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Acidizing Different Formations

ATTACHMENT DETAILS acidizingMuch of the worlds oil and gas comes from limestone (CaCO3) and dolomite (CaMg(CO3)2) formations, either in their relatively pure form or in the form of carbonate or siliceous sands cemented together with calcareous materials (CaCO3).
Dolomites are similar to limestones with the exception that they generally react more slowly with hydrochloric acid.
The primary method of stimulating wells drilled into these formations is to inject an acid treating solution. The acid dissolves part of the formation and may also dissolve other acid soluble material (mud damage, scales etc.), which is restricting or blocking the flow of oil or gas from the formation. Matrix acidizing increases the flow capacity of a producing formation when these restrictions are removed.

read more about Acidizing Concepts

Limestone and Dolomite
When either limestone and/or dolomite formation are stimulated, acid enters the formation through pores in the matrix of the rock or through natural or induced fractures. The type of acidizing used depends on, the injection rate and the number and size of the fractures present. Most limestone and dolomite formations produce through a network of fractures, though both formations can exist in an unfractured state. Normally, an interval will accept acid through the fractures more readily and at lower pressure than through the pore spaces. The acid solution reacts with the walls of the flow channels, increasing the width and conductivity of the fractures.
Most limestones and dolomite formations vary in acid solubility. Acid will attack the surface of the formation at varying rates, leaving an unevenly etched face. The existence of natural fractures, that occur at random intervals and in random sizes, contribute to the final uneven etching configuration.
The type of acid and strength are equally important factors in influencing the etch pattern. . The use of various types of acid (such as chemically retarded or emulsified acid), ensure that the volume of limestone or dolomite dissolved, will occur in an uneven pattern across the face of the fracture.

Gelled and cross-linked acids can also be used effectively. These fluids will create wider fractures and have reduced leak-off, resulting in less “worm holing” and deeper penetration due to the retarded reaction of the acid.
Chemically retarded acids are made effective by preceding the acid treatment with a hydrocarbon preflush containing an oil-wetting surface acting agent (surfactant) Due to the variable composition of the rock, the surfactant leaves a discontinuous oil film on the fracture face. The resulting acid break-through is irregular, creating an
improved etch pattern.
With emulsified acid, the resulting etch patterns are influenced by the rate at which acid penetrates the hydrocarbon outer phase of the emulsion and reacts with the The temperature of the formation should also be considered to ensure that the selection of either chemically retarded acid or delayed reaction acid is the one that is most suitable for the treatment recommended Acid volume and pump rate determine the acid contact time, during which the
fracture faces are exposed to live acid. Contact time has a direct bearing on the amount of etching obtained. However, increasing the volume of an acid treatment does not appreciably increase the depth of penetration. Thus, the benefit of a treatment with a contact time greater than the spending time of the acid, can be attributed to acid etching, which results in additional flow conductivity.
The “shut-in time”, or the length of time a well is closed in after a stimulation treatment, is determined by the type of acid used and by such downhole factors as:
· Type of formation.
· Bottom-hole temperature.
· Bottom hole pressure.
After an acid solution has been neutralised by reaction with the formation, it is no longer a stimulation agent. However, it may become harmful to the formation permeability if allowed to remain downhole.
Hydrochloric acid reacts so rapidly with limestone formations that it is essentially neutralised by the time the acid has been completely placed. This neutralisation generally occurs at all ranges of temperature and pressure. Limestone formations incorporate varying amounts of insoluble impurities, which can plug permeability if allowed to come to rest. Therefore, it is important to remove the neutralised hydrochloric acid as soon as possible. The shut-in time with such formations is zero.
Figures 2 to 5 show the relative reaction rates of 15% hydrochloric acid with limestone and dolomite formations at different temperatures. When chemically retarded acids like super retarded acids (SRA), delayed reaction systems (Super Sol Acid (EQH)), Sta-Live and emulsified acids like SRA-3 are used, the reaction time exceeds the displacement time. This is also true for gelled and cross-linked acids (Gelled Acid, Gelled Acid XL, XL Acid II). Here, the shut-in time may be extended if there is sufficient bottom-hole pressure to promote rapid cleanup.
For reaction times of retarded acids consult the engineering product bulletin pertaining to the acid system used.

