β-cyclodextrin assists salt-resistance polymer flooding: Injectivity improvement

Shengwang Yuan

a Exploration and Development Research Institute of Daqing Oilfield Company Ltd,Daqing,163712,China

b Heilongjiang Provincial Key Laboratory of Reservoir Physics & Fluid Mechanics in Porous Medium,Daqing,163712,China

ABSTRACT Polymer flooding method has attracted wide attentions in petroleum industry because of its relatively simple process equipment,low cost of agents and good performance on enhancing oil recovery.However,common polymer can hardly meet the technical requirements of the oilfields with high salinity.Existing salt-resistance polymer solutions get a large viscosity in high salinity condition by changing polymer molecules aggregation which,however,may result in the poor adaptability between polymer molecules and the pore sizes of reservoir.β-cyclodextrin (β-CD),a kind of chemical agent,is found to enhance the injectivity of salt-resistance polymer solutions by wrapping the hydrophobic groupings of salt-resistance polymer molecules.For a certain reservoir,the addition of β-cyclodextrin into salt-resistance polymer solution can balance the effect of plugging the layers with high permeability and the purpose of diverting to the layers with low and medium permeability.In this paper,viscosity of polymer solution is tested in condition of different concentrations of β-CD.On that basis,experiments on seepage characteristics are carried out to evaluate the injectivity of salt-resistance polymer solution.Finally,an appropriate concentration of β-CD,which is added into salt-resistance polymer solution,is chosen to adapt a certain reservoir with heterogeneity.

Keywords:Salt-resistance polymer solutions Adaptability β-cyclodextrin Heterogeneity

1.Introduction

Polymer flooding has been playing an important role in enhancing oil recovery in Daqing oilfield [1].After decades of water flooding,polymer solutions with a high concentration are prepared to displace residual oil for their capability of improving mobility ratio between oil and water [2,3].For the reservoirs with serious heterogeneity,as polymer molecules adsorb and retend in the pores of the layers with high permeability,the seepage resistance grows and the injection pressure rises [4,5].As a result,the chase fluids divert to the layers with medium-low permeability,enhancing the ultimate oil recovery [6].However,the high salinity of sewage,which is used to prepare polymer solutions (partially hydrolyzed polyacrylamide),eliminates the repulsion of -COONa in the polymer molecules,reducing the viscosity of polymer solutions [7].Therefore,the effect of polymer flooding gets impaired.In the circumstances salt-resistance polymers are discovered to solve the problem relying on their special molecular structures.Hydrophobically Associating Polymer,a kind of salt-resistance polymers,enhances the salt-resistance property through the association of hydrophobic groupings on the molecular chains [8,9].As the supramolecular aggregations form with the association of hydrophobic groupings,another problem follows that the injectivity of hydrophobically associating polymer is worse [10].As the size of pore throats in the formations makes a selection on the polymer molecule size,such polymer flooding can't achieve a good performance on enhancing oil recovery in the reservoirs with serious heterogeneity [11].Therefore,the adaptability between polymer molecule size and pore throats size needs to be investigated to reach a maximum of the benefits.

The objective of this work is to investigate the adaptability of polymer molecule size and pore throats size.The inspiration that certain chemical agent should be chosen to improve the adaptability is confirmed through a series of experiments.β-cyclodextrin (β-CD)is chosen as the agent for its “hydrophobicity of its internal cavity and hydrophilicity of its external rim” [12,13].The special structure wraps the hydrophobic groupings,thus making it difficult to form a molecular aggregation and a dynamic three-dimensional network structure.When the polymer solution flows through the formation with serious heterogeneity,the addition of β-CD may reduce the polymer molecule size,which increases the possibility of the injection into low-medium layers.Finally,the purpose to balance the adsorption and retention of polymermolecules in the formations with high permeability and the injection into formations with low-medium permeability is realized.

Table 1 Brine salinity analysis (mg/L).

2.Experimental methodology and evaluation methods

2.1.Materials and apparatus

The whole investigation is consisted of three parts.V-Salt resistance polymer,which is supplied by Daqing Refining Company,is chosen as the experimental polymer whose relative molecular weight is 700 × 104.And the solid content of the polymer is 90%.β-CD is produced by Sinopharm Chemical Reagent.Brine used in the experiment is supplied by the 1st oil-production plant of Daqing Oilfield.The ion component is shown in Table 1.The oil used in the oil-displacement experiments is the mixture of crude oil and kerosene,whose viscosity is 9.8 mPa s at the experimental temperature(45 °C).Two kinds of artificial cores are made for displacing experiments.One is a cylinder with the size of Φ2.5 cm × 10 cm.Another is a cube with the size of 4.5 cm × 4.5 cm × 30 cm.

2.2.Chemical analysis

Salt-resistance polymer solution (Cp = 1000 ppm)is added by different concentrations of β-CD (0、0.002%、0.005%、0.01%、0.02%、0.03%、0.04%、0.05%、0.06%、0.065%、0.07%).

