**2. Geophysical monitoring strategies**

The intent of the carbon injection is to explore the feasibility of EOR in the tertiary production of an existing oil reservoir, as well as the potential of subsequent CO<sup>2</sup> sequestration. The geophysical monitoring strategy is used to assess the site geostability and potential geohazards. Site geostability for mineral extractions can be associated either with the site geological conditions pertaining to sustained production or with the large geological deformations such as land subsidence or landslides. In the context of the current study, the association is with the latter definition. The key purpose of a geostability analysis is to determine the possibilities of significant geohazards due to formation instabilities, which may result from the CO<sup>2</sup> -oil replacement in the Rodessa formation.

the externally induced pressures. Hence, the total stress within the geomaterial system is equal to the summation of stress within geo-matrix and pore water pressure. The geostability study considers the effective stress, σ', which is defined as the stress carried by geomaterial skeletons and not pore water that causes elastic deformation of the oil-producing layer:

*σ*' = *σ* − *μ* (1)

where σ is the vertical total stress derived from unit weight of material and μ is the pore pressure. Neglecting thermal effects, the effective stress equation is further modified to include

*σ*″ = *σ*′ − (*σinjection* − *μ*) (2)

The effective stress pressure is then used to compute the producing layer elastic deformation (non-permanent settlement) using simplified computation of rock bulk modulus (P-wave

Micro-seismicity tests have been successfully applied to address specific issues in the oil and gas industry [23, 24]. The basic principle of passive seismic monitoring is to detect small movements (regarded as microseismic events) from unknown seismic sources that can be recorded on geophones placed on site. Contrast to active geophysical testing, the passive seismic monitoring is a testing method that does not rely on a source of ground excitation. The main advantage of the passive monitoring is that it can be carried out at any time and does not require regulated field access. The disadvantage of passive sensing is the uncertainty introduced due to the lack of controlled input energy, which can result in both poor data

A modified passive sensing Refractive Microtremor (ReMi) technique, Derivative of ReMi (DoReMi), as discussed below, is used at the Citronelle oil field, Alabama [25]. To improve mobility and avoid the cumbersome wiring, wireless triaxial micro-electro-mechanical system (MEMS) accelerometers have been used for the field testing. The MEMS sensors are encased in hard metal boxes and buried into the ground at sufficient depth to ensure good coupling between the sensor and the surrounding soil (at least 1 ft. (0.3 m) deep with fully compacted soil on top). The wireless sensor unit with the three directional acquisition channels can record seismic energy in three Cartesian directions (vertical and two horizontal directions). The vibration signals obtained by the wireless accelerometer are acceleration time histories, which are processed in spectral domain using p-τ transformation, or slant-stack analysis [26].

*<sup>2</sup>* (3)

is the P-wave velocity derived from geophysical testing con-

Geophysical Monitoring of CO2 Injection at Citronelle Field, Alabama

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131

injection pressure *σinjection*

modulus), M [22]:

ducted at Citronelle.

where ρ is the rock density, *Vp*

:

*M* = *ρ Vp*

**2.1. Geophysical testing at Citronelle field**

sensitivity and poor detection accuracy.

Geo-instability of an oil-producing stratum can result from the collapse of voids during the oil extraction process. The repercussions may include global subsidence, localized straining and possible microtremors to earthquakes. **Figure 3** shows a schematic of the geomaterial straining at an oil extraction well. The hypothesis is that as oil is being depleted, the surrounding geomedium may experience straining due to interfacial shear stresses resulting from the settlement and collapse of the strata. Geostability in a narrow oil field within a deep stratum, such as the Citronelle oil field, is typically not a major concern since relatively small settlement is anticipated. The geo-instability concerns in such cases can be generalized as compressibility potential assessment as well as localized stability projection.

