Casing Design By Dr. Khaled El-shreef 1 Casing Design CONTENTS Function of Casing Casing Types & Tools Strength Properties Casing Specification Casing Design 2 1
RUNNING AND CEMENTING CASING Reasons for Running Casing Provide a means of controlling well pressures. Permit circulation. Prevent collapse of hole. Prevent fluid migration. Isolate troublesome zones. Facilitate control of a production well. 3 Types of Casing and Common Size Conductor 20: 24 Surface Casing 13 3/8 Intermediate String 9 5/8 Production String 9 5/8: 7 Liner 7: 5" 4 2
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Conductor Casing Characteristics: Casing is usually large: 20 in. to 30 in. diameter. The hole may be severely eroded. The setting depth of the conductor can vary from as little as 20 ft to as much as a few hundred feet. The most common pipe and hole sizes are a 16 in. pipe in a 20 in. hole and 20 in. pipe in a 26 in. hole. 7 Conductor Casing Conductor Casing is set to : Prevent washing out under rig. Provide elevation for flow line. Provide support for part of the wellhead. A BOP is usually not attached to conductor casings. 8 4
9 Surface casing Surface casing is usually the second string of pipe set in the well. when a conductor casing is not set because, the surface pipe becomes the first string set. 10 5
Surface casing Surface casing is set to: Protect fresh water sands. Case unconsolidated formations. Provide primary pressure control (BOP usually nippled up on surface casing). Support future casings. Case off potential loss circulations zones. Casing sizes normally range from 13-3/8in. on shallow wells to 20 in. on deep, multistring wells. Guide shoe, or float shoe, float collar and centralizers are commonly used 11 12 6
13 Intermediate Casing The intermediate casing strings extend from the surface to formations able to hold the expected mud weights at greater depths. This depth can vary several thousand feet in a single stage job. When a second intermediate string is set, the casing is run to just below the weak zone to a firm formation and cemented at that point. 14 7
Intermediate Casing Intermediate casing is set to: Separates hole into workable drilling segments and cases off loss circulation zones, water flows, etc. isolates salt sections. protects open hole from increases in mud weight. prevents flow from high pressure zones if mud weight must be reduced. Basic pressure control casing. BOP always installed. Supports subsequent casings. 15 Production Casing The production casing is the last full string of pipe set in the well. It extends from below the deepest producing formation to the surface- Production tubing, downhole pumps, and other equipment needed to produce oil and gas are housed in this casing. The production-casing cement must give a pressure-tight seal between the formations and the production casing. It is essential to isolate the reservoir from fluids both within the producing zone itself and from other zones. 16 8
Production Casing Since the production casing may extend from the total depth of the well to the surface, the setting depth can vary from a few thousand feet to as much as 14000 feet. Below 14000 ft, liners may be set because of cost savings and less pipe weight. The size of the casing depends upon the number of strings of production tubing to be run into the well and the size of production equipment used. 17 Production Casing Characteristics Common size 4-1/2 in., 5-1/2 in., and 7 in. casing. Drilling mud usually of good condition. Usually not circulated. Generally cemented back to intermediate casing. Good cement job is vital to successful completion. 18 9
Production Casing Production casing is set to: Complete well for production. Effect zonal isolation. Protect pay zones from unwanted fluids. Provide pressure control. Cover worn or damaged intermediate casing. 19 Liners String A liner is a string of casing that does not reach the surface. They are hung on the intermediate casing by use of suitable arrangement of a packer and slips called a liner hanger. 20 10
Types of Liners String Drilling liners, used to isolate lost circulation or abnormally pressured zones to permit deeper drilling. Production liners, run instead of a full casing to provide isolation across the production or injection zones. Tie back liner, a section of casing extending upwards from the top of an existing liner to the surface. 21 Types of Liners String The scab liner, a section of casing that does not reach surface. It is used to repair existing damaged casing. It is normally sealed with packers at top and bottom and in some cases is also cemented. The scab tie back liner, a section of casing extending from the top of an existing liner but does not reach the surface and is usually cemented in place. 22 11
Liner String The liner string consists of: The cementing head with a drill pipe wiper dart in place. A drill pipe swivel if movement is considered. Drill pipe. Liner-setting mechanism. Latch-in liner wiper plug. A mechanical set or hydraulic set liner hanger. Floating equipment. 23 Liner hangers are set mechanically or hydraulically. The float shoe and float collars are spaced two to five joints apart. The liner plug landing collar is located some distance above the float collar. Most liner hangers are equipped with a tie-back receptacle should the need arise to run a tie-back string to surface or to run a scab liner. The tie-back sleeve is usually a minimum of six feet in length and fluted for easier entry. Production liners are many times equipped with a polished bore receptacle to serve as a seal assembly placement facility when production string is run in the well. 24 12
Hole Conditions Sloughing In many cases, this is the purpose of setting an intermediate casing and it can create several cement problems, such as bridging the annulus, sticking the casing, and increasing the annulus hydrostatic pressure. Drill Pipe Drag Where the drag is occurring, and exactly why it exists, could be important. This condition may indicate the need for centralized casing or the use of fluid loss control cements. 25 Low Pressure Zone: Hole Conditions One of the most persistent problems is incompetent formations that will not support effective columns of cements. Mud Condition: A well-conditioned mud greatly increases the mud removal capability of the flushes and cement slurry. 26 13
