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Pore scale Investigation of Crude Oil/CO2 Compositional Effects on Oil Recovery by Carbonated Water Injection Mojtaba Seyyedi, and Mehran Sohrabi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04743 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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Industrial & Engineering Chemistry Research

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Pore scale Investigation of Crude Oil/CO2 Compositional Effects on Oil Recovery by

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Carbonated Water Injection

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Mojtaba Seyyedi1,2* and Mehran Sohrabi1

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1. Centre for Enhanced Oil Recovery and CO2 Solutions, Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK

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Abstract

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Through coreflood and micromodel studies, it has been shown that carbonated water injection

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(CWI) can improve oil recovery compared to conventional waterflood. However, in most of early

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studies either a refined oil or dead crude oil had been used, which is not representative of a real oil

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reservoir where the oil has significant dissolved gases. In such studies oil swelling and oil viscosity

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reduction had been introduced as the main mechanisms of additional oil recovery by CWI. However,

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in our direct flow visualisation (micromodel) studies reported here, we have used live crude oil and

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we have observed the formation and growth of a new gaseous phase inside the oil when it comes in

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contact with carbonated water (CW). The aim of this work is to visually study the effect of this

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phenomenon on oil recovery by CWI at pore scale.

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In this paper, we present the results of two high-pressure high temperature direct flow visualization

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(micromodel) experiments which have been performed using a live crude oil sample. These include a

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tertiary (post-waterflood) and a secondary (pre-waterflood) CWI experiments performed at 2500

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psia and 100 °F.

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The results of our secondary and tertiary CWI showed that CWI can improve the oil displacement

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and recovery compared to conventional waterflood. Although both secondary and tertiary CWI

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improved oil recovery significantly, the performance of CWI was better when it was injected instead

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of conventional watdflood (secondary) rather than after conventional waterflood (tertiary). Based

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on our study, the predominant mechanism that led to this additional oil recovery was the formation

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and growth of a new gaseous phase within the oil. Formation of the new phase improved the oil

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recovery through; (i) reconnection of the trapped oil and oil displacement, (ii) creating a favourable

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three phase flow region with less residual oil saturation, and (iii) restricting the flow path of CW and 1 *Corresponding Author Email: [email protected]

2. Danish Hydrocarbon Research and Technology Centre, Technical University of Denmark, Copenhagen, Denmark

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diverting it toward unswept areas of the porous medium. Formation of the new phase happened

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faster and stronger when CW was injected as secondary and its final saturation, for a fixed period of

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CWI, was higher than its final saturation in tertiary CWI. We also show that the nucleation and

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growth of the new gaseous phase is directly proportional to the amount of hydrocarbon gas

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dissolved in the oil which is a function of oil properties and saturation pressure and temperature.

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Keywords: New gaseous phase, Carbonated Water Injection, Pore scale

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1. Introduction

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It has been shown that carbonated (CO2 enriched) water injection (CWI) can improve oil recovery.

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Oil swelling and oil viscosity reduction have been suggested amongst the main mechanisms of CWI

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by several researchers. Sohrabi et al.1–4 studied the pore-scale mechanisms of oil recovery by CWI

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through a high pressure flow visualization system. Experimental conditions were 2000 psia and 100

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°F. Based on their study, CWI improved oil recovery for both light oil and heavy oil compared to

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unadulterated waterflooding. The results showed that CWI, compared to conventional water

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injection, improved oil recovery in both secondary (pre-waterflood) and tertiary (post-waterflood)

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injection modes. However, the results showed a stronger potential of CWI when it is injected as a

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secondary recovery mode. The main mechanisms of oil recovery under the conditions of those

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experiments in which dead oil (either refined oil or crude oil) had been used were oil swelling that

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causes coalescence of trapped oil ganglia and therefore local flow diversion and oil viscosity

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reduction as a result of CO2 diffusion from carbonated water (CW) into the oil. Riazi et al. 5,6 studied

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performance of CWI by using a high pressure flow visualization system. Experiments were performed

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at pressure of 2000 psia and temperature of 100 °F. The results of their studies, in which dead oil

