CO2 Sequestration [Geomechanics]

1. How do we ensure that the pressure changes induced by CO2 injection operations (as a function of CO2 injection-rate/pressure) would subsequently change the local spatial and temporal distribution of aquifer/reservoir stresses, which would possibly lead to the formation and/or reactivation of fractures/faults to slip (because of shear) within the aquifer/reservoir; and eventually, which could possibly act as a least resistive pathway for CO2 leakage (or providing a probability of CO2 injection induced seismicity)?

Whether the details on the existing state of stress at a given aquifer/reservoir; and the expected alteration of the reservoir stress conditions remain to be mandatory requirements before CO2 injection – towards informing any decision-making from site characterization to the determination of operational parameters?

2. How important are the determination of the following factors before, during and after CO2 injection?

(a) Critically oriented fractures [fractures that remain oriented to maximize the possibility of slip]

(b) Critically stressed fractures [fractures that would require only a minor increase in fluid pressure to slip]

3. In the absence of knowledge on 2a and 2b, can we still go ahead the following activities?

(a) Placing and orientating CO2 injection wells;

(b) Determination of CO2 injection rate and Maximum allowable injection pressure.

4. In the absence of quantifying uncertainties, can we still go ahead with estimating the magnitudes and directions of the principal stresses associated with the aquifer? Unquantified Uncertainties remain valid in CO2 Sequestration (CO2 Leakage) applications?

5. Due to lack of data (associated with CO2 storage formation, or, with aquifer’s regional stress regime, or with elastic properties of the CO2 injection zone), if the estimated stress tensor components of the aquifer/reservoir remain to be incorrect, then, would it most probably lead to fault reactivation and threatening seal integrity (which may eventually jeopardize the viability of the CO2 storage project)?

6. What is the significant drawback associated with the following widely used stress estimation methods (although, it appears that both the methods involves the estimation of state of stress at any given depth as a function of the weight of the over-burden, the poro-elastic properties and the horizontal strain)?

(a) Stress polygon approach (assumes that for a given pore pressure and coefficient of friction, the differential stress magnitudes cannot exceed the stress required to cause shear failure on pre-existing, optimally oriented faults)

(b) Tectonic-elastic approach (assumes that one of the stresses remains to be vertical, and this vertical stress at a given depth of interest is equal to the weight of the overlying material, while the two horizontal stresses are to be estimated by solving the linear poro-elasticity)

Suresh Kumar Govindarajan

08-Dec-2024