Well Design – Spring 2011 Prepared by: Tan Nguyen Well Design PE 413.

Well Design – Spring 2011

Prepared by: Tan Nguyen

Well Design

PE 413

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Introduction

To obtain the most economical design, casing strings often consist of multiple

sections of different steel grade, casing depths, wall thickness, and coupling types.

Such a casing string is called a combination string. Additional cost savings

sometimes can be achieved by the use of liner combination strings instead of full

strings running from the surface to the bottom of the hole. However, the potential

savings must be weighted against the additional risks and costs of a successful,

leak-free tieback operation as well as the additional casing wear that results from a

longer exposure of the upper casing to rotation and translation of the drill string.

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Selection of Casing Setting Depths

The selection of the number of casing strings and their setting depths generally is

based on a consideration of the pore pressure gradients and fracture gradients of

the formations to be penetrated.

The pore pressure and fracture pressure are expressed as an equivalent density

and are plotted vs. depth. A line representing the planned-mud-density program also

is plotted. The mud densities are chosen to provide an acceptable trip margin above

the anticipated formation pore pressure to allow for reductions in mud weight caused

by upward pipe movement during tripping operation. A commonly used trip margin is

0.5 lbm/gal or one that will provide 200-500 psi of excess bottomhole pressure over

the formation pore pressure.

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Point a: to prevent the formation fluid into the well and to reach the desired depth.

Point b: to prevent the fracture of formation --> intermediate casing need to run at

this depth.

Point c: Fluid density is reduced until it reaches to margin of the curve

Point d: casing shoe of the surface casing

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Example

A well is being planned for a location in Jefferson Parish, LA. The intended well

completion requires the use of 7’’ production casing set at 15,000 ft. Determine the

number of casing strings needed to reach this depth objective safely, and select the

casing setting depth of each string. Pore pressure and fracture gradient, and lithology

data from logs of nearby wells are given in Fig 7.21. allow a 0.5 lbm/gal trip margin,

and a 0.5 lbm/gal kick margin when making the casing seat selections. The minimum

length of surface casing required to protect the freshwater aquifers is 2000ft.

Approximately 180 ft of conductor casing generally is required to prevent washout on

the outside of the conductor. It is general practice in this are to cement the casing in

shale rather than in sandstone.

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Example

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Selection of Casing Sizes

To enable the production casing to be placed in the well, the bit size used to drill the

last interval of the well must be slightly larger than the OD of the casing connectors.

The selected bit size should provide sufficient clearance beyond the OD of the coupling

to allow for mud cake on the borehole wall and for casing appliances, such as

centralizers and scratchers. The bit used to drill the lower portion of the well also must

fit inside the casing string above.

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Selection of Casing Sizes

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Selection of Weight, Grade, and Couplings

In general, each casing string is designed to withstand the most severe loading

conditions anticipated during casing placement and the life of the well. The loading

conditions that are always considered are burst, collapse, and tension. Because the

loading conditions in a well tend to vary with depth, it is often possible to obtain a less

expensive casing design with several different weights, grades, and couplings.

The casing design usually is based on an assumed loading condition. the assumed

design load must be severe enough that there is a very low probability of a more severe

situation actually occurring and causing casing failure.

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The high-internal pressure loading condition used for the burst design is based on a

well control condition assumed to occur while circulating out a large kick.

The high-external pressure loading condition used for the collapse design is based on

a severe lost-circulation problem.

The high-axial tension loading condition is based on an assumption of stuck casing

while the casing is run into the hole before cementing operations.

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The burst design should ensure that formation fracture pressure at the casing seat will

be exceed before the burst pressure is reached. Thus, this design uses formation

facture as a safety pressure release mechanism to ensure that casing rupture will not

occur at the surface.

The pressure with the casing is calculated assuming that only formation gas is in the

casing.

The external pressure outside the casing that helps resist burst is assumed to be equal

to the normal formation pore pressure for the area.

Burst Design

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The collapse design is based either on the most severe lost-circulation problem

that is felt to be possible or on the most severe collapse loading anticipated when

the casing is run. For both cases, the maximum possible external pressure that

tends to cause casing collapse results from the drilling fluid that is in the hole when

the casing is placed and cemented.

Collapse Design

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If a severe lost circulation zone is encountered near the bottom of the next interval of

hole and no other permeable formations are present above the lost circulation zone, the

fluid level in the well can fall until the BHP is equal to the pore pressure of the lost

circulation zone.

lcpmlc DDD 052.0052.0 max

where Dlc is the depth of the lost circulation zone; gp is the pore-pressure gradient of the

lost circulatio zone; max is the maximum mud density anticipated in drilling to D lc; and

Dm is the depth to which the mud level will fall.

Collapse Design

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Well Design – Spring 2011 Prepared by: Tan Nguyen Well Design PE 413.

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