RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 3, June 2015 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(3): 390–396.

Received date: 16 Sep. 2014; Revised date: 05 Mar. 2015. * Corresponding author. E-mail: wanggaofeng@petrochina.com.cn Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05016) and the PetroChina Science and Technology Major Project (2014E3606). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

A new screening method of low permeability reservoirs suitable for CO2 flooding

WANG Gaofeng1,2,3,*, ZHENG Xiongjie4, ZHANG Yu4, LÜ Wenfeng1,2,3, WANG Fang1,2,3, YIN Lina4

1. State Key Laboratory of EOR, Beijing 100083, China; 2. PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China; 3. National Energy CO2 Flooding and Storage (Experiment) R & D Center, Beijing 100083, China; 4. PetroChina Jilin Oilfield Company, Songyuan 138000, China

Abstract: Since the existing gas flooding reservoir screening criteria lack economic indexes reflecting future dynamic production per-formances, indexes reflecting economic profits are added into the exiting criteria (related to well oil production rate) to form a new method of selecting low permeability reservoirs suitable for CO2 flooding. Reservoir engineering methods of average well peak oil pro-duction rate prediction for gas drive is given by employing the concept of “Oil production rate multiplier due to gas flooding”. Based on the principles of technical economics, the method calculating the economical limit well oil production rate of CO2 flooding is also pre-sented. On this basis, a new screening criterion of reservoirs suitable for CO2 flooding is proposed: if the peak well oil production rate predicted by reservoir engineering method is higher than the economic limit well oil production rate, the target reservoir is suitable for CO2 flooding. Furthermore, a four-step reservoirs screening method is advanced: technical screening, economic screening, fine feasibility evaluation, recommendation of optimal gas flooding blocks. The new screening criteria were applied to evaluate the CO2 flooding poten-tial of seventeen blocks in an oilfield, which ended up with only 32.4% of the original oil in place from conventional method suitable for CO2 flooding. It is recommended blocks suitable for CO2 flooding be selected according to the new procedure to ensure economic suc-cess.

Key words: CO2 flooding; low permeability reservoirs; screening criterion; oil production rate multiplier due to gas flooding; reservoir engineering method; economic limit well oil production rate

Introduction

Gas flooding has been widely used in North America. Ac-cording to statistics, about 80% of CO2 injection projects in America is less than 50×10−3 m2 in permeability[1,2], and since 2000, 71% of CO2 flooding projects in China have been aimed at low permeability reservoirs. Almost all gas injection projects worldwide have achieved increase of oil production rate of various extents, and a number of practical screening criteria of reservoirs suitable for gas flooding have been emerged, such as Geffen[3], National Petroleum Council (NPC)[4], Carcoana[5]. All these criteria are based on reaching miscible flooding so as to give a full play of carbon dioxide’s ability to extract lighter components of crude oil. Thus, the content of screening criteria are mainly formation and reser-voir fluids parameters closely related to miscibility, for exam-ple, the reservoir temperature should not be too high (upper limit of 121 °C in NPC1976[4]) , reservoir pressure should not

be too low or reservoir depth should not be too shallow, oil density and viscosity should not be too high, and oil saturation should not be too low. Although some researchers made dis-cussion on the economy of gas injection, the research results, not universal, have not made into standard[6,7]. In a word, the existent screening criteria have no index to judge the eco-nomic benefit of a gas injection project. Some popular screening software represented by CO2 Prophet of the United States Department of Energy integrates streamline generation, stream tube simulation and economic evaluation. However, the core of the software is ‘1-D Black Oil Model’, different from the real CO2 flooding process. Both theoretical analysis with probability theory and gas injection practices show the multi-components numerical simulation method of low per-meability reservoirs is unreliable with the relative error be-tween the predicted results and the real results reaching over 50% in general[8]. Therefore, the ‘Bow-tie’ risk control mode would fail during development planning stage to evaluate gas

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 391 −

flooding feasibility. This explains why 20%−30% gas injec-tion projects in North America have poor or no economic profit.

