CO2SINK Project Research Articles

Cross-well seismic waveform tomography for monitoring CO2 injection: a case study from the Ketzin Site, Germany

Geological storage of CO2 is one means of mitigating the effects of continued burning of fossil fuels for power generation. An important component in the storage concept is the monitoring of the CO2 distribution at depth. Seismic methods can play a significant role in this monitoring, in particular cross-well methods are of interest due to their high resolution. For these purposes, a series of cross-well seismic surveys were acquired within the framework of the CO2SINK project at Ketzin, Germany, at various stages of an injection test. We study here the potential of applying cross-well seismic waveform tomography to monitor the CO2 injection process. First, we test the method on synthetic data having a similar geometry to that of the real data. After successful application on the synthetic data, we test the method on the real data acquired at the Ketzin Site. Traveltime tomography images of the real data show no observable differences between the surveys. However, seismic waveform tomography difference images show significant differences. A number of these differences are artefacts that can probably be attributed to inconsistent receiver coupling between the different surveys. However, near the injection horizon, below the caprock, a velocity decrease is present that is consistent with that expected from the injection process.

Dynamic simulation and history matching at Ketzin (CO2SINK)

Abstract Since the end of June 2008, carbon dioxide (CO 2 ) is being injected in the Stuttgart Formation at Ketzin, Germany as part o f the European Union’s CO2SINK project. The injection well (Ktzi 201) is roughly 50 and 100 m away from observation wells Ktzi 200 and Ktzi 202, respectively. CO 2 was detected at the closest observation well on the 15th of July 2008, approximately 20 days after injection commenced. Breakthrough at the Ktzi 202 well was recorded on March 21, more than 8 months after injection was initiated. Dynamic simulations of the injection and flow of CO 2 into the subsurface at Ketzin will be described. The three dimensional (3D) geological model was built based on a 3D seismic survey as well as logging and core analysis data. The formation consists of fluvial sandstone channels within a muddy flood plain at mean depth of 650 m within an anticlinal structure. The geological model uses a cell size of 20×20 m and a layer thickness of 0.5 m, resulting in a total of ∼7 million cells. The model is upscaled (i.e., coarsened) and the grid is refined locally in the zone comprising the injection and observation wells. This upscaled model has corner point geometry with 78×74×91 (525, 252) grid cells in X, Y, and Z directions. X and Y direction grid sizes around the wells are respectively 4 m and 5 m. Average grid size in Z direction is about 0.5 m. Following the upscaling of the geological or static model, a blackoil commercial streamline simulator was used to simulate the flow of injected CO 2 according to the actual injection rate history. The phase behavior of CO 2 and brine were described by blackoil pressure-volume-temperature (PVT) tables. The blackoil PVT was modeled by imposing brine properties to the simulator oil model and CO 2 properties to the simulator gas model. In this manner, solubility of CO 2 into water was taken into account. Salinity of the brine is represented by the appropriate density. Fluid properties (density, viscosity) are pressure dependent and isothermal (at reservoir temperature). The outcome of the simulation is the time for injected CO 2 to arrive at the observation wells and the history of bottom-hole pressures (BHP) as a function o f time that can be compared to measurements taken in the bottom of the injection well. History matching was performed by adjusting the permeability (along and across bedding) until model BHPs agreed with the measured ones. Permeability was chosen as history matching parameter because of its high degree of uncertainty and relies on the porosity/permeability relation derived from core measurements. Good agreement was obtained with a multiplicator of 0.1 applied to the permeability across and along bedding. Breakthrough time for the closest observation well was in good agreement with reality. However the breakthrough time at the most distant well was underestimated by several months. We believe that geological features at distances greater than 50 m from the injection well may be responsible for the mismatch and should be investigated further.

Time-lapse seismic surface and down-hole measurements for monitoring CO2 storage in the CO2SINK project (Ketzin, Germany)

Ketzin is an European pilot site for onshore CO2 storage in a saline aquifer. Injection started in summer 2008 and to date, more than 37.000 tons of CO2 have been injected into the Stuttgart Formation at approximately 650 m depth. A wide range of monitoring methods are being applied at Ketzin, among which are active seismic observations at various scales. 3D reflection seismic, combined surface-downhole measurements and crosshole tomographic surveys were performed before injection and after the start of injection in order to image the reservoir and to track the CO2. Time-lapse signatures of the injected CO2 were observed by all active seismic methods. The CO2 could be detected by increased reflectivity at the top of the injection reservoir, by a change in the attenuation behaviour and also by reduced propagation velocity within the reservoir. The ongoing injection of CO2 during the next years will be followed by further repeat surveys. Current investigations focus on the still difficult problem of the quantification of the CO2 imaged by the geophysical measurements.

