*Performance Evaluation and Mechanism Study of a Silicone Hydrophobic Polymer… DOI: http://dx.doi.org/10.5772/intechopen.90811*

OSSF solution increased from about 8 to 93%; then, with the distilled water, the permeability recovery was 77%. Both increases were at 0.06 MPa and 20 PV. At 0.03 MPa and 20 PV, permeability recovery of 0.20% OSSF solution increased from about 7 to 86%, with the distilled water 67% by contrast. These results illustrate that the increase of flow-back volume and gas pressure can improve the permeability recovery and reduce the water blocking damage of cores. In addition, OSSF is beneficial to the permeability recovery of the reservoirs damaged by external fluids.

**Figure 12.** *Size and distribution of OSSF aggregate versus weight percentage (T = 25°C, free pH).*

**Figure 13.** *Flow patterns of OSSF aggregate in the reservoir pores.*

*3.3.3 Permeability recovery of cores*

*21st Century Surface Science - a Handbook*

**Figure 10.**

**Figure 11.**

**176**

*Pore volumes (PV) versus permeability recovery rate of artificial cores (D).*

*Gas flow-back volumes versus remaining water saturation (Sw) of artificial cores.*

As shown in **Figure 10**, the permeability recovery of the cores gradually increases with flow-back PV. Meanwhile, the recovery rate gradually decreases. When gas pressure was 0.06 MPa and the flow-back PV is 20, the cores' permeability recovery changes from 7 to 77%. It is evident that the higher the displacing pressure, the higher the permeability recovery. A permeability recovery of 0.20%

#### *3.3.4 Retained water saturation of cores*

Accumulation of external fluids in gas reservoir is the principle factor responsible for water blocking damage [23, 24]. As shown in **Figure 11**, the remaining water saturation decreased with PV, but the decreasing rate reduced gradually. Retained water saturation of 0.20% OSSF solution decreased from about 83 to 30%; moreover, with the distilled water, the water saturation decreased from 90 to 34% by contrast. The results reveal that increasing flow-back time can reduce the water saturation and the water blocking damage of the reservoir. Besides, OSSF is also beneficial to the flow back of invasive external fluid in a reservoir.

#### **3.4 Mechanism analysis**

#### *3.4.1 Physical blocking*

**Figure 12** shows that aggregates can be formed when the concentration exceeds its critical micelle concentration (CMC). Adjusting the OSSF concentrations to match reservoir needs with varied pore and throat sizes is necessary when the aggregates invade reservoir with external fluids. This causes friction to form on the surface in which pores or throats and fluid come in contact with each other.

When the aggregates move into the pores or throats of reservoirs, as shown in **Figure 13(b)**, the interfacial tensions on the two contact surfaces are even. When the aggregates flow from the pores to the reservoir throats (**Figure 13(c)**), friction can decrease the flow rate. When aggregates move from the throats to the pores (**Figure 13(d)**), the interfacial tension between aggregates and external fluid becomes smaller. In the process of production, the fluid begins to flow back, the aggregates move back from the throats to the pores, and then, the differences in the interfacial tensions between two contact surfaces could become the driving force for flowing, which could accelerate these fluids to flow back.

*3.4.2 Wettability alteration*

**Figure 15.**

**179**

group represents the nonpolar part.

ties of the gas reservoir change [17].

When the alkaline solution invades into the reservoir, the structural groups of the silicon-oxygen bond (Si-O-Si) on the surface of the silicate minerals can produce silanol groups (Si-OH) by hydrolysis. The density can reach up to 6–7 nm per square. These silanol groups can adsorb the water molecules owing to the hydrogen bonding and the Van Der Waals force upon the water-pore interface. Within both forces, hydrogen bonding plays an important role. It is needed to overcome the adhesion force between rock and water, thereby decreasing the remaining water in the formation [25]. The basic structural unit of the OSSF is polydimethylsiloxane. The silicon-oxygen chain is the polar part of the OSSF composition, and the methyl

*SEM image and X-ray spectrum of artificial cores. (a) Untreated and (b) treated by 0.20 wtT OSSF.*

