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Petroleum Treatment Equipment Books

a collection of free books about the equipment used in oil and natural gas industry

Introduction to Pressure Vessels
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Gravity Separation
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Pressure Vessels
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Packed Towers
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Air Compressor System
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Strainers
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Separators
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  Flare Stacks power point
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  Flare System
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Flares
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Adsorption & Stripping
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Wet Crude Treatment Equipment

Crude Oil Desalter
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 Crude Oil Dehydration
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Crude Oil Treatment
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 Desalting Crude Oil
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Desalter Effluent Treatment
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Optimum Temperature in the Electrostatic Desalting of Crude Oil
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Engineering Design Guidelines of Crude Unit Desalter System
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Mechanisms of Crude Oil Demulsification
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Other Treatment Equipment

Pressure Relief, Flares, Flame Arrestors
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Protect your system with Flame Arrestor
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Flame Arrestor Maintenance
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Flame Arrestor
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the Selection and Use of Gas Detector
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Heat Transfer Equipment
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 Air Compressor
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Separator Design
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Separator Vessel Sizing & Selection
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Coalescer Engineering Design Guidelines
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Gas -Liquid & Liquid-Liquid Separators
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Separation, Treatment & Storage
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 3-phase separator
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Three-Phase Separator – Gas Internals
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Oil & Gas Facilities PowerPoint
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2 Phase Oil – Gas Separator PowerPoint
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 Oil and Gas Separator
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Oilfield Equipment & Services
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Oil and Gas Surface Facilities
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installing mist eliminator in oil seprator
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Mist Elimination
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a Working Guide to Process Equipment
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Phase Separation Internals 
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Surface Facilities in Oil & Natural Gas Production Part.1

المنشآت السطحية لأنتاج النفط والغاز الطبيعي – الجزء الأول

Surface Facilities in Oil & Natural Gas Production Part.1

أن آبار النفط أو الغاز تنتج مزيج من الغازات الهيدروكاربونية والمكثفات، أو النفط ، والماء مع بعض المعادن الذائبة و كمية من الأملاح وبعض الغازات الأخرى، مثل النتروجين، ثاني أوكسيد الكربون (CO2)، وربما كبريتيد الهيدروجين (H2S)، والمواد الصلبة، بما في ذلك الرمل والشوائب من تآكل الأنابيب.

من أجل الحصول على النفط والغاز بكميات تجارية ليتم بيعها، يجب فصلها عن الماء والمواد الصلبة وبيعها ونقلها عبر خط أنابيب أو الشاحنات أو السكك الحديدية، أو ناقلات النفط الى المستخدم النهائي أما تصدير الغاز فيقتصر عادة على خطوط الأنابيب ولكن يمكن أيضا أن يشحن في الشاحنات او الناقلات وعربات السكك الحديدية بعد ضغطه وتحويله الى الغاز الطبيعي المسال (LNG). 

أن الهدف من هذه المنشآت هو انتاج النفط الذي يلبي مواصفات المستهلك والتي تحدد الحد الأقصى المسموح به من الماء والأملاح، أو غيرها من الشوائب. وبالمثل، لا بد من معالجة الغاز لتلبية مواصفات المستهلك من نسبة بخار الماء المسموحة ونقطة الندى Dew Point  للحد من التكثيف أثناء النقل.  أما الماء المنتج Produced Water فيجب أن يكون مطابقاً للمواصفات البيئية للتخلص منها في المحيط في حالة الآبار البحرية Off-Shore wells أو الحقن الى المكمن بالشكل الذي يضمن عدم حصول أنسداد في مسامات المكمن. كما يمكن أستخدامه لاستخدامات أخرى، مثل تجهيزه الى المراجل الحرارية Boilers  أو أستخدامه لأغراض الري أو ماء للشرب في بعض الأحيان .

وتسمى هذه المعدات بين الآبار وخطوط الأنابيب، أو غيرها من نظام النقل Transportation System تسمى منشأت سطحية لحقول النفط. 

