As the industrialization of the world is growing, both energy consumption and
demand are steadily increasing. Hydrocarbons have been an important energy
provider for several decades, but the production from mature oil and gas producers
is now declining. In order to meet this rise in energy demand, new hydrocarbon
deposits must be found and produced. Because large oil and gas reservoirs are
associated with salt formations, this may be an important energy source for the
future. Salts low permeability and ability to deform under stress and temperature,
makes it an ideal hydrocarbon-trap.
The initial objective of this thesis was to establish which challenges are related to
drilling for pre-salt hydrocarbons, and propose solutions for how to overcome these
challenges. When drilling towards these oil and gas reservoirs, the first problems
may occur already in the formations above the salt structures. The density of salt
does not increase with burial depth. When its density becomes lower than of the
surrounding formations, salt will start to migrate and push through the overlying
rocks. Due to the combined effect of compaction disequilibrium and salt tectonics,
complex stress patterns can be created in the formations surrounding the salt
structures. This may lead to many hazardous scenarios, created by rubble zones
and recumbent beds. When drilling inside salt formations, it is salts ability to
creep and flow that may cause problems. Salt flow may cause borehole deformation that impedes with the drilling and casing operations. When a wellbore has been
drilled, salt can creep into the removed rock volume. This may cause situations
such as stuck-pipe, hole instability, and high levels of shock and vibration when
drilling. Salt flow is a positive function of time, so minimizing the time factor
will decrease the possibility of salt flow related problems. One of the measures to
minimize exposure time is to perform drilling operations quickly. Hence, a high
rate of penetration (ROP) is beneficial when drilling in salt.
As part of proposing solutions to the drilling challenges in salt, two new drillbit
technologies have been evaluated. These bits may be beneficial in order to overcome
many of the problems related to salt drilling. Based on the results obtained
in previous studies, these bits are capable of reducing the shock and vibration
levels while drilling. This is because these bits are able to drill smoother than
conventional PDC bits. Further, the reduced shock and vibration levels will allow
an increase in WOB and rotary speed. Based on previous studies, these are
the two of the most important parameters to ROP. Thus, these two new drillbit
technologies might be able to increase ROP when drilling in salt.
Another goal for this thesis was to establish which parameters have the most effect
on ROP when drilling in salt formations. Knowing this could help minimize the
challenges faced due to salt creep and flow. Therefore, a modelling attempt was
performed using Bourgoyne and Youngs ROP model. This model uses multiple
linear regressions to calculate a straight line that best fits the data used in the
model. In this attempt, data acquired from a well drilled in salt formation was
used. Due to lack of variation in the drilling data, several parameters had to
be discarded from the model in order to obtain physically meaningful results.
This led to only three variables being used in the model. This was weight on bit
(WOB), rotary speed of the drillstring, and jet impact force. It was found that
the parameter that had most effect on ROP in salt was WOB, followed by rotary
speed, and last, jet impact force.
Institutt for petroleumsteknologi og anvendt geofysikk , 2014. , 159 p.