Hole Conditions Fluid Movement Zone isolation fails any time fluid movement is allowed to move during the time a cement slurry is between the fluid and set state. If the cement moves during the hardening, the cement will not set. Formation Movement Are you located in an active fault zone? The most common formation movement occurs with salt intrusions. 27 Physical Properties Length Ranges a. Casing comes in three range lengths: R I R II R III 10 to 25 feet 25 to 34 feet > 34 feet b. There are different 'grades' of casing which indicate the strength of the sheet. These are color coded: 28 14
These are color coded: 29 30 15
Casing Grades 31 32 16
33 Couplings API couplings STC- Short round thread casing LTC- Long round thread casing BTC- Buttress thread casing XL- Extreme line casing Non-API couplings Hydril super EU, TS Valorec Vam See Halliburton Book.. 34 17
Basic Design Features of Coupling Form: V-shaped or squared shaped Taper: change in thread diameter in inch per foot of thread length Height: distance between crest and root Lead: distance between one point on the thread and the corresponding point on the other one. Pitch diameter: diameter on an imaginary cone that bisects each thread midway between its crest and root 35 Basic Element of Threads 36 18
API Coupling Configurations 37 Strength Properties Yield strength Collapse strength Burst strength 38 19
Yield strength (Tensile Loading ) Tensile loading is applied to casing as a result of its own weight and is at a maximum underneath the casing hanger at the surface. Buoyancy reduces the tensile loading on casing. Tensile loading on the casing is increased as a result of running it in directional hole. 39 Yield strength (Tensile Loading) A critical factor is the outside diameter of the casing which, if reduced, reduces the tensile loading on the casing. It is for this reason that smaller sizes of casing are selected to be run on the build up sections of directional hole, particularly if rapid changes of angle are expected. API defines it as the tensile stress required to produce a total elongation of 0.5 % of the gauge length 40 20
Yield strength (Tensile Loading ) 41 Collapse Loading The maximum external pressure required to collapse a specimen of casing. If the casing is emptied of fluid completely the worst collapse situation exists. With no internal hydrostatic pressure of the mud, the full formation pressure is exerted on the casing at the shoe. At the surface the collapse pressure is clearly zero since only atmospheric pressure is acting on the casing. 42 21
Collapse Loading Where exceptional circumstances occur such as casing run in salt formations or in earthquake areas, extra collapse resistance is required and must be designed for. Types Elastic collapse Plastic collapse Transition collapse 43 Burst Loading (or Internal Yield strength) The maximum value of internal pressure required to cause the steel of casing to yield. Burst loading is the net internal pressure load exerted on the casing. The worst case of burst loading usually occurs when a gas kick is taken and the well is shut in. The net burst load is the difference between the pressure inside the casing and the pressure outside. The point of maximum burst loading in this case is therefore at the top of the casing string where there is a high gas pressure. 44 22
Biaxial Loading Collapse and burst loading on casing are both affected by tensile loading. Tensile loading tends to reduce the collapse resistance of casing. This is a particular problem in deep wells with long casing strings. Tensile loading has the reverse effect on burst resistance. Burst resistance is increased due to the tensile loading. Temperature effects must also be considered as the elongation of the casing can effect all loadings. The Drilling Engineer will make use of standard tables and equations to allow for the effect of tension and temperature. 45 46 23
Casing Design 47 Casing Design Casing design is required to ensure that casing run in the well will withstand the various loads applied to it. The principle loadings casing is subjected to are: Burst Collapse Tensile The worst case loading is considered in each case. Safety Factors Burst pressure 1-1.1 Collapse pressure 0.85-1.125 Tensile force 1.6-1.8 48 24
49 Graphical Method for Casing Design Collapse Line Pc = Pout - Pin, P = 0.052* (ppg)*d (ft) psi Calculate P surface, P shoe (empty casing). Draw collapse line 50 25
Graphical Method for Casing Design Burst Line Burst pressure = Internal pressure External pressure Internal pressure = Pf (TD CSD)*G External pressure = CSD* Gm Where Pf, formation pressure at total depth TD, total depth, CSD, casing setting depth G, formation fluid gradient (0.1 psi/ft) Gm, mud gradient (0.465 psi/ft) 51 Graphical Method for Casing Design Tensile force Tensile force = weight of casing in air Buoyancy force 52 26
Graphical Method for Casing Design Assume full reservoir pressure all along the wellbore. Hydrostatic pressure increases with depth Tensile stress due to weight of string is highest at top 53 Worst Possible Conditions 1. For Collapse design, assume that the casing is empty on the inside (p = 0 psig), and assume no buoyancy effect 2. For Burst design, assume no backup fluid on the outside of the casing (p = 0 psig) 3. For Tension design, assume no buoyancy effect The casing string must be designed to stand up to the expected conditions in burst, collapse and tension. Above conditions are quite conservative. They are also simplified for easier understanding of the basic 54 concepts. 27
Running Casing Condition hole. Casing shoe and first two or three joints. Check float equipment. Remove wear bushing. While running casing, check calculated displacement to trip tank. Keep pipe moving to avoid sticking and circulate. Rig up cement head and plugs. Pump water or chemical spacer to remove mud cake. Drop bottom plug. Load top plug (if not already in cement head). 55 Refer to Examples in the text The End 56 28