أن المنشأت السطحية لحقول النفط تختلف عن المصافي أو مصانع المواد الكيميائية في أمور كثيرة. حيث تكون أبسط منها، وتتألف من وحدات الفصل Phase Separation وتغير درجة الحرارة وتغييرات الضغط، ولكن تختلف معها في التفاعلات الكيميائية لصنع جزيئات جديدة. 

وفي المصافي يجب معرفة معدل التدفق المغذي للمصفى ومكوناته قبل التصميم. أما في الحقول النفطية فأن المكونات يتم تقديرها إستناداً الى  اختبارات الأنتاج من الآبار الاستكشافية Exploration Wells أو من آبار موجودة في حقول مشابهة.

وتقدر معدلات التدفق التصميمية من سجلات الآبار ومن خلال محاكاة المكامن Reservoir Simulation . وحتى لو كانت التقديرات جيدة، فأن معدلات التدفق (الغاز والنفط، والماء) والضغوط ودرجات الحرارة تتغير طوال عمر المكمن مع تقادم الآبار وحفر آبار جديدة.

ويتم مراعاة أقصى معدل تخميني للتدفق عند التصميم على أساس عدد الآبار، ومخططات الإنتاج Production Profile ومجموع النفط أو الغاز التي يمكن أن تنتج من المكمن.

أن معدلات الإنتاج الفعلي للمنشآة النفطية ستزيد مع إكمال الآبار وصولاً الى المعدلات التصميمية. ويتم الحفاظ على هذا المعدل لأطول فترة ممكنة من خلال حفر آبار إضافية  وبعد ذلك تبدأ معدلات النفط والغاز بالتراجع، وأنتاج الماء في الزيادة، بالأضافة الى أنخفاض ضغوط الجريان مع استنزاف عمر المكمن. لذا يجب تصميم المعدات في المنشآة النفطية للتعامل مع مدى واسع من معدلات التدفق ، ودرجات الحرارة والمكونات المتغيرة.

تعريف لبعض المصطلحات :

النفط الخام Crude Oil هو مجموعة الهيدروكربونات السائلة المنتجة من المكمن.

المكثفات Condensate هي السوائل الهيدروكربونية المتكثفة من الغاز بأنخفاض الضغط ودرجات الحرارة عند أنتاج الغاز من المكمن خلال الأنبوب والصمام الخانق في الآبار. وتكون المكثفات عادة أفتح لونا وأقل في الوزن الجزيئي واللزوجة من النفط الخام. ويمكن أن يكون للنفط الخام الخفيف خصائص مشابهة للمكثفات.

وتتكون الهيدروكربونات من العديد من العناصر المختلفة أو جزيئات من الكربون وذرات الهيدروجين. بدءا من ذوات الوزن الجزيئي الأقل ، وهي الميثان CH4 الإيثان(C2H6) والبروبان (C3H8) والبيوتان (C4H10)، البنتان (C5H12)، الهكسان (C6H14) صعوداً. وبصعود نسبة ذرات الكربون إلى ذرات الهيدروجين فأن الجزيئات تصبح “أثقل” ويكون لها ميل أكبر في الوجود كسائل بدلا من الغاز.

المنشأت السطحية في حقول النفط Oil Field Facility هي عبارة عن مجموعة من المعدات التي تستخدم لفصل السوائل المنتجة من البئر النفطي أو الغازي الى مكونات مختلفة يمكن بعد ذلك بيعها وإرسالها إلى معمل معالجة الغاز Gas Plant أو المصفى Refinery للمزيد من المعالجة. 

محاكاة العملية Process Simulation وهي حسابات تتم عادة بواسطة برنامج كمبيوتر يتنبأ بالمكونات المنتجة من البئر ويتفاعل هذا البرنامج مع التغيرات في الضغط ودرجة الحرارة التي يتم معالجتها في هذه المنشآة. 

وهذا ليس تفاعل كيميائي، بل تغير بسيط في الأطوار من خلال تحول السوائل إلى بخار قد تتكثف الى سائل. ومع تقليل الضغط وزيادة درجة الحرارة فأن الجزيئات الأخف وزنا مثل الميثان والإيثان تميل إلى التحول الى الطور البخاري أما معظم الجزيئات الأثقل فتستقر على شكل سوائل.

