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RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 36, Issue 2, April 2009 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2009, 36(2): 216–220.

Received date: 30 December 2008; Revised date: 10 January 2009 * Corresponding author. E-mail: [email protected] Foundation item: Supported by the 973 Project (2006CB705800) and the Ministry of Education New Century Talents Support Program. Copyright © 2009, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Methodology for estimation of CO2 storage capacity in reservoirs

Shen Pingping1,*, Liao Xinwei2, Liu Qiujie1

1. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China 2. China University of Petroleum, Beijing 102249, China

Abstract: The CO2 storage in reservoirs is one of the most effective ways of reducing the greenhouse gas emission, which is based on the mechanisms of structural and stratigraphic trapping, residual gas trapping, dissolution trapping and mineral trapping. The CO2 storage capacity in oil reservoirs includes theoretical, effective, practical and matched storage capacities. In the estimation of the CO2 storage ca-pacity in both waterflooding and CO2 flooding oil reservoirs, theoretical and effective storage capacities can be obtained by the material balance and analogy methods. The theoretical storage capacity represents the physical limit of what the reservoir system can accept. The effective storage capacity represents a subset of the theoretical capacity and is obtained by applying a range of technical cut-off limits to a storage capacity assessment which incorporate the cumulative effects of reservoir and fluid parameter. When the material balance method is used, the amount of CO2 dissolution is not negligible. In using the analogy method, the key is to determine CO2 utilization factor. Ex-amples show that the method is simple and convenient for the estimation of the CO2 storage capacity in China.

Key words: carbon dioxide; storage; waterflooding; carbon dioxide flooding; dissolution; algorithm

Introduction

As a result of excessive dependence on fossil fuels, emis-sion of greenhouse gases (mainly carbon dioxide) from indus-trial production and human activities is increasing. The re-sulting air pollution and greenhouse effect is seriously threat-ened to the environment where human survives. One of the most effective ways to reduce GHG emission is to store CO2

in geologic bodies, such as coalbed, deep saline aquifers and oil and gas reservoirs. Comparing with coalbed and deep sa-line aquifers, oil reservoir is much better explored and devel-oped, with clearer characteristics and more data available. Moreover, CO2 storage in oil reservoirs can not only reduce the GHG emission, but also improve the oil recovery. There-fore, CO2 storage in reservoirs is the most economical and reliable technology at present. To apply it, the CO2 storage potential must be evaluated first to determine the maximum theoretical storage capacity and effective storage capacity of CO2 in reservoirs.

Predecessors have studied the calculation methods (mainly for depleted reservoir) for CO2 storage capacity in reservoirs [1-11], and have made evaluation on the study area. The authors make a systematical study on the calculation method for CO2

storage capacity after calibrating existing method and consid-ering the practical waterflooding operations in Chinese reser-

voirs and the CO2 dissolution in crude oil and water. This pa-per will introduce the methods in terms of theoretical storage capacity and effective storage capacity, thus to provide a tech-nical tool to evaluate the CO2 storage potential in reservoirs in China.

1 CO2 storage mechanism and capacity classifi-cation in reservoirs

Fig. 1 shows that CO2 storage in reservoirs[3-5] is realized

Fig. 1 Mechanism of CO2 storage in reservoir

Shen Pingping et al. / Petroleum Exploration and Development, 2009, 36(2): 216–220

through structural space and bond space, dissolution trapping and mineral trapping, of which first two are the most impor-tant storage modes. Therefore, it is the key for calculating theoretical storage capacity of CO2 in reservoirs to determine the geometrical space providing for CO2 storage. With CO2

storage duration increases, the dissolution of CO2 in crude oil and water is not negligible. It should be considered in calcula-tion.

Based on literatures [3-5], CO2 storage potential in geologic bodies includes four levels theoretical, effective, practical and matched storage capacities. Theoretical storage capacity repre-sents the physical limit of what the geological system can accept, and it constitutes the entire resource pyramid. Effective storage capacity considers the factors technically, such as property of reservoir, seal ability, storage depth, pressure systems and pore volume; it is a subset of theoretical storage capacity. Practical storage capacity is a subset of effective storage capacity by con-sidering technical, legal and regulatory, infrastructural and eco-nomic conditions. Matched storage capacity is a subset of prac-tical storage capacity by considering CO2 sources, injectivity and supply. Depending on evaluation purposes and storage modes, different calculation methods are required in order to estimate potential CO2 storage capacity. Generally, practical storage capacity and matched storage capacity should be calcu-lated only for a given reservoir and relevant conditions. The key is to determine theoretical storage capacity and effective storage capacity when CO2 storage sites in reservoirs are screened ini-tially. This paper will discuss the calculations methods for these two capacities.

