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Experimental investigation on the operation parameters of carbon dioxide huff-n-puff process in ultra low permeability oil reservoirs
Accepted Manuscript Experimental investigation on the operation parameters of carbon dioxide huff-n-puff process in ultra low permeability oil reservoirs Xiyi Peng, Yanyong Wang, Yuqian Diao, Liang Zhang, Iddi M. Yazid, Shaoran Ren PII:
S0920-4105(18)31069-6
DOI:
https://doi.org/10.1016/j.petrol.2018.11.073
Reference:
PETROL 5545
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 2 March 2018 Revised Date:
17 October 2018
Accepted Date: 28 November 2018
Please cite this article as: Peng, X., Wang, Y., Diao, Y., Zhang, L., Yazid, I.M., Ren, S., Experimental investigation on the operation parameters of carbon dioxide huff-n-puff process in ultra low permeability oil reservoirs, Journal of Petroleum Science and Engineering (2018), doi: https://doi.org/10.1016/ j.petrol.2018.11.073. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental investigation on the operation parameters of carbon dioxide
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huff-n-puff process in ultra low permeability oil reservoirs
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Xiyi Peng, Yanyong Wang**, Yuqian Diao, Liang Zhang, Iddi M. Yazid, Shaoran Ren*
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School of Petroleum Engineering, China University of Petroleum (East China), Qingdao,
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Shandong 266580, People’s Republic of China
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Corresponding author: [email protected] (S. Ren); [email protected] (Y. Wang)
Abstract: For oil reservoirs featured with ultra low permeability, CO2 huff-n-puff can
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be a promising approach for enhanced oil recovery. To understand its production
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performance and the effects of different factors in field operation, a series of CO2
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huff-n-puff experiments under various conditions were conducted using a 1D
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sandpack model, and the extraction effect of CO2 and its influence on oil production
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have been analyzed. The experimental results show that, as cyclic CO2 injection
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quantity increases, the cyclic oil recovery factor is improved, but the CO2 utilization
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factor is reduced, and there will be an optimum cyclic injection quantity from
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economic point of view. Both cyclic oil recovery factor and CO2 utilization factor are
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improved with the increasing reservoir temperature, while injection of low
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temperature CO2 may impose an adverse influence on oil recovery performance. In
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addition, the cyclic oil recovery factor and CO2 utilization factor firstly rise and then
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fall with the increasing soaking period. The results presented in this study are
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expected to provide some guidance to the field implementation of CO2 huff-n-puff
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technique in ultra low permeability oil reservoirs.
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Keywords: Low permeability oil reservoirs; CO2 huff-n-puff; Enhanced oil recovery;
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Oil swelling; CO2 injection
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Nomenclature
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EOR
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MMP
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IFT
interfacial tension
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GC
gas chromatography
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enhanced oil recovery minimum miscible pressure
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1 Introduction With the rapid depletion of oil production from conventional reservoirs and the
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rising energy demand, the petroleum industry has shifted their attention to the
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exploitation of low permeability oil reservoirs. A typical classification of the low
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permeability oil reservoirs can be found in Table 1. In consideration of the abundant
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undeveloped reserves at present as well as reserves newly discovered (Hu, 2009), oil
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production from ultra low permeability formations will become an important
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substitute to that from conventional reservoirs in future.
Reservoirs Permeability, mD
Tight
Ultra low
formation
permeability
<0.1
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Table 1 Classification of low permeability reservoirs in China. Extra low
Ordinary low
permeability
permeability
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0.1-1
1-10
10-50
Water injection is the primary technique for the development of conventional oil
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reservoirs, while injection of water into ultra low permeability formations is generally
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faced with an unfavorable high injection pressure as a result of the small diameters of
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pore throats. In such context, gas injection has been widely employed in low
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permeability reservoirs due to the better injectivity of gas. Among common gases
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used, carbon dioxide (CO2) is one of the most effective displacing fluids for enhanced
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oil recovery (EOR), which can dissolve into the oleic phase to promote oil swelling,
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reduce oil viscosity and increase oil mobility. CO2 also has an advantage of low
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minimum miscible pressure (MMP) over other gases, such as methane, nitrogen and
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air (Wang et al., 2017a, 2018a, 2019), and therefore, it can be easier to achieve a
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miscible flooding mode in field operation (Gozalpour et al., 2005). In addition, a
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portion of injected CO2 can be stored in the reservoir, which is conducive to reducing
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carbon emissions and mitigating global warming (Abedini and Torabi, 2014b; Zhang
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et al., 2017; Wang et al., 2018b). Huff-n-puff and continuous flooding are two typical
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modes for CO2 injection, and CO2 flooding has been extensively investigated and
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applied in different oil reservoirs for EOR and/or geological storage (Cao and Gu,
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2013a; Luo et al., 2012, 2017; Ren et al., 2015, 2016; Zhang et al., 2015a, 2016;
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of water and CO2, CO2 foam injection) have been thoroughly studied in order to
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mitigate the unexpected early breakthrough of CO2 in production wells and improve
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the sweeping efficiency (Zhang et al., 2015b; Li et al., 2015, 2017; Chen and
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Reynolds, 2016; Wang et al., 2017b).
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In comparison with the complex well pattern for CO2 flooding, CO2 huff-n-puff
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can be implemented more readily on site using single well, and it is a preferred
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approach for ultra low permeability reservoirs or can be adopted as a precursor
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technique for CO2 flooding. In CO2 huff-n-puff process, a certain volume of CO2 is
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firstly injected into the oil layer, and then the well will be shut in for a period of
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soaking and pressure build up, in which CO2 will interact with crude oil in place.
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After soaking, the well will be opened for oil production. The EOR mechanisms of
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CO2 huff-n-puff can be attributed to the improved oil properties (e.g., viscosity
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reduction and swelling), and pressure build up (for gas drive). The degree to which
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the oil properties can be improved mainly depends on the nature of CO2 and crude oil
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systems, which is also a function of time (e.g., the magnitude of molecular diffusion
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coefficient of CO2 in oil phase). In production phase, the oil will be expelled from
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porous media by swelling and solution gas drive.
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Abedini and Torabi (2013, 2014a) experimentally investigated the effects of
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different parameters (i.e., injection pressure, injection time, soaking period) on the
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performance of cyclic CO2 injection in light oil systems (with core permeability in the
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order of 70 mD), and they found that the ultimate oil recovery factor with operation
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pressure below MMP is quite low, while the oil recovery factor can be substantially
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improved with operation pressure increasing from immiscible condition to
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near-miscible condition. However, a further increase of operation pressure beyond
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MMP cannot improve the recovery factor. Then Abedini and Torabi (2014b) evaluated
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the storage potential of CO2 in cyclic injection mode via laboratory experiments.
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Abedini et al. (2015) has also studied the performance of cyclic CO2 injection in a
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of CO2 huff-n-puff in tight formations and shales has also been widely investigated in
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recent years (Sheng, 2015, 2017). Chen et al. (2014) studied CO2 huff-n-puff in a
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shale matrix using numerical simulation method, and investigated the effect of
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reservoir heterogeneity on oil recovery. Using laboratory experiments and numerical
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simulation, Li et al. (2017) studied the effect of injection pressure on the performance
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of CO2 huff-n-puff in shale, and they concluded that the injection pressure should be
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higher than the MMP estimated by slimtube tests to obtain a high oil recovery. By
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numerical simulation, Alfarge et al. (2017) found that natural fracture intensity and
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conductivity of oil-pathways were the two main factors controlling the success of CO2
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EOR in shale oil formations. With regard to tight oil, Yu et al. (2014, 2015) studied
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CO2 huff-n-puff process in tight formations for enhanced oil recovery using reservoir
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numerical simulation, and examined the impacts of a series of factors. Zuloaga et al.
