PRELIMINARY DESIGN

Preliminary design consists of four individual stages:

Examination of the geological data and offset data

Investigation of the well objectives

Examination of additional constraints

Fitting of a suitable casing scheme to the well requirements

The first two of these items are provided directly by the operating company.

The third item, additional constraints, may come from the operating company, governmental departments, regulatory bodies or may be imposed by physical or economic conditions. For an offshore well, such conditions may require that the well be drilled from an existing structure, requiring some form of directional profile. For an onshore well, the surface location directly above the target may be in the middle of a town, lake, swamp or other physical restriction. There may be an existing drilling facility from which the well can (or must) be drilled. Surface facilities, including road and rail, electrical power supply, water, oil and gas pipelines, and communications can all play a part in surface location positioning and of the well path.

The well objectives will primarily control the size of the casing through the reservoir.

For a purely exploration well and in general, the smaller the hole diameters to be drilled, the better. Smaller hole sizes, for the same hole depth, require lower rated drilling rigs, less lifting and rotating power, smaller mud systems, less cuttings volume removal, smaller (and hence cheaper) casing strings. (Against these must be set the possible requirement to change out drillstring components for different sections, BHA component and logging tool availability, and increased risk with reduced kick tolerance.) For a production well the final casing (or liner) size will be determined by the estimated production profile and the completion required.

Once all of the requirements and constraints have been examined, a well course and preliminary casing design may be determined.

10.1.1 Input Data

Figure 10.1 shows an example of a Well Design Data Summary Sheet, as may be produced for a particular well. The main information contained on this, from the basis of casing design, are the Pore Pressure and Fracture Gradient curves.

EXAMPLE CASING DESIGNPage 4 of 66

Figure 10.1 - Design Data Summary Sheet

Originator

Chrono Strat Formations Trajectory Targets Casing Lithology and Faults Hydrocarbons

of prediction. Faults)

To be drilled as exploration well but may possibly be completed for production if tests show long term potential

T D / A b a n d o n m e n t D e c i s i o n Well site geologist

1 9 9 7 )

Side Wall Samples / Ditch Cuttings

R F T

Drill Stem Tests / Production Tests A drill stem test using annular pressure operated tools will be performed on any potentially productive zone. ShaleLimestoneShaleInterbedded Sand and Shale

Additional information required for the design (and the assumptions used for this example) are:

A vertical land well

Target depth: 12,000ft TVD

The geothermal gradient: 18F per 1,000ft – surface temperature 60F x bottom hole temperature = 276F

Bottom hole pressure (Shut-in) = 8,600psi (13.78ppg EMW)

The final casing or liner size: 7in (based on possible production and the availability of DST tools)

Use of a liner over the reservoir section, with a 500ft overlap

5in liner as contingency, in case the 7in has to be set high

The cement mix water has a density of 9.0ppg

Although the well is vertical, a bending load of 2/100ft has been assumed for Fbend, to allow for deviations from verticality

Once the data described above is analysed, the first requirement is to convert the Pore Pressure and Fracture Gradients to equivalent mud weights versus depth and to plot these, as shown in the schematic in Figure 10.2, for the equivalent mud weight profiles.

Parallel to the Pore Pressure Gradient, we draw the minimum weight line, offset by the trip margin (commonly either 500psi or 0.5ppg mud equivalent). Parallel to the Fracture Gradient, we can draw a ‘Design’ Fracture Gradient, allowing a safe margin for additional cementing loads and a possible gas kick.

These two lines control the acceptable load ranges, both maximum and minimum, which can be safely imposed on the formation at any depth. The segmented section indicates the load lines for casing shoe location. This indicates that, with the liner shoe at 12,000ft, the production casing shoe must be deeper than ±7900ft, and the intermediate casing shoe must be deeper than ±3800ft. The conductor string must be set sufficiently deep to be able to support any mechanical drilling, or casing loads imposed on it. For the sake of this example we will assume a conductor setting depth of 400ft but such depth would depend on the nature of the near-surface formations.

Similarly, the surface BOP stack dimensions and availability, plus any additional external loads, would determine the actual diameter of the conductor. For this example we will assume a 20in, H-40, 94 lb/ft conductor.

The remaining part of the casing design process is to calculate the forces for each of the potential loads and select appropriate weights and grades of casing, within acceptable safety margins. It is possible, under extreme circumstances, that during the design process no combination of weight and grade exists which will be acceptable, or that such a combination is available but creates mechanical difficulties for another casing string. In such a case, the load cases should be examined in order to determine if an alternate casing design is necessary. Senior personnel must ultimately make such decisions.

EXAMPLE CASING DESIGNPage 6 of 66

Figure 10.2 - Mud Weight Equivalent Profiles

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

8 10 12 14 16 18 20

A

Production Liner Production

Casing Intermediate Casing Fracture Gradient

Design Fracture Gradient Including Kick and Cementing Margin Mud Weight Curve

Pore Pressure Gradient

Equivalent Mud Weight, ppg

True Vertical; Depth (TVD), ft

E

Note that the casing design has started ‘Bottom Up’ to allow for production constraints.

The detailed design proceeds from the top down. This system is referred to as ‘Top Down, Bottom Up’ design.

We have thus determined that the desired casing strings are as follows:

PROPOSED CASING SCHEMEDEPTH – TVD (feet) CEMENT TOP TVD (feet)HOLE SIZE (inches) MAXIMUM MUD WEIGHTS (ppg) PORE PRESSURE EMW (ppg)

FRACTURE PRESSURE EMW (ppg) Conductor – 20in, H-40, 94 lb/ft – Shoe400Surface (15ppg)269.509.00– Intermediate – 13-3/8in – Shoe4,500Surface (16ppg)17-1/210.009.0012.50 Production – 9-5/8in – Shoe8,0004,000 (16ppg, 500 into the 13-3/8in shoe)12-1/411.4010.8715.00 Production Liner – 7in – Liner Top7,500To Liner Hanger (16ppg)–––– Production Liner – Shoe12,000–8-1/214.3013.7817.60