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Oil Well Logging

عمليات الجس في الابار

 نهير قاسم جبار – رئيس جيولوجيين أقدم


عند الانتهاء من حفر مقطع جيولوجي معين وقبل انزال البطانة تجري عمليات جس البئر والمقصود بها هي تلك العمليات التي تتضمن انزال اجهزة كهربائية الكترونية وصوتية وتسجيل صفات صخور المقطع الجيولوجي وما يحتويه من سوائل وكذلك ظروف البئر ابتداءا من قعر البئر وحتى اخر بطانة . وهناك بعض انواع المجسات ممكن تسجيلها للبئر المبطن كذلك . 
وهذا وان المعلومات التي نحصل عليها من المجسات تعتبر احسن وسيلة لتقييم الطبقات الجيولوجية من حيث المسامية والنفاذية والشواهد الهيدروكاربونية ونسبة السجيل اضافة الى صفات اخرى وتعتبر المجسات ارخص بمقدار ( 500 ) مرة من اللباب وهي ارخص ب( 5 ) مرات من المعلومات المشتقات من طين الحفر . 
واهم انواع المجسات هي :-

مجس الكثافة   FDC  Density Log

وهذا المجس يقيس كثافة الصخور والتي لها علاقة عكسية مع المسامية اذا كلما ازدادت المسامية كلما قلت الكثافة ويعتبر الانهيدرايت من اكثر الصخور كثافة اما اقلها كثافة فهي الحجر الكلسي المسامي والدلومايت ذو الفجوات . 

 

مجس النيوترون  CNL  Neutron Log   

وهذا المجس يقيس المسامية ايضا ولكن بصورة غير مباشرة اذا انه يقيس عدد ذرات الهيدروجين في الصخرة والتي لها علاقة بالمسامية عن طريق مصدر سيل من النيوترونات التي تصطدم بالهيروجين الموجود في الصخرة وكل ذرة هيدروجين موجودة في الصخرة تؤدي الى اصطياد نيوترون يصطدم بها وهكذا من معرفة عــدد النيوترونات التي اصطيدت نستطيع تقديــر عدد ذرات الهيدروجين في الصخرة وبالتالي المسامية لتلك الصخرة .

 

مجس اشعة كاما  x – ray  Gamma ray 

ويعتبر من اهم المجسات لانه يعبر عن مدى احتواء الصخرة على المواد المشعة (السجيل) Shale  او نظافتها . تختلف الصخور في اشعاعها لاشعة كاما بمقدار احتوائها على السجيل والمعادن الطينية الاخرى حيث ان الانهايدرايت والحجر الكلسي تعتبر نظيفة مقارنة مع السجيل والطفــل والســلت التي تعتبر محتوية على مواد مشعة . ويفيد مجس اشعة كاما في اجراء المقارنات الجيولوجية لمعرفة تتابع الطبقات وكذلك يفيد في تثبيت اعماق الحفر عند اجراء عملية التثقيب ( Perforation ) ويسجل اشعاع كاما بوحـدات (APT) التي تتراوح بين صفر الى 100 . 

 

مجسات المقاومة  Resistivity Logs 

وهذه المجسات تسجل مقاومة الصخور لمرور التيار الكهربائي عبرها وتعتبر السوائل الموجودة فـــي مسامات الصخور ( نفط , غاز , ماء ) ذات مقاومة مختلفة لمرور التيـــار الكهربائي فالنفط والغاز اكثر مقاومة لمرور التيــار الكهربائي من المـاء وكذلك الماء العــذب اكثر مقاومة للتيــار من المــاء المـالح وهناك ثلاثة انواع من مجس المقاومة . 

 

مجس المقاومة بعيد المدى  Deep Laterolog  

وهو يقيس المقاومة في المنطقة البعيدة عن جدار البئر وهي المنطقة التي لم تتأثر باختراق طين الحفر لها  uninvaded zone 

مجس المقاومة قريب المدى  Shallow Laterologوهو يقيس المقاومة في منطقة قريبة من جدار البئر وهي المنطقة المتأثرة براشح طين الحفـــر  ( Flushed zone  . 