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(2017) compared the effectiveness of CO2 huff-n-puff and continuous injection for
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tight oil formations through field scale numerical simulation, and found that CO2
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huff-n-puff performed better than continuous injection when the permeability was
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lower than 0.03 mD. Song and Yang (2017) studied the performance of CO2
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huff-n-puff in tight formations by lab scale experiments and numerical simulations,
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and they found that near miscible or miscible huff-n-puff could result in a better oil
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recovery than immiscible huff-n-puff. Hejazi et al. (2017) studied cyclic CO2 EOR in
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Bakken Formation by numerical simulation paired with experimental design method,
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and they found that fracture spacing, fracture half length, operation start time, oil
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gravity, and injection pressure were the most influential variables for oil recovery,
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CO2 utilization, and CO2 retention factors. Zhang et al. (2018) studied the effects of
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CO2 molecular diffusion, nanopore confinement, and stress-dependent deformation on
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CO2 huff-n-puff process in tight formations. Oil recovery factor and CO2 utilization
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factor are two main parameters to evaluate the performance of CO2 huff-n-puff in
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ultra low permeability reservoir, nevertheless the effects of some operation conditions
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on these two parameters are still unclear. In this study, a series of laboratory experiments have been carried out to
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investigate the impacts of various influencing factors on the performance of CO2
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huff-n-puff in ultra low permeability reservoirs (with permeability in the range of
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0.1-1 mD). The extraction effect of CO2 and its influence on the production
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performance have also been analyzed. The results presented in this study are expected
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to provide some guidance for the field implementation of CO2 huff-n-puff process in
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ultra low permeability oil reservoirs.
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2 Experimental
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2.1 Materials
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The original light crude oil was collected from a low permeability oil reservoir
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(with permeability ranging from 0.1 to 10 mD), and then the oil sample was cleaned
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using a centrifuge to remove any brine and sands. The density and viscosity of the
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light oil (oil sample #1, see Table 2) were 0.855 g/cm3 (34.0 API°, 20°C) and 23 cP
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(50°C) at atmospheric pressure, respectively. A viscous oil (oil sample #2) with
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density of 0.880 g/cm3 (29.3 API°, 20°C) and viscosity of 115 cP (50°C) at
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atmospheric pressure was used to study the effect of oil properties on the production
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performance. The purity of CO2 (Qingdao Tianyuan Gas Manufacturing Co., Ltd.)
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used in this study is equal to 99.9%. The viscosities of the crudes at atmospheric
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pressure and different temperatures were measured using a regular rotational
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viscometer and then an empirical viscosity correlation (Li et al., 2012) was adopted to
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predict oil viscosities at higher temperatures after fitting the measured data, as shown
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in Fig. 1a, and the viscosities of CO2 at various pressures and temperatures are shown
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in Fig. 1b, which were obtained from the NIST (National Institute of Standards and
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Technology) Chemistry WebBook, SRD 69. Fig. 2 shows the MMP for CO2 and light
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crude oil under different experimental temperatures, and the MMP was predicted
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using the correlations proposed by Chen et al. (2013). There are several methods to
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determine the MMP between crude oil and CO2 system (Abedini et al., 2014;
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chosen since they can be used conveniently and the predicted results are very close to
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the experimental results. It can be seen that the MMP for CO2 and the light oil
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increases from 15.04 MPa to 28.93 MPa with the reservoir temperature increasing
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from 50°C to 110°C. The dissolution of CO2 in oil phase can result in viscosity
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reduction and oil swelling, and hence the CO2 solubility in oil becomes a key
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parameter that affects the performance of CO2 huff-n-puff (Mosavat et al., 2014). Fig.
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3 illustrates the solubility of CO2 in the light oil under different experimental
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conditions, which was calculated using empirical correlations proposed by Xue et al.
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(2005). Xue’s model is very simple and just needs several inputs. It can be observed
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that the CO2 solubility in crude oil decreases with reservoir temperature and increases
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with reservoir pressure.
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Table 2 Physical properties of the used oil samples. Density@20°C, g/cm3
API°
Viscosity@50°C, cP
#1
0.855
34.0
23
#2
0.880
29.3
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(a) Viscosity temperature profiles of the used crudes at atmospheric pressure.
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(b) Viscosity temperature profiles of CO2 at different pressures.
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Fig. 1. Viscosity temperature profiles of crudes used and CO2.
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Fig. 2. Effect of temperature on the MMP of CO2 and the used light oil.
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Fig. 3. Solubility of CO2 in the light crude oil under different reservoir conditions.
2.2 Experimental equipment and procedure
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CO2 huff-n-puff experiments were conducted utilizing an experimental system as
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shown in Fig. 4, which mainly consists of a fluid injection system (with an accuracy
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of 0.01 mL/min), a physical sandpack model, a fluid collection and separation system,
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a data acquisition system (with a pressure accuracy of 0.01 MPa and a temperature
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accuracy of 0.1°C), and an air bath for maintaining constant temperature (with an
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accuracy of 0.1°C). The maximum working pressure and temperature are 60 MPa and
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150°C, respectively. The ultra low permeability formation was simulated using the
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sandpack model. The length of sandpack tube is 370 mm with the inner diameter of
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76.5 mm, and the inner wall of the sandpack tube was roughened to eliminate fluid
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channeling along the wall.
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ACCEPTED MANUSCRIPT Notes: 1-distilled water, 2-constant flow pump, 3-oil container, 4-brine water container, 5-CO2 container, 6-produced oil-gas-water separator, 7-pressure transducer, 8-temperature transducer, 9-sandpack model, 10-high temperature oven, 11-data acquisition system, 12-personal computer, 13-back pressure regulator, 14-measuring cylinder. (a) Schematic diagram of the experimental setup.
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(b) Real experimental system.
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Fig. 4. Experimental setup for simulating CO2 huff-n-puff process in low permeability oil
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reservoirs.
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Quartz sands of 100-140 mesh (109-150 µm) and more than 140 mesh (<109 µm) were mixed with a volume ratio of 1:3, and then these sand mixtures were packed into
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the sandpack tube and compressed manually in both radial and axial directions. The
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sandpack model was then placed into the high temperature oven to mimic the
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reservoir temperature conditions. Saturation of the sandpack with water was
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conducted at a flow rate of 1 mL/min after vacuum, with the sandpack model
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positioned vertically, and water was injected from the bottom. Water saturation
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process was stopped when there was a constant water flow from the top production
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side of the sandpack. Porosity of the sandpack was measured during water saturation
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process. The sandpack was then placed horizontally for permeability measurement
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through water flooding (1 mL/min). Saturation of the sandpack with oil was
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conducted through oil injection at a low drainage rate (0.5 mL/min), and the oil
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injection volume was measured and the oil saturation can be obtained. The average
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porosity and permeability of the sandpack for huff-n-puff experiments are about 25.99%
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and 0.45 mD, respectively, and the oil saturation is of 85.06%.
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was closed to mimic the huff-n-puff operation. CO2 was firstly injected into the
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physical model utilizing a constant flow pump, with an injection rate of 2 mL/min (in
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reservoir condition). When the injection volume/pressure reached the predetermined
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value, the injection valve was closed for a period of soaking. After the soaking phase,
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the same valve was opened for oil production. In the production phase, oil production
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was proceeded intermittently with a pressure drop of about 0.5 MPa each time, and
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stopped when the reservoir pressure dropped to 4 MPa. The produced fluid mixtures
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were collected and separated using an oil-gas separator, and the produced oil was
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measured by an electronic balance (with a full-scale of 300 g and an accuracy of
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0.001 g). During the experiments, formation pressures at both ends of the sandpack
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model were measured and recorded by the data acquisition system.
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In this study, the impacts of CO2 injection quantity (or injection pressure),
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reservoir temperature, soaking time, and oil properties on oil production performance
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have been investigated. The oil production performance was evaluated by cyclic oil
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recovery factor (in terms of weight percent) and CO2 utilization factor. The CO2
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utilization factor is equal to the mass ratio of cyclic oil production to cyclic CO2
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injection. The mass of CO2 injected was calculated according to the injection rate (for
215
volume) and injection pressure (for density, with constant temperature). Cyclic CO2
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injection quantity was characterized using the mass ratio of injected CO2 to saturated
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oil in sandpack.
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3 Results and discussions
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3.1 Pressure propagation behavior analysis
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Fig. 5 shows the change of pressure at both ends of the sandpack for two
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representative experiments. We can see that there was not apparent pressure drop
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along the sandpack tube in gas injection and soaking phase, and pressure transmission
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from gas injection side to the closed side was very fast, which is because the system is
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nearly incompressible with an initial reservoir pressure of 4 MPa. In such case, the
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of CO2 in porous media will be dominated by molecular diffusion (dependent on
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concentration difference).
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(b) Pressure variation for test #3.
Fig. 5. Variation of pressures at both ends of the sandpack model as a function of time.
3.2 Effect of cyclic CO2 injection quantity
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To study the effect of cyclic injection quantity (i.e., mass ratio of injected CO2 to
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saturated oil in sandpack) on oil production performance, a series of four experiments
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have been conducted, and the results can be found in Table 3 and Fig. 6. The
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respective cyclic oil recovery factors for injection quantity of 4.20 wt%, 6.95 wt%, 11
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means that the cyclic oil recovery factor rises with the increase of injection quantity,
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while the CO2 utilization factor falls with the increase of injection quantity as shown
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in Fig. 6b. With the increase of CO2 injection quantity, the reservoir pressure is
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significantly boosted (see Table 3) and hence more CO2 will dissolve into the oleic
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phase, which will promote oil swelling and increase the mobility of crude. When
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reservoir pressure approaches the MMP, the interfacial tension (IFT) of CO2 and
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crude oil system will be greatly reduced, which is also helpful to improve oil
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displacement efficiency (Cao and Gu, 2013b). However, the CO2 utilization factor
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drops with the increasing CO2 injection quantity, which means a declining economic
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benefit of CO2 huff-n-puff project, and therefore, both cyclic oil recovery factor and
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CO2 utilization factor should be taken into consideration in the design of CO2
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injection quantity in field application.
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Table 3 Experimental conditions and pressure changes during cyclic CO2 operation. Temperature, °C
#1
108
#2
108
#3
108
#4
108
#5
20
#6
35
#7
60
#8
80
#11 #12 #13
quantity, wt%
Pressure change
Soaking time,
Pressure change
during injection, MPa
hr
during soaking, MPa
9.54-13.23
24
13.23-12.70
6.95
10.60-20.16
24
20.16-19.38
12.30
10.88-25.01
24
25.01-22.96
20.05
10.50-33.89
24
33.89-32.85
4.49
9.88-11.54
24
11.54-11.77
4.44
10.39-12.74
24
12.74-10.37
4.33
10.50-14.18
24
14.18-13.36
4.55
10.78-15.67
24
15.67-14.68
108
4.07
8.38-11.92
24
11.92-10.70
108
3.67
8.71-14.64
0
-
108
4.24
9.59-13.18
11.9
13.18-12.82
108
4.10
8.38-11.92
61.1
11.92-10.70
108
4.24
9.26-16.27
90.4
16.27-14.96
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(a) Oil production vs pressure.
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(b) Cyclic oil recovery factor and CO2 utilization factor.
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Fig. 6. Effect of CO2 injection quantity on the production performance with reservoir temperature
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of 108°C and soaking time of 24 hrs.
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3.3 Effect of reservoir temperature To investigate the influence of reservoir temperature on oil production
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performance in CO2 huff-n-puff process, five experiments with different operation
262
temperatures of 20, 35, 60, 80, 108°C have been conducted, and the results are
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presented in Table 3 and Fig. 7. It can be clearly seen that, when reservoir temperature
264
increases from 20°C to 108°C, the cyclic oil recovery factor is improved from 2.15 wt%
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to 5.67 wt%, and the CO2 utilization factor is also enhanced from 0.479 t oil/t CO2 to
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1.394 t oil/t CO2. With the increase of reservoir temperature, the oil viscosity will 13
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diffusion of CO2 in oleic phase can be improved with increasing temperature, which is
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of great benefit to promoting the interaction between CO2 and oil in place and
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achieving viscosity reduction and oil swelling. In field operation, the CO2 injected
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into oil layer is usually featured with a relatively low temperature, which will reduce
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the reservoir temperature near the wellbore and result in an unfavorable oil production
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performance. This will be discussed in detail in the next section.
(a) Oil production vs pressure.
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(b) Cyclic oil recovery factor and CO2 utilization factor.
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Fig. 7. Effect of temperature on the production performance of CO2 injection with cyclic injection
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quantity of 4.1 wt%-4.5 wt% and soaking time of 24 hrs.
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3.4 Effect of soaking time Cyclic oil recovery performance with different soaking periods for CO2
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huff-n-puff has been shown in Fig. 8, where the cyclic oil recovery factor and CO2
283
utilization factor both firstly rise and then fall with the increase of soaking time.
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When soaking time is prolonged from 0 to 24 hrs, much more CO2 will dissolve into
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the oleic phase, which can contribute to oil swelling, oil viscosity reduction and etc.,
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so the cyclic oil recovery factor and CO2 utilization factor are both improved.
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However, with the soaking time prolonged from 24 to 90.37 hrs, more CO2 transfers
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from injection side of sandpack model to the closed side, and this portion of CO2
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cannot contribute to oil recovery in oil production phase due to the relatively short
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production period. As a result, the oil recovery performance gets poor when the
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soaking time exceeds 24 hrs. The experimental results demonstrate that there is an
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optimum soaking time in CO2 huff-n-puff process (about 24 hrs), and preliminary
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field results show that a soaking period of two to four weeks can be favorable in field
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operation (Mohammed-Singh et al., 2006). For specific ultra low permeability oil
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reservoirs, the soaking time can be optimized through reservoir numerical simulation.
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(a) Oil production vs pressure.
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(b) Cyclic oil recovery factor and CO2 utilization factor.
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Fig. 8. Effect of soaking time on the production performance of CO2 injection with cyclic
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injection quantity of 4.1 wt%-4.2 wt% and reservoir temperature of 108°C.
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3.5 Effect of oil properties
The effect of oil properties on the production performance of CO2 huff-n-puff
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process was investigated using different oil samples, and the results are presented in
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Table 4. It can be seen that, the production performance of CO2 huff-n-puff for
306
viscous oil (oil sample #2) is lesser than that of light oil (oil sample #1), which means
307
that oil viscosity also plays an important role for CO2 huff-n-puff. Table 4 Cyclic oil production performance of CO2 huff-n-puff process for different oil samples.
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CO2 injection
Cyclic oil recovery
CO2 utilization
quantity, wt%
factor, wt%
factor, t oil/t CO2
6.95
7.29
1.049
7.55
1.05
0.140
3.6 CO2 extraction effect and its influence on oil production To explore the extraction effect of CO2, hydrocarbon distributions of the used
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light and heavy crudes before and after CO2 injection were analyzed using gas
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chromatography (GC), and corresponding results have been illustrated in Figs. 9 and
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10. It can be seen that, medium fractions (C11-C25) in the produced oil have increased
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to some extent, and because the CO2 produced was released directly to atmosphere in
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experiments, some light fractions contained in CO2 are not collected and thus C7-C10 16
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unit can be used to capture the lighter stripped hydrocarbons in a light oil CO2
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huff-n-puff process.
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Fig. 9. Hydrocarbon distributions of the light crudes (oil sample #1) before and after CO2 injection
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(injection quantity-6.95%, soaking time-24 hrs, operation temperature-108°C).
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Fig. 10. Hydrocarbon distributions of the heavy crudes (oil sample #2) before and after CO2
324
injection (injection quantity-7.55%, soaking time-24 hrs, operation temperature-108°C).
325
In CO2 huff-n-puff process, due to the extract effect of CO2, the oil produced can
326
be much lighter than the original crude, whilst the residual will get heavier and thicker
327
and more difficult to be recovered (Abedini and Torabi, 2014a). In addition, the
328
composition change of in situ crudes will affect the stability of asphaltene in crude oil, 17
ACCEPTED MANUSCRIPT which can lead to the asphaltene precipitation. The deposited particles on the surface
330
of pore network can induce the permeability reduction (Abedini and Torabi, 2014a;
331
Shen and Sheng, 2018) and wettability alternation (from water wet to oil wet), which
332
may adversely affect the oil production performance, especially for ultra low
333
permeability reservoirs. It is suggested that the operation of CO2 huff-puff in ultra low
334
permeability formations should be no more than three cycles (Pu et al., 2016).
335
4 Effect of CO2 injection temperature in field operation
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In field operation, CO2 is generally stored and transported in liquid form for the
337
sake of convenience, which means that the CO2 injected into wellbore is featured with
338
a relatively low temperature. Injection of low temperature CO2 into oil layer can
339
lower the reservoir temperature and further deteriorate oil production performance.
340
The bottom hole temperature of CO2 becomes a key factor to analyze the effects of
341
operation parameters during CO2 injection. A series of calculations have been
342
conducted to explore the influences of different operation parameters on bottom hole
343
temperature of CO2. The model and method for the calculations can be found
344
elsewhere (Zhang, 2011). Corresponding results have been presented in Table 5 and
345
Fig. 11. In the calculation, the well depth is set to 2000 m, and the geothermal
346
gradient is 0.051°C/m, and the flowing bottom hole pressure is set to 24 MPa.
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347
Injection
Injection
Injection
Bottom hole
Temperature
rate, t/day
mass, t
temperature, °C
temperature, °C
increased, °C
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Table 5 Bottom hole temperature of CO2 under different injection modes.
1
50
200
-20
33.63
53.63
2
50
200
0
42.03
42.03
3
50
200
40
58.82
18.82
4
50
100
-20
39.03
59.03
5
50
300
-20
30.89
50.89
6
20
200
-20
67.48
87.48
7
70
200
-20
23.80
53.80
348
It can be seen that the bottom hole temperature of CO2 rises with the increasing
349
injection temperature (Fig. 11a), but falls with the increase of cyclic injection quantity
350
and gas injection rate (Figs. 11b and c). Although the bottom hole temperature of CO2 18
ACCEPTED MANUSCRIPT can be elevated to some extent (due to heat exchange between CO2 and geothermal
352
energy), it is still much lower than that of the oil layer. When these CO2 enters the oil
353
layer, it will reduce the reservoir temperature around the wellbore, which can cause
354
some adverse effects on oil recovery performance. In such condition, a relative longer
355
soaking period can be better to oil recovery, which is conducive to improving the
356
reservoir temperature via heat transfer from the region away from the operation well.
357 358 359
(a) Effect of injection temperature with cyclic injection quantity of 200 t and injection rate of 50
360 361 362
(b) Effect of cyclic injection quantity with CO2 injection temperature of -20°C and injection rate
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t/day.
of 50 t/day.
19
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(c) Effect of injection rate with cyclic injection quantity of 200 t and injection temperature of
366
Fig. 11. Temperature profiles of CO2 in wellbore under different operation conditions.
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-20°C.
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5 Conclusions
This study presents a research of CO2 huff-n-puff process for enhanced oil
369
recovery in ultra low permeability reservoirs. The effects of different operation
370
parameters on the recovery performance, including cyclic CO2 injection quantity,
371
temperature, soaking time and oil viscosity, have been investigated, associated with
372
analyses of related EOR mechanisms. The main conclusions can be drawn as follows.
373
(1) The cyclic oil recovery factor increases along with the rising cyclic CO2 injection
374
quantity, while the CO2 utilization factor falls with the increase of CO2 injection
375
quantity, and there is an optimum cyclic injection quantity from economic point of
376
view.
377
(2) The cyclic oil recovery factor and CO2 utilization factor are both improved with
378
the increase of reservoir temperature, and injection of low temperature CO2 may
379
impose an adverse effect on oil recovery performance, and a relatively longer soaking
380
period will be better for oil production in such condition.
381
(3) The cyclic oil recovery factor and CO2 utilization factor firstly rise and then fall
382
with the increasing soaking period in the experimental conditions, and a soaking
383
period of two to four weeks can be favorable in field operation, and the optimum
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value can be determined through reservoir numerical simulation for specific oil
385
reservoir.
386
Acknowledgements This research is partly supported by the National Major S&T Project
388
(2016ZX05056004-003), the Graduate Innovation Program of China University of
389
Petroleum (YCX2018011), and the Program for Changjiang Scholars and Innovative
390
Research Team in University (PCSIRT, IRT_14R58).
391
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ACCEPTED MANUSCRIPT
Highlights:
experimentally studied. 2. The effects of different operation parameters were investigated.
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1. The performance of CO2 huff-n-puff in ultra low permeability oil reservoirs was
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3. The effect of bottom hole temperature of CO2 on oil production was analyzed.
