Peter Bergmann

Geophysical Monitoring Of CO2 at the Ketzin Storage Site - The Results of the Second 3D Repeat Seismic Survey

Various geophysical methods applied at the Ketzin storage site have successfully imaged migration of the injected CO2 within the target reservoir zone of the ~ 650-680 m deep saline aquifer. Results from the first 3D repeat seismic survey conducted in 2009, after about 15 months of injection (~22,000 t), showed that the CO2 plume was concentrated around the injection well with a lateral extent of approximately 300-400 m and a thickness of about 5–20 m. The plume, however, was not radially symmetric, but had a rather westerly trending tendency, revealing the heterogeneous nature of the reservoir. A second 3D repeat seismic survey was acquired in the Summer/Fall of 2012, when ~ 61,000 tons of CO2 had been injected. Preliminary results show further growth and migration of the anomaly which has been interpreted to be induced by the CO2 injection. It is similar in shape to the one observed at the time of the first repeat survey, but larger by approximately 100-200 m and much stronger with the highest amplitudes nearly centered at the injection well. There is still a pronounced westward propagating tendency. The new seismic data show no indication of upward migration into the caprock.

The First Post-injection Seismic Monitor Survey at the Ketzin Pilot CO2 Storage Site: Results from Time-lapse Analysis

The injection of CO2 at the Ketzin pilot CO2 storage site started in June 2008 and ended in August 2013. During the 62 months of injection, a total amount of about 67 kt of CO2 was injected into a ...

Fluid injection monitoring using electrical resistivity tomography — five years of CO2 injection at Ketzin, Germany

ABSTRACT Between the years 2008 and 2013, approximately 67 kilotons of CO2 have been injected at the Ketzin site, Germany. As part of the geophysical monitoring programme, time‐lapse electrical resistivity tomography has been applied using crosshole and surface‐downhole measurements of electrical resistivity tomography. The data collection of electrical resistivity tomography is partly based on electrodes that are permanently installed in three wells at the site (one injection well and two observation wells). Both types of ERT measurements consistently show the build‐up of a CO2‐related resistivity signature near the injection point. Based on the imaged resistivity changes and a petrophysical model, CO2 saturation levels are estimated. These CO2 saturations are interpreted in conjunction with CO2 saturations inferred from neutron‐gamma loggings. Apart from the CO2–brine substitution response in the observed resistivity changes, significant imprints from the dynamic behaviour of the CO2 in the reservoir are observed.

Preliminary Seismic Time-lapse Results from the First Post-injection Survey at the Ketzin Pilot Site

Since the CO2SINK project start in April 2004, different methods involving seismics, geoelectrics andpressure-temperature monitoring have been applied at the Ketzin pilot site to map geological str ...

Volumetric Bounds on Subsurface Fluid Substitution Using 4D Seismic Time-shifts with an Application at Sleipner

A method for volumetric estimation of subsurface fluid substitution is presented that relies on the analysis of 4D seismic time-shifts. Since time-shifts cannot resolve for fluid saturation and layer thickness simultaneously without additional constraints, mass estimates are derived from the complete set of possible fluid saturations and layer thicknesses. The method considers velocity-saturation relationships that range from uniform saturation to patchy saturation. Based on a generalized velocity-saturation relationship that is parameterized by the degree of patchiness, explicit upper and lower fluid mass bounds are provided. We show that the inherent ambiguity between fluid saturation and layer thickness has a severe impact on the convergence of these mass bounds. That is, roughly linear velocity-saturation relationships with patchy saturation tend to provide significantly better accuracy in a mass interpretation than the strongly non-linear velocity-saturation relationships associated with homogeneous saturation. The method is validated at the Sleipner storage site, where injected fluid masses are known. Moreover, a linear relationship between 4D time-shifts and injected mass is observed, suggesting that the evolving patterns of fluid saturation and fluid mixing in the CO2 plume at Sleipner have remained roughly constant with time.

