Article pubs.acs.org/EF
Evaluation Method of CO2 Sequestration and Enhanced Oil Recovery in an Oil Reservoir, as Applied to the Changqing Oilfields, China Xiaoliang Zhao* and Xinwei Liao China University of PetroleumBeijing, Mail Box 269, Changping, Beijing 102200, People’s Republic of China ABSTRACT: Injecting CO2 into oil reservoirs was proven to have a great CO2 sequestration capacity to reduce greenhouse gas emission and economic potentials via enhanced oil recovery (EOR). This paper proposes a CO2 sequestration estimation method based on the material balance method, which considered the CO2 displacement efficiency, CO2 sweep efficiency, CO2 dissolution, and some reservoir and fluid properties. The CO2 EOR estimation method is also proposed and refers to the traditional petroleum engineering method. In the evaluation method of CO2 sequestration and EOR potential, the sequestration coefficient and recovery factor are two important parameters. In this study, the stream tube simulation method is introduced to determine them. The evaluation method is applied to estimate the CO2 sequestration capacity and EOR potential in the Changqing oilfield of China. The Changqing oilfield province lies in the Ordos Basin of western China. Published data indicate that the Changqing oilfield includes about 22 oilfields, and the majority of oil reservoirs are low-permeability reservoirs. The estimation results of CO2 sequestration and EOR potential show that the Changqing oilfield is suitable for CO2 sequestration and EOR and has great potential. Detailed evaluation of sequestration and EOR is worth further study.
1. INTRODUCTION CO2 sequestration is one of the most effective methods of reducing greenhouse gas emissions. There are five major geologic systems for CO2 sequestration: oil and gas reservoirs, saline formations, unmineable coal areas, shales, and basalt formations.1 Relatively speaking, oil reservoirs are the simplest and most straightforward geologic system for estimating CO2 sequestration capacity because the characteristics of oil reservoirs are generally better known as a result of the extensive history of exploration and production.2 The technoeconomic resource−reserve pyramid of CO2 sequestration capacity2,3 is shown in Figure 1. In the pyramid, theoretical
depends upon technical, legal and governmental, and economic barriers to CO2 sequestration. Matched sequestration capacity is a subset of the practical capacity. It is based on the capacity and the rate of injection and supply of the geological sequestration site. This capacity is the smallest in the pyramid. The carbon sequestration leadership forum (CSLF)4 and the United States Department of Energy (DOE)5 proposed a calculated set of methods for quantifying sequestration capacity. These methods, which are all based on the material balance theory, are used to analyze sequestration capacity. In China, most reservoirs have high water saturation after waterflooding. Therefore, CO2 solubility in water and oil and CO2 sweep efficiency in a reservoir should be considered in calculating CO2 sequestration capacity. In this study, on the basis of previous data sets, a method for estimating the effective sequestration capacity for a highly water-saturated reservoir is derived and applied to the Changqing oilfields. The Changqing oilfield province is one of the largest oilfields in China. Published data indicate that the Changqing oilfield actually includes 22 oilfields covering an aerial extent of about 37 km2 (Figure 2).6,7 Exploration and production began in 1970. Most reservoirs in this oilfield are low-permeability reservoirs. Injecting CO2 into these reservoirs has the added benefit of enhanced oil recovery (EOR) while sequestering CO2 to reduce warm gas emissions.
Figure 1. Techno-economic resource−reserve pyramid for CO2 sequestration capacity.
2. EVALUATIONS OF CO2 SEQUESTRATION AND EOR POTENTIAL
sequestration capacity is the greatest. It represents the physical limit of what a geological system can accept, and it occupies the whole of the resource pyramid. The effective sequestration capacity represents a subset of the theoretical capacity and depends upon technical (geological and engineering) cutoff limits. The effective sequestration capacity usually changes with the introduction of new technology and new data. Practical sequestration capacity is a subset of the effective capacity that © 2012 American Chemical Society
2.1. Calculating Sequestration Capacity. The capacity for CO2 sequestration in oil reservoirs is calculated on the basis of reservoir properties, such as original oil in place (OOIP), recovery factor, temperature, pressure, rock volume and porosity, as well as in situ CO2 Received: May 7, 2012 Revised: July 6, 2012 Published: July 9, 2012 5350
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SCO2 = Ce{[(1 − Spw )R f + SwR w ] + Ef Spw(1 − R w )mCO2,in water + Ef (1 − Spw)(1 − R w)mCO2,in oil }
(8)
SCO2, the sequestration factor, is introduced here. From eq 8, this sequestration factor can reflect the CO2 solubility in oil and water, CO2 sweep efficiency, CO2 recovery factor of oil and water, etc. The sequestration factor can be derived from local CO2 EOR experience or reservoir simulation. In this study, SCO2 is derived by the stream tube simulation method. 2.2. Calculating EOR Potential. The estimation of EOR potential is a mature technique based on many years of development, research, and experience in oilfields.8,9 EOR potential can be given by MCO2 = R f Ahϕ(1 − Sw)
Figure 2. Location of Changqing oilfields in China.