1
S0920-4105(18)31069-6
DOI:
https://doi.org/10.1016/j.petrol.2018.11.073
Reference:
PETROL 5545
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 2 March 2018 Revised Date:
17 October 2018
Accepted Date: 28 November 2018
Please cite this article as: Peng, X., Wang, Y., Diao, Y., Zhang, L., Yazid, I.M., Ren, S., Experimental investigation on the operation parameters of carbon dioxide huff-n-puff process in ultra low permeability oil reservoirs, Journal of Petroleum Science and Engineering (2018), doi: https://doi.org/10.1016/ j.petrol.2018.11.073. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental investigation on the operation parameters of carbon dioxide
2
huff-n-puff process in ultra low permeability oil reservoirs
3
Xiyi Peng, Yanyong Wang**, Yuqian Diao, Liang Zhang, Iddi M. Yazid, Shaoran Ren*
4
School of Petroleum Engineering, China University of Petroleum (East China), Qingdao,
5
Shandong 266580, People’s Republic of China
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1
6
Corresponding author: [email protected] (S. Ren); [email protected] (Y. Wang)
Abstract: For oil reservoirs featured with ultra low permeability, CO2 huff-n-puff can
8
be a promising approach for enhanced oil recovery. To understand its production
9
performance and the effects of different factors in field operation, a series of CO2
10
huff-n-puff experiments under various conditions were conducted using a 1D
11
sandpack model, and the extraction effect of CO2 and its influence on oil production
12
have been analyzed. The experimental results show that, as cyclic CO2 injection
13
quantity increases, the cyclic oil recovery factor is improved, but the CO2 utilization
14
factor is reduced, and there will be an optimum cyclic injection quantity from
15
economic point of view. Both cyclic oil recovery factor and CO2 utilization factor are
16
improved with the increasing reservoir temperature, while injection of low
17
temperature CO2 may impose an adverse influence on oil recovery performance. In
18
addition, the cyclic oil recovery factor and CO2 utilization factor firstly rise and then
19
fall with the increasing soaking period. The results presented in this study are
20
expected to provide some guidance to the field implementation of CO2 huff-n-puff
21
technique in ultra low permeability oil reservoirs.
22
Keywords: Low permeability oil reservoirs; CO2 huff-n-puff; Enhanced oil recovery;
23
Oil swelling; CO2 injection
24
Nomenclature
25
EOR
26
MMP
27
IFT
interfacial tension
28
GC
gas chromatography
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enhanced oil recovery minimum miscible pressure
1
ACCEPTED MANUSCRIPT 29
1 Introduction With the rapid depletion of oil production from conventional reservoirs and the
31
rising energy demand, the petroleum industry has shifted their attention to the
32
exploitation of low permeability oil reservoirs. A typical classification of the low
33
permeability oil reservoirs can be found in Table 1. In consideration of the abundant
34
undeveloped reserves at present as well as reserves newly discovered (Hu, 2009), oil
35
production from ultra low permeability formations will become an important
36
substitute to that from conventional reservoirs in future.
Reservoirs Permeability, mD
Tight
Ultra low
formation
permeability
<0.1
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Table 1 Classification of low permeability reservoirs in China. Extra low
Ordinary low
permeability
permeability
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0.1-1
1-10
10-50
Water injection is the primary technique for the development of conventional oil
39
reservoirs, while injection of water into ultra low permeability formations is generally
40
faced with an unfavorable high injection pressure as a result of the small diameters of
41
pore throats. In such context, gas injection has been widely employed in low
42
permeability reservoirs due to the better injectivity of gas. Among common gases
43
used, carbon dioxide (CO2) is one of the most effective displacing fluids for enhanced
44
oil recovery (EOR), which can dissolve into the oleic phase to promote oil swelling,
45
reduce oil viscosity and increase oil mobility. CO2 also has an advantage of low
46
minimum miscible pressure (MMP) over other gases, such as methane, nitrogen and
47
air (Wang et al., 2017a, 2018a, 2019), and therefore, it can be easier to achieve a
48
miscible flooding mode in field operation (Gozalpour et al., 2005). In addition, a
49
portion of injected CO2 can be stored in the reservoir, which is conducive to reducing
50
carbon emissions and mitigating global warming (Abedini and Torabi, 2014b; Zhang
51
et al., 2017; Wang et al., 2018b). Huff-n-puff and continuous flooding are two typical
52
modes for CO2 injection, and CO2 flooding has been extensively investigated and
53
applied in different oil reservoirs for EOR and/or geological storage (Cao and Gu,
54
2013a; Luo et al., 2012, 2017; Ren et al., 2015, 2016; Zhang et al., 2015a, 2016;
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ACCEPTED MANUSCRIPT Huang et al., 2016). In addition, mobility control methods (e.g., alternative injection
56
of water and CO2, CO2 foam injection) have been thoroughly studied in order to
57
mitigate the unexpected early breakthrough of CO2 in production wells and improve
58
the sweeping efficiency (Zhang et al., 2015b; Li et al., 2015, 2017; Chen and
59
Reynolds, 2016; Wang et al., 2017b).
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In comparison with the complex well pattern for CO2 flooding, CO2 huff-n-puff
61
can be implemented more readily on site using single well, and it is a preferred
62
approach for ultra low permeability reservoirs or can be adopted as a precursor
63
technique for CO2 flooding. In CO2 huff-n-puff process, a certain volume of CO2 is
64
firstly injected into the oil layer, and then the well will be shut in for a period of
65
soaking and pressure build up, in which CO2 will interact with crude oil in place.
66
After soaking, the well will be opened for oil production. The EOR mechanisms of
67
CO2 huff-n-puff can be attributed to the improved oil properties (e.g., viscosity
68
reduction and swelling), and pressure build up (for gas drive). The degree to which
69
the oil properties can be improved mainly depends on the nature of CO2 and crude oil
70
systems, which is also a function of time (e.g., the magnitude of molecular diffusion
71
coefficient of CO2 in oil phase). In production phase, the oil will be expelled from
72
porous media by swelling and solution gas drive.
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Abedini and Torabi (2013, 2014a) experimentally investigated the effects of
74
different parameters (i.e., injection pressure, injection time, soaking period) on the
75
performance of cyclic CO2 injection in light oil systems (with core permeability in the
76
order of 70 mD), and they found that the ultimate oil recovery factor with operation
77
pressure below MMP is quite low, while the oil recovery factor can be substantially
78
improved with operation pressure increasing from immiscible condition to
79
near-miscible condition. However, a further increase of operation pressure beyond
80
MMP cannot improve the recovery factor. Then Abedini and Torabi (2014b) evaluated
81
the storage potential of CO2 in cyclic injection mode via laboratory experiments.
82
Abedini et al. (2015) has also studied the performance of cyclic CO2 injection in a
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ACCEPTED MANUSCRIPT tight carbonate reservoir under immiscible and miscible scenarios. The performance
84
of CO2 huff-n-puff in tight formations and shales has also been widely investigated in
85
recent years (Sheng, 2015, 2017). Chen et al. (2014) studied CO2 huff-n-puff in a
86
shale matrix using numerical simulation method, and investigated the effect of
87
reservoir heterogeneity on oil recovery. Using laboratory experiments and numerical
88
simulation, Li et al. (2017) studied the effect of injection pressure on the performance
89
of CO2 huff-n-puff in shale, and they concluded that the injection pressure should be
90
higher than the MMP estimated by slimtube tests to obtain a high oil recovery. By
91
numerical simulation, Alfarge et al. (2017) found that natural fracture intensity and
92
conductivity of oil-pathways were the two main factors controlling the success of CO2
93
EOR in shale oil formations. With regard to tight oil, Yu et al. (2014, 2015) studied
94
CO2 huff-n-puff process in tight formations for enhanced oil recovery using reservoir
95
numerical simulation, and examined the impacts of a series of factors. Zuloaga et al.
