aes1330 02 casing design

8/10/2019 AES1330 02 Casing Design

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CASING DESIGN

What will be covered in this chapter Standard strength calculations for a casing string

1.

The Design of casing and tubing strings The basic philosophy in the design of a casing or tubing string is that nowhere yield shouldtake place. Detailed stress-analyses for each casing are not usually performed. A series oftables containing the strength characteristics of available casing types are published andregularly updated by the API in their API bulletin 5C2. An example is given in Table 1

Factors we have to take into account when designing strings are: collapse burst tension (of both pipe and couplings) compression

Many other factors are also important, such as bending stresses, corrosion, damage (duringtransport, making connections), fatigue, wear, buckling, chemical environment. Althoughthese factors must be taken into account, they are of secondary importance.

Due to the factors mentioned above, the load- and stress conditions for a casing are highlycomplex, poorly known, or unknown. It is therefore common practice to use a set of standarddesign rules that are applied to a simplified casing string model. For this model a generalpicture of the load and stress conditions (depth, rock type, mud weights, etc.) is sufficient.

In view of unknowns that cannot be evaluated properly, it is customary to introduce a number

of design factors. These design factors take into account: the probability that the assumed load and stress conditions are being surpassed the reliability of the casing-material and the manufacturing process uncertainties with respect to corrosion, wear and damages the consequences (financially, morally, etc.) if a casing should fail past experience

Design factors are in principle different from safety factors; the latter assume that load andstress conditions are exactly known and that a specific safety-margin is being built in. In thedesign of a casing or tubing one should therefore not use the term safety factor but only theterm design factor.


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1000 psi 40.00 630 859 91643.50 942 1005 138147.00 1018 1086 149353.50 1166 1244 1710

Mill hydrostatic testpressure

36.00 3000

psi 40.00 3000 4900 530043.50 5400 5800 800047.00 5900 6300 860053.50 6800 7200 9700

2. Standard Design Rules for a Casing

Every company uses its own design rules. The most commonly used are summarized below.These design rules are formulated so that the casing will be strong enough under the worstpossible conditions that can be imagined. So the scenarios are not likely scenarios, butextreme cases. Of course, we want to design a well that is safe at the lowest possible cost, butif something goes wrong (for example, a major blowout) we do not want to be worried aboutcasing strength when tackling the blowout. The design rules guarantee that the casing will bestrong enough under the most extreme conditions that it is possible to imagine.Collapse The external pressure on a casing is assumed to result from the column of mud in the

casing/borehole annulus at the time the casing will be cemented; the pressure due tocement column is usually not taken into account.

The internal pressure in a casing during drilling and production is 1 bar at bottom, whichcorresponds with complete evacuation. (By exception, in the case of deep intermediatecasings partial evacuation may be assumed)

Burst Surface & Intermediate Casings The external pressure on the casing is assumed to be the pressure caused by the column of

mud in the casing/borehole annulus at the time the casing will be cemented. It is assumed that the casing is fully filled with gas and that the gas pressure is determined

by the break-down pressure of the formation at the casing shoe. The pressure at surface isthis breakdown-pressure minus the static pressure of the gas column. (Again forintermediate casings installed at great depth different assumptions may be made; theinternal pressure may be based on a gas column from the bottom of the hole, assumingcomplete evacuation, and taking into account the full reservoir pressure).

Burst Production Casings The external pressure on the casing is assumed to be the pressure caused by the column of

mud in the casing/borehole annulus at the time the casing was cemented.

It is assumed that the full closed-in tubing head gas pressure (via a leaking wellhead) ispushing on the column of completion fluid which fills the annulus of thetubing/production casing

In a well in which it is planned to carry out hydraulic fracturing, the design of theproduction casing should take this into account.

In wells with artificial lift (gaslift, pumping) the design of the production casing should beadapted to these conditions.


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Tension The axial force experienced at a given point of the casing is assumed to be equal to the

weight (in air) of the casing hanging below this point. The casing must be able to withstand pressure testing. The allowable test pressure takes

into account the buoyancy of the casing (in mud). Design factors Design factors and design rules are fully dependable; a design is not acceptable if the designfactors have not been applied. Design factors which are frequently used by operators are:

Collapse: 1.0 and 1.125 Burst: 1.0 and 1.1 Tension: 1.6 and 1.8 Compression: 1.0These design factors may look high, particularly those for tension. However, one should notforget that the string is not designed only for its landed position. The string should be able towithstand the running (or sticking and subsequent jarring) while running in.

3. Design procedure for casing strings

1. Determine the depth of the hole and in case of a deviated well also its length. Establishwhich hole-diameter will be required at the final depth of the well.

2. Estimate the formation breakdown gradient and the fracture propagation pressure at thedepth (or depths if it is a deep hole) where the intermediate casing(s) will be installed

3. Establish the minimal installation depth of the intermediate casing string(s) when we canassume that hydrocarbon bearing formations are being drilled through. Assume cases of"gaskicks" in relation to formation breakdown pressures.

4. Make a provisional estimate of the diameters and depths of production, intermediate,surface and conductor strings.

5. Design the strings mentioned above in the sequence: intermediate string; productionstring; surface string; conductor string.