2 Calculation methods of CO2 storage capacity in reservoirs

The calculation method of CO2 storage capacity in reservoir depends on the storage mechanism, and also the reservoir status. There are two types of reservoirs in China for CO2

storage, namely post-waterflooding reservoir and CO2 flood-ing reservoir (for reservoir that is hard to produce by water-flooding). The following sections will discuss the calculation methods and matters of theoretical and effective storage ca-pacities for these reservoirs.

2.1 Calculation method of theoretical storage capacity

Current calculation methods of CO2 theoretical storage ca-pacity and effective storage capacity in reservoirs are estab-lished on the basis of material balance equation with the as-sumption of all geometrical space of produced oil and gas can be used for CO2 storage. The relevant researchers from US-DOE, European Commission and the Carbon Sequestration Leadership Forum (CSLF) have further investigated the cal-culation methods for CO2 theoretical storage capacity in res-ervoir, which were established on the basis of an assumption that reservoirs are not in contact with an aquifer, or that the reservoir is not floodout during secondary and tertiary oil re-coveries. In China, most oilfields are developed by water-flooding. The paper proposes the calculation methods for CO2

theoretical storage capacity, which considers CO2 dissolution, according to the characteristics of waterflooding reservoir development in China and CO2 storage mechanism.

2.1.1 Calculation method of theoretical storage capacity for waterflooding reservoirs

The method assumes reservoir pressure to build up the original reservoir pressure when CO2 is injected to the reser-voir, i.e., the free space left by the produced oil and gas is wholly occupied by CO2, and CO2 may dissolve into crude oil and water during injection. CO2 storage capacity can be ex-pressed by the following equation.

wspwiwwiR9r

t )1(10

CVVSAhEM

wi iw pw os R w1 1Ah S V V C E Ah S (1) Where

Mt——theoretical storage capacity of CO2 in reservoir, 106 t; r——density of CO2 under reservoir condition, kg/m3;

ER——the oil recovery factor, f; A——reservoir area, m2;h——reservoir thickness, m; ——reservoir porosity, f; Swi——irreducible water saturation, f; Viw——injected water volume, m3; Vpw——produced water volume from reservoir, m3; Cws——dissolution coefficient of CO2 in water, f; Cos——dissolution coefficient of CO2 in crude oil, f.

In Eq. 1, CO2 theoretical storage capacity consists of three parts, i.e. the part stored by structural space (including bond space), the part dissolved in water and the part dissolved in crude oil.

2.1.2 Calculation method of theoretical storage capacity for co2 flooding reservoir

CO2-EOR technology is used in the USA more than other countries. US experience shows that about 40% of injected CO2 is produced. Based on this, Bachu S et al[7] proposed the calculations methods of CO2 theoretical storage capacity be-fore and after CO2 breakthrough when CO2 is used for EOR. In this respect, the authors establish the equation taking into account the injected water, produced water and CO2 dissolu-tion in reservoir.

Before CO2 breakthrough:

wspwiwwiRb9r

t )1(10

CVVSAhEM

wi iw pw os Rb wi1 1Ah S V V C E Ah S (2)

After CO2 breakthrough

iwwiRhRb9r

t )1()6.04.0(10

VSAhEEM

pw ws w iw pw os 1V C Ah S V V C

Rb Rh wi0.4 0.6 1E E Ah S (3)

Where


ERb——oil recovery before CO2 breakthrough, f; ERh——oil recovery when certain CO2 is injected, f.

In the above equations, the key is to determine the oil re-covery factor. Generally, it can be achieved by the reservoir numerical simulation or empirical formula [11-15].

2.2 Calculation method of effective storage capacity

The effective storage capacity considers such factors as reservoir property, sealing ability, storage depth, pressure systems and pore volume based on theoretical storage capacity, namely it will be impacted by the fluid mobility, gravity sepa-ration reservoir heterogeneity and underground water body etc. Therefore, it cannot reach to the ideal theoretical storage ca-pacity.

The effective storage capacity is given by following equa-tion by taking into account buoyancy, gravity override, mobil-ity ratio, reservoir heterogeneity, water saturation and strength of the underground water body etc, based on theoretical stor-age capacity calculation method.

