Page 314 - Petrophysics 2E

FORMATION EVALUATION 285

The sidewall coring device contains a hollow bullet which, when fired
from an electric control panel at the surface, embeds itself in the
formation wall. With the core sample caught in the bullet, a flexible
steel cable retrieves the bullet and its contained core (approximately 1
in. in diameter and 1 in. in length) to the surface. Sidewall diamond
coring is necessary in hard rocks. A relatively new technique, known
as directionally oriented coring, involves the scribing of grooves along
the axis of the core in a gyroscopically controlled orientation [42]. This
method requires periodic stops to take a measurement of orientation
and is accomplished by replacing the conventional inner core barrel sub
with the scribe shoe sub. The main purpose of oriented coring is to
allow visualization of rock in its exact reservoir condition orientation,
which may be useful in predicting reservoir continuity, especially in
fluvial deposition systems [43].
The early methods of core analysis were more an art than a science, and
the results were not taken seriously. The practice of breaking the core
into small pieces to smell and taste for the presence of hydrocarbons was
widespread, even though it was well known that the sweet gases, i.e.,
gases that do not contain hydrogen sulfide, have no apparent odor or
taste [41].
Consequently, many gas formations were diagnosed as water
productive because of the inability physically to detect gas. Today, core
analysis is a highly specialized phase of petroleum reservoir engineering.
Analysis of sidewall cores provides far more geological information than
bit cuttings. Core data play an important role in exploration programs,
well completion, and reservoir evaluation programs. Core analysis makes
it possible to recognize the structure of the reservoir trap, determine its
physical characteristics such as porosity and permeability, and estimate
production possibilities of exploratory wells. Core data allows wells to
be properly completed by selecting intervals for drillstem testing and
evaluating the effectiveness of completion. In the field development
stage, core measurements are employed to estimate hydrocarbon
reserves, determine contacts between reservoir fluids such as water-oil
contact line, and their variations across the field [44, 451. Table 4.18
shows typical data obtained from core analysis and their use. Routine
core analysis results are usually presented in tabular or in graphical form
as shown in Figure 4.49. For the purpose of recognizing the stratification
effect, the graphical form is preferred.

WELL LOG ANALYSIS

Well logging can be defined as a tabular or graphical portrayal of
any drilling conditions or subsurface features encountered that relate to