Monitoring petrophysical, mineralogical, geochemical and microbiological effects of CO2 exposure — Results of long-term experiments under in situ conditions

Abstract To study the long term effects of geochemically and microbially induced mineral dissolution and precipitation during CO 2 storage and their impact on storage reliability and efficiency, laboratory experiments under in situ P-T conditions were performed. Long term CO 2 exposure experiments were initiated in September of 2007 using fresh core material from the reservoir horizon at the CO2SINK project in Ketzin, Germany. Formation water-filled sandstone samples, obtained from a depth of 627 to 633 m, have been incubated individually in high pressure vessels together with sterile synthetic formation brine under simulated reservoir P-T conditions of 5.5 MPa and 40 °C. After 15, 21, and 24 months of CO 2 exposure, rock and fluid samples were analyzed with respect to geochemistry, microbial community, as well as mineralogical and petrophysical characteristics. The analysis of fluid samples revealed the dissolution of mineral phases during CO 2 exposure, indicated by increasing concentrations of Ca 2+ , Mg 2+ , K + and SO 4 2− . First mineralogical results are in consistence with the fluid chemical data and showed dissolution of anorthitic plagioclase, K-feldspar and anhydrite. However, the mineral dissolution can only partly be attributed to the influence of CO 2 . A proportion is due to the equilibration between rock and synthetic brine. The dissolution of minerals led to increasing porosities, as observed with NMR measurements and mercury injections. Decreasing porosities after 24 months of incubation together with decreasing inorganic fluid component concentrations, and precipitation of albite suggest secondary mineral precipitation. A 16S rDNA based fingerprinting method was used to identify the dominant species in DNA extracts of pristine sandstone samples. So far sequences belonging to facultative anaerobic, chemoheterotrophic bacteria were identified. During CO 2 exposure minor changes in the microbial community composition were observed. The majority of microbes were able to adapt to the changes in environmental conditions.

Assessing drilling mud and technical fluid contamination in rock core and brine samples intended for microbiological monitoring at the CO 2 storage site in Ketzin using fluorescent dye tracers

The CO 2SINK project in Ketzin represents a field laboratory for the storage of CO 2 in a 650-m deep saline aquifer. The project is accompanied by a microbiological monitoring programme to characterise the composition and activity of the autochthonous microbial community in rock and brine samples and their changes in response to CO 2 storage. A prerequisite of these studies is the acquisition of samples free of contamination from microorganisms and organic and inorganic components. Drilling mud and technical fluids are the main sources of contamination. This study describes the application of the fluorescent dye tracers fluorescein and rhodamine B as contamination controls for rock core and brine samples. Fluorescein was added to drilling mud that was used during the coring phase of the Ketzin wells Ktzi 200, 201 and 202. In addition, total organic carbon (TOC) concentrations, reflecting the carboxymethyl cellulose (CMC) component of the drilling mud, were determined to verify the tracer results. The fluorescence and TOC analyses revealed that drilling mud filtrate penetrated the outer 20 mm of mildly permeable sandstone cores. Rhodamine B was added to brines that were used to displace the drilling mud and to flush the wells after completion. The tracer monitoring during the discharge of drilling mud and displacement brines from the wells during hydraulic tests and nitrogen lifts enabled the quantification of reservoir fluid quality. After the production of 140–190 m 3 (16–21 borehole volumes) of fluid, the drilling mud concentration was reduced to about 0.05%. The use of fluorescein emerged as a field-capable, sensitive and reliable method during the sampling of rock core and formation brine samples.