Me=Ce Mt=Cm Cb Ch Cw Ca Mt (4) Where

Me——effective storage capacity of CO2 in reservoir, 106 t; Ce——effective storage coefficient affected by all factors, f; Cm——effective storage coefficient affected by mobility, f; Cb——effective storage coefficient affected by buoyancy, f; Ch——effective storage coefficient affected by reservoir het-erogeneity, f; Cw——effective storage coefficient affected by water saturation, f; Ca——effective storage coefficient af-fected by underground water body, f.

The determination of above coefficients will be generally achieved by numerical simulation or empirical methods[7].Eq.4 is suitable for calculating CO2 effective storage capacity for both waterflooding reservoir and CO2 flooding reservoir.

2.3 Calculation method of CO2 storage capacity based on analogy method

The analogy method is used to calculate CO2 storage capac-ity based on the experiences of CO2 flooding EOR demonstra-tion projects on sites and by analogy analysis of the reservoir parameters and development status. The method is mainly used for the CO2 storage calculation for CO2 flooding EOR reservoirs. Many countries and organizations including USA and EU have conducted a lot of activities regarding CO2-EORand obtained valuable experiences. On this basis, the CO2

utilization coefficient is also introduced to calculate CO2 stor-age capacity by:

2

6e p CO10M N R (5)

Where Np——produced oil because of CO2 injection, m3;

2COR ——CO2 utilization coefficient, i.e., the ratio of injected CO2 to. produced oil, t/m3.

How to determine CO2 utilization coefficient and incre-mental oil is the key to calculate CO2 storage capacity by the analogy method. The unitization coefficient can be gained by

Table 1 CO2 utilization coefficients used in overseas CO2 pro-jects[12]

Project CO2 Utilization coeffi-

cient/(t·m-3)Ref.

Weyburn 0.9435 Wilson, 2000

Willard-Wasson 1.0693-1.3838 Stalkup, 1984

SAROC in Main CO2

injection 1.6354

Stalkup, 1984

SACROC pilot 1.7612-4.9062 Stalkup, 1984

Little Creek 4.7804 Stalkup, 1984

Permian, North Sea 1.8870 Espie,

2000

Aver. Value of Miscible 2.1134 Stevens, 1999

Aver. Value of Immis-cible

3.5161 Stevens, 1999

referring to projects conducted or the numerical simulation method. Table 1 shows CO2 utilization coefficients [12] used in major international CO2 projects.

It can be seen in Table 1 that, the CO2 utilization coefficient vary by areas, ranging from 0.9-5.0 t/m3. It is proposed in literature [12] that CO2 utilization coefficients are divided into three grades: maximum, moderate and minimum, with their values setting at 5.0 t/m3, 3.0 t/m3 and 1.0 t/m3 respectively.

If the produced oil because of CO2 injection is unknown in Eq. (5), following Eq. (6) may be used.

NP=ERe No C (6) Where

ERe——Recovery increment amplitude after CO2 injection, f; No——OOIP, 109 m3; C——contact coefficient of CO2 and crude oil, f, being 0.75 normally[6].

EOR after CO2 injection can be calculated with analogy, reservoir numerical simulation or empirical formula. Stevens [6] investigated the data on CO2 -EOR in seven Permian basins and defined the empirical correlation between crude gravity and recovery increment amplitude after CO2. In literature [12], recovery increment amplitude after CO2 injection is divided into three grades: maximum, moderate and minimum, with their values setting at 0.20, 0.12 and 0.05 respectively.

3 Case calculations

There are 21 block in an oilfield in China that can be used for CO2 storage, with current OOIP of 119 212×104 t, ultimate oil recovery factor of 0.2, average formation pressure of 22.0 MPa, average temperature of 93 , CO2 and crude oil densi-ties of 750.0 kg/m3 and 856.0 kg/m3 respectively under reser-voir conditions, crude oil volume factor being 1.17, and the dissolution factor of CO2 in crude oil and water being 0.35 and 0.05 respectively.

3.1 Calculation of theoretical storage capacity

According to Eq.1, theoretical storage capacity of 24 186 × 104 t is stored in the free space, the theoretical storage capacity of water dissolution is 8 062 × 104 t, the theoretical storage


capacity of crude oil dissolution is 44 632 × 104 t, therefore, total theoretical storage capacity of CO2 is 76 880 × 104 t. The calculation shows that the CO2 dissolution in crude oil takes higher percentage.

3.2 Calculation of effective storage capacity

According to Eq.4, effective storage coefficients of all the factors should be obtained by reservoir numerical simulation. If as defined in literature [1], 0.25 is taken as the effective storage coefficient affected by all factors, the effective storage capacity is 19 200 × 104 t.

3.3 Calculation by analogy method

Two manners are used to determine CO2 utilization coeffi-cient in the analogy method. One takes value referring to for-eign literatures, and another one uses numerical simulation results of typical blocks in this region.