 

أن الرواسب والماء (BS & W) هو النسبة المئوية للماء والشوائب الموجودة في النفط. ويجب أن تتراوح بين 0.1 الى 3% ، وتصل النسبة الى 1% حجماً في خليج المكسيك Gulf of Mexico .

نقطة الفقاعة bubblepoint هو الضغط التي تبدأ فيه أول قطرة من السائل الهايدروكاربوني بالظهور في الطور السائل بصعود درجة الحرارة أو خفض الضغط. أن نقطة الفقاعة للسوائل الهيدروكاربونية دالة لدرجة الحرارة والضغط، ومكونات السائل.

 ضغط ريد البخاري Reid Vapor Pressure هو الضغط الذي يبدأ فيه الهيدروكربون السائل بالتحول الى بخار في ظل ظروف محددة. ويمكن قياس ذلك في الحقل وفقا لقاياسات الجمعية الأمريكية للأختبارات والمواد القياسية ASTM وأحتساب النتائج في ضغط أقل من الضغط البخاري الحقيقي.

نقطة الندى للهيدروكاربونات Hydrocarbon Dewpoint : هي النقطة التي يبدأ فيها تكثف الهيدروكاربونات السائلة من عينة الغاز عندما يتم خفض درجة الحرارة أو يتم زيادة الضغط، ويعتمد ذلك على تركيب الغاز. ويتم تعيين نقطة ندى الماء لخطوط أنابيب الغاز  للسيطرة على تكون الهيدرات لهيدرات والحيلولة دون التآكل .

الهيدرات Hydrates : هي بلورات شبيهة بالثلج تتكون في وجود الغازات الهيدروكربونية والماء . ويمكن أن تتشكل الهيدرات في درجات حرارة أعلى من درجة الأنجماد للماء ويمكن أن تتسبب في حصول أنسدادات في المعدات وخطوط الأنابيب.

وظيفة المنشآت النفطية :

العمليات الرئيسية Main Process

وهي عبارة عن عمليات فصل النفط والغاز والماء والمواد الصلبة ، ومعالجة النفط للوصول الى المواصفات التسويقية (على سبيل المثال، BS & W، محتوى الماء والأملاح، وضغط البخار ..ألخ) وقياس ونمذجة النفط لتحديد نوعه ومن ثم نقله الى واسطة النقل (أنبوب التصدير – الناقلة البحرية – الشاحنات – أو سكك الحديد).

كما يجب أن تتم معالجة الغاز لأغراض البيع أو التخلص منه (التي كانت تتم في الماضي عن طريق حرقه ) أما الآن فالغاز الذي لا يمكن نقله فيتم كبسه لغرض إعادة الحقن في المكمن. قد تتضمن معالجة الغاز فصله من السوائل فقط ، أو قد تشمل عمليات الكبس والتجفيف Dehydration  وإزالة غازات H2S و CO2، أو معالجة الغاز لتحويله الى غاز سائل ليسهل نقله.

 العمليات الثانوية Secondary Process :

 بالإضافة إلى معالجة النفط والغاز الطبيعي لأغراض البيع، فهناك معالجة الماء المنتج والذي يجب معالجته للتخلص من المواد الصلبة للتمكن من تصريف هذا الماء وعادة ما تتضمن المعالجة إزالة المواد الهيدروكربونية الذائبة وبالإضافة إلى فصل النفط عنه باتخدام قاشطات Skimmers أو مرشحات filters أو وحدات إزالة الايونات deionization  ووحدات الضخ Pumping Stations.

 

وإذا كان المطلوب معالجة المواد الصلبة فقد تشمل ماء الغسيل Wash Water وتحريك المواد الصلبة لإزالة النفط ومن ثم فصل الماء منها.

المنظومات المساعدة Auxiliary Systems :

 بالإضافة إلى العمليات الرئيسية والثانوية يجب توافر المنظومات المساعدة مثل عمليات التسخين والتبريد التي قد تحتاجها المنشآة النفطية . وعادة ً تكون هناك حاجة لعمليات التسخين لعملية لمعالجة النفط وغيرها ، اما عملية التبريد فتكون مطلوبة في محطات كبس الغاز مثلاً.

وإذا لزم الأمر، يمكن تشغيل المنشآت النفطية بدون الطاقة الكهربائية حيث أن توليد الطاقة والكهرباء يكون ضمن نفس المنشآة بالأضافة الى توفير مستلزمات السكن للعاملين في هذه المنشآة النفطية.

أن جميع المنشآت النفطية تتطلب نظم السلامة، بما في ذلك أجهزة ومعدات السلامة ومنظومات التوقف الأضطراري Shut Down systems ومتحسسات الكشف عن الحريق والغاز Fire & gas detectors ومعدات مكافحة الحريق، وسائل الإخلاء، مثل قوارب النجاة في المنصات البحرية ومعدات أخرى اعتمادا على الموقع ومدى تعقيد المنشأة .

المصادر :

Petroleum Engineering Handbook – Part 3 – Kenneth E. Arnold

Surface Production Operations – Ken Arnold/ Maurice Stewart

Gas Treating

Introduction to Gas Treating

When talking of gas treating, it is most often implied that natural gas is the focus. The natural gas industry is one of the World’s largest. However, there is also treatment of gas in the synthesis gas industry, and there are a number of processes used for this that are similar to those used for natural gas treatment.

Finally, there is a large industry devoted to separate air to make nitrogen, oxygen and argon, and to an extent, krypton and xenon. Such plants would be cryogenic distillation outfits for large capacities while adsorption and membranes are also in use for smaller units. There is also an emerging interest in CO2 removal from flue gases caused by the focus on CO2’s role in global warming.

The term gas treating is normally used to cover CO2 removal, H2S removal, water removal, hydrocarbon dew pointing and gas sweetening. Gas sweetening is a generic term for sulfur removal. Sometimes the term ‘gas conditioning’ is used instead of gas treating.

The key question is ‘why treat gas?’

This is a multi-faceted issue. The gas is produced from a well at a location where there is usually only a negligible market for it. Transport of the gas to the market is the first challenge. Traditionally this has been achieved by pipelines.

A lot could be said about pipelines, but here it will suffice to say that these are constructed in some steel material, and the properties of these materials are such that the presence of certain gases must be kept low to ensure the integrity of the pipeline. Hydrogen sulfide is a key component as it may cause stress corrosion cracking.

Pipeline specifications may vary, but its content is commonly kept below 3–5 ppm. In the US the number 0.25 grain per 100 SCF is often used, but there is no standard in this matter. It must be remembered that the flow velocity in gas pipelines will be too high to allow integrity for a protective sulfide film on

the inner pipe wall. For flow assurance reasons the dew point of the gas must be engineered before entering the pipeline. If the temperature is reduced, both water and hydrocarbons may condense. Water could form hydrate crystals with methane and these could block the flow of gas. Such hydrates are hard and time consuming to get rid of.

Clearly no pipeline operator would want this to happen. Liquid water could also cause corrosion when acidified by CO2 that is likely to be present in the gas. This is also undesirable. Finally hydrocarbon condensate could amass to quantities that would cause flow problems if left unchecked.

It must be remembered that pipelines follow landscapes where its elevation goes up and down repeatedly. CO2 is usually also kept below a certain limit, say 1–4%, depending on the local situation.

There is also another reason other than the pipeline considerations to treat gas. Downstream of the transport system, that could be complex, there is a multitude of customers that will use the gas. Their equipment will have been made with certain gas specifications in mind. Here, the gas heating value will an issue, theWobbe number is often specified and there will be limits on H2S and CO2. Corrosion issues apart, H2S would end up as SO2 in the flue gas and this would be an environmental problem.

Process Categories

There is limited attention paid to processing gases in a typical chemical engineering curriculum. This is, however, a huge field where many chemical engineers find employment. In general terms, there are four principally different main methods that may be used to separate gases. They include (in alphabetical order):

  • Absorption
    • Adsorption
    • Cryogenics: liquefaction and distillation
    • Membrane permeation.

Ab- and adsorption are often mixed up in write-ups, probably because their spelling is so similar. Process-wise there is a huge difference though. Adsorption is essentially a surface phenomenon while absorption involves something being dissolved.

Cryogenics involves gases being cooled until they condense after which they may be separated by distillation. Some such processes could also be argued to lean towards absorption and/or desorption. A nitrogen wash unit, sometimes used for synthesis gas treatment, is an interesting case with respect to that kind of discussion.

Membrane technology used for gas separation is in general based on so-called dense membranes that separate gases based on different permeation rates. Small volume niche

products within inorganic membranes may be different, but a discussion of this is beyond

the present scope.

 Absorption

Absorption is a much used process for separating gases, removing undesired gas components or to prevent pollution from stacks. The process is, by its nature, run at supercritical temperature with respect to the main gas component(s). There is no boiling like that seen in distillation columns. The mass transfer process is generally rate controlled. All components

are in principle undergoing mass transfer between gas and liquid, but all does not need to be accounted for and/or may be neglected. Mass transfer rates and mass transfer coefficients may differ in different directions for different components.

If a lot of gas needs to be absorbed, large absorbent flows will be needed. This represents an operational cost that, in the end, may be a show stopper for using absorption.

It is a very interesting process, and is in many ways the main focus for this treatise. A separate sub-chapter is dedicated to a preliminary discussion of absorption into alkanolamines in view of these absorbents’ commercial importance.

Adsorption

Practical adsorption processes use a granulated material with affinity for the component, or components that are desired to be removed. This material is referred to as the adsorbent,

while the material adsorbed is referred to as the adsorbate. There are four categories of adsorbents commonly used:

  • Molecular sieve zeolites
  • Activated alumina
  • Silica gel
  • Activated carbon.

There is also a carbon molecular sieve that is used for making moderate quantities of nitrogen from air, but we shall leave that aside. Also liquids may be treated by adsorption. In gas treating with absorbents, there is usually an adsorption treatment of this absorbent.

Regeneration of adsorbent may be done by both pressure swing and thermal swing, or a combination. Pressure swing alone is a commercial process that is applied to air separation and at least to hydrogen recovery from streams of synthesis gas. In gas treating contaminant removal by a combination process involving both temperature and pressure variation is mainly used. It could be used to remove water from the gas, and it is used as pretreatment

upstream of liquefied natural gas (LNG) trains to ensure sufficiently low dew points.

Pressure swing implies that the pressure is changed, and temperature swing implies that the temperature is changed.

Adsorption processes are semi-continuous. By this we mean that they continuously treat the gas without a buffer volume, the discontinuity comes from the need to switch between two or more parallel units. The unit not adsorbing is being regenerated offline at a lower pressure and increased temperature with a heat carrying dilution gas flowing through.

This dilution gas may need to be a part of the product gas that most likely will need to be recirculated. In big units more than two parallel columns are often used, sometimes in intricate process stages to emulate some counter-current action while regenerating.

Isotherms for commercial adsorbents are hard to come by. There used to be a couple of companies that handed out leaflets with such content, but such information is certainly not offered on their web sites. In this context it has to be kept in mind that these products are forever being developed such that isotherms may be improved. It is, however, nice to be able to make the odd order of magnitude estimate. This is by no means the result of a thorough

review, but the isotherms used have been published by a number of people and such publications are summarized in Table 2.1 to provide a quick reference as a starting point.

When both CO2 and water are adsorbed, it should be clear from Table 2.1 that water is significantly more strongly adsorbed and will push CO2 away as they compete for adsorption sites. In pretreatment of air for air separation plants this means that there will be a CO2 front moving through the adsorption bed in the direction of flow with a water front pushing from behind. A practical aspect of this is that there is no real need to check for water breakthrough.

It is actually easier to handle a CO2 detector and a water break-through would, in any case, be worse for a cryogenic plant.

On the practical side molecular sieve zeolites could catch 10 g water per 100 g of zeolite in a practical cycle. (Suggested as a quick first approach by a sales engineer a long time ago.

The capacity for CO2 is less. This has implications for adsorption column design. When the gas quantity to be treated is large and the contaminant concentration is significant, the amount of adsorbent needed could be become very large.

