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ROTARY DRILLING.

The most common form of drilling is rotary drilling this is accomplished by rotating a drill pipe by means of a power driven rotary table or hydraulic powered sub, more commonly referred to as a top drive This drive a string of hollow pipe "know as the drill string with a bit attached to the bottom. The action of the bit can "cuts, scrap, chip or break" up formation as it penetrates. 

Drilling fluid " mud" is pumped through the rotating drill pipe and through holes in the bit. "Jets" This fluid washes the bottom of the hole picking up material broken by the bit, the fluid and the cutting are then forced upward in the space outside the drill string, to the surface and clearing the hole. The drill pipe and bit move down deepening the hole as the operation proceeds. 

At the surface, drilling mud flows over the shakers and into a to settling pit where it is cleaned of all the small debris that passed through the screens at the shakers. From the settling pit the fluid overflows into another pit where it will be conditioned before it is picked up and re-circulated back through the drill string. 

The walls of the hole are held in place by the pressure of the mud pump against the sides of the hole until the hole section is complete. The hole is then cased off and cement is pimped down the casing and into the annuals. It is then given time to set before the next section of hole is drilled 

Drilling mud has a number of functions in rotary drilling operations. These are as follows: - 

1. Cooling the bit. This is the simplest and one of the most essential duties of drilling mud. Practically any type of mud that can be pumped will perform this function. Otherwise the task of keeping the bits from burning might be a difficult one. 

2. Removing cuttings. This is another essential of drilling mud. The removal of cuttings of a given size and density requires an upward mud velocity of sufficient pressure to force the cuttings out of the hole. 

3. Preventing caving. By providing a hydrostatic pressure the drilling mud tends to hold loosely consolidated, cave - in formations in place. A column of fluid or mud in water well exerts pressure equally in every direction at any point. In consolidating loose or cavy formations such as gravel, sand, glacial drift, etc., the drilling mud should surround the particles of the formation and consolidate them by holding. 

4. Lubricating drill string. Good drilling mud covers everything with a slippery film that is an excellent lubricant.

5. Checking corrosion and rust. When prepared muds are not available, native clays can be used for drilling purposes in emergencies but are inferior. Most of them contain too much sand, which interferes with the sealing properties of the mud.

6. It is the primary well control and is designed by use of weight material to counteract and formation pressures that could be leaking down hole.  The Exploration Drilling for hydrocarbons is aims to identify if any oil and gas is present, and if there is, to then try and figure out how much.

There are essentially four step in this process, things can either go wrong or get better at each step. It is important that as many evaluation steps as possible are carried out and that any further money that is invested is based on the results of each step. Hydrocarbon Shows This step refers to the recovery of rock and fluid samples as part of the drilling process and the evaluation of these samples to see if they contain traces of oil or gas.

The rock samples are examined under a microscope for traces of visible oil and to see if they are porous. They are also looked at under an ultraviolet light, as oil is strongly fluorescent. This is usually reported in the sample description in the daily report, with strong fluorescence being good, weak fluorescence being more ambiguous.

The drilling fluid (mud) is also sampled for any gas and/or volatile hydrocarbons which come back up to the surface with the mud as it is circulated. These are known as gas shows and are usually reported in "units". They are also broken down by their constituents. C1 being methane (natural gas), up to C5 and C6 which are more oily.

Even though there might be a good gas show with strong fluorescence in the cuttings there is no guarantee the oil or gas can be produced. It could be "residual" i.e. only traces left from where oil and gas has moved through the rocks or the rocks could be tight, i.e. they do not have sufficient porosity and permeability to let the oil or gas flow out. To find out if the shows are significant it is necessary to log the hole

 

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