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Water-Base Muds

 

 

Drilling Muds

A water-base drilling fluid is one that has water as its continuous or liquid phase. The types of drilling fluids are briefly described in the following sections.

Freshwater muds are generally lightly treated or untreated muds having a liquid phase of water, containing small concentrations of salt, and having a pH ranging from8.0 to 10.5. Fresh water muds include the following types.

Spud Muds These muds are prepared with available water and appropriate concentrations of bentonite and/or premium commercial clays. They are generally untreated chemically, although lime, cement, or caustic soda is occasionally added to increase viscosity and give the mud a fluff to seal possible lost return zones in unconsolidated upper hole surface formations.

Spud muds are used for drilling the surface hole. Their tolerance for drilled solids and contaminants is very limited.

Natural Mud Natural or native muds use native drilled solids incorporated into the mud for viscosity, weight, and fluid loss control. They are often supplemented with bentonite for added stability and water loss control. Surfactants can be used to aid in controlling mud weight and solids buildup. Natural muds are generally used in top hole drilling to mud-up or to conversion depth. They have a low tolerance for solids and contamination.

Saltwater Muds Muds ordinarily are classified as saltwater muds when they  contain more than 10,000 mg/L of chloride. They may be further classified according to the amount of salt present and/or the source of makeup water (see Table below):

Amount of chloride in mg/L

  1. Saturated salt muds (315,000ppm as sodium chloride)
  2. Salt muds (over 10,000 mg/L chloride but not saturated) Source of make-up water
  3. Brackish water
  4. Sea Water

Saltwater muds may be purposely prepared, or they may result from the use of salty makeup water, from drilling into salt domes or stringers, or when saltwater flows are encountered. Saltwater muds include the following types.

sea water composition

Seawater or Brackish Water Muds These muds are prepared with available makeup water, both commercial and formation clay solids, caustic soda, and lignite and/or a lignosulfonate. CMC is usually used for fluid loss control, although concentration of lignites and lignosulfonates are also often used for this purpose. Viscosity and gel strength are controlled with caustic soda, lignosulfonate, and/or lignites. Soda ash is frequently used to lower the calcium concentration. CMC or lignosulfonates are used for water loss control, and pH is controlled between 8.5 to 11.0 with caustic

soda. Seawater muds and brackish or hard water muds are used primarily because of the convenience of makeup water, usually open sea or bays. The degree of inhibitive properties varies with the salt and calcium concentration in the formulated fluid.

Saturated Salt Muds Saturated salt water (natural or prepared) is used as makeup water in these fluids. Prehydrated bentonite (hydrated in freshwater) is added to give viscosity, and starch is commonly used to control fluid loss. Caustic soda is added to adjust the pH, and lignosulfonates are used for gel strength control. Occasionally, soda ash may be used to lower filtrate calcium and adjust the pH. Saturated salt muds are used to drill massive salt sections (composed mainly of NaCl) to prevent washouts and as a work-over or completion fluid. Freshwater bentonite suspensions are converted by adding NaCl to reach saturation. Conversion is carried out by diluting the freshwater mud to reduce the viscosity “hump” seen in breakovers. Saturated salt muds usually are used at mud weights below 14.0 lb/gal.

Composition of NaCl mud

  • Brine NaCl
  • Density —salt, barite, calcium carbonate or hematite
  • Viscosity — CMC HV, Prehydrated bentonite, XC-polymer (xanthan gum)
  • Rheology—lignosulfonate
  • Fluid Loss—CMC LV or PAC (polyanionic cellulose)
  • pH – Pf (alkalinity) —caustic potash or caustic soda

Chemically Treated Mud (No Calcium Compounds) This type of mud is made up of a natural mud that has been conditioned with bentonite and treated with caustic soda and lignite or lignosulfonate (organic thinner). No inhibiting ions are found in this type of fluid.

Lignite/Lignosulfonate Mud This fluid is prepared from fresh water and  conditioned with bentonite. Lignosulfonate is added as a thinner and lignite for filtration control and increased temperature stability. CMC or PAC may be used for additional filtration control when the bottom-hole temperature does not exceed 121◦C (250◦F). This type of mud is applied at all mud weights and provides a relatively low pH system (pH values for calcium lignosulfonates will be 10.0–11.0). This type of fluid is stable at reasonably high temperatures (325◦F) and has good resistance to contamination.

Calcium Treated Muds Calcium-treated fluids are prepared from any low or high pH mud by the addition of appropriate amounts of lime or gypsum, caustic soda, and thinner (lignite or lignosulfonate). Calcium-treated drilling muds include lime and gypsum drilling muds.

