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ISSN:2157-7463
Journal of Petroleum & Environmental Biotechnology
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Research and Practice of the Early Stage Polymer Flooding on LD Offshore Oilfield

Ting Sun*, Ma Kuiqian and Li Yanlai

China National Offshore Oil Corporation (CNOOC) Ltd., Tianjin, China University of Petroleum-Beijing, P.R. China

*Corresponding Author:
Ting sun
China National Offshore Oil Corporation (CNOOC) Ltd.
Tianjin, China University of Petroleum-Beijing
China
Tel: (852) 2213 2500
E-mail: [email protected]

Received date: April 24, 2017; Accepted date: July 31, 2017; Published date: August 07, 2017

Citation: Sun T, Kuiqian M, Yanlai L (2017) Research and Practice of the Early Stage Polymer Flooding on LD Offshore Oilfield. J Pet Environ Biotechnol 7: 337. doi: 10.4172/2157-7463.1000337

Copyright: © 2017 Sun T, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Literature survey shows that polymer flooding was generally conducted during high water-cut stage (WCT>80% to 90%). Even the first China Offshore polymer flooding project was carried out in SZ when water cut was 60%. By then, conduction of polymer flooding in early phase (WCT<10%) was just discussed in theory. For offshore oilfield, the treatment of water could be costly. Because polymer improves mobility ratio of replacement fluid over oil and sweep efficiency, less water is injected and less water is produced. So, we did enormous research about the polymer flooding on early stage by theoretical analysis, series of experiments and chemical flooding simulation. Based on these researches, we carried out the first field test of polymer flooding on early stage in LD. Single well polymer injection test was started in Mar 2006 when the water cut in the pattern was lower than 10%. After the trial, there were other 5 water injectors being converted to polymer injectors from 2007 to 2009. The polymer flooding controlled reserve was about 25,250,000 m3. For the early stage polymer flooding, the characteristics of the responses on producers were different from the case in which polymer flooding was conducted during high water cut stage. The water producing of the producers continued to rise up after polymer flooding, but the simulation research showed that the water cut increasing rate was lower than the rate during merely water flooding. In addition, we observed the drop-down on the water cut in some wells, such as A11, A12, A13, A15, etc. For the well A11, the highest water cut reduction reached 41% after the injectors (A5/A10) profiles controlled, and net incremental oil for A11 even reached 154,510 m3. By Dec 2014, the total incremental oil by polymer flooding was about 754,650 m3, and the stage oil recovery efficiency was enhanced by 3.0%. The polymer flooding is still effective, and we will get more oil from the polymer flooding.

Keywords

EOR; Polymer Flooding; Offshore oil field

Abbreviations: RRF: Residual Resistance Factor; RF: Resistance Factor; PORFT: Accessible Pore Volume; FREQFAC: Reaction Frequency Factor; ADMAXT: Maximum Adsorption Capacity; ADRT: Residual Adsorption Level; PI: Pressure Index; WCT: Water Cut.

Introduction

With the improvement of offshore exploration extent, there are more and more heavy oil found. The oil recovery was only 18% to 25% by the conventional oil production method of water injection. So more challenges rise up, such as how can we get more oil? How can we improve the oil recovery? For the offshore oilfield, a method which can be used to develop oilfield more efficiently must be found due to the life time of offshore platform is limited (with the life of 25 years to 30 years).

Polymer flooding was already a mature set of technology to improve oil recovery on the onshore oilfields in China. Literature survey shows that polymer flooding was generally carried out during high water-cut stage (WCT > 80% to 90%). Even the first China Offshore polymer flooding project was carried out in SZ when water cut was 60%. By then, conduction of polymer flooding in early phase (WCT< 10%) was just discussed in theory. So, we did enormous research about the polymer flooding on early stage by theoretical analysis, series of experiments and chemical flooding simulation, as shown in (Figures 1 and 2). Gel high permeability, severe heterogeneity, high crude oil density (0.947 g/ cm3) and medium oil viscosity (7.2 cp to 19.4 cp). LD oilfield was put online in January of 2005, and started to inject water on September of 2005. Based on well understanding of the mechanism and effect of the early polymer flooding, we carried out the single well polymer injection pilot test from 2006 when the water cut in the pattern was lower than 10%. After the trial, there were other 5 water injectors were converted to polymer injectors from 2007 to 2009. The polymer flooding controlled reserve was about 25,250,000 m3. For the early stage polymer flooding, the characteristics of the responses on producers were different from the case in which polymer flooding was conducted during high water cut stage. The water producing of the producers continued to rise up after polymer flooding, but the simulation research showed that the water cut increasing rate was lower than the rate during merely water flooding. Of course, we also observed the drop down on the water cut in some wells, such as A11, A12, A13, A15, etc. For the well A11, the highest water cut reduction reached 41% after the injectors (A5/ A10) profiles controlled, and net incremental oil for A11 even reached 154,510 m3. By December 2014, the total incremental oil by polymer flooding was about 754,650 m3, and the stage oil recovery efficiency was enhanced by 3.0%. The polymer flooding is still effective, and we will get more oil from the polymer flooding.

petroleum-environmental-biotechnology-numerical-simulation

Figure 1: The numerical simulation of the mechanism research.

petroleum-environmental-biotechnology-EOR

Figure 2: The EOR in different time.

Experiment Tests and Polymer Flooding Scheme Design

Mechanism study on early stage polymer flooding

Geometric size of the core model is 30 cm × 4.5 cm × 4.5 cm, and the parameters of experiments were shown in Tables 1 and 2). The results of lab test experiments were shown in Table 2. From the results of the lab test, the difference of the EOR between cases in which polymer flooding conducted at different time was not obvious. But for offshore oilfield, the platform life was about 25 years to 30 years. We must produce more oil in limited time. From numerical simulation of the mechanism research, we know that the larger enhanced recovery value can be obtained, if polymer can be injected earlier (Table 3) [1-3].

Method of displacement The water cut start to the polymer injection (%) Viscosity of the oil(cp) Polymer concentration (mg/l) Core size Other lab test conditions
Polymer 0 70 1750 L:30 cm
W:4.5cm H:4.5cm
Temperature: 65°C
Rate of displacement: 1m/d
Polymer injection PV: 0.25PV
28
53
97
Water --

Table 1: The geometric parameters of the model.

Method of displacement The water cut start to the polymer injection (%) The water flood recovery before polymer flood (%) The ultimate recovery factor(%) Improve oil recovery by polymer flooding (%)
Polymer 0 0 51.35 6.23
28 22.2 51.08 5.96
53 23.1 50.75 5.63
97 45.1 50.35 5.23

Table 2: The results of lab test experiments.

The water cut start to the polymer injection (%) 0 20 40 60 80
Rw (%) 39.86
Rp (%) 49.28 48.98 48.73 48.3 46.71
ΔR (%) 9.39 9.09 8.84 8.42 6.82

Table 3: Statistical recovery efficiency in the development of 30 years.

Experiment tests and mechanistic models of gel

The relationship between viscosity and concentration of gel and polymer in early stage was shown in Table 4. The experiment about the crosslinked polymer viscosity in the case of different polymer concentrations and ratios of the polymer to Cr3+ was done. As is shown in the Table 5, the viscosity also increased with time going by. The results of RF and RRF test were shown in Table 6. RRF of the polymer is 1.8 to 3.2 from the experiment, and the gel is 39.3. According to the experiment results and the SZ oilfield mature experience, RRF of LD gel is 5, and the polymer is 2.5. The static adsorption and dynamic adsorption tests were shown in Tables 7 and 8.

The polymer
Polymer concentration (mg/L) 500 700 1000 1200 1500
Polymer viscosity (cp) 3.5 4.5 6 8 9.5
The cross-linked polymer (P:Cr³=60:1)
Polymer concentration (mg/L) 500 700 1000 1200 1500
Polymer viscosity (cp) 3 4 5.5 7 8.5

Table 4: The relationship between viscosity and concentration of gel and polymer.

Polymer concentration (P:Cr³) Viscosity of the crosslinked polymer (cp)
Initial After 2 days After 5 days After 10 days After 15 days
1200mg/L, (P:Cr³=40:1) 8.2 420 582.3 613 602.3
1200mg/L, (P:Cr³=60:1) 8.1 312 452.3 523 572.3
1000mg/L, (P:Cr³=60:1) 5.9 62.9 87.5 137 210.5
1000mg/L, (P:Cr³=40:1) 6.2 154 231.4 336 341.2

Table 5: The viscosity of the cross-linked polymer.

Core number Kg ('10-³mm²) Displacement system RF RRF
R3-9 1350 The crosslinked polymer Cp=1200mg/l(P:Cr³⁺=20:1) 45.6 58.2
12-4-5 1355 The crosslinked polymer Cp=1200mg/l(P:Cr³⁺=40:1) 39.9 48.4
12-3-5 1360 The crosslinked polymer Cp=1200mg/l(P:Cr³⁺=60:1) 34.9 39.3
L12-1-1 1345 The crosslinked polymer Cp=1200mg/l(P:Cr³⁺=80:1) 32.9 35.6
12-4-2 1320 The crosslinked polymer Cp=800mg/l(P:Cr³⁺=20:1) 38.9 46.7
12-3-4 1380 The crosslinked polymer Cp=800mg/l(P:Cr³⁺=40:1) 33.9 42.9
L12-1-2 1346 The crosslinked polymer Cp=800mg/l(P:Cr³⁺=60:1) 29.9 36.3
12-4-10 1368 Polymer Cp=800mg/l 12.9 1.8
12-4-10 1349 Polymer Cp=1000mg/l 16.2 2.5
12-4-13 1342 Polymer Cp=1200mg/l 21.3 3.2

Table 6: The RF and RRF of the cross-linked polymer.

