Available online at http://ijcpe.uobaghdad.edu.iq and www.iasj.net Iraqi Journal of Chemical and Petroleum Engineering Vol.23 No.2 (June 2022) 35 – 42 EISSN: 2618-0707, PISSN: 1997-4884 Corresponding Authors: Name: Amel Habeeb Assi, Email: amel@coeng.uobaghdad.edu.iq IJCPE is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Geological Considerations Related to Casing setting depth selection and design of Iraqi oil wells (case study) Amel Habeeb Assi Petroleum Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq Abstract Well integrity is a vital feature that should be upheld into the lifespan of the well, and one constituent of which casing, n ecessity to be capable to endure all the interior and outside loads. The casing, through its two basic essentials: casing design and casing depth adjustment, are fundamental to a unique wellbore that plays an important role in well integrity. Casing set depths are determ ined based on fracturing pressure and pore pressure in the well and can usually be obtained from well-specific information. Based on the analyzes using the improved techniques in this study, the following special proposition can be projected: The selection of the first class and materials must be done correctly and accurately in accordance with the depth of casing preparation and the strategy in the considered field that must be taken into account definitely in the drilling and completion period, nevertheless corresponding ly in production and upkeep, conversion to an injection well or the opposite, the plug in addition to the closing stage. Features that control the depth of the casing seat have been studied, which consist of fracture gradient, pore pressure with other issues are the s urviving lithology's of rocks. Subsequently defining the casing seat can be sustained with an investigation of the determination of the suitable drilling fluid. According to the consequences of the fracture pressure and pore pressure investigation and the findings of casing setting depth by means of the bottom-up technique, the consequences are gotten to each casing for the 4 studied wells. reference point designed from the rotating table RT. For well A, the conductor casing depth is 47m, the casing surface depth is 533m , the intermediate casing setting depth is 1882 m. Finally, for the production casing depth is 3441 m. Compared to the collapse pressure method, it was found that the bottom-up method gave results that are close and similar to the real results. The results of other wells are included in the search consequences Keywords: Casing, formation, design oil well, setting depth, formation pressure Received on 08/03/2022, Accepted on 30/04/2022, published on 30/06/2022 https://doi.org/10.31699/IJCPE.2022.2.5 1- Introduction The first strategic duty in developing a well plan is to choose the depths to which the casing is to be installed and fixed. The drilling engineer must take into account geological conditions for example, fracture gradients, formation pressures, and other well problems, as well as the company's plan [1]. The results of the program should allow the well to be drilled safely without the need to build a "steel monument" to the casing chains. Inappropriately, numerous well strategies provide significant thoughts to the real pipe project, with that, just give a quick attention to the depth of tube adjustment [2]. It cannot be exaggerated if it is said and emphasized the importance of choosing the appropriate depths to adjust the casing. A basic detailed drilling application with initial information of the geological conditions in a part can help in organizing where to set the casing strings to ensure that drilling can proceed with minimal effort, as will be explained in detail in this research [3]. The choice of casing string depth of adjustment depends on the fracture gradient values of the well as well as pore formation pressure and geological factors [4]. Well integrity is a vital feature that should be upheld into the lifespan of the well, and one constituent of which casing, necessity to be capable to endure all the interior and outside loads. Casing program design includes assembling depth settings, casing ranks, and sizes that allow for secure drilling, and well completion in order to prepare for required production [5]. A variety of casing string and their respective location depths are constructed according to geologic conditions and the fresh aquifers they contain. Casing setting depth means founded from the seat sector casing depending on the fractures gradient and pore pressure information commencing to the offset wells [6]. Assortment of the casing string sizes is usually well-ordered by three main issues which are: (1) production tubing string size, (2) the number of casing strings essential for reaching the ultimate depth, and (3) the other drilling circumstances and geological aspects [7]. http://ijcpe.uobaghdad.edu.iq/ http://www.iasj.net/ mailto:amel@coeng.uobaghdad.edu.iq http://creativecommons.org/licenses/by-nc/4.0/ https://doi.org/10.31699/IJCPE.2022.2.5 A. H. Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 36 Then the number of casing strings necessary to include the hole is released, their particular setting depths and its outside diameters, steel grade, the nominal weight, and couplings of each of these strings necessity to be designated. The existing casing installed in some wells depends only on the formation of the drill weight statement mainly without reference to fracture pressure and pore pressure. Then, during production, the casing disappointment was revealed. [8]. This research aims to find the fastest and most accurate ways to determine the depth of fixing casings and compare them with what is available through the study of four wells in one of the fields in southern Iraq. 2- Methodology 2.1. Bottom Up Method Aimed at the casing setting depth purpose, the fracture gradient and pore pressure are typically termed in pound per gallon(ppg) as shown in Fig. 1. [9] Fig. 1. Fracture Pressure with pore pressure values apposite depth [9] The Eaton equation, equation 1is used for estimating the fracture gradient: F= S/D* α+(1- α) * P/D (1) where: F = fracture gradient, psi/ft. D = depth, m S = overburden stress, psi P = bottom hole static pressure, psi α = V / (1-V), α that diverges between 0.3 and 0.6. V = Poisson’s ratio, dimensionless .The thick lines in Fig. 1 are not a safety aspect; Thus, the first stage of casing preparation depth design, the safety factor should be acceptable. 0.3 ppg. is added as a safety factor for fracture gradient and pore pressure as in Fig. 2. The safety factor should be complementary to the formation pressure to maintaining the wellbore pressure between the supreme value which does not principal to fracture of the formation and the pressure of the fluid within the formations, since the pressure of the wellbore surpasses the pressure of the fracture, damage to the formation happens, that subsequently leads to the loss of circulation difficulties [9]. Fig. 2. Safety limits for fracture pressure and pore pressure [9] Two methods are used for determining the casing setting depths which are: top-down method and bottom- up. In this paper the Bottom Up Casing Design is used. This strategy will flinch on or after the bottom of the well up to superficial on the other hand, the setting depths are calculated surrounded by the safety feature restrictions (scattered lines). The techniques are as follow: Preliminary at the bottom or formation pressure intermittent line at Point A), then make a perpendicular line rising to fracture pressure intermittent line at Point B as shown in Fig. 3. Thus, the casing must be adjusted from 4,500 ft. Total Vertical Depth (TVD)to 12,000 ft. total vertical depth to cause 12,000 ft. total vertical depth to be reached with a maximum equivalent mud density. in other words, we will not be broken down the formation at shallow depth (4,500 TVD), and the same concept to another string will be applied [9]. Fig. 3. The first step of the bottom design [9] A. H. Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 37 The subsequent casing string is founded by sketch a straight line from Point B for intersecting the pore pressure intermittent line through Point C. At that point make a perpendicular line from the Point C until the fracture gradient intermittent line by the side of Point D as shown in Fig. 4. By conclusion Casing should be established from 1,800 ft. TVD to 4,500 ft. TVD [9]. Fig. 4. The second stage of the bottom design [9] Using the above steps, determine the next casing chain by sketch a straight line from point D to point E and an orthogonal line from point E to point F as shown in Fig. 5. So the casing should be adjusted from the surface to 1800 TVD[9]. Fig. 5. The third stage of the bottom design [9] Depending on the aforementioned bottom-up design principles, 3 series of casing will be required at 1800 ft. TVD, 4500 ft. TVD and finally 12000 ft. TVD as shown in Fig. 6 [16]. Fig. 6. The final step of the bottom design [9] 2.2. Top Down Method This method of design starts from the surface of the well downwards, where the depths are designed, including the limits of the safety factor, as shown in Fig. 7. Fig. 7. Top Down Method Final design (all steps) [9] Method Pressure Collapse 2.3. In order to determine the depth of installation in a section other than the lower section, the effect of axial tension on the buckling pressure must be taken into consideration, and this involves the use of either a trial and error solution or schematic solutions [10]. At some point from the upper end of the well, the buckling resistance control stops as a major and controlling factor in the design from this point to the surface and begins to control the durability of the joint and the longitudinal compliance, [11] as they are the first consideration in the design, so the lining pipes must achieve the equation 2: 𝐋𝐬 = 𝐏𝐜 𝟎.