Directional Drilling

Introduction

Many problems such as fishing, restricted surface location, salt domes and water coning are faced during drilling in addition to, weakening of oil prices and the need of reducing drilling costs. A suitable way to overcome these dilemmas is to use directional drilling.
Directional drilling is a technique used for controlling the direction and deviation of a wellbore along a predetermined course to a subsurface target located at a certain distance and direction from a surface location.



Directional drilling applications:

Directional drilling is used for many reasons as:



  • Sidetracking existing wells: a fish may become stuck at the bottom of the wellbore during drilling so, no progress can be made if the isn’t pulled out of the wellbore. To remedy this; a cement plug is placed on top of the fish then, the wellbore is deflected around the fish. The wellbore is continued down to the target. Also, sidetracking may be carried out for re-drilling if theoriginal well didn’t locate at the required formation or is producing from a depleted zone.
  • Inaccessible surface locations:oilfields may be located under natural or man-made obstructions. If it is not permitted to drill in such areas, it may be possible to use directional drilling from the surface location outside the restricted area.
  • Geological requirements:salt domes may be directly above the reservoir, so that a vertical well will have to penetrate them which would cause certain drilling problems such as large washouts–enlarged area of the wellbore caused by the removal of formation grains during drilling–, lost circulation–the flow of mud into a formation–, and corrosion of drill pipes. Directional drilling can be used to avoid these salt formations.
  • Offshore development drilling: by reducing the number of offshore platforms. A fixed platform is installed on the seabed from which directional well can be drilled.
  • Horizontal drilling:to increase productivity, produce from low-permeability reservoirs, andinject steam or CO2.
  • Reach multiple targets with a single wellbore reduces rig moves, field area and grantees easy and cheap well completion and production
  • Drilling a relief well which is used to reduce the pressure of a well when blowout occurs. Relief well is drilled at a safe distance from the original well and it is directionally drilled to intersect the originalone, then a heavy drilling mud “kill mud” is pumped into the relief wellbore to reduce the high pressure in the original wellbore.
Types of well profiles:

Directional wells are first drilled vertically until they reach a point called kick off point which is defined as the point below the surface at which the well is deflected from the vertical position. Total vertical depth (TVD) is the vertical distance from the surface (rotary table or sea level) to the point at which survey is done.
Buildup rate is change in inclination per measured length drilled by the use of deflection tools used to deflect the wellbore.
To build an angle is to increase the deviation of the well and to drop it is to decrease the well’s deviation.



True Vertical Dip

Type one
Build and hold trajectory.
Type two
S-Shaped trajectory.
Type three
Deep kick off trajectory.

Well Types


Mechanical methods of building or dropping angle

To deflect the wellbore, the following mechanisms are used:


Whipstocks

The whipstock method is one of the earliest tools. It is the most reliable method, but it is time-consuming. It is a wedge-shaped steeltool used to bend the drillstring. The whipstock is run into the well on a drillstring and oriented by survey instruments. Weight is then applied to the drillstring to break a shear pin and separate the whipstock from the drillstring, which is then pulled out. A small-diameter bit is then run in the hole on a drillstring to drill a pilot hole. The pilot hole is then surveyed. If it has the right orientation, the hole is then enlarged with a normal bit.
Whipstock

Jetting

Jetting bit which is a tricone bit with one large nozzle ”toolface” and two small nozzles is another method of deviating the trajectory of a wellbore. Water or mud is pumped through the large nozzle that is oriented in the direction of the desired trajectory change.When the orientation is correct, mud is circulated at maximum possible flow rate without rotating the drillstring. The hydraulic action of mud jetting out of the large nozzle erodes the well out in that direction. The drillstring is then pulled, and the pilot hole is surveyed. If it is oriented correctly, the pilot hole is then drilled out with a normal bit. Jetting is affected by formation hardness. Medium and soft sandstones are suitable for jetting but very soft unconsolidated sands aren’t and they will erode very much.

Downhole mud motors

Another method to deviate the wellbore is to use a steerable downhole assembly which is a combination of a bent sub, downhole mud or turbine motor, stabilizers to centralize the assembly, and a diamond or PDC ”polycrystalline diamond compact” bit. The motor consists of a rotor which is a spiral shaft and a stator than surrounds the rotor. Mud flowing down the center of the drillstring through the space between the rotor and the stator rotates the shaft causing it to turn the drill bit.
The steerable downhole assembly is run into the well and oriented in the right direction. The drillstring remains stationary as the mud motor is activated to kick off the deviated well.

Positive Displacement Motor
Conclusion

Directional drilling is a technique used to overcome some problems such as sidetracking, inaccessible surface location and geological consideration by deviating the vertical wells using certain tools such as jetting, Whipstocks and downhole mud motors.



References:

  • Inglis, T. A. : Directional Drilling Petroleum Engineering and Development Studies, Vol.2, Published by Graham and Trotman 1985
  • Nontechnical Guide to Petroleum Geology, Exploration, Drilling & Production, 3rd Edition
  • Rabia.H “well engineering and construction”
  • Http://www.petrowiki.org/