Giuliana Rossi

Quantitative analysis of time-lapse seismic monitoring data at the Sleipner CO2 storage operation

The CO2 storage operation at Sleipner in the Norwegian North Sea provides an excellent demonstration of the application of time-lapse surface seismic methods to CO2 plume monitoring under favorable conditions. Injection commenced at Sleipner in 1996 with CO2 separated from natural gas being injected into the Utsira Sand, a major saline aquifer of late Cenozoic age. CO2 injection is via a near-horizontal well at a depth of about 1012 m below sea level (bsl) some 200 m below the reservoir top, at a rate approaching 1 million tonnes (Mt) per year, with more than 11 Mt currently stored.

Time-lapse tomography

In time-lapse analysis, we have to distinguish the seismic response changes due to oil and gas production at a reservoir over the years from several other causes, such as the recording signature and random noise. In this paper, we focus our attention on the velocity macromodel provided by seismic tomography, which is a basic tool for the data regularization, its depth or time migration, and a possible final subtraction among different vintages. We show first that we cannot use just a single velocity model for all data sets, because of seasonal variations of the overburden velocity (which is mainly due to seawater temperature in marine cases and to the water table depth in land cases). However, we can exploit the basic assumption of time-lapse analysis for constraining reflection/refraction tomography, i.e., by imposing the constraint that the layer structure and the local velocities do not change outside the reservoir (and in the shallowest part) over time. We thus get coupled models that are physically consistent, with a better spatial coverage and higher information redundancy. The new method is illustrated by a marine case history from the North Sea.

Tomographic imaging by reflected and refracted arrivals at the North Sea

We discuss some processing steps of a marine 3-D data set from the Oseberg field, North Sea. We compare the prestack depth‐migrated images obtained by the velocity fields provided by different tools: velocity spectra, reflection tomography, and joint tomographic inversion of reflected and refracted arrivals. The last ones are definitely better. We also produced a synthetic example by modeling the estimated earth structure and the actual recording geometry, and we reached similar conclusions. The correlation between reflected and refracted signals may be unclear for later arrivals because of their reciprocal interference and multiple reflections. We adopt a technique based on a surgical mute in the τ-p domain, which allows coupling the signals coming from the same elastic interface.

Seismic modeling to monitor CO2 geological storage: The Atzbach- Schwanenstadt gas field

[1] We develop a petro-elastical numerical methodology to compute realistic synthetic seismograms and analyze the sensitivity of the seismic response when injecting carbon dioxide (CO2) in a depleted gas reservoir. The petro-elastical model describes the seismic properties of the reservoir rock saturated with CO2, methane and brine, and allows us to estimate the distribution and saturation of CO2 during the injection process. The gas properties, as a function of the in-situ pressure and temperature conditions, are computed with the Peng-Robinson equation of state, taking into account the absorption of gas by brine. Wave attenuation and velocity dispersion are based on the mesoscopic loss mechanism, which is simulated by an upscaling procedure to obtain an equivalent viscoelastic medium corresponding to partial saturation at the mesoscopic scale. Having the equivalent complex and frequency-dependent bulk (dilatational) modulus, we include shear attenuation and perform numerical simulations of wave propagation at the macroscale by solving the viscoelastic differential equations using the memory-variable approach. The pseudo-spectral modeling method allows general material variability and provides a complete and accurate characterization of the reservoir. The methodology is used to assess the sensitivity of the seismic method for monitoring the CO2 geological storage at the Atzbach-Schwanestadt depleted gas-field in Austria. The objective of monitoring is the detection of the CO2 plume in the reservoir and possible leakages of CO2. The leakages are located at different depths, where the CO2 is present as gaseous, liquid and supercritical phases. Even though the differences can be very subtle, this work shows that seismic monitoring of CO2 from the surface is possible. While the identification of shallow leakages is feasible, the detection of the plume and deep leakages, located in the caprock just above the injection formation, is more difficult, but possible by using repeatability metrics, such as the normalized RMS (NRMS) images. Considering real-data conditions, affected by random noise, a reference detection threshold for deep leakages and the CO2 plume in the reservoir corresponds to a signal-to-noise ratio of about 10 dB.

