Monitoring casing corrosion of legacy wells using CSEM: implications for large-scale energy and CO2 storage projects in shut-down hydrocarbon fields

Large-scale CO2 and energy storage is a mandatory part of the green shift to reduce CO2 emissions and limit consequences of climate change. Large-scale storage will require the use of shut-down depleted hydrocarbon fields to take advantage of well-characterized reservoirs and cap rocks. Thanks to extensive data from historical hydrocarbon production, the uncertainties related to storage capacity, injectivity, and containment are limited. However, legacy exploration and production infrastructure, and especially legacy wells, are the main threat for possible fluid leakage toward the surface. Such legacy wells are numerous and penetrate the full rock column. In this paper, we describe a workflow to screen and monitor legacy wells in the shut-down Frigg Field in the North Sea. By using numerical modeling of electromagnetic (EM) field propagation in one of the Frigg Field wells, we explore the complex interactions of fields, currents, and well structure in the presence of corrosion. The corrosion is implemented as a change in the electrical conductivity of the innermost steel casing at different depths along the structure. To enhance probing depth, we plug the dipole source (1 km long) into the casing at the seafloor and excite the casing as an antenna. We find that at moderate levels of corrosion, the current distribution is significantly modified with respect to the uncorroded case. This generates a signal that propagates and can be observed at the seafloor in the numerical results. Other elements of the well geometry (e.g., concentric overlapping cement casings) have their own effect on the signal. This leads the possibility of estimating the location of the corroded area within the well geometry. These results suggest that by relaxing some of the model's approximations and implementing realistic transmitters, it will be possible to evaluate and optimize controlled-source EM survey strategies for detecting and monitoring corrosion levels.publishedVersio

Inter-source seismic interferometry by multidimensional deconvolution (MDD) for borehole sources

Seismic interferometry (SI) is usually implemented by crosscorrelation (CC) to retrieve the impulse response between pairs of receiver positions. An alternative approach by multidimensional deconvolution (MDD) has been developed and shown in various studies the potential to suppress artifacts due to irregular source distribution and intrinsic loss. Following previous theories on SI by MDD, we extend it to retrieve the impulse response between pairs of source positions by invoking source and receiver reciprocity. We verify the theory using a simple two-layered model and show that the retrieved response by MDD is more accurate than that by CC, and furthermore, it is free of free-surface multiples. We discuss the necessary pre-processing required for this method. This inter-source SI approach creates a virtual acquisition geometry with both borehole sources and receivers without the need to deploy receivers in the borehole, which might be of interest to applications such as seismic while drilling (SWD)

Apport de l'inversion des formes d'onde sismique - approximation 2D élastique - à la caractérisation des milieux de la proche surface : cavités souterraines, glissements de terrain