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(either refined oil or crude oil) had been used, showed strong potential of CWI either as a secondary

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or tertiary injection mode. Based on their study, oil swelling and oil viscosity reduction are

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responsible mechanisms for additional oil recovery by CWI. They also studied potential benefit of a

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subsequent depressurisation period on oil recovery after the CWI. Sohrabi et al.7 studied the

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potential of CWI for improving recovery from oil reservoirs with the added benefit of safe storage of

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CO2 through conducting high-pressure flow visualization as well as coreflood experiments at

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reservoir conditions. Crude oil was used in their study. In their micromodel test CWI led to 16%

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additional oil recovery and in core flood test CWI led to 9% addition oil recovery. Kechut et al.

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studied tertiary CWI by conducting a series of high pressure flow visualization and core flood

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experiments as well as compositional simulation. Dead oil had been used in the experiments and oil

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swelling and oil viscosity reduction, due to CO2 transfer from CW into the oil, were proposed as the

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main responsible mechanisms for additional oil recovery by tertiary CWI.

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Sohrabi et al.9 studied the potential of CWI through performing a series of coreflood experiments at

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pressure and temperature of 2500 psia and 100 °F. Based on their study both secondary and tertiary

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CWI showed significant potentials for improving oil recovery. Furthermore, the water breakthrough

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time for the secondary CWI took place later than water breakthrough time during plain

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waterflooding. Crude oil was used in this study as the oil phase. Kechut et al. 10 presented the results

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of a series of core flood experiments and numerical investigations on the oil recovery and CO2

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storage by CWI in both secondary and tertiary recovery modes. Their experimental results showed a

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good performance for CWI and also a good delivery of CO2 to the oil by CW front. Based on their

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study CO2 was moving a head of CW front instead of behind it as predicted by commercial

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simulators. Dead decane and crude oil were used in these experiments. Sohrabi et al.

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series of core flood experiments studied the effects of oil viscosity, rock wettability and brine salinity

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on CWI performance. Their study shows that CO2 front is moving ahead of CW front which shows

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good delivery of CO2 by CW. light oil (n-decane), refined viscous oil and a stock-tank crude oil were

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used in this study. Furthermore, they observed some evidence of wettability alteration by CW that

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could also influence the oil recovery. Seyyedi et al. 12 studied the possibility of wettability alteration

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by CW through performing a series of contact angle measurements at high-pressures and high

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temperature. Based on their study, CW has a significant impact on wettability. They showed that the

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extent of wettability alteration is a direct function of CO2 concentration in brine (pressure) and also

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initial wettability stage of the rock. Seyyedi et al.

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through a

studied the potential of CW for enhancing the 3

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rate of water imbibition in both sand stone and carbonate rocks through a series of high pressure

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imbibition experiments. Experiments were performed at pressure of 2500 psia and crude oil was

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used. Their results revealed high potential for carbonated water to enhance water imbibition rate

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and oil recovery.

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Torabi and Mosavat14 studied the performance of CWI through a series of core flood experiments.

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Results of their study show that the recovery for both secondary and tertiary CWI was higher than

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conventional waterflooding. They studied the effect of pressure and temperature on CWI

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performance. They used dead crude oil sample in their study. Alizadeh et al.15 studied CO2 exsolution

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by dropping the pressure of the system during CWI. They dropped the pressure of back pressure

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from around 180 psig to 0 psig at specific period of time to study effect of in situ degassing on

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mobilization and recovery of trapped oil. In their study, they used Soltrol 170 (a mineral oil) as the oil

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phase. Based on their study, oil recovery increased through gas exsolution formed by pressure

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dropping. Guanli et al.16 studied the potential of CW on improving the CO2 EOR performance in

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water-wet Berea sandstones. They observed that injecting one pore volume CW before CO2 flooding

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can alleviate the negative effect of water shielding and thereby led to better recovery during CO2

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injection in water-wet oil reservoirs. In their study they used a light crude oil as the oil phase. Zuo et

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al.17,18 studied gas exsolution from CW using micromodel investigations. Pressure drop led to CO2

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exsolution from CW which led to water flow blockage and local flow diversion into oil filled pores

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and therefore better oil recovery. A 10% incremental oil recovery was achieved by lowering the

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pressure by 2 MPa below the CO2 liberation pressure. Mineral oil was used as the oil phase in these

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studies.