Lime Muds Lime muds include low- and high-lime muds. They are prepared from available drilling muds by adding calcium lignosulfonate, lignite, caustic soda or KOH, lime, and a filtration-control material, PAC or starch. Caustic soda is used to maintain the filtrate alkalinity (Pf values) and lime to control the mud alkalinity (Pm values) and excess lime. Lime drilling muds offer resistance to salt, cement, or anhydrite contamination even at high mud weights.

Gypsum Mud Commonly called “gyp muds,” they are prepared from freshwater and conditioned with bentonite or from available gel and water mud. Caustic soda is added for pH control. Gypsum, lignosulfonate, and additional caustic soda are added simultaneously to the mud. CMC may be added for filtration control. This fluid is used for drilling in mildly reactive shale or where gypsum or anhydrite must be drilled. It resists contamination from cement or salt. Use is limited by the temperature stability of the filtration control materials, CMC (250◦F ±).

Drilling Fluid Additives

 

 

 

drilling fluidsEach drilling fluid vendor provides a wide array of basic and specialty chemicals to meet the needs of the drilling industry. The general classification of drilling fluid additives below is based on the definitions of the International Association of Drilling Contractors (IADC):

  1. Alkalinity or pH control additives are products designed to control the degree of acidity or alkalinity of a drilling fluid. These additives include lime, caustic soda, and bicarbonate of soda.
  2. Bactericides reduce the bacteria count of a drilling fluid. Paraformaldehyde, caustic soda, lime, and starch are commonly used as preservatives.
  3. Calcium removers are chemicals used to prevent and to overcome the contaminating effects of anhydride and gypsum, both forms of calcium sulfate, which can wreck the effectiveness of nearly any chemically treated mud. The most common calcium removers are caustic soda, soda ash, bicarbonate of soda, and certain polyphosphates.
  4. Corrosion inhibitors such as hydrated lime and amine salts are often added to mud and to air-gas systems. Mud containing an adequate percentage of colloids, certain emulsion muds, and oil muds exhibit, in themselves, excellent corrosion-inhibiting properties.
  5. Defoamers are products designed to reduce foaming action, particularly that occurring in brackish water and saturated saltwater muds.
  6. Emulsifiers are used for creating a heterogeneous mixture of two liquids. These include modified lignosulfonates, certain surface-active agents, anionic and nonionic (negatively charged and noncharged) products.
  7. Filtrate, or fluid loss, reducers such as bentonite clays, sodium carboxymethyl cellulose (CMC), and pregelatinized starch serve to cut filter loss, a measure of the tendency of the liquid phase of a drilling fluid to pass into the formation.
  8. Flocculants are used sometimes to increase gel strength. Salt (or brine), hydrated lime, gypsum, and sodium tetraphosphates may be used to cause the colloidal particles of a suspension to group into bunches of “flocks,” causing solids to settle out.
  9. Foaming agents are most often chemicals that also act as surfactants (surface-active agents) to foam in the presence of water. These foamers permit air or gas drilling through water-production formations.
  10. Lost circulation materials (LCM) include nearly every possible product used to stop or slow the loss of circulating fluids into the formation. This loss must be differentiated from the normal loss of filtration liquid and from the loss of drilling mud solids to the filter cake (which is a continuous process in an open hole).
  11. Extreme-pressure lubricants are designed to reduce torque by reducing the coefficient of friction and thereby increase horsepower at the bit. Certain oils, graphite powder, and soaps are used for this purpose.
  12. Shale control inhibitors such as gypsum, sodium silicate, chrome lignosulfonates,

as well as lime and salt are used to control caving by swelling or hydrous disintegration of shales.

  1. Surface-active agents (surfactants) reduce the interfacial tension between contacting surfaces (e.g., water—oil, water—solid, water— air); these may be emulsifiers, de-emulsifiers, flocculants, or deflocculents, depending upon the surfaces involved.
  2. Thinners and dispersants modify the relationship between the viscosity and the percentage of solids in a drilling mud and may further be used to vary the gel strength and improve “pumpability.” Tannins (quebracho), various polyphosphates, and lignitic materials are chosen as thinners or as dispersants, because most of these chemicals also remove solids by precipitation or sequestering, and by deflocculation reactions.
  1. Viscosifiers such as bentonite, CMC, Attapulgite clays, sub-bentonites, and asbestos fibers are employed in drilling fluids to ensure a high viscosity–solids ratio.
  2. Weighting materials, including barite, lead compounds, iron oxides, and similar products possessing extraordinarily high specific gravities, are used to control formation pressures, check caving, facilitate pulling dry drill pipe on round trips, and aid in combating some types of circulation loss.

