مطالعات موردی موفق در استفاده از پلیمر به منظور ازدیاد برداشت نفت از میادین نفتی به‌روش سیلاب‌زنی پلیمری

نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشکده مهندسی نفت و پتروشیمی، دانشگاه حکیم سبزواری، سبزوار، ایران

2 دانشکده مهندسی نفت و پتروشیمی، دانشگاه حکیم سبزواری، سبزوار، ایران / دانشکده مهندسی نفت، دانشگاه صنعتی امیرکبیر، تهران، ایران

چکیده

در برداشت اولیه هیدروکربن‏ها که با استفاده از سازوکار‏های طبیعی مخزن صورت می‌گیرد، به دلایل مختلف از جمله کاهش فشار ناشی از تولید، در بهترین حالت کمتر از یک سوم نفت تولید می‏گردد. همچنین با اتخاذ روش‏های ثانویه ازدیاد برداشت مانند تزریق آب نیز با وجود کاربرد گسترده، به‌دلیل تحرک بالای آب نسبت به نفت، بخش عظیمی از منابع قابل تولید نخواهد بود. از این رو جهت تأمین تقاضای نفت خام که به‌صورت پیوسته در حال افزایش است، همواره نیاز به استفاده از روش‏های ثالثیه ازدیاد برداشت جهت افزایش ضریب بازیافت پس از شروع تولید وجود دارد. از جمله روش‏های کارآمد جهت افزایش میزان بازدهی در فرآیند جاروب‏زنی نفت خام، تزریق (سیلاب‌زنی) پلیمر است که به‌عنوان یکی از روش‏های شیمیایی مؤثر ازدیاد برداشت از سال‏ها پیش تا کنون مورد استفاده قرار گرفته است. از آنجا که مطالعه موردی به‌صورت گسترده، ژرف و جزئی‌نگر به مطالعه یک مورد خاص می‌پردازد و در نتیجه به محدوده وسیعی از دانش برای تحلیل سیستم‌های پیچیده دسترسی می‌یابد، این اطمینان در پژوهش مطالعه موردی حاصل می‌شود که تمام اجزا مورد بررسی قرار گرفته‌اند. از این رو در این مقاله پس از بررسی نظام‌مند بسیاری از مطالعات موردی موفق ازدیاد برداشت توسط عملیات سیلاب‌زنی پلیمر در میادین مختلف نفتی جهان، به بررسی عوامل و شرایط تأثیر‏گذار در هر یک از مراحل انتخاب پلیمر، عملیات تزریق و سپس تأثیر آن برروی میزان افزایش نرخ تولید و همچنین میزان ضریب بازیافت نفت مخزن پرداخته شده و چالش‌های موجود در این عملیات‌ها به دقت مورد بحث و بررسی قرار گرفته است. نتایج حاصل از تحقیق نشان می‌دهد افزودن پلیمر مناسب به آب در فرآیند سیلاب‌زنی و در شرایط مختلف، از پتانسیل بالقوه‏ای برای کاهش میزان تولید آب و همچنین کاهش میزان اشباع نفت باقی‌مانده مخازن و در نتیجه افزایش ضریب بازیافت نفت برخوردار است.
 

کلیدواژه‌ها


عنوان مقاله [English]

Successful Case Studies on the Use of Polymers to EOR by Polymer Flooding

نویسندگان [English]

  • Ashkan Kyani 1
  • Abbas Hashemizadeh 2
1 Petroleum Engineering Department, Petroleum and Petrochemical Engineering School, Hakim Sabzevari University, Sabzevar, Iran
2 Petroleum Engineering Department, Petroleum and Petrochemical Engineering School, Hakim Sabzevari University, Sabzevar Petroleum Engineering Department, Amirkabir University of Technology, Tehran, Iran
چکیده [English]

