Laboratory experiments for CO2 geological

characterisation

Katriona Edlmann

Aim to characterise the input parameters for input into the CO2

injection and storage reservoir model

• Elements of the CO2 storage system – Caprock – Storage reservoir rock – Fluids: formation and injected CO2 – Fractures

• Laboratory experiments for geological characterisation – Rock properties – Rock mechanical properties – Fluid properties – Rock / fluid interactions

• Summary of the experimentally derived parameters controlling the CO2 storage system

Presentation Outline

Geological storage of CO2

CO2 storage mechanisms

• Structural trapping

• Residual trapping

• Solubility trapping

• Mineral trapping

• Adsorptive trapping

Primary geological elements of the CO2 storage system

• Overburden • Caprock • Storage reservoir

rock • Fluids: formation

and injected CO2 • Fractures

Caprock properties

• Structural storage reservoir seal

• Mudstones, claystones, shale and evaporites

• Limited clay and other mineral reactivity.

• Low permeability /barrier to flow.

• Small pores and pore throats – capillary sealing.

• Ductile so less prone to faulting and fracturing

• Lack of fractures

• Lateral seal continuity

• Thick multi layered deposits

Storage reservoir rock properties

• Under impermeable layer (caprock) with a trapping structure.

• Porous and permeable rock

• Sandstones and limestones

• Silicate and carbonate minerals and cements

• Deeper than potable water / usable aquifers

• Thick and extensive deposits

Fluids • Multiphase system

– Formation brines

– Hydrocarbons: gas and oil

– CO2 (generally in supercritical state)

7

Miscibility of oil and CO2 – an overview

68 bar – 1000 psi

Immiscible CO2

102 bar – 1500 psi

Miscibility begins to develope

170 bar – 2500 psi

CO2 has developed miscibility

Higher hydrocarbons (dark spots)

begins to condense

Final stage: Higher HC forms

continuous phase- CO2 immiscible

Fracture networks

• Reactivation of existing faults

• Sealing or non sealing faults

• Pre-existing micro-fractures within the caprock

• Hydraulic fracturing

Geological characterisation

• Provide data for the storage site reservoir model

• Each grid block can be over 100m3

• Differences in scale

– Micron to cm in lab

– m’s in wireline logs

– 100’s km in field

Upscaling

• Upscaling statistics

• Fine-scale geological model must be upscaled to a coarser grid suitable for fluid flow simulations

mm km 100km microns m

Laboratory Experiments

Rock (matrix)

properties

Mechanical properties

Fluid properties

Rock / fluid interactions

Laboratory experiments to determine the parameters needed for geological characterisation

Geological data available

Rock (matrix) properties

• Porosity

• Pore diameters

• Grain shape, sorting and distribution

• Permeability

• Bulk density

• Rock mineralogy

• Rock heterogeneity

• Fracture profiling

Porosity • A measurement of the pore volume available

within the rock. Defined as the percentage of the bulk rock volume (Vb) not occupied by solid material.

• Easier to measure grain volume (Vg) of a sandstone:

Porosity = ((Vb – Vg)*100)/Vb

• Gives no indication of pore size, distribution or connectivity as rocks with identical porosity can have very different physical properties.

Porosity Triple weighing method

immersedsaturated

drysaturated

MM

MM

100

immersedsaturated

dry

MM

M

Immersed sample Dry sample (in vaccum)

With three weighing, we can calculate the water available porosity and the sample density.

Measuring porosity • Helium gas expansion porosimeter is used for

direct grain volume and pore volume measurement. It is based on the Boyle's law of expansion of helium gas where:

• Under conditions of fixed gas quantity and constant temperature, the product of the pressure and volume stay constant.

• Boyle's law is expressed as follows:

P1V1 = P2V2:

Pore (and pore throat) diameters • Pore throat diameter

influences:

– Capillary entry pressures

– Flow through of the sample (permeability) especially in multiphase systems

0

10000

20000

30000

40000

50000

60000

70000

0 0.1 0.2 0.3 0.4 0.5

Pore diameter (µm)

Intr

usi

on

pre

ssu

re (p

sia)

Série1

0

5

10

15

20

25

0 0.02 0.04 0.06 0.08 0.1

Pore diameter (µm)

Incr

emen

tal v

olu

me

(mL/

g)

Série1

The total injected mercury volume represents the connected porosity (down to pore diameter of ~1nm)

Mercury intrusion porosimetry

Method: -Mercury is injected into the sample -Mercury intrusion pressure is increased to access to smaller pore diameters

Measuring pore diameters

Measuring porosity, pore size distribution and pore diameters

From thin sections / optical microscope using 2D images

Determination of total porosity on 2D images using blue epoxy on thin section by microscopy technique.

