**4. Conclusions**

(7)

(8)

(9)

gas may

velocity change ratio function (∆*v*/*v*) has been suggested as a means to establish the detection of geomechanical condition changes due to oil production or fluid injection [29] and has been successfully implemented in a study to synchronized field measurements to localized

To determine the stress wave speed changes, the velocity change functions are computed for

**Figure 11** shows (∆*v*/*v*) for different stages of the injection process at Citronelle field indicating different strata stress plays: for both Line 1 and Line 2, it is shown that the stress waves have

have migrated at this stage. The velocity change functions for Layers 8–10 (corresponding to

**Figure 11.** Velocity change functions vs. strata layers for (a) injection histories for Line 1; (b) injection histories for Line 2

injection indicating that the CO<sup>2</sup>

injection:

reduced in the injection layer (Layer 14) after CO<sup>2</sup>

and (c) injection histories for Line 1 and Line 2.

microtremors [30].

before, during and after CO<sup>2</sup>

140 Carbon Capture, Utilization and Sequestration

Carbon sequestration through injection into a depleted oil field is an effective method to reduce atmospheric CO<sup>2</sup> . However, proper monitoring of the CO<sup>2</sup> injection process is essential in order to ensure the geomechanical stability of the storage reservoir and to minimize risks of potential geohazard to the terrestrial and sub-terrestrial environments. This chapter reports the use of a passive microseismic sensing technique to monitor the CO<sup>2</sup> injection process at the Citronelle oil field, Alabama. The ability of the passive DoReMi technique to monitor the CO<sup>2</sup> sequestration process in the heterogeneous oil reservoir is demonstrated through analysis of the wave speed profiles indicating that there are strata stress build-ups during and after the injection of CO<sup>2</sup> , which resulted in the pressurization of the Rodessa oil-bearing layers. Clear demarcation of the shear-wave velocity profile is shown for before, during, and after the CO<sup>2</sup> injection in the field. The detection of geomechanical deformation within the overburden of the reservoir is important for monitoring the long-term CO<sup>2</sup> storage—continued monitoring may provide information on possible reservoir breakthroughs and possible pathways for CO<sup>2</sup> leakage.

The COV value associated with the shear-wave velocity changes is suggested as a measure of the conditions at the oil field and is observed to drop in value during the CO<sup>2</sup> injection process, indicating that the stress state in the oil-bearing layer has reached a stable state. Thus, the COV values can be used as an indication of oil field stability during the CO<sup>2</sup> injection operation and have the potential for long-term monitoring of the strata stress change throughout the field operations. Further studies are needed to develop the COV value into risk index that can be used to indicate geohazard. The strata stressing is especially important to the City of Citronelle, where the oil wells (potential CO<sup>2</sup> leak sites) are in close vicinity to humans and livestock. Continued geophysical monitoring of the strata stress changes can help mitigate potential geohazards due to the CO<sup>2</sup> injection operation.