The most common commercially available drilling mud additives are published annually by World Oil. The listing includes names and descriptions of more than 2,000 mud additives.

References:
1. Drilling Operations.
2. Drilling Equipment and Operation.

Oil-Base and Synthetic-Base Muds

Drilling Mud Tests

The field tests for rheology, mud density, and gel strength are accomplished in the same manner as outlined for water-based drilling mud. The main difference is that rheology is tested at a specific temperature, usually 120◦F or
150◦F. Because oils tend to thin with temperature, heating fluid is required and should be reported on the API Mud Report.
see our Drilling Fluids Books section

Sand Content
Sand content measurement is the same as for water-base drilling mud except that the mud’s base oil instead of water should be used for dilution. The sand content of oil-base mud is not generally tested.
HPHT Filtration The API filtration test result for oil-base drilling mud is usually zero. In relaxed filtrate oil-based muds, the API filtrate should be all oil. The API test does not indicate downhole filtration rates. The alternative high-temperature–high pressure (HTHP) filtration test will generally give a better indication of the fluid loss characteristics of a fluid under downhole temperatures The instruments for the HTHP filtration test consists essentially of a controlled pressure source, a cell designed towithstand a working pressure of at least 1,000 psi, a system for heating the cell, and a suitable frame to hold the cell and the heating system. For filtration tests at temperatures above 200◦F, a pressurized collection cell is attached to the delivery tube.
The filter cell is equipped with a thermometer well, oil-resistant gaskets, and a support for the filter paper (Whatman no. 50 or the equivalent). A valve on the filtrate delivery tube controls flow from the cell. A nonhazardous
gas such as nitrogen or carbon dioxide should be used as the pressure source. The test is usually performed at a temperature of 220 – 350◦F and a pressure of 500 psi (differential) over a 30-minute period. When other temperatures, pressures, or times are used, their values should be reported together with test results. If the cake compressibility is desired, the test should be repeated with pressures of 200 psi on the filter cell and
100 psi back pressure on the collection cell. The volume of oil collected at the end of the test should be doubled to correct to a surface area of 7.1 inches.

read also Testing of Drilling Systems

Electrical Stability
The electrical stability test indicates the stability of emulsions of water inoilmixtures. The emulsion tester consists of a reliable circuit using a source of variable AC current (or DC current in portable units) connected to strip electrodes . The voltage imposed across the electrodes can be increased until a predetermined amount of current flows through the drilling mud emulsion-breakdown point. Relative stability is indicated as the voltage at the breakdown point and is reported as the electric stability of the fluid on the daily API test report.

Liquids and Solids Content
Oil, water, and solids volume percent is determined by retort analysis as in a water-base drilling mud. More time is required to get a complete distillation of an oil mud than for a water mud. The corrected water phase volume, the volume percent of low-gravity solids, and the oil-to-water ratio can then be calculated.

The volume oil-to-water ratio can be found from the procedure below:

Oil fraction 100 × % by volume oil or synthetic oil / (% by volume oil or synthetic oil−% by volume water)

Chemical analysis procedures for nonaqueous fluids can be found in the API 13B bulletin available from the American Petroleum Institute.

Alkalinity and Lime Content (NAF)
The whole mud alkalinity test procedure is a titration method that measures the volume of standard acid required to react with the alkaline (basic) materials in an oil mud sample.
The alkalinity value is used to calculate the pounds per barrel of unreacted, “excess” lime in an oil mud. Excess alkaline materials, such as lime, help to stabilize the emulsion and neutralize carbon dioxide or hydrogen sulfide
acidic gases.

Total Salinity (Water-Phase Salinity [WAF] for NAF)
The salinity control ofNAFfluids is very important for stabilizing water-sensitive shales and clays. Depending on the ionic concentration of the shale waters and of the drilling mud water phase, an osmotic flow of pure water from the weaker
salt concentration (in shale) to the stronger salt concentration (in mud) will occur. This may cause dehydration of the shale and, consequently, affect its stabilization

Specialized Tests
Other, more advanced laboratory-based testing is commonly carried out on drilling fluids to determine treatments or to define contaminants. Some of the more advanced analytical tests routinely conducted on drilling fluids include:

Advanced Rheology and Suspension Analysis
FANN 50 — A laboratory test for rheology under temperature and moderate pressure (up to 1,000 psi and 500◦F).
FANN 70 — Laboratory test for rheology under high temperature and high pressure (up to 20,000 psi and 500◦F).
FANN 75 — Amore advanced computer-controlled version of the FANN 70 (up to 20,000 psi and 500◦F).

High-Angle Sag Test (HAST)
A laboratory test device to determine the suspension properties of a fluid in high-angle wellbores. This test is designed to evaluate particle setting characteristics of a fluid in deviated wells.