density. For some reasons, processes and reservoir characteristics, such as buoyancy, mobility ratio, inhomogeneity, water saturation, and the nature of aquifers, can all reduce the actual volume available for CO2 sequestration.2 CSLF proposed a method of effective sequestration capacity3 MCO2,e = CeMCO2,t = CmC bC hCwCaMCO2,t
(9)
In the above equation, Rf is the recovery factor, and A, h, f, and Sw are the reservoir area, thickness, porosity, and water saturation, respectively. 2.3. Determination of Key Factors. Estimation methods for sequestration capacity and EOR potential involve two key factors: Rf and SCO2. These factors must be determined prior to the estimation. Rf and SCO2 can be derived from local CO2 EOR experience or reservoir simulation. In this study, a stream tube simulation method9−11 is adopted to determine them. This method has a track record of reliable results from rapid simulations.10 The basic assumptions of the stream tube simulation method are as follows:11,12 (1) In the process of fluid flow, oil and water do not evaporate to the gas phase. The miscible form of the injected gas and crude oil is the first-contact miscible. (2) Flooding is isothermal. (3) The viscous fingering is described by the Koval factor. (4) Water and CO2 are injected with a certain proportion. (5) No large fractures exist in the reservoir; the injected CO2 does not leak. (6) There is no free gas in the reservoir for miscible displacement. According to the material balance law, we can determine an equation for the component concentration and fractional flux
(1)
where MCO2,t is the theoretical sequestration capacity based on the material balance theory, MCO2,e is the effective reservoir capacity for CO2 sequestration, where the subscripts m, b, h, w, and a stand for mobility, buoyancy, heterogeneity, water saturation, and aquifer strength, respectively, and the coefficient Ce is a single effective capacity coefficient that incorporates the cumulative effects of all of the others. MCO2,t can be divided into three parts that consider the volume of displacement and a solution for high water saturation reservoirs. They are (a) CO2 displacement, (b) CO2 solution in oil, and (c) CO2 solution in water, on the basis of the material balance theory
∂Ci ∂Fi + =0 ∂t D ∂XD
(10)
where i = 1 are the water components, i = 2 are the oil components, and i = 3 are the injected gas components. XD = X/L is the dimensionless distance of the system. tD = ∫ t0qdt/dVp is dimensionless time in the pores. Ci is the overall concentration of component i
MCO2,displace + MCO2,in oil + MCO2,in water
(2)
MCO2,displace = ρCO ,r (R f POIP − Viw + Vpw)
(3)
MCO2,in water = Ef ρCO ,r (PWIP + Viw − Vpw)mCO2,in water
(4)
Ci = Ci1S1 + Ci2S2 + Ci3S3
(11)
MCO2,in oil = Ef ρCO ,r POIP(1 − R f )mCO2,in oil
(5)
Fi = Ci1f1 + Ci2f2 + Ci3f3
(12)
Vpw = AhϕSpwR w
(6)
2
2
2
where Cij is the concentration of component i in the j phase. According to the assumption, oil and water do not evaporate to the gas phase; therefore, C33 = 1 and C13 = C23 = 0. Similar to i, j = 1 is the water phase, j = 2 is the oil phase, and j = 3 is the gas phase. Sj is the saturation of the j phase, and f j is the fractional flow of the j phase. The model described above is the immiscible model. For the miscible model, the terms Ci3S3 in eq 5 and the terms Ci3 f 3 in eq 6 are all 0, and then i = 1 and 2. In this modeling method, calculations of the gas breakthrough time and oil recovery are based on the revised fractional flow theory that considers several factors, including viscous fingering, an area sweep factor, vertical heterogeneity, and gravity segregation, to solve the model by the characteristics method.12 The Koval factor is used to describe the impact of viscous fingering, vertical heterogeneity, and other factors in the model.11 The injected gas tends to flow to the top of the reservoir, and overriding or slipstreaming occurs in the lower part of the reservoir. Increasing the Koval factor can change the effect of gravity in the model.12,13 The stream tube simulation method can transform a threedimensional displacement calculation into a one-dimensional displace-
