The term drilling indicates the whole complex of operations necessary to construct wells of circular section applying excavation techniques.
To drill a well it is necessary to carry out simultaneously the following actions (drilling process):
- to overcome the resistance of the rock, crushing it into small particles measuring just a few mm;
- to remove the rock particles, while still acting on fresh material;
- to maintain the stability of the walls of the hole;
- to prevent the fluids contained in the drilled formations from entering the well.
This can be achieved by using rotary drilling rigs which are the ones operating today in the field of hydrocarbons exploration and production.
The drilling rigs are complexes of mobile equipment which can be moved (onshore and offshore) from one drill site to another, drilling a series of wells.
In rotary drilling the rock is bored using a cutting tool called the bit, which is rotated and simultaneously forced against the rock at the bottom of the hole by a drill string consisting of hollow steel pipes of circular section screwed together.
The cuttings produced by the bit are transported up to the surface by a drilling fluid, usually a liquid (mud or water), or else a gas or foam, circulated in the pipes down to the bit and thence to the surface.
The rotation is transmitted to the bit from the surface by a device called the rotary table or, in the modern rigs, by a top drive motor with the rotary table as backup; additional rotation can be added by downhole motors located directly above the bit.
After having drilled a certain length of hole, in order to guarantee its stability it has to be cased with steel pipes, called casings, joined together by threaded sleeves.
The space between the casing and the hole is then filled with cement slurry to ensure a hydraulic and mechanical seal.
The final depth of the well is accomplished by drilling holes of decreasing diameter, successively protected by casings, likewise of decreasing diameter, producing a structure made up of concentric tubular elements.
Planning the drilling of a well
The planning of a well is a fundamental part of the drilling process – it is the basis for making all the important technical choices, for assessing the costs and organizing the actual construction of the well in the most efficient way.
The well planning starts the moment when the interpretation of the seismic data and the reconstruction of the geology of the area reveal the presence of a structure favourable to the accumulation of hydrocarbons.
These data also enable an estimate to be made of:
- depth of the productive levels; stratigraphic and lithological sequences; problems that could emerge in an operational phase.
- Geologists prepare a document containing estimates and proposals for drilling the well on the basis of the information in their possession obtained from:
- geological surface campaigns; regional geological studies; studies conducted on seismic maps; profiles of wells already completed in the area.
This document is the introduction to the drawing up of the proper drilling program to give the data necessary for:
well location, drilling objectives, foreseen of the final depth, forecasts of the litho-stratigraphic profile, possible drilling problems that could be encountered, etc.
The analysis and processing of all this large mass of information provides the drilling engineers with the essential elements for drawing up the drilling program.
These data are available from existing documents such as:
Geological profiles; Well Logs; Pressure reports; Drilling reports; Drilling fluid reports; Cementing reports; etc
Every well has its own specific features (depth, exploratory, development or appraisal well, onshore or offshore, vertical, directional or horizontal..) but the basic criteria are the same for all well types.
The first step to be taken by engineers to work out the drilling programme is to construct, the trend of the pressure gradient curves according to depth (geostatic, pore pressure and fracture gradients) to design the well casing profile.
As experts in oil and gas downhole drilling and completion tools, the Vigor team has a number of experienced professional engineers who have been providing on-site service on the field for many years and are familiar with every aspect of oil extraction. It is precisely because of this strong resource that we dare to assure all our customers that as long as you choose Vigor, we will provide you with the best quality product solutions and the most advanced product support. If you are also interested in Vigor's products, please do not hesitate to get in touch with us to get the most professional products and the best quality service. Our team is always looking forward to hearing from you, communicating with you, and cooperating with you.
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