جس المقاومة الدقيقة  Micro Resistivity 

وهذا المجس يقيس المقاومة عند جدار البئر اي مقاومة طين الحفر المترسب على جدران البئر  ( Mud cake ) ومنطقة قريبة جدا من جدار البئران فائدة مجسات المقاومة هي تعيين المقاطع الجيولوجية المحتوية على شواهد الهيدروكاربونية وهي مناطق الصخور المسامية ذات المقاومة العالية وكذلك تفيدنا في معرفة مستوى تلامس النفط – الماء oil- water contact حيث نلاحظ نزول قيم المقاومة بصورة تدريجية في مقطع الجيولوجي تتسم صخوره بثبات ومجانسة المسامية. 

كذلك يمكن من مجس المقاومة التعبير(بصورة غير مباشرة) عن مدى صلادة الصخرة compactness حيث ان الصخور عديمة المسامية تعطي مقاومة عالية جدا مثل الانهايدرايت والحجر الكلسي الصلد بينما يعتبر السجيل والطفل والسلت صخورا مسامية وذلك تكون مقاومتها قليلة.

 

المجس الصوتي Sonic log  

وهذا المجس يفيدنا في تقدير المسامية الاولية للصخرة primary porosity الغير متاثرة بعمليات لاحقة مثل التشقق والتصدع.

وهذا المجس يعتمد على فكرة ان الصخور الصلدة الكثيفة عديمة المسامية تكون فيها سرعة الصوت اكثر من الصخور المسامية الخفيفة حيث يتم استحداث موجة صوتية توجه للصخور وتستلم في مكان اخر ومن حساب وقت ذهاب الموجة وعودتها يمكن حساب مسامية الصخرة بصورة غير مباشرة.

كذلك يفيد المجس الصوتي في التبوء بالضغوط غير الاعتيادية خاصة اذا كان المقطع الجيولوجي يحتوي على السجيل حيث ان للسجيل تدرج ضغط معين بزيادة العمق وعند اختلال هذا التدرج نستطيع التنبوء باننا دخلت منطقة ضغط غير اعتيادي Abnormal Pressure ويعتبر المجس الصوتي من احسن المجسات لتمييز الطبقات الملحية Rock Salt.

مجس قطر البئر Caliper Log

وهذا المجس يتكون من عدة اذرع اربعة او اثنان تفتح بقطر معين يتناسب مع قطر البئر في تلك النقطة وهذا الذراع تحول مقدار فتحته الى اشارة كهربائية تسجل على ورق الجس على شكل خط يمثل قطر البئر تبعا للعمق.

فاذا كان المقطع سجيل متهدم فان اذرع المجس تنفتح باقصى ما يمكنها لان قطر البئر واسع وهذا الفتح يسجل على المجس على انه توسع في قطر البئر اما مناطق الصخور المسامية والنفاذية حيث يرسب طين الحفر طبقة رقيقة على جدران البئر mud cake نتيجة لترشيح سائل الحفر الى داخل التكوين فان قطر البئر يضيق في هذه الحالة فتنكمش اذرع المجس مما يعطي اشارة على المجس بان هنالك تضييق في البئر. ان التوسع والتضييق في قطر البئر يقاس بالنسبة الى قطر ثابت وهو قطر الدقاقة bit sise  التي حفرت البئر.

يفيد مجس قطر البئر في معرفة مناطق التهدم او ذوبان الملح وبالتالي يفيد في حساب كمية السمنت اللازمة لتبطين البئر وكذلك يفيد معرفة مناطق التضييق لاجراء تشذيب reaming لها قبل لنزال البطانة.

مجس ميلان الطبقات Dipmeter 

وهذا المجس عبارة عن قياس المقاومة الدقيقة Micro Resistivity بمسافات مختلفة عن جدار البئر لنفس الطبقة. 

فاذا كانت الطبقة مائلة وهذا ما يحدث في اطراف التركيب الجيولوجي فان التعبير عن ذلك يكون ان كل المجسات وعددها اربعة تسجل نفس المقاومة ولكن باختلاف بسيط في العمق حيث ان المجس الاول يخترق الطبقة بعمق اعلى من المجس الثاني والثاني بعمق اعلى من الثالث وهكذا ذلك نتيجة لميل الطبقة لان هذه المجسات تسجل المقاومة بمسار عمودي على جدار البئر وليس بمقدار مائل بنسبة ميلان الطبقة.

