Sajad Asadizadeh,Shahab Ayatollahi,Bahman ZareNezhad,

1 Faculty of Chemical,Petroleum and Gas Engineering,Semnan University,Semnan,Iran

2 Chemical and Petroleum Engineering Department,Sharif University of Technology,Tehran,Iran

Keywords:Petroleum Excess water Nanocomposite Polymer gel Nanoparticle EOR

ABSTRACT A new nanocomposite polymer gel is synthesized for reduction of excess water production in petroleum reservoirs at real operating conditions.This new nanocomposite gel contains SiO2 nanoparticles,partially hydrolyzed polyacrylamide (HPAM)and chromium triacetate.High pressure and high temperature tests using porous carbonate core are carried out to evaluate the effects of nanoparticles on the synthesized polymer gel performance.It is shown that the residual resistance factor ratio of water to oil using the synthesized polymer gel nanocomposite in this work is much higher than that of the ordinary polymer gels.The presented results confirm the high performance of the synthesized nanocomposite polymer gel for decreasing the water flow through porous carbonate bed.A mathematical model for description of oil and water flow behavior in the presence of synthesized nanocomposite polymer gel is also presented.The presented nano polymer gel leads to considerable cost saving in enhanced oil recovery(EOR)processes.?2021 The Chemical Industry and Engineering Society of China,and Chemical Industry Press Co.,Ltd.All rights reserved.

1.Introduction

One of the most important impediments in enhanced oil recovery processes and petroleum industry is excess production of water[1].Based on the global information,for each barrel of extracted oil,three water barrels are recovered[2].Among different technologies for excess water production control,polymer gel has been found to be more effective[3–6].In the recent years,polymers such as polyacrylamides and polysaccharides which are soluble in water,have been generally applied for water shut-off processes.These kinds of polymers are interacted with phenol–formaldehyde [7] chromium (III) salt [8] and polyethyleneimine (PEI) as metallic and organic cross-linkers to create three-dimensional network of hydrogels [9,10].Different parameters are important in the polymer gel selection.Reservoir operating conditions such as hardness,temperature,pH and salinity of water are the important parameters.Additional parameters that can be investigated for the suitable choice of a polymer gel system are the permeability and characteristic of the formation rock [11–13].Polymers gel can selectively increase the effective permeability of oil compared to the effective permeability of water[14,15].

Polymer gels which are metallically cross-linked usually have small gelation time and low thermal stability at temperatures higher than 60–70°C.Thus these type of polymer gels are not suitable for high temperature reservoirs.However,polymer gels with organically cross-linker have higher gelation time and thermal stability compared to the metallically cross-linked gels [16].

The nanoparticle due to high reaction activity can be employed for improving the structural strength of polymer gels [17,18].Effects of silica nanoparticles has been investigated on the polyacrylamide (PAM)/hydroquinone (HQ)–hexamethylenetetramine(HMTA)composite gel strength.It is proved that based on the rheological measurements,the addition of silica nanoparticles reduces the gelation time and enhances the gel strength[19–24].It should be noted that water can move and breakthrough the fractured reservoirs such that the well production will encounter higher water production as compared to the non-fractured ones[25–31].This problem is more serious regarding the hydraulic fractured oil wells,especially in the last period of production due to the existence of large fractures near the wellbore zoon [32–38].

In the present work,a new nanocomposite polymer gel is synthesized and performance of this polymer gel system is evaluated using the high pressure core flooding tests regarding fractured carbonate cores in the presence of formation water and sea water at real operating conditions.The impacts of different parameters such as temperature,salinity of water and concentration of SiO2nanoparticle are also evaluated.A theoretical computational framework for description of the oil and water flow through the high pressure carbonate core using the synthesized nannocomposite polymer gel is also presented.

In this model,the fluid flow equations regarding two phases of water and oil are considered [39].The physical properties of the synthesized nanocomposite are also incorporated in the presented model.These equations form a system of differential equations that are solved using numerical methods.

