Experimental Investigation of CO 2-Water-Rock Interactions during CO 2 Flooding in Carbonate Reservoir

Injecting CO2 into underground reservoir to displace oil is a viable means of reducing greenhouse gas emission to the atmosphere and enhancing oil recovery. To evaluate the effect of CO2-water-rock interactions on the characteristics of carbonate reservoir at high pressure, the mineralogy of calcite, the ion concentration in the reacted solution, the surface texture of calcite, the permeability of calcite after reacted with injected CO2 and deionized water was investigated by X-ray diffraction (XRD), inductive coupled plasma-atomic emission spectrometry (ICP-AES), scanning electronic microscope (SEM), and sand-packed model at pressure of 5.0 MPa. The results show that the mineral dissolution of calcite would occur when interacting with injected CO2 and water. The mineral dissolution of calcite caused the change of surface texture of calcite and increase in 2 Ca + , 3 HCO − ion concentration in the solution. With the increase of CO2 pressure, the surface dissolution of calcite appeared more obvious. With the increase of reaction temperature, the surface dissolution of calcite also appeared more obvious and 2 Ca + , 3 HCO − ion concentration in the solution increased first, then decreased. The mineral dissolution of calcite caused the improvement in water permeability of calcite/quartzsand-packed model.


Introduction
The emission of carbon dioxide (CO 2 ) is increasing natural greenhouse gas ef- fects.It has been achieved a broad consensus to reduce CO 2 emissions on a global scale.One of technological solutions to reduce CO 2 emissions is to inject CO 2 into underground reservoirs to displace oil, which can enhance oil recovery effectively and sequestrate CO 2 simultaneously, mitigate global warming consequently [1] [2].CO 2 flooding will yield remarkable social and economic benefits [3] [4].
The CO 2 -water-rock interactions caused by CO 2 flooding of reservoirs are complex [5] [6] [7].These interactions are highly reservoir specific and cannot easily be generalized [8] [9].Injected CO 2 dissolved in formation water to produce carbonic acid which will react with the reservoir rock [10] [11] [12].The porosity and permeability of the reservoir [13] [14] [15] [16] [17], the wettability of the rock surface and the characteristics of the crude oil will be changed by the reaction between carbonic acid and the rock [18] [19], which will affect the efficiency of CO 2 flooding and geological sequestration of CO 2 [20] [21] [22].And the carbonate minerals in the reservoir can especially easily react with CO 2 and water to affect reservoir property.Carbonate reservoir is mainly composed of calcite, dolomite, ankerite and magnesite, and these minerals can easily dissolve in carbonate solution which leads to corrosion of the rock and increase of reservoir permeability.Ross [23] found that after the reaction of CO 2 , water and rock with carbonate mineral, carbonate minerals in the rock dissolved, and it formed a large number of secondary dissolution channels.Ross also analysed ion concentration of the displacement solution and found that the Ca 2+ concentration in the solution increased significantly.Izgec [24] pointed out that a large number of loop hole formed in the process of the dissolution of carbonate mineral, it obviously improved the connectivity of pore, and so caused the permeability improved significantly.Raistrick [25] found that after the reaction of carbonate sample, CO 2 and water, the concentration of Ca 2+ , Mg 2+ and 3 HCO − in effluent solution increased significant which indicating the carbonate mineral dissolution.In addition, some scholars also pointed out that the change of physical property of carbonate reservoir was related to formation water pressure, temperature, and rock mineral composition and CO 2 partial pressure [26].
Therefore, in order to carry further study on the main controlling factors which result in the change of reservoir physical properties in the process of CO 2 flooding in carbonate reservoir, calcite (main ingredient is CaCO 3 ) was selected to react with CO 2 and water in this paper.Then, the surface texture of calcite, the ion concentration in the reacted solution and the permeability of calcite/quartz packed model were investigated after their reaction.The mechanism of interaction of CO 2 -water-calcite was discussed, which provided the valuable reference for CO 2 flooding and geological sequestration of CO 2 in the carbonate reservoir.

Materials and Preparation of Sample
Two types of calcite materials were used in the study, the plate and the grained.
The plate samples were made by cutting calcite material into 8 mm sized blocks, washed with dionized water by ultrasonic vibration, dried at 70˚C for 12 h.The calcite material was crushed to grains and selected the grains with average diameter of 0.45 mm.The calcite material was provided by Shijiazhuang Cuanshi mining company, in China.The purity of CO 2 source provided by Beijing Haipubeifen gas limited company was >99.95%.The water used in the experiments was deionized.

1) Calcite component analysis
The clay mineralogy of the calcite was identified and quantified by X-ray diffraction (XRD) (D/MAX 2500, Rigaku Industrial Corporation, Japan) which can determine the mineral composition and content in calcite.3) The static evaluation method of CO 2 -water-rockchemical interaction The chemical interaction of CO 2 -water-rock was identified by exposing the rock samples in a stainless steel reactor of 100 ml (Figure 1), equipped with a Teflon (PTFE) internal cup, manometer and automatic temperature controller, connected to an electric heating apparatus.The sheet calcite sample was put into the bottom of the high pressure reactor, and 100 mL deionized water was added into the high pressure reactor, then the CO 2 was injected into it at specific pressure and temperature.After 20 days' reaction, the calcite slice was took out and dried.

4) Aqueous sampling and ion concentration determination
The grained calcite was mixed with deionized water by ratio of 1:20, w:w, and

Composition Analysis of Calcite
The calcite used in the experiments is white.The crystal structure of calcite was shown in Figure 2, an obvious rhombus structure.According to the formation of the cubic packing arrangement form,

Surface Texture of Calcite after Reacted with CO2
In order to investigate the dissolution of rock caused by CO 2 -water-rock interactions, the surface of calcite slice dissolved by CO 2 was observed by scanning electron microscope after the calcite slice reacted with CO 2 under different pressure and temperature.The experiment results were shown in Figure 4 and Figure 5.   changed obviously after reaction with CO 2 and water.At 30˚C, with the increase of CO 2 pressure, honeycomb cave phenomenon of the calcite is more obvious caused by the reaction of CO 2 .At the pressure of 0.5 MPa, the surface of calcite appeared only a little corrosion phenomenon caused by the reaction of CO 2 .As the pressure rose to 10 MPa, the surface of calcite appeared obvious corrosion phenomenon, namely the morphology of calcite changed more obvious.This is mainly because the reaction of CO 2 and calcite is greater with the increase of CO 2 pressure, the surface corrosion of calcite appeared more obviously.This result is consistent with the study by Gilfillan and Tang et al. [27] [28] [29].
2) Surface texture of calcite before and after the interactions of CO 2 -watercalcite at different temperature The change of the calcite morphology in different temperature at 2 MPa was shown in Figure 5.As shown in Figure 5, at the temperature of 60˚C and 90˚C, the reaction of CO 2 and calcite is greater than which was at the temperature of 30˚C, and the surface corrosion of calcite appeared more obviously.After the reaction of CO 2 and calcite at 60˚C, many large pores appeared in the calcite surface, namely the calcite was dissolved more.

Ion Concentration in the Solution after Reacted with CO2
The main reason for the changes of the surface morphology of calcite is carbonic acid which is produced when CO 2 dissolved in the water.Carbonic acid dissociated hydrogen ion which can react with calcium carbonate in calcite and pro-duced calcium soluble bicarbonate.That is CO 2 -water-dolomite interactions lead to calcite corrosion.The chemical reaction formula of CO 2 -water-calcite interactions is shown in formula (1).
CaCO H O CO Ca 2HCO There was Ca 2+ ion in the solution after calcite dissolution and the dissolution trace was shown in the surface of the calcite.For proving that, the major Ca 2+ and 3 HCO − concentration of the solution after having reacted with calcite and CO 2 at different pressure and temperature were investigated.The results were shown in Figures 6-9.The type of calcite was grained in this experiment [30] [31] [32].
1) Ion concentration in the reacted solution after the interactions of CO 2 -water-calcite at different pressure Figure 6 and Figure 7 shows that with the increase in reaction time, Ca 2+ and 3 HCO − ion concentration in the solution are also increased; with the increase of reaction pressure, Ca 2+ and   at the pressure of 0.5 MPa, 2 MPa and 5 MPa, but the increment is small.Ca 2+ and 3 HCO − ion concentration in the solution are increased obviously at the pressure of 10 MPa.This is mainly due to the reaction pressure of 10 Mpa is higher than the critical pressure of CO 2 , so the reaction condition is close to supercritical condition, the diffusivity and permeability CO 2 are stronger, which speed up the reaction, and lead to the significant increase of Ca 2+ and 3 HCO − concentration in the solution.
The major chemical component of calcite is CaCO 3 which reacted with carbonic acid to cause the dissolution of calcite.Consequently, Ca 2+ and 3 HCO − ion concentration increased.The variation of Ca 2+ and 3 HCO − ion concentra- tion also proved the dissolution of calcite after the interaction of CO 2 -water-rock which led to forming the new surface texture of the calcite.
2) ion concentration in the reacted solution after the interactions of CO 2 -water-calcite at different temperature Figure 8 and Figure 9 shows the changes of Ca 2+ and 3 HCO − ion concentra- tion in the solution with the reaction temperature at 30˚C, 60˚C, 90˚C respectively.
Figure 8 shows that with the increase in reaction time, Ca 2+ ion concentration     maximum after reacted with CO 2 for 2 days at 90˚C, and then decreased with the increase in the reaction time.
3 HCO − ion concentration in the solution showed the similar trend.The final Ca 2+ ion concentration and 3 HCO − ion concentra- tion at 90˚C ware all lower than those at 60˚C.This is because when the temperature is higher than 50˚C, the dissociation constant of carbonic acid decreases with the increase of temperature [33].So the H + ion concentration in the solution is reduced, the effective H + ion concentration which can react with CaCO 3 is also reduced, which lead to the less corrosion of CaCO 3 , lower concentration of Ca 2+ ion and 3 HCO − ion in the solution.

