XI International Conference on Structural Dynamics, 2020
In this study, we propose and implement a convergence scheme for the numerical wavenumber integra... more In this study, we propose and implement a convergence scheme for the numerical wavenumber integration in the context of 2.5D FE solution and validate it through a numerical example. In brief, the scheme is to estimate the convergence based on the relative difference between two interpolations for a given set of wavenumbers, which is done iteratively by refining the wavenumber sampling until the relative difference becomes no greater than a specified value. We evaluate the performance and convergence of the numerical integration scheme by comparing the results of the responses of a tunnel in a layered ground due to a concentrated harmonic load using both a full 3D model and a 2.5D model, including computational time aspect. Finally, we consider applying the same scheme to other relevant numerical integrations.
This study presents a finite element (FE) framework for simulation of steady state wave motion, c... more This study presents a finite element (FE) framework for simulation of steady state wave motion, combined with a so-called PML approach. The steady state wave motion of current interest are not only time-harmonic load but can also be constant-speed moving load. We adopt an existing PML approach (from low-frequency electromagnetic wave) to apply to the steady state FE simulation of constant-speed moving load vibration as well as temporal harmonic load vibration. In the paper, we will present the PML formulation in both the frequency and steady-state time domains. To validate the approach, then we compare the simulation results with available analytical reference solutions, and with a set of real measured field data (high-speed train data). In addition, we solve more example of steady state vibrations resulting from different train speed (e.g. subcritical, critical and supercritical).
The response of railway tracks and trackside vibration are strongly governed by the quality of th... more The response of railway tracks and trackside vibration are strongly governed by the quality of the track. Defects or nonhomogeneities in the track/substructure/ground can remarkably increase the responses in the system, leading to further deterioration of the track. This issue is more dramatic in high-speed lines. The objective of this paper is to study the impact of two types of non-homogeneities on the track response. In the first case, the effect of hanging sleepers is studied, and in the second case, the effect of locally deteriorated substructure is investigated. Two numerical solutions are used for the simulation of track-substructure-ground response. The first is the frequency-domain solution VibTrain that has been developed for efficient simulation of track/ground response under moving loads using a combination of beam elements for the track/substructure and Green's functions for the layered soil medium. The second model is a FE model in COMSOL Multiphysics enhanced with the absorbing boundary PML (Perfectly Matched Layer) in the time domain. In addition to studying the effect of track defects on rail vibration, the results of the two solutions are compared and practical conclusions are drawn on the potential of using vibration data for detection of track defects.
In this study, we introduce and discuss features and improvements of the well-established stiffne... more In this study, we introduce and discuss features and improvements of the well-established stiffness matrix method that is used in simulation of wave propagation in layered media. More specifically, we present stiffness matrices for an acoustic layer and a vertically transverse isotropic (VTI) viscoelastic soil layer. Combining these stiffness matrices enables a straightforward technique for modeling of acousto-elastic wave propagation in layered infinite media. In addition, we propose a technique to simulate discontinuity seismic sources, which was not used earlier in the context of the stiffness matrix method. Finally, we propose a framework to derive a key parameter of the absorbing boundary domain technique Perfectly Matched Layer (PML). Numerical examples are presented in order to help understanding the features and improvements discussed in the study from the fields of geophysics and soil dynamics. It is believed that the features and improvements discussed herein will make the application of the stiffness matrix method even wider and more flexible.
Bulletin of the Seismological Society of America, 2006
A rigorous method is presented to obtain-directly in the time domain and without the need for an ... more A rigorous method is presented to obtain-directly in the time domain and without the need for an integral transform over frequencies-the seismic response of horizontally layered, viscoelastic half-spaces due to arbitrarily distributed (or concentrated) sources. The method is based on exact expressions for the response of the underlying half-space cast in the wavenumber-time domain, and a complete modal solution in that domain for the layers. The proposed method is highly effective and avoids the problems of the usual propagator matrix method that arise when numerical integrations over both frequencies and wavenumbers are carried out, especially the waviness of the kernels due to resonances and reverberations in the layers when damping is light or even absent. Part I presents the basic methodology for the relatively simple case of antiplane SH sources in two-dimensional space, while Part II generalizes the concept to two-dimensional SV-P line sources and to threedimensional point sources, including seismic couples.
To monitor the integrity of the storage formation in relation to CO2 injection, several methods a... more To monitor the integrity of the storage formation in relation to CO2 injection, several methods and technologies will be employed, including microseismic monitoring. Injection of CO2 leads to increased pore-pressure which in turn can activate existing weaknesses, e.g. faults, at the risk of leakage. Presented is a hydro-mechanical model that describes single-phase fluid flow in a deforming porous media with an existing weakness that can fracture. In the intact porous media, the fluid flow is governed by Darcy's law and the geomechanical behavior is described by Biot's theory of linear poroelasticity. The weakness zone is defined as a zero-thickness domain and the governing equations are obtained by integrating the governing equations of the intact porous media over the virtual thickness of the weakness. This makes possible the seamless transition of an intact porous media formulation to an open fracture formulation as the weakness zone activates, deteriorates and eventually ruptures. The deterioration of the weakness zone is described using the Cohesive Zone Model (CZM). The hydro-mechanical model is used to study the impact of different fluid types on activation and fracturing of a weakness, in particular mode-I, tensile failure, and potential implications for micro-seismicity. Firstly, it was confirmed that fluid type matters, e.g. under the same pressurization, different fluids (different mobility and compressibility) propagates a fracture at different velocities. Additionally, three hypotheses from observations in the literature was tested. One hypothesis was confirmed, as simulation results show that for a high-viscosity fluid, a strong pore pressure gradient occurs ahead of the fracture tip, in the propagation direction, thus driving the fluid flow and the propagation of the fracture, favoring longer fractures. However, for the other two hypotheses, the hydro-mechanical model could neither confirm that low-viscosity fluids will tend to produce smaller but more abundant fractures (as reported in the literature), nor reproduce observations of lower breakdown- and initiation pressure of rocks for low-viscosity fluids. The reason for this discrepancy could be differences in the premises of the observation and the case studied here, or due to other factors not considered in this study, e.g. chemical effect, differences in wetting properties of the rock, or that super critical CO2 is not representative for typical low-viscosity fluids.
It is well known that the Norwegian North Sea has a great potential to store anthropogenic CO2 in... more It is well known that the Norwegian North Sea has a great potential to store anthropogenic CO2 in large scale, captured in Europe, in the geological time frame. Recently, the Norwegian government has also established a plan to develop the first full-scale CCS project, called Northern Lights, in which the firstly-chosen candidate site (named Smeaheia) was the Sognefjord formation, located between the Troll gas field and the Norway west coast. Not surprisingly, the Smeaheia site has a lot of faults of different sizes, including two regional faults of Vette and Øygarden, respectively, in the west and east bounds. We know the locations of the two regional faults, but it is rather challenging to quantify any further details of the two regional faults such as fault-composite structure (core zones, damage zones, etc.), dynamic hydraulic and geomechanical properties, and so on. Since such information is not fully known as of today, there is a high uncertainty to foresee whether or not there is a possibility of injected CO2 leaking through the regional faults for a given injection plan. If (and only if) any leakage would happen at all, we need to be prepared to answer "How could we monitor and detect cost-effectively such unwanted events?", which requires the sensitivity study of geophysical data (e.g. seismic, gravity, EM, etc.), yet with taking into account realistic geological models. In this study, we investigate how, and more precisely, whether or not, we could monitor any CO2 flow through the Vette Fault by means of marine CSEM survey. This type of survey has not yet been done in the North Sea setting. The only evidence of the efficiency of CSEM data to detect CO2 flow through faults/fractures was made through a laboratory test where liquid CO2 was injected to a naturally-fractured shale core (Nooraiepour et al, 2018), while the changes in seismic velocity and electrical resistivity were measured. It was clearly shown that the electrical resistivity is much more sensitivity to the CO2 flow than the seismic velocity. The leakage scenario at the Smeaheia is not yet well defined, either. Nevertheless, it is useful and necessary in advance to learn how the CSEM data may interact with the CO2 flow through the Vette Faults, which is the main target of the present study. Through this study, we also identify the issues and factors for which the CSEM technique should be improved further in order to make CCS projects in the North Sea successful.
Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-crit... more Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-critical state into deep saline aquifers. As CO2 is injected, stress changes due to fluid pressure and temperature changes may result in deformation and potentially may result in microseismicity if existing faults are reactivated. Microseismicity may provide additional information on in situ stress conditions and the progression of the CO2 plume. To assess failure criteria in the reservoir, we must know the mechanical response of reservoir sandstone due to injection of super-critical CO2 (scCO2) under reservoir stress conditions. This includes fracturing processes at the microstructural scale as well as at the macroscopic scale by reactivation of existing fractures. We conducted three triaxial tests on a sandstone analogue to deep North Sea reservoir rock: once pressurized, samples were saturated with either scCO2, brine, or a brine-scCO2 mixture. Each sample was then deformed axially to ind...
On behalf of NGI, under FME SUCCESS, an MSc thesis at the University of Oslo has been conducted t... more On behalf of NGI, under FME SUCCESS, an MSc thesis at the University of Oslo has been conducted to finalize an inversion study of a CSEM dataset acquired in 2008 for the Sleipner CO2 storage project. The aim of the MSc thesis was to establish an inversion strategy that can be applied in order to resolve some of the various challenges related to inverting the dataset. This paper presents some of the work done related to the influence on the CSEM data from a pipeline network at seabed, with the objective of establishing a method to identify and filter out strongly influenced data, in order to improve inversion results.
