Marlim R3D: a realistic model for CSEM simulations - phase I: model building

It is well known that no single geophysical method can provide a completepicture of the earth and its properties and recently integrated interpretationand joint inversion of multiple data types has become a much studied topic(Chen et al, 2007; Harris et al, 2009; Jegen et al, 2009; de Stephano et al,2011). The seismic method is in many situations the tool of choice: it isgeneral and widely applicable, and can provide detailed images of sub-surfacestructure and stratigraphy from which complex geological models can beconstructed. If seismic methods can provide the answer to the question ofinterest then this is undoubtedly the tool to use. However although seismicdata are extremely sensitive to the changes in lithology occurring at theboundaries between geological units, they are less sensitive to fluid changeswithin these units. This is because acoustic and elastic properties of theearth show only small changes when the fluid content or saturation is changed. These changes can in some circumstances be detected and used to provideinformation on fluid distribution. In other situations this is difficult orimpossible to do with certainty, and complementary geophysical methods must beemployed to meet the reservoir characterization goal. In many situations electrical resistivity is driven by the properties anddistribution of fluids in the earth. Commercial hydrocarbon deposits may bemany times more resistive than surrounding lithologies. This change inresistivity caused by variations in fluid content and saturation can, inprinciple, be detected using CSEM tools (See Constable & Srnka 2007 for areview of CSEM technology). However when only CSEM data are considered, structural resolution is poor because of the diffusive nature of the EM fields, and the results can be ambiguous because the effect of an increase in porefluid resistivity cannot be distinguished from the effect of a decrease inporosity. The presence of frustrating resistors in the section (for exampletight carbonates, cemented sandstones or volcanics) can also complicate theinterpretation. Well-log data provide a range of measurements, including both resistivity andacoustic/elastic properties as well as a range of further properties. Apetrophysicist analyzing this well log data will take all of these measurementsand integrate them together to provide an interpretation of the lithology andfluid properties. The resistivity measurement in particular provides keyinformation on the fluid content at the well bore. However such informationcannot provide any constraint on the variation of properties away from thewell, across a reservoir. For any given geophysical question, the most robust answer will be obtained byusing the tool, or combination of tools best suited to the task, andintegrating the resulting data within a rock physics framework, to provide ashared earth model that is geologically reasonable, and consistent with each ofthe geophysical data types available. Careful integration of multiple datatypes can allow the strengths in one method to compensate for the weaknesses inanother. Here we consider three data types: seismic, controlled source electromagnetic(CSEM), and well log data. The workflow applied is shown in figure 1. CSEM dataare first inverted to provide a measure of resistivity. Because of the poorstructural resolution of CSEM data taken in isolation, this inversion isconditioned with seismic structural information. From the results, thetransverse resistance (the vertically integrated resistivity), which is wellconstrained by the CSEM method, can be calculated within the intervals ofinterest.

We have found how the effects of the air wave in marine controlled-source electromagnetic (CSEM) methods gradually vanish in the sea for shallow waters, and how at the same time they gradually grow below the seafloor, in an effort to comprehend existing detectability definitions. The transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea. We consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs, particularly in shallow-water explorations and, specifically, on how the air wave affects detection. Our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1D profile. Two-dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets. Results include the expected growth of the inhibiting effect of the sea as the water depths decrease. However, we also find that this happens up to a point and that from then on its effect decreases to zero. Regarding the resistive layer at depth, its importance grows to a constant as the water depth decreases to zero. As a function of offsets, there appear first the direct current effects. The induction zone is next and is dominated by contributions from the underlying formations. The third zone, which corresponds to the air wave, is largely dominated by contributions from the sea. The fourth and last zone is the plane-wave asymptote. All four classical zones identified in marine CSEM are also present in land CSEM.

SUMMARY Carbon capture and storage (CCS) within sealed geological formations is an essential strategy to reduce global greenhouse gas emissions, the primary goal of the 2015 United Nations Paris Agreement. Large-scale commercial development of geological CO2 storage requires high-resolution remote sensing methods to monitor CO2 migration during/after injection. A geological formation containing a CO2 phase in its pore space commonly exhibits higher electrical resistivity than brine-saturated (background) sediments. Here, we explore the added value of the marine controlled-source electromagnetic (CSEM) method as an additional and relevant geophysical tool to monitor moderate to significant changes in CO2 saturation within a fluid conduit breaking through the seal of a CCS injection reservoir, using a suite of synthetic studies. Our 2-D CSEM synthetic models simulate various geological scenarios incorporating the main structural features and stratigraphy of two North Sea sites, the Scanner Pockmark and the Sleipner CCS site. Our results show significant differentiation of leakage through the seal with CO2 saturation ($S_{{\rm CO}_2}$) ranging between 20 and 50 per cent, while our rock physics model predicts that detection below 20 per cent would be challenging for CSEM alone. However, we are able to detect with our 2-D inversion models the effects of saturation with 10 and 20 per cent CO2 within a chimney with 10 per cent porosity. We demonstrate that simultaneous inversion of Ey and Ez synthetic electric field data facilitates a sharper delineation of a CO2 saturated chimney structure within the seal, whereas Ez synthetic data present higher sensitivity than Ey to $S_{{\rm CO}_2}$ variation, demonstrating the importance of acquiring the full 3-D electric field. This study illustrates the value of incorporating CSEM into measurement, monitoring and verification strategies for optimal operation of marine CCS sites.