where MCO2,displace is the sequestration capacity after CO2 flooding in the oil reservoir, MCO2,in oil is the sequestration capacity by dissolution in the remaining oil, and MCO2,in water is the sequestration capacity by dissolution in the reservoir water. POIP is the present oil in place in the reservoir after waterflooding. PWIP is the present water in place in the reservoir after waterflooding. Viw is the total volume of the injected water and invasion water in the reservoir. Vpw is the total volume of the production water in the reservoir. ρCO2,r is the CO2 density in the reservoir. Rf is the recovery factor by CO2 flooding. Ef is the overall sweep efficiency. MCO2,in water is the CO2 solubility in water. MCO2,in oil is the CO2 solubility in oil. A, h, ϕ, and Spw are the reservoir area, thickness, porosity, and present water saturation, respectively. Rw is the water recovery factor. By applying eq 2 to eq 1, the efficient sequestration capacity can be given by
MCO2,e = ρCO ,r AhϕSCO2 2
(7) 5351
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Table 1. Properties of Oil Reservoirs in the Changqing Oilfields oilfield
reservoir pressure (MPa)
reservoir temperature (°C)
reservoir oil viscosity (MPa)
permeability (mD)
porosity (%)
oil saturation (%)
AS CH WJ WWZ NL FJC BYJ HJZ YC BE HJS HC JA DHZ DSK XF YW WQ LZZ IY MF ML ZL MW YFZ SJ
9.4 12.5 16.2 12.7 12.7 17.6 10.5 12.7 16.4 16.0 16.2 14.2 13.4 16.5 11.8 15.7 12.0 15.0 21.5 11.0 15.4 9.7 19.8 16.2 10.7 14.6
44.6 41.4 57.8 51.5 51.5 67.3 43.1 51.5 55.9 67.0 56.1 48.0 50.4 61.2 56.4 53.9 52.1 58.4 64.5 48.2 47.0 53.6 60.5 52.1 47.4 51.4
4.5 3.7 9.9 9.9 4.2 2.7 5.4 9.9 3.2 3.4 6.0 9.9 7.4 3.4 5.2 9.9 5.3 9.2 1.5 7.3 6.2 14.6 2.1 3.6 6.0 4.4
2.7 1.1 379.1 80.0 142.2 29.3 123.7 84.9 67.4 18.3 97.4 0.1 265.7 133.0 54.2 26.0 42.1 5.4 0.8 80.0 25.7 67.4 10.6 70.6 131.6 64.3
12.2 11.6 16.5 16.0 17.5 14.2 15.9 17.0 15.2 15.5 15.7 11.0 20.9 17.2 15.0 17.0 15.8 15.0 13.0 15.5 13.0 17.8 14.3 15.9 16.4 15.5
55.7 54.9 64.0 60.0 63.0 59.0 62.1 64.0 52.5 49.7 62.1 55.0 60.0 60.0 58.8 50.0 62.4 55.7 60.0 54.4 52.9 50.0 54.3 60.0 63.6 62.4
Table 2. Estimation of CO2-Miscible Reservoirs in the Changqing Oilfields oil field
OOIP (×104, tons)
recovery by water flooding (%)
recovery by CO2 flooding (%)
recovery increment (%)
sequestration coefficient (tons/tons)
oil production increment (×104, tons)
sequestration potential (×104, tons)
DHZ BYJ DSK FJC HJZ JY MF ML NL XF YW WJ YC BB
113.09 478.14 398.20 1599.48 1470.13 7421.49 1069.81 8478.78 2711.01 12683.20 219.92 415.91 1819.23 1415.14
20.45 23.71 21.36 22.06 24.91 25.60 22.97 21.51 19.74 23.65 23.02 19.58 21.64 23.73
34.42 36.67 30.49 35.15 38.10 39.66 36.72 33.90 30.43 33.92 36.87 30.42 32.3 1 35.47
13.97 12.96 9.11 13.09 13.20 14.07 13.76 12.39 10.70 10.28 13.85 10.85 10.67 11.75
0.36 0.24 0.24 0.25 0.25 0.30 0.27 0.26 0.26 0.24 0.29 0.20 0.20 0.23
15.80 61.97 36.28 209.37 194.06 1044.20 147.21 1050.52 290.08 1303.83 30.46 45.13 194.11 166.28
40.71 114.75 93.58 399.87 367.53 2226.45 288.85 2204.48 704.86 3043.97 63.78 83.18 363.85 325.48
ment calculation.13 The reservoir recovery and sequestration factor can be predicted quickly and reliably by this method.
