The Research and Application of CO 2 Flooding in Low-permeability Reservoirs, China

1. The Research and Application of CO2 Flooding in Low-permeability Reservoirs, China Presentedby: Yang Siyu yangsiy@petrochina.com.cn State Key laboratory of EOR (Research Institute of Petroleum Exploration & Development , PetroChina) Canberra, Australia, Sept. 21-23, 2009

2. Outline Typical characteristics of low K reservoirs in China Part1 The advantages of CO2 flooding in low K reservoir Part2 Part3 Analysis of the CO2 flooding effect Part4 Summary

3. Low permeability reservoir account for a large percentage • 30% OOIP deposited in the low-permeability reservoirs in China. • 2/3 undeveloped oil in place deposited in low-permeability reservoirs ( K<10mD). • 95% oil in CNPC produced from waterflooding reservoirs.

4. Reservoir characteristics of low permeability reservoirs in China • Small pore throat，low P&K, poor reservoir quality • lithologic trap ，poor reservoir continuity • thin interbedded sandstone and mudstone • variable reservoir thickness and physical properties • well-developed micro-fracture • strong heterogeneity • higher water saturation Micro-pore with fracture (thin section) great lateral variation thin interbedded sandstone and mudstone

5. Development characteristics of low permeability reservoirs in China • No natural flow，Put into production after fracturing； • weak natural energy ，the production decline quickly，supply artifical energy • strong flow resistance ，high throat pressure gradient，difficult to keep the injection-production balance • Slow water flooding response,low sweep efficiency • Low oil recovery（<20%），low oil production rate（<1%） • High investment ,great economic risk

6. ν Displacement velocity Darcy flow e e Non-Darcy flow d d d 0 0 p/L △ △ a c b 12 10.81 Actual production 10 Computed production with ND method 8.30 Computed production with D method 8 6.34 Oil production(m3/d) 5.95 6 4.76 4.55 4.12 3.71 3.65 4 2.41 2.24 2.21 4.73 4.38 2 3.22 2.50 2.35 2.18 0 C45 C5 C64 C50 C55 C522 Non-linear flow with throat pressure gradient The relationship between TPG & Permeability Displacement pressure gradient The comparison of the oil production between the computed data and the actual data BLOCK

7. Well pattern influences the TPG Startup coefficient: at certain drawdown , the area displaced by injected water is divided by the total well pattern area • the critical well distance decreases with the TPG increases • At the certain drawdown and TPG, the SC increases with well distance decreases 压差25MPa，k=1mD, λ＝0.12MPa/m Well distance:180m SC: 0.23 Well distance:190m SC: 0 Well distance:160m SC: 0.58 Well distance:150m SC: 0.72

8. Part 2 Theadvantages of CO2 flooding in low permeability reservoir

9. Da an oil field, CO2 flooding pilot Daqingzijing oil field, CO2 flooding pilot Xinli oil field ,CO2 flooding pilot Natural gas field with CO2 over 20% In recent years, there is a major breakthrough in natural gas exploration in the deep reservoirs of Songliao Basin. A large amount of natural gas resources has been discovered and CO2 resources are amounted to more than 100 billion cubic meters. CNPC promise “0” CO2 emission in developing the Gas Field

10. Hall curves Cumulative injection (CO2 /water) 1. To be injected easily to supply formation energy，to be used to solve the problems for water injection in low-permeability reservoir • The slope of Hall curves indicates that the capability of gas injection is 6.2 times than water injection. • CO2 absorption index is 5 timesthan water absorption index. Water injection CO2 injection

11. 2. CO2 has strong dissolution and expansion in formation fluids, and has a key role in oil viscosity decrease

12. 3. To greatly improve the oil displacement efficiency in low-permeability reservoir K = 1.43mD Displacement efficiency of long core displacement test

13. Part 3 Analysis of the CO2 flooding effect

14. The CO2-EOR Projects in PetroChina

15. 1.Oil production increases and water cut decreases after CO2 flooding The water cut of producers obviously decreases before the CO2 arrives at the producers, which indicates that it is easier to drive oil with CO2 than water in the case of the co-existence of oil and water in the reservoir, and CO2 can drive oil bypass water to increase oil production and decrease water cut.

16. 1.Oil production increases and water cut decreases after CO2 flooding Silmtube test also confirmed this phenomenon, in the tubules saturated with oil, 0.3PV of water was injected to drive the oil, then CO2 was injected, in the outlet, CO2 was produced firstly with a little bit water, after CO2 broke through, water production increased greatly, which indicates that CO2 can pass through water plug to drive oil.

17. 2.Comparing with the water injection, the displacement pressure for CO2 flooding is low and CO2 flooding is an effective way for low permeability reservoir development From the gas and water injection profiles ,the interval with low water injection has higher gas injection, and all intervals have balanced gas injection. The interval heterogeneity is relatively weakened for gas injection, and the sweep area is expanded.

18. In the pilot area, the average permeability of the reservoir is about 3mD • the average pore throat radius is about 1um; and when the permeability is less than 1mD, the corresponding average pore throat radius is about 0.5um, and the displacement pressure will increase one time than that for permeability of 3mD • Under the condition of formation, interfacial tension for the CO2-oil or CO2-water system is lower than oil-water system • CO2 can get into small throats of low permeability reservoir to drive oil, so the gas injection profile is relatively more uniform than water injection

19. 一期受效井 自喷 二期受效井 抽油产量大于10吨 24.5MPa 24.4MPa 20.6MPa 22.5MPa 19.5MPa 3.Great difference of displacement efficiency among wells mainly caused by the strong lateral heterogeneity of reservoirs

20. Summary • A larger amount of hydrocarbon resources is preserved in the low- permeability reservoirs in China, due to the poor reservoir quality and strong heterogeneity, it is difficult for the conventional water injection to supply the formation energy, which results in low production, poor developing effect and low oil recovery. • 2. CO2 flooding has some advantages for the development of low permeability oilfield, which can effectively supplement the formation energy and improve displacement efficiency.

21. Summary 3. Recent years, many natural gas reservoirs were discovered in China, contained 10~20%mol CO2, the CO2 resources was estimated more than 100 billion cubic meters, which will be used to develop low-permeability oilfield. 4. The oil resources are very big which can be developed with CO2 flooding, only in the Songliao basin, such resources are estimated 2 billion tons, CO2 flooding is very prospective for low-permeability oilfields.

22. Summary 5. Based on the CO2 flooding projects in China, CO2 injection is better and more effective in low water-cut stage than high water-cut stage in the low-permeability reservoirs, CO2 EOR pilots are proven to be a technical success (due to the increase in oil production) , but exists an economic risk in some projects at the current oil price. 6. The sufficient CO2 sources and better understanding of reservoirs are the key important factors for CO2-EOR.

23. Summary 7. From the preliminary results in pilot areas, CO2 flooding as the primary recovery, the oil recovery in low-permeability reservoir with difficulty in development by water flooding, will reach 25% by CO2 miscible flooding, and as the method for enhance oil recovery after water flooding, the oil recovery can be improved 8~13% by CO2 flooding.

24. Thanks