Acidizing Concepts

Acid Types
acidizingAlthough many acid compounds are available to the oil industry, only the following types have been proven economically effective in oil well stimulation:
Inorganic Acids (Strong).
· Hydrochloric Acid (HCl).
· Hydrofluoric Acid (HCl:HF).
Other inorganic acids include Sulphamic, Sulphuric and Nitric acids.
Organic Acids (Weak).
· Acetic Acid and Glacial Acetic Acid.
· Acetic Anhydride.
· Citric Acid.
· Formic Acid.

Inorganic Acids
Hydrochloric Acid (HCl).
Hydrochloric acid is an inorganic acid and is the most commonly used acid in oil well stimulation. Hydrochloric acid has many advantages in its application as follows:
· Low cost and availability.
· Easily inhibited to prevent attack on oil-field tubulars.
· Surface tension can be controlled to aid in :
– Penetration.
– Wetting properties.
– Exhibit detergency.
– Reducing friction pressure.
· Can be emulsified for slower reaction rate.
· Exhibit de-emulsification properties for rapid clean up.
· Most reaction products are water soluble and easily removed.
· Additives to minimise or eliminate insoluble reaction products can be applied.
It has long been recognised that hydrochloric acid is the best field acid for most applications. It is however, not without limitations. Hydrochloric acid is quite reactive; therefore, it will spend quite rapidly on some formations. It is essential with hydrochloric acid to size acid treatments and pump rates to optimise this property.

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The reaction rate also dictates the selection of additives that will perform their functions during the relatively short spending time. These same additives must survive the spending process and function in the spent acid. Certain materials are soluble in hydrochloric acid but not necessarily in the spent acid water. For example, calcium sulphate can be partially solubilised by hydrochloric acid, but will crystallise out as scale when the acid spends. Iron oxide will dissolve in hydrochloric acid but will re-precipitate, as the acid spends, at about a pH of 2.0. These properties require
the selection of additives that will circumvent these problems.
Hydrochloric acid is normally pumped in concentrations ranging from 3.0% to 28%.
The low concentration acids are used for the removal of salt plugs and emulsions. The high concentration acids are selected to achieve longer reaction times and to create larger flow channels. By far the most frequently used strength is 15%, for the following reasons :
· Less cost per unit volume than stronger acids.
· Less costly to inhibit.
· Less hazardous to handle.
· Will retain larger quantities of dissolved salts in solution after spending.
In addition to the above advantages 15% hydrochloric acid will also provide other specific properties such as emulsion control and silt suspension. The general uses for hydrochloric acid are as follows :
· Carbonate acidizing – Fracture and Matrix.
· Sandstone acidizing – Matrix only.
· Preflush for HCl:HF mixtures.
· Post-flush for HCl:HF mixtures.
· Acidizing sandstones with 15% to 20% carbonate content.
· Clean-up of acid-soluble scales.
· Perforation washes.
Pure hydrochloric acid (muriatic acid) is a colourless liquid, but takes on a yellowish hue when contaminated by iron, chlorine, or organic substances. It is available commercially in strengths up to 23.5° Bé (Baumé scale) or 38.7% percent by weight of solution.
Some processes dictate that hydrochloric acid is not the most suitable acid to use. In these cases, alternatives, such as organic acids (acetic and formic) may be used.
These acids are used because of their inherently retarded nature, their ability to be used at higher temperatures and their solvation ability in “dirty” formations. The primary objection to the use of organic acids is their cost and their lack of effectiveness in removing limestone.