𝟎𝟓𝟐∗𝛒∗𝐍𝐬 (2) Where: Ls= Setting depth (ft.). ρ=Mud weight (ppg). Pc= Collapse pressure (psi) Ns= Design factor (dimension less), From the steps bellow, it can be seen that several stages must be calculated as an iterative process in order to calculate the depth of installation, i.e., trial and error [12]. 1- data collection, drilling lithology, pressure and geologic 2- data identification and verification 3- evaluation based on practices and standard 4- analysis: actual casing setting depth and design, and casing failure 5- recalculation casing setting depth and design by using equation 1 Tension load is created from the load of casing and power that formed axially. Established on the maximum force idea, the supreme tension force happens in order to its individual load minus the buoyancy next to casing running then in advance the cementing process [13] and as in equation 3 A. H. Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 38 BF=1- (MW /7848.6) (3) where: BF=buoyancy feature, MW=mud density (kg/m3) The extreme burst weight happens as soon as the cement is pushed through the well. The interior pressure is designed by using equation 4 bearing in mind the hydrostatic pressure related to the cement slurry [14]: Pi =Psur +Gce ×D (4) where: Pi = the internal pressure (MPa), Psur = pumping surface pressure (MPa) Gce = pressure gradient of cement slurry (MPa/m) D= casing shoe depth (m) 3- Data Collection Getting data is one of the basics and implementing to get the necessary data is from the South Oil Company SOC / Rumelia Oil Field, where it helped to prepare this research. The basic data consists of:4 wells in Rumelia oil field, depth (vertical and measured) lithology, casing program, Leak of test LOF, pore pressure and fracture pressure, mud density, formation tops, Fig. 8 shows the lithological columns for well 4 for Rumelia oil field, table 1 represent casing information. Table 1 signify Top, bottom of formation with pore and fracture pressure. Fig. 8. lithological columns for well 4 for Rumelia oil field [15] Table 1. casing information for surface, intermediate and production [15] Tubulars and Casing Hardware surface casing property MD OD Joint Weight ID Grade Collapse Burst Thread m in m lb./ft. in psi psi Prev .Casing 42 20 12.5 94 19.124 K-55 520 2110 BTC Casing 583 13 3/8 12.2 54.5 12.688 J-55 1130 2730 BTC Tubulars and Casing Hardware intermediate casing property MD OD Joint Weight ID Grade Collapse Burst Thread m in m lb./ft. in psi psi Prev. Casing 583 13 3/8 12.2 54.5 12.688 J-55 1130 2730 BTC Tubulars and Casing Hardware surface casing property MD OD Joint Weight ID Grade Collapse Burst Thread m in m lb./ft. in psi psi Prev .Casing 42 20 12.5 94 19.124 K-55 520 2110 BTC Casing 583 13 3/8 12.2 54.5 12.688 J-55 1130 2730 BTC Tubulars and Casing Hardware intermediate casing property MD OD Joint Weight ID Grade Collapse Burst Thread m in m lb./ft. in psi psi Prev. Casing 583 13 3/8 12.2 54.5 12.688 J-55 1130 2730 BTC Casing 2036 9 5/8 12.2 47 8 37/50 L-80 4750 6870 Vam Top Tubulars and Casing Hardware production casing property MD OD Joint Weight ID Grade Collapse Burst Thread m in m lb./ft. in psi psi Prev. Casing 2036 9 5/8 12.2 47 8 37/50 L-80 4750 6870 VAM TOP Casing 2557 7 12.2 29 6.185 0 7030 8160 VAM TOP A. H. Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 39 Table 2. Top, bottom of formation with pore and fracture pressure. Formation Top Md m Bottom Md m Bottom TVD m Top ED SG Frac Bottom ED Frac SG Top ED Pore SG Bottom ED SG Pore Formation 0.0 412.0 412.0 1.34 1.34 1.11 1.11 Sandstone 412.0 694.0 694.0 1.26 1.72 1.11 1.08 Limestone 694.0 794.0 794.0 1.72 1.71 1.08 1.08 Sandstone 794.0 1050.0 1050.0 1.70 1.63 1.08 1.06 Sandstone Dolostone 1.06ذ 1.06 1.73 1.58 1088.0 1088.0 1050.0 1088.0 1530.0 1530.0 1.73 1.72 1.06 1.06 Dolostone 1530.0 1690.0 1690.0 1.72 1.70 1.06 1.07 Dolostone 1690.0 1840.0 1840.0 1.70 1.70 1.07 1.08 Shale 1840.0 2034.0 2034.0 1.71 1.72 1.08 1.11 Dolostone 2034.0 2182.0 2182.0 1.72 1.70 1.11 1.13 