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Present status of EOR in the Chinese petroleum industry and its future
Journal of Petroleum Science and Engmeermg, 6 ( 1991 ) 175-189
175
Elsevier Science Publishers B.V, Amsterdam
Present status of EOR in the Chinese petroleum industry and its future Cheng-zhi Yang and Da-kuang Han Research Institute of Petroleum Exploratton and Development, P 0 Box 910, Beumg, Chma (Received June 7, 1990; accepted after revision March 21, 1991 )
ABSTRACT Yang, Cheng-zhi and Han, Da-kuang, 1991 Present status of EOR in the Chinese petroleum industry and its future. J_ Pet Scl Eng, 6. 175-189 This paper gives a b n e f review of the present status of laboratory studies and pilot tests of thermal drive, chemical flooding and miscible-flooding enhanced oil-recovery techmques, and their future m China.
Introduction The annual oil production in China in 1988 was 13.7 million tons. In order to satisfy the increasing energy d e m a n d required for the modernization of China, new petroleum reserves are being explored, especially in the Northwestern part of China as well as in offshore areas. Although some new oil fields have in recent years been discovered in these areas, considerable investments of capital and a long period of time are required to establish commercial oil fields in these desert regions. Attention is being paid to the EOR technique as a m e t h o d of obtaining more oil from the existing oil fields. Although the currently proven EOR techniques are capital intensive, they may still be attractive since they result in additional reserves and enhanced productivity. Moreover, EOR projects in China have been proven to be economically feasible in some areas. Most of the discovered and developed oil fields are located in continental formations, characterized by complicated geological conditions, a large variation of reservoir types and
great differences in the characteristics of the oil. Most of the reservoirs are being developed by means of water flooding. Many oil fields are currently in a m e d i u m to high water-cut stage. At present, their productivities are being depleted at an ever increasing rate accompanied by an increase in the water cut. However, much of the oil (estimated at up to 65%) is still being left behind in the reservoir, as oil in place. Thus, a study of EOR techniques was started as early as in the 1950's and, on the basis of results of in-house studies, some of these techniques were at that time pilot tested in the Yum e n and Klamayi oil fields (Cheng, 1985; Yang and Han, 1985). A rapid development in the study of the EOR technology has occurred since the late 1970's. Thermal drive and chemical-flooding m e t h o d received much emphasis, and miscible flooding was also studied to some extent. The investigation of EOR techniques is matched to the features of each oil field. Pilot tests are then carried out, based on the results of the in-house studies. Field-wide commercial-scale application of the potential technology starts if it is found to be economically vi-
0920-4105/91/$03.50 © 1991 ElsevierSoence Pubhshers B.V. All rights reserved.
176
CHENG-ZH1YANG AND DA-KUANGHAN
able. The thermal drive technique has been industrialized, and the amount of oil produced by this technique in 1986 was approximately 1.5 million tonnes (Liu, 1987 ). This paper will give a brief review of the present status of in-house theoretical studies, laboratory studies and pilot tests of thermal drive, chemical-flooding and miscible-floodmg techniques, and their development in China, especially during the last decade. The thermal drive technique There are fairly abundant heavy oil resources in China. Numerous heavy oil deposits have been discovered in fifteen m e d i u m to large sedimentary basins. These are largely situated m Meso-Tertiary formations with a depth of burial of less than 2000 m; most of them are buried at 1000-1500 m, with some, in the west, at depths of only hundreds of meters. Most of these heavy oils contain less than 10% hght fractions (generally 5%) and have a high resin (usually over 30%) and a relatively low asphaltene content. The specific gravity of these oils is usually higher than 0.92 and their viscosities exceed 125 cP under reservoir condltions, with some of them up to thousands of centipoises (Hu et al., 1987). Obviously these deposits cannot be effectively recovered by water flooding. The only efficient way to recover these with a relatively high recovery fac-
tor, is through the application of a viscosityreducing heating technique. In-situ combustion In the Yumen and Klamayi oll fields (Western China) there are some relatively shallow heavy oil deposits buried at depths ranging from tens to hundreds meters. The study of insitu combustion through pilot field tests started there early in 1958 (Chen, 1976). Gasoline igrater and electric igniter techniques were developed for igmtion at a depth of approximately 500 m. Based on the results of in-house physical modeling, three types of in-situ combustion techniques were developed: conventional combustion, in-situ combustion followed by water injection; and as above with the cycle being repeated. Momtoring of the mxgration of the fire front and heat transfer were studied systematically. Ten pilot tests were carried out at the Heyushan reservoir (in Klamayi oil field), the Shiyougou reservoir (in Yumen oil field), the Sheng-tuo reservoir (in Sheng li oil field) and the Fu-You reservoir (Fu-You) The Heyushan reservoir in the Klamay~ oil field is taken as an example. In the period 1958-1974, eight pilot tests were carried out (Table 1) (Chen, 1976). Of these pilot projects the largest area covered was 11350 m: (He-4 test area), and included a m a x i m u m of 30 wells (He-4 area). The m a x i m u m depth of
T4BLE 1 In-s~tu combustion pilot
test m
the Klamay~ oll field
Umt
Pattern
Number of well
Time (days)
Depth (m)
Surface tested (m 2 )
EO R (%)
Heyushan Heyushan H-4 H-2 H-4 H-4 H-4 H-4
Surface Surface Surface Surface Line Surface Surface Surface
15 11 18 4 30 11 15 10
25 35 815 1008 748 487 761 733
14 18 85 410 110 110 85 85
179 120 11350 2825 6504 3720 7867 7051
43 2 55.2 64 0 84 0 17 4 31 3 19.0 74 0
PRESENT STATUSOF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
the test wells was 410 m (II-west area), and the m a x i m u m length of time for which testing was continued was 1008 days (II-west area). Results of these pilot tests show that the recovery factor of the tested well pattern is 47.8%, the recovery factor in the volume bounded by the fire front is 64%, while the recovery factor of the burned volume is 100%. The major problem encountered in these tests lay in the improper selection of the test area. Because the reservoir is fairly shallow and a lot of fractures are present, severe bypassing of the injected air occurred. As a result, the air/oil ratio was as high as 6919 m3/t. This process was also not investigated further, on account of the extremely expensive surface facilities required. The steam huff-and-buff and steam-drive techniques Steam stimulation has been used as the principal process for heavy oil recovery in China since 1980. For most of these heavy oil fields the composition of the heavy oils and the mineral composition of their reservoir rocks have been analyzed. Thermal properties such as heat conduction, thermal expansion and
177
heat loss, both in the surface facilities, as well as in well bores are measured and studied; and theoretical calculation pentaining to the flow of the heavy oil in porous media at high temperatures, as well as those relating to the thermodynamics within the well bores are carried out (Yah and Hepler, 1987 ). Based on the results of theoretical analyses and laboratory studies, physical modeling and numerical simulation of some pilot tests were conducted. A heat insulation device, including an insulated tubing string and a high-temperature downhole packer was developed to reduce the heat loss within a well bore to 9-14% (Liu, 1986). A new sand control agent was developed, capable of working under high steam temperatures (Chen and Chen, 1987). A series of theoretical studies relating to the increase in the volumetric performance of the injected steam were made, and the foam technique was suggested to increase the swept efficiency (Isaacs et al., 1987). Moreover, high-temperature foaming and diffusion agents to be used in the steam huff-and-puff process have been developed. In 1982 use of the steam huff-and-puff process was started, and this process has since been
TABLE 2 Reservoir parameters of 5 reservoirs pilot tested for steam mject~on Oll field
Reservoir formaUon Depth ( m ) Reservoir Temp. ( ° C) Viscosity ofdegassed off at reservoir temp. (cP) Av thickness of the pay zone ( m ) Porosity (%) Permeability ( m D ) Initial od saturaUon (%) Current reservoir pressure ( a t m ) Clay content (%) Asphaltene content (%) Pour point (%) Paraffin content ( ° C ) Sulphur content (%)
Gaosheng
Shuguan (Block 1-7-5)
Huan-xlling (Huan-17)
Shanjlas (Shan-2)
Klamayl 9-zone
Llanhua 1510-1700 56 0.94-0 96 67 6 23-30 1000-3000 65 120-150 5-10 44 i0 5 05
Dalinghe 1000-1100 50 0 97-0_98 44 22-30 2500-3000 65 104-110 6-8 47 15-20 27 05
Xlnglongtal 1000-1200 43 0.95-0 98 25-40 24 700-2000 60 104-120 5-13 35 11 5 04
Shanejle 1100-120 56 0.97-0 9 28-85 31-34 900-5000 65 110-120 3-8 28 16-18 2.4 0.7
Qlgu 160-240 18 0_927 16 4 32 700-4000 66 20 25.2 - 16-23 24 0 26
178
CHENG-ZHIYANG AND DA-KUANG HAN
,7y.y 50~0~ "~'-
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Fig_ 1 Pilot test for steam huff-and-puff m the G a o s h e n oil field
continued to be used in the Gaoshen (Li and Cai, 1985), Shuguang (Liu, 1986), Huanxiling (Liu, 1986), and Shanjiashi (Huang, 1987) reservoirs, usually at a depth of 10001600 m. It has also been tested and continued to be used in the Klamayi oil field where the ~7
well depth is approximately 200 m. Steam drive will start in these reservoirs in the near future. The main geological parameters of the above reservoirs are listed in Table 2. For example, the pilot test area in the Gaoshen oil field (Fig. 1 ) covers 0.4 km 2 and includes three 5-spot well patterns in which the well spacing is 150 m, a 9-spot well pattern with a well spacing of 105 m (Li and Cai, 1985). Thus, there are eighteeu wells in this area with depths ranging from 1490 to 1568 m. The total daily production of these eighteen wells is 121 ton with an average of 6.7 t/d/well before the test. In 1983 the first steam injection was started using two sets of steam generators (steam pressure, 175 atm; steam generated; I 1.5 and 23 t/d, respectively; steam property, 60-80%; steam temperature, 343°C). The steam was injected and then the wells were shut in for 57 days after which the wells were again reopened. Production was increased remarkably to 20-40 t / d for a period of about 6 months.