Other acids are also used in limited quantities. An example is citric acid, which can be used both alone, or as a component of an acid blend, or for use as a stabiliser, buffer and iron control agent. Also sulfamic acid has been used in the oil industry on a “do it yourself” basis. Its usage is recommended because of its low corrosivity,
although it is limited by its ability to strip chrome from chrome pumps and by its relatively high cost.

Hydrofluoric Acid (HF).
Hydrofluoric acid, another inorganic acid, is used with hydrochloric acid to intensify the reaction rate of the total system and to solubilise formations, in particular sandstones. In general hydrofluoric acid is used as follows :
· It is always pumped as an HCl:HF mixture.
· Ensure that salt ion contact is prevented.
· Sandstone matrix acidizing.

· Removal of HCl insoluble fines.
· Normal concentrations 1.5% to 6.0%.
· One gallon of 12:3 HCl:HF will dissolve 0.217 pounds of sand.
Hydrofluoric occurs as a liquid either in the anhydrous form (where it is fuming and corrosive), or in an aqueous solution (as used in well stimulation). Hydrofluoric acid attacks silica and silicates, (glass and concrete). It will also attack natural rubber, leather, certain metals such a cast iron and many organic materials.
In well stimulation, hydrofluoric acid is normally used in combination with hydrochloric acid. Mixtures of the two acids may be prepared by diluting mixtures of the concentrated acids with water, or by adding fluoride salts (e.g. ammonium bifluoride) to the hydrochloric acid. The fluoride salts release hydrofluoric acid when dissolved in hydrochloric acid.
Hydrofluoric acid is poisonous, alone or in mixtures with hydrochloric acid, and should be handled with extreme caution.Other Inorganic Acids.
Some consideration has been given to using sulfuric and nitric acids; however, these acids are not used extensively in the oil industry today. The reasons for the lack of use are; sulfuric acid will form insoluble precipitates, and nitric acid often forms poisonous gases during its reaction with certain minerals.

 Organic Acids.
These acids are used in well stimulation basically because they have a lower corrosion rate and are easier to inhibit at high temperatures than hydrochloric acid. Although mixtures of organic acids are considered corrosive to most metals, the corrosion rate is far lower than that of hydrochloric or hydrofluoric acid, therefore, organic acids are used when long acid-pipe contact time is required. An example of this is when organic acid is used as a displacing fluid for a cement job. The organic acids is left in the production string. and is subsequently used as the perforating fluid.
Organic acids are also used when metal surfaces of aluminium, magnesium, and chrome are to be contacted, such as in trying to remove acid-soluble scales in wells with downhole pumps in place. They can also be used as iron control agents for other acid systems. Many organic acids are available, but the four most commonly
used are :
· Acetic Acid.
· Acetic Anhydride.
· Citric Acid.
· Formic Acid.

Acetic Acid (CH3COOH).
Acetic acid is a colourless organic acid soluble in water in any proportion and in most organic solvents. Although mixtures of acetic acid with water are considered corrosive to most metals, the corrosion rate is far lower than that of hydrochloric and hydrofluoric acids. Acetic acid is easy to inhibit against corrosion and is used frequently as a perforating fluid where prolonged contact times are required. With this ability, the acid is sometimes used as a displacing fluid on a well cementing job, where the contact time may be hours or days before perforating takes place. This ability is beneficial in three ways:
· Reduces formation damage. The first fluid two enter the formation will be an acid or low pH fluid which will react with carbonate or the calcareous materials of a sandstone formation.
· Reduces clay swelling.
· Can be used where aluminium, magnesium or chrome surfaces must be protected.
The relation of dissolving power of one gallon of a 15% concentration of acetic acid compared to that of hydrochloric acid and formic acid at the same volume is listed in Table 1, page 3. The cost of acetic acid per unit, based on dissolving power, is more expensive than either hydrochloric acid or formic acid.
Normally, acetic acid is used in small quantities or with hydrochloric acid, as a delayed reaction, or retarded acid. The general uses and properties of acetic acid are as follows:
· Acetic acid is relatively weak.
· Normal concentrations of 7.5% to 10% when used alone.
· Mainly used in hydrochloric acid mixtures.
· Used as an iron control additive.
· Carbonate acidizing.
· Perforating fluid.
· Retarded acids.
Commercially available acetic acid is approximately 99% “pure”. It is called glacial acetic acid because, ice-like crystals will form in it at temperatures of approximately 60° F (16° C) and will solidify at approximately 48° F (9° C). When glacial acetic acid is mixed with water, a contraction occurs. For this reason, the amount of acetic acid and the amount of water normally total more than the required volume.
Care should be exercised when handling acetic acid. This solution in concentrated form can cause severe burns and fume inhalation can harm lung tissue

