Page 16 - Basic Well Log Analysis for Geologist

CHAPTER I

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

Introduction

As logging tools and interpretive methods are developing unfamiliar with petrophysical logging, some confusion may
in accuracy and sophistication, they are playing an develop over the use of the word /og. In common usage, the
expanded role in the geological decision-making process. word /og may refer to a particular curve, a suite or group of
Today. petrophysical log interpretation is one of the most curves, a logging tool (sonde), or the process of togging.
useful and important tools available to a petroleum Rock properties or characteristics which affect logging
geologist. measurements are: porosity, permeability, water saturation,
Besides their traditional use in exploration to correlate and resistivity. It is essential that the reader understand these
zones and to assist with structure and isopach mapping, logs properties and the concepts they represent before
help define physical rock characteristics such as lithology, proceeding with a study of log interpretation.
porosity, pore geometry. and perineability. Logging data is Porosity—can be defined as the percentage of voids to
used to identify productive zones, to determine depth and the total volume of rock. It is measured as a percent and has
thickness of zones, to distinguish between oil, gas, or water the symbol #.
in a reservoir. and to estimate hydrocarbon reserves. Also,
_ volume of pores
geologic maps developed from log interpretation help with
Porosity (6) = ‘total volume of rock
determining facies relationships and drilling locations.
Of the various types of logs, the ones used most The amount of internal space or voids in a given volume of
frequently in hydrocarbon exploration are called open hole rock is a measure of the amount of fluids a rock will hold.
logs. The name open hole is applied because these logs are The amount of void space that is interconnected, and so able
recorded in the uncased portion of the well bore. All the to transmit fluids, is called effective porosity. Isolated pores
different tvpes of logs and their curves discussed tn the text and pore volume occupied by adsorbed water are excluded
are this type. trom a definition of effective porosity.
A geologist’s first exposure to log interpretation can be a Permeubilitv—is the property a rock has to transmit
frustrating experience. This is not only because of its fluids. It is related to porosity but ts not always dependent
lengthy and unfamiliar terminology, but also because upon it. Permeability is controlled by the size of the
knowledge of many parameters, concepts, and connecting passages (pore throats or capillaries) between
measurements is needed before an understanding of the pores. It is measured in darcies or millidarcies and is
logging process is possible. represented by the symbol K,. The ability of a rock to
Perhaps the best way to begin a study of logging is by transmit a single fluid when it is 100% saturated with that
introducing the reader to some of the basic concepts of well fluid is called absolute permeability. Affective permeability
log analysis. Remember that a borehole represents a refers to the presence of two fluids in a rock, and is the
dynamic system: that fluid used in the drilling of a well ability of the rock to transmit a fluid in the presence of
affects the rock surrounding the borehole, and therefore, another fluid when the two fluids are immiscible.
also log measurements. In addition, the rock surrounding Formation water (connate water in the formation) held by
the borehole has certain properties which affect the capillary pressure in the pores of a rock serves to inhibit the
Professional Well Log Analysts, 1975). For the reader otherwise reduce the ability of other fluids to move through
movement of fluids into and out of it.
transmission of hydrocarbons. Stated differently, formation
water takes up space both in pores and in the connecting
The two primary parameters determined from well log
passages between pores. As a consequence, it may block or
measurements are porosity, and the fraction of pore space
filled with hydrocarbons. The parameters of log
the rock.
interpretation are determined both directly or inferred
indirectly, and are measured by one of three general types of
Relative permeability is the ratio between effective
permeability of a fluid at partial saturation, and the
logs: (1) electrical, (2) nuclear, and (3) acoustic or sonic.
permeability at 100% saturation (absolute permeability).
The names refer to the sources used to obtain the
When relative permeability of a formation’s water is zero,
meusurements. The different sources create records (logs)
then the formation will produce water-free hydrocarbons
which contain one or more curves related to some property
in the rock surrounding the well bore (see Society of
(i.e. the relative permeability to hydrocarbons ts 100%).
With increasing relative permeabilities to water, the