**3. Geophysical response analysis and interpretations**

#### **3.1. Shear-wave speed determination**

As mentioned earlier, each test line has 24 measurements points representing a total of 24 channels in data processing. To process the shear-wave velocity data, SeisOpt ReMi software was used [26]. The procedure of stress wave signal processing involves first a wave field data transformation (ReMi Vspect module was used), which converts the time domain data acquired in the field to frequency domain. An interactive Rayleigh-wave dispersion modeling was then conducted with the outcomes is 1-D shear-wave velocity models. At the end, the dispersion curves were generated [28].

**Figure 7** shows the typical averaged shear-wave velocity profiles as a function of depth (measured from Line 1 and Line 2). The shear-wave velocity curve was obtained based on the averaging of the test data sets during each test stage and is shown to have a total of 14 strata. The 14th strata correspond to the measurements of shear-wave velocity to depths at around 12,500 ft. (3810 m), which is about the oil-bearing Donovan sand. As described earlier, most of the injection pressures were retained within the oil layer at around 12,500 ft. (3810 m). Hence, the test results confirmed about the pressurization of the Donovan sand and that the anhydrite layer has maintained its leak prevention integrity.

In order to compare the changes of the shear-wave velocity obtained from the geophysical tests, the data were divided into three groups: before CO<sup>2</sup> injection, during CO<sup>2</sup> injection, and after CO<sup>2</sup> injection. **Figures 8**–**10** show the shear-wave velocity curves from both Line 1 and Line 2 tests for the three stages: **Figure 8** shows the results of average shear-wave velocity versus depth curve for test 1, test 2, and test 3 (before CO<sup>2</sup> injection) for Line 1 and Line 2, respectively. Error bars are used to indicate the deviation of shear-wave velocity in the measurements of each group.

Both Line 1 and Line 2 show that different strata experienced different stress histories: for Line 1, the top seven layers (approximately at 6000 ft. (1829 m) depth) are shown to experience initial increase in wave speed (during CO<sup>2</sup> injection) and then decreasing wave speed (after CO2 injection); the trend reversed after 6000 ft. (1829 m) depth showing increasing wave speeds for both during and after CO<sup>2</sup> injections; and finally, the oil-bearing layer [around 12,000 ft. (3658 m) depth] showed slowly decreasing wave speeds. Line 2, on the other hand, showed an increase decrease trend up to 3000 ft. (914 m) depth; followed by an increasing pattern above the oil-bearing layer; and decreasing wave speeds at the oil-bearing layer. The explanation of the response history is that the oil-bearing layer experienced strata expansion due to the injection pressure and the inability of oil to escape quick enough from the oil sand; hence, the pressure is transferred to the strata above the oil-bearing layer (mostly salient saturated material), which experienced stressing (increasing wave speed). This trend reversed for the upper layer above the salient layers, which is dependent upon the balancing act of the weight of the overburden and the upward lifting of the injection pressure.

**Table 2** shows the *Cv*

For the before CO<sup>2</sup>

and after CO<sup>2</sup>

measurements of each group.

gradually stabilized during the process.

**3.1. Shear-wave speed determination**

dispersion curves were generated [28].

survey lines: for the after CO<sup>2</sup>

136 Carbon Capture, Utilization and Sequestration

cases, *Cv*

values for each layer and each survey line. The table shows that in all

val-

value is shown to be 0.01

value is 0.07 for Line

values are less than 0.2 indicating that the strata responses are slow, and consistent

and that there are no drastic events occurring during the whole injection process. The *Cv*

ues also are consistently dropping during the three stages indicating that the pressurization is

Careful evaluation of **Table 2** indicates that there is a difference between the results from both

for Line 1 and the value is 0.001 for Line 2 (Layer 14). This is an order of magnitude different.

1 and is 0.03 for Line 2. This observation may be detrimental considering the experimental

As mentioned earlier, each test line has 24 measurements points representing a total of 24 channels in data processing. To process the shear-wave velocity data, SeisOpt ReMi software was used [26]. The procedure of stress wave signal processing involves first a wave field data transformation (ReMi Vspect module was used), which converts the time domain data acquired in the field to frequency domain. An interactive Rayleigh-wave dispersion modeling was then conducted with the outcomes is 1-D shear-wave velocity models. At the end, the

**Figure 7** shows the typical averaged shear-wave velocity profiles as a function of depth (measured from Line 1 and Line 2). The shear-wave velocity curve was obtained based on the averaging of the test data sets during each test stage and is shown to have a total of 14 strata. The 14th strata correspond to the measurements of shear-wave velocity to depths at around 12,500 ft. (3810 m), which is about the oil-bearing Donovan sand. As described earlier, most of the injection pressures were retained within the oil layer at around 12,500 ft. (3810 m). Hence, the test results confirmed about the pressurization of the Donovan sand and that the

