Magnetic Resonance Imaging, Vol. 14, Nos. 7/8, pp. 951-953, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved 0730-725X/96 $15.00 + .OO ELSEVIER l Short Communication PII SO730-725X( 96)00192-S THE APPLICATION OF NMR IMAGING TO THE STUDIES OF ENHANCED OIL RECOVERY IN CHINA WANG WEIMIN, LANG DONGJIANG, AND LIU WEI Institute of Porous Flow CNPC, Lang Fang 102801, P.O. Box 44 P.R. China The advantages of the application of NMR imaging to the studies of enhanched oil recovery methods that include polymer slug flooding, combination slug flooding, and thermal soaking are described. Copyright 0 1996 Elsevier Science Inc. Keywords: NMR imaging; Enhanced oil recovery. INTRODUCTION Chinese oil production reaches a high water mark in the next decade, and the problem of the natural fall- off in oil production thereafter must be addressed. Therefore, enhanced oil recovery (EOR) methods will play an important role in the Chinese petroleum indus- try. Several EOR pilot tests that include polymer slug flooding, combination slug flooding, and thermal soak- ing have been conducted in Chinese oil fields in recent years. However, it is important to evaluate EOR effects accurately for the reason of economy. MRI is a power- ful tool for investigating multiphase flow in porous media.‘-4 This paper presents some research work about MRI applied to the EOR studies in the Chinese oil fields. MATERIALS AND METHODS permeability ratio for the three layers was 3:2:1. The size of the core moulds was 3.4 cm in diameter and 28 cm in length Two sand-packed cores with two layers were used in the combination slug flooding ex- periments. The permeability ratios of the two layers were 3:l for one sand-packed core and 1O:l for the other sand-packed core. The size of the two moulds was 14.5 cm in length, 4.5 cm in width, and 1 cm in thickness. We have used natural cores from the Liaohe oil field in thermal soaking experiments. The size of the cores was 2.5 cm in diameter and 7 cm in length. The cores were held on a special core holder made of Teflon. After every thermal soaking conducted outside the MRI system, the core holder was covered with cotton, put inside the magnet, and imaged immediately. The steam soakings were conducted at steam tempera- ture of 230°C. The thermal chemical soakings were conducted at 80°C. The displacing fluid in all experi- ments was 500 ppm MnCl, aqueous solution. The three techniques of polyacrylamide slug optimisa- All experiments were carried out on a Bruker Bio- tion, thermal soaking evaluation, and combination slug spec 47/40 NMR imaging/spectroscopy system. Dif- ( polyacrylamide+alkali+surfactant) flooding evaluation ferent kinds of gradient coils and probes have been have been studied by MRl. All experimental cores were used for the different size of cores used in the experi- selected according to the nature of the experiments, and ments. All images were acquired using a standard two- the geological structure of the reservoirs. dimensional Fourier Transform spin-echo sequence. The cores used for the polymer slug flooding experi- The echo time TE was 4 ms. The recovery time TR ment were sand-packed cores with three layers. The was 500 ms. The time interval between two imaging - Address correspondence to W. Weimin, Institute of Porous Flow CNPC, Lang Fang 102801, P. 0. Box 44 P.R. China. 951
Transcript
Magnetic Resonance Imaging, Vol. 14, Nos. 7/8, pp. 951-953, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved
0730-725X/96 $15.00 + .OO
ELSEVIER
l Short Communication
PII SO730-725X( 96)00192-S
THE APPLICATION OF NMR IMAGING TO THE STUDIES OF ENHANCED OIL RECOVERY IN CHINA
WANG WEIMIN, LANG DONGJIANG, AND LIU WEI
Institute of Porous Flow CNPC, Lang Fang 102801, P.O. Box 44 P.R. China
The advantages of the application of NMR imaging to the studies of enhanched oil recovery methods that include polymer slug flooding, combination slug flooding, and thermal soaking are described. Copyright 0 1996 Elsevier Science Inc.
Keywords: NMR imaging; Enhanced oil recovery.
INTRODUCTION
Chinese oil production reaches a high water mark in the next decade, and the problem of the natural fall- off in oil production thereafter must be addressed. Therefore, enhanced oil recovery (EOR) methods will play an important role in the Chinese petroleum indus- try. Several EOR pilot tests that include polymer slug flooding, combination slug flooding, and thermal soak- ing have been conducted in Chinese oil fields in recent years. However, it is important to evaluate EOR effects accurately for the reason of economy. MRI is a power- ful tool for investigating multiphase flow in porous media.‘-4 This paper presents some research work about MRI applied to the EOR studies in the Chinese oil fields.