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In all of the above studies, either mineral oil or dead crude oil was used which cannot represent the

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condition of real oil reservoirs when oil has dissolved gases. Currently, Sohrabi et al.19 presented a

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study of CWI and its mechanism at the pore and core scales using live and dead crude oil samples.

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The results revealed an important new mechanism of additional oil recovery by CWI. The results

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showed that when CW comes in contact with live crude oil (crude oil with dissolved hydrocarbon 4 ACS Paragon Plus Environment

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gas) a new gaseous phase nucleated inside the oil. The formation of this new phase led to significant

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improvement in the performance of CWI in live oil compared to dead oil. In a recent paper, Seyyedi

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et al.20 through a series of high-pressure and high-temperature fluid characterization tests

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thoroughly studied the phase behaviour of CW-‘’live oil’’ (oil with dissolved gas) system. Based on

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their results CO2 partitioning between CW and live oil led to rapid formation of a new gaseous phase

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inside the system. They analysed the characteristics of the new gaseous phase formed during

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contact of carbonated water with live oil. Based on their results, the new phase is a multi-

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component mixture of hydrocarbons starting with CH4 and CO2 at early stages and becoming richer

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towards the latter contacts. Latter on through a series of integrated coreflooding experiments,

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they21 studied the coupling impacts of gaseous new phase formation and wettability alteration on

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additional oil recovery by CWI for live oil system. Their results showed that formation and continues

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growth of the new gaseous phase is the dominant oil recovery mechanism by CWI.

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In this paper, through using high-pressure micromodel system, we are aiming to reach an in-depth

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understanding of the effects of nucleation and growth of this new phase on the performance of CWI

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at pore-scale. Live crude oil had been used in this study. The experimental conditions were 2500 psia

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and 100 °F.

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2. Experimental Setup

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2.1. Micromodel rig

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Figure 1 shows the schematic of the micromodel rig used in this study. As can be seen, all fluids are

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kept inside fluid’s oven at test temperature and pressure (100 °F and 2500 psia). The micromodel is

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inside a separate oven at same experimental conditions. The micromodel is a transparent porous

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media which is made of two glass plates. The micromodel dimensions are shown in Table 1. Figure 2

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shows the micromodel when it was fully saturated with water.



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Fig. 1. Schematic of micromodel rig

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Table 1. Micromodel dimensions height (cm) 4

width (cm) 0.7

pore volume (cm3) 0.01

porosity 0.5109

average pore depth (µm) 50

pore dia. range(µm) 30-500

132 4 cm

7 mm

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Fig. 2. Micromodel when it is fully saturated with water

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2.2. Fluids Properties

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The properties of the crude oil used in this investigation are shown in Table 2. To prepare the live oil,

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the crude oil was fully saturated with CH4 at our experimental conditions. Table 3 presents the

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properties of the crude oil (Crude P) used in our previous study19. As can be seen from Tables 2 and

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3, crude P is lighter than crude J used in this study. The viscosities of both crude P and J were

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measured at atmospheric pressure and temperature of 100 °F. Furthermore, the amount of

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dissolved gas in crude P is much higher than the one for crude J. Both crudes were fully saturated


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with CH4 at same saturation pressure and temperature (2500 psia and 100 °F). Table 4 presents the

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compositions of our brine in this study. The total brine salinity is 54,500 ppm. CW was made at test

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conditions by mixing brine with CO2.