CO2 flooding projects in China are more likely to have poor or no economic profit due to the following factors: firstly, no sound carbon trading mechanism, immature carbon market and different sources of gas supply[9]; secondly, lower ultimate recovery factor and the higher ratio of gas injected to oil pro-duced caused by harsher conditions of miscibility of crude oil and CO2 and strong heterogeneity of continental deposited reservoirs; finally, larger burial depth of CO2 flooding reser-voirs in China (about five hundred meters deeper than the Permian reservoirs in New Mexico and Texas in the US). Therefore, it is very necessary to perfect existing screening criteria of reservoirs suitable for gas flooding. Since oil pro-duction rate is most important in all production indexes, the authors propose introducing indexes related to average well oil production rate (WOPR) reflecting economic profit of gas flooding into existing screening criterion.

By introducing a new concept of economic limit oil pro-duction rate, combined with reservoir engineering method to predict peak WOPR of gas flooding in low permeability res-ervoirs, a new index to judge economic feasibility of CO2 flooding projects is gained, furthermore, new methodology of screening low permeability reservoirs suitable for CO2 flood-ing is presented.

1. Theoretical basis

1.1. Peak oil production rate prediction of gas flooding

Based on the principle that ultimate recovery factor equals the displacement efficiency multiplies volumetric sweep fac-tor and the intrinsic relationship between recovery percent, annual recovery velocity, and decline rate, the authors propose a universal method of gas flooding oil production rate (OPR) computation by introducing a new concept ‘OPR Multiplier due to gas flooding’[10]. ‘OPR Multiplier’ is defined as the ratio of oil production from gas flooding to that of water flooding in the same period (Assuming no gas injection but continue to injection water), its engineering computation method for low permeability reservoirs is[10]:

og 1 2gw

ow 21Q R RFQ R

−= =−

(1)

where 1 Dgi Dwi 2 e0 Dwi/ /R E E R R E= = According to the definition of ‘OPR Multiplier’, the water

flooding production during the stable period and peak period must be known if the gas flooding OPR during the stable pe-riod or peak period is to be calculated, the water flooding production before gas flooding is known, if the production decline characteristics of water flooding is known, the oil production from water flooding corresponding to gas flooding peak can be worked out. Rich experience has been accumu-lated over 30 years water flooding development of low per-meable reservoirs, so water flooding OPR decline characteris-

tics of the same type of reservoirs can be taken for reference (exponential decline). Assuming the single well oil production from water flooding one year before gas flooding is qow0, the annual production decline rate from water flooding is Dw, the time from the start of gas injection to gas emergence respond-ing is t, the average WOPR of gas flooding during the peak period is: ( )w

ogs gw ow0 gw ow0 we 1D tq F q F q D t−= ≈ − (2)

The decline from start of gas flooding to response can be neglected for two reasons: firstly, gas injection practices of low permeability reservoirs worldwide show the time needed from gas injection to overall gas breakthrough is often several months to one year[9]; secondly, to some extent, formation pressure can be replenished during early gas injection period. Thus Eq. 2 can be simplified as: ogs gw ow0q F q= (3)

Based on Reference[10], the authors add another three CO2 injection projects, North Liu of Jidong oilfield, North Hei79 in Jilin oilfield and Bei14 in Haita basin, so the above equa-tion used for gas flooding OPR prediction of low permeability reservoirs has been verified by twenty one gas flooding pro-jects worldwide (Fig. 1), including extra-low permeable res-ervoirs (1~10 mD) and ordinary low permeable reservoirs (10~50 mD), infill drilling pilots and extended well spacing pilots, miscible or immiscible flooding performance. The concept, ‘OPR Multiplier’ provides a theoretical tool for pre-dicting gas flooding OPR of low permeability reservoirs.

Fig. 1. OPR Multipliers of 21 gas flooding projects.

The decline rate can be worked out from the derivative of oil production rate in Eq. 1 with respect to time:

og owgw

d dd dQ QF

t t= (4)

Eq. 4 shows the OPR decline characteristics of gas flooding is similar to that of water flooding, and theoretically, the ab-solute OPR decline rate of gas flooding equals a constant times that of water flooding, and the constant is ‘OPR Multi-plier’. Of course, decline rate can also be obtained from field gas flooding experience.