Water Permeability of Sand-Packed Model after Reacted with CO2
The dissolution of calcite to Ca(HCO 3 ) 2 in the solution under the interaction of CO 2 -water-rock would affect the permeability of reservoir rock.Figure 10 shows that the change of water permeability of sand-packed model filled with mixed grained calcite and quartz under the presence of CO 2 and water at 65˚C.Before injecting gas, the water permeability of the sand-packed model remained relatively stable during placing for 3 days.After injecting CO 2 , the water permeability of

Conclusions
1) The mineral dissolution of calcite would occur when interacting with injected CO 2 and water, and the dissolution caused the change of surface texture of calcite.
2) The surface dissolution of calcite appeared more obvious with the increase of pressure when reacting with injected CO 2 and water.
3) The dissolution of calcite caused the increase in Ca 2+ , faster at higher reaction temperature.4) When the reaction temperature is lower, the dissolution of calcite increases with the increase of temperature.When the reaction temperature reaches up to a certain value, the dissociation constant of carbonic acid decreases which leads to less dissolution of calcite.
5) The dissolution of calcite caused improvement in water permeability of sand-packed model filled with mixed grained calcite and quartz.

2 )
Determination of the rocks' apparent morphology SEM measurements were carried out on a Leica Cambridge S-360 (Malvern Instruments Ltd., UK) at 25˚C.The acceleration voltage was 20 kV.The resolution of the instrument was 5 nm.The vacuum of the sample room was 1.33 × 10 −3 -1.33 × 10 −4 Pa.First, the calcite was cut and polished into 20 × 20 mm sheet, and the surface of it was cleaned with ultrasonic waves in deionized water.Then the calcite was dried and sticked on specimen holder by conductive adhesive, and it was coated by means of ion sputtering at last.The morphology of coating calcite slice was observed by scanning electron microscope, and then it was placed in the high pressure reactor.After the reaction, the calcite slice was took out and dried, the newly formed or altered surface texture in the reacted rock sample was investigated by SEM again.

then CO 2
was injected into the reactor to 2.0 MPa.The reacted liquid was sampled every 2 days during reacting for 20 days.The major cation concentrations of the sampled solution were determined by inductive coupled plasma-atomic emission spectrometry, ICP-AES (Proflie, Leeman Labs, USA).The resolution of the instrument is 200 nm.The 3 HCO − concentration of the sampled liquid was determined by double-tracer technique.

2 3 CO
− create a diamond surface grid.Most calcite crystal is colorless, transparent.It has smooth plane and straight edge, it aggregates in a variety of forms: flake, fibrous, dense block, membrane and drusy, etc.The result of X-ray diffraction of the calcite was shown in Figure 3. Compared with JCPDS (Joint Committee Powder Diffraction Standard), the major chemical component of the calcite was CaCO 3 .Based on quantitative calculation of intensity and half band width of the diffraction peak, CaCO 3 accounted for 97.8%, so the calcite crystal had high quality.

1 )
Surface texture of calcite before and after the interactions of CO 2 -watercalcite at different pressure The change of the calcite morphology in different pressure and 30˚C was shown in Figure4.As shown in Figure4, the surface morphology of calcite

Figure 4 .
Figure 4. SEM micrographs of calcite samples that pre-and post-reacted with CO 2 under different pressures.

Figure 5 .
Figure 5. SEM micrographs of calcite samples that pre-and post-reacted with CO 2 in different temperature.

3 HCO−
ion concentration vs. reaction time at different reaction pressure.

Figure 8 .
Figure 8. Ca 2+ ion concentration vs. reaction time at different reaction temperature.

Figure 9 shows that the change trend of 3 HCO 3 HCO 3 HCO
Figure 9 shows that the change trend of

3 HCO−
ion concentration in the reacted solution was higher at 60˚C than the ion concentration in the solution at 30˚C at the same reaction time.The results show that the reaction rate was increased and the reaction reached the equilibrium much faster at higher reaction temperature.

Figure 8
Figure 8 also shows that Ca 2+ ion concentration in the solution reached a

3 HCO−
ion concentration vs. reaction time at different reaction temperature.