Brookfield Viscometer is utilized to measure the viscosity of the polymer solution.Viscosity can quantitatively elucidate the effect of the polymer solution on reducing the mobility ratio between water and oil.As a complement,pictures are taken by scanning electron microscope(SEM)to observe molecule structure more concretely.

2.3.Flooding procedure

On the basis of chemical tests,polymer solutions with five concentrations of β-CD are prepared to continue with the following experiments.

2.3.1.Seepage characteristics

The experiments on seepage characteristics evaluate the mobility property of salt-resistance polymer [14].When polymer solution flows through the pores,there exists an adaptability between polymer molecular size and pore throat size.The external manifestation is the fluctuation of the injection pressure.Meanwhile,the pressure values are also needed to calculate resistance factors and residual resistance factors.Therefore,the pressure must be recorded accurately.Experimental apparatus is composed of wobble pump,cylinders for polymer,brine and crude oil,a fractionator,core holders,a displacement pump and a cabinet that is adjusted at the temperature of 45 °C.The workflow is vividly displayed in Fig.1.And the details of the whole experimental process are given as follows: (i)saturate the artificial cores with brine.The porous volume is obtained by the difference between the weight of cores after saturation and that of cores before saturation.(ii)Flood the cores with the brine to get the absolute permeability.When the pressure is steady,record the value for the follow-up calculations.(iii)Flood the cores with polymer solution.The whole process is conducted at a constant injection rate of 0.3 ml/min.Finally,10 PV of polymer solution is injected into the cores.Pressure data is recorded every 30 min.(iv)Chase water is injected at the rate of 0.3 ml/min.In the meanwhile,pressure data is recorded every 30 min until the pressure keeps steady.

Resistance factor (RF)explains the improvement of polymer on mobility ratio.Due to the adsorption and retention of polymer molecules in the porous media,the reduction of the cores permeability is represented as residual resistance factor (RRF)[15].The two factors are calculated according to the following formulas:

2.3.2.Oil-displacement experiment

The oil-displacement experiments are conducted to simulate the subsurface displacement process of polymer solutions.We observe the pressure changes and the oil-water production.These indexes disclose many details of the development,such as oil recovery,the timing for diversion of fluids and the injectivity of injection fluids.In the experiment,the crude oil,whose viscosity is 9.8 mPa s at the temperature of 45 °C,is provided by the 1st oil-production plant of Daqing Oilfield.On the basis of the seepage characteristics experiment,4 kinds of heterogeneous cores are designed to proceed.The permeability of every layer is shown in Table 2.

After all the materials have been prepared,oil-displacement experiment is conducted as following: (i)all the cores should be evacuated,and saturated with the brine.As a result,the porous volume and porosity can be calculated.(ii)The cores are saturated with crude oil to get the initial oil saturation.(iii)Water flooding is conducted at a constant injection rate of 0.3 ml/min until the water cut reaches 98%.Then the oil recovery after water flooding is calculated.(iv)After the water flooding process,0.5 PV polymer is injected followed by chase water flooding.When the water cut reaches 98% again,the whole process ends up.After the experiments,oil recovery in different flooding stages are obtained.There is an optimization of oil recovery increment for certain heterogeneous cores.

3.Results and discussions

3.1.The influence of β-CD on salt-resistance polymer solution

It is already known that β-CD can wrap the hydrophobic groupings of salt-resistance polymer.As a result,the viscosity of salt-resistance polymer solution decreases with the increase of β-CD concentration(Fig.2 and Table 3).

As shown in Fig.2 and Table 3,when the β-CD concentration differs from 0 to 0.01%,the viscosity of the polymer solution declines drastically by 21.9%.The hydrophobic cavities of β-CD have wrapped most of hydrophobic groupings of salt-resistance polymer molecules.When the β-CD concentration sets in the range of 0.01%-0.065%,the viscosity declines within a smaller extent by 8.0%.After the β-CD concentration is above 0.07%,the viscosity becomes a constant,which indicates that all the hydrophobic groupings have been wrapped.On the basis of the first stage investigation,five concentrations of β-CD,which are respectively 0,0.002%,0.005%,0.01% and 0.065%,are chosen to make further research.

In order to view the influence of different concentrations of β-CD on the structure of salt-resistance polymer,pictures are taken by SEM(Fig.3).

The hydrophobic groupings are distributed on the molecular chains of salt-resistance polymer through the association of inter-chains and intra-chains.Therefore,a larger size of aggregation is formed.At the same time,due to the association of inter-chains among the aggregations,the spatial network structure is formed to make the viscosity larger.With the addition of β-CD,the association of inter-chains suffers from being weakened.As a result,a loose figure of kladodromous structure is presented for salt-resistance polymer molecules.

Fig.1.Schematic of experimental set-up for flooding experiments.

Table 2 Cores with different permeability difference.

Fig.2.Relationship between the viscosity of polymer solution and β-CD concentration.

