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. See Flowchart F-3.
FLOWCHART F-3 :DESIGN TREE FOR COLLAPSE LOAD CASES, PRODUCTION PHASE APPLIES TO PRODUCTION CASING/LINER
a) Internal pressure profile 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.
Above the production packer
Casing above the 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. See Figure F-7.
Special cases like gas lift and pump- off are dealt with later.
FIGURE F-7: CONSTRUCTION OF INTERNAL PRESSURE PROFILES ABOVE AND BELOW PACKER FOR COLLAPSE IN PRODUCTION PHASE
b) 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.
i) 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. As indicated in Figure F-2, 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 [3]. 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 formations) 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 spore 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 (see Figure F-8).
Poor cement column
In this case, the cement column no longer acts as an effective seal between the high-permeability formations) 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 to 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 (see Figure F-9). This leads to an annulus level drop or an annulus pressure build-up.
FIGURE F-8 CONSTRUCTION OF EXTERNAL PRESSURE PROFILE FOR COLLAPSE IN PRODUCTION PHASE, WITH QUALITY CEMENT COLUMN AND MULTIPLE
HIGH-PERMEABILITY FORMATIONS
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 (see Figures F-8 and F-9).
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.
ii) 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 [3].
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 (see Figure F-8).
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 (see Figure F-9).
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.
FIGURE F-9A : CONSTRUCTION OF EXTERNAL PRESSURE PROFILE FOR COLLAPSE IN PRODUCTION PHASE, WITH LOW-QUALITY CEMENT COLUMN AND MULTIPLE
HIGH-PERMEABILITY FORMATIONS RESULTING IN ANNULUS FLUID LEVEL DROP
FIGURE F-9b CONSTRUCTION OF EXTERNAL PRESSURE PROFILE FOR COLLAPSE IN PRODUCTION PHASE, WITH LOW-QUALITY CEMENT COLUMN AND MULTIPLE
HIGH-PERMEABILITY FORMATIONS RESULTING IN ANNULUS PRESSURE
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.).
c) 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.
The external pressure profile will be as described in section 2.2.2 b).
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 will therefore be as described in section 2.2.2 b), but with the effects of the salt loading giving 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. This case is dealt with in Chapter N.
The internal pressure profile will be as described in section 2.2.2 a).
Formation compaction
External loading due to formation compaction should replace, where applicable, that resulting from annulus fluid and cement column pressures. This case is dealt with in Chapter N.
The internal pressure profile will be as described in section 2.2.2 a).
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.
The external pressure profile will be as described in section 2.2.2 b).
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.