Acetic Anhydride Acid.
Acetic anhydride is the cold weather version, for use instead of acetic acid due to its lower freezing point of 2.0° F (-17° C). The properties of acetic anhydride are the same for those of acetic acid, the only changes are those in relation to volumes used.
A comparison of acetic anhydride to acetic acid shows that one gallon of acetic anhydride mixed with 0.113 gallons of water is equivalent to 1.127 gallons of acetic acid. Expressed alternatively one gallon of acetic acid is equivalent to 0.887 gallons of acetic anhydride mixed with 0.101 gallons of water.
When mixing acetic anhydride always add it to water or dilute acid. If water or dilute acid is added to acetic anhydride, an explosion will occur due to a rapid increase in temperature caused by the chemical reaction.
As with acetic acid, care should be exercised when handling acetic anhydride as this solution in concentrated form can cause severe burns and fume inhalation can harm lung tissue.

Citric Acid (C6H8O7)
Iron scales are normally found in the casing and tubing in wells and sometimes as the mineral deposits in the formation rock itself. When hydrochloric acid solutions come into contact with these scales or deposits, the iron compounds are partially dissolved and are carried in solution as iron chloride. As the acid becomes spent,
the pH rises above 2.0, allowing the iron chloride to undergo chemical changes and re-precipitate as insoluble iron hydroxide. This re-precipitation can reduce formation permeability and injectivity.
Citric acid (Ferrotrol 300) is a white granular organic acid material. It is used to “tie up” dissolved iron scales and prevent re-precipitation of dissolved iron from spent hydrochloric acid solutions. Normally, citric acid (often referred to as a sequestrant or sequestering agent), is used with X-14 to make the effects of suspension more
stable.
Citric acid is not used alone as an acid treating solution itself but is used in hydrochloric acid solutions known as sequestering acids (SA-systems) for the control of iron.
The amount of citric acid added to the hydrochloric acid system depends upon the amount of iron that is present. The first 50 pounds of citric acid added to 1000 gallons of acid, will sustain 2000 parts per million (ppm) of iron in solution (SA-2).
Each additional 50 pounds of citric acid added will increase its sequestering property by an additional 2000 ppm

Formic Acid (HCOOH)
Formic acid is the simplest of the organic acids and is completely miscible (capable of being mixed) with water. Formic acid is stronger than acetic acid yet weaker than hydrochloric acid. Formic acid is used in well stimulation, most frequently in combination with hydrochloric acid as a retarded acid system for high-temperature wells. The percentage of formic acid used in such applications is commonly between 8.0% and 10%. Formic acid can be easily inhibited, but not as effectively as with acetic acid at high temperatures and long contact times. The properties and uses of formic acid parallel those of acetic acid as stated below:
· Formic acid is relatively weak.
· Seldom used alone.
· Mainly used in hydrochloric acid mixtures.

Corrosion inhibitor aid.
· Hot wells.
· Retarded acids.
Acetic acid, acetic anhydride and formic acid are used when exceptionally retarded acid is needed because of extreme temperature or very low injection rates. At high temperatures, blends of organic and hydrochloric acid are much more successfully inhibited by organic inhibitors, than when hydrochloric acid is used alone. This property minimises the danger of hydrogen embrittlement of steel associated with hydrochloric acid treatments in high-temperature wells. Organic acid concentrations of up to 25% by weight are required, making acid treatment costs increase. Organic acids do not give as much reacting capability as hydrochloric acid treatments.