145 crude ID Crude J 146 crude ID Crude P 147

API 20.87

Table 2. Properties of crude J dead oil viscosity (cp) GOR (scc CH4/cc oil) 85 51

Table 3. Properties of crude P dead oil viscosity (cp) GOR (scc CH4/cc oil) API 28.9 31 106.7 Table 4. Brine composition ion ppm Na 16844 Ca 664 Mg 2279 -2 SO4 3560 Cl 31107 HCO3-1 193

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2.3. Methodology

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Having prepared the CW and live oil samples, they were transferred into the fluid oven and their

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pressure were controlled and monitored. As oil was fully saturated with CH4, to minimise the CH4

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mass transfer between the brine and oil during experiments, the brine was also fully saturated with

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CH4 at the conditions of the experiments. Having transferred the fluids into their storage cells, the

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micromodel was saturated with CH4-saturated brine. Next, the live oil was injected to displace the

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water and establish initial water saturation (Swi). The injection of live oil was continued for several

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pore volumes to be sure that no more water would be produced and also to age the model’s surface.

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Having established the initial water saturation, in the first experiment, the performance of tertiary

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CWI was studied. For this purpose, the model was flooded by CH4-saturated brine with the rate of

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0.05 cc/hr. The injection continued until the distribution of fluids in the model stabilised. Next, CW

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was injected into the model with the same rate. The injection of CW continued for 24 hours during

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which the model was scanned by a high resolution camera to detect any possible oil redistribution in

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the model. After the 24hrs of CWI, the model was flooded by CH4-saturated brine one more time to 7 ACS Paragon Plus Environment


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strip the CO2 out of the oil and determine residual oil saturation after CWI.

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In the second test, the performance of secondary CWI has been studied. The procedure was the

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same as the one used in the tertiary CWI experiment with the only difference being that CWI replace

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WF. The experiments were performed at pressure of 2500 psia and temperature of 100 °F.

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3. Experimental Results and Discussion

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A mentioned above, two high-pressure micromodel experiments have been conducted to study the

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effects of the new phase formation on oil recovery by CWI at the pore scale. In the first experiment

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the effect of nucleation and growth of new gaseous phase on oil recovery during tertiary CWI was

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studied. Furthermore, the results were compared with the results reported in our previous paper19

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to study the impact of crude oil type (see Tables 2 and 3) on the nucleation and growth of the new

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phase during CWI. In the second test, the effect of the formation and growth of the new gaseous

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phase on oil recovery by secondary CWI was studied.

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3.1. Tertiary CWI

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In the first experiment, a tertiary CWI was performed. First, the initial water and oil saturations were

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established in the model (Figure 3A). Next, the model was flooded by CH4 (methane) saturated brine

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to mimic a conventional waterflooding. The direction of flow was from bottom of the model to the

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top. Figure 3B shows the model at the last stage of waterflooding when there was no more oil

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redistribution and production. Oil production and fluid redistribution in the porous medium stopped

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almost immediately after the water breakthrough (BT). The recovery factor of the waterflood was

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46% based on the original oil in place (OOIP). As can be seen from Figure 3B, a large volume of oil, in

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the form of separated oil ganglia, was bypassed by water and remained in the model after

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waterflooding. When we switched to CWI, we noticed that after 21 minutes of injection, a new

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gaseous phase nucleated inside the oil. Figure 4 shows a section of micromodel after 21 minutes of

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tertiary CWI. As CWI continued, the saturation of this gaseous phase increased. Figure 3c shows the

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model after 24 hrs of tertiary CWI. The yellow areas show the gas phase formed inside the oil after

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24 hrs of contact with CW. The gas saturation at this stage was 10%. At this stage, due to dissolution 8 ACS Paragon Plus Environment

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of CO2 into the oil, oil swelled and therefore to determine the actual volume of the remaining oil

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after CWI, the model was flooded by CH4-saturated brine one more time but this time for stripping

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the oil of its dissolved CO2. No oil was produced at this stage. Figure 3d shows the residual oil

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saturation at the end of the test. Clearly the oil saturation reduced compared to waterflood (see the

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rectangular areas). Based on the image analysis, the oil recovery at the end of tertiary CWI was 55%,

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which is 9% higher than the recovery obtained by secondary waterflood (Figure 6). Interestingly, the

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difference between residual oil saturation after waterflooding with the residual oil saturation after

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CWI, which was 9%, was almost equal to the saturation of new phase at the end of CWI (around

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10%, Figure 7). This indicates that the predominant mechanism in here was not oil swelling or oil

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viscosity reduction but it was the formation and growth of the new phase.