*Performance Evaluation and Mechanism Study of a Silicone Hydrophobic Polymer…*

*DOI: http://dx.doi.org/10.5772/intechopen.90811*

Under high temperature and catalyst conditions, polarization occurs in the sili-

directionally adsorbed methyl groups reduce the surface energy of the rock and lead to the hydrophobization on the rock surface, changing the composition of the rock surface. Meanwhile, the capillary force direction of fluids and the physical proper-

As shown in **Figure 15**, the cores treated by OSSF have a higher weight and atom Si number as well as a lower weight and atom O number. This is because the cores are composed of silicon and oxygen, with a chemical structural much like that of OSSF. **Figure 16** shows how the OSSF adsorption on the reservoir core surface can

con oxygen main chain. This leads to multipoint adsorption of silanol groups (Si-OH) and silicon-oxygen bond (Si-O-Si) through chemical bonding and hydrogen bonding. Meanwhile, the methyl groups directionally rotate and sequentially arrange on the rock surface. This process is shown in **Figure 14** [26]. These

**Figure 14.** *Principle diagram of OSSF adsorption on water-pore interface.* *Performance Evaluation and Mechanism Study of a Silicone Hydrophobic Polymer… DOI: http://dx.doi.org/10.5772/intechopen.90811*

**Figure 15.** *SEM image and X-ray spectrum of artificial cores. (a) Untreated and (b) treated by 0.20 wtT OSSF.*

#### *3.4.2 Wettability alteration*

*3.3.4 Retained water saturation of cores*

*21st Century Surface Science - a Handbook*

**3.4 Mechanism analysis**

*3.4.1 Physical blocking*

**Figure 14.**

**178**

Accumulation of external fluids in gas reservoir is the principle factor responsible for water blocking damage [23, 24]. As shown in **Figure 11**, the remaining water saturation decreased with PV, but the decreasing rate reduced gradually. Retained water saturation of 0.20% OSSF solution decreased from about 83 to 30%; moreover, with the distilled water, the water saturation decreased from 90 to 34% by contrast. The results reveal that increasing flow-back time can reduce the water saturation and the water blocking damage of the reservoir. Besides, OSSF is also

**Figure 12** shows that aggregates can be formed when the concentration exceeds

When the aggregates move into the pores or throats of reservoirs, as shown in **Figure 13(b)**, the interfacial tensions on the two contact surfaces are even. When the aggregates flow from the pores to the reservoir throats (**Figure 13(c)**), friction can decrease the flow rate. When aggregates move from the throats to the pores (**Figure 13(d)**), the interfacial tension between aggregates and external fluid becomes smaller. In the process of production, the fluid begins to flow back, the aggregates move back from the throats to the pores, and then, the differences in the interfacial tensions between two contact surfaces could become the driving force

its critical micelle concentration (CMC). Adjusting the OSSF concentrations to match reservoir needs with varied pore and throat sizes is necessary when the aggregates invade reservoir with external fluids. This causes friction to form on the surface in which pores or throats and fluid come in contact with each other.

beneficial to the flow back of invasive external fluid in a reservoir.

for flowing, which could accelerate these fluids to flow back.

*Principle diagram of OSSF adsorption on water-pore interface.*

When the alkaline solution invades into the reservoir, the structural groups of the silicon-oxygen bond (Si-O-Si) on the surface of the silicate minerals can produce silanol groups (Si-OH) by hydrolysis. The density can reach up to 6–7 nm per square. These silanol groups can adsorb the water molecules owing to the hydrogen bonding and the Van Der Waals force upon the water-pore interface. Within both forces, hydrogen bonding plays an important role. It is needed to overcome the adhesion force between rock and water, thereby decreasing the remaining water in the formation [25]. The basic structural unit of the OSSF is polydimethylsiloxane. The silicon-oxygen chain is the polar part of the OSSF composition, and the methyl group represents the nonpolar part.

Under high temperature and catalyst conditions, polarization occurs in the silicon oxygen main chain. This leads to multipoint adsorption of silanol groups (Si-OH) and silicon-oxygen bond (Si-O-Si) through chemical bonding and hydrogen bonding. Meanwhile, the methyl groups directionally rotate and sequentially arrange on the rock surface. This process is shown in **Figure 14** [26]. These directionally adsorbed methyl groups reduce the surface energy of the rock and lead to the hydrophobization on the rock surface, changing the composition of the rock surface. Meanwhile, the capillary force direction of fluids and the physical properties of the gas reservoir change [17].