Limestone 2182.0 2227.0 2227.0 1.70 1.76 1.13 1.12 Shale 2227.0 2557.0 25570 1.80 1.80 1.12 1.11 Limestone 2557 3440 3440 1.81 1.8 1.13 1.12 Limestone 4- Results and Discussion Many methods are used for determining the casing setting depth for instant the bottom-up technique and the collapse pressure method, which were used in this study. In general, the bottom-up technique used in development wells. in some circumstances, the exploration wells also use the down top method when facing complex lithology circumstances and nonstandard pressure. This effort included two methods for choosing the depth of installation, as it was found that both methods gave results that are close to the truth, but the bottom-up method was more accurate and closer to reality than the design factors method as in Table 3. In hydrocarbon wells it collapses, bursts, in addition to the axial tension demands that must be taken into account when choosing a casing adjusting depth. Geological interpretation during the design of the casings is a very important factor, especially knowing the location of the top layer, while sitting the casing and this was proven by Boniface and Marcus,2015. Table 3. Formations with facture, pore pressures and lithology for the studied wells Well Casing Setting Depth (m) from Down up Casing Type Hole Size in casing Size in Casing Setting Depth (m) from collapse pressure Actual Casing Setting Depth (m) A-1 47 Conductor 26 20 44 46.5 583(10m in Top of Dammam) Surface 17.5 13 3/8 530 582 1882 (15m into Top of Sadi) Intermediate 12.25 9 5/8 1881 1882 3441 ( 50m above Upp er Shale) Production 8.5 7 3440 3442 A-2 53 Conductor 26 20 51 52 544(12m in Top of Dammam) Surface 17.5 13 3/8 542 543 1892 (10m into Top of Sadi) Intermediate 12.25 9 5/8 1891 1892 3431 ( 40m above Upp er Shale) Production 8.5 7 3430 3431 A-3 45 Conductor 26 20 44 44 533(11m in Top of Dammam) Surface 17.5 13 3/8 532 532.5 1882 (20m into Top of Sadi) Intermediate 12.25 9 5/8 1882 1881 3440( 33m above Upp er Shale) Production 8.5 7 3439 3440 A-4 56 Conductor 26 20 555 55 543(9m in Top of Dammam) Surface 17.5 13 3/8 541 542 1889 (2m into Top of Sadi) Intermediate 12.25 9 5/8 1887 1888 3441 ( 40m above Upp er Shale) Production 8.5 7 3449 3440 For Table 4, it represents the frac. and pore pressures for the studied formations, where the highest value for frac. was. 1.8 for limestone formation at a depth of 2256 m, the lowest value for frac. It was 1.26 for limestone formation at a depth of 694 meters. As for the pore pressure, the lowest value was 1.06 for the formation of dolomite at a depth of 1088 meters and the highest value for clay at a depth of 2227 by 1.13. This indicates that the density of the mud used should be less than 1.26 and higher than 1.06 to ensure a safe drilling process without losses or kick. Well security is responsible for indicating casing design principles and superior practices to ensure good casings, and as in Table 3 and Table 5 which represent well security for well A-1 as a sample by using CemCADE software. (Certainly, CemCADE is specific to cement, but in this research, it was used to find the security of the well, which is one of the results given by the aforementioned program Table 4. Well Security for well A-1 for production section Security of well Station Explanation Minimum Differential Pressure Depth m Time hr:mn Accomplishment Fracture 657 2036 06:00 Accomplishment Production 352 2182 06:50 Accomplishment Burst 7247 zero 08:00 Accomplishment Collapse 6505 2557 08:40 Table 5. The designing of casings for well A-1 Casing type MD m OD in Joint m Weight lb./ft. ID in Grade Collapse psi Burst psi Thread conductor 42.0 20 12.5 94.0 19.124 K-55 520 2110 BTC surface 583.0 13 3/8 12.2 54.5 12.688 J-55 1130 2730 BTC intermediate 2036.0 9 5/8 12.2 47.0 8 37/50 L-80 4750 6870 Vamp production 3440 7 12.2 29.0 6.185 L-80 7030 8160 BTC A. H. Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 40 Fracture pressure is a serious factor for drilling fluid weight designing in the oil wells manufacturing. Leak-off test information for the used drilling fluid for well A-1 are investigated, and fracture pressure expectation technique for the studied well (drilling can continue drilling under protective casing towards the ahead next casing point, in other words just double checking for the mud density for the next hole). If the drilling fluid pressure surpasses the native tensile failure pressure for the studied formation, in other words, fracture pressure times versus vertical depth, a fracture is founded. For such these cases, the pore pressures frequently are uncharacteristically high and may be surpass