Materi Uts Geologi Well Logging Sem I 2018 2019 (1).pdf

GEOLOGI WELL LOGGING

Dwiharso Nugroho, Dr. Ir. Lab. Sedimentologi, Stratigrafi dan Geologi Bawah Permukaan Fakultas Ilmu dan Teknologi Kebumian (FITB) Institut Teknologi Bandung (ITB) 0811 236 230 [email protected] [email protected]

Outline I. II. III.

Introduction Mud Logging Pre-Analysis (postponed..) A. B.

Environmental Correction Normalization

B.

1.

2. 3.

C.

Picking Parameters (postponed..) 1. 2.

Single well vs Multi well Picking Parameter Process a. b. c.

Gama Ray ρma, ρwet shale Rw

D.

Single Clay Indicator : Gamma Ray SCI : Spontaneous Potential Double Clay Indicator: Neutron - Density

Defining Porosity 1. 2. 3. 4.

IV. Log Analysis A.

Defining Vsh

Concept of Porosity Density Porosity Neutron Porosity Sonic Porosity

Calculating Fluid Saturation 1. 2.

Clean Sand Analysis Shaly-sand Analysis

INTRODUCTION Well Logging : The study of the properties of rocks (& fluids) by petrophysical techniques using electric, nuclear, and acoustical sources

Log curve shapes are determined visually from the appearance of the recorded data when plotted versus depth.

Crain, E.R, 2000, The Log Analysis Handbook, Vol. 1

rOCk pROpERtieS •

rOCk PHYSICAL pROpERtieS:

•

rOCk CHEMICAL pROpERtieS:

– Mineral composition – Atoms/elements comp. – Stability, ..etc

..directly taken from well data

– – – – – – –

Electrical Conductivity Magnetism Spontaneous Potential Natural Radioactivity Density Sound Velocity (sonic) ..etc



rOCk GEOLOGICAL pROpERtieS:      



textures Sedimentary structures Facies Facies successions Fractures Structural dip, ..etc

rOCk (reservOiR) PETROPHYSICAL pROpERtieS:  THICKNESS  POROSITY  WATER SATURATION  PERMEABILITY © NUGROHO 2006

© NUGROHO 2006

Wireline Logging • Log is a continuous recording of a geophysical parameter along a borehole • Wireline logging is a conventional form of logging that employs a measurement tool suspended on a cable or wire that suspends the tool and carries the data back to the surface • Basic physical parameters measured with logs – – – – – – – –

size of the borehole orientation of the borehole temperature Pressure natural radioactivity of the rocks acoustic properties of the rocks electrical properties of the rocks Rock’s response to radioactivity generated by the tool – NMR characteristics of the rocks (Taken from Glover, Paul W.J.)

(Source: Glover, Paul W.J.)

Log Type



Open Hole Logs



Electric Logs



Nuclear Logs



 

  

SP Resistivity Image Log (FMI, EMI)

  

GR Neutron Density

  

Sonic Log NMR / CMR / MRIL Image Log (STAR)

Acoustic Logs

Cased Hole Logs LWD / MWD

© NUGROHO 2006

Log Header •

Data, crucial for the evaluation, can be found in the log header: – – – – – – – – – – – – – –

Well name & -location, date, drill floor elevation (DFE), Ground elevation (GE), bit size, mud -type and -properties, resistivities of the -mud (Rm), -mud filtrate (Rmf) &-mudcake (Rmc), total depth (TD), bottom hole temperature (BHT), weight-, size- . &depth- of previous casings, time of last mud circulation, list of all tools run in this hole section, serial number of tools and logging unit used, name of logging engineer and company representative.