Drilling Mud
Salt Saturation Curves

Dynamic HAST
Laboratory test device to determine the suspension properties of a drilling fluid under high angle and dynamic conditions.

Specialized Filtration Testing
FANN 90 Dynamic filtration testing of a drilling fluid under pressure and temperature. This test determines if the fluid is properly conditioned to drill through highly permeable formations. The test results include two numbers: the dynamic filtration rate and the cake deposition index (CDI).
The dynamic filtration rate is calculated from the slope of the curve of volume versus time. The CDI, which reflects the erodability of the wall cake, is calculated from the slope of the curve of volume/time versus time. CDI and dynamic filtration rates are calculated using data collected after twenty minutes. The filtration media for the FAN 90 is a synthetic core. The core size can be sized for each application to optimize the filtration rate.

Particle-PluggingTest (PPT)
The PPT test is accomplishedwith a modified HPHT cell to examine sealing characteristics of a drilling fluid. The
PPT, sometimes known as the PPA (particle-plugging apparatus), is key when drilling in high-differential-pressure environments.

Aniline Point Test
Determine the aniline point of an oil-based fluid base oil. This test is critical to ensure elastomer compatibility when using nonaqueous fluids.

Particle-Size Distribution (PSD) Test
The PSD examines the volume and particle sizedistribution of solidsinafluid.This test is valuable indetermining
the type and size of solids control equipment that will be needed to properly clean a fluid of undesirable solids.

Luminescence Fingerprinting
This test is used to determine if contamination of a synthetic-based mud has occurredwith crude oil during drilling
operations.

Lubricity Testing
Various lubricity meters and devices are available to the industry to determine how lubricous a fluid is when exposed to steel or shale. In high-angle drilling applications, a highly lubricious fluid is desirable to allow proper transmission of weight to the bit and reduce side wall sticking tendencies.

OilWell Drilling Books Page.5

drillingthe biggest collection of oil well drilling books such as Drilling bits, Drilling Fluids, and Casing, links updated from time to time, all you have to do is to click on the icon under the required book name:

  Casing Dimensions, Materials and Strength

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  Drilling Fluids Presentation   

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  Introduction to Directional Drilling

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  Well head Components

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  BOPE – Description and Selection

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Casing Heads

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Coordinates Systems
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Drill Bits Hydraulics Calculations
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  Drilling Fluids
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  Principles of Rheology
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   SLP Well Coordinates and Directions
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  Bit Hydraulics Optimization
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  BOPE – Testing Procedures

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  Survey Calculations

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Cementing Flashs

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Operation of Downhole Motor
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 Side Track Procedures
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MWD and LWD Measurement Tables
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Anti Collision
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Well Design Fundamentals
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Well Planning
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Drilling Manual Planning
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Well Cementing Part.1
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Well Cementing Part.2
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Horizontal Directional Drilling Guidelines
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Well Control Methods
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Drilling Calculations part.1
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Quick Drilling Calculations
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Technical Data Book for Well Control
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Pressure Control During Oil Well Drilling
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Wellhead Safety
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Acidizing
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Well Control Equations
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Modern Well Test Analysis
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MWD
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Drilling preliminaries
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Subsurface Safety Equipment
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Drilling Data Handbook DDH

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Weatherford Casing Hardware
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What is Wellhead?
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Oil and Gas Well Drilling Illustrated Glossary RAR
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Managing Drilling Operations Ken Fraser

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Field Maintenance of Drilling Fluids


الصيانة الحقلية لوحدة سوائل الحفر

 Field Maintenance of Drilling Fluids

مصطفى عبدالستار


يضم سائل الحفر الية عند دورانة داخل البئر قطع الصخور المحفورة او المنسلخة من على جدار البئر. والتي بالامكان بقائها على شكل قطع صخور او رمل او بالامكان ان تتما او تتشتت داخل السائل مودية الى زيادة لزوجتة. ان عزل الاجزاء الصلبة الغير مرغوب بها تتم عادة بوسائل ميكانيكية مثل الغرابيل الهزازة (shale shakers) عازلات سايكلون (cyclone separators) النابذات المركزية(centrifuges)  وغيرها وفيما يلي عمل كل منها :

  1. الغرابيل الاهتزازية (The Shale Shakers)

 يمر سائل الحفر العائد من البئر الى السطح على الغربال(الغرابيل) الهزازة ان تعزل قطع الصخور المحفورة عن السائل. يتكون الغربال الهزاز من غرابيل دوارة او اهتزازية ذات فتحات مناسبة للسماح بمرور سائل الحفر والمواد الصلبةالعائدة لة مثل البنتونايت والبرايت ونفس الوقت عدم السماح بمرور قطع الصخور المحفورة. يبين الشكل الاتي يبين غربال اهتزازي ذو سطح  واحد . 