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PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
After several cycles of steam huff-and-puff, steam drive is now ready to be carried out in this pilot area. In the No. 9 region of the Klamayi oil field (Fig. 2 ) an area of 1.225 km 2 was selected for a steam-cycling pilot test. Here 64 wells (14 injectors) grouped in 7-spot well patterns and 74 wells ( 15 injectors) in 9-spot well patterns, i.e., a total of 138 wells, are included with an average well spacing of 100 m 2 (Ni, 1987). Starting in 1984, wells were steam injected at an average rate of 210 t / d for an average period of 7.5 d/well. A total of 1300-1800 ton of steam (270-290°C) were injected per well in a huff-and-puff cycle over the period of each cycle (250 days ). A total of 482 thousand tons of oil, equivalent to a recovery factor of 11.4%, were recovered at an overall steam/oil ratio of 1.03 t/t. Surface facilities capable of handling a total production capacity of 1207 t / d of oil have now been completed in this region. Moreover, another pilot test project for steam drive was selected in this region, covering an area of 0.253 km 2 including nine well patterns (a total of 30 wells). This project is currently underway.
Chemical flooding Oil production can be enhanced either by an improvement in the performance of the injected water such as polymer flooding (sometimes this is called improved water flooding) and foam flooding, or by an increase in the displacement efficiency such as surfactant flooding (also called micellar/polymer flooding or microemulsion/polymer flooding) and caustic flooding.
Polymer flooding Most of the Chinese oil reservoir formations are characterized by severe heterogeneity, especially in their vertical profiles. The vertical permeability coefficient reaches 0.7 or more, and in some cases as high as 0.9-1.0. More-
179
over, in most of the reservoirs, the viscosities of the oils are rather high. Thus the oil/water mobility ratio is fairly unfavorable, leading to bypassing of the displacement agent. Because the salinity of the formation waters associated with most of the reservoirs is not high, the application of polymers to thicken the injected water and to improve the mobility ratio, would remarkably increase the recovery of oil. Early in 1962, in the Petroleum Development Department of Beijing Petroleum University, the effect of the mobility of the driving fluid on the recovery efficiency of the oil using one- and two-dimensional physical models was studied systematically (Han and Yang, 1965 ). Radioactive tracer and electric resistivity techniques were used to detect the change in the saturation of the porous media in the models, and some good viscocifiers, such as sodium alginate and polyacrylamide, were screened for improving the mobility ratio. In recent years, the properties of aqueous polyacrylamide solution, and its sensitivity to the salt content were studied and the relations between the viscosity of the polyacrylamide solution and the concentration of mono- and bivalent ions were established and presented (Mei, 1982). The absorption of the polyacrylamide by the porous media and its retention during flow through the porous media were studied. It was pointed out that factors such as the clay content of the reservoir rock, the geometry of the pore structure, and the bivalention concentration in the water seriously affect the retention of the polymers (He, 1981; He and Hu, 1983; Gao and Hu, 1985 ). Moreover, the apparent viscosity of the polyacrylamide solution decreases with increasing permeability of the porous media and its flow velocity (Xu and He, 1985 ). This relationship can be used to adjust the mobility of this solution in the heterogeneous reservoir. Polyacrylamides with various molecular weights and various degrees of hydrolyzation are available from domestic chemical plants on a commercial scale in the form of concen-
180
CHENG-ZHI YANG AND DA-KUANG HAN
trated solution, powder, or emulsion. A study of the relation between the oil displacement efficiency and the amount of the polymer injected in the Daqing oil field (Fig. 3; Wang and Zhang, 1988) shows that an injected quantity (the product of the concentration of the polymer and the s~ze of the injected slug) of 200 ppm × PV [ concentration of polymer (ppm) × pore volume of polymer injected (PV)] will give an optimum result. The recovery factor can theoretically be increased by about 10% and about 200 ton of additional ml can be recovered by the injection of 1 ton of polymer. Of course, the optimum quantity to be injected will differ from one oil field to another. Based on the results of the above theoretical studies, polymer flooding has been pilot tested at various scales in the Daqing, Shengli, Klamayi and Dagang oil fields in five projects (some of which were carried out with the cooperation of Japanese, English and American companies). Most of these tests were successful to varying degrees. Hence, some commercial scale tests are now planned. AWo
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20
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Fig 3 General curves for the selection of the a m o u n t of polymer flooding to be used.
A pilot test (Fig. 4 ) in a closely spaced well pattern area in the Daqing oil field is a typical example where polymer flooding is successful (Zhang, 1984; Wang and Zhang, 1988). A triangular well pattern with six wells spaced at 75 m apart was selected as the testing project. These wells were water flooded to a water cut of 97.5% prior to the injection of 0.164 PV of polyacrylamide solution. Positive effects were successively observed in three producers, 12 days after the polymer solution was again replaced by freshwater. The water cut of these producers decreased from 99.99 and 94.5% to 88, 60.4 and 77.7%, respectively, over an effective period of 200 days. The net incremental od produced is 608.8 ton, equivalent to an increase in the recovery factor of 5.6%, while the additional oil produced as a result of the injection of I ton of polymer is equal to 153.4 ton. Radioactive tracer monitoring showed that the thickness of water intake was increased from 2.4 to 3.8 m, while an analysis of the C1 content in the produced water also showed that the vertical performance is improved remarkably. Field experience shows that where a severe variation of the vertical permeability exists, the injectlvlty profile control prior to the injection of the polymer solution can be modified by injecting a cross-linked polymer. Polymer flooding is a proven economical and feasible technology. It seems likely, therefore, that this will be one of the main techniques to be used m the near future to enhance the oil recovery from water-flooded reservoirs in China. Pilot tests will be extended, such as for example, two projects with an area of 9 acres will be tested further in the Daqing oil field (Chouveteau and Han, 1988). At the same time, some more complicated reservoirs with temperatures exceeding 60°C or with strongly fractured reservoirs will also be pilot tested. Feasibility studies of the pilot tests in these areas are currently under way. It is expected that this technique will come into general use in the near future.
