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PETE 411 Well Drilling Lesson 17 Casing Design

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**Casing Design Why Run Casing? Types of Casing Strings**

Classification of Casing Wellheads Burst, Collapse and Tension Example Effect of Axial Tension on Collapse Strength

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**Read Applied Drilling Engineering, Ch.7**

HW # Due

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**Casing Design What is casing? Casing Cement Why run casing?**

1. To prevent the hole from caving in 2. Onshore - to prevent contamination of fresh water sands 3. To prevent water migration to producing formation

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**Casing Design - Why run casing, cont’d**

4. To confine production to the wellbore 5. To control pressures during drilling 6. To provide an acceptable environment for subsurface equipment in producing wells 7. To enhance the probability of drilling to total depth (TD) e.g., you need 14 ppg to control a lower zone, but an upper zone will fracture at 12 lb/gal. What do you do?

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**Types of Strings of Casing**

Diameter Example 16”-60” ” 16”-48” ” 8 5/8”-20” /8” 1. Drive pipe or structural pile {Gulf Coast and offshore only} 150’-300’ below mudline. 2. Conductor string. 100’ - 1,600’ (BML) 3. Surface pipe. 2,000’ - 4,000’

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**Types of Strings of Casing**

Diameter Example 4. Intermediate String 5. Production String (Csg.) 6. Liner(s) 7. Tubing String(s) 7 5/8”-13 3/8” /8” 4 1/2”-9 5/8” ”

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**Example Hole and String Sizes (in)**

Hole Size Pipe Size 36” 26” 17 1/2 12 1/4 8 3/4 Structural casing Conductor string Surface pipe IntermediateString Production Liner 30” 20” 13 3/8 9 5/8 7

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**Example Hole and String Sizes (in)**

Hole Size Pipe Size 36” 26” 17 1/2 12 1/4 8 3/4 Structural casing Conductor string Surface pipe IntermediateString Production Liner 30” 20” 13 3/8 9 5/8 7

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**Example Hole and String Sizes (in)**

Structural casing Conductor string Surface pipe IntermediateString Production Liner Mudline 250’ 1,000’ 4,000’

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**Classification of CSG. 1. Outside diameter of pipe (e.g. 9 5/8”)**

2. Wall thickness (e.g. 1/2”) 3. Grade of material (e.g. N-80) 4. Type to threads and couplings (e.g. API LCSG) 5. Length of each joint (RANGE) (e.g. Range 3) 6. Nominal weight (Avg. wt/ft incl. Wt. Coupling) (e.g. 47 lb/ft)

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s e

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**Length of Casing Joints**

RANGE ft RANGE ft RANGE > 34 ft.

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**Casing Threads and Couplings**

API round threads - short { CSG } API round thread - long { LCSG } Buttress { BCSG } Extreme line { XCSG } Other … See Halliburton Book...

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**API Design Factors (typical)**

Required 10,000 psi 100,000 lbf Design 11,250 psi 180,000 lbf 11,000 psi Collapse Tension Burst

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Abnormal Normal Pore Pressure Abnormal Pore Pressure psi/ft gp > normal

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Design from bottom

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**X-mas Tree Wellhead Hang Csg. Strings Provide Seals**

Press. Gauge X-mas Tree Wing Valve Choke Box Master Valves Wellhead Hang Csg. Strings Provide Seals Control Production from Well

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Wellhead

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Wellhead

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**Casing Design Tension Tension Depth Burst Collapse Collapse STRESS**

Burst: Assume full reservoir pressure all along the wellbore. Collapse: Hydrostatic pressure increases with depth Tension: Tensile stress due to weight of string is highest at top Burst

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**Casing Design Collapse (from external pressure)**

Yield Strength Collapse Plastic Collapse Transition Collapse Elastic Collapse Collapse pressure is affected by axial stress

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**Casing Design - Collapse**

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**Casing Design - Tension**

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**Casing Design - Burst (from internal pressure)**

Internal Yield Pressure for pipe Internal Yield Pressure for couplings Internal pressure leak resistance Internal Pressure p p

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**Casing Design - Burst Example 1 Design a 7” Csg. String to 10,000 ft.**

Pore pressure gradient = 0.5 psi/ft Design factor, Ni=1.1 Design for burst only.

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**1. Calculate probable reservoir pressure.**

Burst Example 1. Calculate probable reservoir pressure. 2. Calculate required pipe internal yield pressure rating

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Example 3. Select the appropriate csg. grade and wt. from the Halliburton Cementing tables: Burst Pressure required = 5,500 psi 7”, J-55, 26 lb/ft has BURST Rating of 4,980 psi 7”, N-80, 23 lb/ft has BURST Rating of 6,340 psi 7”, N-80, 26 lb/ft has BURST Rating of 7,249 psi Use N-80 Csg., 23 lb/ft

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23 lb/ft 26 lb/ft N-80

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**Collapse Pressure The following factors are important:**

The collapse pressure resistance of a pipe depends on the axial stress There are different types of collapse failure

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Collapse Pressure There are four different types of collapse pressure, each with its own equation for calculating the collapse resistance: Yield strength collapse Plastic collapse Transition collapse Elastic collapse

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**Collapse pressure - with axial stress 1.**

Casing Design Collapse pressure - with axial stress 1. YPA = yield strength of axial stress equivalent grade, psi YP = minimum yield strength of pipe, psi SA = Axial stress, psi (tension is positive)

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**Casing Design - Collapse**

2. Calculate D/t to determine proper equation to use for calculating the collapse pressure Yield Strength Collapse : Plastic Collapse:

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**Casing Design - Collapse, cont’d**

Transition Collapse: Elastic Collapse:

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**Casing Design - Collapse**

If Axial Tension is Zero: Yield Strength Plastic Transition Elastic J N P

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Example 2 Determine the collapse strength of 5 1/2” O.D., #/ft J-55 casing under zero axial load. 1. Calculate the D/t ratio:

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**Example 2 2. Check the mode of collapse**

Table on p.35 (above) shows that, for J-55 pipe, with < D/t < 25.01 the mode of failure is plastic collapse.

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**The plastic collapse is calculated from:**

Example 2 The plastic collapse is calculated from: Halliburton Tables rounds off to 3,120 psi

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Example 3 Determine the collapse strength for a 5 1/2” O.D., #/ft, J-55 casing under axial load of 100,000 lbs The axial tension will reduce the collapse pressure as follows:

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**The axial tension will reduce the collapse pressure rating to:**

Example 3 cont’d The axial tension will reduce the collapse pressure rating to: Here the axial load decreased the J-55 rating to an equivalent “J-38.2” rating

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Example 3 - cont’d …compared to 3,117 psi with no axial stress!

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