One Million Years of Climatic Generated Landslide Events on the Northwestern Barents Sea Continental Margin

Relatively recent, shallow landslides are imaged both on swath bathymetry, sub-bottom and multichannel seismic reflection (MCS) data from the upper-middle continental slope on the Storfjorden and Kveithola Trough Mouth Fans, NW Barents Sea margin. Giant paleo-landslide deposits, detected only by MCS profiles, are characterized by chaotic acoustic units up to about 250 m thick on the lower continental slope. The thickest, oldest landslide, dated between 1 and 0.8 Ma, took place just after the large-scale intensification of glaciation in the Barents Sea. The apparent spatial coincidence of landslides and channels with the boundary between the two fan systems, that are generated due to huge quantities of sediments transported to the continental slope by paleo-ice streams, suggests a common controlling climatic process for their development. Most probably the slides are related to the abundance of basal meltwater beneath the ice sheet, which in addition to determining ice stream motion and lubrication also influences the behavior of mass wasting processes.

Tomographic inversion of the water layer in the 4D analysis

The variations of the seawater velocity in space and time are often uncared for in the conventional processing of marine data, because their influence may be negligible for imaging targets at large depths. However, in the time-lapse monitoring of hydrocarbon reservoirs, subtle effects must be observed, as the variations of the local velocities and the Gas/Oil or Oil/Water contact. We show here by a real example that the velocity anomalies in the seawater should be measured and compensated: otherwise, their effect can distort significantly the time-lapse analysis.

A 3D Seismic Survey For Groundwater Protection

3D seismic surveys are proven cost-effective tools for hydrocarbon exploration and production. This technology can be transferred and adapted to an emerging problem for social life, i.e. the supply of fresh water and its protection from pollution. We carried out a 3D survey in a catchment area close to industrial plants, where new infrastructures could be built. The inversion of P and Rayleigh waves allowed estimating the weathering heterogeneity, with possibly related variations of permeability.

FReDNet: Evolution of a Permanent GNSS Receiver System

Continuous GNSS networks provide unique information about the crustal dis-placements, of use for studies concerning plate motions, tectonic processes, and earthquake cycle understanding. The Istituto Nazionale di Oceanografia e di Geofisca Sperimentale—OGS, since 2002, is responsible for the installation, maintenance, and development of FReDNet (Friuli Regional Deformation Network), the system for crustal deformation monitoring in the Friuli Venezia Giulia Region. The main objective of the network is the detection of tectonic plate movements in this collision area: such movements can give some guidance to quantify the seismic hazard. The infrastructure currently consists of 16 permanent stations located in the Friuli Venezia Giulia and Veneto regions, all equipped with GNSS receiver (Global Navigation Satellite Systems) capable of tracking satellite systems GPS, GLONASS and Galileo. The system includes a central server for collecting, processing and distributing of data and results. FReDNet, furthermore, provides a service for browsing high-precision real-time positioning in the most common differential correction RTK (Real Time Kinematic) modes. The current document will briefly describe the technical implementation of FReDNet in the last years with new stations, new features (high-frequency sampling and monumentation) and upgraded services.

Tomographic inversion of ocean bottom seismograph (OBS) data: Problems and solutions applied to the NW Svalbard Hydratech data set

The paper is aimed at discussing some problems that may arise in the tomographic inversion of data acquired with particular geometries, as vertical seismic profiles (VSP) or implying the use of Ocean Bottom Seismographs (OBS), and presenting the solutions used to solve them. One problem is due to the limited number of sensors (that may be even a single one) and of their sparseness. This affects the information distribution that is concentrated in the vicinity of each sensor. This fact, together with the second problem, i.e. the strong asymmetry of the ray path, may decrease the reliability of the inversion. Two procedures for irregular adaptive gridding of the reflecting interfaces and single ray datuming are proposed as possible solutions to overcome these obstacles, while obtaining a reliable and highly resolved velocity model in depth: the relative procedures are tested through synthetic models and on real data.

3D Adaptive Tomography And Imaging In the Vienna Basin

The use of reflection tomography for building a velocity model has been demonstrated precious by several case histories. Such depth model is the key for a successful pre-stack depth migration. However, there is a trade-off for these applications: pre-stack depth migration is cost effective only for very complex structures, especially in 3D. On the other hand, the picking of traveltimes, necessary for the velocity inversion, may be prohibitive if diffractions, multiples and noise exceed reasonable limits. To verify by a practical experience this trade-off, we processed a 3D survey at the Vienna Basin. The area is characterised by static problems, organised noise and a composite fault system, whose complexity is comparable to the Marmousi model.