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Chemical Additives in Oil Wells’ Drilling

المواد الكيمياوية المستعملة في حفر الآبار النفطية

  Chemical Additives in Oil Wells’ Drilling

هناك العشرات من المواد الكيمياوية التي تستعمل في المراحل المختلفة من صناعة النفط والغاز الطبيعي مثل : الحفر Drilling – الإكمال Completion – ومرحل أنتاج النفط والغاز الطبيعي. وسنحاول أن نلقي الضوء على قسم منها ، وتتضمن هذه المواد الأملاح غير العضوية – المواد المعدنية – البوليمرات Polymersالذائبة في الماء والنفط والمواد النشطة سطحياً . حيث تستخدم أغلب هذه المواد لديمومة الإنتاج في الحقول التي تعاني من النضوب.

 لقد بدأت المواد الكيمياوية تأخذ دوراً كبيراً في صناعة النفط والغاز الطبيعي ، حيث يمكن أستخدامها في تحسين الإنتاج من هذه الحقول ، ويوماً بعد يوم ستأخذ الاعتبارات البيئية سيكون لها التأثير الأهم في أختيار هذه المواد الكيمياوية وخاصة في المنصات البحرية offshore ، وسنحاول أن نسلط الضوء على المواد الكيمياوية التي تستخدم في هذه المجالات.

سوائل الحفر Drilling Fluids:

وتُسمى في بعض الأحيان بطين الحفر Drilling Mud بسبب مظهرها ويعود ذلك الى الطين الذي يضاف الى أغلبها ، فائدة هذه السوائل هي لتبريد وتزييت رأس الحفارة Drill bit بالإضافة الى نقل نواتج الحفر الى السطح ، أن المكون الرئيسي لسوائل الحفر Drilling Fluids هو الماء ، أما الطين ذو الأساس الزيتي oil-based muds يمكن أستخدامه في درجات الحرارة العالية وفي المكامن والطبقات الحساسة للماء وتكون على نوعين الأول ذو مستحلب زيتي خارجي يحتوي على الماء بنسبة تصل الى 50% ، والثاني أساس زيتي يحتوي على القليل من الماء.

وبشكل عام ، كلما زاد عمق المكمن أو البئر يجب أستخدام المزيد من المواد الكيمياوية للحفاظ على خواص الموائع ، ويمكن تقسيمها الى أنواع تبعاً لوظيفتها:

مواد الموازنة Weighting Materials :

 وتستعمل لتعيير كثافة المائع وبالتالي الضغط الهيدروستاتيكي المسلط على المكمن ، والغاية منه منع حدوث تغيرات مفاجئة في الضغط أو حالة Blowouts أثناء الحفر وتجنب تسرب السائل نحو الطبقة في نفس الوقت ، وأكثر هذه المواد شيوعاً هو كبريتات الباريوم (Barite)  كما توجد العديد من المواد الأخرى تستعمل لنفس الغرض مثل : Hematite Siderite – كبريتيد النحاس ، أن أغلب هذه المواد تكون مسببة للتآكل لذا يتوجب أستخدام بعض أنواع مانع التآكل Corrosion Inhibitor .

إضافات فقدان الموائع Fluid Loss Additives : وأهم أمثلتها البوليمرات المُثخنة التي تضاف الى طين الحفر Drilling Mud لكي لتقليل خسارة الموائع من جوف البئر ، ومن أمثلة هذه المواد : البنتونايت Bentonite وغيرها من أنواع الصلصال ، يتم معالجتها بمصادر مختلفة مثل الليكنايت المعالج بالمواد الكاوية أو الأمين – الراتنجات المختلفة – الكلسونايت – رقاقات حامض البنزويك – المايكا. 