The compression or settlement issue may involve both local elastic settlement (non-permanent deformation) and long-term creep (long-term deformation due to sustained loading). Elastic settlement is instantaneous and is a function of the weight of overburden above the layer of interest. For an oil-producing layer, elastic settlement is also a function of the system pressurization, where pressure is kept to ensure the injection fluids remain in the oil layer. Creep is difficult to assess since it is a function of time. Current geostability analysis does not include considerations of thermoelastic effects or micro-poroelastic effects for the pressurized system, for example, the Biot's equations. The rationale is that the difficulties in establishing geomaterial properties based on current geophysical measurements and assumed values only allow a grossly simplified bulk material analysis. As a result, the extensive works by Biot on poroelasticity has been greatly simplified.

Stresses in geomaterials are derived essentially from the self-weight of the overburden materials (predominantly the geo-matrix), the liquid within the voids (pore water pressure), and

**Figure 3.** Poroelastic stresses due to extensive oil extraction (hollow arrows indicate surface strains and line arrows indicate interplanar shear stresses).

the externally induced pressures. Hence, the total stress within the geomaterial system is equal to the summation of stress within geo-matrix and pore water pressure. The geostability study considers the effective stress, σ', which is defined as the stress carried by geomaterial skeletons and not pore water that causes elastic deformation of the oil-producing layer:

$$
\sigma' = \sigma - \mu \tag{1}
$$

where σ is the vertical total stress derived from unit weight of material and μ is the pore pressure. Neglecting thermal effects, the effective stress equation is further modified to include injection pressure *σinjection* :

$$
\sigma'' = \sigma' - (\sigma\_{\text{iqection}} - \mu) \tag{2}
$$

The effective stress pressure is then used to compute the producing layer elastic deformation (non-permanent settlement) using simplified computation of rock bulk modulus (P-wave modulus), M [22]:

$$M = \rho \, V\_p^2 \tag{3}$$

where ρ is the rock density, *Vp* is the P-wave velocity derived from geophysical testing conducted at Citronelle.

#### **2.1. Geophysical testing at Citronelle field**

as land subsidence or landslides. In the context of the current study, the association is with the latter definition. The key purpose of a geostability analysis is to determine the possibilities of significant geohazards due to formation instabilities, which may result from the CO<sup>2</sup>

Geo-instability of an oil-producing stratum can result from the collapse of voids during the oil extraction process. The repercussions may include global subsidence, localized straining and possible microtremors to earthquakes. **Figure 3** shows a schematic of the geomaterial straining at an oil extraction well. The hypothesis is that as oil is being depleted, the surrounding geomedium may experience straining due to interfacial shear stresses resulting from the settlement and collapse of the strata. Geostability in a narrow oil field within a deep stratum, such as the Citronelle oil field, is typically not a major concern since relatively small settlement is anticipated. The geo-instability concerns in such cases can be generalized as compressibility

The compression or settlement issue may involve both local elastic settlement (non-permanent deformation) and long-term creep (long-term deformation due to sustained loading). Elastic settlement is instantaneous and is a function of the weight of overburden above the layer of interest. For an oil-producing layer, elastic settlement is also a function of the system pressurization, where pressure is kept to ensure the injection fluids remain in the oil layer. Creep is difficult to assess since it is a function of time. Current geostability analysis does not include considerations of thermoelastic effects or micro-poroelastic effects for the pressurized system, for example, the Biot's equations. The rationale is that the difficulties in establishing geomaterial properties based on current geophysical measurements and assumed values only allow a grossly simplified bulk material analysis. As a result, the extensive works by Biot on poroelasticity has been greatly simplified.

Stresses in geomaterials are derived essentially from the self-weight of the overburden materials (predominantly the geo-matrix), the liquid within the voids (pore water pressure), and

**Figure 3.** Poroelastic stresses due to extensive oil extraction (hollow arrows indicate surface strains and line arrows

replacement in the Rodessa formation.

130 Carbon Capture, Utilization and Sequestration

indicate interplanar shear stresses).

potential assessment as well as localized stability projection.


Micro-seismicity tests have been successfully applied to address specific issues in the oil and gas industry [23, 24]. The basic principle of passive seismic monitoring is to detect small movements (regarded as microseismic events) from unknown seismic sources that can be recorded on geophones placed on site. Contrast to active geophysical testing, the passive seismic monitoring is a testing method that does not rely on a source of ground excitation. The main advantage of the passive monitoring is that it can be carried out at any time and does not require regulated field access. The disadvantage of passive sensing is the uncertainty introduced due to the lack of controlled input energy, which can result in both poor data sensitivity and poor detection accuracy.