Drilling Fluids Shale Shaker diagram 

يتم جمع قطع الصخور المعزولة في حقل تفريغ وترما كنفايات او ربما تستعمل من قبل الجيولوجي لتحليل التكوين.

ان سائل الحفر الذي يمر من خلال الغربال الاهتزازي يسقط (الشكل الموضح اعلاة) في حفر انحدار تدعي مصيدة الرمل والتي تقع تحت الغربال الاهتزازي مباشرة حيث تعزل بواسطة الترسيب (بسبب الجاذبية) قطع الصخور المحفورة  الصغيرة الحجم والتي تمكنت من العبورخلال الغربال الاهتزازي. توجد في مصيدة الرمل صمامات والتي يتم فتحها بين فترة واخرى لابعاد الجسيمات الصلبة الموجودة فيها تدعى صمامات تفريغ.      

  1. 2.  مزيلات الرمل والغرين (Desanders And Desilters)

 يرسل سائل الحفر العائد من الغربال الاهتزازي الى طاقم من مزيلات الرمل والغرين بواسطة  مضخة نابذة (Centrifugal Pump) صغيرة. ان مزيلات الرمل والغرين عبارة عن هايدروسيكلون (Hydro cyclones) التي تعي وفق مبدا عزل الجسيمات الصلبة عن سائل الحفر بواسطة القوة الطاردة المركزية Centrifugal Pump)) . ان احد المشاكل المهمة في منظومات سوائل الحفر هي درجة تراكم المواد الصلبة المحفورة التي تمر من خلال الغربال الاهتزازي والتي تبقى معلقة في احواض سوائل الحفر وتدور بصورة مستمرة. ان هذة الزيادة المستمرة في محتوى المواد الصلبة تؤدي الى :
– 
تقلل من سرعة الحفر.
– 
تضعف من خواص الجريان (تزداد اللزوجة – مقاومة الجل – الكثافة).
– 
تسبب استهلاك مبكر للمضخة وتقلل من فترة اشتعال الحافرة.

لذالك بين فترة واخرى يجب رمي جزء من سائل حفر المنظومة الى احواض السائل الاحتياطية مع اضافة كمية اضافية من الماء لتحضير الحجم المطلوب. ومن ثم يجب اضافة مواد تثقيل اضافية لاستعادة الكثافة المطلوبة. تم مواد التثقيل في السائل المرمى تمثل خسسارة مادية. ولغرض تفادي هذة الخسارة فقد تم استعمال عازلات سايكلون في اعمال الحفر التي بامكانها ان تنبذ المواد الصلصالية ذات الكثافة المنخفضة فقط (وزن نوعي =2.5 – 2.7) وتستبقي البرايت ذو الكثافة العالية. هذة المعداد ان صممت بصورة صحيحة فانها تنبذ 75%  من الصلصال وتستبقي 80 -90 % من البرايت هذة وان عازلات سايكلون بامكانها ان تستبقي في ظروف عمل ملائمة مواد فقدان سائل الحفر الخشنة. ان عازلات سايكلون لا تحتوي على اجزاء داخلية متحركة وانما تتكون من :-

أ‌  مقطع اسطواني علوي مع انبوب دخول مماسي(الشكل الموضح اسفل ). وان المقطع الاسطواني مزود بانبوب موجة للدوامة (Vortex Finder Pipe) والذي نهايتة السفلى تكون اسفل نهاية انبوب الدخول مباشرة. ان السائل النظيف يخرج من خلال انبوب موجة الدوامة نحو الاعلى.

ب‌  مقطع مخروطي ينتهي بمقطع اسطواني ذو قطر صغير (Apex) والذي خلالة تخرج المواد الصلبة.