المُثخنات والمُشتتات Thinners & Dispersants

تستخدم لمنع حدوث التزغب في جزيئات الطين والحفاظ على قابلية ضخه ، وأمثله هذه المواد هي : رابع فوسفات الصوديوم وأنواع أخرى من الفوسفات والبوليمرات الصناعية مثل البولي ستايرين ، كما يتم أستخدام المواد المُقللة للأحتكاك Friction Reducers مثل بولي أكريلمايد ، حيث أنها تسهل تدوير السائل خلال تجويف البئر مما يقلل الإجهاد على المضخات.

مانع التآكل Corrosion Inhibitors

يستخدم لتقليل الأحتكاك في المعدات والذي قد يسببه أستخدام سوائل الحفر ، حيث تستعمل العديد من المواد مثل: أملاح الأمين – سلقات الأمين – كاربونات الزنك – كرومات الزنك – أحادي اثيل أمين.

قاتل البكتريا Bactericides:

للسيطرة على نمو البكتريا التي قد تسبب التآكل أو قد تسبب انسداد مسمامات المكمن أو تغيير خواص سوائل الحفر مثل : بارافورمالديهايد  – هيدروكسيد الصوديوم – أثيل أمين.

السيطرة على الحامضية PH Control :

ويساعد أيضا على تقليل التآكل ، ويمنع حدوث أي تفاعل بين سوائل الحفر Drilling Fluids ومعادن المكمن وتستعمل المواد التالية: هيدروكسيد الصوديوم – كاربونات الكالسيوم – هيدروكسيد البوتاسيوم – أوكسيد المغنسيوم – أوكسيد الكالسيوم – حامض الفورميك.

السيطرة على التضرر الطبقي Formation Damage Control:وتُضاف لتقليل التضرر النفاذي الذي يحدث عند دخول سوائل الحفر الى المكمن. كما أنه يساعد في منع تآكل تجويف البئر ومن هذه المواد : كلوريد الأمونيوم – كلوريد الصوديوم – سليكات الصوديوم .. وغيرها.

مانع الصدأ Scale Inhibitors : يستخدم لمنع تكون أملاح الكالسيوم غير الذائبة عند تلامس سوائل الحفر مع المعادن الموجودة في المكمن والمياه المالحة في المكمن. وأمثلة هذه المادة هي: هيدروكسيد الصوديوم – كاربونات الصوديوم – بيكاربونات الصوديوم .. وغيرها.

عوامل الاستحلاب Emulsifiers: يستخدم لإعداد مستحلب نفط خارجي في سائل الحفر ، حيث يتم أستخدام المواد النشطة سطحياً Surfactant مثل الأملاح الدهنية ، والأحماض الأمينية.

أن تطوير سوائل الحفر مستمر بشكل كبير ، كما أن هناك العديد من السوائل التي يتم أختبارها وتطويرها مختبرياً في الوقت الحاضر للوصول الى التركيبة الأفضل.

سوائل التسميت Cementing Fluids:  بعد أكمال عملية الحفر ، يتم إنزال بطانة فولاذية الى أسفل البئر ومن ثم ضخ سائل الى الأسفل لإزالة سوائل الحفر ومنع تلامس طين الحفر مع خلطة السمنت أن إزاحة طين الحفر ستساعد على تماسك خلطة السمنت.

السمنت المقاوم للتآكل Corrosion -resistant Cements: لقد تم تطوير هذا النوع من السمنت للاستخدام في الآبار التي يتم حقنها بثاني أوكسيد الكاربون لغرض تحسين إنتاج البئر .

مواد التحميض Acidizing Chemicals :

الغسل بالحامض Acid Washing يستخدم لغرض إذابة الصدأ القابل للإذابة بالحامض من جدار البئر ، وفتح أية انسدادات تسبب بها جزيئات الصدأ. بالإضافة الى نوع آخر من التحميض يسمى Matrix Acidizing وهو حقن الأحماض الى التكوين Formation في ضغط معين أقل من ضغط التكوين (وهو الضغط الذي تُجبر فيها التشققات الطبيعية على الفتح لسوائل الحقن). 

