COCO22 sequestration in sequestration in geological formations – geological formations –

application of application of Computer Tomography Computer Tomography

(CT)(CT)

OutlineOutline

Background of COBackground of CO22 sequestration sequestration Core idea – new researchCore idea – new research Fundamentals of Computer Fundamentals of Computer

Tomography (CT)Tomography (CT) Experimental Setup and Experimental Setup and

MeasurementsMeasurements ResultsResults Future WorkFuture Work

COCO22 sequestration sequestration

The carbon dioxide (COThe carbon dioxide (CO22) concentration in ) concentration in Earth’s atmosphere is continuously and Earth’s atmosphere is continuously and strongly increasing (Mauna Kea strongly increasing (Mauna Kea Observatories).Observatories).

COCO22 increases the average global temperature. increases the average global temperature. This again leads to climate change (Orr 2004).This again leads to climate change (Orr 2004).

One proposed way to mitigate climate change One proposed way to mitigate climate change and reduce atmospheric COand reduce atmospheric CO22 levels is the levels is the storage of COstorage of CO22 in geological formations in geological formations (Holloway and Savage 1993, Orr 2004, Kumar (Holloway and Savage 1993, Orr 2004, Kumar et al. 2005, Spiteri et al. 2005). et al. 2005, Spiteri et al. 2005).

COCO22 sequestration sequestration

Several field scale pilot projects have been Several field scale pilot projects have been running for years now, for instance the running for years now, for instance the Sleipner and the Weyburn project (Korbol and Sleipner and the Weyburn project (Korbol and Kaddour 1995).Kaddour 1995).

The petroleum industry uses COThe petroleum industry uses CO22 for enhanced for enhanced oil recovery (EOR) for many years now (Green oil recovery (EOR) for many years now (Green and Willhite 1998, Jessen et al. 2005). and Willhite 1998, Jessen et al. 2005).

There are still many open questions, e.g. how There are still many open questions, e.g. how much COmuch CO22 can be stored, how fast does the can be stored, how fast does the COCO22 migrate, how much is dissolved, how migrate, how much is dissolved, how much rock is dissolved and precipitated much rock is dissolved and precipitated (because of chemical reactions).(because of chemical reactions).

Direct Benefits of Direct Benefits of ResearchResearch

COCO22 storage capacities can be estimated. storage capacities can be estimated. Multi-phase flow processes on the pore-scale are Multi-phase flow processes on the pore-scale are

analysed; they again determine overall reservoir analysed; they again determine overall reservoir flow behaviour.flow behaviour.

Interfacial areas can be determined; they present Interfacial areas can be determined; they present a fundamental kinetic parameter in a fundamental kinetic parameter in COCO22 dissolution and any following chemical reaction. dissolution and any following chemical reaction.

Grain volume change – porosity change can be Grain volume change – porosity change can be analysed; such a change can dramatically change analysed; such a change can dramatically change permeability and long-term storage behaviour. permeability and long-term storage behaviour.

Improved risk assessment, one of the most Improved risk assessment, one of the most importat issues associated with the legal and importat issues associated with the legal and political side of political side of COCO22 sequestration. sequestration.

Computer Tomography – Computer Tomography – Historic backgroundHistoric background

CT was first used commercially in medical applications.CT was first used commercially in medical applications. Since the 1990’s, CT has become an established Since the 1990’s, CT has become an established

experimental method in the geological and petroleum experimental method in the geological and petroleum engineering sciences.engineering sciences.

Notable ApplicationsNotable Applications Investigation of pore structures of sandstones and Investigation of pore structures of sandstones and

carbonates (Schena and Favretto 2007, Hu 2007),carbonates (Schena and Favretto 2007, Hu 2007), Multi-phase flow in sandstones (Wildenschild 2004 & Multi-phase flow in sandstones (Wildenschild 2004 &

2005, Culligan 2004)2005, Culligan 2004) Estimation of dynamic and static petrophysical Estimation of dynamic and static petrophysical

parameters such as porosity, permeability, NMR signal, parameters such as porosity, permeability, NMR signal, formation factor, saturations, etc. (Arns 2006, Øren and formation factor, saturations, etc. (Arns 2006, Øren and Bakke 2006, Knackstedt 1994 & 2004, Prodanovic and Bakke 2006, Knackstedt 1994 & 2004, Prodanovic and Lindquist 2004).Lindquist 2004).