A modified passive sensing Refractive Microtremor (ReMi) technique, Derivative of ReMi (DoReMi), as discussed below, is used at the Citronelle oil field, Alabama [25]. To improve mobility and avoid the cumbersome wiring, wireless triaxial micro-electro-mechanical system (MEMS) accelerometers have been used for the field testing. The MEMS sensors are encased in hard metal boxes and buried into the ground at sufficient depth to ensure good coupling between the sensor and the surrounding soil (at least 1 ft. (0.3 m) deep with fully compacted soil on top). The wireless sensor unit with the three directional acquisition channels can record seismic energy in three Cartesian directions (vertical and two horizontal directions). The vibration signals obtained by the wireless accelerometer are acceleration time histories, which are processed in spectral domain using p-τ transformation, or slant-stack analysis [26]. Since passive sensing assumes the signals are random in nature and the analysis is done in the spectral domain, time sequence of the sampled data is not considered. Only the vertical direction has been used in the wave motion analysis for this study.

B-19-10 #2 from the beginning of CO<sup>2</sup>

kPA) to 3800 psig (26,200.1 kPA). After CO<sup>2</sup>

In the first month of CO<sup>2</sup>

**Figure 5.** Record of CO<sup>2</sup>

injection to the end of the injection is shown in **Figure 6**.

Geophysical Monitoring of CO2 Injection at Citronelle Field, Alabama

http://dx.doi.org/10.5772/intechopen.78386

injection was resumed on January 27, 2010, the range

injection with geophysical test data.

injection,

133

injection, the well head pressure changed from 2400 psig (16,547.4

The pressure has been normalized in order to compare it with the normalized stresses at the oil-bearing layer quantified based on the geophysical testing results to be presented later.

of well head is between 3800 psig (26,200.1 kPA) and 4200 psig (28,957.9 kPA). Passive tests were

conducted at the Citronelle oil field in December 2009 when the start of significant CO<sup>2</sup>

injection during Phase II at Well B-19-10#2.

**Figure 6.** Normalized well head pressure at Well B-19-10#2 during CO<sup>2</sup>

To monitor the responses of the reservoir throughout the CO<sup>2</sup> injection process, two linear test arrays were conducted at the Citronelle oil field. Each test array consists of 24 measurement points, which are all located near the oil wells. The site test layout is shown in **Figure 4**. The Line 1 is generally aligning with the north to south direction, whereas, Line 2 is in general in the northeast to southwest direction. Line 1 covered approximately a distance of 30,102 ft. (9175 m) in total with approximately 1309 ft. (399 m) for sensor spacing. Line 2 is 25,603 ft. (7804 m) in total span and has a sensor spacing of 1113 ft. (339 m) between pickup points. CO<sup>2</sup> is injected in well No. B-19-10 #2, which is located near the intersection of the two survey lines and is in the top north end of the Citronelle oil field. The sensors were buried at each measurement point, and the recording duration for each set was set at 39.06 s. The sampling frequency was set at 512 Hz.

Background measurement was deployed prior to the start of CO<sup>2</sup> injection in the field. It should be noted that in order to restore the pressure in the well to the level suitable for production, water injection at the well has been conducted since 2007. CO<sup>2</sup> injection in well No. B-19-10 #2 started in December 2009 and at the rate of 46.5 tons/day. The CO<sup>2</sup> injection was stopped from December 30, 2009 to January 26, 2010, due to the triplex pump not being able to maintain the injection pressure. After a thorough problem detection process, the pumping was resumed and, as a result, the average injection rate of CO<sup>2</sup> was stabilized at 31.5 tons/day. The CO<sup>2</sup> injection history in short tons until late September 2010 is presented in **Figure 5**. Final amount of CO2 injected in the pilot well is about 8036 short tons. The record of well head pressure at Well

**Figure 4.** The testing lines at the Citronelle oil field.

B-19-10 #2 from the beginning of CO<sup>2</sup> injection to the end of the injection is shown in **Figure 6**. The pressure has been normalized in order to compare it with the normalized stresses at the oil-bearing layer quantified based on the geophysical testing results to be presented later.