يدخل الحامض الى قنوات الجريان في التكوين ، ويتدفق بشكل شعاعي الى خارج تجويف البئر لإذابة جزيئات المعادن الصغيرة ، وهذه المعادن التي تشكل قنوات الجريان في التكوين تتفاعل مع هذه الأحماض ، وهذه العمليات تؤدي الى زيادة نفاذية التكوين Formation Permeability قرب جدار البئر مما يؤدي الى زيادة إنتاجية البئر بدون زيادة الماء المنتج أو زيادة نسبة الغاز الى النفط Gas Oil Ratio GOR

أما النوع الثالث من التحميض فهو التحميض للتشققات Fracture Acidizing حيث يحقن الحامض بضغط أعلى من ضغط التشققات ويتفاعل مع المعادن على سطح التشقق في عملية تسمى التنميش Etching . وعند أجراء هذه العملية فأن جدرات التشقق لن تُسد عند أنتاج البئر .

كما أن الأحماض تكسر المستحلبات في التكوين أحياناً من خلال تخفيض الدالة الحامضية PH أو من خلال إذابة الجزيئات الصغيرة التي تُثبّت المستحلبات. أن كسر المستحلب ستؤدي الى تقليل اللزوجة أيضاً.

يتم أختيار نوع الحامض إعتماداً على نوع المعاملة المطلوبة للبئر nature of well treatment ونوع المعادن الموجودة في التكوين ، بالأضافة الى حساب كمية المواد المكمنية التي سيتم إذابتها بحجم معين من الحامض ، ثابت التوازن ، ومقدار التفاعل بين الحامض والمعادن الموجودة في المكمن.

الأحماض المعدنية Mineral Acids: ويتضمن حامض الهيدروكلوريك Hydrochloric Acid وخلطة من الهيدروكلوريك والهيدروفلوريك بنسبة (12%HCL ) مع 3% HF ويستخدم حامض الهيدروكلوريك لتحميض المكامن الكاربونية. حيث يتميز بقلة كلفته ، وقابليته العالية على تذويب المعادن الكاربونية مما يقلل التضرر الطبقي، أما مساوئه فهو تسببه للتآكل.

أما حامض الهيدروفلوريك فقد يستخدم بالتخفيف مع محلول مائي مركز أو بالتفاعل مع كمية كافية لثاني فلوريد الأمونيوم مع محلول 15% HCL لتهيئة خلطة (12%HCL ) مع 3% HF . أن خليط حامضي الهيدروكلوريك والهيدروفلوريك ذو فائدة كبيرة في لإذابة المعادن السليكونية ، وهي تسبب التآكل بشكل كبير في نفس الوقت.

الأحماض العضوية Organic Acids: ويستخدم مع الصخور الكاربونية ويضمن أحماض الفورميك – الأستيك – السولفاميك – الكلوروأستيك. وهي أقل تسبباً للتآكل من الأحماض المعدنية ، مما يجعلها مرغوبة عند الأستخدام لفترات طويلة مع الأنابيب ، كما انها لا تتفاعل مع الكاربونات في درجات الحرارة العالية مما يؤدي الى تغلغل الحامض في التكوين ، وتستخدم الأحماض العضوية في نطاق أضيق من الأحماض المعدنية بسبب تكلفتها العالية وتفاعلها غير الكامل مع مع المعادن الكاربونية.

Completion Techniques

by Abass Radhi Abbas

1.General

Communication between the formation and the wellbore will directly affect the productivity of the well. Factors such as hydrocarbon saturation, porosity, permeability, fluid properties and geometry can be measured or inferred from the measurements but they cannot usually be controlled. By contrast, completion can be controlled and thus affect well performance. During the drilling, logging and testing phase of the well, valuable information will have been gained and the relevant completion technique chosen. There are two main categories of completions to consider with, of course, many variations;

–  Open hole Completions.
– Cased Hole Completions.

