Page 207 - Well Logging and Formation Evaluation

Well Deviation, Surveying, and Geosteering 197

4. Pressure/gradient determination. Accurate knowledge of the TVD
at which pressure measurements are made is essential for accurate
determination of gradients and correction of formation/well pressures
to a common datum reference.
5. Legal reasons. When drilling close to a concession or national bound-
ary, it may be essential to avoid accidentally crossing such a boundary.
There may also be implications for equity determination and unitization.
Most government bodies will have minimum requirements regarding
survey accuracy and maintenance of a proper database of survey data.

13.3 GEOSTEERING

Geosteering is the use of information gained while drilling to make real-
time decisions on the trajectory of the well. Such decisions may be essen-
tial to optimize the utility of a well. Geosteering is used (a) in high-angle
deviated wells in thin formations where productivity can be achieved only
if the wellbore remains in a thin permeable zone and (b) in horizontal
wells where it is necessary to remain a fixed distance from either a fluid
contact or an overlying tight formation, as well as during (c) drilling in
close proximity to a fault where it is necessary to establish whether or
not the fault is close and should be crossed and (d) drilling with a fixed
orientation to natural fractures.
Data that may be used in the decision-making process during geosteer-
ing concern (1) deviation; (2) cuttings, including hydrocarbon shows and
gas readings; (3) transmission of LWD (logging while drilling) tools in
real time, typically up/down GR (gamma ray), density, neutron, and resis-
tivity; and (4) drilling parameters, such as losses, kicks, rate of penetra-
tion (ROP), and torque.
Geosteering is often much harder in practice than anticipated, due to
the following factors:

• Tools used in the decision-making process are typically run some way
behind the bit (possibly up to 30m). Therefore, if the bit is not where
you want it to be, you will often not know about it until quite a bit of
formation has been penetrated.
• In high-angle wells, there are often problems with real-time data trans-
mission through mud pulses arising from noise, high ROP, tool failures,
battery life limitations, and bandwidth.
• Cuttings data may take up to 2 hours to reach surface (the “bottoms-
up” time). Where a turbine is used, the cuttings may be very finely