Segmentation of the 2D image to determine the total porosity, which represents the ratio between the number of black pixel and the total pixel of the image. Here: 55.4% of porosity

Advantages: -Easy and rapid method -Total porosity determined and not only the connected porosity

Drawbacks: -2D porosity ( from 3D porosity ) -Depends on the pixel size resolution

Measuring porosity, pore size distribution and pore diameters

Using X-ray microtomography to generate 3D images

Advantages:

-3D images with high resolution pixel

size

-A lot of physical and structural

parameters can be measured or

calculated from the processed images :

porosity (total and connected), specific

surface, tortuosity, permeability, …)

Drawbacks:

-Expensive and time consuming technique

Grain sorting and distribution

• Grain size, shape, sorting will influence porosity

– Grain sorting: porosity is generally found to increase with increased sorting

– Grain packing: porosity will vary depending on how the grains are packed.

– Grain shape; sediments composed of spherical grains will have a lower porosity and very elongate particles can align in a manner to pack tightly

– Grain cement: the amount and distribution of cement has a huge impact on porosity.

Measuring grain sorting and distribution

Using samples whole or in thin section with the aid

of a microscope or magnifying lens.

Permeability • Permeability is a measurement of rocks ability for

gases or fluids to flow through the rock.

• High permeability values mean that fluids and gases can move rapidly through the rock.

• In a storage system you want the reservoir rocks to have a reasonable permeability and the caprock must have very low permeability (impermeable).

Permeability The Darcy flow equation defines permeability, and after some rearrangement, is used to calculate permeability from laboratory measurements.

Q = K * A * (P1 - P2) / (u * L) Where: Q = flow rate K = permeability A = area P1 - P2 = pressure drop L = path length u = mobility

Permeability measurement • Absolute (intrinsic) permeability (Ka)

measured with a nitrogen permeameter using Darcy's equation.

• When water is used as the single fluid, the result is called "liquid permeability" (Kliq).

• Air permeability is usually a little higher than liquid perm.

• The Klinkenberg correction is used to reduce air perm to an equivalent liquid permeability.

Permeability measurement • Effective permeability is the permeability of a

rock to one fluid in a two phase system.

– For example, the effective permeability of oil in an oil-water system (Ko) will be less than absolute permeability.

• Relative permeability is the ratio of the effective permeability of a fluid at a given saturation to some base permeability.

– Base permeability is typically defined as • absolute permeability (Ka),

• air permeability (Kair), or

• effective permeability to non-wetting phase at irreducible wetting phase saturation.

Relative permeability

Résults (Perrin et al., Energy Procedia, 2009)

Measured using a steady state approach

Porosity (main 18.2%) Porosity (main 20.3%)

1.2 cm3.min-1

2.6 cm3.min-1

Bulk Density • Density varies with rock type due to differences in

mineralogy and porosity.

• Density is taken to be the weight in air of a unit volume of a rock at a specific temperature.

• Density is calculated from the weight of grains and cement (solids) (Wg) and the total volume of the grains and cements plus the void space (Vb).

bulk density (b)= Wg / Vb

Vb = plug diameter2*p/4*plug length / 1000

Bulk density (b) = plug weight / Vb

Mineralogy • The minerals that make up the reservoir rock and

caprock are of paramount importance as they provide information about potential rock / fluid reactivity – precipitation / dissolution

• They also influence fluid dynamics through wettability, interfacial tension and contact angle.

• In general thermodynamics favours the dissolution of carbonate phases in limestone and dissolution of silicates and precipitation of carbonates in sandstones.

Mineralogy measurements

• Scanning Electron Microscope (SEM) imaging

– Electron beam interacts with mineral. The mineral electrons lose energy by scattering and absorption within an interaction volume – this provides information on atomic number and density.

