188bet asian Potential of CO2-EOR associated with CCS in VietNam
Summary
CO2-EOR has a potential not only in enhancing oil recovery but also in CO2 storage. It is, however, necessary to understand 188bet asian technical, operational and economic uncertainties by conducting a pilot test for applying CO2-EOR to actual oil fields. This paper describes 188bet asian methods and procedures to design an inter-well CO2-EOR pilot test and discusses 188bet asian potential of CO2-EOR in offshore fields, Vietnam.
We devised screening criteria in terms of saturations and reservoir pressure to select locations for pilot tests based on 188bet asian reservoir model developed and calibrated with 188bet asian actual field data. 188bet asian number of pilot wells was determined to be one (1) injection well and two (2) production wells in order to estimate areal sweep efficiency during CO2 injection at a minimum cost. To confirm 188bet asian effects of CO2-EOR, well interval, cumulative CO2 injection volume and duration were determined to be 300m, 2,000 tons and twoweeks, respectively. 188bet asian oil production rate began to increase after 1 month from 188bet asian beginning of 188bet asian pilot test, and 188bet asian gas production rate and 188bet asian vapour phase CO2 mole fraction also began to increase at 188bet asian same time. Then, 188bet asian oil production rate reached 188bet asian peak, which corresponds to oil bank, after 1.5 months from 188bet asian beginning, and 188bet asian effect of CO2-EOR was confirmed clearly. If oil saturation is high at 188bet asian target reservoir, it would be difficult for confirming 188bet asian oil bank clearly. In such case, pre-water injection before CO2 injection is conducted to reduce 188bet asian oil saturation at 188bet asian target reservoir.
In parallel, a CO2 procurement and a new platform were designed through 188bet asian process optimisation study. Manufactured CO2 was selected as CO2 source for 188bet asian pilot test due to easy availability andminimumcost. 188bet asian packedmanufactured CO2 into tank containers was transported to 188bet asian injection site by trucks and supply vessels. From a CO2 impact study, 188bet asian potential risks related to corrosion and scale formation can bemanageable by suitable countermeasures such as corrosion resistant alloys (CRAs) and inhibitors.
We also estimated 188bet asian CO2 volume for storage in 188bet asian reservoir when CO2 injection was applied. Wefoundabouthalfofthe injected CO2 volume to remain in 188bet asian reservoir at 188bet asian normal operational conditions for CO2-EOR. Through this study we proposed a methodology to devise a pilot test plan of CO2-EOR in any offshore, remote oil field.
Key words: CO2-EOR, pre-water injection, CO2 injection.
1. Introduction
Enhanced oil recovery (EOR) by CO2 has a potential not only in enhancing oil recovery but also in CO2 storage. CO2 is a useful agent due to relatively high density and high solubility to crude oils. Because it is expected that sweep efficiency during CO2 injection is better than that in other gases i.e. hydrocarbon gases. It is also expected that miscible displacement by CO2 that enables to achieve high oil recovery is developed more readily in other gases.
In addition, CO2 is one of 188bet asian greenhouse gasses that address global warming and climate changes. Many efforts have been made to reduce CO2 produced from industrial processes for a decade. Carbon stored underground is an important candidate for preventing CO2 gas from entering 188bet asian atmosphere. 188bet asian potent underground is oil and gas reservoirs. This is because 188bet asian rock caps that are impermeable layer are integrity and 188bet asian volume is large enough to be stored.
To meet incremental oil recovery and carbon storage, CO2-EOR associated with carbon capture and storage (CCS) has been proposed by many organisations. Similar to applications of CO2-EOR, it is necessary to understand 188bet asian technical, operational and economic uncertainties by conducting a pilot test to actual oil fields.
188bet asian Vietnam Oil and Gas Group (PVN) and JOGMEC conducted an international joint study on a design of inter- well CO2-EOR pilot test at an oil field in Vietnam offshore from 2013 to 2015 in order to understand 188bet asian applicability of CO2-EOR associated with CCS. 188bet asian objectives of this study are to understand 188bet asian CO2 injectivity, vertical sweep and areal sweep efficiency, incremental oil production, injected CO2 volume and cost/risks.
To design 188bet asian CO2-EOR pilot test, not only subsurface study but also surface study is important to implement a proper and safe operation. When 188bet asian injected CO2 is produced, potential risks related to CO2 such as corrosion and scale formation on 188bet asian surface facilities will be increased. This paper describes 188bet asian subsurface study including uncertainty analysis and countermeasure and 188bet asian surface study including required CO2 procurement method, optimised surface facility design, and corrosion/ scale formation studies.
2. Subsurface study
Site selection
• Reservoir pressure > 2,600psi
• Oil saturation > 0.2
• Gas saturation = 0
188bet asian key factors to screen a pilot area for achieving 188bet asian miscible displacement are 188bet asian reservoir pressure in 188bet asian pilot area. MMP of oil in this field was estimated to be 2,950psig based on 188bet asian past laboratory studies such as slim-tube test and IFT measurement [1]. To observe 188bet asian sufficient effects of CO2-EOR, 188bet asian reservoir pressure during CO2 injection was set over 2,600psi based on 188bet asian simulation study. A potential hydrocarbon zone was identified by a rule of thumb that porosity multiplied by 188bet asian oil saturation is 0.05. We assume that 188bet asian average porosity around 188bet asian pilot test area is 0.25, thus, 188bet asian oil saturation is estimated to be 0.2 as a screening criterion. If free gas is liberated in 188bet asian reservoir, it leads to an undesirable oil displacement. Therefore, 188bet asian gas saturation is set to 0. Some candidate areas for 188bet asian pilot test were selected using 188bet asian criteria.
Thereafter, a certain area was selected as 188bet asian first option for 188bet asian following reasons. 188bet asian selected area is not close to 188bet asian current producing areas and 188bet asian large faults which may lead CO2 leaking from 188bet asian reservoir, and 188bet asian sufficient incremental oil was clearly estimated to confirm 188bet asian effects of CO2-EOR based on 188bet asian reservoir simulation results. Moreover, because 188bet asian operator has a plan to develop 188bet asian vicinity of this area, synergy between 188bet asian development and 188bet asian pilot test areas in terms of sharing 188bet asian new platform has been expected.
Base case simulation
We designed a pilot test of CO2-EOR based on a reservoir simulation study. A pilot well configuration was selected to be one injection well and two production wells because of 188bet asian minimum number of wells in order to understand CO2 areal sweep [4].
Some parameters such as well spacing, CO2 injection rate and total CO2 injection volume were determined based on 188bet asian sensitivity analysis of reservoir simulation.
For determining these parameters, CO2 utilisation factor (= cumulative incremental oil volume by CO2- EOR/cumulative CO2 injection volume) was used as a performance index. 188bet asian selected simulation settings for 188bet asian pilot test are summarised in Figure 1. For maintaining 188bet asian reservoir pressure, 188bet asian BHP control value in 188bet asian production wells is set so that 188bet asian voidage replacement ratio should be one. To save 188bet asian required CO2 volume for confirming 188bet asian effects of CO2-EOR, both injection well and production wells were perforated in 188bet asian limited zone of reservoir. 188bet asian horizontal grid was refined to investigate 188bet asian fluid advancement in detail.
Production profile predicted by reservoir simulation is shown in Figure 2. 188bet asian oil production rate began to increase 1 month after 188bet asian beginning of 188bet asian pilot test. In addition, 188bet asian gas production rate and 188bet asian vapour phase CO2 mole fraction also began to increase at 188bet asian same time. Then, 188bet asian oil production rate reached 188bet asian peak after 1.5 months from 188bet asian beginning, and 188bet asian effect of CO2-EOR can be confirmed clearly. As a result, 188bet asian cumulative CO2 injection volume is 36,791Mscf (1,948 tons), and 52.8% amount of CO2 is stored in 188bet asian reservoir (19,437Mscf, 1,029 tons). We also simulated 188bet asian stored CO2 with various injection scenarios in 188bet asian full field. 188bet asian ratio of 188bet asian stored CO2 to 188bet asian injected CO2 during CO2-EOR is around 50% and 80% without and with recycling of produced CO2, respectively. Figure 3 shows 188bet asian simulation results in 188bet asian 2nd layer. CO2 sweeps mainly in this layer because 188bet asian permeability is 188bet asian highest in 188bet asian other layers. Reservoir pressure near 188bet asian injection well is higher than MMP during 188bet asian simulation period. These simulation outputs also indicate practical aspects of CO2 flooding as follows. 188bet asian interfacial tension in 188bet asian CO2 swept area is very low (less than 1.0 dyne/cm). In addition, 188bet asian oil viscosity in 188bet asian CO2 swept area except for 188bet asian vicinity of injection well becomes less than that in 188bet asian unswept area. Oil saturation is decreased near 188bet asian injection well, and an oil bank is seen at 188bet asian downstream side of CO2 front. Residual oil saturation after 188bet asian pilot test is small (less than 0.1) near 188bet asian injection well.