ومن الفوائد الثانوية لهذا المجس هو معرفة مقدار الانحراف البئر عن خط الشاقول deviation وكذلك معرفة اتجاه هذا الميل ( الانحراف) وهل هو باتجاه مركز التركيب crest او باتجاه اخر.

مجس تصلب السمنت Cement bond log CBL 

وهذا المجس يفيدنا في معرفة نوعية وكمية السمنت خلف البطانة حيث انه يعبر عن مقدار ارتباط البطانة بجدار البئر اذا كان هناك سمنت جيد او بقاءها معلقة في تجويف البئر اذا لم يكن هناك سمنت جيد.

وهناك مجسات اخرى مثل مجس الحرارة ومجس قياس انتاجية البئر للنفوط flow meter ومجسات تحديد النقطة الحرجة للتفجير عند حدوث استعصاء للانابيب وعملية تثقيب البطانة perforation لاجراء الفحوصات الاكمالية.

Measurement While Drilling MWD

MWD
MWD

The use of measurement and logging while drilling has matured a great deal in the last 10 years.  The use of these tools that have been developed for the oil and gas industry for use in primarily sedimentary depositional environments must be investigated in light of the goals set for EGS systems.  Let us first define what is meant in this section by the terms, realizing that the line between these two areas continue to blur.

1)      Measurement While Drilling (MWD): Tools that measure downhole parameters of the bit interaction with the rock are MWD tool.  These measurements typically include vibration and shock, mudflow rate, direction and angle of the bit, weight on bit, torque on bit, and downhole pressure.

2)      Logging While Drilling (LWD): Tools that measure downhole formation parameters are LWD tools.  These include gamma ray, porosity, resistivity and many other formation properties.  The measurements fall into several categories that are discussed below.  The oldest and perhaps most fundamental formation measurements are spontaneous potential (SP) and gamma ray (GR).  Today one or both of these traces are used mostly for correlation between logs.  Electric or formation resistivity logs are another class of logs used in oil and gas logging.  Because of the long history of these logs, several varieties have evolved.  The electrical basis of this class of logs is to measure the conductivity or resistivity of the various geologic materials and fluids in them.  The resistivity of shales vs that of a clean sand set the limits for an ideal electric log.  The fluids in the formation also are reflected in this measurement as water is conductive when found in boreholes and oil is not.  The basic use of electric logs is to delineate bed boundaries and in combination with other logs to determine gas/oil/water contacts.  Yet another class of logs is density logs.  These logs are indicative of the formation density of the material in the well bore.  These logs require either a neutron or a gamma source, and actually measure gamma ray flux differences.  Porosity tools are another class of common logging tools.  These tools normally use chemically or now more common electrically generated neutron to estimate formation porosity.  Since these logs are normally calibrated in sandstone, limestone or dolomite care has to be taken when measurements are made in different rock types.  Finally in the last few years a number of specialty tools have evolved, these include specialized formation pressure testing tools which can be run while drilling, nuclear magnetic resonance tools, and pulsed neutron spectroscopy tools to list only the most popular.

Rationale for use

In recent years the cost of an average oil and gas hole has increased dramatically, part of this cost increase has been driven by the need to go after much deeper and more complicated reserves.  This increases the risk of failure of holes drilled into these reserves.  As a reaction to increased risk, the use of LWD and MWD technology and techniques has increased.  In the final analysis, the decision to use of LWD and MWD tools depends on managing risk.  The EGS program moves the art of geothermal drilling into a new region of risk, the evaluation of the LWD and MDW technologies must be undertaken to determine the applicability of these technologies to the particular risks faced in this new effort.  It is important to realize in the EGS model, in many cases we are not going to be setting our surface casing into igneous or metamorphic rock as we have in the past.  These deeper holes may look more like the classic oil and gas hole at the shallower depths, with this in mind we begin to examine the possible uses of the LWD and MWD technologies.  We begin by listing what is commonly available.

