2 2 casing design

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Network of Excellence in Training Casing Design Principles © COPYRIGHT 2001, NExT All Rights Reserved Casing Design Principles • Lecture Contents; – Lecture Objectives, – Definitions, – Casing Design Criteria, • Collapse, • Burst, • Tension © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Lecture Objectives – At the end of this lecture YOU will be able to: • Be familiar with the design criteria that is taken into consideration when a casing string is to be designed with respect to: – Collapse, – Burst, – Tension List the design safety factors â COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Definition; – Casing design involves the determination of factors which influence the failure and the selection of the most suitable casing grades and weights for specific operations, – The casing program should also reflect the completion and production requirements, – A good knowledge of the stress analysis and the ability to apply it are necessary for the design of casing strings © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Definition; – The end product of such design is a pressure vessel capable of withstanding the expected internal and external pressures and axial loading, – Hole irregularities further subject the casing to bending forces which must be considered during the selection of casing grades, – In general, the cost of a given casing grade is proportional to its weight, the heaviest weight being the most expensive, – And hence the designer must ensure the cheapest and the best quality © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Casing design criteria are: • Collapse, • Burst, • Tension (weight, bending & shock load), • Triaxial, • Other service loads (Corrosion, wear, H2S, High Temperature etc.) © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Collapse Pressure: • This pressure originates from the column of mud used to drill the hole and acts on the outside of the empty casing, • Since the hydrostatic pressure of a column of mud increases with depth collapse pressure is highest at the bottom and zero at top Pressure Pressure © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Basic Collapse Assumptions: • Casing is empty due to lost circulation at shoe or at TD, • Internal pressure inside casing is zero, CSD • External pressure is caused by mud in which casing was run in, No cement outside casing TD â COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Collapse (C): • C = mud density x depth x acceleration due to gravity, • C = ρgh, • C = 0.052 ρ h • Collapse pressure at shoe: External pressure - Internal pressure • Collapse pressure at surface = © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • The burst criterion is normally based on the maximum formation pressure resulting from a kick during the drilling of the next hole section Pressure 10 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • Gas to Surface: – This is an extreme case, – For added safety it is assumed that the influx fluid (gas) displaces the entire drilling mud, CSD – This will subject the inside casing to bursting effects of formation pressure TD 11 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • Gas to Surface: – At the top of the hole, the external pressure due to hydrostatic head of mud is zero and the internal pressure must be supported entirely by the casing body, – Therefore, the burst pressure is highest at the top and lowest at the casing shoe where internal pressure is resisted by the external pressure originated from fluids outside the casing 12 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • Gas to Surface: – In conventional casing design, it is customary to assume a gas kick, thereby anticipating the worst possible type of a kick, – The gas gradient is of the order of 0.1 psi/ft This gradient causes a small decrease in formation pressure as gas rises up the well, – Casing seat should be selected so that gas pressure at the casing shoe is less than the formation breakdown at the shoe, – In exploration wells where reservoir pressure is not known, formation pressure from the next openhole section is calculated form the maximum mud weight 13 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • Burst pressure at surface (B1) = Pf - G x TD, • Calculate the internal pressure (Pi) at the shoe using the maximum formation pressure at next hole TD, assuming the hole is full of gas: – Pi = Pf - G x (TD – CSD) – Pe = 0.465 x CSD, • Burst pressure at shoe (B2) = Pi - Pe – B2 = Pf - G x (TD - CSD) - 0.465 x CSD 14 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Example: • • • • • • • • 5/8” casing, CSD = 5000 ft, TD = 10000 ft, Pf = 5000 psi, Mud for 5/8” casing = 10 ppg Collapse at shoe = 10 * 0.052 * 5000 = 2600 psi, Burst at surface = 5000 - ( 0.1 * 10000 ) = 4000 psi, Burst at shoe = { 5000 - ( 0.1* [ 10000 - 5000] ) } - { 0.465 * 5000} = 4500 - 2325 = 2175 psi, • Selection L80 40lb/ft, B = 5750 psi, C = 3090 psi 15 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Most of axial tension arises from the weight of the casing itself, • Other tension loadings can arise due to: – Bending, – Drag, – Shock loading and during pressure testing of casing • In casing design, the uppermost joint of the string is considered the weakest in tension as it has to carry the total weight of the casing string 16 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Tensile forces are determined as follows: – Calculate weight of casing in air (positive value) using true vertical depth, – Casing air weight = casing weight (lb/ft) * hole depth (TVD), – Calculate buoyancy for (negative value), – BF = Pe (Ae - Ai) for open-ended casing, – BF = Pe Ae - Pi Ai for closed casing 17 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Pressure area method Ae Ae Ai Ai Pi 18 Pi Pi Open-ended CSG Pe Closed CSG © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Example: – 20”, ID = 18.71 in, 133 lb/ft, open-ended, – CSD = 2800 ft, Mud = 10 ppg • Solution using pressure-area method; – Air weight = 2800 ft * 133 lb/ft = 372,400 lbs, – B Force = Pe (Ae - Ai) for open-ended casing, – BF = 0.052 * 10 * 2800 (314.16 - 274.94) = 57,104 lb, – B weight = 372,400 - 57,104 =315,295 lbs 19 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Solution using Buoyancy Factor method; – Air weight = 2800 ft * 133 lb/ft = 372,400 lbs, – BF = ( 1- 10/65.4 ) = 0.847 – BW = 372,400 * 0.847 = 315,422 lb (315,652 lb from pressure area method) 20 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Tension: • Calculate bending force in deviated wells (positive value), – Bending force = 63 Wn * OD * Θ, Wn = wt of casing (lb/ft) positive force, Θ = dogleg severity (deg/100ft), (use 2deg/100 as an average) • Calculate pressure testing force: – The casing should be tested to the maximum pressure for which it has been designed – Force due to pressure = π/4 (ID2) * test pressure 21 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Safety Factors: • Collapse : 1.0, • Burst : 1.1, • Tension : 1.3 - 1.6 22 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Now You should be able to: – Be familiar with the design criteria that is taken into consideration when a casing string is to be designed with respect to: • Collapse, • Burst, • Tension – List the usable design safety factors 23 © COPYRIGHT 2001, All Rights Reserved Network of Excellence in Training Casing Design Principles End of Lecture © COPYRIGHT 2001, NExT All Rights Reserved .. .Casing Design Principles • Lecture Contents; – Lecture Objectives, – Definitions, – Casing Design Criteria, • Collapse, • Burst, • Tension © COPYRIGHT 2001, All Rights Reserved Casing Design. .. expensive, – And hence the designer must ensure the cheapest and the best quality © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Casing design criteria are: •... from fluids outside the casing 12 © COPYRIGHT 2001, All Rights Reserved Casing Design Principles • Design Criteria; – Burst: • Gas to Surface: – In conventional casing design, it is customary
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