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 392 −

1.2. Economic limit well oil production rate of CO2 flooding

Not all CO2 injection projects have economic profit. The economic limit WOPR in this paper is referred in particular to the average well oil production rate during the stable produc-tion stage when the present value of gas flooding construction investment plus operational cost equals the present value of output, this value can be obtained from technical economic evaluation method. It must be stressed the economic limit WOPR is not a technical boundary obtained by artificially adjusting working system of producers, but the minimal re-quirement of well oil production rate which can meets the target economic goals.

Assuming the stable production years of a CO2 flooding project is Ts, and the annual oil production of the project de-creases exponentially, the total sales revenue in the evaluation period can be written as:

( ) ( ) ( ) ( )c c s

c

c1 o o o co o o o1 1

1 1T T T

j j

j jj j T

I P Q r i P Q iα α+

− −

= = +

= + + + +

( )c s g

c s

( )o o o

1e 1

njj T T D

j T TP Q iα −− − −

= + ++ (5)

If the operation cost of one ton oil from CO2 flooding is Pm, total production and operational cost is:

( ) ( )c1 m o o1

1n

j

jj

O P Q iα −

=

= + (6)

Average fixed investment for a well Pw includes drilling and completion expense, CO2 flooding injection-production engineering investment, surface engineering construction and non-installed equipment investment, and the interest of fixed investment in payback period is included in operating costs, then the total fixed investment loans and interest of construc-tion period is:

( ) ( )cc

c

ow w 0c2

1

10 000 11

TT Tj

j T

n P iO i

T

+−

= +

+= + (7)

The recovered residual value of fixed assets is expressed as: ( )c2 f ow w 1 nI r n P i −= + (8)

Fluid capital is cash or used asset in one year or one oper-ating cycle. Fluid capital, only a small proportion in total in-vestment, spent during early development stage will be re-covered at a later stage, so it is negligible in economic evalua-tion analysis.

Crude oil sales taxes including value added tax, urban maintenance and construction tax and education supplemen-tary tax. Tax of oil and gas resources is levied ad valorem. So, the total crude oil sales taxes to be paid is:

( ) ( )c3 t c1 t o o o1

1n

j

jj

O r I r P Q i −

== = + (9)

In addition, the special oil gain levied is:

( ) ( )c4 s o o1

1n

j

jj

O P Q iα −

=

= + (10)

The net oil price is defined as the oil price after the various taxes, and operating costs is deducted from original oil price: ( )oe t o s m1P r P P P= − − − (11)

Total revenue in the evaluation period of a gas injection project equals crude oil sales revenue plus residual value of fixed assets. Total expenditures include the total cost of pro-duction and operation, fixed investment and interest, total sales tax, resource tax and special oil gain. Total net present profit ( NPV) equals total revenue minus total expenditure: ( ) ( )c1 c2 c1 c2 c3 c4NPV I I O O O O= + − + + + (12)

When gas injection effect is poor enough and the total NPV will be zero, the production, Qo is the economic limit oil pro-duction, Qoel. Let the above equation is zero: ( )oel 0NPV Q = (13)

Economic limit oil production of gas flooding, Qoel, is ob-tained from equations (5) - (13):

( ) ( ) ( )c c

c

0w f

1

oelo ow

11 1

0.0001 /

T T Tj n

j T

iP i r i

TQ

nα ψ

+− −

= +

++ − +

= (14)

where

( ) ( )c c s

c

oe co oe1 1

1 1T T T

j j

j j TP r i P iψ

+− −

= = +

= + + + +

( )g c s

c s

( )oe

1e 1

njD j T T

j T TP i −− − −

= + ++

The relationship between the number of producers and total well number is: ( )ow o 1n n λ= + (15)

If the economic limit daily oil production rate per well is qogel, then: oel o ogel365Q n q= (16)

The mathematic model of economic limit well oil produc-tion rate of CO2 flooding, qogel, is derived by combing Eqs. (14) through (16):

