Page 19 - The Geological Interpretation of Well Logs

2

THE LOGGING

ENVIRONMENT

2.1 Introduction height of the fluid column and the density of the fluid.
The pressure in kg in a column of water can be calculated
Treated simply as an instrument of measurement, a
thus:
logging tool is required to do two things: to give a true,
repeatable reading, and to make the reading of a height of water column (m) x density (g/cm*)

Iepresentative, undisturbed sample of the subsurface
10
formation. For the following reasons, neither of these
= pressure (kg) per sq. cm (1)
ideals can be realized.
The first is that the undisturbed formation environment
For a column of pure water of 2500 m (density of pure
is irrevocably disturbed by drilling a well. The new drill-
water = 1.00 g/cm?)
created conditions are those in which the logging tools
work. A tool can only ‘guess’ at the original states. This
chapter examines what is involved in this guess, in terms 2900x1 250kg /cm? Q)
of drilling pressure, drilling temperature and invasion.
The second reason is that the idea] conditions for a
In oilfield terms, the pressure of a column of fluid may be
perfect geophysical measurement cannot be met in bore-
expressed by its pressure gradient. Thus pure water has a
hole logging methods. Ideal conditions would require
gradient of 1.00 g/cm’. That is, a column of pure water
a logging tool to be motionless for each individual
will show a pressure increase of | kg/cm? per 10 m of
measurement, and to have a sensor of zero dimensions
column (or | g/cm? per cm of column) (Figure 2.1). The
measuring a point sample. Sensors have dimensions
term ‘column of water’ is used as applicable to wells:
and tools move. Tool design acknowledges this, and a
‘depth’ is equally applicable and more understandable
compromise is made between a practical and practicable
when talking about water masses, such as the oceans.
measurement and one that is perfect. This chapter will
As water becomes more saline, its density increases
also examine, in general terms, the effects of the logging
(Figure 2.2). Water which has a salinity of 140,000 ppm
method on the measurements made. The notions of
(parts per million) of solids (mainly NaCl), has a density
depths of investigation, minimum bed resolution and
bed-boundary definition will be discussed.
2.2 The pressure environments of
borehole logging and invasion
height)
The pressure environment during drilling and, inevitably,
during logging, is made up of an interplay between two
column
elements; formation pressure and drilling-mud column
pressure.
The formation pressure is the pressure under which the
subsurface formation fluids, and gases are confined. The (fluid
pressure of the drilling mud is hydrostatic and depends
km
only on the depth of a well, that is the height of the mud
depth,
column, and the mud density. Maintaining the pressure
‘-:
=
exerted by the column of drilling mud at just a little above most oilfield brines
the pressure of the subsurface formations encountered
is one of the necessities for equilibrium drilling: it is a

delicate balance. The two pressure environments are

examined below.
90 200 400 600 800
Hydrostatic pressure pressure, kg/cm 2
Fluids transmit pressure perfectly so that the pressure Figure 2.1 Fluid pressure gradients related to depth, or height
exerted by the column of fluid is dependent simply on the of fluid column.