of less than 6 pTHz-1/2 at 1 kHz, and the electric field noise floor was approximately 20 nVm-1Hz-1/2 at 1 kHz. The −3 dB electric field bandwidth can reach DC −10 kHz. The results of our experiments prove that high-quality CSEM signals can be obtained using this new borehole electromagnetic receiver and that the electric field component exhibits sufficient advantages for measuring the vertical component of the electric field.

Reservoir evaluation method for complex resistivity using the borehole–surface electromagnetic method: A case study of an igneous reservoir in the K exploration area, China

Electrical well-logging techniques, combined with electrical conductivity models, have been widely used to estimate oil saturation levels in hydrocarbon reservoirs. For complicated and heterogeneous petroleum reservoirs, however, evaluation results are inevitably limited by well locations. Recent studies have indicated that the borehole–surface electromagnetic method (BSEM) can detect electrical characteristics distal from a well, and it can be applied to delineate the boundaries of complicated reservoirs. Therefore, in the study described here, a new method to estimate reservoir oil saturation, based on the BSEM, has been developed. In this work, we deployed the BSEM in a designated study area, obtaining complex resistivity and polarizability data, through the constrained inversion of the re-weighted regularized conjugate gradient method. Based on the petrophysical analyses, we developed a new saturation evaluation model, composed of a parallel circuit connection between formation water and the polarization induced by the porous medium containing clay minerals. The effectiveness of the new model was verified with a synthetic dataset generated using Archie's law and the improved Dias model. We found that the target reservoir oil saturation results calculated using the new oil saturation model were in good agreement with applicable nuclear magnetic resonance log data, thus verifying the validity of this new BSEM evaluation technique.

Downhole System Enables Real-Time Reservoir-Fluid-Distribution Mapping

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper IPTC 19807, “A New Downhole System for Real-Time Reservoir-Fluid-Distribution Mapping: E-REMM, the Eni Reservoir Electromagnetic Mapping System,” by Franco Bottazzi, Paolo Dell’Aversana, and Claudio Molaschi, SPE, Eni, et al., prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13-15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. This paper presents the basic concepts and architecture of the Eni Reservoir Electromagnetic Mapping (E-REMM) borehole electromagnetic (EM) mapping system that integrates borehole EM methodology with surface EM methods to provide real-time mapping of reservoir-fluid distribution during production or injection. By helping well teams know how distribution of hydrocarbons and water changes over time and space, the system addresses a fundamental requirement for managing the reservoir to maximize the recovery factor, optimize production, and reduce associated costs. Introduction Several approaches are currently used to map hydrocarbons and other fluids in the reservoir in real time with variable sensitivity, differences in effectiveness, and a wide range of costs. Among the various approaches, EM and electric methods show a high benefit-to-cost ratio and a high sensitivity to the resistivity contrast between oil-saturated and brine-saturated reservoir rocks. These methods can estimate variations in fluid properties at reservoir depth in a distance range of hundreds of meters, although the spatial resolution decreases significantly with the distance between sources and receivers. Furthermore, combining multiple layouts in boreholes and at surface can improve the effectiveness of EM and electric methods significantly while increasing their spatial resolution. The integrated system discussed in the complete paper allows multiscale EM prospecting and deep investigation in a large 3D volume of rocks between multiple wells and surface.

Return flows from beaver ponds enhance floodplain-to-river metals exchange in alluvial mountain catchments

River to floodplain hydrologic connectivity is strongly enhanced by beaver- (Castor canadensis) engineered channel water diversions. The hydroecological impacts are wide ranging and generally positive, however, the hydrogeochemical characteristics of beaver-induced flowpaths have not been thoroughly examined. Using a suite of complementary ground- and drone-based heat tracing and remote sensing methodology we characterized the physical template of beaver-induced floodplain exchange for two alluvial mountain streams near Crested Butte, Colorado, USA. A flowpath-oriented perspective to water quality sampling allowed characterization of the chemical evolution of channel water diverted through floodplain beaver ponds and ultimately back to the channel in ‘beaver pond return flows’. Subsurface return flow seepages were universally suboxic, while ponds and surface return flows showed a range of oxygen concentration due to in-situ photosynthesis and atmospheric mixing. Median concentrations of reduced metals: manganese (Mn), iron (Fe), aluminum (Al), and arsenic (As) were substantially higher along beaver-induced flowpaths than in geologically controlled seepages and upstream main channel locations. The areal footprint of reduced return seepage flowpaths were imaged with surface electromagnetic methods, indicating extensive zones of high-conductivity shallow groundwater flowing back toward the main channels and emerging at relatively warm bank seepage zones observed with infrared. Multiple-depth redox dynamics within one focused seepage zone showed coupled variation over time, likely driven by observed changes in seepage rate that may be controlled by pond stage. High-resolution times series of dissolved Mn and Fe collected downstream of the beaver-impacted reaches demonstrated seasonal dynamics in mixed river metal concentrations. Al time series concentrations showed proportional change to Fe at the smaller stream location, indicating chemically reduced flowpaths were sourcing Al to the channel. Overall our results indicated beaver-induced floodplain exchanges create important, and perhaps dominant, transport pathways for floodplain metals by expanding chemically-reduced zones paired with strong advective exchange.