1.1.Openhole Completions

An open hole completion is when the well is drilled to the top of the target formation and the casing is cemented at this stage. Drilling is continued across the target formation and then the well is completed and produced. Open hole completions are only possible in “competent” rocks that will hold their form and not cave in or crumble – so calledOpen Hole Completion hard rock environments. This technique is generally associated with older, cheaper methods of drilling and completing wells and today would only be used in very low profile applications, if at all. Variations on the straight forward open hole completion include gravel packing with slotted liners used to contain the pack. Whereas this technique offers the least restriction to flow from formation to wellbore and as mentioned is an economical completion, it has many apparent disadvantages;

– No possibility for selectively producing or treating different zones.
–  Limited control of water or gas encroachment.

These two factors alone can play a significant part in the future management of the well, and this coupled with safety issues has lead the industry down the road of cased hole and perforated completions.

1.2.Cased Hole Completions

Cased Hole CompletionA cased hole completion is when the well has been cased and cemented across the target formation and requires shaped charge perforation to achieve communication between the formation and the wellbore. This is the most common form of completing wells today and our discussion will center around this technique.

  1. Completion Types

Completions can be broken down into two main categories;

–  Natural (perforated).

– Stimulated.

In all three the objective is to maximize production through enhancement of some aspect of the near-wellbore reservoir performance.  Of particular importance is the change in flow geometry near the wellbore caused by wellbore damage (from drilling and filtrate invasion), perforations (debris), flow convergence due to partial penetration and deviation. This damage is known as skin (S) and will be discussed in more detail in section 5, but for the time being can be considered as an induced pressure drop across the completion which effects productivity.

Completion

Completion

Pressure Distribution
Pressure Distribution in a Reservoir with Skin

The aim of the completion design engineer and reservoir engineer is to reduce the influence of skin as much as possible.

  1. 1.Natural Completions (perforated)

The natural completion is usually chosen for sandstone reservoirs with permeabilities above 10 md and porosities above 9 p.u. These reservoirs typically have small damaged zones and limited skin, good transmissibility and stable rock mechanics. They generally do not require stimulation or sand control during primary completion. The objectives of the perforation in this case would be depth of penetration and effective shot density, the perforation diameter is generally unimportant if it is larger than 0.25” (0.5 cm). The deepest penetration with the greatest phase distribution is desirable for production enhancement.

2..2        Stimulated Completions

These fall into two broad categories;

–  Hydraulic Fracturing.

–   Acidizing.

Occasionally the two are combined in an “acid frac” job.

2.2.1     Hydraulic Fracturing

Hydraulic fracturing is performed to enhance the effective wellbore radius rw and is usually employed in reservoirs

with small permeabilities (k < 1 md). This is accomplished by injecting fluids and propant at high pressure, in order to create a bi-wing, symmetrical fracture or crack in the reservoir. Hydraulic fracturing is generally a five-step process;

  • Pre-fracturing treatment.
  • Fracture initiation and breakdown.
  • Fracture extension.
  • Proppant injection.
  • Cleanout and production.

    Hydraulic Fracturing
    Hydraulic Fracturing Process
  • 2.2.2     AcidizingAcidizing is a stimulation process used to repair formation damage caused by the drilling or perforating operation. This type of damage is usually associated with plugging of the pore throats around the wellbore. Acidizing removes this damage from the matrix rock by injecting acid into the naturally porous rock at sub-fracturing rates, allowing the acid to dissolve the plugs.
  • Acid frac jobs are used to etch the surface of the hydraulically induced fracture. After the fracture closes, the etched surface cannot form a closed seal. Acid frac jobs are operationally less complicated because no proppant is used thus eliminating the potential for premature frac termination that may be caused by screenout or problems of proppant flowback. The principle disadvantage of this technique is the expense of the acid fluids and non-uniform leak-off resulting in “wormholes.” Acid frac jobs are usually performed on carbonate reservoirs