POROSITY DETERMINATIONFROM LOGS

Most slides in this section are modified primarily from NExT PERF Short Course Notes, 1999.However, many of the NExT slides appears to have been obtained from other primarysources that are not cited. Some slides have a notes section.

Well LogSP Resistivity

OPENHOLE LOG EVALUATION

Oil sand

Gammaray

Resisitivity Porosity

Increasingradioactivity

Increasingresistivity

Increasingporosity

Shale

Shale

POROSITY DETERMINATION BY LOGGING

POROSITY LOG TYPES3 Main Log Types

• Bulk density

• Sonic (acoustic)

• Compensated neutron

These logs do not measures porosity directly. To accurately calculate porosity, the analyst must know:• Formation lithology• Fluid in pores of sampled reservoir volume

DENSITY LOGS• Uses radioactive source to generate

gamma rays• Gamma ray collides with electrons in

formation, losing energy• Detector measures intensity of back-

scattered gamma rays, which is related to electron density of the formation

• Electron density is a measure of bulk density

DENSITY LOGS

• Bulk density, ρb, is dependent upon:– Lithology

– Porosity

– Density and saturation of fluids in pores• Saturation is fraction of pore volume

occupied by a particular fluid (intensive)

GRAPI0 200

CALIXIN6 16

CALIYIN6 16

RHOBG/C32 3

DRHOG/C3-0.25 0.25

4100

4200

DENSITY LOG

Caliper

Density correction

Gamma ray Density

Formation (ρb)

Long spacing detector

Short spacing detector

Mud cake(ρmc + hmc)

Source

BULK DENSITY

( ) φρ+φ−ρ=ρ fmab 1

Matrix Fluids influshed zone

•Measures electron density of a formation•Strong function of formation bulk density•Matrix bulk density varies with lithology

–Sandstone 2.65 g/cc–Limestone 2.71 g/cc–Dolomite 2.87 g/cc

POROSITY FROM DENSITY LOG

Porosity equation

( )xohxomff S1S −ρ+ρ=ρ

fma

bmaρ−ρρ−ρ

=φ

Fluid density equation

We usually assume the fluid density (ρf) is between 1.0 and 1.1. If gas is present, the actual ρf will be < 1.0 and the calculated porosity will be too high.

ρmf is the mud filtrate density, g/ccρh is the hydrocarbon density, g/ccSxo is the saturation of the flush/zone, decimal

DENSITY LOGS

Working equation (hydrocarbon zone)

( )

( ) mashshsh

hcxomfxob

V1V

S1S

ρ−φ−+ρ+

ρ−φ+ρφ=ρ

ρb = Recorded parameter (bulk volume)φ Sxo ρmf = Mud filtrate componentφ (1 - Sxo) ρhc = Hydrocarbon componentVsh ρsh = Shale component1 - φ - Vsh = Matrix component

DENSITY LOGS• If minimal shale, Vsh ≈ 0

• If ρhc ≈ ρmf ≈ ρf, then

• ρb = φ ρf - (1 - φ) ρma

fma

bmad ρ−ρ

ρ−ρ=φ=φ

φd = Porosity from density log, fractionρma = Density of formation matrix, g/cm3

ρb = Bulk density from log measurement, g/cm3

ρf = Density of fluid in rock pores, g/cm3

ρhc = Density of hydrocarbons in rock pores, g/cm3

ρmf = Density of mud filtrate, g/cm3

ρsh = Density of shale, g/cm3

Vsh = Volume of shale, fractionSxo = Mud filtrate saturation in zone invaded by mud filtrate, fraction

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

BULK DENSITY LOG

Bulk DensityLog

RHOC1.95 2.95

NEUTRON LOG

• Logging tool emits high energy neutrons into formation

• Neutrons collide with nuclei of formation’s atoms

• Neutrons lose energy (velocity) with each collision

NEUTRON LOG

• The most energy is lost when colliding with a hydrogen atom nucleus

• Neutrons are slowed sufficiently to be captured by nuclei

• Capturing nuclei become excited and emit gamma rays

NEUTRON LOG• Depending on type of logging tool either gamma

rays or non-captured neutrons are recorded• Log records porosity based on neutrons

captured by formation• If hydrogen is in pore space, porosity is related

to the ratio of neutrons emitted to those counted as captured

• Neutron log reports porosity, calibrated assuming calcite matrix and fresh water in pores, if these assumptions are invalid we must correct the neutron porosity value

NEUTRON LOGTheoretical equation

( )