Computer Tomography Computer Tomography BasicsBasics

C = X-ray sourceV = Sample volumeL = path length inside the objectA = projectionΠ = detector plane

Lambert – Beers law

I = transmitted beam intensityI0 = incident beam intensityµ = linear attenuation coefficientx = sample thicknessi = phase numbern = total number of phases

]exp[1

0

n

iiixII

Experimental SetupExperimental Setup

Sketch of the flow cell.

Picture of experimental setup at Elettra.

Flow Cell PreparationFlow Cell Preparation The cylindrical specimen was a 40mm high, 6mm wide pack The cylindrical specimen was a 40mm high, 6mm wide pack

of unconsolidated quartz sand.of unconsolidated quartz sand. A homogeneous sandpack was compressed and the porosity A homogeneous sandpack was compressed and the porosity

was determined via mass balance (Φ = 38,93).was determined via mass balance (Φ = 38,93). Air was displaced by COAir was displaced by CO2 2 (to eliminate trapping of gas/air). (to eliminate trapping of gas/air). A highly concentrated aqueous KI solution (13 wt%) was A highly concentrated aqueous KI solution (13 wt%) was

injected and a CT scan was performed at Sinjected and a CT scan was performed at Sww1.1. n-Heptane was injected; when no more brine was produced, n-Heptane was injected; when no more brine was produced,

another CT scan was performed at the irreducible water another CT scan was performed at the irreducible water saturation, Ssaturation, Swiwi..

COCO22 was injected again. Gas injection was stopped when no was injected again. Gas injection was stopped when no more liquid production was observed. Another CT scan was more liquid production was observed. Another CT scan was taken.taken.

30 pore volumes (PV) of brine were injected and a final CT 30 pore volumes (PV) of brine were injected and a final CT scan was taken at the residual gas saturation Sscan was taken at the residual gas saturation Sgr gr ..

CT ScanningCT Scanning

We acquired images with a 16bit CCD We acquired images with a 16bit CCD camera. The camera has a high camera. The camera has a high resolution, 4008 x 2672 pixels, with resolution, 4008 x 2672 pixels, with an active input area of 18 x 12 mm.an active input area of 18 x 12 mm.

The nominal resolution reached was 9 The nominal resolution reached was 9 µm. µm.

All CT scans were conducted at an All CT scans were conducted at an energy level of 28 keV. The sample to energy level of 28 keV. The sample to detector distance was 10 cm.detector distance was 10 cm.

Linear mass attenuation Linear mass attenuation coefficients of investigated coefficients of investigated

phasesphases

Brine saturated Brine saturated sandpacksandpack

Brine - blueSand - red ResidueCO2 –yellow

Gas-brine-quartz grayscale histogram.

Light Gray is sand and dark gray is pore volume.

• Residue CO2 is located in the largest pore as expected by capillary pressure theory.• It is almost circular because of surface tension effects.

Sandpack at irreducible Sandpack at irreducible water saturationwater saturation

Sand brine oilsegmentation.

Red volume represents sand, blue brine and yellow oil.

Brine – blueSand – redOil - orange

• Oil penetrates on average mainly into the larger pores as expected by capillary pressure considerations. • Thin water layer is visible on the rock surface as expected for quartz.• Oil has penetrated into the middle of some pores.

Sandpack at maximum Sandpack at maximum gas saturationgas saturation

Sand brine gas segmentation.

• All large pores and almost all medium-sized pores have been filled with non-wetting phase, in agreement with surface free energy considerations. • The thin water layer visible on the rock surfaces is probably a product of the border enhancing effect.

Brine – blueSand – redCO2 - yellow

Sandpack at residual Sandpack at residual gas Saturationgas Saturation

Sand brine gassegmentation.

Red volume represents sand, blue brine and yellow CO2 gas.

Brine –blueSand – redCO2 - yellow

• The largest CO2 ganglia is continuously spread over the largest available pore.• Though overall gas accumulates in the larger pores, a random distribution between large and medium size pores is observable.• Several tiny gas bubbles are randomly distributed in the pore volume. Though they might originate from the segmentation process, it is thought that they are real.