Monitoring CO2 saturation using time-lapse amplitude versus offset analysis of 3D seismic data from the Ketzin CO2 storage pilot site, Germany

The injection of CO2 at the Ketzin pilot site commenced in June 2008 and was terminated in August 2013 after 67 kT had been injected into a saline formation at a depth of 630–650 m. As part of the site monitoring program, four 3D surface seismic surveys have been acquired to date, one baseline and three repeats, of which two were conducted during the injection period, and one during the post-injection phase. The surveys have provided the most comprehensive images of the spreading CO2 plume within the reservoir layer. Both petrophysical experiments on core samples from the Ketzin reservoir and spectral decomposition of the 3D time-lapse seismic data show that the reservoir pore pressure change due to CO2 injection has a rather minor impact on the seismic amplitudes. Therefore, the observed amplitude anomaly is interpreted to be mainly due to CO2 saturation. In this study, amplitude versus offset analysis has been applied to investigate the amplitude versus offset response from the top of the sandstone reservoir during the injection and post-injection phases, and utilize it to obtain a more quantitative assessment of the CO2 gaseous saturation changes. Based on the amplitude versus offset modelling, a prominent decrease in the intercept values imaged at the top of the reservoir around the injection well is indeed associated solely with the CO2 saturation increase. Any change in the gradient values, which would, in case it was positive, be the only signature induced by the reservoir pressure variations, has not been observed. The amplitude versus offset intercept change is, therefore, entirely ascribed to CO2 saturation and used for its quantitative assessment. The estimated CO2 saturation values around the injection area in the range of 40%–60% are similar to those obtained earlier from pulsed neutron-gamma logging. The highest values of 80% are found in the second seismic repeat in close vicinity to the injection and observation wells.

Field Injection Operations and Monitoring of the Injected CO2

Monitoring the fate of the injected CO2 and possible associated effects, such as hydro-mechanical and chemical effects in the target reservoir and its surroundings, is essential for safe operation of a storage facility. In this chapter, we shall first provide an overview of the technologies available and used for monitoring of CO2. We shall then proceed to describe specific methods and finally present some important case studies that will demonstrate the use of the discussed monitoring technologies under specific field settings.

The relationship of soil-moisture saturation and time-lapse static shifts-An example from the Ketzin pilot site

Changes in the near surface are a major problem for land time-lapse seismic projects. Three seismic surveys at the Ketzin pilot site for CO2 storage in Germany demonstrated the importance of removing the variations in the shallow subsurface by applying spatially variable, relative time shifts to the different vintages prior to 4D interpretation. The main reason for these time shifts is a change in seismic velocities in the ground layer above the water table due to different soil-moisture saturation at the times of acquisition. We compared the variation in precipitation, groundwater level and trace-to-trace time shifts between the baseline and two monitor surveys and revealed that delays in reflected energy are in a qualitative sense, proportional to the moisture content in the soil.

4D Result Enhancement with Crosscorrelation-based Time-lapse Static Correction at Ketzin, Germany

A method for correction of time-lapse differences (TLD) in the statics of seismic data from repeated surveys is presented. Such static differences are typically caused by changes in the near-surface velocities between the acquisition repeats and have a deteriorating impact on the time-lapse image. Trace-to-trace time shifts are determined from the pre-stack data sets using cross-correlations. These time shifts are decomposed in a surface-consistent manner, which is providing a static correction that is capable of aligning the repeat data to the baseline data. The approach is demonstrated on a 4D seismic data set from the Ketzin CO2 pilot storage site, Germany, and is compared with results of an initial processing that was based on individual refraction static corrections. It is shown that the proposed TLD static correction reduces 4D noise more effectively than refraction static corrections while being significantly less labor intensive.

4D Imaging of the Ketzin CO2 Storage Site (Germany) Using a Combination of Seismic and Electrical Resistivity Imaging

A combination of seismic and geoelectric processing is presented using a structurally constrained inversion approach. Structural constraints are interpreted from seismic data and inserted into the geoelectric inversion through a local regularisation, which allows inverted resistivities to behave also discontinuously across these a priori constraints. This sequential arrangement of seismic processing and constrained resistivity inversion makes the generic assumption that the petrophysical parameters of both methods change across common lithostructural boundaries. We evaluate the approach using a numerical example and a real 4D data example from the CO2 pilot storage site, Ketzin, Germany. The real data case shows that the time-lapse anomalies, which have independently been imaged by surface seismic and surface-downhole geoelectric methods, correlate well at the CO2-flooded reservoir. However, at some distance to the downhole electrodes the geoelectric images provide a notably lower resolution in comparison to the corresponding seismic images. The results demonstrate the relevance of the presented approach for the combination of both methods in geophysical CO2 storage monitoring operations.