3.2.Seepage characteristics

Five concentrations of β-CD (0,0.002%,0.005%,0.01% and 0.065%)are chosen to be added into the salt-resistance polymer solution.During the experiment process,pressure is recorded to calculate the resistance factor and residual resistance factor.The results are shown in Table 4.

After the analysis of data,we can tell that when the β-CD concentration is a constant,the resistance factor and residual resistance factor both decline with the increase of cores’ permeability.Polymer solution chooses to flow through the big sized pores because such pores exert less resistance on polymer solution.It indicates that if the porous space is big enough,the polymer solution can flow through the pores smoothly,which reduces the adsorption and retention of polymer molecules.As a result,the development degree of the layers with high permeability will also be undermined.When chase water is injected,due to less polymer molecules in the pores,the core permeability will not decline much.Therefore,the residual resistance factor also becomes smaller.The efficiency of polymer flooding does not reach a maximum.From another aspect,in comparison of two cores with same or similar permeability,resistance factor and residual resistance factor decrease with the increase of the β-CD concentration.That is because of the wrapping function of β-CD on the hydrophobic groupings of salt-resistance polymer.If the polymer molecular size is bigger than pore size,the resistance factor and residual resistance factor are also higher.However,the efficiency of polymer flooding does not meet our expectation.The reason is that the polymer mainly accumulates on the injection end,which weakens the effect of amplifying swept volume.

While analyzing the data,we also summarize the relationship between the PV and the pressure.And the results are shown in Fig.4(a-e).

The trend of the curves really attracts our attention.When the β-CDconcentration is a constant (taking the concentration of 0% as an example),the pressure rises during the whole process of polymer flooding on condition that the permeability is below 490mD.Conversely,the pressure is inclined to reach a stable state during the latter period of polymer flooding.Here we define such a permeability value as permeability threshold,which means that the molecular thread size of polymer is just fit for the pore throat size.We can infer that the injectivity gets enhanced with the addition of β-CD because the permeability threshold goes down from 490mD to 160mD.Such results provide us a guidance on the analysis of oil recovery efficiency of polymer flooding.

Table 3 Relationship between the viscosity of polymer solution and β-CD concentration.

Fig.3.SEM pictures of salt-resistance polymer molecules.

3.3.Analysis on the result of oil-displacement experiment

The oil-displacement experiment is conducted as the procedures that have been introduced in 1.3.2 and the results are shown in Table 5.

As seen in Table 5,the β-CD concentration and the permeability difference exert a synergic influence on the ultimate oil recovery.First,we can compare the oil recovery after water flooding in condition of different cores.As the core heterogeneity gets stronger,the increment of oil recovery after water flooding becomes smaller.Water is more inclined to flow through the layers with high permeability because the flow resistance is smaller.If the core permeability difference is low,the layers with medium and low permeability can be exploited to a certain extent.As the core permeability difference becomes higher,there forms a dominant channel for water in high permeability layer,leaving medium and low permeability layer little exploited.

Second,when the β-CD concentration is a constant,the oil recovery presents a different trend with the variation of permeability difference.As the permeability threshold is 490mD for β-CD concentration of 0%,polymer solution cannot flow through the layer with low permeability except for core 1.Therefore,the increment of oil recovery is the largest for core 1 with better homogeneity.As the β-CD concentration increases,the adaptability between the polymer solution and the pores ofcores gets enhanced (the permeability threshold goes down).As a result,the development degree of the cores with poor heterogeneity becomes better.Therefore,core 2 gets the largest increment of oil recovery for β-CD concentration of 0.002% and 0.005%.For the same principle,core 3 gets the largest increment of oil recovery efficiency for β-CD concentration of 0.010% and 0.065%.

Table 4 Resistance factor and residual resistance factor.

Fig.4.Relationship between injection pressure and PV.

Table 5 Oil recovery results.

Finally,when the permeability difference is a constant (taking core 1 as example),the addition of β-CD reduces the retention of polymer molecules in the pores,weakening the effect of fluid diversion.As the permeability difference is larger,the layers with low permeability and medium permeability make a stricter selection on the polymer solution with different β-CD concentrations.When the permeability difference is 3.75 (core 2),polymer solution with β-CD concentration of 0.002% can guarantee the retention of polymer molecules and the swept volume.It is the principle that can elucidate the trend of oil recovery by salt-resistance polymer with β-CD.

4.Conclusions

(1)The addition of β-CD changes the shape of salt-resistance polymer molecules and thus enhances the injectivity of salt-resistance polymer solution.

(2)When the β-CD concentration is 0,0.002%,0.005%,0.01% and 0.065%,the viscosity of the salt-resistance solution is respectively 36.4 mPa·s,34.0 mPa·s,31.2 mPa·s,28.6 mPa·s,and 26.3 mPa·s.The permeability threshold is respectively 490mD,345mD,230mD,195mD and 160mD.

(3)There exists an optimized β-CD concentration for cores with different permeability difference to balance the adsorption and retention of polymer in the formations with high permeability and the injection into formations with low-medium permeability.