In the primary oil recovery, which is done using natural mechanisms in the reservoir, less than one third of the volume of petroleum is produced in the best cases for various reasons, including the process of reducing the pressure caused by production. In addition, by adopting secondary methods of oil recovery, such as water injection, despite the widespread use, due to the high mobility of water relative to oil, a large part of the resources will not be able to be produced. Therefore, in order to meet the demand for crude oil, which is constantly increasing, there is always a need to use EOR methods of overdraft to increase the recycling rate after the start of production. One of the efficient methods to increase the efficiency of the crude oil sweeping process is polymer injection (or flooding), which has been used as one of the chemical methods of oil recovery for many years. Since Case Study is a wide, in-depth, and detailed study of a particular case and thus accesses a wide range of knowledge for the analysis of complex systems, the assurance of case study research that all components have been examined is assured. In this study, after a comprehensive review of successful case studies, EOR by polymer flooding operations, the factors and conditions affecting each of the stages of polymer selection, injection operations and then its impact on the rate of increase in production rate and also the rate of reservoir oil recovery has been studied. The results of this operation in different oil fields show the addition of suitable polymer to water in the flooding process and in different conditions, the potential to reduce water production and also reduce the amount of oil saturation remaining in the reservoirs and thus increases oil recovery factor.
 

کلیدواژه‌ها [English]