• EDS (energy dispersive) X-ray analysis

– The number and energy of x-rays emitted from a mineral allows elemental compositions

• X-Ray Diffraction (XRD) analysis

– Analysis of the scattered intensity of a x-ray beam hitting a mineral allows identification.

Fracture profiling

22/10/2013 PANACEA 32

• Laser scanner used for capturing fracture surface topography

Rock heterogeneity • Geological characterisation requires average

parameter input values

• Averaged over grid block areas of in excess of 100m3

• Rocks are NOT homogeneous (at any scale)

• Statistical up scaling – representative elemental volume.

Rock (matrix)

properties

Mechanical properties

Fluid properties

Rock / fluid interactions

Mechanical properties

Mechanical properties

• As rocks are buried the weight of the overlying material generates stress.

• This stress works on the rock matrix, pores and pore fluids.

• Injection of CO2 creates fluid and thermal stresses that also acts on the rock matrix / pore / fluid system.

• The mechanical properties of the rock categorise how the rocks respond to any changes in stress.

Mechanical properties

• Burial and fluid forces act on the rock mass to create a stress (force per unit area).

• Three principle stresses in a reservoir

s1 (maximum) > s2 (intermediate) >s3 (minimum)

• When stress is applied to a rock (matrix), the rock experiences a change in dimension, volume or shape termed strain (e)

• The fluids exert multi directional loads on the walls of the pore spaces called pore pressure

Mechanical properties - Failure • Elastic region

– If stress is removed sample will return to original state

• Yield point

– Point at which permanent changes occur

• Ductile region

– Sample undergoes deformation but can support load

• Brittle region

– Ability to withstand stress decreases as deformation is increased

Mechanical properties Elastic Moduli

• Measurement of distortion under linear stress

• Modulus of Elasticity (Young's Modulus) (E)

– Samples ability to resist compression

• Poisson’s Ratio (u)

– Measure of the lateral expansion relative to longitudinal contraction

Static elastic moduli testing

• Loading frame (delivers load s1)

• Hoek cell (delivers a confining stress (s2 and s3)

• Strain gauged samples (measure rock deformation)

Elastic moduli testing • Sample loaded into Hoek cell

• Confining pressure applied

• Hoek cell sample loaded hydrostatically (axial sa and radial (or confining stress) sr pressure (stress) set to same values) from 7MPa to 70MPa in incremental steps.

• At each hydrostatic stress level the axial stress is increased and decreased by approx 3kN to induce vertical and horizontal strain.

• The stress strain curves are measured from the strain gauges.

Elastic moduli measurements

• Modulus of elasticity (E)

– Calculated as the ratio of change in axial stress (sa) to change in axial strain (ea) E = Dsa/Dea

• Poisson’s ratio (u)

– Calculated as the ratio of the change in radial strain (er) to change in axial strain (ea) u = Der/Dea

Dynamic / static elastic properties • The static moduli are those directly measured in a

deformational experiment

• The dynamic moduli of rock are those calculated from the elastic wave velocity and density (from wireline data).

• The static and dynamic moduli of the same rock may significantly differ from each other.

• The main reason is likely to be the difference in the deformation (strain) amplitude between the dynamic and static experiments. In dynamic strain is around 10-7 while static strain may reach 10-2.

Dynamic mechanical properties

arrival time of the wave t

acoustic velocities Vp and Vs

bulk modulus K and shear modulus G

other elastic moduli (poisson coeff n, Young’s modulus E)

t

LV

material : a transducer arrangement (propagate waves), a ultrasonic pulse generator and an oscilloscope, measurements on saturated and dried samples

mafp VVV

11

measure

calculate

(Wyllie, 1956) relationship to porosity

nn 21312 KGE

GVs

GK

Vp

3

4

Mechanical properties Strength parameters

• Uniaxial Compressive strength (Co)

– Maximum stress the rock can withstand (yield point)

• Cohesion (So)

– Inherent shear strength

• Angle of internal friction ()

– the angle on the Mohr's Circle of the shear stress and normal effective stresses at which shear failure occurs

• Triaxial stress factor (k)

– Related to the angle of internal friction by:

(1+sin ) / (1-sin)

Strength testing

• Basic compressive test involves loading a Hoek cell sample at a constant rate to failure at a constant value of confining pressure

• This results in a single pair of minimum and maximum principle stresses and the determination of stress at failure (UCS) can be calculated:

UCS = load / cross sectional area of sample

Mechanical properties Strength parameters

Shear

stress

Effective

normal stress

• Generally failure occurs as a shear failure, when the shear stress along some plane in the sample is too large

• Mohr / Coulomb assumed failure as a result of the normal stress across a plane and the shear stress along the plane

Strength testing • To generate a failure envelope multi failure tests

must be done

• Axial stress at a constant confining stress is increases until incipient failure is observed on the load versus axial displacement curve and a reduction in slope occurs – then stopped.

