Yong Yang ,Xiangliang Li,Ping Guo ,Yayun Zhuo ,Yong Sha ,*

1 Research Institute of Exploration and Development,Shengli Oil field Company,SINOPEC,Dongying 257000,China

2 College of Chemistry and Chemical Engineering,Xiamen University,Xiamen 361000,China

1.Introduction

The CO2flooding is one of the most important methods of the enhanced oil recovery(EOR)process.In the CO2flooding,CO2is injected downward into the subterranean reservoir of crude oil,and it can drive crude oil out of the reservoir formation.In recent years,application and development of the CO2flooding grows rapidly since it has a high oil recovery efficiency.Moreover,the CO2flooding can consume a large amount of CO2,and it can effectively reduce CO2emission and the greenhouse effect[1,2].The mechanism of the CO2flooding is that CO2can be either totally or partially miscible with crude oil at the certain temperature and pressure,and this leads to expansion of crude oil volume,decrease of crude oil density and reduction of crude oil viscosity.As a result,recovery of crude oil can be improved greatly[3].

Pressure is an important factor in the CO2flooding.Usually the miscibility between CO2and crude oil increases while pressure increases,and CO2at a higher pressure can result in a higher oil recovery[4-6].While the reservoir pressure is less than the minimum totally miscible pressure(MMP)of CO2and crude oil,CO2and crude oil become immiscible or partially miscible.In this situation,the CO2flooding efficiency is poor,and it is much less than that of the totally miscible system[7,8].It is almost impossible to change the reservoir pressure.Therefore,more CO2might be injected into the subterranean reservoir in order to achieve certain oil recovery.However,due to the low viscosity of CO2fluid in the reservoir,injection of more CO2may result in viscous fingering and low sweeping efficiency[9,10],and it has no obvious effect on improvement of the oil recovery.Therefore,it is necessary to develop an effective method which can improve the miscibility between CO2and crude oil as well as the oil recovery of CO2flooding while CO2and crude oil are immiscible or partially miscible.

In chemistry industry,in order to improve extraction capability of CO2,supercritical CO2is often utilized with joint application of some chemical modifiers such as alcohols.This method is applied widely,and it is verified to have good extractive efficiency and economic benefit[11-14].Similarly,it is also feasible to improve extraction ability of CO2for crude oil by means of chemical modifiers[15-19].In previous studies,it was found that CO2extraction accompanied with the chemical modifier can yield crude oil extracts almost 3 times over the CO2extraction only[17].So far,most studies focused on chemically modified CO2extraction of crude oil,but there was no work about chemically modified CO2flooding displacement.In addition,it is well known that polar chemical modifiers,such as methanol,can improve the extraction ability of CO2for polar crude oil components such as asphaltic.However,most of components in crude oil are nonpolar,so it is possible that addition of nonpolar modifiers with CO2could significantly influence the phase equilibrium of the CO2-crude oil system[20].As a result,it could improve the miscibility of CO2and crude oil as well as the CO2flooding efficiency.For this purpose,it is worthy of investigating influence of nonpolar chemical modifiers on the CO2flooding.

In this work,by means of four different nonpolar chemical modifiers,under the subterranean temperature-pressure condition,a series of experiments were conducted to evaluate the capability of chemical modifiers in enhancing miscibility of CO2and crude oil as well as oil recovery of the CO2flooding.

2.Experimental

Two kinds of experiments were conducted in this work.At first,by means of a PVT phase equilibrium device,experiments were conducted to evaluate the miscibility of different chemicals with CO2at the high temperature and high pressure,and the capability of chemicals to enhance the miscibility between CO2and crude oil was evaluated.Then,the crude oil slim-tube displacement experiments with pre-slug CO2injection and joint CO2injection were carried out.The displacement experiment can simulate the oil recovery in the subterranean pore,and it can evaluate the integrative influence of modifiers on improving CO2extraction for oil,CO2dissolution into oil,expansion of the oil volume,decrease of the oil viscosity as well as mobility of oil in the subterranean pore.In comparison with usual CO2extraction experiment of crude oil,which focuses on CO2extraction for oil,the crude oil slimtube displacement experiment is more close to the actual situation.

2.1.Materials

The 99.99%purity CO2was supplied by the Linde Group,China.Toluene,benzene,methyl carbonate(DMC)and diethyl carbonate(DEC)were analytically pure,and they were provided by Sinopharm Chemical Reagent Co.Ltd.The crude oil came from#Fan-142-9-5 oil well of Shengli Oil field in China.It had little Asphalt and a high paraffin content.

2.2.Apparatus description

The experimental apparatus included a PVT phase equilibrium device and a slim-tube oil displacement test device.

The miscibility evaluation experiment was carried out in the PVT phase equilibrium device as shown in Fig.1.The device mainly consisted of a high pressure phase equilibrium tank,a booster pump,a pressure gage,a sample collector and a gas mass flow meter.The maximum operation pressure of this device was 30 MPa,and the maximum operation temperature was 423 K.The tank temperature was controlled by an oil bath heater.The tank was 0.2 m high,and its effective volume was 0.21 L.A pair of glass windows was sealed in two sides of the tank in order to observe the inner gas-liquid state,and a piston was set on the top of the tank to control the pressure by means of changing the tank volume.In addition,at the bottom of the tank a magnetic stirring was installed to shorten the period of achieving phase equilibrium.

Fig.1.The PVT phase equilibrium device.(1)CO2 tank,(2)modifier tank,(3)oil tank,(4)booster pump,(5)modifier pump,(6)buffer tank,(7)phase equilibrium tank,(8)pressure gauge,(9)liquid sample bottle,(10)gas sample bottle,(11)mass flow meter.

The crude oil displacement experiments were carried out in the slim-tube displacement test device as shown as Fig.2.The maximum operating pressure and temperature of this device were 70 MPa and 453 K respectively.The length of the slim-tube was 16 m,and the inner diameter of the slim-tube was 6.35 mm.The tube was packed with silica sands which had 230-310 mesh size and 32.25%porosity.With respect to the packed tube,the pore volume was 0.165 L,and the gas permeability was less than 10 μm3.

2.3.Experimental procedure

2.3.1.Procedure of miscibility evaluation experiments

In order to evaluate the miscibility of CO2and different chemicals,at first the tank in Fig.1 was flushed with CO2at a speed of 0.2 L·min-1to clean air,then the certain quality of pure chemical was fed into the phase equilibrium tank.Afterwards,with the magnetic stirring and the oil bath heating,CO2was continuously fed into the tank via the booster pump until the required pressure was achieved.At the experimental pressure and temperature,chemical and CO2interacted for 2 h.If it was observed that the chemical was not completely miscible with CO2,then more CO2was fed to raise the pressure by the step of 1 MPa.The above steps were repeated until the chemical and CO2was totally miscible.

To investigate the ability of chemicals in enhancing miscibility of CO2and crude oil,the tank was flushed with CO2at a speed of 0.2 L·min-1to clean air,then the certain quality of crude oil was fed into the tank.At the experimental pressure and temperature,oil and CO2interacted during 4 h to ensure that the phase equilibrium was reached,and then the gas and liquid phase were sampled respectively.At last,the certain quality of chemical was injected into the liquid phase of the tank by the modifier pump,and the gas and liquid phase were sampled again respectively after phase equilibrium was reached.The results with and without chemical mixing can be compared to evaluate the ability of chemicals in enhancing miscibility.

2.3.2.Procedure of crude oil displacement experiments

Before experiment,the slim tube packed with silica sands was cleaned by toluene and dried by air,then its porosity and permeability were measured.The slim tube was saturated with the crude oil at first by the displacement pump.When the system achieved the experimental temperature and pressure,CO2was injected at a speed of 0.17 cm3·min-1to displace crude oil in the tube.With respect to the pre-slug injection,CO2and a chemical were simultaneously injected for 1.5 h from the beginning of the experiment.After 1.5 h,chemical injection stopped,and CO2injection continued.The crude oil which was driven out of the slim tube by CO2was collected and weighed every 15 min,and the experiment was ended until the injected CO2volume reached 1.2 Pore Volume(PV),which means a volume of 0.198 L.In this configuration,there was a CO2and chemical mixing pre-slug with a length of about 1 m during the experiment.