  • Polymer Flooding
  • EOR
  • Recovery Factor
  • Mobility Ratio
  • Case study
[1]. Reynolds D J O, Gas E L (2004) Scarcity and growth considering oil and energy, An Alternative Neo-Classical View, 2nd edition, Edwin Mellen Pr Publisher, 1-232. ##
[2]. Hirsch R, Bezdek R, Wendling R (2005) Peaking of world oil production: impacts, mitigation and risk management, National Energy Technology Laboratory, Department of Energy, US. ##
[3].Hashemizadeh A, Gholizadeh M, Tabatabaeinejad A, Hoopanah M (2014) The possibility of enhanced oil recovery by using magnetic water flooding, Petroleum Science and Technology, 32, 9: 1038-1042. ##
[4]. Puskas S, Vago A, Toro M, Ordog T, Kalman G, Hanzelik P, Lakatos I (2018) Surfactant-polymer EOR from laboratory to the pilot, in SPE EOR Conference at Oil and Gas West Asia, Society of Petroleum Engineers: Muscat, Oman, 15. ##
[5]. Juri J, Ruiz A M, Pedersen G, Pagliero P, Blanco H, Eguia V, Serrano V (2017) Grimbeek2: first successful application polymer flooding in multilayer reservoir at YPF, Interpretation of Polymer Flooding Response, in SPE Latin America and Caribbean Petroleum Engineering Conference, Society of Petroleum Engineers. ##
[6]. Gao S, He Y, Zhu Y, Han P, Peng S, Liu X (2020) Associated polymer ASP flooding scheme optimization design and field test after polymer flooding in daqing oilfield, in SPE Improved Oil Recovery Conference, OnePetro. ##
[7]. Austad T, Matre B, Milter J, Saevareid A, Øyno L (1998) Chemical flooding of oil reservoirs 8. Spontaneous oil expulsion from oil-and water-wet low permeable chalk material by imbibition of aqueous surfactant solutions, olloids and Surfaces A: Physicochemical and Engineering Aspects, 137, 1-3: 117-129. ##
[8]. Wang D, Wang G, Wu W, Xia H, Yin H (2007) The influence of viscoelasticity on displacement efficiency--from micro to macro scale, In SPE Annual Technical Conference and Exhibition, OnePetro. ##
[9]. Daneshmand H, Wang G, Wu W, Xia H, Yin H (2020) Evaluation of polymer wettability alteration and adsorption of modified silica nanoparticles for enhanced oil recovery, Iranian Journal of Polymer Science and Technology,33, 3: 272-285. ##
[10]. Shaban M, Ramazani SaadatAbadi A, Ahadian M M J B (2017) Water soluble polymers as mobility control agents in enhanced oil recovery (EOR): a literature review, 7, 3: 38-49. ##
[11]. Abidin A Z, Puspasari T, W, Nugroho A (2012) Polymers for enhanced oil recovery technology, Procedia Chemistry, 4: 11-16. ##
[12]. Rezk M Y (2018) Nanocomposite formulations for enhanced oil recovery (EOR). ##
[13]. Sheng J J (2010) Modern chemical enhanced oil recovery: theory and practice, Gulf Professional Publishing. ##
[14]. Rellegadla S, Prajapat G, Agrawal A (2017) Polymers for enhanced oil recovery: fundamental and selection criteria, Applied Microbiology and Biotechnology, 101. ##
[15]. Sheng J J, Leonhardt B, Azri N J J o C p t (2015) Status of polymer-flooding technology, 54, 2: 116-126. ##
[16]. باشتی ن، گندم‌کار ا، شریف م (2020) تأثیر قوام‌دهنده پلیمری P-1-D در کنترل تحرک‌پذیری گاز دی‌اکسیدکربن در فرآیند ازدیاد برداشت نفت، پژوهش نفت، 30، 99-2: 103-112. ##
[17]. Pope G A (2007) Overview of chemical EOR, in Casper EOR workshop, The University of Texas at Austin Austin, TX. ##
[18]. Huang W, Dong Y, Zhao J, Liu X, Fan Y, Bai H, Hao (2019) JA/SP alternating flooding, a modified ASP flooding technology, in SPE Russian Petroleum Technology Conference, Society of Petroleum Engineers. ##
[19]. Aitkulov A, Dao E, Mohanty K K (2018) ASP flood after a polymer flood vs. ASP flood after a water flood, in SPE Improved Oil Recovery Conference, Society of Petroleum Engineers. ##
[20]. Li X, Zhang F, Liu G (2021) Review on polymer flooding technology, in IOP Conference Series: Earth and Environmental Science, IOP Publishing. ##
[21]. AlSofi A M, Blunt M J (2014) Polymer flooding design and optimization under economic uncertainty, Journal of Petroleum Science and Engineering, 124: 46-59. ##
[22]. Vermolen E, Pingo Almada M, Wassing B M, Ligthelm D J, Masalmeh S K (2014) Low-salinity polymer flooding: improving polymer flooding technical feasibility and economics by using low-salinity make-up brine, In International petroleum technology conference, OnePetro. ##
