D6.6: Final summary report, Norwegian Site

Ane Lothe et al.

CO₂ migration modelling has been carried out for the Trøndelag Platform area, offshore Mid-Norway (SiteChar D6.2. Lothe et al. 2013) . The Garn Formation has been considered as a promising reservoir for CO₂ storage activities. The north-westwards dipping structure is characterised by a high sand content only moderately buried. Consequently porosity and permeability are excellent for CO₂ storage purposes. Formation thickness has been estimated as between 100 and 150 m, and the number of faults is low on the Platform. In addition, the Garn Formation is overlain by thick shale sequences further reducing fault leakage risk and also suggesting a low risk for cap rock leakage. The modelling has been carried out with different modelling tools:

• Migration modelling using PetroCharge Express (PetroMod), with no loss included.
• Migration modelling using SEMI with loss function implemented for residual and density induced convection within the traps.
• Pressure modelling using Eclipse.

A static geological model of the Trøndelag Platform was built incorporating the available data from wells, seismic interpreted depth horizons and fault polygons. The potential reservoir (Garn Fm.) was simplified and modelled as homogeneous sandstone. For three potential injection sites CO₂ migration scenarios were developed, using Petromod and SEMI. For SEMI additional loss functions were implemented, mimicking the physic-chemical behaviour of the (supercritical) CO₂ phase on the migration path and within trap entities. Pressure build-up was simulated using Eclipse in order to assure that injection scenarios do not cause a seal rock fracturing risk.

In general, storage unit quality and capacity is excellent that under industrial demo size injection volume scenarios of 1 Mt/year (over a period of 40 years) no CO₂ migrates out of the enclosures. If injection volumes are increased by 5-fold to 5 Mt/year (also over a period of 40 years) the CO₂ phase starts to spill from the initial entities. As no loss to account for residual trapping is considered and no density induced convection implemented in Petromod, migration distances are higher in PetroMod than in SEMI where such loss functions are implemented. However, loss functions may require tuning of the intrinsic function Ψ(ξ) according to history matching. Pressure build-up is low to moderate and is not considered to cause any major risk for CO₂ storage on the Trøndelag Platform.

In Subtask 6.3 (SiteChar 6.3: Rinna et al. 2013) the aim was to simulate pressure and stress distribution, both on large scale and in a smaller area in the Halten Terrace area. The study area is situated at the eastern part of the Halten Terrace area, covering a producing gas and condensate field named Mikkel. The gas field is situated close to the Bremstein Fault Complex that defines the border to the Trøndelag Platform area. To avoid confusion, a synthetic dataset named "Fox Field" is used in this study, built on data from the Mikkel Field. Also an extended area around the Fox Field is used to give the boundary conditions to the Fox field model. The aim has been to evaluate CO₂ migration, and pressure build-up. Several injection rates and sites have been tested. Two static models are built:  

• Fox Field, synthetic data set in a small area (12X20 km);
• Extended area, around the Fox field (36X50 km)

Several simulations tools have been used:

• A 'tank model' using storage-DGF (depleted gas fields) and storage-AQUIFER
• Reservoir modelling using Eclipse 300;
• Pressure modelling using Pressim Production.

The tank model reproduces the Flowing Bottom Hole Pressure response of the Eclipse model of the Fox field. The tank model tends to have a lower estimate of the total CO₂ injected in the reservoir compared to the Eclipse model. The reason for this is that the tank model does not model the structure.

The Pressim simulator represents a simplified scheme to quickly calculate the overpressure within the formation during and after injection period. The simulations indicate that Pressim tends to overestimate the pressure during injection period, but can assess the long term (>100 years after injection stop) overpressure distribution within the carrier unit. The fault description is the central and vital input for this approach.

From the techno-economic analysis performed in Task 2.2 the estimated total discounted (8% discount rate) cost for the Trøndelag Platform is 160 M€ corresponding to an equivalent cost of 26.6 €/tonne of CO₂ stored (cf. SiteChar D2.2, Gruson et al., 2013). The main cost drivers for CO₂ storage in the Trøndelag Platform off-shore Mid-Norway are drilling and completion of wells (62.5 M€), the subsea installations (35 M€), the site characterization and engineering (18 M€) and the seismic monitoring (11 surveys at 3.25 M€ each).

A generic monitoring plan has been derived for the Trøndelag Platform with example from the Alpha structure, that is flexible enough to be applicable to other identified storage sites in the Norwegian sector and that would cope with any deviation between observation data and predictive models (SiteChar D6.5 Querendez et al., 2013). The main emphasis is put on repeat 3D seismic and pressure and temperature monitoring at the injection well. A generic remediation plans has also been proposed based on site specific containment risks as well as more generic scenarios for contingency monitoring.