Drilling Fluids Machine Oil well Drilling Fluids

يحفز سائل الحفر بصورة مماسية في دخول الهايدروسايكلون وتقود القوة النابذة المركزية الناتجة الجسيمات الصلبةالى جدار الهايدروسايكلون وفي النهاية يتم تصريفها من الطرف الاسفل Apex مع حجم من سائل الحفر ويعاد تصريفة الى احواض سائل الحفر تحت ظروف الضغط الجوي. اما الجزء السائل من طين الحفر فانة يخرج من الجهة العليا للهايدروسايكلون كفائض ويرسل الى احواض سائل الحفر الاحتياطي. ان السائل ومحتوى الاجزاء الصلبة الذي يمر من الفتحة العليا والفتحة السفلى بالامكان السيطرة عليها بواسطة تغيير هبوط الضغط عبر السايكلون. لقد تم ترتيب الهايدروسايكلونات بحيث يكون بامكانها عزل جسيمات الرمل الكبيرة   µm 74 ≤  . وهذا الطاقم من الهايدروسايكلون يوصف بمزيلات الرمل Desander.  يوجة سائل الحفر من مزيلات الرمل الى مزيلات الغرين Desilter التي تبعد الجسيمات الصلبة الصغيرة والتي قطرها بحدود 2 – 74 مايكروميترµm . وبما ان حجم جسيمات البرايت اقل من 74 µm  فان ذلك يعني عدم امكانية استعمال مزيلات الغرين في سوائل الحفر المثقلة. ان مجال حجم التشغيل لاي نوع من الهايدروسايكلون تعتمد على القطر الداخلي للجزء المخروطي يبين الشكل في الاسفل الحجم المكافئ لسائل الحفر المبعد لاحجام مختلفة من الهايدروسايكلون.

Drilling fluids Hydrocyclone Ordering Information

– منظف سائل الحفر(Drilling Fluid cleaner)  

عند استعمال سوائل الحفر المثقلة  يتم ابدال مزيل الرمل والغرين بمنظف لسائل الحفر بهدف انقاذ البرايت. يتكون منظف السائل من مجموعة من الهايدروسايكلونات ذات قطر داخلي يساوي 4 عقدة موضوع فوق غربال ذو طاقة اهتزازية عالية وفتحات تتراوح اقطارها مابين 40-120 مايكروميتر. في هذة الترتيب يقوم الهايدروسايكلون بابعاد الرمل والبرايت اللذان يجريان نحو الاسفل ويسقطان على الغربال الهزاز. وهنا بامكان البرايت ان يمر من خلال الغربال ويعاد استعمالة ثانية بينما تحتجز جسيمات الرمل ويتم ابعادها كنفايات. هذة المنظومة لها ميزة  توفير المركبات القيمة مثل( البرايت  – KCL – النفط وسائل الحفر) كما ان المواد التي ترمى كنفايات تكون اكثر جفافا الامر الذي يؤدي الى تقليل مشاكل التصريف والابعاد.

drilling fluids cleaner

drilling fluids cleaner components

النابذات(Centrifuges

تستعمل النابذات القوة الطاردة المركزية  ايضا لعزل المواد الصلبة الثقيلة عن السائلوالمواد  الصلبة الخفيفة الداخلة في تركيب سائل الحفر تتكون نابذة تصفيف (ترويق)(Decanting  Centrifuge) السائل من اناء (Bowl) حديدي مخروطي الشكل بوضع افقي يدور بسرعة عالية يحتوي الاناء على ناقل(conveyor) ثنائي اللولب(double screw) يدوران في نفس الاتجاة وفي اتجاة دوران الاناء ايضا ولكن بسرعة ابطا قليلا. يحتوي الناقل على محور دوران مجوف(hollow spindle) الذي يدخل من خلالة سائل الحفر. يدخل سائل الحفر الى نابذة تصفيق السائل من خلال المحور المجوف ويوزع على الاناء وان القوة الطاردة المركزية التي تنتج من دوران الاناء تمسك المزيج على شكل بركة (pond)  مقابل جدار الاناء. في هذة البركة تترسب جسيمات الرمل والغرين مقابل الجدران وتقوم ريش الناقلة بقشط  او دفع المواد الصلبة المترسبة باتجاة النهاية الضيقة للاناء. اين تجمع على شكل جسيمات رطبة ولكن بدون ماء حر (طليق). السائل وجسيمات الصلصال التي يقل اقطارها عن 2مايكروميتر والتي لا يمكن فصلها بواسطة النابذات تجمع كفائض من الفتحات الكبيرة الموجودة في النهاية الكبرى (الواسعة) للاناء. في حالة تغيير لزوجة ونقطة المطاوعة ومقاومة جل سائل الحفر من الضروري معالجتة بواسطة الاضافات الكيمياوية (البنتونايت وغيرها..). بعد ذلك يتم سحب سائل الحفر النظيف بواسطة مضخات التلقيم (charge pumps) ومضخات جهاز الحفر لاستعادة دورته داخل البئر.     

عازلات الغاز (Degasifies)  

بامكان الغازات ان تدخل في سائل الحفر عن طريق ما يلي :

أ‌. من الصخور المحفورة فيما اذا كانت تحتوي على الغازات.

    ب‌. عن طريق التماس الغاز (Gas Effusion)  اذاكان ضغط هذة الغازات اكبر من الضغط الهايدروستاتي لسائل الحفر.