…THICKNESS (overview) • Gross Sand (GS) thickness tebal stratigrafis antara dua top reservoir

• Net Sand (NS) thickness  GS dng cut off Vshale

TAF-6

NP GS

•

Net Effective Sand (NES) thickness  NS dng cut off Porosity

•

Net Pay (NP) thickness  NES dng cut off Sw

NE S

NS TAF-5

LOG ANALYSIS • Matrix

• Pore (filled by fluid)

PETROGRAPHY • Grain • Matrix • Cement • Pore (filled by fluid) • Pore (none fluid within; rare)

!! Log analysis (determin) hanya menggunakan 1 nilai r ma and/or 1 nilai r sh !!! shale fragment !!! Carbonate has an intraparticle porosity (WP)

© NUGROHO 2006

“X” Sd Vsh = 0.39 PHIA = 0.13 Sw = 0.42

“Z” Vsh = 0.25 PHIA = 0.16 Sw = 0.71 “Y” Sd Vsh = 0.17 PHIA = 0.13 Sw = 0.24

“K” Fm Vsh = 0.34 PHIA = 0.24 Sw = 0.80

“L” Fm Vsh = 0.22 PHIA = 0.06 Sw = 0.50

MUD LOGGING

Mud Logging – Mudlogger • A mudlogger is a professional geologist responsible for operating a computerized logging unit at both onshore and offshore drilling sites. • Mudloggers are responsible for : – evaluating the lithology of all strata penetrated by drilling and reporting any hydrocarbon discoveries by analyzing rock cutting samples from drilling mud, – evaluating gas chromatography data used to screen for hydrocarbon in the cuttings, – sometimes analyzing wireline logs, – monitor aspects of rig operations and downhole conditions on a wellsite, and report suspected unsafe conditions to other rig personnel. – Mudloggers provide time-sensitive geological and drilling data to clients in the form of daily reports and logs, often transmitted live via satellite. – write longer summary reports upon well completion

Principle While drilling a well, the following information about the drilled formations is recorded as a function of depth: • •

• •

•

The drilling rate or rate of penetration (ROP). All important parameters which influence the drilling speed, e.g. type of bit, rotation speed (RPM), weight on bit (WOB), pump speed (SPM), pump pressure (SPP), etc. The lithology and texture of the cuttings, which are sampled at regular intervals (~5meter). The total combustible gas content in the air above the returning mud from the well bore. The relatively simple gas detector can be supplemented with a gas chromatograph to analyze the gas composition. Hydrocarbon staining on the cuttings.

Accuracy • The information related to the formation and its fluid content is available on the moment that the mud with the cuttings come to surface. • The lag time between the moments of drilling and sampling (varying from 0 - 2 hours), depends on the volume of the annulus and the circulation rate. The depth of drilling is for this reason corrected, using a lag-time estimate and an average ROP. The depth accuracy is about +/- 5meter. • Variations in specific gravity and shape of the cuttings from various lithologies, causes differences in slippage. As a result a sample taken from the flowline may originate from a range of depths and will consist of a mix of the lithologies present. • In some cases formations (often shales) higher up in the well bore are not stable. Cavings (flakes) of this particular formation can "contaminate" the cutting samples of lower intervals.

Evaluation objective • Monitoring of the bit performance. • Early indication ofthe well's position within the predicted stratigraphy. This is of particular importance as a basis for operational decissions, e.g. at what depth to set casing, or where to core a well. • Determination of lithology. • Indication of fluid type. • Indication of pressure conditions.

Evaluation Technique • Every formation has a signature on the ROP log. The depth of formations can be determined, by correlating this log with the mud log from a nearby well. • The lithology of the cuttings are given as percentages of the total sample. An accurate interpretation can be made in combination with the wireline logs, which have a far better vertical resolution. • The oil" staining on the cuttings is analysed using several small chemical and fluorescence tests, which can differentiate the light and heavy hydrocarbons. – In water bearing formation no HC staining is expected. – The presence of only light HC's indicates gas. – In the case of an oil bearing formation more heavy HC's will be present.

Evaluation Technique Lithology Analysis • Every formation has a signature on the ROP log. The depth of formations can be determined, by correlating this log with the mud log from a nearby well. • The lithology of the cuttings are given as percentages of the total sample. An accurate interpretation can be made in combination with the wireline logs, which have a far better vertical resolution.

Evaluation Technique Fluid Analysis : Oil Cutting • Stain • Flourescence

• Cut Flourescence

• Odor

Evaluation Technique Fluid Analysis : Gas detection

WIRELINE LOGGING

Principle • After a section of a well has been drilled , measuring sondes are lowered into the open hole at the end of an electrical cable. • Whilst pulling the tools out of the well, various properties of the formations are measured continuously as a function of depth. • These physical properties can be interpreted in terms of lithology, porosity, hydrocarbon saturation, etc.