Full waveform inversion has shown a great suitability in the acoustic case for imaging large and deep kilometric structures. Contrary to first traveltimes tomography, this imaging technique allows to process the whole seismic signal in order to produce a high resolution image of the medium. In the particular context of subsurface prospecting, the elastic approximation needs to be considered to take into account the surface waves which constitute the most part of the seismic signal, with the offset range used for such studies. In our approach, the forward problem is numerically solved in the frequency domain in order to take into account a multi-source and multi-receiver seismic device efficiently. In front of the difficulties inherent to the finite difference method with regard to complex topographies, a formulation based on finite elements discontinuous Galerkin technique is finally used. The inverse problem requires an initial guess of the inverted parameters to minimize the objective function in an iterative way. The method is first applied to simple numerical models to assess the role of the different parameters which control the inversion process. A synthetic model of the Super-Sauze earthflow, characterized by strong contrasts and a complex topography is afterwards considered to evaluate the performances of the code. This study allows us to highlight the impact of the optimisation technique used, the acquisition device, the quality of the initial model and the contribution of the preconditioning strategy used in the inversion process. The approach is then applied to examples using real data acquired over the Jargeau site (45) in the presence of a cavity, and over the Super-Sauze earthflow (04). The difficulties inherent to real conditions require an important data pre-processing. The source estimation is also a critical issue that directly impacts the convergence of the inversion. In the first case, the proposed methodology succeeds to reconstruct an image of the seismic parameters distributions (Vp and Vs) with a relatively good accuracy, the final quality of these images being dependant on the efforts produced to constrain the inversion. The second case reveals the difficulties related to complex media ; seismic data show a low signal to noise ratio, the topography is very sharp and generates a strong level of diffracted surface waves, the seismic source varies from one shot to another. Despite the complexity of the context, the inversion results are in good agreement with the geotechnical knowledge of the site.L'inversion des formes d'onde sismique a démontré toute sa pertinence dans le cas acoustique pour imager des structures kilométriques. Contrairement aux méthodes de tomographie des premiers temps d'arrivée, cette technique d'imagerie à l'avantage d'exploiter l'ensemble du signal sismique pour produire une image haute résolution du milieu. Dans le contexte de la proche surface, l'approximation élastique devient nécessaire afin de prendre en compte les ondes de surface, qui prédominent le signal sismique avec les gammes de déports utilisées. Dans notre approche, le problème direct est résolu numériquement dans le domaine fréquentiel afin de prendre en charge efficacement des acquisitions multisources et multi-récepteurs. Devant les complications générées par une approche en différences finies pour traiter les topographies complexes, une formulation en éléments finis Galerkin discontinus est finalement utilisée. Le problème inverse nécessite un modèle initial pour les paramètres inversés et permet de minimiser la fonction objectif de façon itérative. Cette méthode est appliquée dans un premier lieu à des modèles numériques simples afin de contrôler le rôle des différents paramètres de l'inversion. Un modèle synthétique du glissement de Super-Sauze, caractérisé par de forts contrastes et une topographie complexe, est par la suite utilisé pour évaluer les performances du code. Cette étude met en évidence l'importance de la méthode d'optimisation utilisée, du dispositif d'acquisition, de la qualité du modèle initial et de l'apport des stratégies de pré-conditionnement utilisées dans le processus d'inversion. La méthode est ensuite appliquée à deux jeux de données réelles, acquis respectivement au-dessus d'une cavité maçonnée à Jarjeau (45), et sur le glissement de terrain de Super-Sauze (04). Les difficultés relatives aux conditions réelles nécessitent un prétraitement important des données. L'estimation de la source constitue aussi un problème majeur qui impacte directement la convergence de l'inversion. Dans le premier cas, la méthode proposée permet de produire une image plus ou moins fidèle des distributions des vitesses du milieu, la qualité finale de ces images variant suivant les efforts mis en oeuvre pour contraindre l'inversion. Le deuxième cas illustre quant à lui toutes les difficultés relatives aux milieux complexes : les données montrent un faible rapport signal sur bruit, la topographie très découpée génère un fort taux de diffraction des ondes de surface, la signature de la source admet de fortes variations d'un tir à l'autre. Malgré la complexité du contexte, les résultats de l'inversion des formes d'onde sont en bon accord avec la connaissance géotechnique du site

CSEM for CO2 Storage – Feasibility Study at Smeaheia to Optimise Acquisition

In this work, we evaluate the use of controlled-source electromagnetics (CSEM) for CO2 monitoring at Smeaheia, a possible candidate for future phases of the Norwegian full-scale CCS project. CSEM is sensitive to electrically resistive material replacing conductive pore water in the pore space, which enables to infer volumetric estimates of the injected CO2 in the formation. CSEM is often used in combination with high-resolution seismic reflection data due to the sensitivity of the two methods to complementary physical properties. Here, we present a technique to optimise the CSEM survey parameters for efficient 4D surveying. Realistic synthetic models prior to and after injection are derived from reservoir modelling and converted to electrical resistivities. Inversion tests are carried out in 2D for the baseline and monitor cases considering realistic data errors. We show that the resistivity changes due to CO2 injection can be monitored using CSEM. We discuss the optimal orientation of the receivers, frequency range and transmitter-receiver offset. We finally discuss a strategy for optimal survey design based on the sensitivity to the CO2 plume.publishedVersio

Apport de l'inversion des formes d'onde sismique - approximation 2D élastique - à la caractérisation des milieux de la proche surface (cavités souterraines, glissements de terrain)

PARIS-BIUSJ-Physique recherche (751052113) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Offset dependence of overburden time-shifts from ultrasonic data

