Since production began in the HP/HT Kristin Field off mid-Norway, reservoir pressure in each of the three mid to late Jurassic reservoir units (the Garn, Ile and Tofte formations) has declined significantly more rapidly than was initially predicted. In the Garn Formation, the Tofte Formation and to some extent also the Ile Formation, this has occurred at least partly because an unusual distribution of reservoir properties led to bias in the four-well appraisal dataset and this in turn resulted in an overestimation of reservoir properties. Of particular importance to this bias was the fact that very good but unrepresentative reservoir properties were encountered in all three reservoir zones in the discovery well located in the centre of the field. These, it is now realized, are not even typical of most of the central part of the field but are, instead, restricted within one, small, anomalous area. Study of cores and thin sections indicates that in each reservoir unit this directly reflects a concentration of more energetic depositional facies in the area while less energetic facies are present on three sides. This pattern was not predictable from the original dataset and seems to have arisen because there was structural control upon facies positioning during accumulation of the reservoir section. This influenced the distribution of cleaner, coarser grained, more proximal depositional facies and, ultimately, reservoir quality distribution and pressure development. What is interesting about Kristin Field is that the structural influence upon sedimentation is observed within the footwall stratigraphy of a major relay structure where the primary provenance direction was on the hanging-wall side. This pattern is the reverse of what is normally reported in tectono-stratigraphic studies.

Large-scale underground storage of CO2 has the potential to play a key role in reducing global greenhouse gas emissions. Typical underground storage reservoirs would lie at depths of 1000m or more and contain tens or even hundreds of millions of tonnes of CO2. A likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety. A diverse portfolio of potential monitoring tools is available, some tried and tested in the oil industry, others as yet unproven. Shallow-focused techniques are likely to be deployed to demonstrate short-term site performance and, in the longer term, to ensure early warning of potential surface leakage. Deeper focused methods, notably time-lapse seismic, will be used to track CO2 migration in the subsurface, to assess reservoir performance and to calibrate/validate site performance simulation models. The duration of a monitoring programme is likely to be highly site specific, but conformance between predicted and observed site performance may form an acceptable basis for site closure.

CO2 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of CO2 in the reservoir, image the CO2 plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of CO2 trapped beneath intra-reservoir shales. A prominent vertical ‘chimney’ of CO2 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude – thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected CO2 is concentrated in the layers. The remainder is interpreted to occupy the feeder ‘chimneys’ and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the CO2 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by CO2 present only at high saturations in the thin layers; a minor proportion of low saturation CO2 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.

Abstract The HPHT Kristin field is located offshore Norway in 350 m water depth and is developed with four sub-sea templates. Since production began in November 2005, reservoir pressure has dropped around 4 times faster than predicted. The main Kristin field statistics include: an initial pressure of 910 bar, a reservoir temperature of 170°C, a dew point of around 400 bars (rich fluid accounts for 50% of income) and in-place volumes of around 100 GSm3 of gas and 100 MSm3 of condensate. The Kristin reservoir section comprises three separate Jurassic sandstone units and the field is segmented by faults. This paper describes the process of continuously history matching pressure decline during the first two years of production in Kristin using: production data, RFT pressures from wells drilled following production start-up, shut-in wellhead pressures, and data from two bottom-hole gauges. Close multidisciplinary cooperation between geophysicists, geologists and reservoir engineers has been key to ‘re-understanding' the Kristin reservoirs since production start-up and has facilitated the process of tuning the simulation model to obtain a better history matched model. The most important modifications have been the introduction of horizontal pressure barriers to match RFT pressures, and a significant reduction of pressure support from the eastern part of the field. The poor pressure support is interpreted to result from enhanced fault seal in one of the main reservoir units, and a rapid easterly decrease in reservoir properties in the other (main) reservoir unit. These changes have been directly input into the simulation model and have resulted in a good match of the pressure history from the 11 producing wells. Directly input into the simulation model has shortened the updating cycle to a few minutes and saved several months in the history matching process. Work is currently ongoing to refine a more detailed geological model based on these results, and the additional knowledge gained by testing different solutions directly in the simulation model. This improved understanding of the reservoir behaviour also gives valuable input to the uncertainty study required before updating official reserves.

This chapter highlights that the quantitative assessment of leakage risk and leakage rates from planned underground CO2 storage sites is a primary requirement for public acceptance, formal site approval, and credit for stored CO2 quantities under CO2 emission schedules. Leakage through the top seal can basically occur by three processes: diffusion through the pore system, capillary transport through the pore system of the seal, and multiphase migration through a fracture network; or by a combination of any of these. Diffusion results in very low leakage rates; maximum rates typically attained after several 100 000 years, being in the ppm range. Multiphase capillary migration is characterized by two main parameters: capillary breakthrough pressure and effective permeability to the non-wetting phase. The dependence of effective permeability to CO2 on capillary pressure, which in turn is a function of CO2 column height, is hysteretic in character with generally higher effective permeability during pressure decrease than during increase, at the same capillary pressure. Leakage is likely to stop at approximately 20 to 50% of the breakthrough pressure as suggested by the snap-off theory. Capillary breakthrough pressure and effective permeability is very difficult to measure for low-permeable rocks.