96
(2017) compared the effectiveness of CO2 huff-n-puff and continuous injection for
97
tight oil formations through field scale numerical simulation, and found that CO2
98
huff-n-puff performed better than continuous injection when the permeability was
99
lower than 0.03 mD. Song and Yang (2017) studied the performance of CO2
100
huff-n-puff in tight formations by lab scale experiments and numerical simulations,
101
and they found that near miscible or miscible huff-n-puff could result in a better oil
102
recovery than immiscible huff-n-puff. Hejazi et al. (2017) studied cyclic CO2 EOR in
103
Bakken Formation by numerical simulation paired with experimental design method,
104
and they found that fracture spacing, fracture half length, operation start time, oil
105
gravity, and injection pressure were the most influential variables for oil recovery,
106
CO2 utilization, and CO2 retention factors. Zhang et al. (2018) studied the effects of
107
CO2 molecular diffusion, nanopore confinement, and stress-dependent deformation on
108
CO2 huff-n-puff process in tight formations. Oil recovery factor and CO2 utilization
109
factor are two main parameters to evaluate the performance of CO2 huff-n-puff in
110
ultra low permeability reservoir, nevertheless the effects of some operation conditions
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on these two parameters are still unclear. In this study, a series of laboratory experiments have been carried out to
113
investigate the impacts of various influencing factors on the performance of CO2
114
huff-n-puff in ultra low permeability reservoirs (with permeability in the range of
115
0.1-1 mD). The extraction effect of CO2 and its influence on the production
116
performance have also been analyzed. The results presented in this study are expected
117
to provide some guidance for the field implementation of CO2 huff-n-puff process in
118
ultra low permeability oil reservoirs.
119
2 Experimental
120
2.1 Materials
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The original light crude oil was collected from a low permeability oil reservoir
122
(with permeability ranging from 0.1 to 10 mD), and then the oil sample was cleaned
123
using a centrifuge to remove any brine and sands. The density and viscosity of the
124
light oil (oil sample #1, see Table 2) were 0.855 g/cm3 (34.0 API°, 20°C) and 23 cP
125
(50°C) at atmospheric pressure, respectively. A viscous oil (oil sample #2) with
126
density of 0.880 g/cm3 (29.3 API°, 20°C) and viscosity of 115 cP (50°C) at
127
atmospheric pressure was used to study the effect of oil properties on the production
128
performance. The purity of CO2 (Qingdao Tianyuan Gas Manufacturing Co., Ltd.)
129
used in this study is equal to 99.9%. The viscosities of the crudes at atmospheric
130
pressure and different temperatures were measured using a regular rotational
131
viscometer and then an empirical viscosity correlation (Li et al., 2012) was adopted to
132
predict oil viscosities at higher temperatures after fitting the measured data, as shown
133
in Fig. 1a, and the viscosities of CO2 at various pressures and temperatures are shown
134
in Fig. 1b, which were obtained from the NIST (National Institute of Standards and
135
Technology) Chemistry WebBook, SRD 69. Fig. 2 shows the MMP for CO2 and light
136
crude oil under different experimental temperatures, and the MMP was predicted
137
using the correlations proposed by Chen et al. (2013). There are several methods to
138
determine the MMP between crude oil and CO2 system (Abedini et al., 2014;
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140
chosen since they can be used conveniently and the predicted results are very close to
141
the experimental results. It can be seen that the MMP for CO2 and the light oil
142
increases from 15.04 MPa to 28.93 MPa with the reservoir temperature increasing
143
from 50°C to 110°C. The dissolution of CO2 in oil phase can result in viscosity
144
reduction and oil swelling, and hence the CO2 solubility in oil becomes a key
145
parameter that affects the performance of CO2 huff-n-puff (Mosavat et al., 2014). Fig.
146
3 illustrates the solubility of CO2 in the light oil under different experimental
147
conditions, which was calculated using empirical correlations proposed by Xue et al.
148
(2005). Xue’s model is very simple and just needs several inputs. It can be observed
149
that the CO2 solubility in crude oil decreases with reservoir temperature and increases
150
with reservoir pressure.
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Table 2 Physical properties of the used oil samples. Density@20°C, g/cm3
API°
Viscosity@50°C, cP
#1
0.855
34.0
23
#2
0.880
29.3
115
152 153
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(a) Viscosity temperature profiles of the used crudes at atmospheric pressure.
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154
(b) Viscosity temperature profiles of CO2 at different pressures.
156
Fig. 1. Viscosity temperature profiles of crudes used and CO2.
158
Fig. 2. Effect of temperature on the MMP of CO2 and the used light oil.
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Fig. 3. Solubility of CO2 in the light crude oil under different reservoir conditions.
2.2 Experimental equipment and procedure
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CO2 huff-n-puff experiments were conducted utilizing an experimental system as
163
shown in Fig. 4, which mainly consists of a fluid injection system (with an accuracy
164
of 0.01 mL/min), a physical sandpack model, a fluid collection and separation system,
165
a data acquisition system (with a pressure accuracy of 0.01 MPa and a temperature
166
accuracy of 0.1°C), and an air bath for maintaining constant temperature (with an
167
accuracy of 0.1°C). The maximum working pressure and temperature are 60 MPa and
168
150°C, respectively. The ultra low permeability formation was simulated using the
169
sandpack model. The length of sandpack tube is 370 mm with the inner diameter of
170
76.5 mm, and the inner wall of the sandpack tube was roughened to eliminate fluid
171
channeling along the wall.
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ACCEPTED MANUSCRIPT Notes: 1-distilled water, 2-constant flow pump, 3-oil container, 4-brine water container, 5-CO2 container, 6-produced oil-gas-water separator, 7-pressure transducer, 8-temperature transducer, 9-sandpack model, 10-high temperature oven, 11-data acquisition system, 12-personal computer, 13-back pressure regulator, 14-measuring cylinder. (a) Schematic diagram of the experimental setup.
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(b) Real experimental system.
180
Fig. 4. Experimental setup for simulating CO2 huff-n-puff process in low permeability oil
181
reservoirs.
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Quartz sands of 100-140 mesh (109-150 µm) and more than 140 mesh (<109 µm) were mixed with a volume ratio of 1:3, and then these sand mixtures were packed into
184
the sandpack tube and compressed manually in both radial and axial directions. The
185
sandpack model was then placed into the high temperature oven to mimic the
186
reservoir temperature conditions. Saturation of the sandpack with water was
187
conducted at a flow rate of 1 mL/min after vacuum, with the sandpack model
188
positioned vertically, and water was injected from the bottom. Water saturation
189
process was stopped when there was a constant water flow from the top production
190
side of the sandpack. Porosity of the sandpack was measured during water saturation
191
process. The sandpack was then placed horizontally for permeability measurement
192
through water flooding (1 mL/min). Saturation of the sandpack with oil was
193
conducted through oil injection at a low drainage rate (0.5 mL/min), and the oil
194
injection volume was measured and the oil saturation can be obtained. The average
195
porosity and permeability of the sandpack for huff-n-puff experiments are about 25.99%
196
and 0.45 mD, respectively, and the oil saturation is of 85.06%.
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198
was closed to mimic the huff-n-puff operation. CO2 was firstly injected into the
199
physical model utilizing a constant flow pump, with an injection rate of 2 mL/min (in
200
reservoir condition). When the injection volume/pressure reached the predetermined
201
value, the injection valve was closed for a period of soaking. After the soaking phase,
202
the same valve was opened for oil production. In the production phase, oil production
203
was proceeded intermittently with a pressure drop of about 0.5 MPa each time, and
204
stopped when the reservoir pressure dropped to 4 MPa. The produced fluid mixtures
205
were collected and separated using an oil-gas separator, and the produced oil was
206
measured by an electronic balance (with a full-scale of 300 g and an accuracy of
207
0.001 g). During the experiments, formation pressures at both ends of the sandpack
208
model were measured and recorded by the data acquisition system.
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In this study, the impacts of CO2 injection quantity (or injection pressure),
210
reservoir temperature, soaking time, and oil properties on oil production performance
211
have been investigated. The oil production performance was evaluated by cyclic oil
212
recovery factor (in terms of weight percent) and CO2 utilization factor. The CO2
213
utilization factor is equal to the mass ratio of cyclic oil production to cyclic CO2
214
injection. The mass of CO2 injected was calculated according to the injection rate (for
215
volume) and injection pressure (for density, with constant temperature). Cyclic CO2
216
injection quantity was characterized using the mass ratio of injected CO2 to saturated
217
oil in sandpack.
218
3 Results and discussions
219
3.1 Pressure propagation behavior analysis
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Fig. 5 shows the change of pressure at both ends of the sandpack for two
221
representative experiments. We can see that there was not apparent pressure drop
222
along the sandpack tube in gas injection and soaking phase, and pressure transmission
223
from gas injection side to the closed side was very fast, which is because the system is
224
nearly incompressible with an initial reservoir pressure of 4 MPa. In such case, the
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226
of CO2 in porous media will be dominated by molecular diffusion (dependent on
227
concentration difference).
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230 231 232 233
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(a) Pressure variation for test #2.
(b) Pressure variation for test #3.
Fig. 5. Variation of pressures at both ends of the sandpack model as a function of time.
3.2 Effect of cyclic CO2 injection quantity
234
To study the effect of cyclic injection quantity (i.e., mass ratio of injected CO2 to
235
saturated oil in sandpack) on oil production performance, a series of four experiments
236
have been conducted, and the results can be found in Table 3 and Fig. 6. The
237
respective cyclic oil recovery factors for injection quantity of 4.20 wt%, 6.95 wt%, 11
ACCEPTED MANUSCRIPT 12.30 wt%, and 20.05 wt% are 5.83 wt%, 7.29 wt%, 9.19 wt% and 10.61 wt%, which
239
means that the cyclic oil recovery factor rises with the increase of injection quantity,
240
while the CO2 utilization factor falls with the increase of injection quantity as shown
241
in Fig. 6b. With the increase of CO2 injection quantity, the reservoir pressure is
242
significantly boosted (see Table 3) and hence more CO2 will dissolve into the oleic
243
phase, which will promote oil swelling and increase the mobility of crude. When
244
reservoir pressure approaches the MMP, the interfacial tension (IFT) of CO2 and
245
crude oil system will be greatly reduced, which is also helpful to improve oil
246
displacement efficiency (Cao and Gu, 2013b). However, the CO2 utilization factor
247
drops with the increasing CO2 injection quantity, which means a declining economic
248
benefit of CO2 huff-n-puff project, and therefore, both cyclic oil recovery factor and
249
CO2 utilization factor should be taken into consideration in the design of CO2
250
injection quantity in field application.
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Table 3 Experimental conditions and pressure changes during cyclic CO2 operation. Temperature, °C
#1
108
#2
108
#3
108
#4
108
#5
20
#6
35
#7
60
#8
80
#11 #12 #13
quantity, wt%
Pressure change
Soaking time,
Pressure change
during injection, MPa
hr
during soaking, MPa
9.54-13.23
24
13.23-12.70
6.95
10.60-20.16
24
20.16-19.38
12.30
10.88-25.01
24
25.01-22.96
20.05
10.50-33.89
24
33.89-32.85
4.49
9.88-11.54
24
11.54-11.77
4.44
10.39-12.74
24
12.74-10.37
4.33
10.50-14.18
24
14.18-13.36
4.55
10.78-15.67
24
15.67-14.68
108
4.07
8.38-11.92
24
11.92-10.70
108
3.67
8.71-14.64
0
-
108
4.24
9.59-13.18
11.9
13.18-12.82
108
4.10
8.38-11.92
61.1
11.92-10.70
108
4.24
9.26-16.27
90.4
16.27-14.96
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(a) Oil production vs pressure.
255 256
(b) Cyclic oil recovery factor and CO2 utilization factor.
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Fig. 6. Effect of CO2 injection quantity on the production performance with reservoir temperature
258
of 108°C and soaking time of 24 hrs.
259 260
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3.3 Effect of reservoir temperature To investigate the influence of reservoir temperature on oil production
261
performance in CO2 huff-n-puff process, five experiments with different operation
262
temperatures of 20, 35, 60, 80, 108°C have been conducted, and the results are
263
presented in Table 3 and Fig. 7. It can be clearly seen that, when reservoir temperature
264
increases from 20°C to 108°C, the cyclic oil recovery factor is improved from 2.15 wt%
265
to 5.67 wt%, and the CO2 utilization factor is also enhanced from 0.479 t oil/t CO2 to
266
1.394 t oil/t CO2. With the increase of reservoir temperature, the oil viscosity will 13
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268
diffusion of CO2 in oleic phase can be improved with increasing temperature, which is
269
of great benefit to promoting the interaction between CO2 and oil in place and
270
achieving viscosity reduction and oil swelling. In field operation, the CO2 injected
271
into oil layer is usually featured with a relatively low temperature, which will reduce
272
the reservoir temperature near the wellbore and result in an unfavorable oil production
273
performance. This will be discussed in detail in the next section.
(a) Oil production vs pressure.
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(b) Cyclic oil recovery factor and CO2 utilization factor.
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Fig. 7. Effect of temperature on the production performance of CO2 injection with cyclic injection
279
quantity of 4.1 wt%-4.5 wt% and soaking time of 24 hrs.
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3.4 Effect of soaking time Cyclic oil recovery performance with different soaking periods for CO2
282
huff-n-puff has been shown in Fig. 8, where the cyclic oil recovery factor and CO2
283
utilization factor both firstly rise and then fall with the increase of soaking time.
284
When soaking time is prolonged from 0 to 24 hrs, much more CO2 will dissolve into
285
the oleic phase, which can contribute to oil swelling, oil viscosity reduction and etc.,
286
so the cyclic oil recovery factor and CO2 utilization factor are both improved.
287
However, with the soaking time prolonged from 24 to 90.37 hrs, more CO2 transfers
288
from injection side of sandpack model to the closed side, and this portion of CO2
289
cannot contribute to oil recovery in oil production phase due to the relatively short
290
production period. As a result, the oil recovery performance gets poor when the
291
soaking time exceeds 24 hrs. The experimental results demonstrate that there is an
292
optimum soaking time in CO2 huff-n-puff process (about 24 hrs), and preliminary
293
field results show that a soaking period of two to four weeks can be favorable in field
294
operation (Mohammed-Singh et al., 2006). For specific ultra low permeability oil
295
reservoirs, the soaking time can be optimized through reservoir numerical simulation.
