Lecture 4 - GRACE & Sleipner CCS case study

Question 1

How does GRACE work to measure gravity?

Answer 1

Measures changes in gravity by measuring continual orbits. Covers Earth once a month.
Accelerates towards areas of high density on the Earth Surface
Very sensitive to density changes at the Earth’s surface

Question 2

How do you calculate mass change?

Answer 2

Using changes in gravity

Mass excess, Me = (1/2piG)*volume under surface

Question 3

What can you use mass change for?

Answer 3

Plot mass change

Highlights area of mass loss (e.g greenland ice sheet) and mass gain (e.g. Canada isostatic uplift)

Question 4

What is leakage in terms of mass change?

Answer 4

Long wavelength of gravity data leads to the underestimation of mass change.
Must account for

Question 5

How do you calculate ice sheet mass balance?

Answer 5

Mass balance = SMB - D

SMB = Surface Mass balance = snow + rain - sublimation - runoff

Runoff = Condensation + rain +melt - refreezing - retention

D = Ice discharge at grounding line

Question 6

How is the SMB-D estimate for ice mass change done?

Answer 6

Estimate parameters (from ice cores, on site monitoring, aerial photos and satellite data).
Input parameters into regional climate model
Parameters difficult to estimate

Question 7

What is the average mass loss estimation of GIS and what is the corresponding SLR?

Answer 7

~270+- 20 Gt/yr

0.75mm/yr SLR

Question 8

How can GRACE be used for water management?

Answer 8

1. Estimating water storage
2. Assessing, modelling and predicting water fluxes
3. Remote sensing of water quality

Question 9

How is water lost by irrigation?

Answer 9

Evaporation & transpiration

Question 10

Why is the Sleipner CCS an important case study?

Answer 10

It is the test case for any future CO2 injection projects

Question 11

What is done at the Sleipner site to monitor CO2?

Answer 11

1. 3D seismic reflection surveys (pre-injection to present)
2. Sea-bottom gravity survey (x3)
3. CSEM survey

Question 12

Why were the Utira sands chosen for CO2 injection?

Answer 12

• High porosity (~37%)
• Depth of 800-1000m so CO2 supercritical fluid and
  occupis a smaller volume
• Sands 150-300m thick
• Sands filled with brine
• Thick, local antiform section of the sands chosen

Question 13

What is the effect of CO2 injection on petrophysical properties?

Answer 13

1. Lowers density
2. Lowers Vp
3. Lowers bulk modulus, K
4. Increases resistivity
5. Large, negative impedance contrast

Question 14

What are the aims of monitoring at the Sleipner injection site?

Answer 14

1. Are there any leaks?
2. Where is the CO2 currently stored
3. What is the CO2 density
4. How fast is the CO2 migrating from the injection site?
5. Can we use data to make predictions?

Question 15

What information does seismic reflection data give on the CO2 injection site?

Answer 15

Locate CO2 using impedance & velocity

Insensitive to density and saturation

Question 16

What information does gravity survey data give on the CO2 injection site?

Answer 16

Density of CO2

Question 17

What information does CSEM survey data give on the CO2 injection site?

Answer 17

Determines resistivity which is sensitive to saturation

Question 18

What causes the velocity push down effect on seismic reflection data?

Answer 18

Decreased Vp due to CO2 injection gives effect below injection site

Question 19

What are the following in the Gassmann’s equations?:

1. K(fluid)
2. S(co2)
3. a
4. ρmineral
5. Φ
6. K(Φ)

Answer 19

1. Bulk modulus of rock fluid
2. Saturation of CO2 fluid
3. crack aspect ratio
4. Density of rock matrix
5. Porosity
6. K(Φ) = aKmineral - sensitivity to pore fluids