     ج. وبدرجة اقل عن طريق انتشار  الغاز (Gas Diffusion) بسبب فرق نسبة التركيز بالغاز.

 بصورة عامة يوجد الغاز في سائل الحفر على شكل فقاعات حرة او ممتص على سطح الجسيمات الصلبة. اذا كانت الفقاعات كبيرة فانها تتطاير او تنفصل عندمرور سائل الحفر بالغرابيل الاهتزازية بالمجرى وفي احواض سائل الحفر. اما فقاعات الغاز الصغيرة التي سرعة صعودها تكون قليلة فانها تبقى مع سائل الحفر.اذا كانت كمية الغاز الموجودة في السائل تؤثر على خواص السائل واهمها زيادة لزوجتة اضافة الى انها تؤثر سابيا على عمل المضخة. ففي هذة الحالة من الضروري استعمال وسائل خاصة لازالة هذا الغاز والتي تدعى بعازلات الغاز  (Degasifies). ان عزل الغازعن سائل الحفر مهم اثناء الحفر المتوازن (عندما يكون ضغط عمود السائل يساوي ضغط الموائع في التكوين) او عند الحفر تحت التوازن (Under Balance Drilling) اي عندما يكون ضغط عمود سائل الحفر اقل من ضغط الطبقات المحفورة بهدف الحصول على سرعة حفر عالية. في مثل هذة الحالات يدخل الغاز الى البئر بصورة مستمرة من الطبقات عند توقف الحفر وفي بعض الاحيان اثناء عملية الحفر ايضا.

المصدر( هندسة الحفر/ د.حسين ربيعة)

Drilling Fluids

Drilling Fluid Definitions and General Functions

Results of research has shown that penetration rate and its response to weight on bit and rotary speed is highly dependent on the hydraulic horsepower reaching the formation at the bit. Because the drilling fluid flow rate sets the system pressure losses and these pressure losses set the hydraulic horsepower across the bit, it can be concluded that the drilling fluid is as important in determining drilling costs as all other “controllable” variables combined. Considering these factors, an optimum drilling fluid is properly formulated so that the flow rate necessary to clean the hole results in the proper hydraulic horsepower to clean the bit for the weight and rotary

Drilling Muds and Completion Systems

speed imposed to give the lowest cost, provided that this combination of variables results in a stable borehole which penetrates the desired target. This definition incorporates and places in perspective the five major functions of a drilling fluid.

Cool and Lubricate the Bit and Drill String

Considerable heat and friction is generated at the bit and between the drill string and wellbore during drilling operations. Contact between the drill string and wellbore can also create considerable torque during rotation and drag during trips. Circulating drilling fluid transports heat away from these frictional sites, reducing the chance of premature bit failure and pipe damage. The drilling fluid also lubricates the bit tooth penetration through the bottom hole debris into the rock and serves as a lubricant between the wellbore and drill string, reducing torque and drag.

Clean the Bit and the Bottom of the Hole

If the cuttings generated at the bit face are not immediately removed and started toward the surface, they will be ground very fine, stick to the bit, and in general retard effective penetration into uncut rock.

Suspend Solids and Transport Cuttings and Sloughing to the Surface Drilling fluids must have the capacity to suspend weight materials and drilled solids during connections, bit trips, and logging runs, or they will settle to the low side or bottom of the hole. Failure to suspend weight materials can result in a reduction in the drilling fluids density, which can lead to kicks and potential of a blowout.

The drilling fluid must be capable of transporting cuttings out of the hole at a reasonable velocity that minimizes their disintegration and incorporation as drilled solids into the drilling fluid system and able to release the cuttings at the surface for efficient removal. Failure to adequately clean the hole or to suspend drilled solids can contribute to hole problems such as fill on bottom after a trip, hole pack-off, lost returns, differentially stuck pipe, and inability to reach bottom with logging tools.

Factors influencing removal of cuttings and formation sloughing and solids suspension include

  • Density of the solids
  • Density of the drilling fluid
  • Rheological properties of the drilling fluid
  • Annular velocity
  • Hole angle
  • Slip velocity of the cuttings or sloughings

Stabilize the Wellbore and Control Subsurface Pressures

Borehole instability is a natural function of the unequal mechanical stresses and physical-chemical interactions and pressures created when supporting material and surfaces are exposed in the process of drilling a well. The drilling fluid must overcome the tendency for the hole to collapse from mechanical failure or from chemical interaction of the formation with the drilling fluid. The Earth’s pressure gradient at sea level is 0.465 psi/ft, which is equivalent to the height of a column of salt water with a density (1.07 SG) of 8.94 ppg.