PRESENT STATUSOF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE 2
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Foamflooding The apparent viscosity of a foam flowing through a porous medium is usually proportional to the diameter of the pore, two-phase gas/liquid flow exhibits non-Newtonian fluid rheological properties. Thus, injection of a foam in the water-oil frontal region can improve the performance of the driving fluid. Furthermore, a foaming agent, an anionic surfactant, also serves to decrease the interfacial tension between oil and water, which may improve the displacement efficiency. An in-house study of foam flooding was started early in the Seventies with emphasis on the stability of the foam (Jia, 1986), the loss of foaming agent, and its inhibition (Yang and Cheng, 1988 ), the mechanism of foam flooding (Yang, 1985; Jia, 1986; Liu et al., 1986; Yang and Han, 1988),
TABLE 3 Data relating to the area pdot tested for foam drive
Reservoir formation Depth ( m ) Thickness of the pay zone ( m ) Permeability ( m D ) Porosity (%) Cementing matenal Well pattern No. of wells Area (ha) Well spacing ( m )
Je-88 well area. Klamayl od field
L3 reservoir area. Laojumlas od field
$7 550
L3 674
15.7 200-900 19 9 montmonllomte, hydromlca etc. 7 spot l m.I and 6 prod
49 1310 20 montmonilonlte, dhte etc. irregular 5 spot 8 lnj and 18 prod 89 150
-
148
and the form of foaming (in the reservoir or at the surface). The results showed that: (1) some of the anionic surfactants (such as alkyl-
18 2
CHENG-ZHI YANG AND DA-KUANGHAN •
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F,g. 5 Well p a t t e r n for the f o a m f l o o d i n g pilot test m the Y u m a n oll field. 1 = I n j e c t i o n wells, 2 = test wells, 3 = wells o u t pdot. 4 = p r o d u c n v e well m L, zone, 5 = p r o d u c t i v e well in M zone.
sulfates and alkyl-benzene sulfonates ) are good foaming agents; (2) the addition of a proper a m o u n t of polymers can increase the stability of the foam; and ( 3 ) that the loss of foaming agent can be reduced to some extent by the addition of polyphosphates. Two pilot tests were carried out in the Yumen and Klamayi oil fields in 198 I. Reservoir characteristics and some of the parameters of the tests in the well Je-88 area, Klamayi oil field (Liu et al., 1986 ), and the L3 zone in the eastern part of the Laojunmiao reservoir, Y u m e n oil field (Yang et al., 1988), are summarized in Table 3. The pilot test in the L3 reservoir (Fig. 5 ) in Laojunmiao field, was larger. A total a m o u n t of 93,000 m 3 of foam (with a concentration of the foaming agent of 0.6%) was injected followed by 160,000 m 3 of air eqmvalent to 28.9% of the reservoir volume over the period October 1979-December 1980. A total of 654 ton of foaming agent (industrial grade) and 172 ton of stabilizer were injected. But only six of the eighteen producing wells showed an increase in production (and then to varying de-
grees accompanied by a decrease in water cut after such an injection; no obvious effect was observed in the other producing wells tested. The failure of this pilot test was attributed to the high clay content (up to 13.3%, mostly montmorillonite) found to cover the grain surfaces of the reservoir rock. This high clay content together with the high specific area of the reservoir rock which reaches 3.55 m2/g, would have caused severe adsorption loss o f the foaming agents (Yang et al., 1988 ) (resulting in a loss of its foaming ability). In the Je-88 well pattern area, Klamayi oil field, a total a m o u n t of 933 m 3 liquid (containing 1% foaming agent) and 8082 m 3 of air (reservoir condition equivalent to about 5% reservoir volume) were injected from May 1971 to June 1973. The water intake capacity of the highpermeability zones, S{, S 2 and $27-3 decreased, while that of the low-pertneability zones $73-2 and $37-s , increased. The average daily production of these efficient wells increased by 48%, a c c o m p a m e d by a decrease in water cut by 27.7%; over an effective period of 26.6 months.
PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
/ii\.
18 3
/
Fig. 6 P d o t test o f f o a m f l o o d i n g in t h e K l a m a y a oll field.
An increase in the recovery factor of 6-8% occurred. This thus represents a successful pilot test (Liu et al., 1986).
Surfactant flooding Since both polymer and foam flooding can only improve the performance of the driving agent, the recovery of oil can only be enhanced to a limited extent. Surfactants, inspite of their high cost, have a greater potential for enhancing oil recovery by improving the displacement efficiency of the displacing agent. Many Chinese oil fields are adaptable to surfactant flooding because ( 1 ) the reservoir temperatures are moderate, (2) the salinity of the formation waters is not too high ( ~ 10,000 ppm), (3) the permeability of the reservoir rocks is medium to high and the viscosity of most of the oils (in the reservoir) is not too great, and (4) some oils contain a relatively high aromatic fraction, even though most of the Chinese oils are paraffin based. Therefore, surfactant flooding has been studied since the 1970's. Emphasis has been placed on the synthesis and development of surfactants, the properties of the surfactant solution and its phase behavior, the absorption, and retention of the surfactant, and its inhibition, and the
mechanism of the oil displacement by surfactant solutions. In recent years, a technology of film sulfonation by SO3 for the synthesis of petroleum sulfonates for field use has been developed and industrialized; a-olefin sulfonates for use with high-salinity reservoir formation waters has also been developed. Because most oils in Chinese oil fields are paraffinic in nature, a lot of work was done on the synthesis of alkyl carboxylate by oxidation using paraffinic hydrocarbons as raw material. Industrial by-products will be cheaper and one kind of lignosulphonate has been identified as a suitable surfactant for use in chemical flooding. The EACNs of crude oils are studied systematically for the selection of surfactants suitable for specific crude oils and vice versa (Jiang and Jiang, 1986; Huang and Li, 1987). The effects of alcohol (Miao and Gao, 1988), hydrocarbon (Li, 1986; Li and Yang, 1986b), salt content (Li and Wang, 1986), additives (Han et al., 1989), temperature (Li and Wang, 1986) etc. on the phase behavior of the surfactantbrine-oil system have been studied and relations between interfacial tension, solubilization parameters and optimum salinity have been established. A dilute system (without the addition of alcohol) has been studied (Li,
184
CHENG-ZHI~ANG AND DA-KUANG HAN
40-
0
2
4
6
F~g. 7 Sulfonate adsorption isotherm
1982, 1986) in order to decrease the chromatographic separation of the micellar system. Results show that in this case high solubilizatlon parameters can also be obtained, even at lower surfactant concentrations. The dynamic lnterfacial tension between caustic solutions and highly acidic oils and its variation with the addition of polymers (Huang et al., 1987 ), as well as the interaction between surfactants and polymers (Yang, 1986) have been studied in order to provide a theoretical basis for the development of the MAP (Micelle-Alkali-Polymer) technology. Studies of surfactant loss in reservoxrs show that the main loss of surfactant is due to adsorption on the rock surfaces (Yang et al., 1985, 1988; Yang, 1985), reaction with multivalent ions (Yang and Huang, 1986; Yang and Han, 1988; Yang and Tang, 1988; Tang and Yang, 1989 ), and partitioning into the immobilized displaced fluids (Yang and Han, 1988; Han et al., 1989). A dynamic adsorption model (Gao et al., 1988) and a flowing retention model (Liu, 1983 ) were constructed on the basis of in-depth study of the mechanism of the adsorption of the surfactants. One of the main results of our studies has been the development of a rational technique for inhibiting the loss of surfactants (Fig. 7 ) ( H a n and Yang, 1986; Yang and Han, 1988; Yang and Tang, 1988).
Displacement tests using surfactant solutions (Jiang and Lu, 1982; Han et al., 1989) and studies of the mechanism of displacement using mlcromodels (Han et al, 1989) show that chemical flooding can be applied and result in h i g h recovery efficiencies without preflush even in cases where the clay content of the reservoir exceeds 10%. The surfactant-saltwater-hydrocarbon system has been employed in the treatment of single well bores. Early in 1974 this system was used in the Shengli (Yang, 1987), Daqing (Jiang et al., 1975), and Yumen oll fields for well stimulation. One hundred sixty eight wells were worked with such a system with an overall success ratio of 66.7% (Table 4). This demonstrates that ultra-low interfacial tension can be achieved under reservoir conditions. A successful surfactant pilot test was completed in the Daqing oil field (Meng et al., 1989), while feasibility studies of other two projects (Han et al., 1989) were completed (one of these has been done in cooperation with the Institut Fran~als du P6trole). The reservoir tested (in the Daqlng oll field) is D 4+ 5 formation. This pay zone has a thickness of 3.4 m and a permeabdity of 220-500 m d There are one injector, two producing and one observation well in an area of 1.7 acres, with a well spacing of 75 m (Meng et al., 1989 ); 1500 m 3 of micellar solution (soap of paper pulp ligno-cellulose/aliphatic alcohol/water) equivalent to 0.1 PV was rejected followed by a polymer solution of 2040 m 3 (0.13 PV) actmg as a buffer. Water injection was then continued and a clear effect due to surfactant flooding was observed after cumulative injection of 0.26 PV of water. The incremental oil produced by the end of 1988 was 3680 ton, and the recovery efficiency as indicated by a numerical simulation now reaches 70%, which is 35.5% higher than the conventional water flooding practiced previously. A pilot test for surfactant flooding in the Yumen oil field characterised by the high clay content of its reservoir is currently at the tech-
PRESENT STATUSOF EOR IN CHINESEPETROLEUMINDUSTRY AND ITS FUTURE
185
TABLE 4 Well stimulation by microemulslon N°
Number of well
Thickness ( m )*
Quantity of mj ection (m 3/ d )
Incremental oil production (m 3/d well )
Efficl ** (%)
1 2 3 4 5 6
14 13 4 3 6 60
129 6 143.3 32.0 42 5 161.2 709 6
168 309 36.8 35 118 666.8
26 7 21 8 182 78 81.1 42 3
92 8 75.8 25 100 66.5 76.5
1.3 2 15 1.55 0 82 0 73 1.31
*Total thickness of pay zone treated. **Efficmncy= number of effective well/number of well treated
nical design stage, based on a series of laboratory and simulation studies and will be put into practice in the near future.