Uncertainty analysis
188bet asian uncertainties of reservoir characteristic in 188bet asian pilot test area are relatively high because it is an undeveloped area. For evaluating 188bet asian uncertainties, based on 188bet asian previous study [2, 3], sensitivity analysis using reservoir simulation and Monte Carlo simulation was conducted to address 188bet asian key geological uncertainties which have significant impacts on 188bet asian pilot performance, and make suitable countermeasures for a successful pilot implementation. In 188bet asian sensitivity study, six geological parameters involving oil saturation, reservoir pressure, horizontal permeability, Kv/ Kh, Sorw and Sorg were selected. For each parameter, low and high cases were set based on 188bet asian representative values from 188bet asian reservoir characteristics. This sensitivity analysis was conducted by changing 188bet asian parameter one by one while keeping 188bet asian other parameters same as 188bet asian base case. Figure 4 shows 188bet asian sensitivity results for CO2 breakthrough
timing and cumulative production oil volume. 188bet asian bar chart shows 188bet asian difference over 188bet asian base case. If 188bet asian horizontal permeability or reservoir pressure is lower than 188bet asian base case, 188bet asian CO2 breakthrough timing delayed about 1 month. If 188bet asian oil saturation is higher than 188bet asian base case, 188bet asian cumulative oil production volume increases significantly.
Monte Carlo simulation was conducted to estimate 188bet asian range of pilot performance. In order to apply 188bet asian Monte Carlo simulation for evaluating 188bet asian range of 188bet asian pilot performance, 188bet asian response surface equation (RSE) and 188bet asian probability distributions were constructed. 188bet asian modelling of response surface equation for 188bet asian cumulative oil production (COP) and 188bet asian CO2 breakthrough timing (BTT) were conducted based on 188bet asian regression of 188bet asian sensitivity analysis results. 188bet asian following equations were modelled as a function of geological uncertainties.
Figure 5 showsthecorrelationsbetweentheprediction results by RSE and 188bet asian reservoir simulation output. 188bet asian figure suggests that 188bet asian predicted value by RSE is high correlation with that of 188bet asian reservoir simulation.Figure 6 shows representative probability distributions of 188bet asian geological parameters in 188bet asian pilot area based on 188bet asian simulation model. Figure 7 shows 188bet asian output of Monte Carlo simulation for CO2 breakthrough timing and cumulative oil recovery for each probability. It is predicted that more than 41,000bbl oil will be recovered with 90% probability. In other words, 188bet asian probability to be less than 41,000bbl is only 10%. Those results depend on 188bet asian assumed probability distribution significantly. Table 1 shows 188bet asian estimated geological parameters for each probability based on 188bet asian most likely scenario.
188bet asian pilot production performance for each probability was predicted by reservoir simulation based on 188bet asian estimated geological parameters. Figure 8 shows 188bet asian oil production rate and CO2 concentration in production gas for P10, P50 and P90 cases, respectively. 188bet asian left figures show 188bet asian prediction results of 188bet asian base
case. They indicate a wide range of 188bet asian performance. It means that pilot production performance has a high uncertainty. Furthermore, it is found that 188bet asian incremental oil production is not confirmed clearly in P10 case because of 188bet asian shortage of CO2 injection volume over 188bet asian large amount of oil in 188bet asian reservoir.
Countermeasure for successful pilot test
If a large amount of CO2 can be prepared, we will confirm 188bet asian oil bank readily even in P10 case. CO2 procurement volume is, however, limited in 188bet asian offshore field.We consider pre-water injection before CO2 injection as an effective countermeasure for a successful pilot test implementation. 188bet asian right figures in Figure 8 show 188bet asian prediction results based on 188bet asian base case scenario plus pre-water injection for 5 months. If pre-water injection is applied, not only 188bet asian range of uncertainty decreases but also incremental oil by CO2-EOR is confirmed readily. If 188bet asian actual oil saturation is higher than 188bet asian expected oil saturation around 188bet asian newly drilled pilot wells, 188bet asian pre-water injection will be conducted for understanding 188bet asian effects of CO2-EOR.
3. Surface study
CO2 procurement
We purchase liquefied CO2 which is 188bet asian enough CO2 volume required for CO2-EOR pilot test from 188bet asian supplier.
It is assumed that liquefied CO2 in 188bet asian tank containers is transported to 188bet asian offshore platform by trucks and vessels. Manufactured CO2 is filled up into 188bet asian tank containers and carried by trucks to temporary storage area in 188bet asian nearby port. 188bet asian tank containers are moved to 188bet asian supply vessels on 188bet asian sea and transported to 188bet asian pilot test area.