[23]. Sieberer M, Jamek K, Clemens T (2016) Polymer flooding economics, from pilot to field implementation at the example of the 8 TH reservoir, Austria. in SPE Improved Oil Recovery Conference, Society of Petroleum Engineers. ##
[24]. Mohsenatabar Firozjaii A, Saghafi H R (2020) Review on chemical enhanced oil recovery using polymer flooding: Fundamentals, experimental and numerical simulation, Petroleum, 6, 2: 115-122. ##
[25]. Shehata A M, Ghatas A, Kamel M, Aly A, Hassan A (2012) Overview of polymer flooding (EOR) in North Africa fields - elements of designing a new polymer/surfactant flood offshore (Case Study), in North Africa Technical Conference and Exhibition.Society of Petroleum Engineers: Cairo, Egypt, 11. ##
[26]. Huifen X, Ye J, Kong F, Wu J (2004) Effect of elastic behavior of HPAM solutions on displacement efficiency under mixed wettability conditions, In SPE Annual Technical Conference and Exhibition, OnePetro. ##
[27]. Clarke A, Howe A M, Mitchell J, Staniland J, Hawkes L (2016) A How viscoelastic-polymer flooding enhances displacement efficiency, SPE Journal, 21, 03: 0675-0687. ##
[28]. مغانی رحیمی ع، صادقی ا، کعبی ع، فلاحیه اصل (2019) بررسی اثر دما و پلیمر پلی اکریل آمید هیدرولیز شده برروی چگالی جذب سورفکتانت تریتون ایکس- 100 در سنگ‌های کربناته. پژوهش نفت، 29 98-5: 107-118. ##
[29]. مالمیر پ، هاشمی ع، سلطانی سولگانی ب (2019) بررسی آزمایشگاهی تزریق پلیمر بر ازدیاد برداشت از مخازن نفت سنگین و تعیین غلظت بهینه تزریق. پژوهش نفت، 29، 98-3: 120-130. ##
[30]. چهاردولی م، سیم‌جو م، صیادی ص (1400) مطالعه‌ مکانیسم‌های موثر بر بازیافت نفت سنگین در فرآیند تزریق متناوب پلیمر و گاز کربن‌دی‌اکسید با استفاده از میکرومدل، پژوهش نفت، 31، 116: 19-3. ##
[31]. گلشکوه س، حکمت‌زاده م، خرمیان ر (2017) طراحی میکرومدل‌های جدید سه‌بعدی برای بررسی عملکرد نانوذرات سیلیکا در افزایش اثربخشی تزریق آب. پژوهش نفت، 27، 96-5: 55-46. ##
[32]. کریمان مقدم ع، صادق‌‌نژاد س، جعفری آ (2015) شبیه‌سازی فرآیند تزریق پلیمر در میکرو مدل شکاف‌دار حاوی نفت سنگین. پژوهش نفت، 25، 83: 15-4. ##
[33]. کرم‌بیگی م (2018) بهینه‌سازی چندهدفه با تلفیق روش‌های طراحی آزمایش، الگوریتم ازدحام ذرات و منطق فازی: مطالعه موردی برای ازدیاد برداشت به کمک تزریق پلیمر. پژوهش نفت، 28، 97-4: 138-126. ##
[34]. Crowe S, Cresswell K, Robertson A, Huby G, Avery A, Sheikh A (2011) The case study approach, BMC medical research methodology, 11, 1: 1-9. ##
[35]. Olajire A A (2014) Review of ASP EOR (alkaline surfactant polymer enhanced oil recovery) technology in the petroleum industry: Prospects and challenges, Energy, 77: 963-982. ##
[36]. Pu W, Shen C, Wei B, Yang Y, Li Y (2018) A comprehensive review of polysaccharide biopolymers for enhanced oil recovery (EOR) from flask to field, Journal of Industrial and Engineering Chemistry, 61: 1-11. ##
[37]. Choi B, Jeong M S, Lee K S (2014) Temperature-dependent viscosity model of HPAM polymer through high-temperature reservoirs, Polymer Degradation and Stability, 110: 225-231. ##
[38]. Ghoumrassi- Barr S, Aliouche D (2013) Characterisation and rheological study of xanthan polymer for enhanced oil recovery (EOR) application, in Offshore Mediterranean Conference and Exhibition. ##
[39]. Abbas S, Sanders A W, Donovan J C (2013) Applicability of hydroxyethylcellulose polymers for chemical EOR, in SPE Enhanced Oil Recovery Conference. ##
[40]. Han M, Xiang W, Zhang J, Jiang W, Sun F (2006) Application of EOR technology by means of polymer flooding in bohai oilfields, in International Oil and Gas Conference and Exhibition in China, Society of Petroleum Engineers: Beijing, China. 6. ##
[41]. Dong H, Fang S, Wang D, Wang J, Liu Z L, Hou W (2008) Review of practical experience and amp; management by polymer flooding at daqing, in SPE Symposium on Improved Oil Recovery. ##
[42].Tamsilian Y, Agirre A, Fernandez M, Sheng J J, Tomovska R (2020) High-molar mass acrylamide-co-diacetoneacrylamide graft copolymers as viscosity enhancer for polymer flooding oil recovery, Polymer Testing,  82: 106332. ##
[43]. Su X, Feng Y (2018) Thermoviscosifying smart polymers for oil and gas production: state of the art, ChemPhysChem, 19, 16: 1941-1955. ##