ConclusionsConclusions Computer Tomography is a suitable method to visualize fluid Computer Tomography is a suitable method to visualize fluid

saturations in unconsolidated quartz sandpacks in 2D and 3D.saturations in unconsolidated quartz sandpacks in 2D and 3D. Quartz and brine highly concentrated (13 wt%) with potassium Quartz and brine highly concentrated (13 wt%) with potassium

iodide had distinctly different CT numbers and could readily be iodide had distinctly different CT numbers and could readily be distinguished in the greyscale histogramdistinguished in the greyscale histogram

n-Heptane has a low CT number, which again was clearly n-Heptane has a low CT number, which again was clearly distinguishable from quartz and doped brine.distinguishable from quartz and doped brine.

Oil penetrated mainly into larger pores of the water-wet sand as Oil penetrated mainly into larger pores of the water-wet sand as expected, but also to a minor extent into medium-sized pores. The expected, but also to a minor extent into medium-sized pores. The brine displacement in the medium pores might have been a kinetic brine displacement in the medium pores might have been a kinetic effect due to external pressure fluctuations generating high effect due to external pressure fluctuations generating high pressure spots in the fluid system.pressure spots in the fluid system.

A water layer is visible between sand and oil and rock and gas. This A water layer is visible between sand and oil and rock and gas. This phenomenon nevertheless cannot be substantiated with the phenomenon nevertheless cannot be substantiated with the presented CT images, as these layers might very well be a product presented CT images, as these layers might very well be a product of the border enhancing effect during image processingof the border enhancing effect during image processing

Gas penetrates into the larger pores.Gas penetrates into the larger pores. At the residual gas saturation, gas is trapped in the large pores, At the residual gas saturation, gas is trapped in the large pores,

but also in some medium-sized pores – the trapping in the medium but also in some medium-sized pores – the trapping in the medium sized pores might again be due to kinetic effects.sized pores might again be due to kinetic effects.

Future WorkFuture Work

Evaluate different consolidated Evaluate different consolidated sandstone, dolomite and limestone sandstone, dolomite and limestone samples in terms of their COsamples in terms of their CO22 storage storage capacity.capacity.

A new flow cell is designed for this A new flow cell is designed for this purpose.purpose.

Saturations and interfacial areas will be Saturations and interfacial areas will be calculated for each saturation state and calculated for each saturation state and compared to our petrophysical data from compared to our petrophysical data from column experiments.column experiments.

ContributorsContributors

• Prof. Martin Blunt • Dr. Stefan Iglauer• Stefano Favretto• Andrea Soldan

Observation – SObservation – Sww11

Residue COResidue CO22 is located in the largest is located in the largest pore as expected by capillary pore as expected by capillary pressure theory as it constitutes the pressure theory as it constitutes the non-wetting phase against the non-wetting phase against the strongly water-wet quartz sand. It is strongly water-wet quartz sand. It is also almost circular, apparently also almost circular, apparently because of surface tension effects – because of surface tension effects – the pore walls distort the gas bubble the pore walls distort the gas bubble from its ideal spherical geometry. from its ideal spherical geometry.

Observations -- SObservations -- Swiwi

Oil penetrates on average mainly into the larger pores as Oil penetrates on average mainly into the larger pores as expected by capillary pressure considerations. Nevertheless expected by capillary pressure considerations. Nevertheless medium-sized pores are filled with oil before all larger pores are medium-sized pores are filled with oil before all larger pores are filled. This might be due to end effects or experimental pressure filled. This might be due to end effects or experimental pressure instabilities, which might have introduced a non-uniform instabilities, which might have introduced a non-uniform pressure front at the inlet. Another explanation for this pressure front at the inlet. Another explanation for this behaviour might be the heterogeneity of the sand, which behaviour might be the heterogeneity of the sand, which contains lignite to a small extent (ca. 0,17 wt%). As lignite is oil-contains lignite to a small extent (ca. 0,17 wt%). As lignite is oil-wet, it could imbibe oil into smaller pores, if it is (coincidentally) wet, it could imbibe oil into smaller pores, if it is (coincidentally) distributed that way.distributed that way.