Reservoir characterization of a CO 2 storage aquifer: The Upper Triassic Stuttgart Formation in the Northeast German Basin

Ketzin, in the Northeast German Basin (NEGB), is the site for pilot injection of CO 2 (CO 2SINK project) into a saline aquifer (the Upper Triassic Stuttgart Formation) situated at a depth of about 630–700 m. This paper reports the baseline characterization of the reservoir formation based on new core material and well-logs obtained from one injection well and two observations wells, drilled at a distance from 50 m to 100 m from each other. The reservoir is lithologically heterogeneous and made up by fluvial sandstones and siltstones interbedded with mudstones showing remarkable differences in porosity. The thickest sandstone units are associated with channel sandstone, whose thickness varies over short lateral distances. In-depth petrographic, mineralogical, mineral-chemical, and whole-rock geochemical analysis were performed focusing on the sandstone intervals, which display the best reservoir properties for CO 2 injection. The dominantly fine-grained and well to moderately-well sorted, immature sandstones classify as feldspathic litharenites and lithic arkoses. Quartz (22–43 wt.%), plagioclase (19–32 wt.%), and K-feldspar (5–13 wt.%) predominate mineralogically. Muscovite plus illite and mixed-layer minerals are omnipresent (4–13 wt.%). Quartz, feldspar, as well as meta-sedimentary and volcanic rock fragments comprise the most abundant detrital components, which often are rimmed by thin, early diagenetic coatings of ferric oxides, and locally of clay minerals. Feldspar grains may be unaltered and optically clear, partially to completely dissolved, partially altered to sheet silicates (mainly illite), or albitized. Analcime and anhydrite constitute the most widespread, often spatially associated pore-filling cement minerals. Authigenic dolomite, barite, and coelestine is minor. The percentage of cements ranges in total from about 5 vol.% to 32 vol.%. Except of samples intensely cemented by anhydrite and analcime, total porosities of the sandstones range from 13% to 26%. The fraction of intergranular porosity varies between 12% and 21%. About 1–5% porosity has been generated by dissolution of detrital plagioclase, K-feldspar, and volcanic rock fragments. The comparatively large modal abundance of feldspars, micas, chlorite, clay minerals, Fe–Ti-oxides, and analcime account for the richness in Ti, Al, Fe, Mg, Na, and K, and the paucity in Si, of the Stuttgart sandstones relative to mature sandstones. Altogether, these sandstones are comparatively rich in minerals that may potentially react with the injected CO 2.

Modelling the geoelectric and seismic reservoir response caused by carbon dioxide injection based on multiphase flow simulation: Results from the CO 2SINK project

Results from crosshole geoelectric and surface seismic monitoring for geological storage of carbon dioxide (CO 2) were investigated by forward modelling within the framework of the CO 2SINK project. Selected geological and petrophysical parameters reflect the conditions of the CO 2SINK site. CO 2 saturation distributions were derived from multiphase flow modelling, whereas the alteration of geophysical rock properties by CO 2 was obtained from laboratory experiments. Crosshole geoelectric modelling was performed for three electrode combinations and three time-dependent CO 2 migration scenarios with different reservoir permeabilities. The magnitude and alteration of modelled resistances were analyzed in the pre-inversion domain. Time-lapse alterations were observable on the synthetic data, with diverse characteristics dependent on applied electrode configuration. Analysis of the alterations showed the opportunity to differentiate migration scenarios within the constraints of the ambient noise level. The synthetic time-lapse seismic reflection experiment was performed for the anticline used for CO 2 storage. The geological model incorporates the structural framework, as determined from the seismic interpretation, and velocities derived from seismic processing and velocity logs. Common depth point (CDP) processing of synthetic shot gathers of a baseline and repeat experiment provided the data for a difference stack section exhibiting a CO 2 induced time-lapse signature. Interpretation of the signature in conjunction with the underlying CO 2 distribution has shown that lateral extent of the plume may be accurately detected. The vertical plume extent is concealed in the waveform coda and is unlikely to be retrievable from standard seismic processing.

The CO2SINK boreholes for geological CO2 -storage testing

This paper reports the well design, drilling and completion operation as well as the coring technique applied in the CO2SINK project. Three boreholes, one injection well and two observation wells have been drilled to a total depth of about 800 m. 200 m of recovered 6” core material has been real-time analysed in a research field lab. The wells have been completed as “smart” wells, containing a variety of permanently installed down-hole sensors for the continuous monitoring of the CO2 in the reservoir. All wells were cased with stainless final casings equipped with pre-perforated sand filters in the reservoir zone and wired on the outside with fiber-optical and multi-conductor copper cables. The reservoir casing section is externally coated with a fiber-glass-resin wrap for electrical insulation.

