Page 159 - Hydrocarbon Exploration and Production Second Edition

146 Data Gathering

samples
rock
coring bit

Figure 6.35 Sidewall coring tool.

extensively to sample microfossils and pollen for stratigraphic analysis (age dating,
correlation, depositional environment). Qualitative inspection of porosity is
possible, but very often the sampling process results in a severe crushing of the
sample, thus obscuring the true porosity and permeability.
In a more recent development a new wireline tool has been developed that
actually drills a plug out of the borehole wall. With sidewall coring (Figure 6.35),
some of the main disadvantages of the SWS tool are mitigated, in particular the
crushing of the sample. Up to 20 samples can be individually cut and are stored in a
container inside the tool.

6.3.4. Wireline logging
Wireline logs represent a major source of data for geoscientists and engineers
investigating subsurface rock formations. Logging tools are used to look for
reservoir quality rock, hydrocarbons and source rocks in exploration wells, support
volumetric estimates and geological/geophysical modelling during field appraisal
and development, and provide a means of monitoring the distribution of remaining
hydrocarbons during the production lifetime.
A large investment is made by oil and gas companies in acquiring openhole log data.
Logging activities can represent between 5 and 15% of total well cost. It is important
therefore to ensure that the cost of acquisition can be justified by the value of
information generated and that thereafter the information is effectively managed.
Wells can be broadly divided into two groups in terms of how logging
operations should be prioritised: information wells and development wells.
Exploration and appraisal wells are drilled for information and failure to acquire
log data will compromise well objectives. Development wells are primarily drilled as
production and injection conduits and whilst information gathering is an important