Adsorption is mostly used for trace quantity removal. Specific applications will be discussed as they arise. They could include water removal from gas, and a big application is combined water and CO2 removal from air feed to ASUs (ASU=Air Separation Unit).

There is a lot of research going on in the hope of finding a solution that may be used for CO2 removal from flue gas. Recent adsorbents have been reviewed by Hedin and co-workers (2013). They point out that rapid cycling is necessary and believe that some form of structured adsorbent is necessary for success. Treatment of flue gas by vacuum-pressure swing adsorption (VPSA) has been studied (Xiao et al., 2008).

They tested three-bed designs using up to 12 steps in the adsorption-desorption cycle to improve CO2 recovery and purity. Recoveries reported were in the range 70–82%, and purities 82–96%. In the air separation industry VSA is used when oxygen purity does not need to be high, typically 90% although higher can be provided. Argon follows oxygen and is one reason why the purity is that low for reasons of economics.

References:
1. Gas Treating – Absorption Theory and Practice – DAG A. EIMER
2. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish

Natural Gas Hydrates

what are Hydrates

  1. Introduction to Hydrate

Natural gas hydrates are ice-like materials formed under low temperature and high pressure conditions. Natural gas hydrates consist of water molecules interconnected through hydrogen bonds which create an open structural lattice that has the ability to encage smaller hydrocarbons from natural gas or liquid hydrocarbons as guest molecules.

Interest in natural gas hydrates as a potential energy resource has grown significantly in recent years as awareness of the volumes of recoverable gas becomes more focused. The size of this resource has significant implications for worldwide energy supplies should it become technically and economically viable to produce. Although great efforts are being made, there are several unresolved challenges related to all parts in the process towards full scale hydrate reservoir exploitation. Some important issues are: 1) Localize, characterize, and evaluate resources, 2) technology for safe and economic production 3) safety and seafloor stability issues related to drilling and production. Thisarticle gives a brief introduction to natural gas hydrate and its physical properties. Some important characteristics of hydrate accumulations in nature are also discussed.

read also What is Natural Gas

Experimental results presented in this chapter emphasis recent work performed by the authors and others where we investigate the possibilities for producing natural gas from gas hydrate by CO2 replacement. By exposing the hydrate structure to a thermodynamically preferred hydrate former, CO2, it is shown that a spontaneous conversion from methane hydrate to CO2 hydrate occurred. Several experiments have shown this conversion in which the large cavities of hydrates prefer occupation by CO2.

read also Hydrate and Hydrate Prevention

  1. Structures and Properties

There are three known structures of gas hydrates: Structure I (sI), structure II (sII) and structure H (sH). These are distinguished by the size of the cavities and the ratio between large and small cavities. SI and sII contain both a smaller and a larger type of cavity, but the large type cavity of sII is slightly larger than the sI one. The maximum size of guest molecules in sII is butane. SH forms with three types of cavities, two relatively small ones and one quite large.

hydrate
hydrate

The symmetry of the cavities leaves an almost spherical accessible volume for the guest molecules. The size and shape of the guest molecule determines which structure is formed due to volumetric packing considerations. Additional characteristics are guest dipole and/or quadropole moments, such as for instance for H2S and CO2. The average partial charges related to these moments may either increase the stability of the hydrate (H2S) or be a decreasing factor in thermodynamic stability (CO2). SII forms with for instance propane and iso-butane and sH with significantly larger molecules, as for instance cyclo-hexane, neo-hexane. Both methane and carbon dioxide form sI hydrate. SI hydrates forms with guest molecules less than 6 Å in diameter. The cages and the number of each cage per unit cell are shown in Figure 1. SI cages are shown at the top of the figure. The unit cell of sI hydrate contains 46 water molecules and consists of 2 small and six large cages.

The unit cell is the smallest symmetric unit of sI. The two smaller cavities are built by 12 pentagonal faces (512) and the larger of 12 pentagonal faces and two hexagon faces (51262). The growth of hydrate adds unit cells to a crystal.