Caustic flooding Some of the oils in China such as those of the Gudao, Yansanmu, Xinglungtai oil fields, etc., are relatively acidic, and may reach a level as high as 1 mg K O H / g oil. These oils when brought into contact with caustic solutions may result in in-situ surfactants. In-house studies by some institutions of the Chinese Academy of Science and some universities, as well as those by the petroleum industry have been in progress since 1981 (Li, 1984; Huang and Yang, 1984, 1985; Bian and Yu, 1986; Huang et al., 1989). It has been found that within a certain range of the salt content, the interfacial tension between oil and water can be lowered to a level of 10-~-10 -2 d y n e / c m by the addition of an alkalic and that the spontaneous emulsification of water and oil does occur. The factors affecting these reactions and the caustic consumption have been studied extensively. The dynamics of caustic consumption has been studied with the cooperation of the National Institute for Petroleum and Energy Research and a model describing the caustic consumption process when a caustic solution flows through a porous m e d i u m has been constructed, thus providing a basis for the mathematical modeling of caustic flooding (Li, 1984;
Liu, 1986). So far no pilot test has been attempted, due to the complexity of the mechanism of caustic flooding, and very high caustic consumption which cannot be inhibited or reduced effectively at present. However, because of the fact that all alkalies can react with some of the substances in oil to form carboxylate-surfactants, and that an alkaline/polymer/acidic oil system can produce 'low interfacial tension, we are now studying the surfactants/alkaline/polymer flooding (combination flooding) (Yang, 1986). A series of studies based on this concept are currently underway and some interesting results have been obtained.
Miscible and immiscible gas drive Early in 1970's, CO2-water injection and C O 2 water cycle injection had been tested in the Daqing and Klamayi oil fields. However, these can hardly be considered as effective miscible displacement process. During the 1980's, the m i n i m u m miscible pressure ( M M P ) of oils from different reservoirs has been studied in the laboratory by the slim-tube technique. It was found that some of the oil fields are adaptable to CO2 miscible displacement while some of the oil fields with lighter oils should be adaptable to enriched gas miscible drive. However, the many crude oil of the oil fields, due to the parafflnic nature of the oils, have a M M P which is too high for the
186
miscible displacement process to be apphed. CO2 miscible displacement is more economical than enriched gas drive. However, only small deposits of natural COz have been discovered in our country and it is costly to use CO2 from power plants or refineries. But COz miscible drive would still have a certain potential for increasing oil recovery if more CO2 reserves are discovered. A feasibility study of the process of N2 displacement which is the immiscible drive process which operates in nature, is now being carried out. Some mention should also be made of the microbial methods which have been studied (Wang and Wang, 1980). The preparation of bio-polymers and bIo-surfactants as well as the screening of strains of bacteria for in-situ oil recovery are included in this subject. The production of polysaccharides has been industrialized. Their rheological properties and techniques for-improving their temperature tolerance are being studied. Some bio-surfactants have been tested on physical models with encouraging results. Moreover, some strains of microbes have been identified and selected for the in-situ enhancement of od recovery and have given very promising results in laboratory tests. Their microbiological features are being studied thoroughly. It is expected that a field pilot test with these microbes will be carried out in the near future.
Conclusions The following achievements in Chinese EOR technology have been obtained: ( 1 ) Thermal drive has been industrialized, and the additional annual production resulting from the use of this technique amounts to millions of tons. (2) Both theoretical studies and pilot tests show that polymer flooding is technically possible and economically feasible, and field-wide operations are currently under consideration. (3) In-house studies and field pilot tests show that surfactant flooding has a high po-
CHENG-ZHIYANG AND DA-KUANG HAN
tentlal in EOR, but more studies are needed before it can be applied on an industrial scale. (4) Other EOR techniques, such as foam flooding, caustic flooding and miscible displacement have been studied or tested to some extent. These techniques will be further studled and tested in order to establish parameters for their o p t i m u m use. (5) A complete and systematic evaluation process, which includes In-house studies (theoretical and laboratory), physical and mathematical modeling, feasibility studies, technical design, pilot field tests, the monitoring analysis of the test results, and the eventual industrial scale applicability of an EOR technique has been established and is now followed in China. (6) Most of the reagents required for EOR can be synthesized locally while most of the equipment can also be manufactured domestically. Thus, it is expected that EOR will be apphed in various oil fields depending on their reservoir characteristics and their adaptability to various EOR technologies.
Acknowledgements The authors wish to express their appreciation to Messrs. Tong-Luo Qin, Qing-Shan Yue, Pu-Hua Yang, Jing-Chun Zhang for valuable Information and suggestions.
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PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
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187
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LI, Xlan-hong and Ca1, Chuan-fu, 1985. Preliminary study of steam soak test in the plot area in Gao sheng Pet_ Explor Dev, 2- 41-44 (m Chinese) L~, Zhi-pmg, 1982. Reserches on feaslblhty of applying alkylsulfonate to tertiary recovery of oll Oll Field Chem, 3 204-210 (in Chinese) L1, Zh~-pmg, 1986 The phase behavolr and the adsorption of sodium alkyl sulfonate with high purity Presented at the sixth International Symposium on Surfactants in Solution, New Delhi, August 18-22 Lm, Ci-qun, 1983. Adsorption m the flow of chemical solution through porous medias. Pet Explor Dev., 5- 6366 (in Chinese). L~u, Cl-qun, 1986. The adsorption problems in the lnfiltratlonofchemlcalsolutlon Pet Explor Dev, 3 6366 (m Chinese) Llu, Jm-kul, Yang, Shl-yuan and He, Wu-kui, 1986 Experiment m foam drive processes for exploiting highviscosity crude oll in Conglomeratic reservoir Acta Pet Sin, 6( 1 ) 61-70 (in Chinese). Lm, Wen-zhang, 1986. Pilot steam soak operation m deep wells in China. SPE 14865 Lm, Wen-zhang, 1987 Standards for heavy oil classification, and thermal recovery sieving. China-Canada Heavy Oil Technology Symposmm Proceedings, Zhuo Zhou, China, pp 3_1-3 17 Mel, You-qlan, 1982. The effect of senslblhty to sahmty for partially hydrolyzed polyaccrylamlde Acta Pet_ Sin., I 41-49 (in Chinese) Meng, Fan-ru et al., 1989_ A pilot of micellar/polymer flooding m Daqmg oll field. Pet Geol. Oll Field Dev Daqmg, 4 (m Chinese). Mmo, Xl-ren and Gao Shu-tang, 1988 Experimental study of abnormal phase behavoxr of mtcroemulslon with Daqmg o11. SPE 17840 Tlan Jmg, China. NL Rhuo-shi, 1987 Production pattern of the systyle steam pilot at Kalamayl, No. 9 oll field. China-Canada Heavy Oll Technology Symposmm Proceedings, Zhuo Zhou, China, pp. 3 I. 1-31 13 QI, Hou-fa and Dai, hang-xmg, 1981. Discussion on dislnbutlon and origin of the gas pools containing high percentage of carbon dioxide in China Pet. Explor_ Dev, 1:34-42 (m Chinese) Tang, Shan-yu and Yang, Cheng-zhl, 1989 A thermodynamic model of precipitation and redlssolutlon of alkylbenzenesulfonate m bnne and the effects of the addition of polyelectrolytes. J Surface Sci. Technol., 3 81-90. Wang, Xln-yan and Wang, Chuan-zhu, 1980 Microbial polysacchande produced from crude oll and its appllcabdlty m secondary oll recovery. Acta Pet Sin., 4 7785 (In Chinese) Wang, Zhl-wu and Zhang, Jlng-chun, 1988 Evaluation of polymer flooding m Daqlng ollfield and analysis of it favorable condmons. SPE 17848, Tlan Jlng, China Xu, Yuan-ze, LI, ping and He, Qlngong, 1985 Flow be-
CHENG-ZHI Y&NG AND DA-KUANG HAN
