Transcript *** 1

BASIC WELL LOGGING ANALYSIS
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Hsieh, Bieng-Zih
Fall 2009
OUTLINES
Introduction
 Borehole Environment
 Invaded Zone, Flushed Zone, Uninvaded Zone
 Invasion and Resistivity Profiles
 Basic Information Needed in Log Interpretation
 Exercises (#1A, #1B, #2A, #2B)

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INTRODUCTION

Well log, Wireline Log,
Geophysical well logging, Log

A continuous measurement of
formation properties with
electrically powered instruments
to infer properties and make
decisions about drilling and
production operations.
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INTRODUCTION (CONT.)

The record of the
measurements, typically a
long strip of paper, is also
called a log.
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INTRODUCTION (CONT.)

In wireline measurements, the logging
tool (or sonde) is lowered into the open
wellbore on a multiple conductor, contrahelically (反螺旋) armored wireline.

Once lowered to the bottom of the
interval of interest, the measurements are
taken on the way out of the wellbore.
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INTRODUCTION (CONT.)

This is done in an attempt to maintain tension on the cable
(which stretches) as constant as possible for depth
correlation purposes.

(The exception to this practice is in certain hostile
environments in which the tool electronics might not survive
the temperatures on bottom for the amount of time it
takes to lower the tool and then record measurements
while pulling the tool up the hole. In this case, "down log"
measurements might actually be conducted on the way
into the well, and repeated on the way out if possible.)
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INTRODUCTION (CONT.)

Most wireline measurements are recorded continuously
even though the sonde is moving.

Measurements include electrical properties (resistivity and
conductivity at various frequencies), sonic properties,
active and passive nuclear measurements, dimensional
measurements of the wellbore, formation fluid sampling,
formation pressure measurement, wireline-conveyed
sidewall coring tools, and others.
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INTRODUCTION (CONT.)

Certain fluid sampling and pressure-measuring tools
require that the sonde be stopped, increasing the chance
that the sonde or the cable might become stuck.

Logging while drilling (LWD) tools take measurements in
much the same way as wireline-logging tools, except that
the measurements are taken by a self-contained tool near
the bottom of the bottomhole assembly and are recorded
downward (as the well is deepened) rather than upward
from the bottom of the hole (as wireline logs are
recorded).
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BOREHOLE ENVIRONMENT
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BOREHOLE ENVIRONMENT

Where a hole is drilled into a formation, the rock plus the
fluids in it (rock-fluid system) are altered in the vicinity of
the borehole.

A well’s borehole and the rock surrounding it are
contaminated by the drilling mud, which affects logging
measurements.

Fig. 1 is a schematic illustration of a porous and
permeable formation which is penetrated by a borehole
filled with drilling mud.
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FIG. 1
Exercise:
You have 15
min. to fill in
your answer
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FIG. 1
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THE DEFINITION OF SYMBOLS USED IN FIG. 1
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DIAMETER
dh – hole diameter
 di – diameter of invaded
zone (inner boundary,
flushed zone)
 dj – diameter of invaded
zone (outer boundary,
invaded zone)
 Δrj – radius of invaded
zone (outer boundary)

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HOLE DIAMETER

A well’s borehole size is described
by the outside diameter of the drill
bit.
But, the diameter of the borehole
may be larger or smaller than the
bit diameter because of
 (1) wash out and/or collapse of
shall and poorly cemented porous
rocks
 (2) build-up of mudcake on porous
and permeable formation

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HOLE DIAMETER (CONT.)

Borehole sizes normally vary from
7 7/8 inches to 12 inches, and
modern logging tools are designed
to operate within these size ranges.

The size of the borehole is
measured by a CALIPER LOG.
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MUD
hmc – thickness of mudcake
 Rm – resistivity of the drilling mud
 Rmc – resistivity of the mudcake
 Rm – resistivity of mud filtrate

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DRILLING MUD

Today, most wells are drilled with rotary bits and use
special mud as a circulating fluid.

The mud helps remove cuttings from the well bore,
lubricate (潤滑) and cool the drill bit, and maintain an
excess of borehole pressure over formation pressure.

The excess of borehole pressure over formation pressure
prevents blow-outs.
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BLOW-OUT
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DRILLING MUD (CONT.)

The density of the mud is kept high enough so that
hydrostatic pressure in the mud column is always greater
than formation pressure.

This pressure difference forces some of the drilling fluid
to invade porous and permeable formations.

As invasion occurs, many of the solid particles (i.e. clay
minerals from the drilling mud) are trapped on the side
of the borehole and form MUDCAKE.
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DRILLING MUD (CONT.)

Fluid that filters into the formation during invasion is
called MUD FILTRATE.

The resistivity values for drilling mud, mudcake, and mud
filtrate are recorded on a LOG HEADER.
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LOG HEADER
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RESISTIVITY
Rw – resistivity of formation water
 Rs – resistivity of shale
 Rt – resistivity of uninvaded zone
(true resistivity)
 Rxo – resistivity of flushed zone

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SATURATION
Sw – water saturation of
uninvaded zone
 Sxo – water saturation of
flushed zone

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INVADED ZONE
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INVADED ZONE

The zone which is invaded by mud
filtrate is called the invaded zone.

It consists of a flushed zone (Rxo)
and a transition or annulus (Ri) zone.