Pre-existing and induced fractures and faults can play a role as bypass conduits and fast leaking... more Pre-existing and induced fractures and faults can play a role as bypass conduits and fast leaking channels in CO2 storage sites. They should therefore be well characterized during site selection, and monitored thoroughly during operation to track the movement and fate of the CO2 plume. Despite to date extensive research on the geophysical properties of brine- and CO2-saturated porous reservoir rocks, changes in acoustic velocity and electrical resistivity during a sole fracture fluid displacement are, however, rather little investigated. Hence, we herein present a laboratory study of core-scale geophysical monitoring during drainage-imbibition cycles of the brine-CO2 system through a shale caprock core sample with a vertical fracture. The experiments were conducted using both gaseous and scCO2 with 4 and 9 MPa pore pressures, respectively, at 12 MPa confining pressure. The tests were performed at 40°C during the loading and unloading stages in order to look into the hysteresis effect. We used a fractured core sample from the Upper Jurassic organic-rich shales of the Draupne Formation, which is the primary caprock for the Smeaheia CO2 storage site – a full-scale CCS project in Norway. The primary objective of the experiment was to compare the geophysical measurements using gaseous and scCO2 drainage-imbibition cycles during the tests in a core-scale experiment. Moreover, we were interested to see how sensitive acoustic velocity and electrical resistance techniques are to the fracture fluid displacement using different CO2 phase states. The outcomes of our high-pressure high-temperature experiment of simultaneous measurements of fracture flow and geophysical properties indicate that potential leakage of injected CO2 through the fractured-shale caprock can be detected in the core-scale laboratory experiments. The performed drainage-imbibition cycles using gaseous and scCO2 resulted in different behaviors in P-wave velocity (Vp) and electrical resistance in axial and radial directions for these two phase states. The measured Vp during the displacement of fracture fluid, CO2-brine subsequent cycles, showed a limited sensitivity in terms of magnitude and relative change. The electrical resistance, on the other hand, shows higher sensitivity and larger variation during fluid displacement along the fracture. It was also observed that the crossplot of Vp versus electrical resistance could detect and even differentiate the different phases during the loading and unloading stages.
Structure-from-motion (SfM) photogrammetry enables the cost-effective digital characterisation of... more Structure-from-motion (SfM) photogrammetry enables the cost-effective digital characterisation of seismic- to sub-decimetre-scale geoscientific samples. The technique is commonly used for the characterisation of outcrops, fracture mapping, and increasingly so for the quantification of deformation during geotechnical stress tests. We here apply SfM photogrammetry using off-the-shelf components and software, to generate 25 digital drill core models of drill cores. The selected samples originate from the Longyearbyen CO2 Lab project’s borehole DH4, covering the lowermost cap rock and uppermost reservoir sequences proposed for CO2 sequestration onshore Svalbard. We have come up with a procedure that enables the determination of bulk volumes and densities with precisions and accuracies similar to those of such conventional methods as the immersion in fluid method. We use 3D printed replicas to qualitatively assure the volumes, and show that, with a mean deviation (based on eight samples)...
International Journal of Greenhouse Gas Control, 2018
This paper presents surface movement (uplift and subsidence) analysis around three CO 2 injection... more This paper presents surface movement (uplift and subsidence) analysis around three CO 2 injection wells at In Salah, Algeria. Analysis of surface movement is based on the InSAR data acquired throughout the injection period from 2004 to 2011. This paper focuses on the detail surface deformation patterns for all three injectors throughout the injection period, and eventually correlates such patterns to the CO 2 injection history. Surface uplift has a higher rate in the beginning of the injection period but levels off later on and reaches a final uplift of about 15-22 mm above different wells. We will explore whether surface movement data can reveal signatures of injection regime (i.e. matrix and fracture injections) in the reservoir. Surface movement, for a few selected points at different distances from the wellhead, and the injection pressure are plotted versus time to explore their relationship. Analysis of the plots show that surface subsidence coincide with fracture injection for some points and at certain times whereas it coincides with matrix injection for others. We also tried to map out possible time delay between injection shut-in and surface subsidence. It is learned that quantification of such a delay as well as monitoring the response of surface to fracturing in the reservoir requires high resolution surface movement data. This study shows the potential of satellite survey for monitoring the injection regime and behavior of reservoir and overburden.
Monitoring microseismic activity provides a window through which to observe reservoir deformation... more Monitoring microseismic activity provides a window through which to observe reservoir deformation during hydrocarbon and geothermal energy production, or CO 2 injection and storage. Specifically, microseismic monitoring may help constrain geomechanical models through an improved understanding of the location and geometry of faults, and the stress conditions local to them. Such techniques can be assessed in the laboratory, where fault geometries and stress conditions are well constrained. We carried out a triaxial test on a sample of Red Wildmoor sandstone, an analogue to a weak North Sea reservoir sandstone. The sample was coupled with an array of piezo-transducers, to measure ultrasonic wave velocities and monitor acoustic emissions (AE)-sample-scale microseismic activity associated with micro-cracking. We calculated the rate of AE, localised the AE events, and inferred their moment tensor from P-wave first motion polarities and amplitudes. We applied a biaxial decomposition to the resulting moment tensors of the high signal-to-noise ratio events, to provide nodal planes, slip vectors, and displacement vectors for each event. These attributes were then used to infer local stress directions and their relative magnitudes. Both the AE fracture mechanisms and the inferred stress conditions correspond to the samplescale fracturing and applied stresses. This workflow, which considers fracture models relevant to the subsurface, can be applied to large-scale geoengineering applications to obtain fracture mechanisms and in-situ stresses from recorded microseismic data.
