Pore geometry and pore-fluid types: Effects on seismic properties of carbonate rocks under a compaction disequilibrium scenario

Gautier Njiekak and Douglas SchmittDept of Physics, University of Alberta, Edmonton, Alberta, T6G 2E1, Email: njiekak@ualberta.ca

6. Sample characterization: results and interpretations

Samples OL and SD have similar pore volumes. This suggests that differences in the behaviors of the ultrasonic ‘dry” velocities between the two samples are mainly due to the pore types and less to the bulk porosity (and the mineralogy).

1. Motivations

2. Goal of this study

4. Some definitions

Compaction disequilibrium

• Growing use of time lapse seismology in monitoring surveys during hydrocarbon production and CO geostorage in carbonate reservoirs over the last years (e.g., 2

White, 2013; Arts et al., 2008).

• Correlations between the traditional pore-type classifications and elastic properties in carbonates still far to be well defined.

• Assess overpressure mechanisms in carbonate rocks.

• Investigate the influence of pore geometry and pore-fluid types on the variation of seismic properties of carbonate samples for the case of a compaction disequilibrium

Theory of elastic properties and wave velocities - Isotropic media -

• Compressional velocity (Vp)

• Shear velocity (Vs)

scenario, i.e., with pore-fluid pressure and confining pressure changing incrementally at the same rate.

Fig. 1

Research Theme: Secure Carbon Storage

CMC Project Nr.: C05

Pressure-depth profile and wireline-log responses anticipated where overpressure is caused by compaction (e.g., Ramdhan and Goulty, 2011)

disequilibrium

r

m3/4+=

KVp

r

m=

sV

K= Bulk modulusµ = Shear Modulus = Density

5. Experimental conditions for the ultrasonic measurements

(see Njiekak et al., 2013 for the experimental setup)

• Samples plug sizes: 3.81 cm in diameter and 4.77 to 5.63 cm in length

• Uncertainty in the measured velocities = 0.1%

• Sequence of the ultrasonic measurements (frequency = 1 MHz):Measurements on the dry samples under vacuum --> 'Nitrogen-saturated' measurements --> Vacuum --> 'Dry' measurements --> 'Water-saturated' measurements

• A differential pressure (confining pressure minus pore-fluid pressure) of 25 MPa was used during the fluid-saturated measurements

• Figure 2 shows the change in the properties of the fluids (nitrogen and water) used in study

2200

2240

2280

2320

2360

0

4

8

12

16

20

0 10 20

K (

MP

a)

Confining Pressure (MPa)

Bulk Modulus, K

975

1000

1025

0

50

100

150

0 10 20

N2

H2O

Confining Pressure (MPa)

Density,

3(K

g/m

)

Fig. 2

- Under N2 saturation

· Regardless of the pore type and the pore volume, effects of the increased bulk modulus and increased density cancel each other, hence the non variation of Vp

· Vs slightly decreases in sample OL (up to 0.3% decrease) because of the increased density

- Under water saturation

· No change of Vp and Vs in samples VL and OL. Vp and Vs values are similar within the experimental error (= 0.1%).

· Small change of Vp and Vs (up to 0.6% increase) on sample SD likely due to:- the presence ments in its fabric and/or- local flow mechanisms

of more compliant ele

- Oolithic Limestone

- Intergranular (sub-rounded) pore Spaces

- Grain density = 2.65 g/cm3

- Porosity (He) = 17 %

- Air permeability < 2 mD

- Main pore throat size = 35 µm

Sample OL

- Sucrosic Dolomite

- Intercrystalline porosity

3- Grain density = 2.78 g/cm

- Porosity (He) = 15 %

- Air permeability < 2 mD

- Main pore throat size = 0.7 µm

Sample SD

Cal

Dol

20 µm

20 µm

5 mm

- Vuggy Limestone

- Intraparticle (see thin section), vugs (see SEM picture on the bottom right) and intercrystalline porosity

- Grain density = 2.71 g/cm3

- Porosity (He) = 11 %

- Air permeability ~ 16 to 35 mD

- Main pore throat size = 0.55 µm

Sample VL

4000

4100

4200

4300

4400

4500

4600

4700

4800

4900

5000

0 5 10 15 20 25 30 35 40 45 50 55

P-Wave, dry samples

Sample OL

Sample SD

Sample VL

Velo

city

(m

/s)

2350

2400

2450

2500

2550

2600

2650

2700

2750

2800

2850

0 5 10 15 20 25 30 35 40 45 50 55

Confining Pressure (MPa)

S-Wave, dry samples

Sample OL

Sample SD

Sample VL

Velo

city

(m

/s)

7. ‘Fluid-saturated’ velocities: results and interpretations

4850

4900

4950

5000

5050

0 5 10 15 20 25 30 35 40 45 50 55

Vuggy Limestone (sample VL)

Dry

N2-saturated

Water-saturated

Vp (

m/s

)

2650

2700

2750

2800

2850

0 5 10 15 20 25 30 35 40 45 50 55

Confining Pressure (MPa)

Vuggy Limestone (sample VL)

Dry

N2-saturated

Water-saturated

Vs

(m/s

)

4525

4575

4625

4675

4725

4775

4825

4875

0 5 10 15 20 25 30 35 40 45 50 55

Oolitic Limestone (Sample OL)

Dry

N2-saturated

Water-saturated

2500

2550

2600

2650

2700

0 5 10 15 20 25 30 35 40 45 50 55

Confining Pressure (MPa)

Oolitic Limestone (Sample OL)

Dry

N2-saturated

Water-saturated

4050

4100

4150

4200

4250

4300

4350

4400

4450

4500

4550

0 5 10 15 20 25 30 35 40 45 50 55

Sucrosic Dolostone (Sample SD)

Dry

N2-saturated

Water-saturated

2350

2375

2400

2425

2450

2475

2500

2525

2550

0 5 10 15 20 25 30 35 40 45 50 55

Confining Pressure (MPa)

Sucrosic Dolostone (Sample SD)

Dry

N2-saturated

Water-saturated

Pressure SonicPorosity

Depth

Lithostactic stress

Hyd

rosta

ctic stress

Constanteffective stress

Constanttransit time

Constantporosity

σ ’v