Classification of Hydrate Deposits

hydrate classes

Boswell and Collett, 2006, proposed a resource pyramid to display the relative size and feasibility for production of the different categories of gas hydrate occurrences in nature. The top resources of the gas hydrates resource pyramid are the ones closest to potential commercialization. According to Boswell and Collett, these are occurrences that exist at high saturations within quality reservoirs rocks under existing Arctic infrastructure.

This superior resource type is estimated by US geological survey (USGS) to be in the range of 33 trillion cubic feet of gas-in-place under Alaska’s North Slope. Prospects by British Petroleum and the US DOE anticipate that 12 trillion cubic feet of this resource is recoverable. Even more high-quality reservoirs are found nearby, but some distance away from existing infrastructure (level 2 from top of pyramid). The current USGS estimate for total North Slope resources is approximately 590 Tcf gas-in-places. The third least

challenging group of resources is in high-quality sandstone reservoirs in marine environments, as those found in the Gulf of Mexico, in the vicinity of existing infrastructure.

There is a huge variation in naturally occurring hydrate reservoirs, both in terms of thermodynamic conditions, hosting geological structures and trapping configurations (sealing characteristics and sealing geometry). Hydrates in unconsolidated sand are considered as the main target for production. For the sake of convenience, these types of hydrate occurrences have been further divided into four main classes,

Class 1 deposits are characterized with a hydrate layer above a zone with free gas and water. The hydrate layer is composed with either hydrate and water

(Class 1W) or gas and hydrate (Class 1G). For both, the hydrate stability zone ends at the bottom of the hydrate interval. Class 2 deposits exist where the hydrate bearing layer, overlies a mobile water zone. Class 3 accumulations are characterized by a single zone of hydrate and the absence of an underlying zone of mobile fluids. The fourth class of hydrate deposits is widespread, low saturation accumulations that are not bounded by confining strata that may appear as nodules over large areas. The latter class is generally not regarded as a target for exploitation.

Proposed Production Schemes

Hydrate

The three main methods for hydrate dissociation discussed in the literature are (1) depressurization, where the hydrate pressure is lowered below the hydration pressure PH at the prevailing temperature; (2) thermal stimulation, where the temperature is raised above the hydration temperature TH at the prevailing pressure; and (3) through the use of inhibitors such as salts and alcohols, which causes a shift in the PH-TH equilibrium due to competition with the hydrate for guest and host molecules. The result of hydrate dissociation is production of water and gas and reduction in the saturation of the solid hydrate phase.

Environmental Aspects of Gas Hydrates

  1. Climate change

The natural gas produced from hydrates will generate CO2 upon combustion, but much less than conventional fuel as oil and coal per energy unit generated. The global awareness of climate change will most likely make it more attractive in relation to oil and coal if fossil fuels, as anticipated, continue to be a major fuel for world economies the next several decades. However, increased global temperatures have the potential of bringing both permafrost hydrates and subsea hydrates out of equilibrium. As a consequence, huge amounts of methane may be released to the atmosphere and accelerate the greenhouse effect due to feedback. In general hydrate is not stable towards typical sandstone and will fill pore volume rather than stick to the mineral walls. This implies that if there are imperfections and leakage paths in the sealing mechanisms the hydrate reservoir will leak. There are numerous small and large leaking hydrate reservoirs which results in methane fluxes into the ocean. Some of these fluxes will be reduced through consumption in biological ecosystems or chemical ecosystems. The net flux of methane reaching the atmosphere per

year is still uncertain. Methane is by far a more powerful greenhouse gas than CO2 (~20 times). hypothesized that major release from methane hydrate caused immense global warming 15 000 years ago. This theory, referred to as “clathrate gun” hypothesis is still regarded as controversial, but is supported in a very recent paper by Kennedy et al. (2008). The role of gas hydrate in global climate change is not adequately understood. For hydrate methane to work as a greenhouse gas, it must travel from the subsurface hydrate to the atmosphere. Rates of dissociation and reactions/destruction of the methane gas on its way through sediment layers, water and air are uncharted.