In this paper, we revisit the marine controlled-source electromagnetic (CSEM) data, acquired abov... more In this paper, we revisit the marine controlled-source electromagnetic (CSEM) data, acquired above the Sleipner CO2 storage, in order to further study the dataset and conclude the feasibility of marine CSEM for offshore CCS monitoring. There are some challenges with respect to CSEM in this particular area: 1) strong airwave influence (due to regional shallow water depth); 2) potential of weak resistivity anomaly; 3) seabed pipeline network; and 4) rather shallow target depth. We are yet able to extract useful information; interpret further the CSEM inversion results by combining seismic data; and to extract the in situ resistivity and saturation of CO2 in the Utsira formation by applying a rock physics model. In addition, to minimize the influence of the seabed pipeline on the CSEM data, we have muted some of data and receivers near the seabed pipeline network. The results show a good agreement with seismic data, and the estimated total mass of CO2 agrees well with the injection data. This current study confirms that the marine CSEM can be an important and essential tool for offshore CO2 storage monitoring, yet not alone but when combined with both seismic and gravity. Finally, near-future large-scale CCS projects in the North Sea would require extensive infra-structures such as seabed pipeline, etc. This study demonstrates that CSEM may work even with such infra-structures in place.
Models of reduced dimensionality have been found to be particularly attractive in simulating the ... more Models of reduced dimensionality have been found to be particularly attractive in simulating the fate of injected CO 2 in supercritical state in the context of carbon capture and storage. This is motivated by the confluence of three aspects: the strong buoyant segregation of the lighter CO 2 phase above water, the relatively long time scales associated with storage, and finally the large aspect ratios that characterize the geometry of typical storage aquifers. However, to date, these models have been confined to considering only the flow problem, as the coupling between reduced dimensionality models for flow and models for geomechanical response has previously not been developed. Herein, we develop a fully coupled, reduced dimension, model for multiphase flow and geomechanics. It is characterized by the aquifer(s) being of lower dimension(s), while the surrounding overburden and underburden being of full dimension. The model allows for general constitutive functions for fluid flow (relative permeability and capillary pressure) and uses the standard Biot coupling between the flow and mechanical equations. The coupled model retains all the simplicities of reduced-dimensional models for flow, including less stiff nonlinear systems of equations (since the upscaled constitutive functions are closer to linear), longer time steps (since the high grid resolution in the vertical direction can be avoided), and less degrees of freedom. We illustrate the applicability of the new coupled model through both a validation study and a practical computational example.
SEG Technical Program Expanded Abstracts 2015, 2015
We present a synthetic-model-based study on applicability of borehole-surface electromagnetic (BS... more We present a synthetic-model-based study on applicability of borehole-surface electromagnetic (BSEM) to CO2 injection monitoring and discuss the result. A conceptual yet realistic 3D resistivity model is created, based on available seismic horizons and resistivity well logs for a mature oil field in, onshore Brazil. The synthetic EM data is calculated by means of finite element (FE) method, including cased well structure in simulation. We analyze the near-top-surface results generated from 5 vertical electric dipole sources located at the well bottoms. It is found that the EM field provides sensitive and stable timelapse response, showing promising aspects of BSEM application.
ABSTRACT We combine gravity and CSEM data acquired at the Sleipner field in order to estimate the... more ABSTRACT We combine gravity and CSEM data acquired at the Sleipner field in order to estimate the actual thickness and in situ resistivity of the CO2 plume when the CSEM data was collected in 2008. For the sake of simplicity, we assume the CO2 plume as a homogeneous elliptical cylinder, and apply the physical properties of the CO2 plume from literature review. The result is within a range of our expectation. Through this exercise, we can see the feasibility of combining the CSEM and gravity data to improve the time-lapse interpretation for monitoring CO2 storage sites, as complementary to time-lapse seismic data.
ABSTRACT Modelling plays an important role in providing capacity estimates and analysing injectiv... more ABSTRACT Modelling plays an important role in providing capacity estimates and analysing injectivity, long-term safety and risk factors for future storage sites. Hundreds and thousands of potential sites will have to be screened for suitability before more detailed studies are done. In this context, fast yet reliable methods will have to be developed involving simplified models but without losing too much of the accuracy. One key parameter for choosing a storage site is the injectivity; how fast can the CO2 be injected. The basics of hydro-mechanical modelling is explained and a conceptual model for single-phase fluid flow is defined. Various levels of complexity of the governing equations are derived, by considering standard methods and using constant and non-linear hydro-mechanical properties these models are compared. Results show that well defined input data makes the coupling to the geomechanical processes redundant, resulting in considerable savings in computational effort to get reliable and accurate estimates of injection pressure.
We attempt to correlate a set of surface heaving (or uplift) data, well head pressure and injecti... more We attempt to correlate a set of surface heaving (or uplift) data, well head pressure and injection rate with microseismic data. This correlation may give a high-confidence indication of fracture development or fault reactivation during injection or production operation. The dataset analysed here is acquired during the CO2 injection in In Salah, Algeria. This attempt is a first trial and done in rather qualitative manner than quantitative. In the current study, we focus only on the data from Well KB502. More advanced and detailed analysis will be required in order to achieve information that is more conclusive. Some preliminary result is presented herein. It is expected that the approach in this study can be applied to monitor production operation (including unconventional resources), not only injection.