Depth Measurement • The depth is measured along hole(AHO) in meters below derrick floor (mbdf). • When the bottom of the tool-string touches the drill floor the depth measurement is set at zero. • The distance between the various tool detectors and the bottom of the tool-string is automatically compensated by the computer in the surface logging unit. • The length of cable in the hole is measured with an accuracy of around 0.1 %. • In vertical wells the AHD is equal to the true vertical depth (TVD). • In deviated wells, a deviation survey is needed to calculate the TVD from the AHD. • The TVD is often expressed in meters below a local datum, e.g. meters subsea (mss).

Log Header •

Data, crucial for the evaluation, can be found in the log header: – – – – – – – – – – – – – –

Well name & -location, date, drill floor elevation (DFE), Ground elevation (GE), bit size, mud -type and -properties, resistivities of the -mud (Rm), -mud filtrate (Rmf) &-mudcake (Rmc), total depth (TD), bottom hole temperature (BHT), weight-, size- . &depth- of previous casings, time of last mud circulation, list of all tools run in this hole section, serial number of tools and logging unit used, name of logging engineer and company representative.

Log types • Logs, which are used to quantify the hydrocarbon in place, can be classified into three families: – Reservoir Thickness (Gamma Ray, Spontaneous Potential) These logs discriminate reservoir from non-reservoir.

– Porosity (Density, Neutron, Sonic.) These logs are used to calculate porosity, identify lithologies, and differentiate oil from gas. – Resistivity (Laterolog, Induction, Microresistivity.) These logs, together with porosity logs, are used to calculate hydrocarbon saturations. – Other types of wireline tools are: • Side wall sampler (Takes small rock samples, which are used forlithology and fluid type confirmation.) • Formation tester (Measures formation pressures and can retrieve fluid samples.) • Dipmeter & FMS (Measure dip and azimuth ofthe layers) • Well shoot & VSP (Used tocalibrate seismic.)

PRE-ANALYSIS (postponed)

PICKING PARAMETERS • There are petrophysical parameters that must be obtained to perform petrophysical calculation. • Those parameters are – GR value of shale and clean sand (GRmax, GRmin) – Matrix and shale density (ρma, ρsh) – Matrix and shale neutron porosity – Matrix and shale sonic (∆tma, ∆tsh) – Formation water resistivity (Rw) petrophysical parameters

Vsh : Vsh (linear) =

∅=

𝜌𝑚𝑎

v v − 𝜌𝑏 − 𝑉𝑐𝑙 × (𝜌𝑚𝑎 − 𝜌c𝑠ℎ ) v v c v 𝜌𝑚𝑎 c − 𝜌c𝑓𝑙 v v v c v

v v cv vc v

L O G A N A LY S I S F O R D E F I N I N G R E S E RVO I R / R O C K P R O P E R T I E S ( Q U A N T I TAT I V E L O G A N A LY S I S )

DEFINING VOLUME OF SHALE (VSHALE)

LOG ANALYSIS • Matrix

• Pore (filled by fluid)

PETROGRAPHY • Grain • Matrix • Cement • Pore (filled by fluid) • Pore (none fluid within; rare)

!! Log analysis (determin) hanya menggunakan 1 nilai r ma and/or 1 nilai r sh !!! shale fragment !!! Carbonate has an intraparticle porosity (WP)

Log Analysis generally distinguish three distribution type of shale

– Laminar Shale • consists of thin laminations of shale which separate stringers or beds of clean sandstone. • the occurrence (of these lamination) is not accompanied by a reduction in the porosities of the sandstone stringers, but overall could be reducing the bulk porosity (of the reservoir)

– Structural Shale • the term for shale fragments, diagenetic altered mineral, etc. which be the grains of sandstone • is not necessarily matched by any reduction in porosity (Doveton, 2005)

– Dispersed Shale • Consists of pore-filling clay minerals • Leads to a progressive reduction in porosity

Shale : • Clay + silt + other • Clays – Plate-like form – Large surface area Clay – Contain Al+3 and Si+4 Crystal +2 – Substitution by Mg – Negative charge results – Attraction by water and cations

