Gas Blowout &

Casing Design

• Eng: Tawfik A. Tawfik

•In drilling a high pressure gas bearing zone which was

encountered at depth 12,000 ft. Where a kick occurred, a part of

drilling fluid was blown-out of the well. The blown out fluid was

occupied a portion of the casing string 3000 ft long. The reservoir

pressure of gas bearing zone was 10,000 psi and the density of

drilling fluid was 80 lb/cu.ft.

* Calculate: -

•The pressure held on the BOP.

•The min. length of intermediate casing shoe which is necessary

to prevent subsurface blowout, (assume formation fracture

gradient was 0.875 psi/ft)

•Pressure just below casing shoe.

TVD = h 12,000 ft

hc ?

he 3000 ft

Ph

Pit gain

PCB psiP

ftG

hPhc

hcGfr

hehcP

hchPP

psi

hehPPPP

CB

rf

f

BOP

fCB

fhfBOP

9310)1043012000(*69.10*052.010000

1043069.10*052.0875.0

12000*69.10*052.010000

*052.0

**052.0

*

)(**052.0

)(**052.0

5000)300012000(*69.10*052.010000

)(**052.0

Casing String Design

Eng: Tawfik A. Tawfik

Surface

Conductor casing

Surface casing

Immediate casing

Production casing

Production liner

Stove casing

Conductor casing

Surface casing

Intermediate casing

Production Liner

Hole Size

30”

20”

13 3/8

9 5/8

7

5

Casing Size

36”

26”

17 1/2

12 1/4

8 ½

6

API Casing Specifications

1. Length of joint: average 22 – 31 – 42 ft

2. Outside diameter: OD 36” – 4.5” tubing less than 4.5”

3. Nominal Weight per foot & wall thickness & inside diameter

& drift diameter: 53.5 lbs./ft

4. Grade of steel, e.g. L-80: yield, collapse and burst

5. Thread connection: Round, Buttress, ..

• Short thread connection (STC) • Long thread connection (LTC) • Buttress thread connection (BTC) • Extreme line (EL) - Hydril – VAM – new VAM

Casing Loads

External

Internal

Tension

Designing A Combination Casing Strings

Design a 7" combination string to be run to the depth

of 3500 m.

•External pressure is created by a mud with the

density 1.25 g/cc;

•Internal pressure corresponding to the hydrostatic

head of the mud;

•The casing is empty;

•The design factor are :

fc = 1.125 (for collapse)

ft = 1.80 (for joint strength)

fi = 1.10 (for burst)

•The buoyancy is neglected;

•The effect of tensile load on collapse resistance is

neglected ;

•The string should not contain more than 4 different

grades.

Grade N.W kg/m Collapse Resisitance, kg/cm2

Joint Strength, Kg

Internal Pressure, kg/cm2

C-75,32 47.6 505 Pc1 288000 -

C-75,29 43.2 430 Pc2 255000 -

C-75,26 38.7 351 Pc3 222000 -

C-75,23 34.2 275 Pc4 189000 420

1- Collapse Design Pc min = 0.1 H fc = 0.1 * 1.25 * 3500 * 1.125 = 492 kg/cm2

So the first grade will be C-75,32 with length of L1

Pc2 = 0.1 (H – L1) fc

L1 = H - ---------------- = 440 m

L2 = H - L1 - ---------------- = 560 m L3 = 545 m

L4 = H – L1 – L2 – L3 = 1955 m

Pc2

0.1 fc Pc3

0.1 fc

2- Check for tensile

sec

length, m

N.wt, kg/m wt, kg cum wt

Joint strength ft

L1 440 47.6 20944 20944 288000 13.8 OK

L2 560 43.2 24192 45136 255000 5.6 OK

L3 545 38.7 21092 66228 222000 3.4 OK

L4 1955 34.2 66861 133089 189000 1.4 No

L4’ 1133 34.2 38749 104977 189000 1.8 OK

L5 822 38.7 31812 136790 222000 1.6 No

L5' 472 38.7 18344 123321 222000 1.8 OK

L6 348 43.2 15034 138355 255000 1.8 OK

'445

4

)332211(4'4

LLL

ftw

ftLwLwLwPjL

3 – Check for bursting For the weakest section of casing grades

Fi = --------------------------- > 1.1 safe

Internal pressure

0.1 Hi

Hi

Interval m

Grade

0-348 C-75,29

348-822 C-75,26

822-1955 C-75,23

1955-2500 C-75,26

2500-3060 C-75,29

3060-3500 C-75,32

Grade Effective Collapse Resistance, psi Joint

strength, lb

T.L, lb 0

T.L, lb 100,000

T.L, lb 200,000

T.L, lb 300,000

N-80, 32 lb 7400 6880 6280 5600 505,000

N-80, 26 lb 5620 4850 4290 3600 402,000

N-80, 23 lb 4300 3850 3310 2690 350,000

J-55, 23 lb 3290 2850 2250 1500 300,000

J-55, 20 lb 2500 2100 1550 980 254,000

H-40, 20 lb 1900 1490 960 ------ 191,000

•Design 7" graduated production casing string to be set at

depth of 10,000 ft in a hole full of 90 lb/cu.ft mud, using

conventional safety factor. The effective collapse resistance at

different tensile loads (T.L) is given in the following table for

different casing grades:

Collapse resistance

Ten

sile

str

ess

1- Collapse Design

32

))1(211(2

1

211

1

PcL

LHnwLnwBFW

L

PcHnwBW

BFs

m

Pc min = 0.1 H fc

sec

depth, ft

length, ft

N.wt, lb/ft wt, lb cum wt

B * cum wt

Joint strength ft

1 2735 32 87520 87520 71460 505,000 7.067

2 1901 26 49426 136946 111816 402,000 3.595

3 1631 23 37513 174459 142446 350,000 2.457

4 1218 23 28014 202473 165319 300,000 1.815

5 990 20 19800 222273 181486 254,000 1.400

6 1525 20 30500 252773 206389 191,000 0.925

4' 1289 23 29647 204106 166652 300,000 1.800

5 2444 23 56212 260318 212550 350,000 1.647

5' 1479 23 34017 238123 194427 350,000 1.800

6 965 26 25090 263213 214913 402,000 1.871

2- Check for tensile

Drill String Design

Eng: Tawfik A. Tawfik

•Derive an equation for determining the length of drill

collar required during drilling a well. If weight on bit

50,000 lb, total well depth to be drilled is 10,000 ft, mud

density 12 ppg.

Calculate lengths of D/C and D/P to be used.

Steel density = 489.5 lb/ft3

D/P (19.5 lb/ft) 5 in O.D. 4.276 in I.D.

D/C (150 lb/ft) 8in O.D. 2.812 in I.D.

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