reservoir pressure, it is assumed to be a CO2-miscible reservoir; otherwise, it is a CO2-immiscible reservoir. The MMP can be calculated by the following empirical method:13
3. SEQUESTRATION AND EOR ESTIMATION FOR CHANGQING OILFIELDS The Changqing oilfield province lies in the Ordos Basin in western China. Exploration began in 1970, and numerous wells have been drilled.6,7 In this study, 26 reservoirs within the Changqing oilfields, constituting 261 production layers, are evaluated. The properties of these oil reservoirs are summarized in Table 1. Before estimation, these reservoirs should be divided into two categories, CO2-miscible reservoirs and CO2immiscible reservoirs, to compare the difference of displacement effects. Minimum miscibility pressure (MMP) is used to determine the displacement state. If MMP is lower than the
MMP = [− 329.558 + (7.727MW1.005T ) − (4.377MW)] /145 MW =
(13)
⎛ 8864.9 ⎞0.988 ⎜ ⎟ ⎝ G ⎠
where MW is the molecular weight of C5+, G is the oil American Petroleum Institute (API) gravity, and T is the reservoir temperature. According to the MMP calculation, of the 26 reservoirs, 14 are CO2-miscible flooding reservoirs and the other 12 are CO25352
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Table 3. Estimation of CO2-Immiscible Reservoirs in the Changqing Oilfields oil field
OOIP (×104, tons)
recovery by water flooding (%)
recovery by CO2 flooding (%)
recovery increment (%)
sequestration coefficient (tons/tons)
oil production increment (×104, tons)
sequestration potential (×104, tons)
AS CH HIS HC JA WWZ WQ LZZ YFZ SJ ZL MW
26037.8 1107.48 1664.53 4477.83 29323.3 355.35 2332.58 149.59 2334.26 1526.48 1528.98 31.91
21.36 20.69 20.51 21 21 20.65 20.1 19.73 20.02 21.89 21.21 21.2
27.61 30.81 27.52 27.76 29.05 29.8 27.31 23.55 28.72 25.97 25.19 28.3
6.25 10.12 7.01 6.75 8.06 9.15 7.21 3.81 8.7 4.09 3.36 5.1
0.18 0.24 0.18 0.18 0.21 0.22 0.2 0.12 0.19 0.18 0.06 0.32
1627.36 112.08 116.68 302.25 2363.46 32.51 168.18 5.70 203.08 62.43 51.37 1.63
4686.80 265.80 299.62 806.01 6157.89 78.18 466.52 17.95 443.51 274.77 91.74 10.21
immiscible flooding reservoirs. The sequestration capacity and EOR potential can be calculated by eqs 7 and 9. Results are shown in Tables 2 and 3.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 0086-13910274825. Fax: +86-10-89733223. Email:
[email protected].
4. DISCUSSION
Notes
In this study, oil recovery from water and CO2 flooding has been compared to demonstrate the strengths of CO2 flooding. From Tables 2 and 3, for miscible reservoirs, the average oil recovery with CO2 flooding is 34.6%, almost 12% more than with water flooding. Also, the total increment in oil production and CO2 sequestration capacity can reach 48 × 106 and 103 × 106 tons, respectively. For immiscible reservoirs, the average oil recovery with CO2 flooding is 27.6%, almost 6.6% more than with water flooding. Furthermore, the total increment in oil production and CO2 sequestration capacity can reach 50 × 106 and 136 × 106 tons, respectively. From the results, CO2-miscible flooding is shown to result in more oil production and sequestration capacity. The assessment results indicate that injecting CO2 into the Changqing oilfield can provide both significant CO2 sequestration capacity and EOR.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program, Grant 2011CB707302) and the Chinese National Major Science and Technology (Projects 2011ZX05016-006 and 2011ZX05009-004-001). We are grateful to all staff involved in this project and also thank the journal associate editor and the reviewers.
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REFERENCES
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5. CONCLUSION The estimation methods presented here were based on a material balance method and were applied to the Changqing oilfield province in China. The following conclusions were made: (1) The methods proposed in this paper can estimate both the sequestration capacity and EOR potential efficiently and reliably considering reservoir features, fluid features, and some CO2 flooding features. (2) The CO2 sequestration and EOR potential of the Changqing oilfields are estimated in this paper. In general, oil reservoirs can provide much larger sequestration capacity, and CO2 flooding can improve oil recovery effectively. (3) Widespread application of geological sequestration in oil or gas reservoirs is presently constrained by economic and political considerations. CO2 injection into oil reservoirs can be a favorable option for the petroleum industry in the near future, and detailed evaluation of sequestration and EOR is worth further study. (4) In the evaluation method presented here, an empirical calculation of MMP is used; however, such a method is limited. A more accurate MMP calculation method should be determined and introduced to improve assessments of CO2 sequestration and EOR potential. 5353
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