the horizontal extremities of a surrounding
is more difficult. floa-
ter or jack-up rig might be used to drill an isolated satellite well that could be
tied in with sea-bed production pipe to the central platform, but to drill the
required twenty, thirty or even sixty development wells from mobile rigs and
then tie them in by pipelines would be inordinately expensive. Numerous
directional holes are therefore usually drilled from fixed production/drilling
platforms to tap the fields around them, and the techniques then employed
are basically the same as those used when a floater has to drill directionally
for some reason. This might be, for example, to penetrate a reservoir at a
certain angle to take advantage of natural formations, or to by-pass an obsta-
cle blocking the path of the vertical well, or to drill a relief well into a well
that has suffered a blow-out. Directional drilling required special skills,
techniques and equipment, and very often it is done on a floater under the
guidance of a highly trained and skilled directional driller who will be spe-
cially hired by the operator.
Directional drilling can be employed for a number of purposes. In most cases from a floater it is to sidetrack an obstacle.
Normally a directional well is drilled vertically for a short initial distance which is cased before deviation is begun from the ‘kick-off point’. Any of a variety of drilling tools could be used from this point, depending on the characteristics of the hole required. Most frequently offshore a ‘downhole drilling motor’ is used in conjunction with a ‘bent sub’, but in some cases a long, tapered steel wedge called a ‘whipstock’ is set on the bottom to start the hole in the desired direction.
The drilling motor converts the hydraulic energy of the mud stream in the drill pipe into mechanical energy to turn the bit. Two types of motor are used, one a turbine, the other a positive-displacement motor, both of which force the drill bit to turn without any rotation of the drill string being needed. However, the rotary is normally kept turning slowly whilst the
‘mud motor’ or ‘turbo-drill’ are being used in order to prevent any sticking of the drill pipe to the wall of the hole. Interestingly, the Russians are reckoned to be the most common users of mud motors.
Some of a directional driller’s tools. Left: A mud motor. Right: A drilling turbine in three sec- tions.
A whipstock can veer a bit away from the vertical and make it drill a window through casing.
The tapered shape of a whipstock is designed to force the bit away from as the bit drills downwards. This long tool is attached to and car- ried down to the bottom of the hole by the drill string, then unlatches automatically, allowing the bit to travel on past it and make the ‘pilot hole’ for the deviation. Once this has been done the whipstock can be recovered, and the motorised bit makes the new directional hole unaided. Special direc- tion-seeking instruments are incorporated in the bottom hole assembly to orientate the whipstock in the desired direction and later, following its with- drawal, to check the course of the hole from time to time.
The correct make up of the bottom hole assembly is most important, and numerous variations can be employed to either make the bit maintain direc- tion or build up angle as required. Where there is room in the hole, stabiliz- ers are used to provide a pivot at the desired point, while ‘packed hole assemblies’ that effectively fill the space created by the bit ensure that the BHA stays rigid and straight. Frequent surveys are made with the magnetic or gyroscopic compass-like instruments, and the bit can be steered with amazing precision to its target or round an obstacle. If magnetic survey instruments are used, the drill collars that they are run inside can not be made of magnetic material, so special non-magnetic collars, sometimes cal- led ‘monels’ are used. These are shiny, like stainless steel.
Top: A or non-magnetic drill collar. Bottom: This gyro instrument package an out- side diameter of only 2% inches.
There is a wide range of survey tools on the equipment market, many of them employing a technique in which a miniature compass card looking rather like a small fairground rifle target is fitted to and punched by a narrow projectile which is dropped down the hole. This is attached to and retrieved by a thin wire called a ‘sand line’ which runs down through the drill string, and from the card the angle of inclination and the direction of the bit’s path can be read. One such instrument commonly used to check the drift of vertical holes offshore is called a ‘Totco’ after the name of its manufacturer.
Left: A gyro survey instrument. Right: A measurement-while-drilling (MWD) tool sends pulses back to the rig through drilling mud.
Once the bit is on its deviated path, the directional tools may only be needed occasionally to correct large deviations from the desired course of the well. Alternatively, the build-up of angle continues towards the desired maximum, which may be 30 or 40 degrees or more, the rate of build-up of angle being about 2 or 2% degrees per 100 feet. Once the desired angle is obtained it is usually maintained steady until the well is completed, as this allows a ‘stiff’ assembly to be used which results in faster drilling.
When directional holes are drilled to relieve wells blowing out, the target area for the bit is usually at the base of the blowing well just above the penet- rated reservoir, and drilling begins from a location a thousand or more feet away horizontally on the surface.
When the blowing well is finally penetrated, large quantities of
weighted drilling fluid are pumped down the relief well to kill the flow in the main well. Piloting a wandering drilling bit towards a narrow hole thousands of feet down and to one side is a technical operation that calls for the highest degree of skill and experience, and some of the names of these drillers are almost legendary in the industry.
Another type of direction drilling, called ‘slant drilling’, is done by a small number of specially-equipped jack-up rigs. Here the derrick is slanted to an angle of about 30 degrees, so that the hole is deviated from the surface and not after some depth of vertical hole has been drilled. This allows grea- ter angles to be ultimately obtained and enables deviated wells to be drilled in relatively shallow depths. Slant drilling might be useful, therefore, in exploiting a wide reservoir lying under shallow water with the minimum number of platforms being built.
So that the slanted well is begun from the position of what will eventually be a production platform, the jack-up rig is positioned close alongside the new platform’s basic structure, or ‘jacket’, and ‘skids’ its whole drilling pac- kage onto the jacket, through a corner of which drilling then takes place. On completion of the first wells, the drilling package can be skidded around to another corner on the jacket to drill other slanted wells, and can also be straightened up to drill vertical wells before finally being skidded back over to the jack-up rig. Once back on the jack-up the drilling package can drill
in the normal manner from its cantilever beams.
The cost of drilling a directional well from a mobile unit or a platform is normally greater than that of drilling a vertical well of the same hole depth. This is because of the slower drilling rate and the time required to make sur- veys of the course of the well, and to correct the course where necessary.
This MWD log shows the decrease in weight on the bit as a result of sharp dog legs in its path.