Measurements available from current LWD/MWD oil field tools.

Mention of companies and tool or service names does not imply endorsement by Sandia National Laboratories; it appears that most companies involved in MWD and LWD have a version of these tools.  Searching the internet is a reasonable way to get most of this information.

Measurement Name: Downhole Weight On Bit Abbreviation:  DWOB
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
This trace allows the determination of the actual weight on bit at the bit.
Potential Geothermal  Use:
The DWD program has shown that this measurement can be used to detect bit-damaging events and prolong bit life
Special Conditions : None Example Tool: Schlumberger TeleScope
Baker Hughes Inteq CoPilot (service)
Measurement Name: Downhole Torque On Bit Abbreviation: DTOB
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
This trace allows the determination of the actual torque on bit.
Potential Geothermal  Use:
The DWD program has shown the use of this measurement in prolonging bit life and providing an effective drilling program.
Special Conditions : None Example Tool: Schlumberger TeleScope
Baker Hughes Inteq CoPilot (service)
Measurement Name:  Downhole flow rate Abbreviation:
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
This measurement allows the determination of the mudflow rate at or near the bit.
Potential Geothermal  Use:
The rolling float meter program has shown that this measurement in combination with a good measurement of return flow is critical in detecting lost circulation events.  The DWD program has shown the use of this measurement in detecting pipe washout and bit plugging conditions.
Special Conditions : None Example Tool: Schlumberger TeleScope
Measurement Name:  3-D Shock Abbreviation:
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
This trace used in combination with 3-D vibration is used to monitor bit conditions.  Avoiding shock loads has been shown to increase bit life
Potential Geothermal  Use:
Same as oil field  but more critical in harder formations.
Special Conditions : None Example Tool: Schlumberger TeleScope
Baker Hughes Inteq VSS (service)
Measurement Name: 3-D Vibration Abbreviation:
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
This trace used in combination with 3-D shhock is used to monitor bit conditions.  Avoiding damaging vibrations  has been shown to increase bit life, and increase ROP. Also used to determine RPM at bit
Potential Geothermal  Use:
Same as oil field  but more critical in harder formations.  RPM determination critical to avoiding several bit damaging situations.
Special Conditions : None Example Tool: Schlumberger TeleScope
Baker Hughes Inteq CoPilot (service)
Measurement Name: Direction and Inclination Abbreviation:  D&I
Class: MWD Measurement Function:
Max Temp: 175ºC Length: 25’
Current Oil Field Use :
These traces are used in directional drilling.  Both are required to control bit position
Potential Geothermal  Use:
Would be used in directional drilling applications.
Special Conditions : None Example Tool: Schlumberger TeleScope
Measurement Name: Azimuthal Natural Gamma Ray Abbreviation: GR
Class: LWD Measurement Function: Gamma Ray
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This trace measures the naturally occurring gamma radiation in several directions from the borehole. The trace is used to identify shales and clays as opposed to sands in lithologic sequences.  Processed trace is a primary correlation  trace between logs run at differing times.
Potential Geothermal  Use:
Gamma ray is primary correlation trace particularly in cased hole.
Special Conditions : None Example Tool: Schlumberger EcoScope
Measurement Name: Multi-frequency resistivity Abbreviation:
Class: LWD Measurement Function: Electric
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This measurement in oil and gas logging provide bed boundary and gas/oil/water contact information.
Potential Geothermal  Use:
Could be used in sedimentary sequence for bed boundary identification.  Usefulness in metamorphic and igneous formation is undocumented.
Special Conditions : None Example Tool: Schlumberger EcoScope
Measurement Name: Sonic Abbreviation:
Class: LWD Measurement Function: Sonic
Max Temp: 150ºC Length: 23’
Current Oil Field Use :
This measurement set provides information on porosity, mechanical rock properties and borehole stability.
Potential Geothermal  Use:
The borehole stability information would be a greatest value
Special Conditions : None Example Tool: Schlumberger sonicVision
Baker Hughes Inteq SoundTrak
Measurement Name: Multi-frequency, multi-depth resistivity Abbreviation:
Class: LWD Measurement Function:  Special/Electrical
Max Temp: 150ºC Length: 11’
Current Oil Field Use :
This tool is intended formation imaging.  Used for fracture identification and finding stress orientation.  Use in non-sedimentary formation is undocumented.  Also provides temperature data.
Potential Geothermal Use:
This tool may find use in advanced directional drilling applications.  If fracture imaging can be done in non-sedimentary geologies may be useful for fracture mapping and stress orientation
