تعیین وزن بهینه گل برای حفاری چاه قائم در حضور شکستگی‌ها: مطالعه موردی چاه SIE-04 در میدان نفتی سیری

نوع مقاله : مقاله پژوهشی

نویسندگان

1 معدن، مکانیک سنگ، صنعتی امیر کبیر، تهران، ایران

2 دکتری ژئومکانیک، دانشگاه ارومیه

3 اداره زمین شناسی، شرکت نفت فلات قاره، تهران، ایران

چکیده

ارزیابی پایداری چاه در مرحله اول به‌منظور تعیین پنجره ایمن وزن گل حفاری است. تعیین حد پایین فشار گل حفاری به‌منظور جلوگیری از وقوع شکست برشی در دیواره چاه و حد بالای فشار گل به‌منظور جلوگیری از وقوع شکست کششی ( شکست هیدرولیکی)، بر اساس گرادیان شکست برشی (SFG) و گرادیان شکستگی (FG) انجام می‌شود. در این مقاله به‌منظور بررسی اثر حضور شکستگی‌ها در مکانیسم‌های شکست چاه و تعیین پنجره ایمن گل حفاری، روابط تحلیلی و مدل‌سازی عددی به روش المان‌مجزا ارائه‌شده است. برای این منظور از معیار موهر کولمب – جیگر به‌منظور بررسی شکست برشی با در نظر گرفتن اثر حضور ناپیوستگی‌ها در مجاورت دیواره چاه استفاده‌شده است. نتایج حاصل از این بررسی نشان داد که در بازه‌های مشخصی از (β) که معرف نحوه جهت‌گیری ناپیوستگی در مقایسه با امتداد تنش افقی بیشینه است، حضور شکستگی‌ها اثری بر مقاومت نهایی توده سنگ نخواهد داشت. همچنین برخلاف این تصور که شکست برشی در امتداد تنش افقی کمینه رخ می‌دهد (در این مقاله منظور برای حالتی است که θ=0)، در حالتی که اثر حضور ناپیوستگی‌ها در نظر گرفته شود، بیشترین فشار گل موردنیاز به‌منظور جلوگیری از شکست و لغزش در امتداد چاه در زاویه (β=45+ϕ_w/2=52.25) و(θ=84.25) است. همچنین در بازه‌ی (17.5≤θ≤46.5) شکستگی‌ها اثر در مقاومت نهایی شکست نخواهند داشت. فشار گل عملیاتی برای چاه موردنظر بر اساس گزارش‌های حفاری شرکت نفت فلات قاره ( Mpa) است. بر اساس روابط تحلیلی این فشار ( Mpa) تعیین‌شده است. نتایج حاصل از مدل‌سازی عددی به روش المان‌مجزا نشان داد که گسترش زون شکست پلاستیک، بیشترین جابجایی دیواره، لغزش در امتداد شکستگی‌ها و شکست‌های کششی در دو فشار تعیین‌شده تقریباً مشابه است. در حالتی که نسبت تنش افقی بیشینه به کمینه برابر با (σ_H/σ_h =1.65) در نظر گرفته شود، لغزش در امتداد شکستگی‌ها و گسترش شکستگی‌ها (زون شکست پلاستیک) در مجاورت چاه بیشتر است.

کلیدواژه‌ها


عنوان مقاله [English]

Determination of optimized mud weight for vertical wellbore drilling in fractures formation : A case study for SIE-04 wellbore in Persian Gulf oilfield

نویسندگان [English]

  • mohammad komeilian 1
  • Omid Saeidi 2
  • Mahdi Rahbar 3
1 Department of Mining & Metallurgical Engineering
2 Geomechanic Ph.D., University of Urmia
3 Geology Department, Iranian Offshore Oil Company, Tehran, Iran
چکیده [English]

Wellbore stability modeling for drilling operations is primarily to create a safe mud weight (mud pressure) window. The designed mud density will be high enough to ensure borehole stability and low enough to not fracture the formation (i.e., mud losses do not occur) Therefore, the safe mud weight should be greater than the pore pressure gradient and shear failure gradient and less than the fracture gradient. In this article in order to investigate the effect of fracture in wellbore failure mechanism and determination of safe mud window, analytical and numerical approach was conducted. In order to shear failure analysis in wellbore in fractured formation, Mohr–Coulomb, Jaeger failure criterion was used. For numerical simulation the distinct element method was used. The simulation of a wellbore in Persian Gulf for fracture formation was implemented in order to understanding the effect of discontinuity in wellbore failure mechanism. The results showed, in a specific angle (β), that represents the direction of fracture orientation relative to the horizontal stress, the fractures do not have any effect on strength of the rock mass. Furthermore, in fractured formation, shear failure do not occur in the direction of minimum horizontal stress, but the orientation of discontinuity specifies the failure and slips direction in wellbore. In this article the maximum mud pressure to prevent failure in wellbore occurs in (β=45+ϕ_w/2=52.25). Also in the range of (17.5≤θ≤46.5) the fractures do not have any effect on strength of the rock mass. The practical mud pressure which was used in drilling operation in real condition in Siri oil field was (40.55 Mpa) and the mud pressure determine in analytical solution is (36.92 Mpa). The numerical result (distinct element method) shows, the overall condition of normalized yield zone, shear failure slips along the fractures and tensile failure in two different pressure (36.92 - 40.55 Mpa) are almost similar. Parametric study results showed that in higher horizontal stress ratio (σ_H/σ_h ), wellbore is more instable and in an isotopic stress regimes the shear and slips along the discontinuity was eliminated.

کلیدواژه‌ها [English]

  • Wellbore stability analysis
  • Fracture
  • Safe mud weight window
  • Fracture gradient
  • Shear failure gradient Double
  • Distinct element method
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