This chapter gives an overview of the CASTOR (CO2, from Capture to Storage) R and D project, funded by the European Union (EU) under the 6th Framework Program. With a partnership involving Industry and Research organizations, CASTOR aims at developing new technologies for post-combustion capture and at studying 4 new European storage sites. The main goal of this project is to develop and validate, in public/private partnerships, all of the innovative technologies needed to capture CO2 at the post-combustion stage and store CO2. The CASTOR R and D target aims to enable the capture and geological storage of 10% of the CO2 emissions of Europe that corresponds to about 30% of CO2 emitted by European power and industrial plants. To achieve this goal, CASTOR would need to improve current techniques and develop, validate, and generalize previously nonexistent methodologies and technologies for the capture of CO2 and its subsequent secure underground storage. © 2005 Elsevier Ltd. All rights reserved.

The paper aims to draw some generic conclusions on reservoir characterization based on the Sleipner operation where CO2 is being injected into the Utsira Sand. Regional mapping and petrophysical characterization of the reservoir, based on 2D seismic and well data, enable gross storage potential to be evaluated. Site-specific injection studies, and longer-term migration prediction, require precision depth mapping based on 3D seismic data and detailed knowledge of reservoir stratigraphy. Stratigraphical and structural permeability barriers, difficult to detect prior to CO2 injection, can radically affect CO2 migration within the aquifer.

Abstract C0 2 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of C0 2 in the reservoir, image the C0 2, plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of C02 trapped beneath intra-reservoir shales. A prominent vertical 'chimney' of C0 2 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude-thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected C0 2 is concentrated in the layers. The remainder is interpreted to occupy the feeder 'chimneys' and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the C0 2 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by C0 2 present only at high saturations in the thin layers; a minor proportion of low saturation C0 2 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.

Since October 1996, Statoil and its Sleipner partners have injected CO2 into a saline aquifer, the Utsira Sand, at a depth of approximately 1000 m. The aquifer has a thickness of more than 200 m near the injection site and is sealed by thick shales. A multi-institutional research project SACS (Saline Aquifer CO2 Storage) was formed to predict and monitor the migration of the injected CO2. To this end two time-lapse seismic surveys over the injection area have been acquired, one in October 1999, after 2.35 million tonnes of CO2 had been injected, and the second in October 2001, after approximately 4.26 million tonnes of CO2 had been injected. Comparison with the baseline seismic survey of 1994 prior to injection provides insights into the migration of the CO2. In this paper the results of the seismic interpretation will be shown, supported by synthetic seismic modelling and reservoir flow simulation of the migrating CO2 at the two different time-steps.

The paper aims to draw some generic conclusions on reservoir characterization based on the Sleipner operation where CO2 is being injected into the Utsira Sand. Regional mapping and petrophysical characterization of the reservoir, based on 2D seismic and well data, enable gross storage potential to be evaluated. Site specific injection studies however require precision depth mapping based on 3D seismic data and detailed knowledge of reservoir stratigraphy. Stratigraphical and structural permeability barriers, difficult to detect prior to CO2 injection, can radically affect CO2 migration within the aquifer.

Capture of C02 from large point sources and subsequent long-term underground storage is at present the most promising option to reduce greenhouse-gas emissions without major sudden changes of our energy system. Two main types of subsurface CO2 reservoirs exist: emptied hydrocarbon fields and (saline) aquifers. In addition, some minor quantities of CO2 can be stored in enhanced oil recovery (EOR) projects. In all these cases, the existente of a tight seal is mandatory, to restrict buoyancy-driven upward migration of the CO2 into the atmosphere.

In the ongoing aquifer CO2 disposal project in the Sleipner license (North Sea), underground CO2 is being monitored by time-lapse seismic. The CO2 is being injected close to the base of a high permeable, highly porous sand unit, the Utsira Sand. In an iterative process between seismic surveys and reservoir simulations, a reservoir model featuring the major controlling heterogeneities has been developed. Well-data and seismic data prior to injection shows that the sand is divided by nearly horizontal, discontinuous shales. From the 3-D seismic image after three years of injection, strong reflectors can be interpreted as CO2 accumulations identifying the major shale layers that control the vertical migration of CO2 from the injection point to the top of the formation. By modelling this flow in reservoir simulations, it can be inferred that the CO2 is transported in distinct columns between the shales rather than as dispersed bubbles over a large area. Improvement of the geological model increases the confidence of predictions based on simulation of the long-time fate of CO2. A possible natural aquifer flow can have a pronounced effect on the location of CO2 accumulations due to the relatively flat topography of the trapping shales. This effect has been quantified by simulation and this phenomenon was used to adjust the localisation of the CO2 bubbles to better fir the seismic images.