In order to compare the changes of the shear-wave velocity obtained from the geophysical

and Line 2 tests for the three stages: **Figure 8** shows the results of average shear-wave veloc-

2, respectively. Error bars are used to indicate the deviation of shear-wave velocity in the

Both Line 1 and Line 2 show that different strata experienced different stress histories: for Line 1, the top seven layers (approximately at 6000 ft. (1829 m) depth) are shown to experience

injection. **Figures 8**–**10** show the shear-wave velocity curves from both Line 1

injection, during CO<sup>2</sup>

injection) for Line 1 and Line

injection,

injection (initial water pumping) stage, where the *Cv*

resolution of the geophysical testing method, which is discussed below.

**3. Geophysical response analysis and interpretations**

anhydrite layer has maintained its leak prevention integrity.

tests, the data were divided into three groups: before CO<sup>2</sup>

ity versus depth curve for test 1, test 2, and test 3 (before CO<sup>2</sup>

injection stage, it is seen where the *Cv*

**Figure 9** shows the results of average shear-wave velocity versus depth curve for test 4, test 5, test 6, and test 7 (during CO<sup>2</sup> injection) for Line 1 and Line 2, respectively. Wave speeds results of the last four layers shown in **Figure 9** are higher than the corresponding results shown in **Figure 8**. The increase in shear-wave velocity is associated with CO<sup>2</sup> injection, which caused an increase in the effective stresses in layers above the injection zone (pressurization). **Figure 10** shows the results of average shear-wave velocity versus depth curve for test 8, test 9, and test 10 (after CO<sup>2</sup> injection) for Line 1 and Line 2, respectively. The deviations on the graphs shown in **Figure 10** are significantly smaller when compared to **Figures 8** and **9** indicating that the strata pressurization has stabilized.

**Figure 7.** Average shear-wave velocity profiles versus depth from sensor survey Line 1 (left) and Line 2 (right), September 8–9, 2010.

**Figure 8.** Average shear-wave velocity profile versus depth before CO<sup>2</sup> injection, average of test 1, test 2, and test 3 (Line 1 and Line 2).

On the other hand, Line 2 has wave velocity reaching 12,667 ft./s (3,861 m/s) during water

It is noticed that the strata pressure above the oil-bearing layer is slow in building up as it takes time for the pressure to dissipate into the upper strata. To study this effect, the wave speed responses above the oil-bearing layers are studied: it is shown for Line 1, the wave speed above the oil-bearing layer has increased from 8144.6 ft./s (2,482.5 m/s) initially, to 9512.9 ft./s

tion. This indicates a slow building up of pressure. For Line 2, the wave speed immediately above the oil-bearing layer has increased from 8207.6 ft./s (2,501.7 m/s) before injection to

injection. The interpretation of this observation is that the oil pressure is pushing against the strata above the Donovan sand and has resulted in the strata pressurization. It is concluded that the pressure build-ups are almost identical in both directions indicating uniform build-

Geophysical testing has been applied to projects similar to the Citronelle field study for the purposes of determining production induced stress changes in the oil-bearing strata and site anisotropy changes. In most high-resolution seismic detections, the tests are performed with controlled excitations such as the use of explosions, seismobile vibrations, or gun shots. The results have sensitivities that can indicate possible migration of injected fluids. However, the interpretation of strata stress changes based on wave speed changes is inherently challenging, as a result of the constrained temporal and spatial resolutions. As a result, the

injection. Again, there is a possibility of mobilization of oil/CO<sup>2</sup>

injection, and finally, increased to 9963.7 ft./s (3,036.9 m/s) post-injec-

 **injection studies**

injection, and finally, to 9935.8 ft./s (3,028.4 m/s) post-

injection, which has dropped to 11,236 ft./s

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139

injection, average of test 8, test 9, and test 10

flow—a

injection, 11,570 ft./s (3,527 m/s) during CO<sup>2</sup>

likelihood of enhanced oil production in the months to come.

**Figure 10.** Average shear-wave velocity profile versus depth during CO<sup>2</sup>

up of pressures at all directions at the Citronelle oil field.

**3.2. Discussion on geophysical sensing for CO2**

(3,425 m/s) post CO<sup>2</sup>

(Line 1 and Line 2).