In the first month of CO<sup>2</sup> injection, the well head pressure changed from 2400 psig (16,547.4 kPA) to 3800 psig (26,200.1 kPA). After CO<sup>2</sup> injection was resumed on January 27, 2010, the range of well head is between 3800 psig (26,200.1 kPA) and 4200 psig (28,957.9 kPA). Passive tests were conducted at the Citronelle oil field in December 2009 when the start of significant CO<sup>2</sup> injection,

**Figure 5.** Record of CO<sup>2</sup> injection during Phase II at Well B-19-10#2.

Since passive sensing assumes the signals are random in nature and the analysis is done in the spectral domain, time sequence of the sampled data is not considered. Only the vertical

test arrays were conducted at the Citronelle oil field. Each test array consists of 24 measurement points, which are all located near the oil wells. The site test layout is shown in **Figure 4**. The Line 1 is generally aligning with the north to south direction, whereas, Line 2 is in general in the northeast to southwest direction. Line 1 covered approximately a distance of 30,102 ft. (9175 m) in total with approximately 1309 ft. (399 m) for sensor spacing. Line 2 is 25,603 ft. (7804 m) in total span and has a sensor spacing of 1113 ft. (339 m) between pickup

survey lines and is in the top north end of the Citronelle oil field. The sensors were buried at each measurement point, and the recording duration for each set was set at 39.06 s. The

be noted that in order to restore the pressure in the well to the level suitable for production,

December 30, 2009 to January 26, 2010, due to the triplex pump not being able to maintain the injection pressure. After a thorough problem detection process, the pumping was resumed

tion history in short tons until late September 2010 is presented in **Figure 5**. Final amount of

injected in the pilot well is about 8036 short tons. The record of well head pressure at Well

is injected in well No. B-19-10 #2, which is located near the intersection of the two

injection process, two linear

injection in the field. It should

injection in well No. B-19-10 #2

was stabilized at 31.5 tons/day. The CO<sup>2</sup>

injection was stopped from

injec-

direction has been used in the wave motion analysis for this study.

To monitor the responses of the reservoir throughout the CO<sup>2</sup>

Background measurement was deployed prior to the start of CO<sup>2</sup>

water injection at the well has been conducted since 2007. CO<sup>2</sup>

started in December 2009 and at the rate of 46.5 tons/day. The CO<sup>2</sup>

points. CO<sup>2</sup>

CO2

sampling frequency was set at 512 Hz.

132 Carbon Capture, Utilization and Sequestration

and, as a result, the average injection rate of CO<sup>2</sup>

**Figure 4.** The testing lines at the Citronelle oil field.

**Figure 6.** Normalized well head pressure at Well B-19-10#2 during CO<sup>2</sup> injection with geophysical test data.

and during steady CO<sup>2</sup> injection in March 2010, May 2010, and September 2010, respectively. Water injection was switched back immediately after CO<sup>2</sup> injection was completed. In addition, measurements were made after CO<sup>2</sup> injection in November 2010, March 2011, and May 2011, respectively. A summary of the monitoring history at the Citronelle oil field is shown in **Table 1**.

#### **2.2. Injection history analysis**

Since the monitoring process involved the three injection stages, namely, water injection (pressure building), CO<sup>2</sup> injection, and post-injection, it is of interest to interpret the results according to the stages. For each stage, at least three monitoring tests were performed. Hence, there are three test group data. To compare the field pressure responses at different injection stages, statistical parameters have been adopted including average shear-wave velocities and coefficient of variations.