As shown in **Figure 15**, the cores treated by OSSF have a higher weight and atom Si number as well as a lower weight and atom O number. This is because the cores are composed of silicon and oxygen, with a chemical structural much like that of OSSF. **Figure 16** shows how the OSSF adsorption on the reservoir core surface can

Here, represents the system energy in kJ/mol when silicon-

represents the system energy when silicon-oxygen tetrahedron and dimethyl siloxane adsorb water in kJ/mol. is the energy of silicon-oxygen tetrahedron or dimethyl siloxane in kJ/mol. represents the energy of water or methane

*Performance Evaluation and Mechanism Study of a Silicone Hydrophobic Polymer…*

As shown in **Figure 17** and **Table 4**, the distances of the dimethyl siloxane models between CH4 and H2O are larger or similar than that of silicon-oxygen tetrahedron. The adsorption energy of silicon-oxygen tetrahedron and the dimethyl siloxane models to CH4 is 11.21 and 0.46 kJ/mol, respectively, which indicates that the adsorption of methane on the core surfaces is nonhydrogen-bonding physical adsorption. The adsorption energy of silicon-oxygen tetrahedron and the dimethyl siloxane models to H2O is 81.78 and 52.18 kJ/mol, respectively, suggesting the adsorption of water is hydrogen-bonding adsorption. Since the adsorption binding energy of CH4 and H2O on the silicon-oxygen tetrahedron model is higher than that on the dimethyl siloxane model, the stability of CH4 and H2O adsorbed on the pore surface of cores treated by OSSF is lower than that on the untreated cores. At the same flow-back pressure, CH4 and H2O adsorbed on the surface of treated cores are often more easily desorbed. Under the same temperature and pressure conditions, after the adsorption of OSSF, the affinity and adhesion of the rock surface to CH4 and H2O are reduced, leading to the reduction of the shear stress of CH4 and H2O on the pore surfaces of the reservoir, when gas or external fluids flow in the reservoir [29]. This can increase the fluidity of CH4 and H2O and finally improve the permeability of CH4 and H2O, thereby decreasing water saturation of the reservoir.

*Optimized structure of the models absorbed the CH4 as shown in (a) and (b) and H2O as shown in (c) and*

**Adsorption model Adsorbate re/ (nm) Ee/ (kJmol<sup>1</sup>**

H2O 3.32 81.77

H2O 3.17 52.18

Silicon-oxygen tetrahedron (a) CH4 4.33 11.21

Dimethylsiloxane (b) CH4 5.17 0.46

*Adsorption equilibrium distance (re) and adsorption energy (Ee) of the absorbed the CH4 and H2O*

**)**

oxygen tetrahedron and dimethyl siloxane adsorb methane.

*DOI: http://dx.doi.org/10.5772/intechopen.90811*

in kJ/mol.

**Figure 17.**

**Table 4.**

**181**

*(d) (T = 273.15 K, P = 1 atm).*

*(T = 273.15 K, p = 1 atm).*

**Figure 16.** *SEM image and X-ray spectrum of reservoir cores. (a) Untreated and (b) treated by 0.20 wt% OSSF.*

significantly increase the weight and number of atom Si and Ca but slightly decrease that of atom O. Since we use carbonate cores, their surface components change greatly as a result of OSSF adsorption. Both artificial and reservoir core surfaces show more Si atoms. The outermost areas of the core are composed of Si-CH3, and the cores have become hydrophobic.

#### *3.4.3 Adsorption energy*

The weak interaction among the molecules can be calculated accurately by the computational technique based on quantum chemistry. In this paper, (a) silicon-oxygen tetrahedron and (b) dimethyl siloxane were applied to simulate the surface of the untreated cores and OSSF treated cores, respectively. The oxygen atoms at the edge were saturated with hydrogen atoms.

