1 Evacuation during drilling
Collapse loads occurring during drilling are usually the result of borehole evacuation due to natural or induced losses. There are however other cases to be considered.
1.1. Internal pressure profile
In a losses situation, the mud column will drop until the pore pressure at section TD is just balanced by the pressure due to the mud column.
The evacuation level should always be the deepest that can occur and which gives the lowest evacuation level.
1.2. External pressure profile
The external pressure profile for collapse during drilling should be constructed in two sections:
- cement column
- annulus fluid column.
1.2.1. Cement column
Set cement behaves as a porous matrix of low permeability (micro to milliDarcy) containing a pore fluid at a certain pressure. The permeability of the cement around the casing is usually intermediate between those of a high-permeability and of a low-permeability formation. Where the cement column is set across a high-permeability formation (milliDarcy and above), the pressure in the cement will be equal to the pore pressure in the formation. Where the cement column is set across a low-permeability formation (microDarcy and below), the pressure will depend on its quality. Local experience will determine whether to choose a good-cement-column or a poor-cement-column scenario.
It is assumed below for the sake of simplicity that the cement column only passes through one high-permeability formation. If it passes through more than one, the procedure described for external pressure profiles should be followed.
Good cement column
Here the cement column acts as an effective seal between the high-permeability formation and the top of cement. The cement pore-pressure profile in the segment of cement column across the low-permeability interval will then be such as to connect the pore pressure at the top of the high-permeability formation with the pressure at the top of cement due to the hydrostatic pressure of the annulus fluid. The cement pore-pressure profile across the low-permeability interval is thus semi-static.
Poor cement column
In this case, the cement column no longer acts as an effective seal between the high-permeability formation and the top of cement. The pressure gradient in the cement across the low-permeability interval will then be equal to the cement mixwater gradient. The pressure at the top of cement is therefore determined by drawing a pressure line with this gradient upwards from the pressure at the top of the high-permeability formation. As a result, the annulus pressure line will be shifted to lower pressures in low-pressure reservoirs and to higher pressures in high-pressure reservoirs. This leads to an annulus level drop or an annulus pressure build-up.
General cement column
No matter whether the cement column is good or bad, the cement pore-pressure profile below the high-permeability formation is given by a line of slope equal to the cement mixwater gradient extending downwards from the pressure at the bottom of the high-permeability formation to the casing shoe.
For the determination of the cement pore-pressure profile in the cement column opposite a previous casing, this previous casing should be treated as a low-permeability formation.
In the event that the cement column does not pass through a high-permeability formation anywhere, the cement mixwater gradient may be assumed to extend downwards from the top of cement to the casing shoe, no matter whether the quality of the cement is high or low. The pressure at the top of cement will be equal to the hydrostatic pressure of the annulus fluid.
1.2.2. Annulus fluid column
In view of the relatively short duration of the drilling phase, deterioration of the annulus fluid during drilling should not be taken into account, either for exploration or for development wells. The pressure gradient in the annulus fluid will therefore be determined by the density of the fluid used at the time of the cement job.
In the case of a high-quality cement column over a high-permeability formation, the annulus-fluid pressure line extends downwards with the above-mentioned gradient from zero pressure at the wellhead to the top of cement. For a low-quality cement column across a high-permeability formation, the annulus-fluid pressure line extends upwards with the same gradient from the pressure at the top of cement towards the wellhead. This can lead to annulus pressure in a high-pressure reservoir, or to annulus fluid drop in a low-pressure reservoir.
If the cement column does not pass through any high-permeability formations, the annulus-fluid pressure line extends downwards from zero pressure at the wellhead to the top of cement, no matter what the quality of the cement
1.3. Special cases
1.3.1. Air, foam or aerated drilling
When air drilling is applied, the wellbore pressure could become atmospheric in the event of system failure. Similarly, foam drilling is subject to the hazard that the foam can lose stability and the liquid phase can drop out. If these scenarios are considered likely, the casing should therefore be designed to withstand full internal evacuation - unlike the base case, where evacuation is likely to be only partial.
For aerated drilling, the designer should consider the internal evacuation level that can be expected based on the pore-pressure profile in the event of a system failure preventing fluid supply.
1.3.2. Salt loading
Salt loading is modelled as if it were an external fluid pressure equal to the overburden pressure at the depth of the salt formation. The external pressure profile with the salt loading gives rise to a step change in the external pressure profile at the top and bottom of the salt formation.
Salt loading is a time-dependent phenomenon but since its onset cannot be accurately predicted, the loading should always be assumed when designing for collapse in the drilling phase.
1.3.3. Formation compaction
External loading due to formation compaction should replace, where applicable, that resulting from annulus-fluid and cement-column pressures.
1.3.4. Blowout
If the casing design is to cater for a blowout scenario, full evacuation of the string to atmospheric pressure must be assumed for the internal pressure profile. This condition represents a blowout where the open hole formation bridges and the gas pressure at surface is allowed to bleed to zero.
It should be noted, however, that during the actual blow-out preceding the full evacuation, the casing integrity might be reduced. To make the design for this scenario fit for purpose, a realistic wear margin should be taken into account when selecting the casing.
2 Evacuation during production
Collapse loads during the production phase generally occur as a result of evacuation resulting from natural or induced losses during workover of the well. There are also, however, a number of special cases to be considered. The base case and the special cases will be addressed in this section.