In most drilling areas, the fresh water plus the solids incorporated into the water from drilling subsurface formations is sufficient to balance the formation pressures. However, it is common to experience abnormally pressured formations that require high-density drilling fluids to control the formation pressures. Failure to control downhole pressures can result in an influx of formation fluids, resulting in a kick or blowout. Borehole stability is also maintained or enhanced by controlling the loss of filtrate to permeable formations and by careful control of the chemical composition of the drilling fluid.

Most permeable formations have pore space openings too small to allow the passage of whole mud into the formation, but filtrate from the drilling fluid can enter the pore spaces. The rate at which the filtrate enters the formation depends on the pressure differential between the formation and the column of drilling fluid and the quality of the filter cake deposited on the formation face. Large volumes of drilling fluid filtrate and filtrates that are incompatible with the formation or formation fluids may destabilize the formation through hydration of shale and/or chemical interactions between components of the drilling fluid and the wellbore.

Drilling fluids that produce low-quality or thick filter cakes may also cause tight hole conditions, including stuck pipe, difficulty in running casing, and poor cement jobs.

Assist in the Gathering of Subsurface Geological Data and Formation Evaluation

Interpretation of surface geological data gathered through drilled cuttings, cores, and electrical logs is used to determine the commercial value of the zones penetrated. Invasion of these zones by the drilling fluid, its filtrate (oil or water) may mask or interfere with interpretation of data retrieved or prevent full commercial recovery of hydrocarbon.

Other Functions

In addition to the functions previously listed, the drilling fluid should be environmentally acceptable to the area inwhich it is used. It should be noncorrosive to tubulars being used in the drilling and completion operations. Most importantly, the drilling fluid should not damage the productive formations that are penetrated.

The functions described here are a few of the most obvious functions of a drilling fluid. Proper application of drilling fluids is the key to successfully drilling in various environments.

Classifications

a generalized classification of drilling fluids can be based on their fluid phase, alkalinity, dispersion, and type of chemicals used in the formulation and degrees of inhibition. In a broad sense, drilling fluids can be broken into five major categories.

Freshwater Muds—Dispersed Systems

The pH value of low-pH muds may range from 7.0 to 9.5. Low-pH muds include spud muds, bentonite-treated muds, natural muds, phosphatetreated muds, organic thinned muds (e.g., red muds, lignite muds, lignosulfonate muds), and organic colloid–treated muds. In this case, the lack of salinity of the water phase and the addition of chemical dispersants dictate the inclusion of these fluids in this broad category.

Inhibited Muds—Dispersed Systems

These are water-base drilling muds that repress the hydration and dispersion of clays through the inclusion of inhibiting ions such as calcium and salt. There are essentially four types of inhibited muds: lime muds (high pH), gypsum muds (low pH), seawater muds (unsaturated saltwater muds, low pH), and saturated saltwater muds (low pH). Newer-generation inhibited-dispersed fluids offer enhanced inhibitive performance and formation stabilization; these fluids include sodium silicate muds, formate brine-based fluids, and cationic polymer fluids.

Low Solids Muds—Nondispersed Systems

These muds contain less than 3–6% solids by volume, weight less than 9.5 lb/gal, and may be fresh or saltwater based. The typical low-solid systems are selective flocculent, minimum-solids muds, beneficiated clay muds, and low-solids polymer muds. Most low-solids drilling fluids are composed of waterwith varying quantities of bentonite and a polymer. The difference among low-solid systems lies in the various actions of different polymers.

Nonaqueous Fluids

Invert Emulsions Invert emulsions are formed when one liquid is dispersed as small droplets in another liquidwith which the dispersed liquid is immiscible. Mutually immiscible fluids, such as water and oil, can be emulsified by shear and the addition of surfactants. The suspending liquid is called the continuous phase, and the droplets are called the dispersed or discontinuous phase. There are two types of emulsions used in drilling fluids:

oil-in-water emulsions that have water as the continuous phase and oil as the dispersed phase and water-in-oil emulsions that have oil as the continuous phase and water as the dispersed phase (i.e., invert emulsions). Oil-Base Muds (nonaqueous fluid [NAF]) Oil-base muds contain oil (refined from crude such as diesel or synthetic-base oil) as the continuous phase and trace amounts of water as the dispersed phase. Oil-base muds generally contain less than 5% (by volume) water (which acts as a polar activator for organophilic clay), whereas invert emulsion fluids generally have more than 5% water in mud. Oil-base muds are usually a mixture of base oil, organophilic clay, and lignite or asphalt, and the filtrate is all oil.

References:
1. Drilling Equipment and Operation.
2. drilling Operation.