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It should be noted that the onset of the new phase reported in our previous publication19, where a

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slightly lighter crude oil had been used, was 6 minutes. This indicates the effect of the amount of

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dissolved gas (solution gas) in the oil on the nucleation and growth of the new phase during CWI. For

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a given pressure and temperature, the amount of dissolved gas in oil is a direct function of oil

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properties. It means that the lighter the oil the higher the amount of dissolved gas (GOR) and vice

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versa.

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To illustrate this effect, in Figure 5, we have shown the distribution of fluids in the micromodel after

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around 20 minutes of CWI as reported in our previous study19. Figure 4, shows fluid distribution in

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the micromodel obtained in the current study almost after the same time during CWI with the only

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difference being oil type (or in another word amount of dissolved gas in oil). Comparing saturation of

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new gaseous phase in this figure with the one from Figure 5, confirms that nucleation and growth of

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new gaseous phase is a direct function of the amount of the gas dissolved in the oil which is a

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function of saturation pressure and temperature and also oil type.

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In the next sections we will discuss in details how the formation of this new phase lead to better oil

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recovery.



B

C

D

Oil Injection

A

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WF and CWI

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Fig. 3. Oil distribution in the micromodel at different time periods. A) Initial oil saturation. B) End of waterflooding. C) After 24 hours of CWI. D) After final seawater injection to strip the CO 2 out of the oil and have its realistic volume. The yellow colour shows the new gaseous phase formed inside the oil during CWI period.


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1 2 3 4 Un-etched glass 5 6 7 Live Oil 8 9 10 CW 11 12 13 14 15 New gaseous phase 16 17 18 19 20 21 22 23 24 218 25 26 219 Fig. 4. Nucleation of new gaseous phase inside the oil after 21 min of tertiary CWI in a system where 27 220 live crude J was used 28 29 30 31 32 33 34 35 36 37 38 39 40 New gaseous phase 41 42 43 44 45 46 47 48 49 50 51 52 221 53 54 222 Fig. 5. Nucleation of new gaseous phase inside the oil after 24 min of tertiary CWI in a system where 55 223 live crude P was used19 56 57 58 59 60



60 55 50 45

RF (%OOIP)

40 35 30 25 20 15 10 5 0

WF

224 225

Tertiary CWI

Fig. 6. Recovery factor (%OOIP) during both secondary waterflooding (WF) and tertiary CWI 55 50

Oil/ Gas Saturation (%)

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Soil after Secondary Soil after Tertiary CWI Sgaseous phase at the WF end of tertiary CWI

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Fig. 7. Residual oil saturations at the end of WF and tertiary CWI and gas saturation at the end of tertiary CWI

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3.2. Secondary CWI

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In the next experiment the performance of CWI as a secondary injection scenario was studied. For

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this purpose, after establishing the initial water and oil saturations (Figure 8A), CW was injected into

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the model for 24 hrs. Figure 8B shows the model after one hour of CWI and Figure 8C shows the

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model at the end of CWI period. The yellow areas in this image show the new gaseous phase formed

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inside the oil. The gas saturation at the end of CWI was 11% which is only around 1% higher than the


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gas saturation at the end of tertiary CWI. Furthermore, we noticed that almost immediately after

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CW breakthrough, the new gaseous phase started to nucleate inside the oil which was faster than

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the time it took for the formation of new phase during tertiary CWI (Figure 9). The reason for the

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observed later formation of the new gaseous phase inside oil during tertiary CWI was the presence

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of large volume of plain water in the model which had been injected during the preceding

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conventional waterflooding. Figure 9 shows the new phase formed inside the oil just after the BT of

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CW. The oil recovery during the secondary CWI was 58% of the OOIP, which was around 3% higher

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than the recovery during tertiary CWI and around 12% higher than the oil recovery obtained by

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secondary WF (Figure 10). Although both secondary and tertiary CWI were effective in improving oil

245

recovery compared to plain (conventional) waterflood, the secondary CWI was more efficient. This is

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because of the higher oil saturation and better oil connectivity during secondary CWI compared to

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tertiary CWI. Furthermore, the saturation of the new gaseous phase at any given time during CWI

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was higher in secondary CWI than in tertiary CWI. The reason for the observed stronger and faster

249

formation of new gaseous phase during secondary CWI is the lower saturation of water in the model

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during secondary CWI compared to tertiary where the model had been flooded with plenty of plain

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water beforehand.