CO2 injection impairment due to halite precipitation

Abstract The injection of dry supercritical CO 2 into brine aquifers has the potential to dry saline formation waters, due to evaporation effects [1] , leading to severe increases in salinity and salt precipitation. This can significantly impair injection rates, as has been noted in gas-storage reservoirs. [2] This is of interest for CO 2 storage in saline aquifers. An injection impairment study was performed for the CO2SINK Project, a European Union research project on testing geological carbon storage near Ketzin, Germany [3] . Core flood experiments showed that halite precipitates due to brine evaporating in dry super-critical CO 2 . The phenomenon was studied with two research codes, TOUGH2 and a streamline-based simulator. Both codes predict substantial salt deposition close to the injection point, with associated severe injection impairment. Our simulations also suggest that simple reservoir engineering measures, such as a brief (hours) preflush with fresh water, can mitigate adverse effects.

Shallow velocity–depth model using first arrival traveltime inversion at the CO 2SINK site, Ketzin, Germany

The CO 2SINK project, a carbon dioxide storage and monitoring project, has been in operation in the Ketzin area, Germany since 2004. Goals are to investigate the techniques and understand the processes related to geological storage of CO 2 in a saline aquifer. A key element of the project is comprehensive monitoring and the development of verification methods to track the long term spread of the injected CO 2. Time lapse seismic surveys in 3D and 2D are important monitoring components for tracking the movement of CO 2. Prior to the 3D baseline seismic survey in 2005, a pilot seismic survey was performed in 2004 in order to provide input parameters for the 3D survey and information on target depth. This pilot study, consisting of two perpendicular 2.4 km long profiles confirmed the geological structure based on 1960s vintage seismic data. However, the image is rather poor in the uppermost parts of the seismic sections. In order to obtain additional structural information and to improve the velocity function estimates, first arrival traveltimes were used to image the near-surface structure and to provide an improved velocity function for interpretation of reflection data. We used a generalized linear inversion (GLI) method, based on iterative least-squares inversion, to reconstruct the velocity–depth model. A simple and smooth starting velocity model was derived from traveltime plots and the stacked reflection seismic sections. Reliability of the resulting velocity models were estimated from analysis of the traveltime residuals (RMS error) and qualitative analysis of ray coverage. There appears to be a good correlation of layer boundaries in our model and higher amplitude reflections observed on the stacked sections. Comparison of the seismic depth sections computed using stacking velocities with the GLI inverted velocity shows that the depth sections obtained from the latter agrees better with the velocity models. The combination of borehole data and the depth converted stacked sections provides key constraints for interpretation of the velocity models. The estimated investigation depth of the seismic profiles is on the order of 400 m with rays penetrating the surface cover and sedimentary sequences. These sedimentary rocks are characterized by a gradual increase in the velocity field with depth without strong contrasts and insignificant lateral velocity variations.

Baseline characterization of the CO2SINK geological storage site at Ketzin, Germany

Since April 2004, preparatory work prior to CO2 injection has been conducted in the CO2SINK Project, the European Union's first research and development activity on the in-situ testing of geological storage of CO2 near the town of Ketzin, Germany. Carbon dioxide will be injected into a saline aquifer of the Triassic Stuttgart Formation in an anticlinal structure of the northeast German Basin. The drilling of one injection and two observation wells will commence at the end of 2006. The predrilling phase focuses on the baseline geological parameters of the anticline. The Stuttgart Formation is lithologically heterogeneous; it consists of sandy channel-(string)-facies rocks, with good reservoir properties alternating with muddy flood-plain-facies rocks of poor reservoir quality. Playa-type rocks form the immediate cap rock above the CO2SINK reservoir. A geostatistical approach has been applied to describe the reservoir architecture between and beyond well control. This model forms the basis for the generation of reservoir-dynamic models of CO2 injection that assist in the planning of injection operations and in the understanding of CO2 plume evolution. A verification of the geometry of the reservoir and the structural situation of its overburden is expected from a three-dimensional baseline seismic survey that was conducted in the autumn of 2005. Laboratory experiments under simulated in-situ conditions were performed to evaluate the geophysical signature of rocks saturated with CO2. The chemical composition of the groundwater and the CO2 flux in the soil were analyzed across the Ketzin anticline, providing the baseline for a monitoring program during and after injection of CO2, targeted at the detection of potential CO2 leakage from the storage reservoir.