Domain decomposition based integral equation modeling of 3-dimensional topography in frequency domain for well electromagnetic field

As an efficient geophysical exploration technology, well electromagnetic method is particularly applicable to oil and gas exploration in China's complex terrain areas (deserts, mountains, etc.). A serious influence of topographic relief area on the electromagnetic response of well is inevitable but challenging. To the best of our knowledge, there is no literature on modeling the electromagnetic response of three-dimensional (3D) topography with well electromagnetic method. Based on the domain decomposition, an integral equation method is presented to simulate the electromagnetic response of 3D topography in frequency domain via the well electromagnetic method. Compared with the finite difference and finite element method based on partial differential equation, this method is very efficient in simulating topographic response without huge computation or truncation boundary error accumulation or special boundary condition requirements. Firstly, an induction coefficient is defined according to the topographic relief situation. Then the computational domain consisting of the target body, background medium and 3D topography is divided into reference model, background medium and the distribution of target body medium area. According to the characteristics of each sub-region, Anderson algorithm is an analytic solution based on Gaussian filtering, which is used to provide the primary field from the excited sources in surface. And then, the stable double conjugate gradient-fast Fourier transform is incorporated into integral equation algorithm to obtain the fast 3D terrain shaft frequency domain electromagnetic responses. By comparing the calculation results using the new algorithm presented in this paper with the analytical solutions of Anderson algorithm for half-space model with surface electromagnetic method, the precision and the efficiency of this new algorithm are demonstrated. And the ability to model the electromagnetic responses of 3D topography is shown by comparing with the published results of 3D boundary integral equation. Thus, the high accuracy and high efficiency of the new algorithm presented in this paper are validated. Finally, the influence of 3D valley topography on electromagnetic field response of surface to borehole electromagnetic (SBEM) observation system is presented and analyzed. It is observed that the response of SBEM is seriously disturbed by the field of 3D valley topography which is necessarily removed. The research results presented in this paper are of significance for guiding the identification and correction of electromagnetic topographic effect from 3D SBEM.

Aquistore Project Measurement, Monitoring, and Verification: From Concept to CO2 Injection

Abstract As an independent research and monitoring project, Aquistore intends to demonstrate that storing liquid carbon dioxide (CO 2 ) deep underground (in a brine and sandstone water formation), is a safe, workable solution to reduce greenhouse gases (GHGs). Managed by the Petroleum Technology Research Centre, Aquistore is built upon the learnings of the IEAGHG Weyburn-Midale CO 2 Monitoring and Storage Project, where over 22MT of CO 2 have been stored in an oil field during EOR operations. As a global leader, PTRC has over a decade of experience in CO 2 storage and monitoring work. Aquistore is Canada's first dedicated CO 2 storage project, and is an integral component of SaskPower's Boundary Dam Integrated Carbon Capture and Storage (CCS) Demonstration project - the world's first fully integrated CCS demonstration project from a coal-fired power plant. As an unique CCS project, Aquistore is providing buffer storage to this commercial CO 2 capture plant and active oilfield EOR operations. In collaboration with SaskPower, Aquistore will be the first integrated project globally to demonstrate deep saline CO 2 capture, transport, and storage on a commercial scale from a coal fired power plant. CO 2 will be captured at unit 3 of the Boundary Dam power- station (BD3), transported via underground pipeline to the Aquistore site, and injected to a depth of 3.4 km. The Winnipeg and Deadwood formations, which constitute the deepest units within the Williston Basin and were chosen as the target zone for CO 2 injection. These geological formations have much greater capacity for storing CO 2 than any oil reservoir in western Canada. The suitability of the storage complex was investigated through 3D characterization using high-resolution 3D seismic images and data from the injection and observation wells. These data show that: there are no significant faults in the immediate area of the storage site; the regional sealing formation is continuous in the area; and lastly, that the reservoir is not adversely affected by knolls on the surface of the underlying Precambrian basement. In the Aquistore project PTRC has brought together internationally recognized expertise and diverse interests. Research in monitoring methods is central to the Aquistore Project. A permanent areal seismic monitoring array has been deployed which comprises 650 geophones installed at 20 m depth on a 2.5x2.5 km regular grid. The objective of this array is to test “sparse array” seismic imaging and to provide continuous passive monitoring. This array is being utilized in conjunction and comparison with time-lapse 3D seismic imaging, continuous microseismic monitoring, vertical seismic profiling, cross-well seismic tomography, and broadband seismographs. Injection-related surface deformation monitoring is provided by InSAR analysis in conjunction with a network of tiltmeters and GPS stations. Electromagnetic surveys and gravimeters are also being used. In an effort to minimize surface impact these monitoring technologies - where possible - have been collocated and solar-powered. Surface electromagnetic methods and time-lapse gravity monitoring will be tested in conjunction with deployment of a fibre-optic distributed acoustic sensing (DAS) line. A fibre-optic distributed temperature sensing (DTS) line is also installed. A unique fluid recovery system (FRS) as well as casing conveyed pressure and temperature gauges will contribute to the project's down-hole monitoring techniques. Aquistore's MMV program will test and develop effective methods for monitoring CO 2 storage sites and ensure conformance of the storage process through continuous monitoring. This program is seeking to minimize the risk associated with any potential leakage of CO 2 from the storage reservoir through early detection. Aquistore's MMV program has included an added focus on integrated monitoring methods such as the inclusion of non-seismic methods and constraints from reservoir flow simulations. Aquistore will provide an efficient, cost effective, and field-tested basis for designing effective MMV programs for other similar projects worldwide.