Conductor casing stringsFor conductor casing strings, collapse and burst are not important and the conductor istherefore not designed for collapse and burst. It is designed for tension and also forcompression. The weight (submerged in the respective fluids/muds) of the surface casing, the

intermediate casing is usually landed on top of the conductor string. If the conductor is cemented up to surface calculations for compression are not necessary.

If the conductor is partly free-standing (or in the case of a riser), then the top-part will bein compression until the yield value is be surpassed or until buckling of the conductor (orriser) takes place.

Related topics that have not been covered in this chapter Similar design techniques can be used for determining the required strength of other

tubulars, such as the tubing. Non-uniform loading of casing strings, caused by subsidence, squeezing salts or swelling

shales or tectonic stresses. A paper dealing with this topic is Effect of nonuniformloading on conventional casing collapse resistance, P.D. Patillo, N.C. Last and W.T.Asbill, SPE Drilling & Completion September 2004, 156 160.


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EXERCISES CASING DESIGN

Question 1A vertical well in the Zuidwal field in the Waddenzee, The Netherlands, produces gas fromthe interval 6050-6200 ft. A 9 5/8" intermediate casing string is set at a depth of 6000 ft. A 7"liner is hung-off with a packer in the 9 5/8" casing at 5900 ft and landed at 6200 ft. The well iscompleted with a 5 tubing, free-hanging in the 7" packer at 6000 ft.1. Sketch the completion2. Design the 9 5/8 production casing for collapse and burst. Make use of the following data

and the API tables in the lecture notes Data

The specific gravity of the mud down to 6000 ft is 1.45. The pressure of the gas reservoir at 6050 ft is 3146 psi (gradient 0.52 psi/ft) The pressure gradient for water is 0.4335 psi/ft The tubing/casing annulus will be filled with MgCl2 brine, relative density. 1.50. Design factors: collapse 1.0, burst 1.1 The gas has relative density to air 0.60. Use the following table giving the ratio between

surface pressure and downhole pressure for the gas

Depth (m) Pressure at surface/Pressure at depth250 0.983500 0.9661000 0.9341500 0.9022000 0.8722500 0.8433000 0.8153500 0.787

4000 0.7614500 0.7365000 0.7115500 0.6866000 0.663

Question 2In a given field, hydrocarbons are found over the interval 7000' - 10800'. All reservoirs arehydrostatically pressured (the pressure gradient in water is 0.4335 psi/ft). The bottom reservoiris a gas reservoir occupying 9800' -10800', with GWC at 10800'. A gas well is producing fromthis reservoir. The gas has specific gravity 0.6.

The completion details are:Tubulars diameter depthStovepipe 22" driven 50'

Conductor 18 5/8" cemented to surface 250'Surface casing 10 3/4" cemented to surface 4000'Production casing 7" cemented to surface 10500'Production tubing 3 " in packer 8000'


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Fluids Specific gravity depthDrilling mud used 1.05 0' - 4000'

1.06 4000'-9500'1.20 below 9500'

Completion fluid 1.05Cement 1.65 Pozmix 0' - 4000'

1.88 Class E 4000'-10500'1.00

(1) Draw the completion(2) Calculate the closed in gas pressure at the tubing head (use table in Question 1).(3) Calculate the collapse load for the production casing with design factor 1.0. Which 7"

casing can be used for the lowest part of the casing string (see extract from API Tablesbelow). Design a production casing string using C-75 grade casing.

(4) Calculate the burst load for the production casing with design factor 1.0. Design a casingstring satisfying the collapse and burst criteria using C-75 grade casing.

(5) For this casing string calculate the tensile load with design factor 1.60. Design a casingstring satisfying the collapse, burst and tensile criteria using C-75 grade casing.

PERFORMANCE PROPERTIES OF 7CASING (API)

Nominalweightlbs/ft

K55 C-75 N-80 P-110

Minimum collapse pressure 20.00 2270psi 23.00 3270 3770 3830

26.00 4320 5250 5410 621029.00 6760 7020 851032.00 8230 8600 1076035.00 9710 10180 1301038.00 10680 11390 15110

Maximum burst pressure 20.00 3740psi 23.00 4360 5940 6340

26.00 4980 6790 7240 996029.00 7650 8160 1122032.00 8490 9060 1246035.00 9340 9960 1370038.00 10120 10800 14850

Maximum allowable tensileload

20.00

1000 lbs wt 23.00 632 632 66626.00 641 641 675 84429.00 685 721 90232.00 761 801 100235.00 850 895 111838.00 917 965 1207


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Question 3In the Tambaredjo field in Suriname, the reservoir is about 1000 deep. The casing program isas follows 2 joints of 14 conductor pipe driven to about 36 Surface hole drilled to 210 and cased with 6 joints of 8-5/8 24lb/ft K-55 casing 7-7/8 hole drilled to TD (5 below the base of oil sand) and cased with 4 9lb/ft K55

casings, with maximum collapse pressure 3320 psi, burst pressure 4380 psi and tensileload 264 1000lbs force.

Mudweight are typically 9.9 10ppg during surface hole drilling and 8.8 9.2 ppg thereafter(1ppg = 1 pounds/US gallon = 119.8 kg/m 3)Class C construction cement is used, with density 16.48 ppgDraw the completion. Check the design of the 4 casing

aes1330 02 casing design

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