Page 149 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

118 Reservoir Engineering

minerals, or gas) can be determined with combined neutron, density, and sonic
logs. This technique will be discussed later. More complex lithologies can be
determined with the aid of special logging tools and computers.
Since well logs infer lithology from physical and chemical parameters, certain
rocks will look the same on logs though they differ in their geologic classifica-
tion. Sandstone, quartz, and chert are all SiO, and appear the same on porosity
logs. The same is true of limestone and chalk. Dolomite, anhydrite, and salt
have very distinct characteristics and are easily distinguished from other rock
types. Shales arc composed of clay minerals. The type and amount of different
clay minerals, which vary widely between shales, can affect their bulk properties.
Permeablllty. Permeability is one of the essential properties used in evaluation
of a potentially producing formation. Unfortunately there are no logging devices
that read permeability. This is because permeability is a dynamic property. Most
logging tools spend only a few seconds in front of any one point of a formation,
therefore it is impossible to measure any time-dependent parameter. There are
methods to estimate permeability from well logs, but they are based on general
assumptions. From a practical standpoint, log parameters only provide an “order
of magnitude” approximation. Several methods of inferring permeability with
well logs arc discussed where applicable in each section.
Two relationships between porosity and irreducible water saturation (SJ are
used to estimate permeability:

1. The Timur relationship [198] (Figure 5-64) for granular rocks (sandstones
and oolitic limestones), which generally gives a more conservative estimate
of permeability.
2. The Wyllie and Rose relationship [43] modified by Schlumberger [199]
(Figure 5-65), which generally gives a higher estimate of permeability.
To enter these charts, porosity and irreducible water saturation (S,) must be
known. Porosity can be obtained from cores or any porosity device (sonic,
neutron, or density). Irreducible water saturation must be found from capillary
pressure curves or it can be estimated. The permeabilities from these charts
should be considered “order of magnitude” estimates.

Influences on Logs
The purpose of well logging is to determine what fluids are in the formation
and in what quantity. Unfortunately the drilling process alters the fluid satura-
tions by flushing the pores near the borehole and filling them with the fluid
fraction of the drilling mud (mud filtrate). To correct for these influences, the
invasion profile must be identified. Figure 5-66, an idealized cross-sectional view
of the borehole and formations, shows an invasion profile and the appropriate
symbols for each part of that profile [199].

Mud Relationships. Since the borehole is filled with mud and the adjacent
portion of the formation is invaded with mud filtrate, mud properties must be
accurately known so they can be taken into account. Mud has a minor influence
on most porosity tools; however, it can have a large effect on the resistivity tools.
In general:
(5-87)