what otherwise are innocuous drilling fluids pressures. For the well-studied, the drilling fluid used is considered safe as in Fig. 9 and Fig. 10. The red line in Fig. 9 is designed for safety issue; Thus, the first stage of casing design is the safety factor, considering the test limits of borehole drilling. The goal is to avoid drilling difficulties anytime drilling fluid is circulated and this has been proven by Syazwan et.al.2016 [16]. As for the blue line, it was designed without relying on safe limits, and thus leads to damage to the casings during the rotation of the drilling mud and during later production processes, and this was confirmed by Zhang and Yin,2017 [17] about the importance of taking into consideration the impact of fracturing pressure and fluid pressure in the formation. Fig. 9. Fracture and pore pressure limits by leak-off tests Fig. 10. Fracture and pore pressure values with depth for well A-1 5- Conclusions 1- This effort presents bottom down method to select casing setting depth for 4 wells. The bottom-up technique is in general used for conventional drilling, nevertheless, not wholly development wells depends on the bottom-up technique. 2- Casing evaluation and substantial collection should be directed correctly and exactly as stated by the formation type of the studied Field because of their important at choosing setting depth processes. 3- Depending on casing set depth investigation by using the bottom down method. the subsequent specific suggestion may be projected: the mud density should be selected 0.41 to 0.25 pounds/gallon above the value needed to create a hydrostatic pressure that balances the pressure of the fluids in the penetrating layers. 4- Decision-creation process and is predominantly beneficial for which method is give the exact setting depth is important issues. It was found that the down- up method is possible and successful to be used and applied in the fields of southern Iraq, such as the Rumaila field in southern Iraq, which was studied in this research. Recommendations For future works, the use of the bottom-up technique for directional and horizontal drilling can be tried as it was used for vertical drilling and proved successful. It is also possible to recommend the use of the top-down method and compare its results with the results of the top-down method. References [1] American Petroleum Institute (API), "American Petroleum Institute (API)", API RP 7G: Recommended Practice for Drill Stem Design and Operating Limits, sixteenth ed. American Petroleum Institute (API), Washington, D.C. 1998 [2] American Petroleum Institute (API), "API Spec 10 a: Specification for Cements and Materials for Well Cementing", twenty-third ed. American Petroleum Institute (API), Washington, D.C. 2005. 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Assi / Iraqi Journal of Chemical and Petroleum Engineering 23,2 (2022) 35 - 42 42 االعتبارات الجيولوجية المتعلقة بتحديد عمق الغالف واختيار عمق آبار النفط العراقية )دراسة حالة( امل حبيب عاصي العراق ,بغداد ,جامعة بغداد,قسم هندسة النفط الخالصة حيوية يجب الحفاظ عليها في العمر االفتراضي للبئر ، ومن أحد مكونات الغالف ، سالمة البئر هي ميزة أساسيتين: خالل من ، الغالف يعتبر والخارجية. الداخلية األحمال جميع تحمل على قادًرا يكون أن يجب المة البئر. تصميم الغالف وتعديل عمق الغالف ، وهو أساسًيا اثناء ثقب البئر حيث يلعب دوًرا مهًما في س عليها الحصول ويمكن البئر في المسام وضغط التكسير ضغط على بناًء الغالف مجموعة أعماق تحديد يتم عادًة من معلومات محددة جيًدا. بناًء على التحليالت باستخدام التقنيات المحّسنة في هذه الدراسة ، يمكن توقع الدرجة اختيار يتم أن يجب التالي: الخاص إعداد االقتراح لعمق وفًقا ودقيق صحيح بشكل والمواد األولى الغالف واالستراتيجية في المجال المدروس الذي يجب أن يؤخذ في االعتبار بشكل قاطع في فترة الحفر و بعد االنتهاء منها ، مع ذلك في المقابل في اإلنتاج والصيانة ، والتحويل إلى بئر الحقن أو العكس ، باإلضافة إلى ا ، مرحلة الكسر تدرج من تتكون والتي ، الغالف مقعد عمق في تتحكم التي الميزات دراسة تمت إلغالق. وضغط المسام مع مشكالت أخرى هي القطع المتبقية من الصخور. يمكن الحفاظ على تحديد مقعد الغالف من تحديد سائل الحفر المناسب. وفًقا لنتائج فحص ضغط الكسر وضغط الم عمق خالل التحقيق في سام ونتائج ضبط الغالف عن طريق تقنية من أسفل إلى أعلى ، تم الحصول على النتائج على كل غالف لآلبار األربعة 47، يبلغ عمق غالف االعلى A.بالنسبة للبئر المدروسة. من النقطة المرجعية المصممة من المنضدة الدوارة متًرا. أخيًرا ، لعمق غالف 1882متًرا ، وعمق إعداد الغالف المتوسط 533متًرا ، وعمق الغالف السطحي هو اعلى 3441اإلنتاج اسفل التصاعدية الطريقة أن وجد فقد ، االنهيار ضغط طريقة مع بالمقارنة متر. حقيقية. تم تضمين نتائج اآلبار األخرى في نتائج البحث أعطت نتائج قريبة ومماثلة للنتائج ال الغالف ، التكوين ، تصميم بئرالنفط ، ضبط العمق ، ضغط التكوين الكلمات الدالة: