Page 208 - Well Logging and Formation Evaluation

198 Well Logging and Formation Evaluation

ground and difficult to interpret. Also, highly deviated wells are often
drilled with OBM (oil-based mud), making hydrocarbon differentiation
difficult.
• Areal variation in the formation is usually much greater than that
expected from the working geological maps. It is very often the case
that subseismic faults of a few meters are encountered, which cause the
well to suddenly go out of the target zone. Often it is not clear whether
one has exited the top or base of the target zone, so one does not know
whether to drill up or down to get back in. Even where faulting is not
present, there may be thinning or deterioration of reservoir properties
that were not envisaged.
• Even where the right geosteering decisions can be made, control of
deviation in the well itself may be a problem. When one is entering a
thin horizon at a steep angle, it may be impossible to avoid immedi-
ately exiting the horizon on the other side. There may also be a ten-
dency for the bit to drop or turn to the right or left, which cannot easily
be controlled. In very long horizontal wells, one may be limited by the
need to keep the drillpipe in tension and have sufficient weight on bit
(WOB) to be able to make further progress.
• Where a horizontal well accidentally penetrates a water-bearing zone,
there may be significant practical difficulties in preventing a large pro-
portion of the well’s production from originating in the water zone. The
possibilities of isolating certain zones in long horizontal wells are very
limited.

In spite of the above limitations, geosteering can be immensely valu-
able in drilling very highly productive wells and can make the difference
between a field being economically viable or not. It may also be the case,
if drilling in a permeable formation surrounded by tight formations or in
a long horizontal well, that the bit will naturally follow a path of least
resistance and steer itself within the most permeable layer, effectively
“bouncing off” the harder layers. An example of a typical geosteered well
through a thin formation is shown in Figure 13.3.1.
With respect to the decisions made by the petrophysicist in the plan-
ning and execution of a geosteered well, it is worthy of consideration that
while one would ideally want as many tools in the hole as possible, with
both up and down measurements of all parameters, one is necessarily
limited by constraints as to what the drillers are prepared to have in the
toolstring (a greater number of tools and their proximity to the bit affect
drillers’ ability to steer the well) and what data can be captured within the