Technical inspection of pipeline groups using surface electromagnetic methods

ABSTRACTIn this paper, we present an electromagnetic surface technique for the integrity inspection of buried pipelines groups. The inspection includes the determination of the pipeline plan location and depth, assessment of insulation‐coating quality and cathodic protection conditions. This technique is based on the approximation of a metallic pipeline by a heterogeneous transmission line. In the transmission line model, the degree of coating damage (disbanded or cracked insulation) corresponds to the leakage resistance that is defined by the coating resistance and environmental resistivity. Using the transmission line model, we simulated the voltage and current distributions along a pipeline created by the cathodic protection system or an external generator in a low‐frequency range. The calculations were performed for both a single pipeline and interconnected pipelines with different insulation‐coating quality (model of a common cathodic protection system). We describe the field operations, which consist of measurements of the magnetic field created by the current flowing into a pipeline, voltages on the cathodic protection control posts and soil resistivity. To find the pipeline position, depth and current we apply the interactive inversion of the magnetic‐field data obtained along the profiles orthogonally oriented to the pipeline‐group route. The interpretation of the field data includes: calculating the leakage current (the current difference on the measuring interval) for each pipeline, determining the leakage resistance (the voltage to the leakage‐current ratio) and assessing the insulation‐coating resistance taking into account the surrounding soil resistivity. We present practical examples of the integrity inspection of interconnected pipelines group in Mexico. The successful results achieved allow us to recommend this electromagnetic technique for quantitative inspection of the technical conditions of interconnected pipelines.

3D modeling and born approximation inversion for the borehole surface electromagnetic method

We present a 3D approach to numerical modeling of the borehole-surface electromagnetic (BSEM) method. The 3D electromagnetic response created by a vertical line current source in a layered medium is modeled using the 3D integral equation method. The modeling results are consistent with analytical solutions. 3D Born approximation inversion of BSEM data is also conducted for reservoir delineation. The inversion method is verified by a synthetic reservoir model.

A helicopter‐borne electromagnetic survey to delineate groundwater recharge rates

Groundwater recharge is one of the most difficult components of the water balance to measure. For this reason, electromagnetic methods have been used to infer its variability from measurements of apparent electrical conductivity. In this study, groundwater recharge was estimated at 20 sites using unsaturated zone chloride methods. Interpolation between drill sites was accomplished with the aid of a helicopter‐borne electromagnetic survey (DIGHEMIV). Correlations between recharge and apparent electrical conductivity were only significant (R2 = 65%) at the highest frequency (56,000 Hz). Using these single‐frequency data, variations in recharge were mapped over an area of 32 km2. Recharge, as inferred from the electromagnetic data, appears to be lognormally distributed, and varies from less than 1 to more than 50 mm yr−1. Within the study region, spatially averaged recharge can be estimated from the electromagnetic data, with an accuracy of approximately −60%, +140% (90% probability). This is comparable to the estimation accuracy when surface electromagnetic methods are used. Aerial electromagnetic methods appear very useful for identifying areas of high and low recharge over large regions.