Absorbed Water Sodium Ion Water Hydration Water

xH

Outer Helmholtz Plane

Schematic Water Molecule

The distribution of clay within porous reservoir formations Laminated

Laminated Laminated – replaces both matrix and porosity – reduces porosity-permeability – common – e.g., intercalations – assume similar to nearby shale

Structural Shale – replaces matrix – may not affect porositypermeability – e.g., lithic/rock fragments (altered metamorphic and/or volcanic), rip-up clasts, etc.

f

e

Shale

Dispersed – – – –

replaces pore space very common forms in situ may differ greatly from nearby shales – porosity-permeability reduction depends on form

• Dispersed Clay Forms – Kaolinite: • moderate perm effects • may dislodge, block throats – Chlorite: • significant perm loss • traps water – Illite: • chokes pores and throats • significantly reduce the porosity

Shaly Sands Swt VSH Sh

Sw

Oil Gas

Vma

Sb

Free Bound Dry Water Water Clay

fe ft fz

Matrix Solids

DEFINING VSHALE • Vshale : ratio of shale volume to matrix volume • Define lithology : Sand – non sand. • Methods to obtain Vshale – SingleClay Indicator: Gamma ray methods – Double Clay Indicator: • Neutron – Density • Sonic- Density • Neutron - Sonic

Defining Vshale – Single Clay Indicator Determination of Shale Content (Vshale) • The gamma ray log values can be used to calculate the shaliness or shale volume Vsh of the rock.

where: IGR GRlog GRmin GRmax =

= the gamma ray index = the gamma ray reading at the depth of interest = the minimum gamma ray reading. the maximum gamma ray reading.

• Many petrophysicists then assume that Vsh = IGR. However, to be correct the value of IGR should be entered into “the chart” from which the corresponding value of Vsh may be read (Asquith and Krygowski, 2004).

Defining Vsh: Gamma Ray (GR) • Shale volume is calculated in the following way: – First the gamma ray index IGR is calculated from the gamma ray log data using the relationship :

where: IGR = the gamma ray index GRlog = the gamma ray reading at the depth of interest GRmin = the minimum gamma ray reading. (Usually the mean minimum through a clean sandstone or carbonate formation.) GRmax = the maximum gamma ray reading. (Usually the mean maximum through a shale or clay formation.)

– Many petrophysicists then assume that Vsh = IGR. However, to be correct the value of IGR should be entered into “the chart” from which the corresponding value of Vsh may be read.

Defining Vsh: Gamma Ray (GR)

GAMMA RAY LOG • The gamma ray log (GR) measures the total natural gamma radiation emanating from a formation. • This gamma radiation originates from potassium-40 and the isotopes of the Uranium-Radium and Thorium series. • Once the gamma rays are emitted from an isotope in the formation, they progressively reduce in energy as the result of collisions with other atoms in the rock (compton scattering). Compton scattering occurs until the gamma ray is of such a low energy that it is completely absorbed by the formation. • The gamma ray intensity that the log measures is a function of (Glover, MSc Course Notes): – –

The initial intensity of gamma ray emission, which is a property of the elemental composition of the rock. The amount of compton scattering that the gamma rays encounter, which is related to the distance between the gamma emission and the detector and the density of the intervening material.

Total Gamma Ray Log • Principle of Measurement  The tool consists simply of a highly sensitive gamma ray detector in the form of a scintillation counter.  The scintillation counter is composed of a thalium activated single sodium iodide crystal backed by a photomultiplier.  The gamma ray measurement device accepts gamma rays from almost a hemisphere that includes the formation and the drilling mud between the formation and the sensor.