Special Conditions: Requires use of conductive mud system. Example Tool: Schlumberger GeoVision
Baker Hughes Inteq StarTrak; AziTrak
Measurement Name: Annular pressure Abbreviation: APWD
Class: LWD Measurement Function: Pressure
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This trace measures the pressure near the BHA-open hole interface.
Potential Geothermal  Use:This trace would be used to determine areas where lost circulation may be occurring.
Special Conditions : None Example Tool: Schlumberger EcoScope
Baker Hughes Inteq PressTEQ (service)
Measurement Name: Azimuthal Density Abbreviation:
Class: LWD Measurement Function: Density
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This trace measures the formation density in multiple  directions out from the bore hole.  Used in combination with other measurements for formation lithology identification
Potential Geothermal  Use:
This trace could be used in directional holes to drill along a bed boundary
Special Conditions : None Example Tool: Schlumberger EcoScope
Baker Hughes Inteq LithoTrak
Measurement Name: Compensated Neutron Abbreviation: CN
Class: LWD Measurement Function: Porosity
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This measurement is used to estimate porosity.  This trace is integrated in most logging tools as a part of a triple combo.
Potential Geothermal  Use:
Same as oil field use
Special Conditions : None Example Tool: Schlumberger EcoScope
Baker Hughes Inteq APLS
Measurement Name: Photoelectric Factor Abbreviation:
Class: LWD Measurement Function:  Special
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This trace measures the average atomic number of the formation constituents, used with density to determine mineralogy.
Potential Geothermal  Use:
Same as oil field use
Special Conditions : None Example Tool: Schlumberger EcoScope
Measurement Name: Ultrasonic Caliper Abbreviation:
Class: LWD Measurement Function: Borehole
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
Measures the hole size directly behind the bit.  Used to determine size of hole and rugosity
Potential Geothermal  Use:
Same as oil field use.
Special Conditions : None Example Tool: Schlumberger EcoScope
Baker Hughes Inteq LithoTrak
Measurement Name: Porosity Abbreviation:
Class: LWD Measurement Function: Porosity
Max Temp: 150ºC Length: 26’
Current Oil Field Use :
This trace measures the apparent porosity of the formation based on fast neutrons emitted by a neutron source.  Neutron source may be chemical or electrical in nature.
Potential Geothermal  Use:
This trace could be used to in the estimation of formation porosity
Special Conditions : None Example Tool: Schlumberger EcoScope
Baker Hughes Inteq LithoTrak
Measurement Name: Sigma Abbreviation: Ѕ
Class: LWD Measurement Function: Special
Max Temp: 150ºC Length: 26’
Current Oil Field Use : This trace is a measure of the macroscopic absorption cross section for thermal neutrons, used to determine formation water saturation. Potential Geothermal  Use:
Special Conditions : None Example Tool: Schlumberger EcoScope
Measurement Name: Pulsed neutron spectroscopy Abbreviation:
Class: LWD Measurement Function: Special
Max Temp: 150ºC Length: 26’
Current Oil Field Use : This measurement utilizes a pulsed neutron source, and gamma ray detectors to estimate formation oil content, salinity, lithology, porosity and clay content.   Potential Geothermal  Use:This trace could be used to in the estimation of porosity and the determination of lithology
Special Conditions : None Example Tool: Schlumberger EcoScope
Measurement Name: Nuclear magnetic resonance Abbreviation:
Class: LWD Measurement Function: Special
Max Temp: 150ºC Length: 39’
Current Oil Field Use : This measurement is used to determine free and bound fluid volumes, fluid type, porosity and permeability estimation. Potential Geothermal  Use:This trace could be used to in the estimation formation porosity and permeability.
Special Conditions :
Informal conversations with persons who have used this tool indicate that its use is best understood in sandstone.
Example Tool:
Sperry MRIL-WD
Baker Hughes Inteq MagTrak
Measurement Name: Seismic Abbreviation:
Class: LWD Measurement Function: Special
Max Temp: 150 ºC Length: 14’
Current Oil Field Use : Data derived from this tool is used to look-ahead for formation changes, pore pressure changes and faults. Potential Geothermal  Use:
Same as oilfield uses
Special Conditions : Requires active seismic source on surface Example Tool:Schlumberger  seismic Vision
Measurement Name: Formation Pressure Abbreviation:
Class: LWD Measurement Function: Pressure
Max Temp: 150ºC Length: 31.5’
Current Oil Field Use : This tool measures the formation pressure and fluid mobility while drilling.  This measurement can replace some drill-stem (DST) formation test measurements.  Device seals against borehole wall and isolated formation from drilling fluids for testing.  This measurement is also used to drilling optimization. Potential Geothermal  Use:Sam
e as oil field use
Special Conditions : None Example Tool: Schlumberger Stethoscope
Baker Hughes Inteq TesTrak

