Well Design – Spring 2011
Well Design
PE 413
Prepared by: Tan Nguyen
Well Design – Spring 2011
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Casing Setting Depths
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Casing Setting Depths
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
Prepared by: Tan Nguyen
Well Design – Spring 2011
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Example
Prepared by: Tan Nguyen
Well Design – Spring 2011
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Casing Sizes
Prepared by: Tan Nguyen
Well Design – Spring 2011
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Weight, Grade, and Couplings
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Weight, Grade, and Couplings
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Weight, Grade, and Couplings
Burst Design
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Weight, Grade, and Couplings
Collapse Design
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.
Prepared by: Tan Nguyen
Well Design – Spring 2011
Selection of Weight, Grade, and Couplings
Collapse Design
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.
0.052 max Dlc  Dm   0.052 p Dlc
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 Dlc; and
Dm is the depth to which the mud level will fall.
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