Bottomhole assembly

Bottomhole Assemblies. In general, the drill pipe, drill collars and, in particular, bottomhole assemblies for air (or gas) drilling operations are the same as those in mud drilling. However, because the penetration rate is much greater in air (or gas) drilling operations due to the lack of confining pressure on the bit cutting surface, care must be taken to control unwanted deviation of the borehole. Thus, for air (or gas) drilled boreholes, a packed-hole or stiff bottom-hole assembly is recommended. 

Run the drill string complete with desired bottomhole assembly and bit to bottom. 

We are using a mud motor which rotates at 500 rpm and develops a useful power of 100 hp. Assuming a constant flowrate of 500 gal/min, no pressure loss in the bit nozzles and 80% motor efficiency, compute the total bottomhole assembly AP at 0.2- and 0.5-in. valve opening. 

Downhole Shocks Measurements. An accelerometer in the MWD telemetry tool measures transverse accelerations, or shocks, that may be damaging for the bottomhole assemblies. When acceleration exceeds a certain threshold, the event is signaled to the surface as being a shock. These events versus time or depth are displayed as shock count. This information is used as a warning against excessive downhole vibrations and to alert the driller to change the rpm or weight on the bit [106]. 

A typical MWD bottomhole assembly used in rotary drilling is as follows from bottom to top drill bit, stabilizer, resistivity, WOB torque, directional and telemetry system, neutron-density Pe. The typical distances are seen in Figure 4-296a. When a mud motor is inserted between the lower stabilizer and the drill bit, the distances are increased as shown in Figure 4-296b. 

To meet the challenges posed by horizontal drilling in particular, a system has been developed to make the measurements at or near the bit and transmit them to the mud telemetry section of the MWD bottomhole assembly. The resistivity at the bit tool is similar to the toroidal resistivity tool described in the section titled Resistivity Logs. As shown in Figure 4-297, the Anadrill geosteering package includes below the Power Pack mud motor ... 

Figure 4-296. Typical MWD bottomhole assemblies (a) rotary drilling (b) mud motor drilling.

Packed-hole technique Utilizes the hole wall to minimize bending of the bottomhole assembly. 

Monel (K monel) A permanently nonmagnetic alloy used in making portions of downhole tools in the bottomhole assembly (BHA) where the magnetic survey tools are placed for obtaining magnetic direction information. Monel refers to a family of nickel-copper alloys. 

Near-hit stabilizer A stabilizer placed in the bottomhole assembly just above the bit. 

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