In most cases, a thin water layer is visible on the rock surface as In most cases, a thin water layer is visible on the rock surface as expected for quartz; nevertheless the existence of these layers is expected for quartz; nevertheless the existence of these layers is not evidenced by the presented CT images as these potential not evidenced by the presented CT images as these potential layers cannot be separated from the phase contrast/border layers cannot be separated from the phase contrast/border enhancing effect, which itself produces a “layer-like” interface enhancing effect, which itself produces a “layer-like” interface between two phases.between two phases.

Moreover, in some pores mainly filled with brine, oil has Moreover, in some pores mainly filled with brine, oil has penetrated into the middle of the pore, completely in agreement penetrated into the middle of the pore, completely in agreement with theory which states that oil should appear there first with theory which states that oil should appear there first because of surface free energy minimisation (Anderson 1987).because of surface free energy minimisation (Anderson 1987).

Observations -- SObservations -- Sgmaxgmax

All large pores and almost all medium-All large pores and almost all medium-sized pores have been filled with non-sized pores have been filled with non-wetting phase, in agreement with wetting phase, in agreement with surface free energy considerations. Gas surface free energy considerations. Gas has partly filled some of the medium-has partly filled some of the medium-sized pores in their middle as expected.sized pores in their middle as expected.

Again the thin water layer visible on the Again the thin water layer visible on the rock surfaces is probably a product of rock surfaces is probably a product of the image processing, i.e. border the image processing, i.e. border enhancing effect.enhancing effect.

Obseravtions -- SObseravtions -- Sgrgr

The largest COThe largest CO22 ganglia is continuously spread over the largest available pore ganglia is continuously spread over the largest available pore connected to two other large pores. Gas blobs are also located in two further large connected to two other large pores. Gas blobs are also located in two further large pores, and in several medium-sized pores. Several tiny gas bubbles are randomly pores, and in several medium-sized pores. Several tiny gas bubbles are randomly distributed in the pore volume. Though they might originate from the segmentation distributed in the pore volume. Though they might originate from the segmentation process, it is thought that they are real. They appear in relatively large pores in the process, it is thought that they are real. They appear in relatively large pores in the water phase, in accordance with theoretical thermodynamic considerations. The water phase, in accordance with theoretical thermodynamic considerations. The problem to decide whether these small gas bubbles are real or an artefact of the problem to decide whether these small gas bubbles are real or an artefact of the segmentation process is currently one of the most interesting questions associated segmentation process is currently one of the most interesting questions associated with CT image analysis.with CT image analysis.

Though overall gas accumulates in the larger pores, a somewhat random Though overall gas accumulates in the larger pores, a somewhat random distribution between large and medium size pores is observable. This might again distribution between large and medium size pores is observable. This might again be due to the injection process, which might not have produced a stable and be due to the injection process, which might not have produced a stable and uniform brine pressure front. Therefore this phenomenon could be attributed to a uniform brine pressure front. Therefore this phenomenon could be attributed to a kinetic effect, i.e. gas was pressed into smaller, medium-sized, pores than expected kinetic effect, i.e. gas was pressed into smaller, medium-sized, pores than expected by capillary pressure theory by external pressure, but would relocate into the larger by capillary pressure theory by external pressure, but would relocate into the larger pores over time. There was insufficient CT time to study this kinetic behaviour as it pores over time. There was insufficient CT time to study this kinetic behaviour as it is estimated that this process takes several days to weeks.is estimated that this process takes several days to weeks.

It is not unlikely that a thin oil film has developed between the gas and the brine It is not unlikely that a thin oil film has developed between the gas and the brine phase as this phenomenon had been observed in previous work (Zhou 1997, phase as this phenomenon had been observed in previous work (Zhou 1997, Anderson 1987). But as the CT equipment used in this study is limited to a Anderson 1987). But as the CT equipment used in this study is limited to a resolution of 9 μm, films with a smaller thickness cannot be observed, even if the resolution of 9 μm, films with a smaller thickness cannot be observed, even if the segmentation process is faultless.segmentation process is faultless.

There is a visible water layer on the rock surfaces separating quartz from gas and There is a visible water layer on the rock surfaces separating quartz from gas and maybe a thin oil layer. As before, the existence of this layer cannot be validated maybe a thin oil layer. As before, the existence of this layer cannot be validated with the CT images as it overlaps with the border enhancing effect.with the CT images as it overlaps with the border enhancing effect.