3.3.1 Determination of CO2 storage capacity by referring to foreign literatures

As stated above, CO2 utilization coefficient can be divided into three grades: maximum, moderate and minimum, with their values setting at 5.0 t/m3, 3.0 t/m3 and 1.0 t/m3 respec-tively, EOR after CO2 injection is divided into three grades: maximum, moderate and minimum, with values setting at 0.20, 0.12 and 0.05 respectively. Eq. 5 and 6 are used to calculate CO2 storage capacity (Table 2). Total storage ca-pacity for such three grades is 89 412×104 t, 32 186 ×104 t and 4 470×104 t respectively. CO2 storage capacity for the maximum grade is equal to theoretical storage capacity ob-tained in above calculation, and the effective storage capac-ity ranges between the moderate grade and the minimum grade.

3.3.2 Determination of storage capacity based on reser-voir cases

CO2 utilization coefficient is determined by reservoir nu-merical simulation of a typical block in this oilfield, and EOR is obtained by potential prediction model of CO2 flooding. There are 21 blocks assessed by case analysis, with results shown in Table 3. It is confirmed that eleven blocks (A to K) are suitable for CO2 immiscible flooding to enhance oil re-covery, with EOR ranging from 7.0% to 9.0%; ten blocks (L to U) are suitable for CO2 miscible flooding to enhance oil recovery, with EOR ranging from 9.0% to 15.0%. Also, based on the CO2 utilization coefficient obtained by numerical stimulation for block H (1.5935 t/m3), total CO2 storage ca-pacity is defined as 14 988×104 t by the calculation of Eq.5 and 6. The value is equal to the effective storage capacity ob-tained above (19 220×104 t).

4 Conclusions and recognitions

The paper presents that calculation of CO2 storage capacity should consider reservoir space and must take into account the fluid dissolution in reservoir, according to characteristics that

Table 2 Calculation of effective storage capacity by referring to foreign literatures

Block OOIP/ 104 t

Max. Stor./ 104 t

Mid. Stor./ 104 t

Min. Stor./ 104 t

A 7 332 5 499 1 980 275 B 341 256 92 13 C 724 543 195 27 D 9 808 7 356 2 648 368 E 19 558 14 669 5 281 733 F 567 425 153 21 G 4 549 3 412 1 228 171 H 2 227 1 670 601 84 I 106 80 29 4 J 4 211 3 158 1 137 158 K 140 105 38 5 L 7 292 5 469 1 969 273 M 13 999 10 499 3 780 525 N 12 306 9 230 3 322 461 O 1 105 829 298 41 P 3 177 2 383 858 119 Q 9 807 7 355 2 648 368 R 1 502 1 127 405 56 S 5 838 4 379 1 576 219 T 12 602 9 452 3 402 473 U 2 021 1 516 546 76

Total 119 212 89 412 32 186 4 470

Table 3 Calculation of effective storage capacity based on res-ervoir evaluation

Effective storage potential for CO2

immiscible flooding Effective storage potential for CO2

miscible flooding

BlockOOIP/104 t

EOR/%Eff. Stor.

/104 tBlock

OOIP/ 104 t

EOR/%Eff.

Stor./104 tA 7 332 8.0 701 L 7 292 12.0 1046 B 341 7.0 29 M 13 999 12.0 2008 C 724 8.0 69 N 12 306 14.0 2 059 D 9 808 9.0 1055 O 1105 15.0 198 E 19 558 7.0 1 636 P 3 177 12.0 457 F 567 7.0 47 Q 9 807 12.0 1 406 G 4 549 8.0 435 R 1 502 12.0 215 H 2 227 9.0 240 S 5 838 12.0 837 I 106 8.0 10 T 12 602 12.0 1 907 J 4 211 8.0 403 U 2 021 9.0 217 K 140 8.0 13

most Chinese oilfields adopt the development by waterflood-ing. A systematic study is made on calculation method of theoretical storage and effective storage capacities of CO2 in reservoir, providing reference for CO2 storage potential evaluation for oilfields in China.

Calculation method of CO2 storage capacity in reservoir can be determined mainly by the material balance and analogy methods, for which the key parameters are to be obtained by reservoir numerical simulation or empirical methods.

When the material balance method is used, the amount of CO2 dissolute in oil has much percentage, which can not be neglected. When the analogy method is used, CO2 utilization coefficient is a key parameter. The results prove that the analogy method can obtain more credible storage potential of CO2 when CO2 utilization coefficient is determined through case analysis.

References


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