Total Gamma Ray Log NATURAL RADIOACTIVITY OF ROCKS, NATURAL GAMMA RAY ACTIVITY • Natural radioactivity is spontaneous decay of a certain isotope into another isotope, characterized by emission of radiation (α, β, γ). • The dacay is a statistical process, mostly described as a poissondistribution. GAMMA RADIATION • Can be described as an electromagnetic wave • Frequencies are in the range v= 1019 -- 1021 Hz • Energy is E = h v in the order of keV to Mev (h is the Plank constant)

GAMMA RAY LOG EXAMPLE

SPECTRAL GAMMA RAY LOG EXAMPLE

Principles • •

•

•

The tool consists of a highly sensitive gamma ray detector in the form of a scintillation counter. The scintillation counter is composed of a thalium activated single sodium iodide crystal backed by a photomultiplier. When a gamma ray strikes the crystal a small flash of light is produced. This flash is too small to be measured using conventional electronics. So, it’s amplified by a photomultiplier (consists of a photocathode and a series of anodes held at progressively higher electrical potentials (are arranged serially in a high vacuum)). The flash of light hits the photocathode and causes a number of primary electrons to be produced. These few electrons still represent too small a signal to be measured. The primary electrons are accelerated towards the first anode. For every electron that hits the anode, a number of secondary electrons are emitted (between 4 and 8 usually).



These electrons are accelerated towards the next anode, where each of the secondary electrons produce even more secondary electrons.



This process is repeated for each of say 10 anodes.



If 6 electrons are emitted at each anode for each incident electron, we can see that a single incident gamma ray ultimately produces 610 = 60,466,176 electrons, which represents a current that can be amplified further by conventional amplifiers.

Depth of investigation •

•

•

• •

At certain distance from the emitting atom increases, the energy of the gamma rays decreases but compton scattering until they are too low to be measured by the scintillation counter. Therefore, there is a maximum depth of investigation for the tool that depends upon formation and mud density. For average values of drilling mud and formation density, we can say that approximately 50% of the gamma ray signal comes from within 18 cm (7 inches) of the borehole wall, increasing to 75% from within 30 cm (1 foot). Hence, the depth of investigation, if defined at 75% of the signal, is 30 cm. However, this will decrease for denser formations of the same radioactivity, and increase for less dense formations of the same radioactivity.

Note that the zone of sensitivity is almost hemispherical, so the 30 cm depth of investigation applies both horizontally (perpendicular to the borehole wall) and subvertically (sub-parallel with the borehole wall). This has implications for the vertical resolution of the tool..

Total Gamma Ray Log

Total Gamma Ray Log

Gamma ray log correction chart for a 3.75 inch tool in an 8 inch hole with a KCl-free drilling mud with a mud weight of rf g/cm3 as a function of borehole diameter (courtesy of ReevesWireline Figure Effect of caving on the gamma ray log Ltd.). (Glover, MSc Course Notes).

Total Gamma Ray Log USES OF THE TOTAL GAMMA RAY LOG  Determination of Lithology  Determination of Shale content  Depth Matching  Cased Hole Correlations  Recognition of Radioactive Mineral Deposits  Recognition of Non-Radioactive Mineral Deposits  Radio-isotope Tracer Operations  Facies and Depositional Environment Analysis

Gamma Ray and Lithology

GR Response Reservoir = sand Non-reservoir = shale GR reservoir = ? Low GR Reservoir = porous limestone Non-reservoir = tite limestone GR reservoir = ? No difference Reservoir = granite wash Non-reservoir = tite limestone GR reservoir = ? High GR

Reservoir = porous dolomite radioactive /not Non-reservoir = tite dolomite GR reservoir = ? No difference

Radioactive dolomite

Gamma Ray Values of Minerals

GR Borehole Effects

• Calibrated for: – 8 inch hole – 10 ppg mud – 3-5/8 inch tool eccentered in hole

• GR What happens if holecould washes out? decreases, so shales look clean GR high in borehole, GR high in invaded formations

• Must account for potassium muds

Lag on Old Analog GR Slide 105

Old GR logs showed zone shallower than its actual depths

New digital GR logs don’t have this problem

29-January-2008

Spectral GR Logs • Contributions by Th, U, K are presented • Used for: – – – –

Better Vshale Distinguishing radioactive dolomite from shale Locating source rock Locating steamed-out zones

• Total GR and (Th + K) are usually plotted in Track 1

Spectral GR Logs

Spectra for K, Th, and U

Clay Minerals Identification

Clay and Mineral Identification

Take core and look at the rock !! Petrographic analyses including SEM, XRD

Spectral GR Log

w/ Spontaneous Log (SP)

DEFINING VSH :

Volume shale dapat dihitung dengan menggunakan beberapa alternatif log sebagai berikut : a. Log Gamma Ray (GR)

