BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
In general a mud weight window will exist. The “width” of the window will depend on a
number of operator controlled factors, but primarily well inclination. Increased well inclination
will usually reduce the width of the mud weight window (Figure 7 - 2), thus increasing the risk
of straying from the region of safe mud weights. Hence, Extended Reach Drilling (ERD) wells
are typically more prone to instability than other more conventional wells drilled in the region.
Another factor strongly influencing the integrity of the hole is the open hole time. Even stable
shales are seldom stable for an indefinite period and the longer the open hole time the greater
the risk that instability will occur. This is particularly the case when using water based drilling
fluids. Increased hole section length and therefore increased open hole time are a natural
consequence of drilling ERD wells. Where possible, a gradual increase in drilling fluid density
can be effective in combating this time element and can stabilize the formation for a longer
period.
When assessing ERD options in a region previously drilled with conventional wells, the
primary hole stability consideration is to assess the impact of trajectory on the mud weight
window. If conventional drilled wells have proven difficult to drill due to a narrow mud weight
window, then serious thought must be given as to whether a casing program can be designed to
combat the increased risks projected in the ERD well.
Preventative Action – Mechanical
Pre Drilling
The purpose of any data collection is to attempt to define the optimum drilling window for
offset wells and to project that window to planned wells. Without any offset well data then
there is little value in any wellbore stability study.
The data of most value are:
• Drilling and Completion reports from offset wells (which may contain much of the other
information listed below).
• Details of any formation stress tests including Leak Off Test (LOT) and Formation
Integrity Test (FIT).
• Daily mud properties.
• Details of any drilling fluid losses encountered.
• Details of any pipe sticking and/or excess reaming.
• Composite logs, dipmeter or borehole geometry logs, any caliper logs, density logs and
sonic logs.
• Description of any major faulting in the region (normal, strike-slip, etc).
Planning Stage
Well Inclination
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-4