havoir of aqueous solutions or polyacrylamlde through mlcroporous media Oil Field Chem, 2 58-67 (in Chinese ) Yan, Hal-ke and Hepler, L.G, 1987_ Heats of combustion of hght and heavy ods_ China-Canada Heavy Oil Technology Symposmm Proceedings, Zhuo Zhou, China, pp 10_ 1-10 5 Yang, Chang-zhl et al, 1985 A laboratory study on partltlonmg of foamer in the foam flooding. Pet Explor Dev, 3 57-64 (in Chinese) Yang, Cheng-zhl, 1978_ Mlcellar solution and its apphcation on ollfield. Pet_ Explor. Dev, 2 44-58 (in Chinese ) Yang, Cheng-zhl, 1986. A juctment of surfactant/pol~cmer interaction m surfactant/polymer flooding with polyelectrolytes. SPE/DOE 14931, Tulsa, Okla Yang, Cheng-zhl, Huang Yan-hua et al., 1986. Precipitation/dissolution de sulfonate petroller en presence de cations polyvalents en solution. Rev. Inst. Fr du Pet, 2 (in French) Yang, Cheng-zhl and Cheng, Yun-xm, 1988 In J.K Borchardt and T F Yen (Editors), Study on loss of surfactants m physical-chemical flow processes Advances m oil field chemistry ACS, 33( 1)- 108-113 Yang, Cheng-zhl and Han, Da-kuang, 1988_ Maltnse de adsorption de I'argils par la polyelectrolyte 3rd European Meeting en Improved Oll Hoodlng, Rome, AGIP proceeding (in French ) Yang, Cheng-zhl and Tang, Shan-yu, 1988. The behavior of sulfonate solution in the presence of polyelectrolyres. Presented at 7th International Symposmm of Surfactants m Solution, Ottawa, Canadla, October 27
Yang, Cheng-zhl, Huang Yan-hua et al, 1985. Influence of properties of rock forming minerals on the adsorption of sodium dodecylbenzene sulfonate Pet. Geol OIl Field Dev Daqlng, 3 27-28 (in Chinese). Yang, Cheng-zhl et al, 1986 Adsorption balance of petroleum sulfonate solution on kaohmte and the relationship between the balance of adsorption and zeta potential_ Presented at the Sixth International Symposium on Surfactants, New Delhi. Yang, Cheng-zhl, Bazln, B e t al, 1988 Reduction ofsurfactant retention with polyphosphates m surfactant flooding process. SPE 1748 l, Tlanjing, China. Yang, Cheng-zhi, Ham Da-kuang et al_, 1988_Analysis and explanation to industrial pilot foam-flooding results on the Lao Jun Miao oil field in China_ SPE/DOE 17387, Tulsa, Okla Yang, Cheng-zhl, Jao, Wen-luo et al, 1988. The mechamsm of adsorption and a new method to reduce the surfactant loss m chemical flooding J Pet Scl. Eng, 3 97-109 Yang, Pu-hua and Ham Da-kuang, 1985_ A review of enhanced oil recovery and its forecast m China China oil, winter, Severth lssuevy No. 4, 22-26
PRESENTSTATUSOF EOR IN CHINESEPETROLEUMINDUSTRYAND ITS FUTURE Yuan, Xh~-yl and Van Quy, N., 1988 The numerical simulation of chemical flooding Acta Pet. Sin., 1 51-60 (in Chinese).
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Zhang, Jmg-chun et al., 1984. A field test rejecting polymer at the period of high water-out m D a q m g oll field. Oll Field C h e m , 1" 106-115 (in Chinese)
175
Elsevier Science Publishers B.V, Amsterdam
Present status of EOR in the Chinese petroleum industry and its future Cheng-zhi Yang and Da-kuang Han Research Institute of Petroleum Exploratton and Development, P 0 Box 910, Beumg, Chma (Received June 7, 1990; accepted after revision March 21, 1991 )
ABSTRACT Yang, Cheng-zhi and Han, Da-kuang, 1991 Present status of EOR in the Chinese petroleum industry and its future. J_ Pet Scl Eng, 6. 175-189 This paper gives a b n e f review of the present status of laboratory studies and pilot tests of thermal drive, chemical flooding and miscible-flooding enhanced oil-recovery techmques, and their future m China.
Introduction The annual oil production in China in 1988 was 13.7 million tons. In order to satisfy the increasing energy d e m a n d required for the modernization of China, new petroleum reserves are being explored, especially in the Northwestern part of China as well as in offshore areas. Although some new oil fields have in recent years been discovered in these areas, considerable investments of capital and a long period of time are required to establish commercial oil fields in these desert regions. Attention is being paid to the EOR technique as a m e t h o d of obtaining more oil from the existing oil fields. Although the currently proven EOR techniques are capital intensive, they may still be attractive since they result in additional reserves and enhanced productivity. Moreover, EOR projects in China have been proven to be economically feasible in some areas. Most of the discovered and developed oil fields are located in continental formations, characterized by complicated geological conditions, a large variation of reservoir types and
great differences in the characteristics of the oil. Most of the reservoirs are being developed by means of water flooding. Many oil fields are currently in a m e d i u m to high water-cut stage. At present, their productivities are being depleted at an ever increasing rate accompanied by an increase in the water cut. However, much of the oil (estimated at up to 65%) is still being left behind in the reservoir, as oil in place. Thus, a study of EOR techniques was started as early as in the 1950's and, on the basis of results of in-house studies, some of these techniques were at that time pilot tested in the Yum e n and Klamayi oil fields (Cheng, 1985; Yang and Han, 1985). A rapid development in the study of the EOR technology has occurred since the late 1970's. Thermal drive and chemical-flooding m e t h o d received much emphasis, and miscible flooding was also studied to some extent. The investigation of EOR techniques is matched to the features of each oil field. Pilot tests are then carried out, based on the results of the in-house studies. Field-wide commercial-scale application of the potential technology starts if it is found to be economically vi-
0920-4105/91/$03.50 © 1991 ElsevierSoence Pubhshers B.V. All rights reserved.
176
CHENG-ZH1YANG AND DA-KUANGHAN
able. The thermal drive technique has been industrialized, and the amount of oil produced by this technique in 1986 was approximately 1.5 million tonnes (Liu, 1987 ). This paper will give a brief review of the present status of in-house theoretical studies, laboratory studies and pilot tests of thermal drive, chemical-flooding and miscible-floodmg techniques, and their development in China, especially during the last decade. The thermal drive technique There are fairly abundant heavy oil resources in China. Numerous heavy oil deposits have been discovered in fifteen m e d i u m to large sedimentary basins. These are largely situated m Meso-Tertiary formations with a depth of burial of less than 2000 m; most of them are buried at 1000-1500 m, with some, in the west, at depths of only hundreds of meters. Most of these heavy oils contain less than 10% hght fractions (generally 5%) and have a high resin (usually over 30%) and a relatively low asphaltene content. The specific gravity of these oils is usually higher than 0.92 and their viscosities exceed 125 cP under reservoir condltions, with some of them up to thousands of centipoises (Hu et al., 1987). Obviously these deposits cannot be effectively recovered by water flooding. The only efficient way to recover these with a relatively high recovery fac-
tor, is through the application of a viscosityreducing heating technique. In-situ combustion In the Yumen and Klamayi oll fields (Western China) there are some relatively shallow heavy oil deposits buried at depths ranging from tens to hundreds meters. The study of insitu combustion through pilot field tests started there early in 1958 (Chen, 1976). Gasoline igrater and electric igniter techniques were developed for igmtion at a depth of approximately 500 m. Based on the results of in-house physical modeling, three types of in-situ combustion techniques were developed: conventional combustion, in-situ combustion followed by water injection; and as above with the cycle being repeated. Momtoring of the mxgration of the fire front and heat transfer were studied systematically. Ten pilot tests were carried out at the Heyushan reservoir (in Klamayi oil field), the Shiyougou reservoir (in Yumen oil field), the Sheng-tuo reservoir (in Sheng li oil field) and the Fu-You reservoir (Fu-You) The Heyushan reservoir in the Klamay~ oil field is taken as an example. In the period 1958-1974, eight pilot tests were carried out (Table 1) (Chen, 1976). Of these pilot projects the largest area covered was 11350 m: (He-4 test area), and included a m a x i m u m of 30 wells (He-4 area). The m a x i m u m depth of
T4BLE 1 In-s~tu combustion pilot
test m
the Klamay~ oll field
Umt
Pattern
Number of well
Time (days)
Depth (m)
Surface tested (m 2 )
EO R (%)
Heyushan Heyushan H-4 H-2 H-4 H-4 H-4 H-4
Surface Surface Surface Surface Line Surface Surface Surface
15 11 18 4 30 11 15 10
25 35 815 1008 748 487 761 733
14 18 85 410 110 110 85 85
179 120 11350 2825 6504 3720 7867 7051
43 2 55.2 64 0 84 0 17 4 31 3 19.0 74 0
PRESENT STATUSOF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
the test wells was 410 m (II-west area), and the m a x i m u m length of time for which testing was continued was 1008 days (II-west area). Results of these pilot tests show that the recovery factor of the tested well pattern is 47.8%, the recovery factor in the volume bounded by the fire front is 64%, while the recovery factor of the burned volume is 100%. The major problem encountered in these tests lay in the improper selection of the test area. Because the reservoir is fairly shallow and a lot of fractures are present, severe bypassing of the injected air occurred. As a result, the air/oil ratio was as high as 6919 m3/t. This process was also not investigated further, on account of the extremely expensive surface facilities required. The steam huff-and-buff and steam-drive techniques Steam stimulation has been used as the principal process for heavy oil recovery in China since 1980. For most of these heavy oil fields the composition of the heavy oils and the mineral composition of their reservoir rocks have been analyzed. Thermal properties such as heat conduction, thermal expansion and
