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Master's thesis

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Steinar Kolstø

on 30 June 2010

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Transcript of Master's thesis

A Sensitivity Study of Cap Rock Integrity on CO2-Storage Key Questions

How and to what extent will CO2 influence the wettability of the cap rock?
What willl be the effects of this wettability alteration on the storage capacity of the storage site?
Are these effects insignificant enough to neglect for a real CO2 storage project?

In order to elucidate these uncertainties the following has been investigated:

The effects of CO2 pressure on the pH of the brine.
The effect of pH on the wettability of the sandstone and its cap rock.
The impact of this wettability alteration on the capillary pressure and relative permeability.
And last but not least; applying these effects on a realistic sector model likely to be found on the NCS.
Motivation Masters Theses in Petroleum Technology Reservoir Physics Steinar Midthun Kolstø Theoretical background Alteration of key parameters Sector model Atmospheric concentration of carbon dioxide increased from the pre-industrial age level of 280 ppm to 380 ppm at the present time
CCS is an important emission mitigation strategy
Large quantities need to be stored annually
Saline aquifers are promising storage sites for carbon storage

Carbon Capture and Storage Leakage of carbon dioxide Effects of leakage can be both local and global
Leakage may occur according to three different physical properties:
Diffusion of carbon dioxide-molecules in the water saturated caprock
Hydraulic or thermal fracuring of the caprock
Capillary breakthrough of the carbon dioxide phase

Storage capacity determined by maxiumum overpressure
in CO2 phase
The CO2-water capillary pressure is hence an important
constraint to consider when planning a CCS-project
The capillary pressure is controlled by the interfacial tension,
the pore throat radius and the wettability.

pH and the point of zero charge The pH variation is greater for low pressures, and will eventually level off
The point of zero charge for a given mineral surface is the pH at which that particular surface has a net neutral charge

Experimental evidence of wettability alteration Contact angle alteration in the pressure regime relevant for CCS Area of investigation Johansen Formation, located on the Horda Platform in the vicinity of the Troll field
The sector model is a 900x1500x95 m, one vertical injection well, ten vertical dummy wells.
The top depth of the model is between 2003-2065 m
The thickness of the shale layers range between 8-48 m, the reservoir is approximately 80 m
30 grid block Eclipse model 990 grid block CMG model Data input 36000 grid block CMG model

Porosity of sector model Permeability of sector model Pc and kr data Network model Capillary pressure data Capillary pressure curve sandstone base case Capillary pressure curve shale base case Relative permeability data Relative permeability curves sandstone base case Relative permeability curves shale base case Simulations and results 1 Base case scenario Parameter Value Unit
Johansen Fm.kv/kh 0.1 -
Shale kv/kh 0.1 -
Shale horizontal permeability 0.01 mD
Shale porosity 10 %
cosθ 1 -
Qinj 2 MtCO2/y
Sgc 0.05 -
Siw 0.15 -
Injection period 110 Years
Initial hydrostatic pressure 210 Bar
Temperature 94 ˚C
Sensitivity study overview Parameter Low High Unit
Johansen Fm.kv/kh 0.01 3.0 -
Shale kv/kh 0.01 0.3 -
Shale vertical permeability 8.1E-7 0.001 mD
Shale porosity 6 26.5 %
Shale mean pore radius 3.1 1403.4 nm
Qinj 1 3 MtCO2/y
Hysteresis Dynamic wettability alteration model Simulations and results 11 Worst case scenario results Discussion Shale not pure mica, sandstone not pure quartz
Great uncertainty in PSD, and co-ordination number
Shale has been considered homogeneous

Summary and conclusion Further work It has been observed that the migration of CO2 will be strongly dependent on the heterogeneity of the aquifer
A larger amount of CO2 accumulates underneath the cap rock when dissolution is ignored
Static and dynamic wettability alteration proved to generate very different results
Effects of wettability alteration significant for static model.
Results indicate that the effects of the wettability alteration upon cap rock integrity will be negligible.
Obtain realistic indications of the co-ordination number and pore size distribution of the specific formation investigated
Apply a different network model, considering that some difficulties, especially in trying to generate imbibition curves, were encountered.
Obtaining the mineralogy of the rocks involved will also be of great importance in estimating the isoelectric point Relative importance of wettability alteration
One parameter changed, all other at base values
Block saturation measured for first shale layer
Water imbibition after injection end.
Trapping by snap-off and by pass by imbibition.
Kr depends on saturation and saturation history,
Reduced wettability Reduced potential for imbibition
Hysteresis included after injection end
Effects of hysteresis significant after 300 years after injection end
Trapping of CO2 predominantly in lowest layers
Sw and krw higher in these regions
No significant difference in gas saturation can be observed in the cap rock
Hysteresis unfortunately not included for capillary pressure.
Pc,entry for base case 291000 Pa.
At 210-230 bar Pc,entry was reduced by a factor ten compared to base case.
For the pressure increase from 210 bar to about 330 bar, Pc,entry was reduced from 29700 Pa to 11798 Pa. Difference in free gas vs. trapped gas in the two scenarios.
Explained by the critical gas saturation.
For the near neutral wettability scenario more free gas is present in the reservoir.
Thank you Static model:
Spontaneous wettability reduction

Dynamic model:
Threshold gas saturation (TGS)
Sandstone=Reduced wettability as long as block saturation exceeds or equals TGS.
Shale=If block saturation significantly high, Pc and kr assigned corresponding to pressure intervall.

Restart files each ten year.
Rock property number assigned each block in sector model.
Mica Quartz CNP for both wells
Sector model porosity calculated on basis of well block porosities, weighted by distance.

Permeability heterogeneity
Perm-por correlation made for Fensfjord and Sognefjord Fm.
Effecs of wettability alteration significant on the cap rock integrety
Overestimate importance of wettability alteration
Investigate further in dynamic wettability alteration model. Increased trapping for hysteresis model
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