This article describes the wellhead technical specifications, Operating Envelope  and Operating Philosophy.

The operating envelope

When defining the type of equipment and service for a project the information given should cover virtually all areas of concern. Therefore the production envelope must cover the total lifespan of the project. If, for example, there was a need for water injection or gas injection on a project, the expected flow rates and water/gas properties would also be included.

For example, this envelope should include:

• Fluid Properties: oil, gas, water steam etc.
• Maximum surface pressures and temperatures
• Maximum and minimum flow rates
• Flow rates for oil/condensate and gas at differing (maximum and minimum) water cuts
• Solid content in terms of sand (including an example of a short term production if for instance a gravel pack fails)
• Maximum CO2 content over the lifespan of the field
• Maximum H2S content over the lifespan of the field
• Gas lift volumes, dewpoint, composition, CO2 and H2S concentrations for the lifespan of the field
• Drilling fluid properties including spudmud, KCl polymer mud etc.
• Completion fluid properties
• Well clean up and stimulation fluid properties
• Formation produced and possibly injected water compositions
• Potential for producing wax (asphaltenes).

The most effective tree design can be determined with the input of the users and maintainers (Operating Philosophy), in combination with the information contained in the operating envelope. All information obtained should be treated as a range. In using these ranges it may be cost effective to move up or down in sizes of equipment.

The data appearing in the following sub-paragraphs is representative of a typical Operating Envelope. However, this information should not be used as a basis for equipment selection.

Oil properties

 Maximum Surface Temperature

Maximum Surface Pressure (Flowing / Closed in)

Maximum Oil Production / % water

Typical Solid Content (Sand)

Maximum Solids Content (Short Term, Gravelpack Failure)

Total gas (reservoir and gaslift)

 Water cut (%)

Reservoir gas (MMscft/d)

Gaslift gas (MMscft/d)

Total (MMscft/d)

Maximum CO2 Concentration (Reservoir + Gaslift Gas) % by Volume.

Maximum H2S Concentration (Reservoir + Gaslift Gas) ppm (V).

The maximum CO2 and H2S concentrations will be experienced towards the end of the field lifespan.

Gas lift

Maximum Surface Temperature

Expected Maximum Surface Pressure

Flow RateA s Indicated Above

Water Dew Point °C at x barg

Expected C02 Concentration % (V)

Expected H2S Concentration ppm (V)

Properties of other fluids

The well will also come into contact with one or more of the different types of drilling mud that are available on the market, e.g. Gypsum (lignosulphonate mud), Polymer Spudmud, KCI Polymer Mud and possibly chalk mud to drill the reservoir section.

For well completion and workover operations Calcium Chloride brines and inhibited sea water (corrosion inhibitor and biocide) will be used. For well clean up and stimulation HCl and/or HF acid will be used (HCl acid concentration is 10%, HF acid 7.5%, HCl 1.5% concentration).

Formation water analysis

Formation water (mg/l) Sea water (mg/l)

Sodium, Potassium, Calcium, Magnesium, Iron, Barium, Strontium, Boron, Chloride, lodine, Sulphate, Sulphide, Bicarbonate, pH, Formate, Acetate, Propionate

n-Butyrate.

Operations philosophy

Designing an operations philosophy

It is always advisable to start the design process with an operating philosophy. This should take into account the needs of the production operator, the well services personnel and other functions involved in the asset management of the well.

•  List all the internal and external factors acting upon the well.
• List all subjects (Criticality, Manning, Availability, Sparing, Maintenance and Inspection etc.).
• Formulate 'options' for each of the subjects.
• Identify the equipment required.
• Select the equipment that meets the preferred option.

Clearly defined standards should be available. These standards should be adhered to, unless the well is of a new type or there is a clearly documented justification for deviating from the standard model.

Space requirements

The limits of the available space for the wellhead equipment should be defined at the initial stages of a project. Preferably as soon as possible after project initiation and certainly before detailed design commences. If not, problems may be encountered at a later stage and mistakes may prove costly.