(1) Mass balance equation

In this equation J is mass flux,Q is mass flow and p subtitle indicate phase.

(2) Flow equations for two-phase flow

Two-dimensional structure is considered and the equations are written in two dimensions.Mass flux is defined as below:

In which,st indicate the standard condition,ρ is density,B is volumetric factor and v is Darcy velocity which is defined as below:

Mpis the phase mobility which is ratio of relative permeability to viscosity:

Saturation degree of a phase is ratio of phase volume to empty volume:

By combining Eqs.(1),(2) and (5),the conservation of mass equation is obtained in two directions of x and y:

(3).Mathematical model for synthesized nano polymer gel flow

The process of gel formation,absorption and physical properties of the gel are described as follows.

(4) Convection-dispersion equations

The mass balance equation considering gel reaction rate can be written as follows:

where Ci:Mass concentration of components,ρR:Rock density,D:Penetration coefficient,vi:Darcy velocity,ρ:Density of liquid phase,Ri:The gel reaction rate and CiR:Mass concentration of material that is absorbed on the surface of the solid phase.According to Hennery low,the absorbed mass concentration is:

That Γiis Hennery coefficient for absorption process.

Initial conditions for Eq.(9) are as follows:

For HPAM:at t=0 Ci=1 wt%

For Cr+3:at t=0 Ci=0.2 wt%

For gel:at t=0 Ci=0

Also boundary conditions for this equation are:

(5).Gel formation reaction rate

The gel formation reaction can be written as follows:

(6).Pressure equations

Phase potential (φ) is defined as below:

In the above equation K is the absolute permeability which is obtained from the following equation:

Based on above equations,the conservation equation is:

In order to create the phrase ‘‘oil pressure”in the above equations,the capillary pressure is defined as the difference between the pressure of water and oil:

where pcowis the capillary pressure of oil.

By replacing Eq.(16) in Eq.(14),the continuity equation for water and oil will be as follows.

αoand αwrepresents the gravity and capillary pressure of each phase:

2.Experimental

2.1.Materials

HPAM polymer (Partially hydrolyzed polyacrylamide) is purchased from SNF Company of France.The hydrolysis degree of this polymer is 29%and the polymer average molecular weight is about 5000,000.Chromium(III) acetate is purchased by Carlo Erba Company of Italy as cross-linker.Also SiO2nanoparticle is provided by SNF Company of France.

2.2.Experimental methods

2.2.1.Bottle test

Powder of HPAM polymer is mixed by sea water and formation water samples at the desired temperature.To receive a uniform solution and regular distribution,this solution is vigorously agitated using a heater stirrer.Solution of the cross-linker is prepared by mixing powder of Cr(III)-acetate with sea and formation water.The gallant solution is made by combination of the HPAM polymer and cross-linker before starting the experiment at 100 °C.Sea water and formation water compositions are shown in Tables 1 and 2.

The solutions of sea water and formation water are mixed with 0.2 wt% of SiO2nanoparticles by an ultrasonic homogenizer.The mean diameter of employed SiO2nanoparticles is about 15–20 nm.

To evaluate the strength of gel and gelation time,the bottle test is recognized as cost-effective and quick method.According to the bottle tests method,various samples of sea water and formation water are prepared by adding definite concentrations of HPAM polymer and different weight ratio of Cr (III)-acetate/HPAM polymer at 100 °C (near to reservoir temperature).The code of gel strength is explained in alphabetic code of A through I based on Sydansk’s strength code as presented in Table 3 [33].Codes ‘‘A”and ‘‘I”indicate no detectable gel and a rigid gel strength respectively.

Table 1 Sea water composition

Table 2 Formation water composition

Table 3 Gel strength based on Sydansk’s code

2.2.2.Core flooding test

For performance evaluation of the synthesized nanocomposite polymer gel in fractured carbonate reservoirs,a core sample of the reservoir with 3.5 cm diameter and 7.5 cm height is prepared.The picture of the fractured core sample is shown in Fig.1.