( )

cc

c

0w f

1ogel

o

(1 ) (1 ) (1 )

0.036 5 / 1

T TTj n

j T

iP i r iT

qα ψ λ

+− −

= +

+ + − +=

+ (17)

The CO2 price is separated from operating cost, and pro-duced CO2 separation and recycling injection are considered to reflect the features of gas flooding. Assuming the ratio of gas injected to oil produced of CO2 flooding is us, and the volume fraction of CO2 re-injected to the produced CO2 is yc, then the operational cost of per ton oil is:

cm s g mw520

y GORP u P P= − + (18)

As oil field development goes on, produced gas oil ratio and total water cut rise, production and operation costs per ton oil will increase constantly and will be more complex in composition because CO2 and water injected per ton oil pro-duced, dehydration workload, management workload keep increasing, so does the operating cost minus CO2 price, Pmw.

The recovered residual value of fixed assets, usually less than 2.0% of its original value at the end of evaluation period, can be ignored. Combining the summation equation of geo-metric progression and binomial theorem, Eq.17 can be sim-

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 393 −

plified as follows:

( ) ( )( )

( )

w 0 cogel

o c

coe t o s s g mw

1 10.036 5 1 1

1520

P i Tq

i T

y GORP r P P u P P

λα ψ

+ +=

+ +

= − − − − +

(19)

The larger the discount rate, the higher the economic limit WOPR calculated from Eq. (19) will be. The discount rate should be at least taken as the industry internal rate of return (IRR), 12% at present, and 14% is suggested in this paper. It must be pointed out incremental method should be adopted to determine average fixed investment per well for those reser-voirs without recovering production capacity investment

According to gas injection experience at home and abroad, the CO2 injected per ton oil produced for a 15-year evaluation period is taken as 3.0 t CO2 / t oil, the construction period is deemed as one year. According to the latest financial policies, economic limit single well oil production of the following three types of gas injected reservoirs is calculated from Eq.19, and simplified algorithms are regressed.

(1) Weak developed/undeveloped reservoirs, which are not water injected or water injected for a short time, and where gas flooding starts before comprehensive water cut enters into regular rapid rising phase. The economic limit WOPR of this kind of reservoirs is simplified as (with absolute relative error of 4.7%):

( )2wogel g23 17e 0.3 1.6 6 1.3 e

4 000z hPq D x x λ= + + + + −

(20) where

( )( )mw g o g0.01 0.028 0.003 6 1x P P P D= + − +

mw0.0015h P= o0.418 0.000 1 z P= − (2) Oil reservoirs reaching a certain degree of water flood-

ing, which has been injected water for several years, and the comprehensive water cut has entered the stage of regular rapid rise. The calculation method of economic limit WOPR for this kind of reservoirs can be simplified as (with an absolute rela-tive error of 4.4%):

( )2wogel g37 15e 0.3 1.6 6 1.3 e

3 600z hPq D x x λ= + + + + −

(21) (3) Mature water flooding reservoirs, which had been de-

veloped by water flooding for many years, and start gas injec-tion after the end of regularly rapid rising phase of compre-hensive total water cut. The calculation method of economic limit WOPR for this kind of reservoirs can be simplified as (with an absolute relative error of 4.8%):

( )2wogel g50 13e 0.3 1.6 6 1.3 e

3 200z hPq D x x λ= + + + + −

(22) The above three simplified equations for economic limit

single well oil production are effective over a wide range of 3 300 CNY/t<Po<4 800 CNY/t ($75-110/bbl, and lower oil price are difficult to ensure economy), 0.05<Dg<0.30, 1100

CNY/t <Pmw+2.8Pg <2300 CNY/t. The gas price deducted operating cost per ton oil produced is always higher than 500 CNY, and domestic carbon dioxide price is usually over 200 CNY[9], so the gas flooding operation cost per ton oil should be higher than 1100 CNY. According to computation with Eq. 20 through Eq. 22, the economic limit WOPR must exceed 6.0 tons per day when the operation cost per ton oil produced is over 2300 CNY, the upper limit of the gas drive operation cost per ton oil produced is set to 2300 CNY because the av-erage WOPR over 6.0 tons per day is rare in low permeability reservoirs in China.

