Directional Bottom Hole Assemblies
A conventional rotary drilling assembly is normally used when drilling a vertical well, or the vertical or tangent sections of a deviated well. When using a steerable assembly in a deviated well it is of course possible to drill the tangent sections of the well with the steerable assembly.
The BHA of the conventional assembly can also be designed in such a way as to result in an increase or decrease in the inclination of the wellbore but it is very difficult to predict the rate at which the angle will increase or decrease.
The tendency of a conventional BHA to result in an increase or decrease in hole angle is a function of the flexibility of the BHA & WOB. Since all parts of the drillstring are flexible to some degree the BHA will bend when weight is applied to the bit. This will introduce a tilt angle at the bit. It is very difficult to predict the impact of the above variables on the rate at which the angle will increase or decrease.
Three types of drilling assemblies have been used in the past to control the hole deviation:
Packed Hole AssemblyThis type of configuration is a very stiff assembly, consisting of drill collars and stabilizers positioned to reduce bending and keep the bit on course. This type of assembly is often used in the tangential section of a directional hole.
In practice it is very difficult to find a tangent assembly which will maintain tangent angle and direction. Short drill collars are sometimes used, and also reamers or stabilizers run in tandem.
Pendulum AssemblyThe principles behind a Pendulum Assembly is that the unsupported weight of drill collars will force the bit against the low side of the hole. The resulting decrease or drop off in angle depends on WOB, RPM, stabilization and the distance between the bit and the first reamer.
The basic drop off assembly is: Bit - Monel DC - reamer - DC - stab - DC - stab - 90' DC - stab
To increase the tendency to drop angle :
- Apply less WOB (lower penetration rate)
- Apply more RPM and pump pressure in soft formations where jetting and Reaming down is possible
- Use bigger size Monel DC below the reamer, small DCs above.
Fulcrum AssemblyThe principles behind a Fulcrum Assembly is to place a reamer near the bit and apply a high WOB. When WOB is applied, the DCs above the reamer will tend to bend against the low side of hole, making the reamer act as a fulcrum forcing the bit upwards. The rate of build up depends on WOB, size of collars, position of reamer and stabilization above the reamer.
The basic build-up assembly is: Bit - sub
- reamer - Monel DC - DC - stab - DC - stab - 90’DC - stab
To increase the build:
- Add more WOB
- Use smaller size monel (increase buckling effect)
- Reduce RPM and pump rates in soft formations