2.1. Internal pressure profile
2.1.1. Below the production packer
The casing below the production packer must always be designed to withstand full internal evacuation to atmospheric pressure. This is to account for high drawdowns, differential depletion, and back-surging operations.
2.1.2. Above the production packer
Casing above the production packer is usually not subject to critical collapse loading during normal production operations.
During completion and workover, however, mud/brine losses may lead to evacuation of the upper section of the production casing. The deepest possible evacuation level should be calculated based on the pore pressure profile and the fluid density in use.
2.1.3. Special cases
Special cases like gas lift and pump-off are dealt with later.
2.2. External pressure profile
The external pressure profile for collapse during production should be constructed in two sections - that for the cement column and that for the annulus fluid column - as described below.
2.2.1. Cement column
Set cement behaves as a porous matrix of low permeability (in the microDarcy to milliDarcy range) containing a pore fluid at a certain pressure. The permeability of the cement around the casing is usually intermediate between those of a high-permeability and of a low-permeability formation. Where the cement column is set across a high-permeability formation (milliDarcy and above), the pressure in the cement will be equal to the pore pressure in the formation. Where the cement column is set across a low-permeability formation (microDarcy and below), the pressure will depend on its quality. Local experience will determine whether to choose a good-cement-column or a poor-cement-column scenario.
It is assumed below that in the production phase the cement column passes through more than one high-permeability formation.
Good cement column
Here the cement column acts as an effective seal between the high-permeability formation(s) and the top of cement. The pressure profile in the segment of cement column across the low-permeability interval above the shallowest high-permeability formation will then be semi-static, connecting the pore pressure at the top of this high-permeability formation with the pressure at the top of cement due to the hydrostatic pressure of the annulus fluid. The pressure profile in the segment of cement column lying across the low-permeability interval between two high-permeability formations will also be semi-static, connecting the pore pressures at the bottom and top of the high-permeability formations it straddles.
Poor cement column
In this case, the cement column no longer acts as an effective seal between the high-permeability formation(s) and the top of cement. The pressure gradient in the cement across the low-permeability interval above the shallowest high-permeability formation will then be equal to the cement mixwater gradient. The pressure profile in the segment of cement column lying across the low-permeability interval between two high-permeability formations will be semi-static, connecting the pore pressures at the bottom and top of the high-permeability formations it straddles. The pressure at the top of cement will therefore be determined by drawing a pressure line of slope equal to the cement mixwater gradient upwards from the pressure at the top of the shallowest high-permeability formation. This leads to an annulus level drop or an annulus pressure build-up.
General cement column
No matter whether the cement column is good or bad, the cement pore-pressure profile below the deepest high-permeability formation is given by a line of slope equal to the cement mixwater gradient extending downwards from the pressure at the bottom of the high-permeability formation to the casing shoe.
For the determination of the pore-pressure profile in the cement column opposite a previous casing, this previous casing should be treated as a low-permeability formation.
2.2.2. Annulus fluid column
Since casing strings can have a much longer service life in the production phase than in the drilling phase, deterioration of the annulus fluid should be taken into account in production-casing design for development wells. The pressure gradient in the annulus fluid in such cases may thus be determined by the density of the fluid used at the time of the cement job, or by the density of the deteriorated fluid, depending on the elapsed time and on the inherent stability of the annulus fluid. While brines and bentonite/water-based muds are stable with time, the density of oil-based and polymer/water-based muds is liable to drop to that of the base fluid.
In the case of a high-quality cement column over a high-permeability formation, the annulus-fluid pressure line extends downwards with the above-mentioned gradient from zero pressure at the wellhead to the top of cement.
For a low-quality cement column across a high-permeability formation, the annulus-fluid pressure line extends upwards with the same gradient from the pressure at the top of cement towards the wellhead.
Exploration wells
For exploration wells used for short-term production tests, it can be assumed that the annulus-fluid pressure gradient is determined by the fluid density at the time of cementation.
Development wells
For development wells it may be assumed that the annulus-fluid pressure gradient will be equal to that for the base fluid for oil-based or polymer/water-based muds (which are liable to deterioration), but will remain at the value prevailing at the time of the cement job for brines and bentonite/water-based muds (which are inherently stable).
2.3. Special cases
Artificial-lift wells
Gas-lift well production casing above the packer should always be designed for complete internal evacuation to atmospheric pressure, to account for complete venting of the tubing/production-casing annulus as a result of surface-equipment failure.
For artificial lift equipment working in pump-off mode, where usually no downhole packer is installed, the casing should also be designed for complete internal evacuation to account for the low annulus working pressure.
Salt loading
Salt loading is modelled as if it were an external fluid pressure equal to the overburden pressure at the depth of the salt formation. The salt loading gives rise to a step change in the external pressure profile at the top and bottom of the salt formation.
Salt loading is a time-dependent phenomenon but since its onset cannot be accurately predicted, the loading should always be assumed when designing for collapse in the production phase.
Formation compaction
External loading due to formation compaction should replace, where applicable, that resulting from annulus fluid and cement column pressures.
Blowout
If the casing design is to cater for a blowout scenario, full evacuation of the string to atmospheric pressure must be assumed for the internal pressure profile. This condition represents a blowout where the internal pressure due to an uncontrolled gas flow is very low.
It should be noted, however, that during the actual blow-out preceding the full evacuation, the casing integrity might be reduced. To make the design for this scenario fit for purpose, a realistic wear margin should be taken into account when selecting the casing.