2. New screening method

2.1. New screening index for CO2 flooding low permeability reservoirs

When gas flooding peak oil stage is taken as the stable pe-riod, the average WOPR during peak stage of gas flooding, qogs is the average well oil rate of stable period. When the oil rate of stable period is lower than the economic limit oil pro-duction, the gas injection projects will be uneconomic, so a new index judging the feasibility of CO2 flooding projects is raised. If the reservoir engineering predicted average gas flooding WOPR in peak stage is higher than the economic limit WOPR, the gas injection project is economic viable. Thus, the new screening criteria for CO2 flooding low perme-ability reservoirs can be described as: ogs ogelq q> (23)

Substituting Eq. 3 into Eq. 23, we get: ow0 ogel gw/q q F> (24)

Based on Eq. 24, the oil production from water flooding before gas injection must be high enough if the gas flooding project is to be economic. This means the reservoir properties, API gravity of crude oil and oil saturation must not be too low at the same time. In the application of equation (24), miscible gas flooding situation can be adopted to calculate OPR multi-plier if the degree of miscibility is not certain. The initial CO2 miscible displacement efficiency of low permeability reservoirs suitable for gas flooding is 80%, and that of water is 46%−57%.

2.2. New screening method of CO2 flooding low permeability reservoirs

Taber pointed out that the function of screening criteria is to pick out reservoirs suitable for gas injection roughly from large amounts of candidate reservoirs to save the expensive costs of reservoir description and economic evaluation[6]. The rough screening is based on existent criteria, the above under-standing can be developed according to the new screening criterion in this paper. Here, the authors propose gas injection reservoir screening in China should abide by the following procedures.

(1) Primary screening or technical screening: mainly give attention to the possibility of reaching miscible flooding at

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 394 −

Table 1. Primary screening criteria of CO2 flooding reser-voirs[3−7].

Reservoir parameter Suggested value

Burial depth/m > 800

Temperature/°C < 121

Initial formation pressure/MPa > 8.5

Permeability/10−3 m2 > 0.6

Surface oil density/(g·cm−3) < 0.89

Reservoir oil viscosity/(mPa⋅s) < 10

Oil saturation/% > 35

reservoir conditions and forming an effective pressure system of injection and production during gas flooding process, and reservoir fluid properties and reservoir physical properties are mainly examined in this screening. The existing screening standards are used in primary screening (Table 1).

(2) Secondary screening or economic screening: the objec-tives are those candidate reservoirs passing the primary screening. The main concern in this stage is the economic feasibility of miscible gas flooding projects. The gas flooding economic limit WOPR and the average WOPR during peak stage of gas flooding will be examined through the screening criterion of Eq. 23. The average WOPR during peak stage of gas flooding is predicted with Eq. 3, and the gas flooding economic limit WOPR is calculated with one of the equation in Eq. 20 - Eq. 22 according to the type of gas injected reser-voirs. Strict parameters, like higher decline rate and fixed investment should be fed into the equation of secondary screening to ensure economic profitability of the reservoirs.

(3) Fine feasibility evaluation: the objectives are candidate reservoirs passing the secondary screening. The main tasks include reservoir description (focusing on lateral continuity of sand bodies), numerical simulation and comprehensive reser-voir engineering analysis to acquire all the necessary engi-neering parameters and economical indexes of gas injection for fine evaluation.

(4) Optimal block recommendation: the main task is to re-view each gas flooding plan formed in step (3) and to recom-mend the blocks most suitable for gas injection by organizing experts of related disciplines.

The purpose of these four steps is to ensure economic fea-sibility of gas injection plans. The authors name the above

screening scheme ‘four-step method of gas injection reservoir screening’. The second step, economic screening is a missing link in current gas injection reservoir screening, which is likely to cause wrong gas flooding project selection under current administrative system and technical level.