The procedure of the crude oil displacement experiment with CO2joint injection was similar to the pre-slug injection,the only difference was that chemical modifier was continuously injected with CO2during the whole experiment process.The experiment was still ended while the injected CO2volume reached 1.2 PV,i.e.0.198 L.

3.Results and Discussion

3.1.Evaluation of chemical modifiers

3.1.1.Miscibility of chemicals and CO2

Fig.2.The slim tube displacement test device.(1)displacement pump,(2)oil tank,(3)gas tank,(4)slim tube,(5)window,(6)valve,(7)oven,(8)split bottle,(9)gas mass meter.

The experiment was conducted in the condition of 416 K which was the temperature of crude oil in the subterranean reservoir,and the quality of chemical fed into tank was 10 g.It meant a mass fraction of about 10 wt%chemical in the CO2system.Under experimental conditions,the totally miscible pressures of different chemicals and CO2were shown in Table 1.As an example,the toluene-CO2system states under different pressures were shown in Fig.3.

Table 1 The totally miscible pressure of different chemicals with CO2(416 K)

Fig.3.The toluene-CO2 system states at(a)15.04 MPa,(b)15.85 MPa,(c)17.85 MPa,(d)18.29 MPa.

As shown in Fig.3,the miscibility of CO2and toluene increased when pressure rose at 416 K.CO2and toluene became totally miscible at 18.29 MPa since the gas and liquid phase got miscible totally.Other chemicals performed similar to toluene,and the totally miscible pressures of chemicals and CO2were listed in Table 1.It can be concluded that all of the four chemicals were well miscible with CO2at certain pressure,so all of four chemicals were utilized to evaluate their ability of enhancing miscibility of CO2and crude oil.

3.1.2.The ability of chemicals in enhancing miscibility of CO2and crude oil

The experiment was conducted in the condition of 416 K and 20 MPa,and the amount of crude oil and chemical modifier was 20 g and 5 g respectively.The amount of 5 g chemical means approximately 5 wt%of chemical modifier in the CO2-chemical mixture system.In this case,chemical modifiers can be completely miscible with CO2.Samples from the gas and liquid phase were weighed to calculate the mass of oil in the gas sample and the mass of CO2in the liquid sample.For convenience,the mass fraction of oil in the gas sample was expressed by woil,g,and the mass fraction of CO2in the liquid sample was expressed by wCO2,l.For the purpose of comparing ability of chemicals in enhancing miscibility of CO2and crude oil,two dimensionless parameters were introduced,which were vaporization-enhancing indicator(VI)and solubilization-enhancing indicator(SI)as defined below.

where superscript0and*represents experimental results without and with addition of chemical modifiers respectively.

Actually VI represented the enhancing factor of crude oil amount extracted into CO2after addition of chemical modifier,and SI represented the enhancing factor of CO2amount dissolving in crude oil after addition of chemical modifier.In general,the performance of chemical modifier increases with growth of SI and VI,so the chemical modifiers with both large SI and VI should be recommended.

As shown in Table 2,in four chemicals,VI and SI of toluene were biggest,and it meant that the ability of toluene in improving the miscibility of CO2and crude oil was the best.For other chemicals,though VI of benzene was larger than DMC and DEC,its SI was quite small.It meant that benzene can enhance extraction of crude oil into CO2effectively,but it had little effect on dissolving of CO2into crude oil.Therefore,it was an unsuitable chemical modifier in comparison with toluene.The performance of DMC was quite similar to DEC.Both of them performed poorly,and it meant that DMC and DEC were also unsuitable.Fig.4 shows phenomena in the phase equilibrium tank with and without toluene.Crude oil swelled obviously after addition of toluene,and it was the direct visual evidence that toluene can increase the solubility of CO2in crude oil.Meanwhile,the change of phenomena caused by other chemicals was hardly observed,and this was consistent with the results shown in Table 2.