[44]. Abdel-Azeim S, Kanj M Y (2018) Dynamics, aggregation, and interfacial properties of the partially hydrolyzed polyacrylamide polymer for enhanced oil recovery applications: insights from molecular dynamics simulations, Energy and Fuels, 32, 3: 3335-3343. ##
[45]. Rodriguez L, Antignard S, Giovannetti B, Gaillard N, Jouenne S, Bourdarot G, Zaitoun A (2018) A new thermally stable synthetic polymer for harsh conditions of middle east reservoirs: Part ii. Nmr and size exclusion chromatography to assess chemical and structural changes during thermal stability tests, in SPE Improved Oil Recovery Conference, OnePetro. ##
[46]. Ji Y, Wang D, Cao X, Guo L, Zhu,Y (2016) Both-branch amphiphilic polymer oil displacing system: Molecular weight, surfactant interactions and enhanced oil recovery performance. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 509: 440-448. ##
[47]. Sheng J J, Leonhardt B, Azri N (2015) Status of polymer-flooding technology, Journal of Canadian Petroleum Technology, 54, 02: 116-126. ##
[48]. Li X e, Xu Z, Yin H., Feng Y, Quan H (2017) Comparative studies on enhanced oil recovery: thermoviscosifying polymer versus polyacrylamide, Energy and Fuels, 31, 3: 2479-2487. ##
[49]. khalili y, Hashemizadeh A, yasemi S (2021) Study on the efficiency of different polymers used in polymer injection (flooding) operations in EOR of heavy oil reservoirs, Basparesh. ##
[50]. Al-Shakry B, Shiran B S, Skauge T, Skauge (2018) A Enhanced oil recovery by polymer flooding: optimizing polymer injectivity, in Spe Kingdom of Saudi Arabia Annual Technical Symposium and Exhibition, OnePetro. ##
[51]. AlSofi A M, Wang J, Kaidar Z F (2018) Smart water synergy with chemical EOR: effects on polymer injectivity, retention and acceleration, Journal of Petroleum Science and Engineering, 166: 274-282. ##
[52]. Du Y, Guan L (2004) Field-scale polymer flooding: lessons learnt and experiences gained during past 40 years, in SPE International Petroleum Conference in Mexico. ##
[53]. Speight J G (2016) Chapter 8 - nonthermal methods of recovery, in introduction to enhanced recovery methods for heavy oil and tar sands (Second Edition), J.G. Speight, Editor, Gulf Professional Publishing: Boston, 353-403. ##
[54]. Saboorian-Jooybari H, Dejam M, Chen Z (2016) Heavy oil polymer flooding from laboratory core floods to pilot tests and field applications: Half-century studies, Journal of Petroleum Science and Engineering, 142: 85-100. ##
[55]. طهماسبی ع، ریاضی م (1393) مقایسه تاثیر پدیده جذب پلیمر تزریقی برروی سطح سنگ بر میزان تولید نفت از مخازن همگن و ناهمگن، دومین همایش ملی نفت و گاز ایران. ##
[56]. Shoaib M, Quadri S M R, Wani O B, Bobicki E, Garrido G I, Elkamel A, Abdala A (2020) Adsorption of enhanced oil recovery polymer, schizophyllan, over carbonate minerals, Carbohydrate Polymers, 240: 116263. ##
[57].Gaol C L, Wegner J, Ganzer L (2020) Real structure micromodels based on reservoir rocks for enhanced oil recovery (EOR) applications, Lab on a Chip, 20, 12: 2197-2208. ##
[58]. Aladasani A, Bai B (2010) Recent developments and updated screening criteria of enhanced oil recovery techniques, in International Oil and Gas Conference and Exhibition in China. ##
[59]. Liang Y, Wang Z L, Jin Y X, Tian Y Q, Liu X M, Guo Y J, Zhou M Y (2019) Heterogeneity control ability in porous media: Associative polymer versus HPAM, Journal of Petroleum Science and Engineering, 183: 106425. ##
[60]. Torrealba V A, Hoteit H (2019) Improved polymer flooding injectivity and displacement by considering compositionally-tuned slugs, Journal of Petroleum Science and Engineering, 178: 14-26. ##
[61]. Saleh L D, Wei M, Bai B (2014) Data analysis and updated screening criteria for polymer flooding based on oilfield data, SPE Reservoir Evaluation and Engineering, 17, 01: 15-25. ##
[62]. Johansson R (2007) On case study methodology. Open house international. ##
[63]. Noor K B M (2008) Case study: A strategic research methodology, American journal of applied sciences, 5, 11: 1602-1604. ##
[64]. Yang D h, Wang J Q, Jing L X, Feng Q X, Ma X P (2010) Case study of alkali - polymer flooding with treated produced water, in SPE EOR Conference at Oil and Gas West Asia, Society of Petroleum Engineers: Muscat, Oman, 6. ##