Gaussian 09 W software (Gaussian Inc., Wallingford, CT, USA) is used as the simulation tool, and 6-31G is selected as the basic set. Correction is performed using a basis set superposition error (BSSE). In the adsorption system, the distance between the C atoms of methane and the O atoms of water is the equilibrium distance *r*e. The following Eqs. (3) and (4) were used to calculate the adsorption energies (Ee) of water and methane on the core surface [27, 28]:

$$E\_{\mathbf{e}} = E\_{CH\_3-Si/CH\_4/Si-O/CH\_4} - E\_{Si-O/CH\_3-Si} - E\_{CH\_4} \tag{3}$$

$$E\_{\mathbf{e}} = E\_{CH\_3-Si/H\_2O/Si-O\_{//}H\_2O} - E\_{Si-O/CH\_3-Si} - E\_{H\_2O} \tag{4}$$

*Performance Evaluation and Mechanism Study of a Silicone Hydrophobic Polymer… DOI: http://dx.doi.org/10.5772/intechopen.90811*

Here, represents the system energy in kJ/mol when siliconoxygen tetrahedron and dimethyl siloxane adsorb methane. represents the system energy when silicon-oxygen tetrahedron and dimethyl siloxane adsorb water in kJ/mol. is the energy of silicon-oxygen tetrahedron or dimethyl siloxane in kJ/mol. represents the energy of water or methane in kJ/mol.

As shown in **Figure 17** and **Table 4**, the distances of the dimethyl siloxane models between CH4 and H2O are larger or similar than that of silicon-oxygen tetrahedron. The adsorption energy of silicon-oxygen tetrahedron and the dimethyl siloxane models to CH4 is 11.21 and 0.46 kJ/mol, respectively, which indicates that the adsorption of methane on the core surfaces is nonhydrogen-bonding physical adsorption. The adsorption energy of silicon-oxygen tetrahedron and the dimethyl siloxane models to H2O is 81.78 and 52.18 kJ/mol, respectively, suggesting the adsorption of water is hydrogen-bonding adsorption. Since the adsorption binding energy of CH4 and H2O on the silicon-oxygen tetrahedron model is higher than that on the dimethyl siloxane model, the stability of CH4 and H2O adsorbed on the pore surface of cores treated by OSSF is lower than that on the untreated cores. At the same flow-back pressure, CH4 and H2O adsorbed on the surface of treated cores are often more easily desorbed. Under the same temperature and pressure conditions, after the adsorption of OSSF, the affinity and adhesion of the rock surface to CH4 and H2O are reduced, leading to the reduction of the shear stress of CH4 and H2O on the pore surfaces of the reservoir, when gas or external fluids flow in the reservoir [29]. This can increase the fluidity of CH4 and H2O and finally improve the permeability of CH4 and H2O, thereby decreasing water saturation of the reservoir.

#### **Figure 17.**

significantly increase the weight and number of atom Si and Ca but slightly decrease that of atom O. Since we use carbonate cores, their surface components change greatly as a result of OSSF adsorption. Both artificial and reservoir core surfaces show more Si atoms. The outermost areas of the core are composed of Si-CH3, and

*SEM image and X-ray spectrum of reservoir cores. (a) Untreated and (b) treated by 0.20 wt% OSSF.*

The weak interaction among the molecules can be calculated accurately by

(a) silicon-oxygen tetrahedron and (b) dimethyl siloxane were applied to simulate the surface of the untreated cores and OSSF treated cores, respectively. The oxygen

Gaussian 09 W software (Gaussian Inc., Wallingford, CT, USA) is used as the simulation tool, and 6-31G is selected as the basic set. Correction is performed using a basis set superposition error (BSSE). In the adsorption system, the distance between the C atoms of methane and the O atoms of water is the equilibrium distance *r*e. The following Eqs. (3) and (4) were used to calculate the adsorption

ð3Þ

ð4Þ

the computational technique based on quantum chemistry. In this paper,

atoms at the edge were saturated with hydrogen atoms.

energies (Ee) of water and methane on the core surface [27, 28]:

the cores have become hydrophobic.

*21st Century Surface Science - a Handbook*

*3.4.3 Adsorption energy*

**Figure 16.**

**180**

*Optimized structure of the models absorbed the CH4 as shown in (a) and (b) and H2O as shown in (c) and (d) (T = 273.15 K, P = 1 atm).*


**Table 4.**

*Adsorption equilibrium distance (re) and adsorption energy (Ee) of the absorbed the CH4 and H2O (T = 273.15 K, p = 1 atm).*
