ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 21 OF 230 REVISION 0 3. SELECTION OF CASING SEATS
The selection of casing setting depths is one of the most critical factors affecting well
design. These are covered in detail in the ?Casing Design Manual?. The following sections are to provide engineers with an outline of the criteria necessary to enable casing seat selection.
The following parameters must be carefully considered in this selection:
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Total depth of well Pore pressures Fracture gradients
The probability of shallow gas pockets Problem zones
Depth of potential prospects Time limits on open hole drilling
Casing program compatibility with existing wellhead systems
Casing program compatibility with planned completion programme on production wells
Casing availability - size, grade and weight
Economics - time consumed to drill the hole, run casing and the cost of equipment.
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When planning, all available information should be carefully documented and considered to obtain knowledge of the various uncertainties. Information is sourced from:
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Evaluation of the seismic and geological background documentation used as the decision for drilling the well.
Drilling data from offset wells in the area. (Company wells or scouting information).
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The key factor to satisfactory picking of casing seats is the assessment of pore pressure (formation fluid pressures) and fracture pressures throughout the length of the well. As the pore pressures in a formation being drilled approach the fracture pressure at the last casing seat then installation of a further string of casing is necessary. figure 3.b show typical examples of casing seat selections.
ARPO ENI S.p.A. Agip Division
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IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 22 OF 230 REVISION 0
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Casing is set at depth 1, where pore pressure is P1 and the fracture pressure is F1.
Drilling continues to depth 2, where the pore pressure P2 has risen to almost equal the fracture pressure (F1) at the first casing seat.
Another casing string is therefore set at this depth, with fracture pressure (F2). Drilling can thus continue to depth 3, where pore pressure P3 is almost equal to the fracture pressure F2 at the previous casing seat.
This example does not include any safety or trip margins, which would, in practice, be taken into account.
Figure 3.A - Example of idealised Casing Seat Selection
ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 23 OF 230 REVISION 0
Figure 3.B - Example Casing Seat Selection
(for a typical geopressurised well using a pressure profile).
ARPO ENI S.p.A. Agip Division 3.1.
CONDUCTOR CASING
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 24 OF 230 REVISION 0 The setting depth for conductor casing is usually shallow and selected so that drilling fluid may be circulated to the mud pits while drilling the surface hole. The casing seat must be in an impermeable formation with sufficient fracturing resistance to allow fluid circulation to the surface. In wells with subsea wellheads, no attempt is made to circulate through the conductor string to the surface but must be set deep enough to assist in stabilising the subsea guide base to which guide lines are attached.
The driving depth of the conductor pipe is established with the following formula:
Hi = [df x (E+H) - 103 x H]/[1.03 - df + 0.67 x (GOVhi - 1.03)] where:
Hi E H df
3.2.
= = = =
Minimum driving depth (m) from seabed
Elevation (m) distance from bell nipple and sea level Water depth (m)
Maximum mud weight (kg/l) to be used integrated density of sediments (kg/dm3/10m)
GOVhi =
SURFACE CASING
The setting depth of surface casing should be in an impermeable section below fresh water formations. In some instances, where there is near surface gravel or shallow gas, it may need to be cased off shallower.
The depth should be enough to provide a fracture gradient sufficient to allow drilling to the next casing setting point and to provide reasonable assurance that broaching to the surface will not occur in the event of BOP closure to contain a kick.
3.3.
INTERMEDIATE CASING
The most predominant use of intermediate casing is to protect normally pressured
formations from the effects of increased mud weight needed in deeper drilling operations. An intermediate string may be necessary to case off lost circulation, salt beds, or sloughing shales.
In cases of pressure reversals with depth, intermediate casing may be set to allow reduction of mud weight.
When a transition zone is penetrated and mud weight increased, the normal pressure interval below surface pipe is subjected to two detrimental effects:
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The fracture gradient may be exceeded by the mud gradient, particularly if it becomes necessary to close-in on a kick The result is loss of circulation and the possibility of an underground blow-out occurring.
The differential between mud column pressure and formation pressure is increased, increasing the risk of stuck pipe.
However, in general practice, drilling is allowed until the mud weight is within 50gr/l of the fracture gradient measured by conducting a leak-off test at the previous casing shoe. Attempts to drill with mud weight higher than this limit are sometimes successful, but many holes have been lost by attempts to extend the intermediate string setting depth beyond that indicated by the above rule.
ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 25 OF 230 REVISION 0 This can cause either, kicks causing loss of circulation and possibly an underground blow- out or the pipe becomes differentially stuck. Sloughing of high pressure zones can also cause stuck pipe .
Significantly in soft rock areas, the fracture gradient increases relatively slowly compared to the depth of the surface casing string, but the pressure gradients in the transition zones usually change rapidly.
Emphasis is often placed on setting the surface casing to where there is an acceptable fracture gradient. Greater control over potential conditions at the surfaces casing seat is affected by the intermediate casing setting depth decision.
It is often tempting to ?drill a little deeper? without setting pipe in exploratory wells. When pressure gradients are not increasing this can be a reasonably acceptable decision, but, with increasing gradient, the risk is greater and should be carefully evaluated.
To ensure the integrity of the surface casing seat, leak-off tests should be specified in the Drilling Programme.
3.4.
DRILLING LINER
The setting of a drilling liner is often an economically attractive decision in deep wells as opposed to setting a full string. Such a decision must be carefully considered as the intermediate string must be designed for burst as if it were set to the depth of the liner. If drilling is to be continued below the drilling liner then burst requirements for the
intermediate string are further increased. This increases the cost of the intermediate string. Also, there is the possibility of continuing wear of the intermediate string that must be evaluated.
If a production liner is planned then either the production liner or the drilling liner should be tied back to the surface as a production casing.
If the drilling liner is to be tied-back, it is usually better to do so before drilling the hole for the production liner. By doing so, the intermediate casing can be designed for a lower burst requirement, resulting in considerable cost savings. Also, any wear to the intermediate string is spanned prior to drilling the producing interval.
If increased mud weight will be required while drilling hole for the drilling liner, then leak-off tests should be specified in the Drilling Procedures in the programme for the intermediate casing shoe.
Insufficient fracture gradient at the shoe may limit the depth of the drilling liner.
3.5.
PRODUCTION CASING
Whether production casing or a liner is installed, the depth is determined by the geological objective. Depths, hence the casing programme, may have to be altered accordingly if depths run high or low.
The objective and method of identifying the correct depth should also be stated in the programme.