[65]. Thakuria C, Al-Amri M S, Al-Saqri K A, Jaspers H F, Al-Hashmi K H, Zuhaimi K (2013) Performance review of polymer flooding in a major brown oil field of sultanate of Oman, in SPE Enhanced Oil Recovery Conference, Society of Petroleum Engineers: Kuala Lumpur, Malaysia, 13. ##
[66]. Gao C, Shi J, Zhao F (2014) Successful polymer flooding and surfactant-polymer flooding projects at shengli oilfield from 1992 to 2012, Journal of Petroleum Exploration and Production Technology, 4, 1: 1-8. ##
[67]. Morel D, Jouenne S, Zaugg E, Bouger Y (2011) EOR polymer in deep offshore field angola: development strategy and polymer performance surveillance, in 20th World Petroleum Congress, World Petroleum Congress: Doha, Qatar, 12. ##
[68]. Pandey A, Kumar M S, Jha M K, Tandon R, Punnapully B S, Kalugin M, Beliveau D (2012) Chemical EOR pilot in mangala field: results of initial polymer flood phase, in SPE Improved Oil Recovery Symposium, Society of Petroleum Engineers: Tulsa, Oklahoma, USA, 8. ##
[69]. Wang D, Hao Y, Delamaide E, Ye Z, Ha S, Jiang X (1993) Results of two polymer flooding pilots in the central area of daqing oil field, in SPE Annual Technical Conference and Exhibition. OnePetro. ##
[70]. Putz A, Pedron B, Bazin B J G S (1995) London, special publications, commercial polymer injection in the courtenay field, 1993 update, Geological Society, London, Special Publications, 84, 1: 239-249. ##
[71]. Ilyasov I, Gudz A, Podkorytov A, Komarov V, Glushchenko N (2020) Results of the first polymer flooding Pilot at East-Messoyakhskoe Oil Field, in SPE Russian Petroleum Technology Conference. ##
[72]. Zagrebelnyy E, Glushenko N, Komarov V, Nemirovich G, Ilyasov I, Sugaipov D, Iskhakov R (2018) Experience in implementation of polymer flooding on East Messoyakhskoye oil field, in SPE Russian Petroleum Technology Conference. ##
[73]. Li M, Guo J, Peng B, Lin M, Dong Z, Wu Z (2005) Formation of crude oil emulsions in chemical flooding, in Emulsions and emulsion stability, CRC Press, 537-568. ##
[74]. Stoll W M, al Shureqi H, Finol J, Al-Harthy S A, Oyemade S, de Kruijf A, Faber M J (2011) Alkaline/surfactant/polymer flood: From the laboratory to the field, SPE Reservoir Evaluation and Engineering, 14, 06: 702-712. ##
[75]. Zhou W, Zhang J, Feng G, Jiang W, Sun F, Zhou S, Liu Y (2008) Key technologies of polymer flooding in offshore oilfield of Bohai Bay, in SPE Asia Pacific Oil and Gas Conference and Exhibition, OnePetro. ##
[76]. Morel D C, Vert M, Jouenne S, Gauchet R, Bouger Y (2012) First polymer injection in deep offshore field Angola: recent advances in the Dalia/Camelia field case, Oil and Gas Facilities, 1, 02: 43-52. ##
[77]. Needham R B, Doe P H (1987) Polymer flooding review, Journal of Petroleum Technology, 39, 12: 1503-1507. ##
[78]. Seright R S, Wang D, Lerner N, Nguyen A, Sabid J, Tochor R (2018) Can 25-cp polymer solution efficiently displace 1,600-cp oil during polymer flooding? SPE Journal, 23, 06: 2260-2278. ##
[79]. Geremia G, Bennetzen M V (2016) An operational workflow for polymer EOR field trials, in SPE EOR Conference at Oil and Gas West Asia, Society of Petroleum Engineers: Muscat, Oman, p, 10. ##
[80]. Akstinat M H (1980) Polymers for enhanced oil recovery in reservoirs of extremely high salinities and high temperatures, In SPE Oilfield and Geothermal Chemistry Symposium. OnePetro. ##
[81]. Du Y, Guan L (2004) Field-scale polymer flooding: lessons learnt and experiences gained during past 40 years, in SPE International Petroleum Conference in Mexico, OnePetro. ##
[82]. Taber J J, Martin F, Seright R J S r e (1997) EOR screening criteria revisited-Part 1: Introduction to screening criteria and enhanced recovery field projects, 12, 03: 189-198. ##
[83]. Al-Bahar M A, Alajmi A, Gharbi R (2004) Evaluation of IOR potential within Kuwait. in Abu Dhabi International Conference and Exhibition, Society of Petroleum Engineers. ##
[84]. Akstinat MH (1980) Polymers for enhanced oil recovery in reservoirs of extremely high salinities and high temperatures, in SPE Oilfield and Geothermal Chemistry Symposium, Society of Petroleum Engineers. ##
[85]. Mohammad Salehi M, Hekmatzadeh A, Sajjadian V A, Masoumi M (2017) Simulation of polymer flooding in one of the Iranian oil fields, Egyptian Journal of Petroleum, 26, 2: 325-330. ##