Bit Selection Guidelines

Monday, September 21, 2009 by ShoXee

BIT SELECTION GUIDELINES

Bit selection begins with a thorough examination of bit records from offset wells data. the
best and worst performance and dull bit grading in formations comparable to the well being
designed should be examined, analysed and the used to determine the characteristics of the
best performing drill bits. In particular attention should be placed on the details such as the
premature failure of bits, reasons drillbits pulled, dull characteristics of inserts: whether the
inserts were worn or broken, etc. A drill bit that had broken inserts clearly indicate that the
formation should have been drilled with a much harder drillbit. This detailed examination
will be explained in the next sections of this Topic
Data required for the correct bit selection include the following:

1. Prognosed lithology column with detailed description of each formation
2. Drilling fluid details
3. Well profile

Formation characteristics should be studied in detail to assess the type of cutting structure
required to successfully drill the formation. The existence of abrasive and hard minerals such
as chert or pyrite nodules should be identified. This will impact on the aggressiveness of the
selected milled teeth or insert bits and, in the case of PDC bits, the requirement for hybrid
design bits.
Gauge protection (which determines the final hole size) is particularly critical in abrasive
formations where the gauge could be lost very quickly resulting in an undergauge hole which
requires reaming during the next bit run. For highly abrasive sections the use of insert bits
with diamond enhanced gauge protection prevents the occurrence of undergauge hole and
reduces reaming on subsequent bit runs.
When drilling directional wells the Directional Contractor should be asked to provide an
assessment of the required BHA changes, motor requirements and any limitations on bit
operating parameters which may impact on the selection of bits. In addition bit
characteristics in terms of walk, build and drop tendencies will need to be assessed for their
impact on the well path.
When using a mud motor in the assembly all tri-cone bits should have a motor bearing
system which allows extended use at high motor RPM‘s or a fixed cutter bit should be
selected.
Due consideration should always be given to the jet system of the bit. When drilling soft
shale sections where the major limitations on ROP is bottom hole and cutter cleaning, the use
of centre jet, extended jets or lateral jet bits should be considered.