MATERIALS AND METHODS
permeability ratio for the three layers was 3:2:1. The size of the core moulds was 3.4 cm in diameter and 28 cm in length Two sand-packed cores with two layers were used in the combination slug flooding ex- periments. The permeability ratios of the two layers were 3:l for one sand-packed core and 1O:l for the other sand-packed core. The size of the two moulds was 14.5 cm in length, 4.5 cm in width, and 1 cm in thickness. We have used natural cores from the Liaohe oil field in thermal soaking experiments. The size of the cores was 2.5 cm in diameter and 7 cm in length. The cores were held on a special core holder made of Teflon. After every thermal soaking conducted outside the MRI system, the core holder was covered with cotton, put inside the magnet, and imaged immediately. The steam soakings were conducted at steam tempera- ture of 230°C. The thermal chemical soakings were conducted at 80°C. The displacing fluid in all experi- ments was 500 ppm MnCl, aqueous solution.
The three techniques of polyacrylamide slug optimisa- All experiments were carried out on a Bruker Bio- tion, thermal soaking evaluation, and combination slug spec 47/40 NMR imaging/spectroscopy system. Dif- ( polyacrylamide+alkali+surfactant) flooding evaluation ferent kinds of gradient coils and probes have been have been studied by MRl. All experimental cores were used for the different size of cores used in the experi- selected according to the nature of the experiments, and ments. All images were acquired using a standard two- the geological structure of the reservoirs. dimensional Fourier Transform spin-echo sequence.
The cores used for the polymer slug flooding experi- The echo time TE was 4 ms. The recovery time TR ment were sand-packed cores with three layers. The was 500 ms. The time interval between two imaging
- Address correspondence to W. Weimin, Institute of
Porous Flow CNPC, Lang Fang 102801, P. 0. Box 44 P.R. China.
951
952 Magnetic Resonance Imaging l Volume 14, Numbers 7/8, 1996
Fig. 1. The behaviour of the 0.2 pv polymer slug.
acquisitions was 5 s in both the polymer slug flooding and the combination slug flooding experiments.
RESULTS AND DISCUSSION
Several polymer slug sizes 0.2,0.4,0.5,0.6 pv were monitored by NMR kinetic imaging. Figure 1 shows the behavior of the 0.2 pv polymer slug. In the begin- ning (i.e., step 1 and step 2), the polymer slug shows a piston-like displacement. However, the polymer slug was broken through by the water injected behind at 8 cm away from the influx later in step 3 and step 4. NMR kinetic images showed that the water injected behind the polymer slug broke through at an early stage of flooding in the high and medium permeability zones, and resulted in a very low sweep efficiency when the polymer slug size was less than 0.5 pv. The optimum size of polymer slug was 0.5 pv, if we take the economic factors into account.
Fig. 2. Combination slug flooding in the core of permeabil- ity ratio 10: 1.
Fig. 3. The residual oil distributions at the end of the first four steam soaking periods.
Figure 2 shows four stages of combination slug flooding in the core mould with a permeability ratio of 10: 1. Most of the combination agent penetrates into the high permeability layer. The kinetic NMR images showed that the swept efficiency of the combination agent in 3:l core mould was much better than that of 10: 1 core mould. The typical features displayed by the NMR images in combination slug flooding were that an oil bank was formed in front of the combination agent slug and region of high water content was formed between the combination agent and the oil bank, caused by the reduction of interfacial tension and irre- ducible water saturation. The MRI results suggest that a profile control process should be done before combi- nation slug flooding begins in reservoir engineering even though some mobility control material has been added to the combination agent.
Figure 3 shows the residual oil distribution at the end of the first four periods of soaking during the steam soaking experiment. We can observe strong back and forth movement of oil. The main reason for the de- crease in the steam soaking efficiency after four peri- ods of soaking is that the condensed water back drained and broke through the oil bank formed by the injected steam. However, thermal chemical soaking acted more homogeneously. Its effectiveness lasted longer, and no apparent back and forth movement of residual oil was observed.
CONCLUSIONS
NMR imaging technology is useful in the design and evaluation of EOR. Polymer slug size has a large influ- ence on the stability of the polymer front and the swept efficiency of the polymer. The combination flooding
NMR imaging and enhanced oil recovery in China 0 W. WEIMIN ETAL. 953
agent is more likely to get into lower permeability zone. An oil bank was clearly observed during combination slug flooding. A strong back and forth movement of residual oil was observed in steam soaking. However, thermal chemical soaking acted more homogeneously.
2. Chen, J.D.; Dias, M.M.; Patz, S.; Schwartz, L.M. Mag- netic resonance imaging of immiscible fluid displacement in porous media. Phys. Rev. Lett. 61( 13):1489-1492; 1988.
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