Surface facility design
Because 188bet asian pilot test area is undeveloped, it is necessary to build a new platform for 188bet asian pilot test as well as treatment of production liquid, water injection and gas lift pipeline. As shown in Figure 9, liquid CO2 supplied from a supply vessel is send to 188bet asian injection pump through 188bet asian CO2 header, and injected into 188bet asian reservoir from injection well via heater. Liquid CO2 heater is to prevent cracking of pipe and degradation of equipment materials. Injection water is supplied from 188bet asian existing platform when water is injected after CO2 injection. 188bet asian volume of produced gas and liquid from each production well is measured with 188bet asian test separator.
CO2 breakthrough impact
188bet asian CO2 impact study for all 188bet asian related facilities was conducted and divided into 188bet asian following steps:
- Process simulation for all 188bet asian related facilities;
- Corrosion and scaling formation estimation study for 188bet asian related facilities;
- Material selection study based on process simulation and estimation results of corrosion and scaling formation.
Process simulation has been conducted to confirm how much CO2 is distributed to 188bet asian related facilities. Corrosion rates and scaling formation potentials were estimated with a corrosion/electrolyte/scaling simulator, based on a “new fresh” carbon steel corroding surface.
Corrosion
Figure 10 shows 188bet asian total predicted corrosion amount for each production equipment/flowline for carbon steel, for 188bet asian duration of 188bet asian CO2-EOR project life. It was calculated by integrating 188bet asian corrosion rate for each time. It shows that there is a very high total corrosion of 1.3mm of 188bet asian production well tubulars downhole, with a reduced total corrosion of 0.31mm at 188bet asian production wellhead once combined with lift gas, which further reduces to 0.28mm once combined
with 188bet asian produced fluids from other well at 188bet asian inlet to 188bet asian flowline. 188bet asian high predicted total corrosion depth for 188bet asian well tubulars over 188bet asian duration of 188bet asian CO -EOR project for carbon steel is not deemed acceptable. It is noted that total corrosion of 1.35mm is predicted at 188bet asian hydrocarbon gas compressor if 188bet asian carbon steel is used. As high corrosion resistance alloy is used, 188bet asian actual total amount of this equipment should become smaller and be negligible. Scaling
188bet asian predicted potential of BaSO4 and CaCO3 scales in 188bet asian production system is presented in Figure 11.
This shows how 188bet asian scale potential changes through 188bet asian process, for 188bet asian baseline simulation (before CO2 breakthrough) and 188bet asian predicted peak breakthrough of CO2. Only 188bet asian 2 cases have been plotted as 188bet asian extreme cases. All of 188bet asian simulated scale formation potentials for BaSO4 in 188bet asian production process train are less than 2mg/L,
and so are manageable by suitable scale inhibitors. 188bet asian breakthrough of CO2 into 188bet asian production wells eliminates 188bet asian scaling formation potential of CaCO3 in 188bet asian front end of 188bet asian production process train from 188bet asian new well bottoms through to 188bet asian inlet to 188bet asian 1st heater inlet. This is probably due to 188bet asian addition of CO2, shifting 188bet asian pH of 188bet asian produced fluids downwards (more acidic conditions) and therefore keeping 188bet asian scaling ions in solution. With CO2 breakthrough, there is an increase in scaling potential of CaCO3 at 188bet asian outlet of 188bet asian 1st heater, but this level of CaCO3 scale is manageable by a suitable scale inhibitor. At 188bet asian water outlet of 188bet asian 2nd separator and all vessels downstream, there is very little change in 188bet asian scaling potential for CaCO3, with only a 10% increase in 188bet asian predicted amount of CaCO3 scale from 188bet asian baseline simulation to 188bet asian predicted peak breakthrough of CO2. This level of scale increase can easily be managed through 188bet asian existing scale inhibitors used in this oil field.
188bet asian predicted scale formation potential for 188bet asian produced water handling system is presented by scale type in Figure 12. It does not change significantly from 188bet asian baseline simulation to 188bet asian predicted peak breakthrough of CO2. This level of scale in 188bet asian produced water handling system can be easily managed through 188bet asian existing scale inhibitors used in this oil field.
Material selection
Corrosion rate simulation was made using high corrosionresistancealloysasindicatedbelowtodetermine their suitability:
• 13Cr steel alloy;
• Stainless steel 316;
• Duplex stainless 2205;
188bet asian simulated corrosion rates across 188bet asian process for these higher steel grades are presented in Figure 13. It is seen that all of 188bet asian corrosion simulated high corrosion resistance alloys offer a vast improvement in 188bet asian corrosion rate in 188bet asian case of CO2 concentration peak. 188bet asian corrosion rate at 188bet asian well bottom for carbon steel at 10.37mm/year reduces to < 0.03mm/year when 13Cr steel alloy is used. This decrease in corrosion rate indicates 188bet asian permissible corrosion limits that can be easily managed through 188bet asian use of corrosion inhibitors. 188bet asian corrosion rate can be further reduced through 188bet asian use of either stainless steel 316 or duplex stainless 2205, as has been used in some oil fields to successfully mitigate corrosion. It should be noted that both stainless steel 316 and duplex stainless 2205 are very expensive for oil field process equipment.
Mitigation plan
188bet asian mitigation plan for corrosion risk and scale formation risk is summarised in Figure 14. It is quite evident that in 188bet asian case of CO2 concentration peak 188bet asian well completions in carbon steel are not appropriate. It is said that 13Cr steel alloy has typically been used for high CO2 gas wells. Stainless steel 316 appears to have a higher corrosion resistance than 13Cr steel alloy. It will, however, be prone to chloride stress corrosion cracking in higher temperature environments > 50°C in 188bet asian presence of chloride ions. Duplex 2205 should provide even better corrosion resistance, but with very significant cost rise. 188bet asian use of 13Cr steel alloy is suitable for 188bet asian new production wells. It is effective to use also a variety of corrosion and scale inhibitors. As for test separator that will be installed on 188bet asian pilot test platform, lining inside 188bet asian separator by polymers such as polyethylene resin is desirable because fluid containing high CO2 concentration will flow into 188bet asian separator although CO2 partial pressure decreases compared to wellhead and well bottom. In addition, it is necessary that regular pigging of 188bet asian subsea flowlines and flaring of 188bet asian associated produced gas at 188bet asian pilot test platform reduce 188bet asian corrosion potential in 188bet asian topside process equipment and flowlines.
4. Conclusions
188bet asian CO2-EOR inter-well pilot test in 188bet asian offshore oil field was designed to achieve 188bet asian aim in all situations as follows;
• 188bet asian number of pilot well is determined to be 1 injection well and 2 production wells because of 188bet asian minimum number of wells in order to estimate CO2 areal effective countermeasure.
• To reduce 188bet asian procurement cost of required CO2 for 188bet asian pilot test, CO2 was determined to be secured from a CO2 manufactory.
• New platform was designed through 188bet asian process optimisation study.
• There will be increased potential risk of corrosion and scale formation. 188bet asian high predicted total corrosion depth for 188bet asian well tubulars over 188bet asian duration of 188bet asian CO2- EOR project for carbon steel is not deemed acceptable, so 188bet asian use of CRA’s in 188bet asian production wells are essential. In addition, inside lining at 188bet asian test separator, pigging and corrosion/scale inhibitors are necessary.
When 188bet asian CO2-EOR pilot test will be planned in any oil field, it will be possible to make 188bet asian test design with 188bet asian same procedure as this study.
Acknowledgements
This study was implemented as a part of 188bet asian joint study between 188bet asian Vietnam Oil and Gas Group (PVN) and JOGMEC, financially supported by 188bet asian Ministry of Economy, Trade and Industry (METI) in Japan. We would like to thank Petrovietnam and METI for providing permission to publish this paper.
References
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• We also simulated 188bet asian stored CO2 with various injection scenarios in 188bet asian full field. 188bet asian ratio of 188bet asian stored CO2 to 188bet asian injected CO2 during CO2-EOR is around 50% and 80% without and with recycling of produced CO2, respectively.
• Uncertainty analysis indicates that there is a possibility 188bet asian oil bank becomes smeared if 188bet asian oil saturation is high. For confirming 188bet asian effect of CO2-EOR clearly, pre-water injection before CO2 injection is an
2. D.Kuramoto. Uncertainty evaluation and history in concerning oil and natural gas reservoir simulation. Oil and Natural Gas Review. 2010; 44(2).
3. Ali Rawahi, Hafez H.Hafez, Arafat Al-Yafei, Saleem G.Ghori, Kevin L.Putney, Timonthy R.Matthews. Maximize 188bet asian ultimate recovery by designing and optimising a CO2 miscible gas injection pilot in a giant carbonate oil reservoir, Abu Dhabi. International Petroleum Conference and Exhibition, Abu Dhabi, UAE. 11 - 14 November, 2012.
4. Gary F.Teletzke, Robert C.Wattenbarger, John R.Wilkinson. Enhanced oil recovery pilot testing best practices. SPE Reservoir Evaluation & Engineering. 2010; 13(1).