To begin designing the well, a great variety of information is desirable, but it is not always possible to get the complete package. It is worth considerable effort to get as much of it as possible, but sometimes the designer must just go with the best available data. The desirable information includes, but is not limited to, the following parameters.
Purpose of the well: A given well may serve any one of several different functions— production, injection, exploration, or workover—and the well design will be influenced by its purpose. For example, an exploration well might be of smaller diameter than one intended for production and, because it might be scheduled for abandonment once the reservoir is characterized, it might also be completed with less attention to the well’s longevity (different cement, casing material, or the like.) Some considerations for hole diameter in small exploration wells, or “slimholes” are described below under Rig Selection.
• Surface or shallow hole conditions: High, shallow temperatures or geothermal surface manifestations may require setting the surface casing too shallow, complicate the well design, and require more strings of casing to mitigate well control risks. An alternate location with directional drilling to the target is one option to avoid these shallow hazards. If the surface conditions are unknown, it may be cost effective to drill a small- diameter pilot hole to determine the surface conditions, rather than having to move the hole location after a larger rig and drill pad have been installed.
• Reservoir conditions: It is extremely useful to know as much as possible about the prospective reservoir; such information might come from previous temperature and pressure logs in offset wells, nearby thermal gradient holes, or geophysical information. Clearly, temperature and pressure are crucial, but brine chemistry is also very important because it can have a major impact on casing selection and cost.
• Logistical requirements: It is common that, for reasons including the lease, a power sales contract, other financing requirements, or even weather, a drilling project must be completed on a given schedule. If this is the case, it can complicate planning because of factors ranging from drill rig availability to acquisition of the necessary permits. It is also more or less a standard condition that any lease site will have regulatory stipulations that affect drilling fluid disposal, cuttings disposal, possibly water supply, and even air- quality requirements that may necessitate emissions control on the rig’s engines. The well planner has little recourse in dealing with these factors, but it is certainly essential to consider them in the planning process.
• Likely problems in drilling: Experience in similar wells or general knowledge of the reservoir can sometimes offer a prediction of what problems may be encountered in
drilling the well. If this knowledge is available it will guide the preparations in many ways: having lost circulation material for underpressured formations; appropriate drilling fluid additives for corrosive brines or for exceptionally high temperatures; high- temperature logging or steering tools and drilling motors if those tools will be used in a hot hole; preparation for pressure anomalies (either depleted zones or overpressured formations); and stand-by fishing tools and possibly shock absorbers in the BHA if there is likely to be rough drilling with twist-offs. It may also provide better definition of the best operating envelope (weight on bit, rotary speed, hydraulics) for the bit in specific formations.
• Casing requirements: The heart of well design is the specification of the casing program, which will be discussed in more detail in the following Chapter.
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