  1. Geomechanical Stability

Gas hydrates will affect the seafloor stability differently for the different types of hydrate occurrences. All of these hydrate configurations may take part of the skeleton framework that supports overlying sediments, which in turn is the fundament for pipelines and installations needed for production. These concerns have already been established for oil and gas exploitation where oil and gas reservoirs that lie below or nearby hydrate bearing sediments. However, geohazards would potentially be far more severe if gas hydrate is to

be produced from marine hydrate deposits. During melting, the dissociated hydrate zone may lose strength due to under-consolidated sediments and possible over-pressuring due to the newly released gas. If the shear strength is lowered, failure may be triggered by gravitational loading or seismic disturbance that can result in submarine landslides

Several possible oceanic landslides related to hydrate dissociation are reported in the literature. Among these are large submarine slides on the Norwegian shelf in the North Sea  and massive bedding-plane slides and slumps on the Alaskan Beaufort Sea continental margin.

Production of CH4 from hydrates by CO2 exposure

Thermodynamic prediction suggests that replacement of CH4 by CO2 is a favourable process. This section reviews some basic thermodynamics and earlier experimental studies of this CH4-CO2 reformation process to introduce a scientific fundament for the experimental work presented later in this chapter.

Thermodynamics of CO2 and CH4 Hydrate

CO2 and CH4 form both sI hydrates. CH4 molecules can occupy both large and small cages, while CO2 molecules will prefer the large 51262 cage. Under sufficiently high pressures or low temperatures both CO2 and CH4 will be stable, but thermodynamic studies suggest that CH4 hydrates have a higher equilibrium pressure than that of CO2 hydrates for a range of temperatures. A summary of these experiments is presented in Sloan & Koh,

shows the equilibrium conditions for CO2 and CH4 hydrate in a P-T diagram. This plot is produced using the CSMGem software (Sloan & Koh, 2008), which supplies the most recent thermodynamic predictions.

CO2-CH4 exchange in bulk

Based on the knowledge of increased thermodynamic stability it was hypothesized that CO2 could replace and recover CH4 molecules if exposed to CH4 hydrate .

Several early researchers investigated the CO2-CH4 exchange mechanism as a possible way of producing methane from hydrates. These studies emphasized the thermodynamic driving forces that favour this exchange reaction, though many of the results showed significant kinetic limitations. Many of these early

studies dealt with bulk methane hydrate samples placed in contact with liquid or gaseous CO2, where available surfaces for interaction were limited., studied the CO2-CH4 exchange process in a high pressure cell using powdered CH4 hydrate and then exposed it to CO2. They observed a fairly rapid initial conversion during the first 200 minutes, which then slowed down significantly. found remarkable recovery of methane hydrate by using CO2 and N2 mixtures. They found that N2 would compete with CH4 for occupancy of the smaller sI cages, while CO2 would occupy only the larger sI cage – without any challenge of other guests. They also found that sII and sH would convert to sI and yield high recoveries (64-95%) when exposed to CO2 or CO2-N2 mixtures.

An inherent limitation in this experiment is the absence of mineral surfaces and the corresponding impact of liquids that may separate minerals from hydrates. These liquid channels may serve as transport channels as well as increased hydrate/fluid contact areas.

CO2-CH4 Exchange in Porous Media

Lee et al., 2003 studied the formation of CH4 hydrate, and the subsequent reformation into CO2 hydrate in porous silica. CH4 hydrate was formed at 268 K and 215 bar while the conversion reaction was studied at 270 K. The temperatures in the ice stability region could have an impact on the reformation mechanisms since ice may form at intermediate stages of opening and closing of cavities and partial structures during the reformation. Temperatures below zero may also have an impact in the case where water separates minerals from hydrates. Preliminary studies of the CO2 exchange process in sediments showed slow methane production when the P-T conditions were near the methane hydrate stability and at CO2 pressure values near saturation levels .The research presented below revisit the CO2- CH4 exchange process in hydrates formed in porous media, this time in larger sandstone core plugs and well within the hydrate stability for both CO2 and CH4 hydrate, and outside the regular ice stab

References:
1. Gas Treating – Absorption Theory and Practice – DAG A. EIMER
2. Fundamentals of Natural Gas, Arthur J. Kidnay & William R. Parrish