XI International Conference on Structural Dynamics, 2020
In this study, we propose and implement a convergence scheme for the numerical wavenumber integra... more In this study, we propose and implement a convergence scheme for the numerical wavenumber integration in the context of 2.5D FE solution and validate it through a numerical example. In brief, the scheme is to estimate the convergence based on the relative difference between two interpolations for a given set of wavenumbers, which is done iteratively by refining the wavenumber sampling until the relative difference becomes no greater than a specified value. We evaluate the performance and convergence of the numerical integration scheme by comparing the results of the responses of a tunnel in a layered ground due to a concentrated harmonic load using both a full 3D model and a 2.5D model, including computational time aspect. Finally, we consider applying the same scheme to other relevant numerical integrations.
This study presents a finite element (FE) framework for simulation of steady state wave motion, c... more This study presents a finite element (FE) framework for simulation of steady state wave motion, combined with a so-called PML approach. The steady state wave motion of current interest are not only time-harmonic load but can also be constant-speed moving load. We adopt an existing PML approach (from low-frequency electromagnetic wave) to apply to the steady state FE simulation of constant-speed moving load vibration as well as temporal harmonic load vibration. In the paper, we will present the PML formulation in both the frequency and steady-state time domains. To validate the approach, then we compare the simulation results with available analytical reference solutions, and with a set of real measured field data (high-speed train data). In addition, we solve more example of steady state vibrations resulting from different train speed (e.g. subcritical, critical and supercritical).
The response of railway tracks and trackside vibration are strongly governed by the quality of th... more The response of railway tracks and trackside vibration are strongly governed by the quality of the track. Defects or nonhomogeneities in the track/substructure/ground can remarkably increase the responses in the system, leading to further deterioration of the track. This issue is more dramatic in high-speed lines. The objective of this paper is to study the impact of two types of non-homogeneities on the track response. In the first case, the effect of hanging sleepers is studied, and in the second case, the effect of locally deteriorated substructure is investigated. Two numerical solutions are used for the simulation of track-substructure-ground response. The first is the frequency-domain solution VibTrain that has been developed for efficient simulation of track/ground response under moving loads using a combination of beam elements for the track/substructure and Green's functions for the layered soil medium. The second model is a FE model in COMSOL Multiphysics enhanced with the absorbing boundary PML (Perfectly Matched Layer) in the time domain. In addition to studying the effect of track defects on rail vibration, the results of the two solutions are compared and practical conclusions are drawn on the potential of using vibration data for detection of track defects.
In this study, we introduce and discuss features and improvements of the well-established stiffne... more In this study, we introduce and discuss features and improvements of the well-established stiffness matrix method that is used in simulation of wave propagation in layered media. More specifically, we present stiffness matrices for an acoustic layer and a vertically transverse isotropic (VTI) viscoelastic soil layer. Combining these stiffness matrices enables a straightforward technique for modeling of acousto-elastic wave propagation in layered infinite media. In addition, we propose a technique to simulate discontinuity seismic sources, which was not used earlier in the context of the stiffness matrix method. Finally, we propose a framework to derive a key parameter of the absorbing boundary domain technique Perfectly Matched Layer (PML). Numerical examples are presented in order to help understanding the features and improvements discussed in the study from the fields of geophysics and soil dynamics. It is believed that the features and improvements discussed herein will make the application of the stiffness matrix method even wider and more flexible.
Bulletin of the Seismological Society of America, 2006
A rigorous method is presented to obtain-directly in the time domain and without the need for an ... more A rigorous method is presented to obtain-directly in the time domain and without the need for an integral transform over frequencies-the seismic response of horizontally layered, viscoelastic half-spaces due to arbitrarily distributed (or concentrated) sources. The method is based on exact expressions for the response of the underlying half-space cast in the wavenumber-time domain, and a complete modal solution in that domain for the layers. The proposed method is highly effective and avoids the problems of the usual propagator matrix method that arise when numerical integrations over both frequencies and wavenumbers are carried out, especially the waviness of the kernels due to resonances and reverberations in the layers when damping is light or even absent. Part I presents the basic methodology for the relatively simple case of antiplane SH sources in two-dimensional space, while Part II generalizes the concept to two-dimensional SV-P line sources and to threedimensional point sources, including seismic couples.
