Directional drilling

Directional drilling

      The most common applications of directional drilling are illustrated in and discussed briefly below

    - Multiple wells from artificial structures. Today's most common application of directional techniques is an offshore drilling where an optimum number of wells can be drilled from a single platform. This operation greatly simplifies production techniques and gathering systems, a governing factor in the economic feasibility of the offshore industry

   - Fault drilling. Another application is in fault control where the wellbore deflected across or parallel to the fault for better production. This eliminates the hazard of drilling a vertical well through a steeply inclined fault plane, which could slip and shear the casing

   - Inaccessible locations. The same basic techniques are applied when an inaccessible location in a producing zone dictales remote rig location, as in production located under riverbeds, mountains, cities, etc

   - Sidetracking and straightening. This is used as a remedial operation, either to sidetrack an obstruction by decimating the wellbore around and away from the obstruction, or to bring the wellbore back to vertical by straightening out cooked holes

  - Salt dome drilling. Directional drilling programs are also used to overcome the problems of salt dome drilling, to reach the producing formations, which often lie underneath the overhanging cap of the dome

   - Relief wells. Directional drilling, was first applied to this type of well so that mud and water could be pumped in to kill a wild and cratered well

Basic hole patterns

A carefully conceived directional drilling program on geological information, knowledge of mud and casing program, target etc., is used to select a hole Pattern suitable for the operation

   - Type I,  is planned so that the initial deflection is obtained at a shallow depth (approximately 1000 ft), and the angle is maintained as a "locked in," straight approach to the target. This pattern is mainly used for moderate drilling in areas where the producing formation is located in a single zone location and where no intermediate casing is required. It is also used to drill deeper wells requiring a larger internal displacement

   - Type II, called the "S" curve pattern, is also deflected neat the surface. The drift is maintained, as with type I, until most of the desired lateral displacement is obtained. The hole angle is then reduced and/or returned to vertical in order to reach the target

   - Type III, is planned such that the initial deflection is started well below the surface and the hole angle is maintained to buttonhole target. This pattern is particularly suited to special situations, such as fault or salt dome drilling, or to any situation requiring redrilling or repositioning of the bottom part of the hole

Deflection tools

      A prime requirement for directional drilling is suitable deflection tools, along with special bits and other auxiliary tools. A deflection tool is a mechanical device that is placed in the hole to ensure a drilling bit to be deviated from the present course of the hole. There are numerous deflection tools available for deflecting a hole or correcting direction. The selection of a deflection tool depends upon several factors, but principally upon the type of formation at the point where the hole deviation is to start. The most common tools used for deflection are

Jet biT

Down hole hydraulic motor with a "bent sub

Whip stocks

 Down hole hydraulic motors

          The down hole hydraulic motor with a bent sub is the most widely used deflection tool. It is driven by drilling mud flowing down the drill string to produce rotary power down hole, thus eliminating the need for rotating the drill pipe

         The first variation of the down hole motor is the "turbine" type motor, or "turbo drill." It consists of a multistage vane-type rotor and stator, a bearing section, a drive shaft and a bit rotating sub. The first stage is comprised of a rotor and a stator of identical profile. The stator is stationary and deflects the flow of drilling fluid to the rotor, which is locked to the drive shaft and thus transmits the rotary motion to turn the bit

           The second variation of a down hole motor is the "positive displacement" or "helicoids hydraulic" motor. It consists of two-stage helicoids motor, a dump valve, a connecting rod assembly, and bearing and shaft assembly. The helicoids motor has a rubber-lined spiral steel rotor. As the mud is pumped under pressure from above, it is forced downward between the rotor and the spiral cavity. The rotor is thus displaced and turned by the pressure of the fluid column, powering the drive shaft and resulting in a rotational force that is used to turn the bit

        The bent sub is used to import a constant deflection to the tool. Its upper thread is cut concentric to the axis of the sub body, and its lower thread is cut with an axis inclined 1 to 3 degrees in relation to the axis of the upper thread. In addition, the "hydraulic bent sub" can be locked into position for straight drilling, or unlocked and reset for directional drilling

         Both down hole motors can be used with the following assembly which consists of a full-range bit, the down hole motor, a bent sub or hydraulic bent sub, a non magnetic drill collar, and the normal drill string

Jet bits

         

Where subsurface formations are relatively soft, the hole can be deviated by using a jet bit. In this method, all but one of the jet openings in a conventional bit are closed off or substantially reduced in size. The jet left open has a very large nozzle. This nozzle is oriented in the proper direction on bottom and the pumps are started, but the drill string is not rotated. Instead, it is usually worked up and down slowly, approximately 10 feet off bottom. Then weight is applied on the drill string and bit while jetting, and the jetting action literally washes the formation out from under the jet. After the jetting has set a proper course, the drill string is rotated. Since the washed-out section is the path of least resistance, the bit will follows it. Extra weight is then applied to bow the collar, and the drilling continuous until the correct hole angle is attained

 Whipstocks

        The standard "removable" whipstock is used to initiate the deflection and direction of the well, sidetrack cement plugs, or straighten crooked holes. It consists of a long inverted steel wedge that is concave on one side to hold and guide a whipstock drilling assembly. It also has a chisel point at the bottom to prevent the tool from turning, and a heavy collar at the top to help withdraw the tool from the hole

          The "circulating" whipstock is run, set and drilled like the standard whipstock

However, in case, the drilling fluids flow through a passage to the bottom of the whipstock and circulate the cutting out of the hole, ensuring a clean seat for the tool. It is most efficient for washing out bridges and buttonhole fills

          The "permanent easing" whipstock is designed to remain permanently in the well

It is mainly used to bypass collapsed casting or junk in the hole, or to reenter and drill out old wells. After the bit has been drilled below the whipstock, increased weight is applied until approximately 20 feet of pilot hole has been drilled. The whipstock is then retrieved and the pilot hole opened to full range with a pilot bit and hole opener

 Knuckle joint

         Is one of deviation control methods, which employs the principle of universal joint. The owed half of the tool can be rotated at an angle to the centerline of the body. It is made up on the lower end of drill pipe