Drilling an oil or gas well plays a key role in overall field development. Good well planning makes sure the well is drilled quickly, at reasonable cost and with maximum safety. This article gives an overview of the types of wells, the aims of well planning, and the main areas the drilling engineer must plan.
Where Drilling Fits in Field Development
- Exploration to confirm the potential presence of hydrocarbons, normally including seismic surveys among other techniques.
- Drilling a number of wells to confirm and exploit the possible oil and gas deposits.
- Well completion, which installs the necessary production tools.
- Production operations, including processing the produced fluids for consumption or export.
The drilling engineer is responsible for making the hole.
Types of Wells
| Well type | Main objective | Available information |
|---|---|---|
| Exploratory | Confirm that hydrocarbons are present | Little or none |
| Appraisal | Appraise the extent of the field and its reserves | Limited |
| Development (infill) | Produce the field efficiently | Relatively good |
With exploratory and appraisal wells, the objectives are to confirm hydrocarbons and appraise the extent of the field. That includes stratigraphic features and lithology, likely problem zones, reservoir characterisation, hydrocarbon types, reserve estimates and production-versus-pressure relationships. There is little or no information about the block, so the aim is to build a comprehensive data bank for the field. In development or infill wells there is relatively good information about the environment.
Aims of Well Planning
The main objective of effective well planning is to carry out the entire drilling program as fast as possible, at a relatively low cost, and to the highest safety standard. The drilling engineer must forecast potential problems, develop preventive measures to eliminate or reduce them, and prepare strategies to cope with them if they occur.
Safety is the overriding criterion. Safe drilling practices that prevent catastrophic problems need effective planning before the well is spudded. For exploratory and appraisal wells, planning is flexible and is modified as drilling progresses and facts are encountered. For development drilling, planning is simpler because data and experience from the area exist.
Major Areas That Need Planning
- The sizing and trajectory of the hole
- Casing setting depths, casing design and cement programs
- Design of the optimum mud weight, type and properties
- Selection of the drilling rig and rig equipment
- Contingency planning against unknown events
- Knowledge of formation and fracture pressure
These programs require a good knowledge of the formation pressure to be encountered and, above all, the fracture gradient, because these parameters drive the overall safe drilling program. The bit program is also chosen at this stage; see drilling bits. The fracture gradient is measured with a leak-off test.
Frequently Asked Questions
What is the difference between an exploratory and an appraisal well?
An exploratory well tests whether hydrocarbons exist. An appraisal well is drilled after a discovery to find the size and quality of the field.
Why is well planning more flexible for exploratory wells?
Little information exists before drilling, so the plan changes as new data are obtained.
Key Takeaways
- The three basic well types are exploratory, appraisal and development.
- Planning aims for speed, low cost and, above all, safety.
- Formation pressure and fracture gradient drive hole size, casing, mud and rig choices.
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