The core flooding experimental procedure through the fractured carbonate core is as follows:

(1) Before injection of water and nano polymer gel,core sample is vacuumed by vacuum pump to eliminate the excess air of porous media.Also core sample is cut by cutting equipment to simulate the fracture in carbonate core sample in the midpoint.The picture of the fractured core sample is shown in Fig.1.

(2) The fractured core sample is saturated by sea or formation water.

(3) Oil is injected into the fractured core at various flow rates at 100 °C.The relative permeability of oil (kro)1is determined before nano gel injection.

(4) Sea water is injected at different flow rates and the pressure gradients are determined.Therefore before gel injection the relative permeability of water (krw)1is calculated.

(5) Optimum amount of nanocomposite polymer gel solution is prepared and injected into the core sample.

(6) The core flood rig is operated for 20 h at 32 MPa and 100°C.The schematic diagram of experimental core flood setup is shown in Fig.2.

(7) To compute the relative permeability of water and oil after gel injection (krw)2and (kro)2,sea water and oil are injected into the core sample at different flow rates.The experiments have been repeated in the presence of formation water as well.Finally,the residual resistance factor(RRF)is measured and the nano gel performance regarding the decrease in water relative permeability is determined[34].The residual resistance factor is a ratio between the mobility before and after treatment of oil and water.Therefore RRF which is the ratio of oil or water permeability before (kr)1and after(kr)2nano polymer gel injection can be determined according to the following equations:

3.Results and discussion

3.1.Gelation performance

The gallant samples are prepared at different ratio of Cr (III)-acetate/HPAM polymer by using various HPAM polymer concentrations in the range of 0.1 wt%–1 wt% at 100 °C.The gel strength experimental results are displayed in Tables 4 and 5.According to the presented results,the concentration of polymer is a main factor in the gel formation process.According to Tables 4 and 5,a minimum HPAM concentration of 0.75 wt% is required to achieve a moderately deformable non-flowing gel with ‘G’ strength.As shown in the last columns of Tables 4 and 5,a minimum HPAM concentration of 1 wt% is required to keep the gel strength above the‘G’code after 720 h.SiO2nanoparticles with the concentration of 0.2 wt% is employed and the effects of nanoparticle on the gel strength is evaluated.

Fig.1.Fractured carbonate core sample employed in this work.

Fig.2.Schematic diagram of the employed Core-flooding experimental setup.

Table 4 Effect of concentration of polymer(using sea water)on gel strength and gelation time at Cr(III)-acetate/HPAM copolymer ratio of 0.1 and 100 °C

Table 5 Effect of concentration of polymer (using formation water) on gel strength and gelation time at Cr(III)-acetate/HPAM copolymer ratio of 0.05 and 100 °C

3.2.Core flooding results

The results of core flooding experiments through the fractured carbonate core during polymer gel injection are presented in Tables 6–9.These results indicate that the synthesized nanocomposite polymer gel can decrease the water relative permeability of carbonated fractured core sample.According to Table 6,the residual resistance factors (Frr) without presence of nanoparticles(using sea water) for water and oil phases are about 624 and 1.42 respectively.Therefore the residual resistance factor ratio of water/oil for polymer gel injection without nanoparticles using sea water is about 439.

As shown in Table 7,the residual resistance factors upon using the synthesized nanocomposite polymer gel regarding sea water and oil phases are about 1040 and 1.76 respectively.It is interesting to note that the residual resistance factor ratio of water/oil regarding the synthesized nanocomposite polymer gel in this work is enhanced to about 590.9 as compared to the case when no nanoparticle is employed(=439).These values reveal the high performance efficiency of the fabricated nanocomposite polymer gel in this work for enhancing the water shutoff efficiency regarding fractured carbonate reservoirs.