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(a) Oil production vs pressure.
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(b) Cyclic oil recovery factor and CO2 utilization factor.
300
Fig. 8. Effect of soaking time on the production performance of CO2 injection with cyclic
301
injection quantity of 4.1 wt%-4.2 wt% and reservoir temperature of 108°C.
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3.5 Effect of oil properties
The effect of oil properties on the production performance of CO2 huff-n-puff
304
process was investigated using different oil samples, and the results are presented in
305
Table 4. It can be seen that, the production performance of CO2 huff-n-puff for
306
viscous oil (oil sample #2) is lesser than that of light oil (oil sample #1), which means
307
that oil viscosity also plays an important role for CO2 huff-n-puff. Table 4 Cyclic oil production performance of CO2 huff-n-puff process for different oil samples.
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309 310
CO2 injection
Cyclic oil recovery
CO2 utilization
quantity, wt%
factor, wt%
factor, t oil/t CO2
6.95
7.29
1.049
7.55
1.05
0.140
3.6 CO2 extraction effect and its influence on oil production To explore the extraction effect of CO2, hydrocarbon distributions of the used
311
light and heavy crudes before and after CO2 injection were analyzed using gas
312
chromatography (GC), and corresponding results have been illustrated in Figs. 9 and
313
10. It can be seen that, medium fractions (C11-C25) in the produced oil have increased
314
to some extent, and because the CO2 produced was released directly to atmosphere in
315
experiments, some light fractions contained in CO2 are not collected and thus C7-C10 16
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317
unit can be used to capture the lighter stripped hydrocarbons in a light oil CO2
318
huff-n-puff process.
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Fig. 9. Hydrocarbon distributions of the light crudes (oil sample #1) before and after CO2 injection
321
(injection quantity-6.95%, soaking time-24 hrs, operation temperature-108°C).
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Fig. 10. Hydrocarbon distributions of the heavy crudes (oil sample #2) before and after CO2
324
injection (injection quantity-7.55%, soaking time-24 hrs, operation temperature-108°C).
325
In CO2 huff-n-puff process, due to the extract effect of CO2, the oil produced can
326
be much lighter than the original crude, whilst the residual will get heavier and thicker
327
and more difficult to be recovered (Abedini and Torabi, 2014a). In addition, the
328
composition change of in situ crudes will affect the stability of asphaltene in crude oil, 17
ACCEPTED MANUSCRIPT which can lead to the asphaltene precipitation. The deposited particles on the surface
330
of pore network can induce the permeability reduction (Abedini and Torabi, 2014a;
331
Shen and Sheng, 2018) and wettability alternation (from water wet to oil wet), which
332
may adversely affect the oil production performance, especially for ultra low
333
permeability reservoirs. It is suggested that the operation of CO2 huff-puff in ultra low
334
permeability formations should be no more than three cycles (Pu et al., 2016).
335
4 Effect of CO2 injection temperature in field operation
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In field operation, CO2 is generally stored and transported in liquid form for the
337
sake of convenience, which means that the CO2 injected into wellbore is featured with
338
a relatively low temperature. Injection of low temperature CO2 into oil layer can
339
lower the reservoir temperature and further deteriorate oil production performance.
340
The bottom hole temperature of CO2 becomes a key factor to analyze the effects of
341
operation parameters during CO2 injection. A series of calculations have been
342
conducted to explore the influences of different operation parameters on bottom hole
343
temperature of CO2. The model and method for the calculations can be found
344
elsewhere (Zhang, 2011). Corresponding results have been presented in Table 5 and
345
Fig. 11. In the calculation, the well depth is set to 2000 m, and the geothermal
346
gradient is 0.051°C/m, and the flowing bottom hole pressure is set to 24 MPa.
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Injection
Injection
Injection
Bottom hole
Temperature
rate, t/day
mass, t
temperature, °C
temperature, °C
increased, °C
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Table 5 Bottom hole temperature of CO2 under different injection modes.
1
50
200
-20
33.63
53.63
2
50
200
0
42.03
42.03
3
50
200
40
58.82
18.82
4
50
100
-20
39.03
59.03
5
50
300
-20
30.89
50.89
6
20
200
-20
67.48
87.48
7
70
200
-20
23.80
53.80
348
It can be seen that the bottom hole temperature of CO2 rises with the increasing
349
injection temperature (Fig. 11a), but falls with the increase of cyclic injection quantity
350
and gas injection rate (Figs. 11b and c). Although the bottom hole temperature of CO2 18
ACCEPTED MANUSCRIPT can be elevated to some extent (due to heat exchange between CO2 and geothermal
352
energy), it is still much lower than that of the oil layer. When these CO2 enters the oil
353
layer, it will reduce the reservoir temperature around the wellbore, which can cause
354
some adverse effects on oil recovery performance. In such condition, a relative longer
355
soaking period can be better to oil recovery, which is conducive to improving the
356
reservoir temperature via heat transfer from the region away from the operation well.
357 358 359
(a) Effect of injection temperature with cyclic injection quantity of 200 t and injection rate of 50
360 361 362
(b) Effect of cyclic injection quantity with CO2 injection temperature of -20°C and injection rate
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(c) Effect of injection rate with cyclic injection quantity of 200 t and injection temperature of
366
Fig. 11. Temperature profiles of CO2 in wellbore under different operation conditions.
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5 Conclusions
This study presents a research of CO2 huff-n-puff process for enhanced oil
369
recovery in ultra low permeability reservoirs. The effects of different operation
370
parameters on the recovery performance, including cyclic CO2 injection quantity,
371
temperature, soaking time and oil viscosity, have been investigated, associated with
372
analyses of related EOR mechanisms. The main conclusions can be drawn as follows.
373
(1) The cyclic oil recovery factor increases along with the rising cyclic CO2 injection
374
quantity, while the CO2 utilization factor falls with the increase of CO2 injection
375
quantity, and there is an optimum cyclic injection quantity from economic point of
376
view.
377
(2) The cyclic oil recovery factor and CO2 utilization factor are both improved with
378
the increase of reservoir temperature, and injection of low temperature CO2 may
379
impose an adverse effect on oil recovery performance, and a relatively longer soaking
380
period will be better for oil production in such condition.
381
(3) The cyclic oil recovery factor and CO2 utilization factor firstly rise and then fall
382
with the increasing soaking period in the experimental conditions, and a soaking
383
period of two to four weeks can be favorable in field operation, and the optimum
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value can be determined through reservoir numerical simulation for specific oil
385
reservoir.
386
Acknowledgements This research is partly supported by the National Major S&T Project
388
(2016ZX05056004-003), the Graduate Innovation Program of China University of
389
Petroleum (YCX2018011), and the Program for Changjiang Scholars and Innovative
390
Research Team in University (PCSIRT, IRT_14R58).
391
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and
continuous
CO2
injection
in
tight
reservoirs.
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https://doi.org/10.1016/j.energy.2017.06.028.
oil
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Zuloaga, P., Yu, W., Miao, J., Sepehrnoori, K., 2017. Performance evaluation of CO2 Huff-n-Puff
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Highlights:
experimentally studied. 2. The effects of different operation parameters were investigated.
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1. The performance of CO2 huff-n-puff in ultra low permeability oil reservoirs was
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3. The effect of bottom hole temperature of CO2 on oil production was analyzed.
1