Injecting CO 2 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 CO 2-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 CO 2 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 CO 2 and water. The mineral dissolution of calcite caused the change of surface texture of calcite and increase in Ca 2+, HCO - 3 ion concentration in the solution. With the increase of CO 2 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 Ca 2+, HCO - 3 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.
The emission of carbon dioxide (CO2) is increasing natural greenhouse gas effects. It has been achieved a broad consensus to reduce CO2 emissions on a glo- bal scale. One of technological solutions to reduce CO2 emissions is to inject CO2 into underground reservoirs to displace oil, which can enhance oil recovery effectively and sequestrate CO2 simultaneously, mitigate global warming consequently [
The CO2-water-rock interactions caused by CO2 flooding of reservoirs are complex [
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 CO2 flooding in carbonate reservoir, calcite (main ingredient is CaCO3) was selected to react with CO2 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 CO2-water-calcite was discussed, which provided the valuable reference for CO2 flooding and geological sequestration of CO2 in the carbonate reservoir.
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 CO2 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.
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.
3) The static evaluation method of CO2-water-rockchemical interaction
The chemical interaction of CO2-water-rock was identified by exposing the rock samples in a stainless steel reactor of 100 ml (
4) Aqueous sampling and ion concentration determination
The grained calcite was mixed with deionized water by ratio of 1:20, w:w, and then CO2 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
5) Permeability test
Steady state permeability variation caused by the presence of deionized water and CO2 was analyzed on a 30 cm sand-packed model which was a sand pipe filled with grained calcite and quartz with the same diameter mixed by a ratio of 1:1. The original permeability of the model was presented by water permeability. Having placed for 3 days, water permeability of the sand-packed model was measured again. Then CO2 was injected into the model to 5.0 MPa. After that the water permeability of the model was measured every 5 days. The test temperature is 65˚C.
The calcite used in the experiments is white. The crystal structure of calcite was shown in
In order to investigate the dissolution of rock caused by CO2-water-rock interactions, the surface of calcite slice dissolved by CO2 was observed by scanning electron microscope after the calcite slice reacted with CO2 under different pressure and temperature. The experiment results were shown in
1) Surface texture of calcite before and after the interactions of CO2-water- calcite at different pressure
The change of the calcite morphology in different pressure and 30˚C was shown in
changed obviously after reaction with CO2 and water. At 30˚C, with the increase of CO2 pressure, honeycomb cave phenomenon of the calcite is more obvious caused by the reaction of CO2. At the pressure of 0.5 MPa, the surface of calcite appeared only a little corrosion phenomenon caused by the reaction of CO2. 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 CO2 and calcite is greater with the increase of CO2 pressure, the surface corrosion of calcite appeared more obviously. This result is consistent with the study by Gilfillan and Tang et al. [
2) Surface texture of calcite before and after the interactions of CO2-water- calcite at different temperature
The change of the calcite morphology in different temperature at 2 MPa was shown in
The main reason for the changes of the surface morphology of calcite is carbonic acid which is produced when CO2 dissolved in the water. Carbonic acid dissociated hydrogen ion which can react with calcium carbonate in calcite and produced calcium soluble bicarbonate. That is CO2-water-dolomite interactions lead to calcite corrosion. The chemical reaction formula of CO2-water-calcite interactions is shown in formula (1).
There was Ca2+ ion in the solution after calcite dissolution and the dissolution trace was shown in the surface of the calcite. For proving that, the major Ca2+ and
1) Ion concentration in the reacted solution after the interactions of CO2-wa- ter-calcite at different pressure
at the pressure of 0.5 MPa, 2 MPa and 5 MPa, but the increment is small. Ca2+ and
The major chemical component of calcite is CaCO3 which reacted with carbonic acid to cause the dissolution of calcite. Consequently, Ca2+ and
2) ion concentration in the reacted solution after the interactions of CO2-wa- ter-calcite at different temperature
in the solution was improved at 30˚C and 60˚C. But the extent of the change of the concentration at different reaction temperature was different. Ca2+ ion concentration in the solution remained unchanged after reacted for 8 days at 30˚C. It took 6 days to reach a plateau of Ca2+ ion concentration in the solution at 60˚C. Ca2+ ion concentration in the reacted solution was higher at 60˚C than the ion concentration in the reacted solution at 30˚C at the same reaction time.
maximum after reacted with CO2 for 2 days at 90˚C, and then decreased with the increase in the reaction time.
The dissolution of calcite to Ca(HCO3)2 in the solution under the interaction of CO2-water-rock would affect the permeability of reservoir rock.
the model increased with the increasing of the reaction time. The reason caused the results above was that the grained calcite (CaCO3) in the sand pipe reacted with injected CO2 and water, and partial dissolution of the calcite occurred, which led to the appearance of the larger pores and the increase in the permeability of the model. Therefore, the porosity and permeability of reservoir containing the chemical component of CaCO3 will be likely increased due to the mineral dissolution of CaCO3 during CO2 flooding, which will affect the efficiency of CO2 flooding.
1) The mineral dissolution of calcite would occur when interacting with injected CO2 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 CO2 and water.
3) The dissolution of calcite caused the increase in Ca2+,
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.
This project was supported by National Natural Science Foundation of China (41302096) and CNPC Science & Technology Innovation Foundation Project (2015D-5006-0206).
Xiao, N., Li, S. and Lin, M.Q. (2017) Experimental Investigation of CO2-Water-Rock Interactions during CO2 Flooding in Carbonate Reservoir. Open Journal of Yangtze Gas and Oil, 2, 108-124. https://doi.org/10.4236/ojogas.2017.22008