177
heat loss, both in the surface facilities, as well as in well bores are measured and studied; and theoretical calculation pentaining to the flow of the heavy oil in porous media at high temperatures, as well as those relating to the thermodynamics within the well bores are carried out (Yah and Hepler, 1987 ). Based on the results of theoretical analyses and laboratory studies, physical modeling and numerical simulation of some pilot tests were conducted. A heat insulation device, including an insulated tubing string and a high-temperature downhole packer was developed to reduce the heat loss within a well bore to 9-14% (Liu, 1986). A new sand control agent was developed, capable of working under high steam temperatures (Chen and Chen, 1987). A series of theoretical studies relating to the increase in the volumetric performance of the injected steam were made, and the foam technique was suggested to increase the swept efficiency (Isaacs et al., 1987). Moreover, high-temperature foaming and diffusion agents to be used in the steam huff-and-puff process have been developed. In 1982 use of the steam huff-and-puff process was started, and this process has since been
TABLE 2 Reservoir parameters of 5 reservoirs pilot tested for steam mject~on Oll field
Reservoir formaUon Depth ( m ) Reservoir Temp. ( ° C) Viscosity ofdegassed off at reservoir temp. (cP) Av thickness of the pay zone ( m ) Porosity (%) Permeability ( m D ) Initial od saturaUon (%) Current reservoir pressure ( a t m ) Clay content (%) Asphaltene content (%) Pour point (%) Paraffin content ( ° C ) Sulphur content (%)
Gaosheng
Shuguan (Block 1-7-5)
Huan-xlling (Huan-17)
Shanjlas (Shan-2)
Klamayl 9-zone
Llanhua 1510-1700 56 0.94-0 96 67 6 23-30 1000-3000 65 120-150 5-10 44 i0 5 05
Dalinghe 1000-1100 50 0 97-0_98 44 22-30 2500-3000 65 104-110 6-8 47 15-20 27 05
Xlnglongtal 1000-1200 43 0.95-0 98 25-40 24 700-2000 60 104-120 5-13 35 11 5 04
Shanejle 1100-120 56 0.97-0 9 28-85 31-34 900-5000 65 110-120 3-8 28 16-18 2.4 0.7
Qlgu 160-240 18 0_927 16 4 32 700-4000 66 20 25.2 - 16-23 24 0 26
178
CHENG-ZHIYANG AND DA-KUANG HAN
,7y.y 50~0~ "~'-
/
O
producLlonwe
Fig_ 1 Pilot test for steam huff-and-puff m the G a o s h e n oil field
continued to be used in the Gaoshen (Li and Cai, 1985), Shuguang (Liu, 1986), Huanxiling (Liu, 1986), and Shanjiashi (Huang, 1987) reservoirs, usually at a depth of 10001600 m. It has also been tested and continued to be used in the Klamayi oil field where the ~7
well depth is approximately 200 m. Steam drive will start in these reservoirs in the near future. The main geological parameters of the above reservoirs are listed in Table 2. For example, the pilot test area in the Gaoshen oil field (Fig. 1 ) covers 0.4 km 2 and includes three 5-spot well patterns in which the well spacing is 150 m, a 9-spot well pattern with a well spacing of 105 m (Li and Cai, 1985). Thus, there are eighteeu wells in this area with depths ranging from 1490 to 1568 m. The total daily production of these eighteen wells is 121 ton with an average of 6.7 t/d/well before the test. In 1983 the first steam injection was started using two sets of steam generators (steam pressure, 175 atm; steam generated; I 1.5 and 23 t/d, respectively; steam property, 60-80%; steam temperature, 343°C). The steam was injected and then the wells were shut in for 57 days after which the wells were again reopened. Production was increased remarkably to 20-40 t / d for a period of about 6 months.
~oo6 qoo5 qoo~ qoo ooo2 _n....., ~°°t
for6 0
uuu..,,_
o/0 0~/I 9o/~@
awsa,~
1~d^
qoo~ q l l ~ / 7
~.~,,/ -
o produL l i o n well i n j e c t i o n well • o b s e r x a t i o n wet[ FLg 2 Area N ° 9 heavy od thermal recovery p d o t test at Klamayl
,~.. re~J
9/~6
PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
After several cycles of steam huff-and-puff, steam drive is now ready to be carried out in this pilot area. In the No. 9 region of the Klamayi oil field (Fig. 2 ) an area of 1.225 km 2 was selected for a steam-cycling pilot test. Here 64 wells (14 injectors) grouped in 7-spot well patterns and 74 wells ( 15 injectors) in 9-spot well patterns, i.e., a total of 138 wells, are included with an average well spacing of 100 m 2 (Ni, 1987). Starting in 1984, wells were steam injected at an average rate of 210 t / d for an average period of 7.5 d/well. A total of 1300-1800 ton of steam (270-290°C) were injected per well in a huff-and-puff cycle over the period of each cycle (250 days ). A total of 482 thousand tons of oil, equivalent to a recovery factor of 11.4%, were recovered at an overall steam/oil ratio of 1.03 t/t. Surface facilities capable of handling a total production capacity of 1207 t / d of oil have now been completed in this region. Moreover, another pilot test project for steam drive was selected in this region, covering an area of 0.253 km 2 including nine well patterns (a total of 30 wells). This project is currently underway.
Chemical flooding Oil production can be enhanced either by an improvement in the performance of the injected water such as polymer flooding (sometimes this is called improved water flooding) and foam flooding, or by an increase in the displacement efficiency such as surfactant flooding (also called micellar/polymer flooding or microemulsion/polymer flooding) and caustic flooding.
Polymer flooding Most of the Chinese oil reservoir formations are characterized by severe heterogeneity, especially in their vertical profiles. The vertical permeability coefficient reaches 0.7 or more, and in some cases as high as 0.9-1.0. More-
179
over, in most of the reservoirs, the viscosities of the oils are rather high. Thus the oil/water mobility ratio is fairly unfavorable, leading to bypassing of the displacement agent. Because the salinity of the formation waters associated with most of the reservoirs is not high, the application of polymers to thicken the injected water and to improve the mobility ratio, would remarkably increase the recovery of oil. Early in 1962, in the Petroleum Development Department of Beijing Petroleum University, the effect of the mobility of the driving fluid on the recovery efficiency of the oil using one- and two-dimensional physical models was studied systematically (Han and Yang, 1965 ). Radioactive tracer and electric resistivity techniques were used to detect the change in the saturation of the porous media in the models, and some good viscocifiers, such as sodium alginate and polyacrylamide, were screened for improving the mobility ratio. In recent years, the properties of aqueous polyacrylamide solution, and its sensitivity to the salt content were studied and the relations between the viscosity of the polyacrylamide solution and the concentration of mono- and bivalent ions were established and presented (Mei, 1982). The absorption of the polyacrylamide by the porous media and its retention during flow through the porous media were studied. It was pointed out that factors such as the clay content of the reservoir rock, the geometry of the pore structure, and the bivalention concentration in the water seriously affect the retention of the polymers (He, 1981; He and Hu, 1983; Gao and Hu, 1985 ). Moreover, the apparent viscosity of the polyacrylamide solution decreases with increasing permeability of the porous media and its flow velocity (Xu and He, 1985 ). This relationship can be used to adjust the mobility of this solution in the heterogeneous reservoir. Polyacrylamides with various molecular weights and various degrees of hydrolyzation are available from domestic chemical plants on a commercial scale in the form of concen-
180
CHENG-ZHI YANG AND DA-KUANG HAN
trated solution, powder, or emulsion. A study of the relation between the oil displacement efficiency and the amount of the polymer injected in the Daqing oil field (Fig. 3; Wang and Zhang, 1988) shows that an injected quantity (the product of the concentration of the polymer and the s~ze of the injected slug) of 200 ppm × PV [ concentration of polymer (ppm) × pore volume of polymer injected (PV)] will give an optimum result. The recovery factor can theoretically be increased by about 10% and about 200 ton of additional ml can be recovered by the injection of 1 ton of polymer. Of course, the optimum quantity to be injected will differ from one oil field to another. Based on the results of the above theoretical studies, polymer flooding has been pilot tested at various scales in the Daqing, Shengli, Klamayi and Dagang oil fields in five projects (some of which were carried out with the cooperation of Japanese, English and American companies). Most of these tests were successful to varying degrees. Hence, some commercial scale tests are now planned. AWo
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Fig 3 General curves for the selection of the a m o u n t of polymer flooding to be used.