Statistical analysis is performed first by determining the averaged shear-wave velocities at different strata for each test group along each of the test lines. The average wave velocities are defined as [27]:

$$\boldsymbol{\Theta} = \frac{1}{N} \sum\_{l=1}^{N} \mathbf{x}\_{l} \tag{4}$$

The average and standard deviation values are then used to compute the coefficient of varia-

The coefficient of variation illustrates how far a set of numbers deviates from the average value—an indication of the consistency of the layer responses as well as the repeatability of the

oil-bearing layer at the Citronelle oil field. Due to the presence of the anhydrite layer, the CO<sup>2</sup> remains within the oil-bearing rock and will slowly flow into the oil-bearing rock resulting in the stress built-up dissipating throughout the oil-bearing layer. As long as the anhydrite retains its integrity and that there are no break-throughs within the rock medium, the pressure at the oil-bearing layer should be consistently higher than in the strata above. Thus, the stress wave velocity at the oil-bearing layer should be higher than in the strata above. The COV and average values of the wave speed profile will be used to determine the stress state in the strata system.

values from both Line 1 and Line 2 tests. **Table 2** shows that the *Cv*

 values of the wave speed at the oil-bearing layer is an indication of the stabilization of the strata pressurization process: as the oil-bearing layer pressure is building up, a larger

value is expected, which dropped later indicating stable pressure in the oil-bearing rock.

**Line 1 Line 2 Line 1 Line 2 Line 1 Line 2**

 0.05 0.06 0.03 0.07 0.02 0.05 0.06 0.06 0.09 0.11 0.01 0.07 0.08 0.07 0.06 0.18 0.02 0.03 0.16 0.08 0.05 0.20 0.01 0.05 0.15 0.08 0.10 0.17 0.03 0.04 0.13 0.05 0.14 0.15 0.03 0.04 0.16 0.07 0.14 0.14 0.01 0.03 0.14 0.03 0.15 0.14 0.02 0.01 0.13 0.05 0.11 0.11 0.01 0.03 0.17 0.05 0.04 0.09 0.01 0.03 0.16 0.06 0.05 0.06 0.01 0.04 0.08 0.05 0.07 0.06 0.002 0.02 0.04 0.06 0.05 0.03 0.004 0.003 0.07 0.03 0.06 0.05 0.01 0.001

values for results from both Line 1 and Line 2 tests.

each layer are reduced during the injection history, indicating a stressing of the strata.

 **injection During CO2**

*α*

injection process instigated a continuous stress building within the

 **injection After CO2**

*<sup>θ</sup>* (6)

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Geophysical Monitoring of CO2 Injection at Citronelle Field, Alabama

values for

135

 **injection**

tion (COV), *Cv*

:

measurements. The CO<sup>2</sup>

**Table 2** lists the *Cv*

**Layer Before CO2**

**Table 2.** Summary of *Cv*

The *Cv*

*Cv*

*cv* = \_\_

where θ represents the average wave speed, *xi* represents the wave speed data at the corresponding depth for each test group, and *N* represents the number of tests in each test group. After calculating the average shear-wave velocities, the standard deviations, *α*, of the corresponding data are determined as:

$$\alpha = \sqrt{\frac{1}{N} \sum\_{l=1}^{N} (\mathbf{x}\_l - \boldsymbol{\theta})^2} \tag{5}$$


**Table 1.** Summary of monitoring history at the Citronelle oil field.

The average and standard deviation values are then used to compute the coefficient of variation (COV), *Cv* :

and during steady CO<sup>2</sup>

measurements were made after CO<sup>2</sup>

**2.2. Injection history analysis**

134 Carbon Capture, Utilization and Sequestration

(pressure building), CO<sup>2</sup>

coefficient of variations.

defined as [27]:

Water injection was switched back immediately after CO<sup>2</sup>

*θ* = \_\_1

where θ represents the average wave speed, *xi*

*<sup>α</sup>* <sup>=</sup> <sup>√</sup>

**Table 1.** Summary of monitoring history at the Citronelle oil field.

responding data are determined as:

injection in March 2010, May 2010, and September 2010, respectively.

injection, and post-injection, it is of interest to interpret the results

respectively. A summary of the monitoring history at the Citronelle oil field is shown in **Table 1**.

