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applied much more during the past decade because of its advantages that include high rate
of penetration, less fonnation damage, and less fluid treatment. The disadvantages are
less borehole stability and pressure control (Guo, 1995).
It is believed that underbalanced drilling will gain momenrum in the furure and
will be applied where feasible. Better and safer well control and borehole stability are
essential for its increased application.
Optimization tools - SIft software
Optimization tools involve developing software to simulate parts of or the entire
drilling process before acrual drilling. This concept was initiated by a major oil company
in the early to mid-80s in their critical drilling facility (COF), where the engineering
simulator for drilling (ESO) (Millheim, -'1983) was used as a cost-reducing tool in
pre-planning, day-to-day and post-job analysis for lowest cost drilling. The concept of
this facility was a closed circuit system capable of monitoring multiple drilling operations
through satellite co=unication and rigsite cameras with input to a central co=and
center where drilling engineers monitor the operations around the world. Part of this
process was. the use of the ESO that had the capability to simulate any aspect of the
drilling operation, or the total drilling operation ,for better safety and/or lower drilling
cost.
The critical drilling facility (COF) and the engineering simulator for drilling
(ESO) led to the development of another generation of smaller expert systems, the
engineering expert systems (ESS) in which the drilling system was broken down into
many smaller engineering tools. The reason for this was that the ESO was too big to run
simple engineering tools.
Today, engineering computer simulation tools exist for any of the hundreds of
engineering systems used on a drilling rig. The objectives of these tools are to .cut cost.
The concept of developing a geological drilling log (GOL) (Onyia, 1985) from the
first field well and then simulate the drilling of the upcoming wells through a learningcurve
has proven to cut drilling cost. Future operations will utilize better computers and
more user-friendly software programs, which will benefit the drilling engineer in any
aspect of the drilling operation.
3-0 planning tools
The use of 3-0 directional survey tools has become co=on practice in planning
directional wells. Currently, many different analytical survey tools are available. They
include the tangential method, average-angle method, minimum curvature method, radius
of curvature method, mercury method. acceleration method, trapezoidal method, vectoraveraging
method (Bourgouyne. 1991). and constant-curvature method (Guo, 1993).
Utilizing these computerized planning tools in conjunction with the constant MWD
monitoring ensures that the surveys are followed and that the target is penetrated for
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