3. Applications

3.1. Primary screening

After pilot test and expanding test in recent years, CO2 flooding has entered industrialization phase in Jilin Oilfield, and is about to be promoted in 17 blocks. The 17 blocks can be divided into five types according to the differences in res-ervoir properties and recovery factor. Representative blocks for these five types are Block 48, H59, South H79 and North H79 infilling area and H46 (Table 2). All these reservoirs are normal in temperature and pressure, and the crude density varies from 0.855 g/cm3 to 0.870 g/cm3.

According to the primary screening criterion in Table 1, all these five types of reservoir are suitable for CO2 flooding after primary screening, with 28.79 million tons original oil in place(OOIP).

3.2. Secondary screening

3.2.1. Calculation of economic limit well oil production rate of CO2 flooding

The type of gas injected reservoirs is judged according to the stage of total water cut to select the proper equation for economic limit WOPR calculation.

With recovery percent of less than 5.0%, no water flooding or short period of water flooding, and water cut not yet enter-ing the rapid rise stage, Type I and II reservoirs in Table 2 are weak/undeveloped reservoirs, Eq. 20 should be chosen to calculate economic limit WOPR of CO2 flooding for these reservoirs. Having been developed by water flooding for 4 years, with water cut in regularly rapid rise stage and recovery percent of around 10%, Type III reservoirs are reservoirs reaching a certain degree of water flooding, so Eq. 21 should be chosen to calculate economic limit WOPR of CO2 flooding for this type of reservoir. With recovery percent of over 20%, Type IV and V reservoirs are mature water flooding reservoirs, Eq. 22 should be used to calculate economic limit WOPR of CO2 flooding for these reservoirs.

Table 2. Reservoir parameters needed in primary screening.

Reservoir type

Represent-ative pilot

Burial depth/m

Permeabil-ity/10−3 m2

Oil saturation/%

Reservoir oil vis-cosity/(mPa·s)

Reservoir tem-perature/°C

OOIP/104 t

Recovery percent/%

I F48 1 700−1 850 0.7−1.1 53.0−55.0 3.0−4.0 85.1 530 0−1.0

II H59 2 200−2 500 1.5−5.0 54.0−56.0 2.0−2.5 98.9 508 3.0−4.8

III South H79 2 100−2 500 4.0−15.0 50.0−53.0 2.0−2.4 97.3 425 9.0−12.0

IV North H79 2 100−2 400 4.0−12.0 45.5−49.5 2.2−2.6 94.2 690 20.0−22.0

V H46 2 100−2 350 5.0−20.0 45.0−47.5 2.2−2.7 97.8 726 25.0−27.0

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 395 −

Table 3. Economic limit single well oil production of secondary screening.

Reservoir type Ratio of producers

to injectors/f Average fixed investment

per well/104CNY Annual OPR decline rate/f

Operating cost exclusive of gas price per ton oil/CNY

Gas flooding economic limit SWOP/(t·d−1)

I 0.28 400 0.18 667 2.05 II 0.30 450 0.18 640 2.31 III 0.32 340 0.15 790 2.13 IV 0.32 280 0.12 905 2.06 V 0.32 280 0.08 993 2.11

Table 4. Single well oil production during peak stage of gas flooding and economic screening results.

Displacement efficiency/% Single well oil production/(t·d−1) Reservoir type Water

flooding Miscible CO2

flooding

OPR Multi-plier/f Before gas

injection Peak stage of gas flooding

Economic limit of gas flooding

Economic feasibility

I 48.0 80.0 1.67−1.68 0.7−1.1 1.17−1.85 2.05 NO II 55.0 80.0 1.49−1.50 2.5−2.8 3.75−4.20 2.31 YES III 55.1 80.0 1.54−1.58 1.7−2.0 2.61−3.16 2.13 YES IV 55.2 80.1 1.71−1.75 1.0−1.1 1.71−1.93 2.06 NO V 55.5 80.3 1.81−1.87 0.8−1.0 1.68−1.87 2.11 NO Here Type I reservoir is taken as an example to illustrate the

