Evacuation during drilling

Collapse loads occurring during drilling are usually the result of borehole evacuation due to natural or induced losses. There are 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-2.

FLOWCHART F-2 : DECISION TREE FOR COLLAPSE LOAD CASES, DRILLING PHASE APPLIES TO CONDUCTOR CASING, SURFACE OR INTERMEDIATE CASING/LINER

a) Internal pressure profile

As mentioned above, it may be assumed that the internal pressure profile in this case corresponds to a losses situation [2]. The internal pressure profile may be constructed as follows (see Figure F-1). The pore-pressure profile (available to be casing designer as one of the design parameters, see chapter C) determines the pressure in the formation and hence in the borehole down to total depth (TD). 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 (see Figure F-1a). The evacuation level can be found by drawing the mud pressure line (whose gradient is determined by the mud density) back from the pore pressure at TD to the depth axis. The resulting pressure profile is shown in this figure and in all other figures in this chapter by the thick grey line.

To construct the internal pressure profile for losses at a depth above TD, one draws the mad pressure line from the point on the pore-pressure profile corresponding to the depth in question. Such hypothetical mud-pressure lines are represented by a sloping broken line in Figure F-1a and in other figures in this chapter. The solid line represents the actual mud pressure line to be used for the design.

The evacuation level chosen should always be the deepest that can occur due to drilling below the casing shoe. Thus, if the pore pressure in a certain formation through which the borehole passes is sub-normal, e.g. because of a depleted horizon, the mud-pressure line will be drawn from the point on the pore-pressure profile which gives the lowest evacuation level (see Figure F-1b), and not from TD. As Figure F-1c shows, abnormally high pore pressures do not form an exception in this methodology.

b) External pressure profile

The external pressure profile for collapse during drilling 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.

FIGURE F-1 : CONSTRUCTION OF INTERNAL PRESSURE PROFILES FOR COLLAPSE IN DRILLING PHASE

FIGURE F-2 : RELATIVE PERMEABILITIES OF CEMENT COLUMN AND SURROUNDING FORMATION

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 in section 2.2.2 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 (see Figure F-3). 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 am 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 (see Figure F-4). As a result, the annulus pressure line will be shifted to lower pressure 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.

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 (compare Figures F-3 and F-4).

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. See Figure F-5.

ii) 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 [3]. 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 (see Figure F-3). 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. As Figure F-4 shows, 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 (see Figure F-5).

c) Special cases

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.

FIGURE F-3 :CONSTRUCTION OF EXTERNAL PRESSURE PROFILES FOR COLLAPSE IN DRILLING PHASE, WITH HIGH-QUALITY CEMENT COLUMN AND A SINGLE HIGH-PERMEABILITY

FORMATION

FIGURE F-4: CONSTRUCTION OF EXTERNAL PRESSURE PROFILES FOR COLLAPSE IN DRILLING PHASE, WITH LOW-QUALITY CEMENT COLUMN AND A SINGLE HIGH-PERMEABILITY

FORMATION

FIGURE F-5 :CONSTRUCTION OF EXTERNAL PRESSURE PROFILE FOR COLLAPSE IN DRILLING PHASE, WHEN CEMENT COLUMN DOES NOT PASS THROUGH A HIGH-PERMEABILITY

FORMATION

For aerated drilling, the designer should consider the internal evacuation level that can be based on the pore -pressure profile in the event of a system failure preventing fluid supply.

In all these cases, the external pressure profile will be as described in section 2.2. 1 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 2.2.1b), but now with the salt loading giving rise to a step change in the external pressure profile at the top and bottom if the salt formation. See Figure F-6.

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. This case is dealt with in Chapter N.

The internal pressure profile will be as described in section 2.2.1 a.).

FIGURE F-6 : CONSTRUCTION OF EXTERNAL PRESSURE PROFILES FOR COLLAPSE IN DRILLING PHASE, WITH SALT LOADING

Formation compaction

External loading due to formation compaction should replace, where applicable, that resulting from annulus fluid and cement-column pressures as described in section 2.2.1b).

This case is dealt with in Chapter N.

The internal pressure profile wilI be as described in section 2.2.1 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 open hole formation bridges and the gas pressure at surface is allowed to bleed to zero.

The external pressure profile will be as described in section 2.2.1 b).

It should be noted, however, that during the actual blowout 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.