(2,899.5 m/s) during CO<sup>2</sup>

9664.0 ft./s (2,945.6 m/s) during CO<sup>2</sup>

**Figure 9.** Average shear-wave velocity profile versus depth before CO<sup>2</sup> injection, average of test 4, 5, 6, and 7 (Line 1 and Line 2).

It is important to point out that the test line selection is constrained by available monitoring sites and is selected in order to help determine possible directional effects of the CO<sup>2</sup> migration at the oil field. Hence, the first test line would determine the likely CO<sup>2</sup> migration in the north-east direction, and the second test line would determine the flow in the northeast and southwest directions. When comparing the average velocity at the oil-bearing layer, the results from Line 1 indicates that the wave speed has reached 12,392 ft./s (3,777 m/s) during water injection, 11,365 ft./s (3,464 m/s) during CO<sup>2</sup> injection, and has dropped slightly to 11,109 ft./s (3,386 m/s) after the CO<sup>2</sup> injection. This indicates that there is a possibility that the supercritical CO<sup>2</sup> may be migrating slowly in the north-east direction.

**Figure 10.** Average shear-wave velocity profile versus depth during CO<sup>2</sup> injection, average of test 8, test 9, and test 10 (Line 1 and Line 2).

On the other hand, Line 2 has wave velocity reaching 12,667 ft./s (3,861 m/s) during water injection, 11,570 ft./s (3,527 m/s) during CO<sup>2</sup> injection, which has dropped to 11,236 ft./s (3,425 m/s) post CO<sup>2</sup> injection. Again, there is a possibility of mobilization of oil/CO<sup>2</sup> flow—a likelihood of enhanced oil production in the months to come.

It is noticed that the strata pressure above the oil-bearing layer is slow in building up as it takes time for the pressure to dissipate into the upper strata. To study this effect, the wave speed responses above the oil-bearing layers are studied: it is shown for Line 1, the wave speed above the oil-bearing layer has increased from 8144.6 ft./s (2,482.5 m/s) initially, to 9512.9 ft./s (2,899.5 m/s) during CO<sup>2</sup> injection, and finally, increased to 9963.7 ft./s (3,036.9 m/s) post-injection. This indicates a slow building up of pressure. For Line 2, the wave speed immediately above the oil-bearing layer has increased from 8207.6 ft./s (2,501.7 m/s) before injection to 9664.0 ft./s (2,945.6 m/s) during CO<sup>2</sup> injection, and finally, to 9935.8 ft./s (3,028.4 m/s) postinjection. The interpretation of this observation is that the oil pressure is pushing against the strata above the Donovan sand and has resulted in the strata pressurization. It is concluded that the pressure build-ups are almost identical in both directions indicating uniform buildup of pressures at all directions at the Citronelle oil field.

#### **3.2. Discussion on geophysical sensing for CO2 injection studies**

It is important to point out that the test line selection is constrained by available monitoring

the north-east direction, and the second test line would determine the flow in the northeast and southwest directions. When comparing the average velocity at the oil-bearing layer, the results from Line 1 indicates that the wave speed has reached 12,392 ft./s (3,777 m/s) dur-

migra-

migration in

injection, and has dropped slightly to

injection, average of test 4, 5, 6, and 7 (Line 1 and

injection, average of test 1, test 2, and test 3 (Line 1

injection. This indicates that there is a possibility that the

sites and is selected in order to help determine possible directional effects of the CO<sup>2</sup>

may be migrating slowly in the north-east direction.

tion at the oil field. Hence, the first test line would determine the likely CO<sup>2</sup>

ing water injection, 11,365 ft./s (3,464 m/s) during CO<sup>2</sup>

**Figure 9.** Average shear-wave velocity profile versus depth before CO<sup>2</sup>

**Figure 8.** Average shear-wave velocity profile versus depth before CO<sup>2</sup>

11,109 ft./s (3,386 m/s) after the CO<sup>2</sup>

supercritical CO<sup>2</sup>

Line 2).

and Line 2).

138 Carbon Capture, Utilization and Sequestration

Geophysical testing has been applied to projects similar to the Citronelle field study for the purposes of determining production induced stress changes in the oil-bearing strata and site anisotropy changes. In most high-resolution seismic detections, the tests are performed with controlled excitations such as the use of explosions, seismobile vibrations, or gun shots. The results have sensitivities that can indicate possible migration of injected fluids. However, the interpretation of strata stress changes based on wave speed changes is inherently challenging, as a result of the constrained temporal and spatial resolutions. As a result, the 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 microtremors [30].