calculation process of economic limit WOPR of CO2 flooding. During industrial application stage of CO2 flooding, sound CO2 gathering, recycling and re-injection systems should be built in order to achieve zero CO2 emission and ensure safety. Based on preliminary evaluation, the average fixed investment per well is 400×104 CNY, and the gas price deducted operation cost per ton oil produced is 667 CNY for Type I reservoirs, CO2 price is 240 CNY/t, oil price is 4180 CNY/t, the ratio of producers to injectors and the annual OPR decline rate is 0.28 of upper limit and 0.18 of lower limit respectively. The eco-nomic limit WOPR of CO2 flooding of Type I reservoirs cal-culated is 2.05 tons per day by substituting the specific value of the parameters above into Eq. 20. Similarly, the economic limit WOPR of CO2 flooding of the remaining four types of reservoirs can be worked out (Table 3).

3.2.2. Prediction of average well oil production rate dur-ing peak stage of CO2 flooding

Still take Type I reservoir as an example to illustrate how to calculate average WOPR during peak stage of gas flooding. Firstly, the CO2 displacement efficiency of 80%, water dis-placement efficiency of 48%, and recovery factor of 1.0% are fed into Eq. 1 to obtain OPR multiplier, and the result is 1.68. According to Eq. 3, average WOPR at peak CO2 flooding is the product of WOPR of water flooding before gas injection and OPR multiplier, so the average well oil production rate at peak CO2 flooding is 1.17−1.85 tons per day because the av-erage WOPR of water flooding before gas injection of Type I reservoir is 0.7−1.1 tons per day. Similarly, average WOPR at peak CO2 flooding of the remaining four types of reservoir can be calculated (Table 4).

3.2.3. Economic feasibility judgment

Economic feasibility of all types of reservoirs can be dis-cerned with the new screening criterion, that is Eq. (23). Type I, IV and V reservoirs are not suitable for CO2 flooding be-

cause their peak well oil production rate is lower than the corresponding economic limit WOPR. Only Type II and III reservoirs are suitable for CO2 flooding and Type II is most suitable (Table 4). After secondary screening, 933 million tons of original oil in place are suitable for CO2 flooding, only 32.4% of that passing primary or technical screening.

3.3. Fine evaluation and pilot recommendation

After secondary screening, fine evaluation of gas flooding feasibility will be conducted and gas injection program made for candidate reservoirs passing the secondary screening, then an expert group of related disciplines will be organized to review the rationality of the gas injection engineering pa-rameters and production indexes, and pick out the blocks most suitable for CO2 flooding. Following the ‘four-step method of gas injection reservoir screening’, H59 and south H79 were chosen as the blocks most suitable for gas injection. Gas in-jection practices have proved that CO2 flooding development effect of Block H59 and south H79 is the best among the aforementioned five representative gas injection blocks.

4. Conclusions

Existing reservoir screening criterion of gas flooding lacks economic index to judge whether a gas injection project has economic benefits. The numerical simulation results of low permeability reservoir gas flooding are very unreliable. The new concept, 'OPR multiplier of gas flooding' provides a tool for predicting well oil production rate of gas flooding theo-retically. The average well oil production rate of gas flooding at peak stage is the product of average well oil production rate of water flooding one year before gas injection and the OPR multiplier. Economic limit well oil production rate in the pa-per is the well oil production rate during the peak stage when the whole gas injection project is at breakeven point.

If the average well oil production rate at peak stage of gas flooding predicted by reservoir engineering method is below the economic limit well oil production rate, the target reser-

WANG Gaofeng et al. / Petroleum Exploration and Development, 2015, 42(3): 390–396

− 396 −

voir is unsuitable for gas injection. The average well oil pro-duction rate of water flooding before gas injection must be high enough to ensure economic profit. Gas flooding reser-voirs screening theory has been completed based on the new screening index.

A ‘four-step method of gas injection reservoir screening’ procedure suitable for reservoirs in China has been proposed, which includes technical screening, economic screening, fea-sibility fine evaluation and optimal reservoir recommendation. Application results show the potential reservoirs suitable for CO2 flooding selected by the new method are very different from the traditional method. It is suggested to choose gas flooding reservoirs with the new method so as to improve the success rate of gas injection.