252



B

C

D

Oil Injection

A

CWI

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Fig. 8. Oil distribution in the micromodel at different time periods. A) Initial oil saturation. B) After 1 hours of CWI. C) After 24 hrs of CWI, and D) Final seawater injection period to strip the dissolved CO2 out of oil. The yellow colour shows the gaseous phase formed inside the oil during CWI period.


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New gaseous phase

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Fig. 9. Nucleation of new phase inside the oil just after CW breakthrough 60 55 50 45

RF (%OOIP)

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40 35 30 25 20 15 10 5 0

259 260

Secondary WF

Secondary CWI

Tertiary CWI

Fig. 10. Recovery factor (%OOIP) during secondary WF and secondary and tertiary CWI

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Figure 11 shows the residual oil saturations at the end of the secondary CWI and its secondary WF

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counterpart. Furthermore, it shows the saturation of new gaseous phase at the end of secondary

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CWI. Interestingly, the saturation of new gaseous phase at the end of CWI is close to the difference

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between the oil saturations at the end of secondary CWI and secondary WF. This result indicates the

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importance of formation and growth of new gaseous phase on additional oil recovery by CWI.



55 50

Oil/Gas Saturation (%)

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Soil after Secondary WF

Soil after secondary Sgaseous phase at the CWI end of secondary CWI

Fig. 11. Residual oil saturations at the end of secondary CWI and counterpart secondary WF and new gaseous phase saturation at the end of secondary CWI

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3.3. Effects of Nucleation and Growth of the New Gaseous Phase on Oil Recovery by CWI

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As was shown in Figures 4 and 9, as soon as CW comes in contact with live crude oil, a new gaseous

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phase nucleates inside the oil at the interface between the oil and CW and with time the saturation

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of this new phase increases. Based on our direct observation, the formation and growth of the new

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gaseous phase was the main reason for the additional oil recovery during CWI in our micromodel.

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Formation and growth of the new phase inside the oil can help improve oil recovery through:

276

1) Reconnection of isolated oil ganglia. Formation of the new phase leads to a much larger effective

277

oil swelling (compared to the normal swelling of oil) and thereby the isolated oil ganglia are

278

reconnected and redistributed. This reconnection and redistribution leads to movements of some of

279

the bypassed oil ganglia and hence, additional oil recovery. Figure 12 demonstrates how CW can

280

lead to reconnection of an isolated oil ganglion. This figure also shows how the new phase grows

281

with time. Shortly after CW comes in contact with live crude oil, small bubbles of gas forms inside

282

the oil at the interface of the oil and CW (Figures 4 and 9). With time and as CWI continues, more of

283

these gas bubbles are formed inside the oil and as they grow, they merge with their surrounding

284

bubbles and form a larger bubble as can be seen from Figure 12. When the saturation of the new 16 ACS Paragon Plus Environment

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285

phase passes a critical value, it can flow inside the oil and brings about a favourable three phase flow

286

(Figure 14).

287 288 A 289 290 291 292 293 294 295 296 297 298 299 300 301 D 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316

B

C Merge of new gaseous phase bubbles

Oil Reconnection

F

E

Oil Reconnection Merge of new gaseous phase bubbles

Fig. 12. Reconnection of an isolated oil in a dead end pore due to formation and growth of the new gaseous phase

317

To quantify the effect of the formation and growth of the new phase on the total enlargement of oil,

318

the change in the volume of an isolated oil ganglion was measured during CWI. As can be seen from

319

Figure 13, the total increase in the volume of oil in this example was around 35% and more than 60%

320

of that was due to formation of the new phase. As can be seen, shortly after the start of CWI, there

321

is some (normal) oil swelling which stops relatively quickly but the increase in the oil volume due to

322

the formation of the new phase continues.