A pilot test (Fig. 4 ) in a closely spaced well pattern area in the Daqing oil field is a typical example where polymer flooding is successful (Zhang, 1984; Wang and Zhang, 1988). A triangular well pattern with six wells spaced at 75 m apart was selected as the testing project. These wells were water flooded to a water cut of 97.5% prior to the injection of 0.164 PV of polyacrylamide solution. Positive effects were successively observed in three producers, 12 days after the polymer solution was again replaced by freshwater. The water cut of these producers decreased from 99.99 and 94.5% to 88, 60.4 and 77.7%, respectively, over an effective period of 200 days. The net incremental od produced is 608.8 ton, equivalent to an increase in the recovery factor of 5.6%, while the additional oil produced as a result of the injection of I ton of polymer is equal to 153.4 ton. Radioactive tracer monitoring showed that the thickness of water intake was increased from 2.4 to 3.8 m, while an analysis of the C1 content in the produced water also showed that the vertical performance is improved remarkably. Field experience shows that where a severe variation of the vertical permeability exists, the injectlvlty profile control prior to the injection of the polymer solution can be modified by injecting a cross-linked polymer. Polymer flooding is a proven economical and feasible technology. It seems likely, therefore, that this will be one of the main techniques to be used m the near future to enhance the oil recovery from water-flooded reservoirs in China. Pilot tests will be extended, such as for example, two projects with an area of 9 acres will be tested further in the Daqing oil field (Chouveteau and Han, 1988). At the same time, some more complicated reservoirs with temperatures exceeding 60°C or with strongly fractured reservoirs will also be pilot tested. Feasibility studies of the pilot tests in these areas are currently under way. It is expected that this technique will come into general use in the near future.
PRESENT STATUSOF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE 2
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Foamflooding The apparent viscosity of a foam flowing through a porous medium is usually proportional to the diameter of the pore, two-phase gas/liquid flow exhibits non-Newtonian fluid rheological properties. Thus, injection of a foam in the water-oil frontal region can improve the performance of the driving fluid. Furthermore, a foaming agent, an anionic surfactant, also serves to decrease the interfacial tension between oil and water, which may improve the displacement efficiency. An in-house study of foam flooding was started early in the Seventies with emphasis on the stability of the foam (Jia, 1986), the loss of foaming agent, and its inhibition (Yang and Cheng, 1988 ), the mechanism of foam flooding (Yang, 1985; Jia, 1986; Liu et al., 1986; Yang and Han, 1988),
TABLE 3 Data relating to the area pdot tested for foam drive
Reservoir formation Depth ( m ) Thickness of the pay zone ( m ) Permeability ( m D ) Porosity (%) Cementing matenal Well pattern No. of wells Area (ha) Well spacing ( m )
Je-88 well area. Klamayl od field
L3 reservoir area. Laojumlas od field
$7 550
L3 674
15.7 200-900 19 9 montmonllomte, hydromlca etc. 7 spot l m.I and 6 prod
49 1310 20 montmonilonlte, dhte etc. irregular 5 spot 8 lnj and 18 prod 89 150
-
148
and the form of foaming (in the reservoir or at the surface). The results showed that: (1) some of the anionic surfactants (such as alkyl-
18 2
CHENG-ZHI YANG AND DA-KUANGHAN •
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sulfates and alkyl-benzene sulfonates ) are good foaming agents; (2) the addition of a proper a m o u n t of polymers can increase the stability of the foam; and ( 3 ) that the loss of foaming agent can be reduced to some extent by the addition of polyphosphates. Two pilot tests were carried out in the Yumen and Klamayi oil fields in 198 I. Reservoir characteristics and some of the parameters of the tests in the well Je-88 area, Klamayi oil field (Liu et al., 1986 ), and the L3 zone in the eastern part of the Laojunmiao reservoir, Y u m e n oil field (Yang et al., 1988), are summarized in Table 3. The pilot test in the L3 reservoir (Fig. 5 ) in Laojunmiao field, was larger. A total a m o u n t of 93,000 m 3 of foam (with a concentration of the foaming agent of 0.6%) was injected followed by 160,000 m 3 of air eqmvalent to 28.9% of the reservoir volume over the period October 1979-December 1980. A total of 654 ton of foaming agent (industrial grade) and 172 ton of stabilizer were injected. But only six of the eighteen producing wells showed an increase in production (and then to varying de-
grees accompanied by a decrease in water cut after such an injection; no obvious effect was observed in the other producing wells tested. The failure of this pilot test was attributed to the high clay content (up to 13.3%, mostly montmorillonite) found to cover the grain surfaces of the reservoir rock. This high clay content together with the high specific area of the reservoir rock which reaches 3.55 m2/g, would have caused severe adsorption loss o f the foaming agents (Yang et al., 1988 ) (resulting in a loss of its foaming ability). In the Je-88 well pattern area, Klamayi oil field, a total a m o u n t of 933 m 3 liquid (containing 1% foaming agent) and 8082 m 3 of air (reservoir condition equivalent to about 5% reservoir volume) were injected from May 1971 to June 1973. The water intake capacity of the highpermeability zones, S{, S 2 and $27-3 decreased, while that of the low-pertneability zones $73-2 and $37-s , increased. The average daily production of these efficient wells increased by 48%, a c c o m p a m e d by a decrease in water cut by 27.7%; over an effective period of 26.6 months.
PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
/ii\.
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Fig. 6 P d o t test o f f o a m f l o o d i n g in t h e K l a m a y a oll field.
An increase in the recovery factor of 6-8% occurred. This thus represents a successful pilot test (Liu et al., 1986).
Surfactant flooding Since both polymer and foam flooding can only improve the performance of the driving agent, the recovery of oil can only be enhanced to a limited extent. Surfactants, inspite of their high cost, have a greater potential for enhancing oil recovery by improving the displacement efficiency of the displacing agent. Many Chinese oil fields are adaptable to surfactant flooding because ( 1 ) the reservoir temperatures are moderate, (2) the salinity of the formation waters is not too high ( ~ 10,000 ppm), (3) the permeability of the reservoir rocks is medium to high and the viscosity of most of the oils (in the reservoir) is not too great, and (4) some oils contain a relatively high aromatic fraction, even though most of the Chinese oils are paraffin based. Therefore, surfactant flooding has been studied since the 1970's. Emphasis has been placed on the synthesis and development of surfactants, the properties of the surfactant solution and its phase behavior, the absorption, and retention of the surfactant, and its inhibition, and the
mechanism of the oil displacement by surfactant solutions. In recent years, a technology of film sulfonation by SO3 for the synthesis of petroleum sulfonates for field use has been developed and industrialized; a-olefin sulfonates for use with high-salinity reservoir formation waters has also been developed. Because most oils in Chinese oil fields are paraffinic in nature, a lot of work was done on the synthesis of alkyl carboxylate by oxidation using paraffinic hydrocarbons as raw material. Industrial by-products will be cheaper and one kind of lignosulphonate has been identified as a suitable surfactant for use in chemical flooding. The EACNs of crude oils are studied systematically for the selection of surfactants suitable for specific crude oils and vice versa (Jiang and Jiang, 1986; Huang and Li, 1987). The effects of alcohol (Miao and Gao, 1988), hydrocarbon (Li, 1986; Li and Yang, 1986b), salt content (Li and Wang, 1986), additives (Han et al., 1989), temperature (Li and Wang, 1986) etc. on the phase behavior of the surfactantbrine-oil system have been studied and relations between interfacial tension, solubilization parameters and optimum salinity have been established. A dilute system (without the addition of alcohol) has been studied (Li,
184
CHENG-ZHI~ANG AND DA-KUANG HAN
40-
0
2
4
6
F~g. 7 Sulfonate adsorption isotherm
1982, 1986) in order to decrease the chromatographic separation of the micellar system. Results show that in this case high solubilizatlon parameters can also be obtained, even at lower surfactant concentrations. The dynamic lnterfacial tension between caustic solutions and highly acidic oils and its variation with the addition of polymers (Huang et al., 1987 ), as well as the interaction between surfactants and polymers (Yang, 1986) have been studied in order to provide a theoretical basis for the development of the MAP (Micelle-Alkali-Polymer) technology. Studies of surfactant loss in reservoxrs show that the main loss of surfactant is due to adsorption on the rock surfaces (Yang et al., 1985, 1988; Yang, 1985), reaction with multivalent ions (Yang and Huang, 1986; Yang and Han, 1988; Yang and Tang, 1988; Tang and Yang, 1989 ), and partitioning into the immobilized displaced fluids (Yang and Han, 1988; Han et al., 1989). A dynamic adsorption model (Gao et al., 1988) and a flowing retention model (Liu, 1983 ) were constructed on the basis of in-depth study of the mechanism of the adsorption of the surfactants. One of the main results of our studies has been the development of a rational technique for inhibiting the loss of surfactants (Fig. 7 ) ( H a n and Yang, 1986; Yang and Han, 1988; Yang and Tang, 1988).
