Directional Drilling
Usually but not always, the crew tries to drill the hole as straight as possible, but at times it is desirable to deflect the hole from vertical. The most dramatic example of this is the offshore platform. Many wells may be drilled from a single platform without having to move the rig. The technique used is called “directional drilling”.
Only the first hole drilled into the reservoir tray be vertical; each subsequent well may be drilled vertically to a certain depth then kicked-off (deflected) directionally so that the bottoms of the hole ends up away from its starting point on the surface.
By using directional drilling, as many as twenty or more wells may be drilled into the reservoir from one platform. Thus directional drilling has become a routine development operation throughout the world.
Controlled directional drilling has many applications. It is used for inaccessible locations, offshore drilling from shore or sea platforms, geological corrections, relief wells, side-tracking tools lost in the hole, redrills to save surface cased hole and a portion of the open hole and bottom hole re-completion.
Drilling to total depth (td)
The final part of the hole is what the operating company hopes will be the production hole. To drill it the crew picks out the smallest bit, say one of 8 inches. This bit is tripped in, drills out the intermediate casing shoe and heads toward what everyone hopes is “the pay zone”, a formation capable of producing enough oil and/or gas to make it economically feasible for the operating company to complete the well. Once again, several bits will be dulled and many round trips made and soon the “formation of interest” (the pay zone; the oil sand or the formation that is supposed to contain hydrocarbons) will be penetrated by the hole. It is then time to consider whether the well contains enough oil or gas (or strong enough suggestions of ) to make it worthwhile running the final production string of casing and completing the well.
Coring
Besides the above mentioned tests, formation core samples are sometimes taken. Two methods of obtaining cores are frequently used.
Conventional coring:
An assembly called a "core barrel" is made up on the drill string with a special type of bits called ”Core Head” and run to the bottom of the hole. As the core barrel is rotated, it cuts a cylindrical core a few inches in diameter that is received in a tube above the core cutting bit. There are many types of core barrel in use as the conventional, the rubber sleeved, The fibreglass, ..Etc. A complete round trip is required for each core taken.
Sidewall coring
In a sidewall sampler a small explosive charge is fired to ram a small cylinder into the wall of the hole. When the tool is pulled out of the hole, the small core samples come out with the tool.
Either type of core can be examined in a laboratory and may reveal much about the nature of the reservoir.
Tripping
There is a number of occasions when all the drill pipe would have to be removed from the hole and then re-run again.
Imagine a well having reached a depth of say 10,000’. Then bit has become worn and needs replacing. This means that 10,000’ of drill pipe must be pulled from the hole, then the old bit is replaced with a new one.
Once this is done, everything must then be lowered back into the hole until the bit is on the bottom and drilling can recommence.
The pipe would have to come out of the hole to change a worn bit, or to insert casing, or if any part of the bottom hole assembly had to be changed, ....,etc.
This whole process is called a round trip and would take a very long time however, and time means money in the drilling industry.
To save time, the pipe is removed in lengths of three joints of pipe. Each length of three joints, which measures approximately 100’, is called a stand.
This process is done in the steps below:-
Step 1
The drillpipe is suspended in the hole and the kelly is disconnected. (Using slips and tongs as before).
Step 2
The kelly is swung across the rig floor and lowered into the rat hole, then the swivel is unlatched from the travelling block hook. The rat hole is a tube rather like the mouse-hole. It provides a storage receptacle for the kelly, kelly bushings and swivel when they are not in use during the round trip.
Step 3
One of the rig crew - the derrickman, climbs to the monkey board high in the derrick. He secures himself at this working platform using a safety harness. It is his job to handle the top of the stand during the round trip.
Step 4
Elevators are latched around the drill pipe just below the tool joint. The elevators are a set of hinged clamps, which are part of the hook and travelling block assembly. They are connected to links which themselves are attached to the eyes of the hook.
Step 5
The driller can now start to pull the drill string out of the hole. As he starts to raise the string, the slips are removed by the roughnecks on the rig floor. The string is then lifted until the third tool joint is clear of the rotary table and the slips are re-set.
We now have a stand of drill pipe up in the derrick being held by the elevators, while the rest of the string is in the hole suspended from the slips.
Step 6
The next job for the roughnecks, is to disconnect the stand from the drill string. This is done using the tongs and pipe spinner. The lower end of the stand is then swung to one side of the rig floor and stood down.
Step 7
The derrickman job now is to unlatch the elevators having first secured the top of the stand with a rope. With the stand now clear, he can pull the top of the stand into the fingers of the monkey board. The stand is now racked (stored) in the derrick.
Step 8
The driller now lowers the travelling assembly, allowing the roughnecks to latch the elevators round the next tool joint ready to pull another stand.
The procedure which was just described is repeated until all the pipe is out of the hole. Depending on the depth of the hole, this could take an entire day to complete.
The drill collars and bit are the last items to come out of the hole. To unscrew the bit from the bit sub, a device called a bit breaker is placed in the rotary table.
This piece of equipment holds the bit while the tongs are used to break the connection.
We have now seen one half of a round trip, tripping out. The second half of a round trip is called tripping in and is just the reverse procedure. See figure 27