Since the monitoring process involved the three injection stages, namely, water injection

according to the stages. For each stage, at least three monitoring tests were performed. Hence, there are three test group data. To compare the field pressure responses at different injection stages, statistical parameters have been adopted including average shear-wave velocities and

Statistical analysis is performed first by determining the averaged shear-wave velocities at different strata for each test group along each of the test lines. The average wave velocities are

> *<sup>N</sup>* ∑ *i*=1 *N*

sponding depth for each test group, and *N* represents the number of tests in each test group. After calculating the average shear-wave velocities, the standard deviations, *α*, of the cor-

> \_\_1 *<sup>N</sup>* ∑ *i*=1 *N*

**Test no. Injection Monitoring date** Water 8–10 October 2008 Water 21–22 January 2009 Water 15–16 June 2009 CO2 9–10 December 2009 CO2 11–12 March 2010 CO2 18–19 May 2010 CO2 8–9 September 2010 Water 17–18 November 2010 Water 16–17 March 2011 Water 17–18 May 2011

\_\_\_\_\_\_\_\_\_\_\_

injection in November 2010, March 2011, and May 2011,

injection was completed. In addition,

*xi* (4)

represents the wave speed data at the corre-

(*xi* − *θ*)<sup>2</sup> (5)

$$c\_v = \frac{a}{\theta} \tag{6}$$

The coefficient of variation illustrates how far a set of numbers deviates from the average value—an indication of the consistency of the layer responses as well as the repeatability of the measurements. The CO<sup>2</sup> injection process instigated a continuous stress building within the oil-bearing layer at the Citronelle oil field. Due to the presence of the anhydrite layer, the CO<sup>2</sup> remains within the oil-bearing rock and will slowly flow into the oil-bearing rock resulting in the stress built-up dissipating throughout the oil-bearing layer. As long as the anhydrite retains its integrity and that there are no break-throughs within the rock medium, the pressure at the oil-bearing layer should be consistently higher than in the strata above. Thus, the stress wave velocity at the oil-bearing layer should be higher than in the strata above. The COV and average values of the wave speed profile will be used to determine the stress state in the strata system. **Table 2** lists the *Cv* values from both Line 1 and Line 2 tests. **Table 2** shows that the *Cv* values for each layer are reduced during the injection history, indicating a stressing of the strata.



**Table 2.** Summary of *Cv* values for results from both Line 1 and Line 2 tests. **Table 2** shows the *Cv* values for each layer and each survey line. The table shows that in all cases, *Cv* 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* values also are consistently dropping during the three stages indicating that the pressurization is gradually stabilized during the process.

initial increase in wave speed (during CO<sup>2</sup>

speeds for both during and after CO<sup>2</sup>

5, test 6, and test 7 (during CO<sup>2</sup>

test 9, and test 10 (after CO<sup>2</sup>

8–9, 2010.

CO2

injection) and then decreasing wave speed (after

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137

injections; and finally, the oil-bearing layer [around

Geophysical Monitoring of CO2 Injection at Citronelle Field, Alabama

injection) for Line 1 and Line 2, respectively. Wave speeds

injection) for Line 1 and Line 2, respectively. The deviations on

injection, which

injection); the trend reversed after 6000 ft. (1829 m) depth showing increasing wave

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.

shown in **Figure 8**. The increase in shear-wave velocity is associated with CO<sup>2</sup>

indicating that the strata pressurization has stabilized.

**Figure 9** shows the results of average shear-wave velocity versus depth curve for test 4, test

results of the last four layers shown in **Figure 9** are higher than the corresponding results

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,

the graphs shown in **Figure 10** are significantly smaller when compared to **Figures 8** and **9**

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

Careful evaluation of **Table 2** indicates that there is a difference between the results from both survey lines: for the after CO<sup>2</sup> injection stage, it is seen where the *Cv* value is shown to be 0.01 for Line 1 and the value is 0.001 for Line 2 (Layer 14). This is an order of magnitude different. For the before CO<sup>2</sup> injection (initial water pumping) stage, where the *Cv* value is 0.07 for Line 1 and is 0.03 for Line 2. This observation may be detrimental considering the experimental resolution of the geophysical testing method, which is discussed below.