• The confining pressure is increased to next target (postponing failure) and the increase in axial stress is continued.

• Termination at the maximum confining pressure and the sample is allowed to fail.

Strength testing • A Mohr coulomb failure criterion is obtained from

a plot of axial stress (load / cross sectional area) versus confining stress.

• A linear function can be applied to the data expressed as s1 = s0 + s3k

– s1 is maximum principle stress, s3 the confining pressure, so the UCS and k the triaxial stress factor.

• Cohesion (So) is calculated from So = s0 / 2√k

• Angle of internal friction (k) is calculated from

K = (1+sin ) / (1-sin)

Caprock Ductility • Ductility is a solid material's ability to deform

under tensile stress. Desire high ductility in caprock so less likely to fracture.

• Measured in a tensile test.

• Lithology dependant: Salt most ductile

Anhydrite

Organic-rich shales

Silty shales

Calcareous mudstones

Cherts least ductile

Rock (matrix)

properties

Mechanical properties

Fluid properties

Rock / fluid interactions

Fluid properties

Fluid properties • Fluid composition

– Formation brine (in equilibrium with host rock)

– CO2 (water + CO2 = weak carbonic acid)

– Hydrocarbon (oil and gas)

Fluid analyses:

- Element concentrations

- major, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopic)

- minor, ICP-MS (Inductively Coupled Plasma-Mass Spectrometry)

- Gas composition

- in-situ raman or infra-red analyses

- gas chromatography

In batch reactor experiment

Measuring fluid composition

Fluid properties • Transport of fluids depends on how each

property responds to changes in pressure and temperature:

–Density

– Viscosity

– Soluability

–Residual saturation

• Supercritical carbon dioxide at or above its critical temperature (31.1 °C) and critical pressure (7.39 MPa),

• Adopts properties midway between a gas and a liquid.

• Expands to fill its container like a gas but with the density of a liquid.

Supercritical CO2

Phase diagram for CO2

Density of CO2 with depth

IPCC/Angus (assume hydrostatic pressure and

25oC/km geothermal gradient

• Cubes represent relative volume occupied by the CO2

• CO2 density increases rapidly up to 800m, where CO2 reaches supercritical state.

• At depths below 1.5km density and specific volume become nearly constant: • Inc temp at depth

causes low density • Inc pressure results

in higher density

Density of CO2 in relation to temperature and pressure

IPCC/bachu

• Under normal conditions the density of water is constant compared to the density of CO2

• Water containing salt or CO2 is heavier than pure water

• At depth CO2 has a density lower than water and migrates upwards

• This effect becomes stronger as it moves upwards as the dec in pressure results in an even lower density.

• However lowering the temp at same pressure leads to higher density

Viscosity of CO2

IPCC/bachu

• At higher temperatures there is a lower viscosity.

• A lower viscosity means lower resistance to flow, better CO2 injection

• scCO2 is much less viscous than water and oil

• Notable contrast in mobility of CO2 and formation fluids

• High mobility of CO2 • Viscous fingering occurs at

front of injected CO2 where part of the CO2 displaces the formation fluids.

• This can cause CO2 to bypass some of the pore space

Solubility of CO2

IPCC/Kohl and Nielsen

• At 100bar and 50oC, 50kg of CO2 can be dissolved in 1 m3 water

• In brines, CO2 solubility decreases when salinity increases

• It can take a period of tens of years up to 100 year before an equilibrium has been reached

Residual saturation

• Water saturation is the ratio of water volume to pore volume, in an aquifer is 100%.

• Generally the rock mineral surfaces are covered with water.

• When CO2 is injected it will be located in the centre of the pores

• Due to the water covering the mineral surfaces which are very difficult to remove, you will never get 100% CO2 saturation.