Commentary

As one can see from the list, most tools are rated to 150ºC.  Some measurements can be made up to 175ºC.  These tools provide limited bandwidth data to the surface via mud pulse telemetry and higher bandwidth data via large onboard memories, which can be unloaded as a part of a bit trip.

The Sandia DWD program provided a limited MWD tool for testing.  While this tool was much more difficult to operate than the mud pulse telemetry tools listed above it did provide a great deal of data and demonstrated the usefulness of a MWD tool in optimizing drilling parameters with respect to drilling performance and extending bit life.  Extensive reviews of this work have been published and presented by others, and may be of interest to some readers.  One class of measurement that was not present in the DWD tool that is present in most current MWD tool is direction and inclination, which are critical in directional drilling.  The case for MWD tools in EGS is relatively clear.  The costs associated with damaged bits and BHA components clearly indicate the need for MWD.  In the longer term, if the use of directional drilling is contemplated MWD is a requirement.

The case for most LWD tools or traces will have to be made on a well-by-well basis.  As a minimum, one must comply with the individual states requirements for logging and MWD may be a viable option to meet these requirements for deeper holes.  MWD may not be required for the full duration of the drilling effort but certainly may be a requirement in deeper directionally drilled holes to assure that the hole is completed in the target area.   As has been noted most LWD traces and tools are optimized for drilling sedimentary formations; some experience will be required to determine the optimal MWD suite for EGS drilling programs.  With this in mind, the first few EGS programs should be funded and scheduled to provide for the evaluation of a wide variety of available MWD tool.  The basic limitation on the available tools is temperature, and accommodations will need to be made in the mud system and operational procedures to maintain the tools within their operational temperature ranges long enough to allow a reasonable evaluation.  After some experience is gained with the available tools efforts to construct tools suitable for higher temperatures could begin.

While the least complex of these measurements can be telemetered to the surface via mud pulse technology, the more complex, data dense applications will require a higher speed telemetry systems and indeed may require two way communication.  Here the use of a wired pipe system such as the intelliserv system will be required.  Looking back on the previous drilling efforts such as the Long Valley drilling project, there has always been a desire by the scientific community to use these precious drilling opportunities to test new and different ideas and tools.  The adoption of a standard telemetry system such as the intelliserv network early in the life of the drilling program would encourage a well disciplined scientific program and provide a framework on which the scientific community could plan their programs in a way that would minimally interfere with the drilling program.

Summary

Just as it would not be reasonable to try to up the efficiency of the countries truck fleet  by looking solely at replacing all tires in use today with tires that lower rolling friction; rather one needs to step back and look at the entire system including routing and deadheading for a quick example;  the selection of MWD and LWD tools must be driven by the entire EGS system. The selection of tools that is currently available at lower temperatures is more that sufficient.  One needs to look at the needs as generated from a broad system outlook and then upgrade the temperature capabilities of the necessary MWD and LWD tools.  A far more challenging task is to evaluate how these tools mostly designed and used in sedimentary environments will react in the EGS environment.  This project should be undertaken early in the development of EGS wells and will require a theoretical as well as a practical side.  If as is expected that directional drilling is an important part of the overall EGS effort, understanding how to control and direct these drilling tools with the available MWD  and LWD modules will be a major effort.