To monitor the integrity of the storage formation in relation to CO2 injection, several methods a... more To monitor the integrity of the storage formation in relation to CO2 injection, several methods and technologies will be employed, including microseismic monitoring. Injection of CO2 leads to increased pore-pressure which in turn can activate existing weaknesses, e.g. faults, at the risk of leakage. Presented is a hydro-mechanical model that describes single-phase fluid flow in a deforming porous media with an existing weakness that can fracture. In the intact porous media, the fluid flow is governed by Darcy's law and the geomechanical behavior is described by Biot's theory of linear poroelasticity. The weakness zone is defined as a zero-thickness domain and the governing equations are obtained by integrating the governing equations of the intact porous media over the virtual thickness of the weakness. This makes possible the seamless transition of an intact porous media formulation to an open fracture formulation as the weakness zone activates, deteriorates and eventually ruptures. The deterioration of the weakness zone is described using the Cohesive Zone Model (CZM). The hydro-mechanical model is used to study the impact of different fluid types on activation and fracturing of a weakness, in particular mode-I, tensile failure, and potential implications for micro-seismicity. Firstly, it was confirmed that fluid type matters, e.g. under the same pressurization, different fluids (different mobility and compressibility) propagates a fracture at different velocities. Additionally, three hypotheses from observations in the literature was tested. One hypothesis was confirmed, as simulation results show that for a high-viscosity fluid, a strong pore pressure gradient occurs ahead of the fracture tip, in the propagation direction, thus driving the fluid flow and the propagation of the fracture, favoring longer fractures. However, for the other two hypotheses, the hydro-mechanical model could neither confirm that low-viscosity fluids will tend to produce smaller but more abundant fractures (as reported in the literature), nor reproduce observations of lower breakdown- and initiation pressure of rocks for low-viscosity fluids. The reason for this discrepancy could be differences in the premises of the observation and the case studied here, or due to other factors not considered in this study, e.g. chemical effect, differences in wetting properties of the rock, or that super critical CO2 is not representative for typical low-viscosity fluids.
It is well known that the Norwegian North Sea has a great potential to store anthropogenic CO2 in... more It is well known that the Norwegian North Sea has a great potential to store anthropogenic CO2 in large scale, captured in Europe, in the geological time frame. Recently, the Norwegian government has also established a plan to develop the first full-scale CCS project, called Northern Lights, in which the firstly-chosen candidate site (named Smeaheia) was the Sognefjord formation, located between the Troll gas field and the Norway west coast. Not surprisingly, the Smeaheia site has a lot of faults of different sizes, including two regional faults of Vette and Øygarden, respectively, in the west and east bounds. We know the locations of the two regional faults, but it is rather challenging to quantify any further details of the two regional faults such as fault-composite structure (core zones, damage zones, etc.), dynamic hydraulic and geomechanical properties, and so on. Since such information is not fully known as of today, there is a high uncertainty to foresee whether or not there is a possibility of injected CO2 leaking through the regional faults for a given injection plan. If (and only if) any leakage would happen at all, we need to be prepared to answer "How could we monitor and detect cost-effectively such unwanted events?", which requires the sensitivity study of geophysical data (e.g. seismic, gravity, EM, etc.), yet with taking into account realistic geological models. In this study, we investigate how, and more precisely, whether or not, we could monitor any CO2 flow through the Vette Fault by means of marine CSEM survey. This type of survey has not yet been done in the North Sea setting. The only evidence of the efficiency of CSEM data to detect CO2 flow through faults/fractures was made through a laboratory test where liquid CO2 was injected to a naturally-fractured shale core (Nooraiepour et al, 2018), while the changes in seismic velocity and electrical resistivity were measured. It was clearly shown that the electrical resistivity is much more sensitivity to the CO2 flow than the seismic velocity. The leakage scenario at the Smeaheia is not yet well defined, either. Nevertheless, it is useful and necessary in advance to learn how the CSEM data may interact with the CO2 flow through the Vette Faults, which is the main target of the present study. Through this study, we also identify the issues and factors for which the CSEM technique should be improved further in order to make CCS projects in the North Sea successful.
Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-crit... more Large-scale geological carbon storage in the North Sea will involve injecting CO2 in a super-critical state into deep saline aquifers. As CO2 is injected, stress changes due to fluid pressure and temperature changes may result in deformation and potentially may result in microseismicity if existing faults are reactivated. Microseismicity may provide additional information on in situ stress conditions and the progression of the CO2 plume. To assess failure criteria in the reservoir, we must know the mechanical response of reservoir sandstone due to injection of super-critical CO2 (scCO2) under reservoir stress conditions. This includes fracturing processes at the microstructural scale as well as at the macroscopic scale by reactivation of existing fractures. We conducted three triaxial tests on a sandstone analogue to deep North Sea reservoir rock: once pressurized, samples were saturated with either scCO2, brine, or a brine-scCO2 mixture. Each sample was then deformed axially to ind...
On behalf of NGI, under FME SUCCESS, an MSc thesis at the University of Oslo has been conducted t... more On behalf of NGI, under FME SUCCESS, an MSc thesis at the University of Oslo has been conducted to finalize an inversion study of a CSEM dataset acquired in 2008 for the Sleipner CO2 storage project. The aim of the MSc thesis was to establish an inversion strategy that can be applied in order to resolve some of the various challenges related to inverting the dataset. This paper presents some of the work done related to the influence on the CSEM data from a pipeline network at seabed, with the objective of establishing a method to identify and filter out strongly influenced data, in order to improve inversion results.