Table 2 VI and SI of different chemical modifiers

In general,toluene was the only suitable chemical in four chemical modifiers since VI and SI of toluene were much bigger than others.The reason of this result is probably that the polarity of toluene is the smallest in these chemicals.Based on this conclusion,the following experiments focused on toluene only.

Fig.4.Phenomena in the phase equilibrium tank(a)without toluene,(b)with toluene.

3.2.Enhancement of chemical in oil recovery

In the crude oil displacement experiment,oil recovery of the CO2flooding was defined as below.

The mass of saturated oil was the initial mass of crude oil in the slim tube.The mass of oil recovered was the mass of crude oil which was driven out of the slim tube by CO2,and it was actual recovered amount of crude oil.In this work,experimental pressure and temperature were set to 20 MPa and 416 K according to the condition of#Fan-142-9-5 oil well.

Fig.5 represents the change of oil recovery with the increase of PV while curves of different color represents different concentrations of toluene in joint injection.It showed that oil recovery achieved almost maximum when PV equaled to 1.2 in the case without addition of toluene.However,with addition of toluene,oil recovery can be improved effectively.The oil recovery was 61.1%while pure CO2was utilized,and it can increase to 64.6%,67.6%and 71.0%when the concentration of toluene in the toluene-CO2mixture fluid was 2 wt%,4 wt%and 8 wt%respectively.Obviously the increase of the concentration of toluene in the toluene-CO2mixture fluid can improve the final oil recovery.With joint addition of toluene,crude oil in the slim tube was recovered actually by the mixture of CO2and toluene.As shown in Fig.5,toluene can enhance miscibility of CO2and crude oil,and the mixture of CO2and toluene can drive more residue oil out of the slim tube even after 1.2 PV in comparison with the case with CO2only.Therefore,as shown as trend of oil recovery curves in Fig.5,oil recovery can still increase even after 1.2 PV.However,joint injection of toluene needed a big consumption of toluene because toluene was injected during the whole oil recovery process.Therefore,pre-slug injection of toluene was carried out in order to investigate its effects on oil recovery of the CO2flooding.

Fig.5.Oil recovery with joint injection of toluene.

Fig.6 shows results of oil displacement experiments with pre-slug injection.Similar to Fig.5,the oil recovery increases with the increase of the toluene concentration in the pre-slug.The oil recovery with pure CO2was 61.1%,and the oil recovery increased to 67.3%,73.3%,81.3%and 83.6%while the concentration of toluene in the pre-slug was 8 wt%,16 wt%,22 wt%and 29 wt%,respectively.Oil recovery with 22 wt%toluene in the pre-slug was very close to that with 29 wt%of toluene.It suggested that the oil recovery had the maximum with the increase of the toluene concentration,and much more injection of toluene was useless.It should be noted that toluene only existed in the initial about 1 m long pre-slug section.The pure CO2fluid followed this pre-slug section,and drove this pre-slug to go through the slim tube.Oil recovery conducted in this configuration resulted from joint action of the initial pre-slug section and following pure CO2section.Though the concentration of toluene in the pre-slug was high,the total consumption of toluene was less than that in the joint injection because toluene was continuously injected in joint injection operation.

Fig.6.Oil recovery with pre-slug injection of toluene.

4.Conclusions

Results from this work suggested that CO2flooding with addition of chemical modifiers can lead to a great increase of oil recovery.Chemical modifiers appeared to have the capability to substantially improve economics of CO2flooding.CO2modified with toluene was found to be better miscible with crude oil than pure CO2.With addition of toluene,the extraction of oil into CO2and dissolution of CO2into oil had the increment of 251%and 64%respectively.By means of pre-slug injection with addition of high toluene concentration,oil recovery of CO2flooding can increase by 22.5%in comparison with CO2only.CO2flooding modified with toluene can effectively expand application of CO2flooding,especially while CO2and crude oil are immiscible or partially miscible at the subterranean reservoir condition and pure CO2flooding has low oil recovery.With respect to different injection methods,the pre-slug injection was recommended because it consumed less toluene than joint injection.Results from this work could be useful to further development and application of the CO2flooding in oil recovery as well as CO2emission reduction.

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