Acknowledgements

The authors would like to express sincere gratitude to Prof. Qin Jishun for his guidance during the writing of the paper.

Nomenclature

Fgw OPR Multiplier of gas flooding, f; R1 ratio of water displacement efficiency to that of gas dis-

placement efficiency, f; R2 general recovery percent of original reserve, f; Qog oil production from gas flooding at certain point, t/d; Qow oil production from water flooding at the same time, t/d; EDgi initial gas displacement efficiency (when reservoir is unde-

veloped), %; EDwi initial water displacement efficiency, %; Re0 recovery percent when water flooding is converted to gas

flooding, %; qogs average well oil production rate at peak of gas flooding, t/d; qow0 average well oil production rate of normal water flooding

one year before gas injection, t/d; Dw annual decline rate of water flooding oil production, a−1; t time from gas injection to overall gas emergence, a; Ts stable production period, a; Ic1 total sales revenue in the evaluation period, CNY; j implementation time of CO2 flooding projects, a; Tc construction period, a; Po oil price, CNY/t;

o commodity rate of crude oil, f; Qo annual oil production of the block during peak stage of gas

flooding, t; rco ratio of oil production in construction period to that in peak

stage, f; i discount rate, f; n—the evaluation period of a gas injection project, a; Dg annual decline rate of gas flooding oil production, a-1; Oc1—total production and operational cost, CNY /t; Pm operation cost per ton oil produced, CNY /t; Oc2—fixed investment loans and interest of construction period, CNY;

T—loan repayment period, a; now total number of injectors and producers, well; Pw average fixed investment per well, CNY /well; i0—loan interest rate of fix investment, f; Ic2—recovered residual value of fixed assets, CNY; rf —residual rate of fixed assets, f; Oc3 crude oil sales taxes to be paid, CNY; rt total tax rate based on oil prices, f; Oc4 total special oil gain, CNY; Ps windfall tax rate, CNY/t; Poe net oil price, CNY/t; NPV net present value, CNY; Qoel economic limit annual oil production, t/d; no number of producers;

ratio of the number of producers to injectors, f; qogel economic limit well oil production rate of gas flooding, t/d; us CO2 injected per ton oil produced, t CO2 /t oil; yc volume fraction of CO2 in produced gas, f; Pg CO2 gas price, CNY/t; Pmw operating cost exclusive of gas price per ton oil, CNY; GOR produced gas oil ratio, m3/t.

References

[1] Stalkup F I, Stein M H, Lake L W, et al. CO2 Flooding. Texas: Society of Petroleum Engineer, 1998.

[2] Li Shilun, Sun Lei, Guo Ping, et al. Re-discussion of EOR with gas injection in China. Natural Gas Industry, 2006, 26(12): 30–34.

[3] Geffen T M. Improved oil recovery could ease energy short-age. World Oil, 1977, 177(5): 84–88.

[4] National Petroleum Council. Enhanced oil recovery: An anal-ysis of the potential for enhanced oil recovery from Know Fields in the United States. Washington: Richardson, 1976.

[5] Carcoana A N. Enhanced oil recovery in Romania. SPE 10699, 1982.

[6] Taber J J, Martin F D, Seright R S. EOR screening revisited: Part 2: Applications and impact of oil prices. SPE 39234, 1997.

[7] Shen Pingping, Yuan Shiyi, Han Dong, et al. Strategy study and potentiality evaluation of EOR for onshore oilfields in China. Acta Petrolei Sinica, 2001, 22(1): 45–48.

[8] Guo Ping, Huo Lijun, Jiang Bin, et al. Parameter optimization of water alternating gas of Fang 48 CO2 flooding pilot area. Journal of China University of Petroleum: Science & Tech-nology Edition, 2013, 36(6): 89–93.

[9] Qin Jishun. Development status and prospects of CCUS in China. Beijing: Gas flooding EOR Symposium of Sinopec, 2014.

[10] Wang Gaofeng, Hu Yongle, Song Xinmin, et al. New theory of oil production prediction in gas flooding low permeability reservoirs. Journal of Science Technology and Engineering, 2013, 13(30): 8906–8911.