To determine the stress wave speed changes, the velocity change functions are computed for before, during and after CO<sup>2</sup> injection:

$$(\frac{\Delta \nu}{\nu})\_i - \frac{(\nu\_{\text{Dættag}})\_i - (\nu\_{\text{Refres}})\_i}{(\nu\_{\text{Refres}})\_i} \tag{7}$$

salient layer) are negative, most likely indicating a reduction in effective stress. Following Eq. (1), this may be interpreted as an increase in pore water pressure in the salient formation. The last figure in **Figure 11** shows a comparison between Line 1 and Line 2 using Eq. (9). This figure shows that the two trends are in consistent in general and that both show the same trend of velocity increase right above the oil production strata (which shows negative velocity change functions). This further enhances the interpretation that the stress within the injection

plum.

A reduction of the strata pressure (shear-wave velocity) could mean a likely leak occurs within the system, which has not been identified at the Citronelle field. The shear-wave velocities at the Donovan oil-bearing layer are normalized by their average value and are plotted against the normalized well head pressure in **Figure 6**. Assuming the shear-wave velocity is a good representation of the stress level within the oil-bearing stratum, the wellfitting of the two sets of data represents that the geophysical testing method has accurately

Carbon sequestration through injection into a depleted oil field is an effective method to

tial 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

process at the Citronelle oil field, Alabama. The ability of the passive DoReMi technique to

through analysis of the wave speed profiles indicating that there are strata stress build-ups

oil-bearing layers. Clear demarcation of the shear-wave velocity profile is shown for before,

age—continued monitoring may provide information on possible reservoir breakthroughs

The COV value associated with the shear-wave velocity changes is suggested as a measure of

cess, indicating that the stress state in the oil-bearing layer has reached a stable state. Thus, the

tion 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

livestock. Continued geophysical monitoring of the strata stress changes can help mitigate

injection operation.

within the overburden of the reservoir is important for monitoring the long-term CO<sup>2</sup>

leakage.

the conditions at the oil field and is observed to drop in value during the CO<sup>2</sup>

COV values can be used as an indication of oil field stability during the CO<sup>2</sup>

sequestration process in the heterogeneous oil reservoir is demonstrated

injection in the field. The detection of geomechanical deformation

, which resulted in the pressurization of the Rodessa

Geophysical Monitoring of CO2 Injection at Citronelle Field, Alabama

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141

leak sites) are in close vicinity to humans and

injection process is essen-

injection

stor-

injection pro-

injection opera-

. However, proper monitoring of the CO<sup>2</sup>

reports the use of a passive microseismic sensing technique to monitor the CO<sup>2</sup>

may be reduced due to migration of CO<sup>2</sup>

quantified the stresses within the reservoir.

**4. Conclusions**

monitor the CO<sup>2</sup>

reduce atmospheric CO<sup>2</sup>

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

and possible pathways for CO<sup>2</sup>

during and after the injection of CO<sup>2</sup>

Citronelle, where the oil wells (potential CO<sup>2</sup>

potential geohazards due to the CO<sup>2</sup>

$$(\frac{\Delta \nu}{\nu})\_i = \frac{(\nu\_{\mathcal{A}\_{\text{filter}}})\_i - (\nu\_{\mathcal{M}\_{\text{Mitter}}})\_i}{(\nu\_{\mathcal{A}\_{\text{Before}}})\_i} \tag{8}$$

$$(\frac{\Delta \mathbf{v}}{\nu})\_i = \frac{(\mathbf{v}\_{\text{cyt}r})\_i - (\mathbf{v}\_{\text{cytog}})\_i}{(\mathbf{v}\_{\text{drag}})\_i} \tag{9}$$

**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 reduced in the injection layer (Layer 14) after CO<sup>2</sup> injection indicating that the CO<sup>2</sup> gas may 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 and (c) injection histories for Line 1 and Line 2.

salient layer) are negative, most likely indicating a reduction in effective stress. Following Eq. (1), this may be interpreted as an increase in pore water pressure in the salient formation.

The last figure in **Figure 11** shows a comparison between Line 1 and Line 2 using Eq. (9). This figure shows that the two trends are in consistent in general and that both show the same trend of velocity increase right above the oil production strata (which shows negative velocity change functions). This further enhances the interpretation that the stress within the injection may be reduced due to migration of CO<sup>2</sup> plum.

A reduction of the strata pressure (shear-wave velocity) could mean a likely leak occurs within the system, which has not been identified at the Citronelle field. The shear-wave velocities at the Donovan oil-bearing layer are normalized by their average value and are plotted against the normalized well head pressure in **Figure 6**. Assuming the shear-wave velocity is a good representation of the stress level within the oil-bearing stratum, the wellfitting of the two sets of data represents that the geophysical testing method has accurately quantified the stresses within the reservoir.