Displacement tests using surfactant solutions (Jiang and Lu, 1982; Han et al., 1989) and studies of the mechanism of displacement using mlcromodels (Han et al, 1989) show that chemical flooding can be applied and result in h i g h recovery efficiencies without preflush even in cases where the clay content of the reservoir exceeds 10%. The surfactant-saltwater-hydrocarbon system has been employed in the treatment of single well bores. Early in 1974 this system was used in the Shengli (Yang, 1987), Daqing (Jiang et al., 1975), and Yumen oll fields for well stimulation. One hundred sixty eight wells were worked with such a system with an overall success ratio of 66.7% (Table 4). This demonstrates that ultra-low interfacial tension can be achieved under reservoir conditions. A successful surfactant pilot test was completed in the Daqing oil field (Meng et al., 1989), while feasibility studies of other two projects (Han et al., 1989) were completed (one of these has been done in cooperation with the Institut Fran~als du P6trole). The reservoir tested (in the Daqlng oll field) is D 4+ 5 formation. This pay zone has a thickness of 3.4 m and a permeabdity of 220-500 m d There are one injector, two producing and one observation well in an area of 1.7 acres, with a well spacing of 75 m (Meng et al., 1989 ); 1500 m 3 of micellar solution (soap of paper pulp ligno-cellulose/aliphatic alcohol/water) equivalent to 0.1 PV was rejected followed by a polymer solution of 2040 m 3 (0.13 PV) actmg as a buffer. Water injection was then continued and a clear effect due to surfactant flooding was observed after cumulative injection of 0.26 PV of water. The incremental oil produced by the end of 1988 was 3680 ton, and the recovery efficiency as indicated by a numerical simulation now reaches 70%, which is 35.5% higher than the conventional water flooding practiced previously. A pilot test for surfactant flooding in the Yumen oil field characterised by the high clay content of its reservoir is currently at the tech-
PRESENT STATUSOF EOR IN CHINESEPETROLEUMINDUSTRY AND ITS FUTURE
185
TABLE 4 Well stimulation by microemulslon N°
Number of well
Thickness ( m )*
Quantity of mj ection (m 3/ d )
Incremental oil production (m 3/d well )
Efficl ** (%)
1 2 3 4 5 6
14 13 4 3 6 60
129 6 143.3 32.0 42 5 161.2 709 6
168 309 36.8 35 118 666.8
26 7 21 8 182 78 81.1 42 3
92 8 75.8 25 100 66.5 76.5
1.3 2 15 1.55 0 82 0 73 1.31
*Total thickness of pay zone treated. **Efficmncy= number of effective well/number of well treated
nical design stage, based on a series of laboratory and simulation studies and will be put into practice in the near future.
Caustic flooding Some of the oils in China such as those of the Gudao, Yansanmu, Xinglungtai oil fields, etc., are relatively acidic, and may reach a level as high as 1 mg K O H / g oil. These oils when brought into contact with caustic solutions may result in in-situ surfactants. In-house studies by some institutions of the Chinese Academy of Science and some universities, as well as those by the petroleum industry have been in progress since 1981 (Li, 1984; Huang and Yang, 1984, 1985; Bian and Yu, 1986; Huang et al., 1989). It has been found that within a certain range of the salt content, the interfacial tension between oil and water can be lowered to a level of 10-~-10 -2 d y n e / c m by the addition of an alkalic and that the spontaneous emulsification of water and oil does occur. The factors affecting these reactions and the caustic consumption have been studied extensively. The dynamics of caustic consumption has been studied with the cooperation of the National Institute for Petroleum and Energy Research and a model describing the caustic consumption process when a caustic solution flows through a porous m e d i u m has been constructed, thus providing a basis for the mathematical modeling of caustic flooding (Li, 1984;
Liu, 1986). So far no pilot test has been attempted, due to the complexity of the mechanism of caustic flooding, and very high caustic consumption which cannot be inhibited or reduced effectively at present. However, because of the fact that all alkalies can react with some of the substances in oil to form carboxylate-surfactants, and that an alkaline/polymer/acidic oil system can produce 'low interfacial tension, we are now studying the surfactants/alkaline/polymer flooding (combination flooding) (Yang, 1986). A series of studies based on this concept are currently underway and some interesting results have been obtained.
Miscible and immiscible gas drive Early in 1970's, CO2-water injection and C O 2 water cycle injection had been tested in the Daqing and Klamayi oil fields. However, these can hardly be considered as effective miscible displacement process. During the 1980's, the m i n i m u m miscible pressure ( M M P ) of oils from different reservoirs has been studied in the laboratory by the slim-tube technique. It was found that some of the oil fields are adaptable to CO2 miscible displacement while some of the oil fields with lighter oils should be adaptable to enriched gas miscible drive. However, the many crude oil of the oil fields, due to the parafflnic nature of the oils, have a M M P which is too high for the
186
miscible displacement process to be apphed. CO2 miscible displacement is more economical than enriched gas drive. However, only small deposits of natural COz have been discovered in our country and it is costly to use CO2 from power plants or refineries. But COz miscible drive would still have a certain potential for increasing oil recovery if more CO2 reserves are discovered. A feasibility study of the process of N2 displacement which is the immiscible drive process which operates in nature, is now being carried out. Some mention should also be made of the microbial methods which have been studied (Wang and Wang, 1980). The preparation of bio-polymers and bIo-surfactants as well as the screening of strains of bacteria for in-situ oil recovery are included in this subject. The production of polysaccharides has been industrialized. Their rheological properties and techniques for-improving their temperature tolerance are being studied. Some bio-surfactants have been tested on physical models with encouraging results. Moreover, some strains of microbes have been identified and selected for the in-situ enhancement of od recovery and have given very promising results in laboratory tests. Their microbiological features are being studied thoroughly. It is expected that a field pilot test with these microbes will be carried out in the near future.
Conclusions The following achievements in Chinese EOR technology have been obtained: ( 1 ) Thermal drive has been industrialized, and the additional annual production resulting from the use of this technique amounts to millions of tons. (2) Both theoretical studies and pilot tests show that polymer flooding is technically possible and economically feasible, and field-wide operations are currently under consideration. (3) In-house studies and field pilot tests show that surfactant flooding has a high po-
CHENG-ZHIYANG AND DA-KUANG HAN
tentlal in EOR, but more studies are needed before it can be applied on an industrial scale. (4) Other EOR techniques, such as foam flooding, caustic flooding and miscible displacement have been studied or tested to some extent. These techniques will be further studled and tested in order to establish parameters for their o p t i m u m use. (5) A complete and systematic evaluation process, which includes In-house studies (theoretical and laboratory), physical and mathematical modeling, feasibility studies, technical design, pilot field tests, the monitoring analysis of the test results, and the eventual industrial scale applicability of an EOR technique has been established and is now followed in China. (6) Most of the reagents required for EOR can be synthesized locally while most of the equipment can also be manufactured domestically. Thus, it is expected that EOR will be apphed in various oil fields depending on their reservoir characteristics and their adaptability to various EOR technologies.
Acknowledgements The authors wish to express their appreciation to Messrs. Tong-Luo Qin, Qing-Shan Yue, Pu-Hua Yang, Jing-Chun Zhang for valuable Information and suggestions.
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PRESENT STATUS OF EOR IN CHINESE PETROLEUM INDUSTRY AND ITS FUTURE
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CHENG-ZHI Y&NG AND DA-KUANG HAN
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PRESENTSTATUSOF EOR IN CHINESEPETROLEUMINDUSTRYAND ITS FUTURE Yuan, Xh~-yl and Van Quy, N., 1988 The numerical simulation of chemical flooding Acta Pet. Sin., 1 51-60 (in Chinese).
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