Rock (matrix)

properties

Mechanical properties

Fluid properties

Rock / fluid interactions

Rock / fluid interactions

Rock fluid interactions

• Wettability – the relative preference of a rock to be covered by a certain

fluid phase. Rock is described as water-wet if the rock has (much) more affinity for water than for oil or CO2.

• Contact angle – The angle, (conventionally measured through the liquid),

where a liquid interface meets a solid surface.

– It quantifies the wettability of a solid surface by a liquid via the Young equation. Wetting refers to how a fluid in contact with a solid spreads out:

– so a small contact angle = strong wetting.

Rock fluid interactions • Interfacial tension

– The interface between two immiscible fluid phases.

– Measured as the Gibbs free energy per unit area of interface at fixed temperature and pressure.

– Interfacial tension occurs because a molecule near an interface has different molecular interactions than an equivalent molecule within the other fluid.

• Capillary pressure – Capillary pressure pc is defined as the

pressure difference between the

non-wetting phase and the wetting phase

as a function of the (wetting phase) saturation

CO2(aq) + H2O = H2CO3 = HCO3– + H+ = CO32– + 2H+

(Ca,Mg,Fe)2+ + HCO3– = (Ca,Mg,Fe)CO3 + H+

(Ca,Mg,Fe)2+ + CO32– = (Ca,Mg,Fe)CO3

Rock fluid interactions

• Dissolving CO2 in water produces weak carbonic acid, which can react with carbonate or silicate minerals to form bicarbonate ions.

• Continued reaction combines bicarbonate ions with calcium, magnesium and iron dissolved from silicate minerals such as feldspars, olivine, pyroxenes or clays to form solid carbonates

Mineral dissolution: permeability enhancement

Mineral precipitation: permeability reduction

Processes influencing the storage system

CO2 storage system

Thermal processes

Heat transport

Hydraulic processes

Fluid transport

Mechanical processes

Stress strain and deformation

Chemical processes Reactivity of the fluids,

gasses and solids

Determine the parameters required for numerical

reservoir scale models

Rock / fluid interactions Experiments

• Thermodynamic experiments(chemical equilibrium)

• Effective kinetic experiments (pure phases)

• Flow through / percolation experiments

Thermodynamic data (chemical equilibrium)

In the reaction:

ba

dc

eqBA

DCK

dDcCbBaA

][][

][][

is the equilibrium constant

Law of mass action

The equilibrium is attained when the reaction Gibbs energy of the system is zero (Q = Keq)

)ln(0

eqKRTG D

The reaction Gibbs energy:

)ln(0 QRTGG DD

At equilibrium DG = 0, and Q is written as Keq to symbolise equilibrium and is referred to as the equilibrium constant

Titration experiment

Effective Kinetics (pure phases)

Calcite example (dissolution by CO2)

2

3

2

3

3

2

323

3

2

3

3

2

1

2

COCaCaCO

HCOCaCOHCaCO

HCOCaHCaCO

k

k

k

23

2232 3321 COCaOHCOH

n

Hr aakakakakk

kr is the kinetic constant of the global reaction (mol.m-2.s-1)

Sr is the reactive surface area (m2)

km is the intrinsec kinetic constant of the mineral (mol.m-2.s-1)

W is the saturation index

')1( mm

rmrr SkSkdt

dnr W

Mineral dissolution rate r (mol.s-1):

Measurement of km and Sr

Percolation through unconsolidated samples

Singurindy and Berkowitz [2003]; Singurindy etal [2004]

Flow-through percolation system

Method:

-Injection of different fluid composition

and different flow rate

-Measurement of permeability changes

-Fluid sampling at the outlet

High p and T flow through 38mm diameter samples

sandstone and fractured caprock

38mm

Luquot and Gouze (2009), Gouze and Luquot (2011)

Sample size : 9 x 18 mm

6.35 x 13 mm

- In situ conditions

- Permeability measurement

- Outlet fluid sampling (at T and P)

- Raman in situ measurement

Percolation on reservoir rock samples

T 200 °C ; P 200 bar

Gouze et al (2003,2004), Noiriel et al (2004, 2007, 2009)

Measurement of

specific surface area

during dissolution

reaction (depending

on mineral

composition)

Percolation on fractured rock samples