When the formation water is employed,the residual resistance factors (Frr) for water and oil are about 501 and 1.27 respectively(without using nanparticles) such that the residual resistance factor ratio of water/oil is about 394.5 as shown in Table 8.

Table 6 The results of core flooding experiments using the ordinary polymer gel in the presence of sea water

Table 7 The results of core flooding experiments using the synthesized nanocomposite polymer gel in the presence of sea water

Moreover according to Table 9,the residual resistance factors(nFrr) regarding the synthesized nanocomposite polymer gel in the presence of formation water are about 813.5 and 1.567 for water and oil phases respectively.Thus the residual resistance factor ratio of water/oil for the nanocomposite polymer gel injection is enhanced to about 519.14 in the presence of formation water.These results show the high performance of the synthesized nanocomposite polymer gel for reducing the water relative permeability in comparison to the oil relative permeability in fractured carbonate porous media.Therefore the SiO2nanoparticles improve the polymer gel structure and increase the performance of polymer gel even at high temperature and salinity conditions.

Table 8 The results of core flooding experiments using ordinary polymer gel in the presence of formation water

Table 9 The results of core flooding experiments using the synthesized nanocomposite polymer gel in the presence of formation water

As shown in Fig.3,the values of RRFwupon using synthesizednanocomposite polymer gel in this work (in the presence of sea water or formation water)are always higher than those of ordinary polymer gel.It should be noted that the nanocomposite polymer gel have negligible effects on the relative permeability of oil phase as shown in Fig.4.

According to Fig.5,the synthesized nanocomposite polymer gel in this work is always more effective than ordinary polymer gel regarding the water shutoff process in the presence of sea water or formation water.

Also the pressure differential across the core sample considering water and oil injections before and after the nanocomposite polymer gel injection are presented in Figs.6 and 7 respectively.

Fig.3.Residual resistance factors for water.

Fig.4.Residual Resistance factor for oil.

Fig.5.Ratio of residual resistance factors.

Fig.6.Pressure gradients across the core sample before the injection.

Fig.7.Pressure gradients across the core sample after the injection of synthesized nanocomposite polymer gel.

Since the synthesized nanoccomposite polymer gel is considerably hydrophilic,water is absorbed in the polymer gel structure leading to the gel swelling and reduction of water permeability.Thus the water phase transfers hardly through the carbonate core sample such that water pressure gradient across the core sample is much higher than that of the oil phase when the synthesized nanocomposite polymer gel is employed.Application of polymer gel in an oil field regarding a water shutoff project is shown in Fig.8.

As shown there is a sharp decline in oil production rate from 450 bbl to 150 bbl and an increase in water cut in a two years period before polymer gel injection.However employing the polymer gel leads to a significant rise in oil production rate from 150 to 300 bbl/day and a decrease in water cut during 16 months since the start of polymer gel injection suggesting the importance of the aforementioned technique in controlling the excess water production regarding the enhanced oil recovery processes in petroleum industry.

4.Conclusions

A new nanocomposite polymer gel for reduction of water flow in carbonate oil reservoirs is synthesized and examined at high pressure and high temperature.Details description of mathematical modeling regarding the oil and water flow in the porous core media is also presented.Effects of different variable such as temperature,water salinity and concentration of SiO2nanoparticle on the fabricated nanocomposite polymer gel performance are investigated.A significant increase in the residual resistance factor ratio for water/oil leading to high pressure gradient of water phase across the porous core and great reduction in the water phase production is observed.Since the synthesized nanoccomposite polymer gel in this work is considerably hydrophilic,water is absorbed in the polymer gel structure leading to the gel swelling and significant reduction of water phase permeability as compared to the oil phase.The presented results confirm the high performance of the synthesized nanocomposite polymer gel for reduction of the relative permeability of water with respect to oil leading to significant reduction of water flow in carbonate reservoirs and increasing the performance of EOR processes.

Fig.8.Variations of oil production rate and water cut in an oil field before and after polymer gel injection.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.