SummaryThe marine controlled-source electromagnetic (CSEM) method has been evolving into a geophysical imaging tool for increasingly complex geological settings in which multiple resistive bodies can be resolved. An advanced subsurface imaging workflow for 3D CSEM surveys is presented, which reproduces the resistivity distribution to within a spatial resolution determined by the frequencies included. The performance of our advanced-processing workflow is demonstrated using a case study from the Gulf of Mexico (GOM), where a dense 3D grid was acquired over an area of high-quality seismic data and well log control.At the core of our 3D workflow is an inversion methodology with approximate Hessian-based optimization and a fast finite-difference time-domain forward operator. The optimization matches the synthetic to the measured field within 100–200 iterations and is sufficiently robust in three dimensions to avoid expensive regularization schemes. The sensitivity of the gradient-based inversion to the starting model is addressed by investing considerable effort in building 1D inversion-based starting models. At the same time, 3D inversion algorithms for survey layouts, including azimuthal data, demand high-quality data conditioning for which we present a processing sequence from time-domain electromagnetic data acquired by seabed receivers to frequency-domain data and weights for inversion.Detection and delineation of reservoirs in the presence of salt is recognized as a major challenge to CSEM methods. In order to interpret 3D data accurately in such a complex environment, the true-resistivity cube is built from a sequence of constrained inversion-based interpretation steps. Using a data set acquired in 2008 in the GOM, we demonstrate the ability of our 3D technology to resolve small (2×2 km), low-resistivity pay (Δρ:5Ω·m) targets within the vicinity (< 1 km) of large salt bodies. In this case study, the 3D method converged within 1 week, running on 150 parallel nodes.

The controlled-source electromagnetic method (CSEM) has been gaining considerable attention and its main use is the monitoring of hydrocarbon reservoirs, due to its sensitivity in the verification of electrical resistivity contrasts, in addition to being slightly less harmful to marine biota than traditional seismic methods. In this manuscript, a multiphysics modelling of the marine controlled-source electromagnetic method (MCSEM) and the vertical-vertical controlled-source electromagnetic method (VVCSEM) was constructed using a realistic geologic model from the Marlim oilfield, Campos Basin - Brazil. Marlim is an oilfield belonging to the Marlim Complex, located in the Brazilian passive margin, which has large intervals of carbonate rocks that laterally mix with sandy sequences. The basin is dominated by shales intercalated with turbiditic sandstones at greater depths, very similar to the stratigraphic package found in the Potiguar Basin – Brazil. A comparison between the two CSEM methodologies is presented, as well as a survey design proposal. The MCSEM and the VVCSEM methods proved to be efficient in detecting the responses of the hydrocarbon reservoir even in the face of a considerable layer of salt, with VVCSEM achieving a better imaging response in some geological situations in which MCSEM almost did not identify the presence of a reservoir.

Controlled-source electromagnetic (CSEM) method is an important geophysical tool for sensing and studying subsurface conductivity structures. Advanced forward modeling techniques are crucial for the inversion and imaging of CSEM data. In this letter, we develop an accurate and efficient 3-D forward modeling algorithm for CSEM problems, combining spectral element method (SEM) and octree meshes. The SEM based on high-order basis functions can provide accurate CSEM responses, and the octree meshes enable local refinement, allowing for the discretization of models with fewer elements compared to the structured hexahedral meshes used in conventional SEM, while also providing the capability to handle complex models. Two synthetic examples are presented to verify the accuracy and efficiency of the algorithm. The utility of the algorithm is verified by a realistic model with complex geometry.

A scaled experiment for the verification of the SeaBed Logging method

Chapter 14 - Electromagnetic Methods in the Frequency and Time Domains

Marine controlled-source electromagnetic (CSEM) method has proved to be a powerful tool and found its increasing applications for offshore resource exploration and tectonic studies. However, bathymetric variations and anisotropic structures encountered in practical measurements have posed challenges for reliable interpretation of marine CSEM data collected in increasingly complex geologic settings. In this study, we present a Finite element (FE) based forward algorithm for simulating 3-D marine CSEM responses in geologic settings characterized with rough bathymetry and electrical anisotropy. Unstructured tetrahedral meshes are employed to permit precise descriptions of arbitrary seafloor topography. After validating the accuracy of the algorithm, we investigate the effects of bathymetric variations and complicated electric anisotropy separately and jointly on marine CSEM responses. Numerical results have demonstrated that both the bathymetry and electrical anisotropy can cause significant distortions on marine CSEM responses, but to different extent. The effects of full electrical anisotropy and bathymetry have to be taken into consideration for reliable interpretations of marine CSEM data in practice.