The flushed zone occurs close to the
borehole where the mud filtrate has
almost completely flushed out a
formation’s hydrocarbon and/or
water (Rw).
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INVADED ZONE (CONT.)

The transition or annulus zone,
where a formation’s fluids and mud
filtrate are mixed, occurs between
the flushed (Rxo) zone and the
uninvaded (Rt) zone.

The depth of mud filtrate invasion
into the invaded zone is referred to
as the diameter of invasion (dj).
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INVADED ZONE (CONT.)

The diameter of invasion is
measured in inches or expressed as
a ratio:
dj/dh
where dh = borehole diameter
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QUESTION -




General invasion diameters are:
dj/dh = 2 for ？
porosity rocks
dj/dh = 5 for intermediate porosity rocks
dj/dh = 10 for ？ porosity rocks
High or Low porosity? And why?
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INVADED ZONE (CONT.)

The amount of invasion which takes place is dependent
upon the permeability of the mudcake and not upon the
porosity of the rock.

In general, an equal volume of mud filtrate can invade
low porosity and high porosity rocks if the drilling muds
have equal amounts of solid particles.

The solid particle in the drilling muds coalesce (結合) and
form an impermeable mudcake.
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INVADED ZONE (CONT.)

The mudcake then acts as a barrier to further invasion.

Because an equal volume of fluid can be invaded before
an impermeable mudcake barrier forms, the diameter of
invasion will be greatest in low porosity rocks.

This occurs because low porosity rocks have less storage
capacity or pore volume to fill with the invading fluid,
and, as a result, pores throughout a greater volume of
rock will be affected.
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INVADED ZONE (CONT.)




General invasion diameters are:
dj/dh = 2 for high porosity rocks
dj/dh = 5 for intermediate porosity rocks
dj/dh = 10 for low porosity rocks
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FLUSHED ZONE
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FLUSHED ZONE

The flushed zone extends only a few inches from the well
bore and is part of the invaded zone.

If invasion is deep, most often the flushed zone is
completely cleared of its formation water (Rw) by mud
filtrate (Rmf).

When oil is present in the flushed zone, you can
determine the degree of flushing by mud filtrate from the
difference between water saturations in the flushed (Sxo)
zone and the uninvaded (Sw) zone.
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FLUSHED ZONE (CONT.)

Usually, about 70 to 95% of the oil is flushed out.

The remaining oil is called RESIDUAL OIL.
Sro = 1.0 – Sxo
where Sro = residual oil saturation (ROS)
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UNINVADED ZONE
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UNINVADED ZONE

The uninvaded zone is located beyond the invaded zone.

Pores in the uninvaded zone are uncontaminated by mud
filtrate; instead, they are saturated with formation water
(Rw), oil, or gas.

Even in hydrocarbon-bearing reservoirs, there is always a
layer of formation water on grain surfaces.

Water saturation (Sw) of the uninvaded zone is an
important factor in reservoir evaluation.
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UNINVADED ZONE (CONT.)

By using water saturation (Sw) data, a geologist can
determine a reservoir’s hydrocarbon saturation.
Sh = 1.0 – Sw
where Sh = hydrocarbon saturation (i.e., the fraction of
pore volume filled with hydrocarbons)

The ratio between the uninvaded zone’s water saturation
(Sw) and the flushed zone’s water saturation (Sxo) is an
index of HYDROCARBON MOVEABILITY.
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INVASION AND RESISTIVITY PROFILES
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INVASION AND RESISTIVITY PROFILES
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INVASION AND RESISTIVITY PROFILES
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TRANSITION PROFILE – WATER ZONE
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ANNULUS PROFILE – HYDROCARBON ZONE
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BASIC INFORMATION NEEDED IN LOG
INTERPRETATION
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BASIC INFORMATION NEEDED IN LOG INTERPRETATION

Lithology – from cutting
Temperature of formation – Because the resistivities of
the drilling mud (Rm), the mud filtrate (Rmf), and the
formation water (Rw) vary with temperature.
 (Resistivities information can be read from LOG HEADER)

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LOG HEADER
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FORMATION TEMPERATURE CALCULATION

Given:

Surface temp. = 80 F

Bottom hole temp. = 180 F

Total depth (TD) = 10000 ft

Formation depth = 6000 ft
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EXERCISE # 1A

Calculate Formation 1A temperature
Given:
 Surface temp. = 60 F
 Formation 1A depth = 5500 ft

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EXERCISE # 1B

Calculate Formation 1B temperature
Given:
 Surface temp. = 75 F
 Formation 1B depth = 7600 ft

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CORRECT THE RESISTIVITIES TO FORMATION TEMPERATURE
Given: Rm = 1.2 at 75 F, Formation temp. = 160 F
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Rm=0.56 at 160F
START HERE
EXERCISE # 2A

Correct SIX resistivities (Rm, Rmf, and Rmc, in RUN-1 and
RUN-2) to surface temperature
Given:
 Surface temp. = 75 F
 Rm, Rmf, Rmc => from log header RUN-1 and RUN-2

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EXERCISE # 2B

Correct the resistivities (Rm, Rmf, Rmc) to Formation 1B
temperature
Given:
 Formation 1B temp. => From your answer of Ex. #1B
 Rm, Rmf, Rmc => From log header RUN-2

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END OF CHAPTER 1
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