Pre-existing and induced fractures and faults can play a role as bypass conduits and fast leaking... more Pre-existing and induced fractures and faults can play a role as bypass conduits and fast leaking channels in CO2 storage sites. They should therefore be well characterized during site selection, and monitored thoroughly during operation to track the movement and fate of the CO2 plume. Despite to date extensive research on the geophysical properties of brine- and CO2-saturated porous reservoir rocks, changes in acoustic velocity and electrical resistivity during a sole fracture fluid displacement are, however, rather little investigated. Hence, we herein present a laboratory study of core-scale geophysical monitoring during drainage-imbibition cycles of the brine-CO2 system through a shale caprock core sample with a vertical fracture. The experiments were conducted using both gaseous and scCO2 with 4 and 9 MPa pore pressures, respectively, at 12 MPa confining pressure. The tests were performed at 40°C during the loading and unloading stages in order to look into the hysteresis effect. We used a fractured core sample from the Upper Jurassic organic-rich shales of the Draupne Formation, which is the primary caprock for the Smeaheia CO2 storage site – a full-scale CCS project in Norway. The primary objective of the experiment was to compare the geophysical measurements using gaseous and scCO2 drainage-imbibition cycles during the tests in a core-scale experiment. Moreover, we were interested to see how sensitive acoustic velocity and electrical resistance techniques are to the fracture fluid displacement using different CO2 phase states. The outcomes of our high-pressure high-temperature experiment of simultaneous measurements of fracture flow and geophysical properties indicate that potential leakage of injected CO2 through the fractured-shale caprock can be detected in the core-scale laboratory experiments. The performed drainage-imbibition cycles using gaseous and scCO2 resulted in different behaviors in P-wave velocity (Vp) and electrical resistance in axial and radial directions for these two phase states. The measured Vp during the displacement of fracture fluid, CO2-brine subsequent cycles, showed a limited sensitivity in terms of magnitude and relative change. The electrical resistance, on the other hand, shows higher sensitivity and larger variation during fluid displacement along the fracture. It was also observed that the crossplot of Vp versus electrical resistance could detect and even differentiate the different phases during the loading and unloading stages.
Structure-from-motion (SfM) photogrammetry enables the cost-effective digital characterisation of... more Structure-from-motion (SfM) photogrammetry enables the cost-effective digital characterisation of seismic- to sub-decimetre-scale geoscientific samples. The technique is commonly used for the characterisation of outcrops, fracture mapping, and increasingly so for the quantification of deformation during geotechnical stress tests. We here apply SfM photogrammetry using off-the-shelf components and software, to generate 25 digital drill core models of drill cores. The selected samples originate from the Longyearbyen CO2 Lab project’s borehole DH4, covering the lowermost cap rock and uppermost reservoir sequences proposed for CO2 sequestration onshore Svalbard. We have come up with a procedure that enables the determination of bulk volumes and densities with precisions and accuracies similar to those of such conventional methods as the immersion in fluid method. We use 3D printed replicas to qualitatively assure the volumes, and show that, with a mean deviation (based on eight samples)...
International Journal of Greenhouse Gas Control, 2018
This paper presents surface movement (uplift and subsidence) analysis around three CO 2 injection... more This paper presents surface movement (uplift and subsidence) analysis around three CO 2 injection wells at In Salah, Algeria. Analysis of surface movement is based on the InSAR data acquired throughout the injection period from 2004 to 2011. This paper focuses on the detail surface deformation patterns for all three injectors throughout the injection period, and eventually correlates such patterns to the CO 2 injection history. Surface uplift has a higher rate in the beginning of the injection period but levels off later on and reaches a final uplift of about 15-22 mm above different wells. We will explore whether surface movement data can reveal signatures of injection regime (i.e. matrix and fracture injections) in the reservoir. Surface movement, for a few selected points at different distances from the wellhead, and the injection pressure are plotted versus time to explore their relationship. Analysis of the plots show that surface subsidence coincide with fracture injection for some points and at certain times whereas it coincides with matrix injection for others. We also tried to map out possible time delay between injection shut-in and surface subsidence. It is learned that quantification of such a delay as well as monitoring the response of surface to fracturing in the reservoir requires high resolution surface movement data. This study shows the potential of satellite survey for monitoring the injection regime and behavior of reservoir and overburden.
Monitoring microseismic activity provides a window through which to observe reservoir deformation... more Monitoring microseismic activity provides a window through which to observe reservoir deformation during hydrocarbon and geothermal energy production, or CO 2 injection and storage. Specifically, microseismic monitoring may help constrain geomechanical models through an improved understanding of the location and geometry of faults, and the stress conditions local to them. Such techniques can be assessed in the laboratory, where fault geometries and stress conditions are well constrained. We carried out a triaxial test on a sample of Red Wildmoor sandstone, an analogue to a weak North Sea reservoir sandstone. The sample was coupled with an array of piezo-transducers, to measure ultrasonic wave velocities and monitor acoustic emissions (AE)-sample-scale microseismic activity associated with micro-cracking. We calculated the rate of AE, localised the AE events, and inferred their moment tensor from P-wave first motion polarities and amplitudes. We applied a biaxial decomposition to the resulting moment tensors of the high signal-to-noise ratio events, to provide nodal planes, slip vectors, and displacement vectors for each event. These attributes were then used to infer local stress directions and their relative magnitudes. Both the AE fracture mechanisms and the inferred stress conditions correspond to the samplescale fracturing and applied stresses. This workflow, which considers fracture models relevant to the subsurface, can be applied to large-scale geoengineering applications to obtain fracture mechanisms and in-situ stresses from recorded microseismic data.