Controlled-source electromagnetic (CSEM) surveying has been in use for more than ten years and has become a well-established tool for hydrocarbon exploration in marine environments. On land, however, CSEM methods are rarely applied in a similar manner, since challenges are greater for a range of technical, logistical, and numerical aspects. Over the last five years, the Geo-Electromagnetics working group at the German Research Centre for Geosciences Potsdam (GFZ) has been developing the CSEM method for land-based applications, including a three-phase current transmitter allowing for arbitrary source polarizations, new robust data processing concepts, and 1D to 3D modelling and inversion software. The CSEM hardware and software has been successfully applied in a series of surveys on different targets. We show examples from case studies, including 3D inversion results from the Ketzin CO2 injection test site in Northern Germany.

Monitoring geothermal reservoir developments with the Controlled-Source Electro-Magnetic method — A calibration study on the Reykjanes geothermal field

Subsurface resistivity mapping based on Controlled Source Electromagnetic (CSEM) measurements is an attractive technology for exploration as it offers the possibility to distinguish between hydrocarbon and brine bearing prospects where conventional seismic methods prove inconclusive. In Shell, we have applied the CSEM method on a worldwide scale since 2003 to both de-risking and portfolio polarization. Early on in the development of the CSEM technique some compelling results were obtained with single 2D profiles over prospects. Unfortunately, the lack of subsurface coverage of this type of acquisition often leads to ambiguous results because the Earth rarely satisfies the 2D assumption at the scale of the CSEM experiment. To reduce such ambiguities, we have focused efforts on the development of 3D processing and inversion capabilities as well as interpretation workflows that take the complexity of the Earth into account. In this paper, we share some of our motivations behind our approach and illustrate its effectiveness with both real and synthetic data examples.

The Controlled-Source Electromagnetic (CSEM) method is one of the electromagnetic methods utilized in geophysical exploration. This method provides a subsurface image through the resistivity anomalies of materials encountered by electromagnetic waves. The research area is located near a major city, resulting in high electromagnetic noise. Electromagnetic noise can be categorized into two types of the noise namely periodic noise and sporadic noise. Eliminating noise is a crucial objective to enhance data quality, as it can introduce uncertainty into interpretations. Three noise removal techniques are employed: pre-stack to filter the harmonic noise, stacking to remove the sporadic noise, and post-stack for smoothing. The CSEM data used consists of signals in the time domain with a 10-second period and a 50% duty cycle. The results of applying these noise removal techniques indicate that all three methods are highly effective in noise reduction. The pre-stack technique can remove periodic noise, while sporadic noise is addressed by the stacking technique, and signal smoothing can be achieved using the poststack technique.

Offshore energy-transition activities typically require the construction of new subsurface facilities where overburden heterogeneity and deformation patterns may not be fully understood. Anisotropic marine controlled-source electromagnetic (CSEM) and seismic refraction imaging can be adapted to geohazard assessments of the overburden to mitigate safety and cost risks in such areas. This is investigated using an area in deepwater northwest Borneo as the case study. For this proof-of-concept study, a well-established multiphysics joint inversion code is extended to handle marine seismic refraction data from conventional towed streamers. Realistic field-scale synthetic models with anomalous resistivities and velocities in the shallow section (approximately 400 m below mudline) are built and their CSEM and seismic refraction responses are first inverted separately and then jointly in three dimensions using the cross-gradient method. The joint inversion recovers the anomalous bodies better than the separate inversions. The velocity model from the joint inversion indicates improvement from the separate inversion but requires a good starting model. As a further test of the methodology, first-break traveltimes are picked from legacy 3D seismic data and inverted with the available sparse CSEM data. For model verification, resistivity and velocity profiles from separate and joint inversion models are extracted at a well location in the survey area and compared with actual well logs. The joint inversion models match the well logs better than the separate inversion models. Moreover, known geohazards (complex fault sets and lateral inflows) from independent geomatics study are imaged in the joint inversion models. In addition, the resistivity anisotropy ratio tracks anomalous zones that correspond to areas of recent complex faulting and potential lateral inflows in the overburden mass transport systems. Thus, the joint inversion of CSEM and seismic refraction data has the potential to improve the mapping of overburden hazards offshore, with the associated resistivity anisotropy serving as a proxy for active ground deformation.

Marine controlled-source electromagnetic method (CSEM) is increasingly used today in oil exploration in conjunction with seismic data. It helps to decide whether or not a structure displays the resistive anomaly characteristic to the presence of hydrocarbons. In this respect, marine CSEM is intended to provide a non seismic attribute that can be considered as a Direct Hydrocarbon Indicator. However, since resistivity is closely related to the nature of the fluids in the reservoir, one can also expect that changes in the fluid flow can induce resistivity changes that can be measured performing time-lapse measurements. In this paper we simulate a lateral variation of the brine-hydrocarbon contact and give an estimation of the 3D corresponding time lapse signal in a land environment.