Depletion or injection into a reservoir implies stress changes and strains in the reservoir and its surroundings. This may lead to measurable time-shifts for seismic waves propagating in the subsurface. To better understand the offset dependence of time-shifts in the overburden, we have systematically quantified the time-shifts of three different overburden shales in controlled laboratory tests. These experiments may be viewed as an analogue to the time-shifts recorded from seismic field surveys. For a range of different stress paths, i.e. the ratio between the horizontal and the vertical stress changes, the changes of the P-wave velocities in different directions were measured such that the offset dependence of time-shifts for different stress paths could be studied. The time-shifts are stress path dependent, which is particularly pronounced at large offsets. For all stress paths the time-shifts exhibit a linearly decreasing trend with increasing offset, i.e. a negative offset-gradient. At zero offset, for which the ray path is normal to the bedding, the time-shifts are similar for all investigated stress paths. The isotropic stress path is associated with the smallest offset-gradient of the time-shifts. Contrary, the constant-mean-stress path shows the largest gradient with a flip in the polarity of the time-shifts for the largest offsets. The separate contributions from the strain and velocity changes to the time-shift were also quantified. The time-shifts for the isotropic stress path are dominated by the contribution from velocity changes at all offsets. Contrary, the strain contributes significantly to the time-shifts at small offsets for the constant-mean-stress path. This shows that the offset dependence in pre-stack seismic data may be a key to understand the changes of subsurface stresses, pore pressure and strain upon depletion or injection. To utilize this knowledge from laboratory experiments, calibrated rock physics models and correlations are needed to constrain the seismic time-shifts and to obtain an adequately updated geological model reflecting the true anisotropic nature of the subsurface. This may have important implications for improved recovery and safety, particularly in mature fields.publishedVersio

CSEM for CO2 Storage – Feasibility Study at Smeaheia to Optimise Acquisition

In this work, we evaluate the use of controlled-source electromagnetics (CSEM) for CO2 monitoring at Smeaheia, a possible candidate for future phases of the Norwegian full-scale CCS project. CSEM is sensitive to electrically resistive material replacing conductive pore water in the pore space, which enables to infer volumetric estimates of the injected CO2 in the formation. CSEM is often used in combination with high-resolution seismic reflection data due to the sensitivity of the two methods to complementary physical properties. Here, we present a technique to optimise the CSEM survey parameters for efficient 4D surveying. Realistic synthetic models prior to and after injection are derived from reservoir modelling and converted to electrical resistivities. Inversion tests are carried out in 2D for the baseline and monitor cases considering realistic data errors. We show that the resistivity changes due to CO2 injection can be monitored using CSEM. We discuss the optimal orientation of the receivers, frequency range and transmitter-receiver offset. We finally discuss a strategy for optimal survey design based on the sensitivity to the CO2 plume

Inversion Of Surface Waves In Complex Structures

International audienc

Combined geophysical and rock physics workflow for quantitative CO2 monitoring

Safe CO2 storage requires conformance verification, i.e. confirmation that the pressure and CO2 accumulation are consistent with modelling forecasts within a given uncertainty range. Quantitative estimates of relevant reservoir parameters (e.g. pore pressure and fluid saturations) are usually derived from geophysical monitoring data (e.g. seismic, electromagnetic and/or gravity data) and potential prior knowledge of the storage reservoir. We describe a two-step strategy combining geophysical and rock physics inversions for quantitative CO2 monitoring. A Bayesian formulation is used to propagate and account for uncertainties in both steps. We demonstrate our workflow using datasets from the Sleipner CO2 storage project (Norwegian North Sea) and combining seismic Full Waveform Inversion and rock physics inversion. We derive rock frame properties from baseline data and use them as input to obtain 2D spatial distribution of CO2 saturation with uncertainty assessment from monitor data. We also discuss the need for advanced rock physics models, considering the way fluid phases are mixed (uniform to patchy mixing) and the trade-off effects of pore pressure and fluid saturation on geophysical measurements. We consequently recommend a joint rock physics inversion approach, where multi-physics, and multi-parameter inversion can be used for better discrimination of pressure, saturation, and fluid mixing effects towards more quantitative conformance verification.publishedVersio

Inversion Of Surface Waves In Complex Structures

International audienc