In this paper, we revisit the marine controlled-source electromagnetic (CSEM) data, acquired abov... more In this paper, we revisit the marine controlled-source electromagnetic (CSEM) data, acquired above the Sleipner CO2 storage, in order to further study the dataset and conclude the feasibility of marine CSEM for offshore CCS monitoring. There are some challenges with respect to CSEM in this particular area: 1) strong airwave influence (due to regional shallow water depth); 2) potential of weak resistivity anomaly; 3) seabed pipeline network; and 4) rather shallow target depth. We are yet able to extract useful information; interpret further the CSEM inversion results by combining seismic data; and to extract the in situ resistivity and saturation of CO2 in the Utsira formation by applying a rock physics model. In addition, to minimize the influence of the seabed pipeline on the CSEM data, we have muted some of data and receivers near the seabed pipeline network. The results show a good agreement with seismic data, and the estimated total mass of CO2 agrees well with the injection data. This current study confirms that the marine CSEM can be an important and essential tool for offshore CO2 storage monitoring, yet not alone but when combined with both seismic and gravity. Finally, near-future large-scale CCS projects in the North Sea would require extensive infra-structures such as seabed pipeline, etc. This study demonstrates that CSEM may work even with such infra-structures in place.
Models of reduced dimensionality have been found to be particularly attractive in simulating the ... more Models of reduced dimensionality have been found to be particularly attractive in simulating the fate of injected CO 2 in supercritical state in the context of carbon capture and storage. This is motivated by the confluence of three aspects: the strong buoyant segregation of the lighter CO 2 phase above water, the relatively long time scales associated with storage, and finally the large aspect ratios that characterize the geometry of typical storage aquifers. However, to date, these models have been confined to considering only the flow problem, as the coupling between reduced dimensionality models for flow and models for geomechanical response has previously not been developed. Herein, we develop a fully coupled, reduced dimension, model for multiphase flow and geomechanics. It is characterized by the aquifer(s) being of lower dimension(s), while the surrounding overburden and underburden being of full dimension. The model allows for general constitutive functions for fluid flow (relative permeability and capillary pressure) and uses the standard Biot coupling between the flow and mechanical equations. The coupled model retains all the simplicities of reduced-dimensional models for flow, including less stiff nonlinear systems of equations (since the upscaled constitutive functions are closer to linear), longer time steps (since the high grid resolution in the vertical direction can be avoided), and less degrees of freedom. We illustrate the applicability of the new coupled model through both a validation study and a practical computational example.
SEG Technical Program Expanded Abstracts 2015, 2015
We present a synthetic-model-based study on applicability of borehole-surface electromagnetic (BS... more We present a synthetic-model-based study on applicability of borehole-surface electromagnetic (BSEM) to CO2 injection monitoring and discuss the result. A conceptual yet realistic 3D resistivity model is created, based on available seismic horizons and resistivity well logs for a mature oil field in, onshore Brazil. The synthetic EM data is calculated by means of finite element (FE) method, including cased well structure in simulation. We analyze the near-top-surface results generated from 5 vertical electric dipole sources located at the well bottoms. It is found that the EM field provides sensitive and stable timelapse response, showing promising aspects of BSEM application.
ABSTRACT We combine gravity and CSEM data acquired at the Sleipner field in order to estimate the... more ABSTRACT We combine gravity and CSEM data acquired at the Sleipner field in order to estimate the actual thickness and in situ resistivity of the CO2 plume when the CSEM data was collected in 2008. For the sake of simplicity, we assume the CO2 plume as a homogeneous elliptical cylinder, and apply the physical properties of the CO2 plume from literature review. The result is within a range of our expectation. Through this exercise, we can see the feasibility of combining the CSEM and gravity data to improve the time-lapse interpretation for monitoring CO2 storage sites, as complementary to time-lapse seismic data.
ABSTRACT Modelling plays an important role in providing capacity estimates and analysing injectiv... more ABSTRACT Modelling plays an important role in providing capacity estimates and analysing injectivity, long-term safety and risk factors for future storage sites. Hundreds and thousands of potential sites will have to be screened for suitability before more detailed studies are done. In this context, fast yet reliable methods will have to be developed involving simplified models but without losing too much of the accuracy. One key parameter for choosing a storage site is the injectivity; how fast can the CO2 be injected. The basics of hydro-mechanical modelling is explained and a conceptual model for single-phase fluid flow is defined. Various levels of complexity of the governing equations are derived, by considering standard methods and using constant and non-linear hydro-mechanical properties these models are compared. Results show that well defined input data makes the coupling to the geomechanical processes redundant, resulting in considerable savings in computational effort to get reliable and accurate estimates of injection pressure.
We attempt to correlate a set of surface heaving (or uplift) data, well head pressure and injecti... more We attempt to correlate a set of surface heaving (or uplift) data, well head pressure and injection rate with microseismic data. This correlation may give a high-confidence indication of fracture development or fault reactivation during injection or production operation. The dataset analysed here is acquired during the CO2 injection in In Salah, Algeria. This attempt is a first trial and done in rather qualitative manner than quantitative. In the current study, we focus only on the data from Well KB502. More advanced and detailed analysis will be required in order to achieve information that is more conclusive. Some preliminary result is presented herein. It is expected that the approach in this study can be applied to monitor production operation (including unconventional resources), not only injection.
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Papers by Joonsang Park