( ) Nmashshsh

NhcxoNmfxoN

V1V

S1S

φ−φ−+φ+

φ−φ+φφ=φ

φN = Recorded parameterφ Sxo φNmf = Mud filtrate portionφ (1 - Sxo) φNhc = Hydrocarbon portionVsh φNsh = Shale portion(1 - φ - Vsh) φNhc = Matrix portion where φ = True porosity of rockφN = Porosity from neutron log measurement, fraction

φNma = Porosity of matrix fractionφNhc = Porosity of formation saturated with

hydrocarbon fluid, fractionφNmf = Porosity saturated with mud filtrate, fractionVsh = Volume of shale, fractionSxo = Mud filtrate saturation in zone invaded

by mud filtrate, fraction

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

POROSITY FROM NEUTRON LOG

NeutronLog

CNLLC0.45 -0.15

Upper transmitter

Lower transmitter

R1R2R3R4

ACOUSTIC (SONIC) LOG

• Tool usually consists of one sound transmitter (above) and two receivers (below)

• Sound is generated, travels through formation

• Elapsed time between sound wave at receiver 1 vs receiver 2 is dependent upon density of medium through which the sound traveled

μsec50

T0E2

E1

E3

Mud wavesRayleigh

wavesCompressional

waves

Lithology Typical Matrix TravelTime, Δtma, μsec/ft

Sandstone 55.5Limestone 47.5Dolomite 43.5Anydridte 50.0Salt 66.7

COMMON LITHOLOGY MATRIXTRAVEL TIMES USED

ACOUSTIC (SONIC) LOG

Working equation( )

( ) mashshsh

hcxomfxoL

tV1tV

tS1tSt

Δ−φ−+Δ+

Δ−φ+Δφ=Δ

ΔtL = Recorded parameter, travel time read from logφ Sxo Δtmf = Mud filtrate portionφ (1 - Sxo) Δthc = Hydrocarbon portionVsh Δtsh = Shale portion(1 - φ - Vsh) Δtma = Matrix portion

ACOUSTIC (SONIC) LOG• If Vsh = 0 and if hydrocarbon is liquid

(i.e. Δtmf ≈ Δtf), then

• ΔtL = φ Δtf + (1 - φ) Δtma

or

maf

maLs tt

ttΔ−ΔΔ−Δ

=φ=φ

φs = Porosity calculated from sonic log reading, fractionΔtL = Travel time reading from log, microseconds/ftΔtma = Travel time in matrix, microseconds/ftΔtf = Travel time in fluid, microseconds/ ft

DTUSFT140 40

SPHI%30 10

4100

4200

GRAPI0 200

CALIXIN6 16

ACOUSTIC (SONIC) LOG

Sonic travel time

Sonic porosity

Caliper

Gamma Ray

SONIC LOG

The response can be written as follows:

( ) φ+φ−= fmalog t1tt

maf

ma

tttt

−−

=φ log

tlog = log reading, μsec/fttma = the matrix travel time, μsec/fttf = the fluid travel time, μsec/ftφ = porosity

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SONIC LOG

SonicLog

DT150 50us/f

EXAMPLE

Calculating Rock Porosity Using an Acoustic Log

Calculate the porosity for the following intervals. The measured travel times from the log are summarized in the following table.

At depth of 10,820’, accoustic log reads travel time of 65 μs/ft.

Calculate porosity. Does this value agree with density and neutron logs?

Assume a matrix travel time, Δtm = 51.6 μsec/ft. In addition, assume the formation is saturated with water having a Δtf = 189.0 μsec/ft.

GRC0 150

SPCMV-160 40ACAL

6 16

ILDC0.2 200

SNC0.2 200

MLLCF0.2 200

RHOC1.95 2.95

CNLLC0.45 -0.15

DTus/f150 50

001) BONANZA 1

10700

10800

10900

SPHIss45 -15

EXAMPLE SOLUTION SONIC LOG

SPHI

FACTORS AFFECTING SONIC LOG RESPONSE

• Unconsolidated formations

• Naturally fractured formations

• Hydrocarbons (especially gas)

• Rugose salt sections

RESPONSES OF POROSITY LOGS

The three porosity logs:– Respond differently to different matrix

compositions– Respond differently to presence of gas or

light oils

Combinations of logs can:– Imply composition of matrix– Indicate the type of hydrocarbon in pores

GAS EFFECT

• Density - φ is too high

• Neutron - φ is too low

• Sonic - φ is not significantly affected by gas

ESTIMATING POROSITY FROM WELL LOGS

Openhole logging tools are the most common method of determining porosity:

• Less expensive than coring and may be less risk of sticking the tool in the hole• Coring may not be practical in unconsolidated formations or in formations with high secondary porosity such as vugs or natural fractures.