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323 324

Fig 13. Total enlargement of an isolated oil ganglia during CWI period

325 326

2) Restricting the path of CW and diverting it to unswept areas of porous medium. As it is shown in

327

Figure 14, the formation and growth of the new phase can restrict the flow path of flowing water

328

and thereby diverting it to unswept areas of the porous medium. The area highlighted by the circle

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in Figure 14 shows how the growth of the new gaseous phase can block the water path and lead to

330

reconnection of two isolated oil ganglia.

331 332 333 334 335 336 337 338 18 ACS Paragon Plus Environment

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339 A

B

340 341 342 343 344 345 346 C 347

D

Isolated oil ganglia

348 Blocking water path

349 350 351 352 353 E 354

F Oil reconnection

355 356 357 358 359 360 361

Fig. 14. Effect of nucleation and growth of the new phase on restricting the path of CW and diverting it to unswept area of porous media

362

3) Formation of the new phase results in a favourable three phase flow condition. As mentioned

363

earlier, as the injection of CW continues, the new phase formed inside the oil grows in volume and

364

its saturation increases and when its saturation reaches a critical saturation, it starts to flow (Figure

365

15). However, it only flows inside the oil. The flow of this gaseous phase creates a favourable three

366

phase flow region which leads to lower residual oil saturation. Figure 15 demonstrates how the

367

movement of the new gaseous phase inside the oil will affect the oil displacement and leads to

368

lower residual oil saturation. 19 ACS Paragon Plus Environment


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369 A 370 371 372 373 374 375 376 377 378 379 C380 381 382 383 384 385 386 387 388

Page 20 of 24

B

D

389

Fig. 15. Flow of new gaseous phase inside the oil that creates a favourable three phase flow region

390

We noticed that during CWI period the colour of the crude oil became lighter gradually (Figure 16).

391

The colour of a crude oil is an indication of its composition. The heavier the oil the darker its colour

392

and vice versa. The observed change in the colour of oil is attributed to dissolution of CO2 into the

393

oil. As CW comes in contact with oil due to partitioning of CO2 between the CW and oil, CO2 transfers

394

into the oil, which causes reduction in the oil density and viscosity.

395 A

B

396 397

Fig. 16. Change in the oil colour during CWI. A) End of waterflooding period. B) End of CWI

398 20 ACS Paragon Plus Environment

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399

4. Conclusion

400

The flowing conclusion can be drawn from the high-pressure micromodel experiments performed in

401

this study with the aim of visual investigation of pore-scale interactions and fluid compositional

402

effects taking place during oil recovery by CWI.

403

1. Formation of the new phase happened rapidly at very early times during CWI and was

404

stronger for the case of secondary CWI compared to tertiary CWI.

405

2. Formation and growth of the new gaseous phase is a direct function of the amount of

406

dissolved gas in oil which is a function of oil type and saturation pressure and temperature.

407

3. Formation of the new phase led to better oil recovery by; (i) reconnection of the trapped oil

408

and oil displacement, (ii) creating a favourable three phase flow region with less residual oil

409

saturation, and (iii) restricting the flow path of CW and diverting CW toward unswept area of

410

the porous medium.

411

4. The colour of the oil became lighter when it came in contact with CW. This is a good

412

indication of oil density and viscosity reduction due to CO2 transfer from CW to crude oil.

413

5. More than 60% of the total enlargement of the oil volume was due to the formation of the

414

new phase.

415

Acknowledgment

416

This work was carried out as part of the ongoing Enhanced Oil Recovery by Carbonated Water

417

Injection (CWI) joint industry project (JIP) in the Institute of Petroleum Engineering of Heriot-Watt

418

University. The project is equally sponsored by ADCO, BG Group, Eni, Galp Energia, Oil India, and the

419

UK DECC, which is gratefully acknowledged.

420

References

421

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478 479 480 481 482



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483 Graphical Abstract:

484 485 486


CO2 Compositional Effects on

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