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Summary There is a growing interest in developing innovative geophysical methods for monitoring hydrocarbon reservoirs. One emerging technique is based on using the borehole electric current transmitter and a grid of surface electric field receivers for detailed mapping of the subsurface resistivity of oil- and gas-producing fields. This method is often called Borehole to Surface Electromagnetic (BSEM) surveying. We introduce a rigorous method for 3D inversion of BSEM data based on the integral equation approach and adaptive regularization. We have applied our developed method to 3D inversion of a field BSEM dataset collected in a giant oilfield in Saudi Arabia. It was demonstrated that 3D inversion of the BSEM data can be effectively used for mapping and monitoring fluid distribution inside a reservoirs and imaging the oil-water contact.

Recent studies have inferred the feasibility of time-lapse controlled-source electromagnetic (CSEM) methods for the monitoring of offshore oil and gas fields. The time-lapse CSEM inverse problem is highly constrained though inherently 3D since the geometry of the reservoir is established prior to production from high-resolution seismic surveys, rock and fluid properties are measured from well logs, and multiple history-matched production scenarios are contained in dynamic reservoir models. Using Archie’s Law, rock and fluid properties from dynamic reservoir simulations of the Harding field in the North Sea were converted to resistivity, from preproduction in 1996 to decommissioning in 2016. CSEM data were simulated for each state. We demonstrate how 3D inversion can be used for monitoring the oil-water contact from preproduction to the end of oil production in 2011, and for monitoring of the gas-water contact from 2011 to 2016 during gas production. In particular, we show that focusing regularization is able to recover sharp resistivity contrasts across the oil-water and gas-water boundaries, whereas smooth regularization fails to recover an adequate resistivity contrast.

A towed streamer electromagnetic (EM) system capable of simultaneous seismic and CSEM data acquisition has been developed and tested in the North Sea. The towed EM data are processed and delivered as a time-domain impulse response. In this paper, we use 3D modeling and inversion to investigate the ability of the towed EM system to detect and characterize the Harding field, a typical North Sea-type target. The 3D model of the Harding field itself was constructed from dynamic reservoir simulations. We have compared our 3D inversion of time-domain towed streamer EM data with 3D inversion of conventional frequencydomain CSEM data. We observe similarities in the recovered models. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables CSEM data to be acquired simultaneously with seismic over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM.

Recent studies have inferred the feasibility of time-lapse controlled-source electromagnetic (CSEM) methods for the surveillance of offshore oil and gas fields. However, quantitative interpretations have not been shown to ascertain what information about the reservoirs that may be recovered. We present a 3D inversion study of synthetic time-lapse CSEM data for the lateral water flooding of a reservoir unit where the hydrocarbon accumulation is trapped by a thin dome structure. We demonstrate that even with few constraints on the model, the flooding front can be recovered from 3D inversion. In this paper, synthetic time-lapse CSEM responses are simulated with the threshold about the noise floor and subject to multiple 3D inversion scenarios. The time-lapse CSEM inverse problem is highly constrained though inherently 3D since the geometry of the reservoir is established prior to production from high resolution seismic surveys; rock and fluid properties are measured from well logs; and multiple history matched production scenarios are contained in dynamic reservoir models.

Summary The integration of shared earth modeling and robust 3D CSEM modeling and inversion is the key to deriving a reliable quantitative interpretation from marine controlledsource electromagnetic (CSEM) data. Workflows should make use of all available subsurface data and enable the interpreter to select the most geologically relevant resistivity model from the multitude of models that satisfy the same CSEM data. To this end, we present our implementation of an iterative migration method for CSEM data, equivalent to rigorous inversion. Our iterative migration method is based on the 3D integral equation method with inhomogeneous background conductivity and focusing regularization with a priori terms. Here, we will show that focusing stabilizers recover more geologically realistic models with sharper geoelectric contrasts and boundaries than traditional smooth stabilizers. Additionally, we will show that focusing stabilizers have better convergence properties than smooth stabilizers. Our method is implemented in a fully parallelized code, which makes it practical to run large-scale 3D iterative migration on multi-component, multi-frequency and multi-line CSEM surveys for models with millions of cells. We present a suite of interpretations obtained from different migration scenarios for a 3D CSEM feasibility study computed from a detailed model of the Shtokman gas field in the Russian sector of the Barents Sea.

Marine controlled-source electromagnetic (MCSEM) technology has been successfully established as an effective tool for offshore hydrocarbon (HC) exploration. In this paper we consider another application of the MCSEM method for HC reservoir monitoring. We demonstrate that EM methods can also be used for the monitoring of producing wells in connection with the enhanced recovery of hydrocarbons. We have developed a powerful new EM modeling technique based on the integral equation method with an inhomogeneous background conductivity (IE IBC). This new method and the corresponding computer software make it possible to model the EM response over a realistic complex model of a sea-bottom HC reservoir. The numerical modeling results demonstrate that the MCSEM method has the ability to map changes in resistivity caused by the production of hydrocarbons over time. In addition, the EM data help to visualize changes in the location of the oil-water contact within the reservoir. This result opens the possibility for practical application of the EM method in HC reservoir monitoring.