If porosity measurements are very important, both coring and logging programs may be conducted so the log-based porosity calculations can be used to calibrated to the core-based porosity measurements.

Influence Of Clay-Mineral DistributionOn Effective Porosity

Dispersed Clay• Pore-filling• Pore-lining• Pore-bridging

Clay Lamination

Structural Clay(Rock Fragments,

Rip-Up Clasts,Clay-Replaced Grains)

φe

φe

φe

ClayMinerals

Detrital QuartzGrains

φe

eφ

FlowUnits

Gamma RayLog

PetrophysicalData

PoreTypesLithofaciesCore

1

2

3

4

5

CorePlugs

CapillaryPressureφ vs k

GEOLOGICAL AND PETROPHYSICAL DATA USED TO DEFINE FLOW UNITS

Schematic Reservoir Layering Profilein a Carbonate Reservoir

Baffles/barriers

3150

SA -97ASA -251

SA -356 SA -71 SA -344 SA -371SA -348

SA -346SA -37

3200

3250

3300

3350

3100

3150

3250

3300

3250

3150

3200

3100

3150

3200

3250

3200

32503250

3350

3300

3150

3200

3250

3300

3100

3200

3250

3300

3350

3150

3200

3250

Flow unit

From Bastian and others

I. ROCK POROSITY VI) Subsurface measurement of porosity A. Types of logs from which porosity can be derived 1. Density Log—Principle

a) A radioactive source, such as Cobolt-60, is applied to the borehole wall in a shielded sidewall skid and emits medium-energy gamma rays into the formations. These gamma rays may be thought of as high-energy particles that collide with the electrons in the formation. At each collision a gamma ray loses some, but not all, of its energy to the electron, and then continues with diminished energy. This type of interaction is known as Compton scattering. The scattered gamma rays reaching the detector, at a fixed distance from the source, are counted as an indication of formation density.

b) The number of Compton-scattering collisions is related directly to the number of electrons in the formation. Consequently, the response of the density tool is determined essentially by the electron density (number of electrons per cubic centimeter) of the formation. Electron density is related to the true bulk density, ρb, which, in turn depends on the density of the rock matrix material, the formation porosity, and the density of the fluids filling the pores.

2. Sonic Log—Principle

a) A sonic tool consists of a transmitter that emits a sound pulse and a receiver that picks up and records the pulse as it passes the receiver. The sonic log is simply a recording versus depth of the time, t, required for a sound wave to traverse one foot of a formation. Known as the interval transit time, Δt or slowness, t is the reciprocal of the velocity of the sound wave. The interval transit time for a given formation depends upon its lithology and porosity. This dependence upon porosity, when the lithology is known, makes the sonic log very useful as a porosity log.

b) The propagation of sound in a borehole is a complex phenomenon. It is governed by the mechanical properties of several separate acoustical domains—the formation, the borehole fluid column, and the logging tool itself.

3. Neutron Log—Principle a) Neutrons are electrically neutral particles, each having a mass almost

identical to the mass of a hydrogen atom. High-energy (fast) neutrons are continuously emitted from a radioactive source in the sonde. These neutrons collide with nuclei of the formation materials in what may be thought of as elastic “billiard-ball” collisions. With each collision, the neutron loses some of its energy.

b) The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greater energy loss occurs when the neutron strikes a nucleus of practically equal mass—i.e., a hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron very much. Thus, the slowing of neutrons depends largely in the amount of hydrogen in the formation.

c) Within a few microseconds the neutrons have been slowed by successive collisions to thermal velocities, corresponding to energies of around 0.25 eV. They then diffuse randomly, without losing more energy, until they are captured by the nuclei of atoms such as chlorine, hydrogen, or silicon.

d) The capturing nucleus becomes intensely excited and emits a high-energy gamma ray of capture. Depending on the type of neutron tool, either these capture gamma rays or the neutrons themselves are counted by a detector in the sonde.

e) When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons are slowed and captured within a short distance of the source. On the contrary, if the hydrogen concentration is small, the neutrons travel farther from the source before being captured. Accordingly, the counting rate at the detector increases for decreased hydrogen concentration, and vice versa.