Well Logs

by Ahmed Imad  

Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled. Traditionally Logs are display on girded papers shown in figure1. Now a days the log may be taken as films, images, and in digital format.

 

HISTORY

  •   1912 Conrad Schlumberger give the idea of using electrical measurements to map subsurface rock bodies.
  •    in 1919 Conrad Schlumberger and his brother Marcel begin work on well logs.

    Logging Unit
    Logging Unit
  •    The first electrical resistivity well log was taken in France, in 1927.
  •    The instrument which was used for this purpose is called SONDE, the sound was stopped at periodic intervals in bore hole and the and resistivity was plotted on graph paper.
  •    In 1929 the electrical resistivity logs are introduce on commercial scale in Venezuela, USA and Russia
  •    For correlation and identification of Hydrocarbon bearing strata.
  •    The photographic – film recorder was developed in 1936 the curves were SN,LN AND LAT
  •    The dip meter log were developed in 1930
  • the Gamma ray and Neutron Log were began in 1941.

       LOGGING UNITS

  •     logging cable
  •     winch to raise and lower the cable in the well
  •     self-contained 120-volt AC generator
  •     set of surface control panels
  •     set of downhole tools (sondes and cartridges)
  •    digital recording system

GR (gamma ray) logs measure radioactivity to determine what types of rocks are present in the well. Because shales contain radioactive elements, they emit lots of
gamma rays. On the other hand, clean sandstones emit very few gamma rays.

SP (spontaneous potential) logs indicate the permemabilities of rocks in the well by measuring the amount of electrical current generated between the drilling fluid and the formation water that is held in pore spaces of the reservoir rock. Porous sandstones with high permeabilities tend to generate more electricity than impermeable shales. Thus, SP logs are often used to tell sandstones from shales

Resistivity logs determine what types of fluids are present in the reservoir rocks by measuring how effective these rocks are at conducting electricity. Because fresh water and oil are poor conductors of electricity they have high resistivities. By contrast, most formation waters are salty enough that they conduct electricity with ease. Thus, formation waters generally have low resistivities. There are many different types of resistivity logs, which results in a confusing array of acronyms.

BHC (borehole compensated) logs, also called sonic logs, determine porosity by measuring how fast sound waves travel through rocks in the well. In general, sound waves travel faster through high-density shales than through lower-density sandstones.

 FDC (formation density compensated) logs, also called density logs, determine porosity by measuring the density of the rocks. Because these logs overestimate the porosity of rocks that contain gas they result in “crossover” of the log curves when paired with Neutron logs (described under CNL logs below).

  CNL (compensated neutron) logs, also called neutron logs, determine porosity by assuming that the reservoir pore spaces are filled with either water or oil and then measuring the amount of hydrogen atoms (neutrons) in the pores. Because these logs underestimate the porosity of rocks that contain gas they result in “crossover” of the log curves when paired with FDC logs (described above).

NMR (nuclear magnetic resonance) logs may be the well logs of the future. These logs measure the magnetic response of fluids present in the pore spaces of the reservoir rocks. In so doing, these logs measure both porosity and permeability, as well as the types of fluids present in the pore spaces.

  Dipmeter logs determine the orientations of sandstone and shale beds in the well, as well as the orientations of faults and fractures in these rocks. The original dipmeters did this by measuring the resisitivity of rocks on at least four sides of the well hole. Modern dipmeters actually make a detailed image of the rocks on all sides of the well hole. Borehole scanners do this with sonic (sound) waves, whereas FMS (formation microscanner) and FMI (formation micro-imager) logs do this by measuring the resisitisvity. These modern, essentially 3D logs are known as image logs since they provide a 360°ree; image of the bore hole that can show bedding features, faults and fractures, and even sedimentary structures, in addition to providing basic dipmeter data on the orientations of bedding.

  References:

 1-Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series
  2-Schlumberger, Log Interpretation Charts, Houston, TX (1995) 
  3-Western Atlas, Log Interpretation Charts, Houston, TX (1992