Prediction of the distribution Of CO2 injected in the Sleipner area (North Sea) is a major topic of the international research project SACS. We report here a summary of detailed geological interpretations of the depository (the Mio-Pliocene Utsira Sands), based on seismic, wireline-log and sample data. We expect that CO2 will ultimately migrate to the top of the Utsira Sands which is formed by the base of a Pliocene shale unit, or into an eastward thickening sand wedge closely above the base of the Pliocene shales. Therefore, and since CO2 migration is primarily buoyancy driven, we consider the topography of the tops of these two alternative reservoirs to be of utmost importance for the medium-term migration pattern.The two barrier surfaces were mapped at 3D seismic resolution and were used in various representations (different interpreters, time domain, depth domain etc.) as input for migration simulations employing the secondary migration simulator SEMI. Migration below the top Utsira Sands is predicted to take place in a north-westward direction and will probably be not more than ca. 12 km for a total injected quantity of 20 Mill. metric tons CO2. Migration below the top sand wedge will take place in a north-eastward direction with the consequence that CO2 will leave the studied area when up to 25% of the planned injection volume will have been trapped. Our simulation results highlight the importance of subtle topography differences (0.3degrees difference in regional dip between the two barrier horizons) on the migration pattern. The large migration distances and the necessary high topography resolution for storage sites in (shallow) aquifers demand modelling capacities not yet available with standard simulation tools.

SUMMARY

From all five wells, the interval directly overlying the reservoir units (the Utsira Sand and a sand wedge in the lowermost part of the Nordland Shales) has been analysed, which is expected to provide the primary seal for upward migrating CO2 both local and regional (‘shale drape’). Quartz content of the samples has been related to pore throat radius according to the empirical relationship of Krushin (1997), and the necessary CO2 column height to cause capillary failure of the seal has been calculated to be approx. 860 m. This is much above expected column heights and capillary failure is thus considered unlikely. Observed migration through shale layers in the Utsira Sand and through a 7 m thick shale package between the Utsira sand and a sand wedge above contrasts to this prediction.

Since October 1996 Statoil has started to inject CO2 coming kom the Sleipner Vest Fieldin the southem Viking Graben area into a saline aquifer at a depth of approximately 900 m. This is the first case of industrial scale CO2 storage in the world (1 million tons per year). Careful monitoring of the behavior of the storage facility is hence required. To this end different time-lapse seismic surveys have been planned (presented in a companion paper: Brevik et al., this volume). In this paper the interpretation of the base survey acquired before injection is presented. The most likely pathways for CO2 migration in the vicinity of the injection point have been indicated

Subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the Nicoya segment of the convergent plate margin off Costa Rica. Based on new results from ODP drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the Nicoya coastline. High seismic velocities in the outermost part of the forearc wedge off the Nicoya Peninsula below the BOSS (bottom‐of‐slope‐sediment) reflector indicate the seaward continuation of the Nicoya ophiolite complex into the basement of the forearc wedge. ODP Site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement, progradation of the sedimentary succession, and landward migration of the coastline. Tilted block structures are explained by substantial fore‐arc extension. In our interpretation, the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline. Landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation.

The convergent plate margin off the Osa peninsula in southern Costa Rica is characterized by the indentation of the Cocos ridge at 4–5 Ma. The indentation causes the uplift of the Osa mélange which we interpret to represent an exhumed major channel for the transport of tectonically eroded material down into the subduction zone. We present evidence that, similar to the Nicoya segment of the Costa Rica convergent margin, subduction erosion rather than accretion has been the dominant process along the plate boundary. The composition of the Osa mélange is dominated by tectonized material of the upper‐plate Nicoya ophiolite complex (basalt, radiolarite, limestone). Strong deformation is concentrated in numerous discrete shear zones and produced the layered fabric of large rock volumes, which partly experienced temperatures > 200°C. We thus interpret the Osa mélange to be a product of subduction erosion at the base of the outer arc wedge structure.

The regional pattern of contraction and extension directions and the evolution of the strain field from Paleogene to Neogene times enabled a reconstruction of the migration path of the Carpathian collision front. The Carpathian nappes were thrust around the Moesian Plate during Paleogene and Early Neogene times and protruded into a small oceanic embayment between the Moesian and European plates. The arc structure of the Carpathian fold-thrust belt was formed in Late Neogene times as a result of the eastward-escaping Tisza–Dacia block, due to N-directed convergence of the Adriatic plate and the retreating subduction of an oceanic slab. Brittle deformation structures in the Romanian Carpathians suggest three tectonic events related to major plate motions: (1) Paleogene to Middle Miocene NE to ENE contraction caused right-lateral curved strike-slip faults; (2) Middle Miocene to Pliocene fan-shaped orientations of contraction directions were caused by right-lateral oblique convergence in the Southern Carpathians, frontal convergence in the southern Eastern Carpathians and left-lateral convergence in the northern Eastern Carpathians; (3) Pleistocene to Holocene general E–W extension and N–S contraction in the Carpathian arc and local ESE–WNW contraction in the Vrancea area is related to the late roll back stage and break-off of the subducted slab in the bend area.