Number Polymer concentration (mg/L) Static adsorption (mg/g)
1 600 2.51
2 800 2.82
3 1000 3.13
4 1200 4.31
5 1400 5.16

Table 7: The static adsorption experiment result.

Displacement solution Core permeability (10-³mm²) Polymer concentration (mg/l) Dynamic adsorption (µg/g)
DQKY polymer 1096 1400 126
1103 1200 95
DQGF gel 1133 1200 332
1108 1000 226
DQGF polymer 1202 1200 104
1100 1000 72

Table 8: The dynamic adsorption experiment result.

Injection scheme design

The size of injection slug was 0.163 PV, and the period of the slug injection was 5 years as shown in Table 9. Injection system was the polymer and Chromium ion (Cr3+) crosslinking agent. The concentration of the polymer was 1200 mg/L (Polymer: Cr3+=600:1- 60:1). The polymer injection rate was 0.033 PV per year. The predicted improve recovery factor was 6.1%.

Injecting slug Displacement pattern Time (month) Displacement agent Slug concentration (mg/L) Mother liquor concentration (mg/L) Injecting pressure (Mpa) Implementation date
First slug (0.0275PV) Gel 10 DQGF+crosslinking agents (P:Cr³=600:1~60:1) 1200 5000 to 6200 ˂10 The polymer injection trial was conducted on the A23 well in March of 2006. After the trial, there were other 5 injectors were converted to polymer injectors from 2007 to 2009.
Second slug (0.11PV) Polymer 40 DQKY 1600
Third slug (0.0275PV) Gel 10 DQGF+crosslinking agents (P:Cr³=600:1-60:1) 1200

Table 9: The dynamic adsorption experiment result.

The Result of the Field Test

From March of 2006, the polymer injection trial has been conducted on the A23 well when the water cut of the pattern was lower than 10%. After the trial, there were other 5 injectors (A1\A5\A10\A14\A18M) were converted to polymer injectors from 2007 to 2009. The polymer flooding controlled reserve was about 25,250,000 m3.

Characteristic of gel-injection well effectiveness is shown in Figure 3 and Table 10. From the table we can conclude: injecting pressure rise, while water injectivity index decline and the values of RF and RRF remain above 1.

petroleum-environmental-biotechnology-polymer-injector

Figure 3: Dynamic characteristic curve of the polymer injector.

For the early stage polymer flooding, the characteristics of the responses on producers were different from the case in which polymer flooding was conducted during high water cut stage. The water producing of the producers continued to rise up after polymer flooding, but the simulation research showed that the water cut increasing rate was lower than the rate during merely water flooding (Figures 4-6). Of course, we also observed the drop down on the water cut in some wells, such as A11, A12, A13, A15, etc. For the well A11 (Figure 5), the highest water cut reduction reached 41% after the injectors (A5/A10) profiles controlled, and net incremental oil for A11 even reached 154,510 m3.

petroleum-environmental-biotechnology-field-histor

Figure 4: LD field history match curves of water cut.

petroleum-environmental-biotechnology-Production-curve

Figure 5: Production curve of producer A11.

petroleum-environmental-biotechnology-polymer-flooding

Figure 6: The effectiveness of the early stage polymer flooding.

By December 2014, the total incremental oil by polymer flooding was about 754,650 m3, and the stage oil recovery efficiency was enhanced by 3.0%. And the polymer flooding is still effective now, and the polymer will be injecting until 2017. So we will get more oil from the polymer flooding. And the reasonable expectation of recovery beyond 2014 will be 2.8%. Totally we can get the oil recovery enhanced by polymer in LD oil field will be 5.8%. The actual EOR from polymer will be less than the plan of 6.1% EOR. The reason was that there were some wells blocked by polymer and sand. And we are doing lots works on solving the sand and polymer blockage [4,5].

Discussion and Conclusion

a. For the offshore oilfield, the platform life was about 25 years to 30 years. From lab test and numerical simulation research, we both know that the larger enhanced recovery value can be obtained, if polymer can be injected earlier.

b. We designed the polymer flooding on early stage, and the field test was carried out on the LD heavy oilfield of China offshore when the water cut was less than 10%.

c. By December 2014, the total incremental oil of polymer flooding was about 754,650 m3, and the stage recovery was improved by 3.0%. The polymer flooding is still effective, and more oil will be produced by the polymer flooding.

Acknowledgment

The data presented in this paper was obtained from many other departments of CNOOC Ltd.-Tianjin, such as the production department and offshore department etc. And the polymer and gel suppliers also provided some laboratory test data. We would like to thank them all for their contributions to this work.

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