IGR= (GR – GRmin)/(GRmax – GRmin) GR

= bacaan log GR pada zona interest

GR min = bacaan log GR pada zona 100% bersih lempung GR max = bacaan log GR pada zona 100% lempung

b. Log Spontaneous Potential (SP) Vsh = (SSP – PSP) / SSP = 1 – (PSP/SSP)

SSP PSP

= = = =

static spontaneous potential of a thick clean sand or carbonate the deflection from the shale/base line to the clean line pseudo static spontaneous potential (SP of shaly formation) the deflection from the shale/base line to the curve reading

LOG : SPONTANEOUS POTENTIAL • The SP is a record of direct current (DC) voltage differences between the naturally occurring potential of a moveable electrode in the well bore, and the potential of a fixed electrode located at the surface. • measured in millivolts • Normally, the SP log is recorded on the left hand track, and is used to: (1) detect permeable beds, (2) detect boundaries of permeable beds, (3) determine formation water resistivity(Rw), and (4) determine the volume of shale in permeable beds.

• Influence Factor – Bed thickness: • <10 feet, need a correction • general rule : whenever the SP curve is narrow and pointed in shape, the SP should be corrected for bed thickness. – Bed resistivity: Higher resistivities reduce the deflection of the SP curves. – Borehole and invasion: The effects of borehole diameter and invasion on the SP log are very small and, in general, can be ignored.

• Shale content: The presence of shale in a permeable formation reduces the SP deflection. – –

In water-bearing zones the amount of SP reduction is proportional to the amount of shale in the formation. In hydrocarbon-bearing zones the amount of SP reduction is greater than the volume of shale

• The SP response of shales is relatively constant and follows a straight line called a shale baseline. SP curve deflections are measured from this shale baseline. • Permeable zones are indicated where there is SP deflection from the shale baseline.

•

The SP log can be used to calculate the volume of shale in a permeable zone by the following steps:

– Establish the shale/ base line – Obtain the SSP – Obtain the PSP – Compute the shale fraction (VSh): Vsh = (SSP – PSP) / SSP = 1 – (PSP/SSP) Where: – –

Vsh = volume of shale PSP = pseudo static spontaneous potential (SP of shaly formation) = the deflection from the shale/base line to the curve reading

–

SSP = static spontaneous potential of a thick clean sand or carbonate = the deflection from the shale/base line to the clean line

– SSP = -K x log (Rmf / Rw) •

K = 60 + (0.133 x Tf)

Defining Vshl – Double Clay Indicator

• Neutron - Density – Clay volume can be obtained from crossplot Neutron data vs density data – Clay volume is defined as how close a particular point inthe cross plot to the clay point

Defining Vshl – Double Clay Indicator • • • • • •

•

This Figure illustrates this method graphically. The graph is a triangle, whose apices are at the following points: The clean matrix point: fN = 0% rb = 2.65 g/cm3 The fluid point: fN = 100% rb = 1.00 g/cm3 The clean matrix point: fN and rb obtained from an adjacent clay The linear effective porosity scale is shown on the matrix-water side and the clay-water side. The iso- Vsh lines are drawn across the triangle. Each pair of fN and rb values obtained from the logs can be entered into the graph and the relevant effective porosity and Vsh can be read off. Alternately, a more accurate figure can be obtained by solving Eqs. (20.1 and 20.2).

2.8469

0

fe 

RHOB  r ma  ( r sh  r ma ) Vsh r fl  r ma

ft  fe  Vsh ftsh

rma

rma

Calculating Porosity

LOG ANALYSIS • Matrix

• Pore (filled by fluid)

PETROGRAPHY • Grain • Matrix • Cement • Pore (filled by fluid) • Pore (none fluid within; rare)

!! Log analysis (determin) hanya menggunakan 1 nilai r ma and/or 1 nilai r sh !!! shale fragment !!! Carbonate has an intraparticle porosity (WP)

a. Log density ρ rock (ρb) = (1- Ф) ρ matriks + Ф ρ fluid Ф = (ρma - ρb) / (ρma- ρf) ρma

= nilai densitas matriks

ρb

= bacaan log density (bulk density) pada zona interest

ρf

= nilai densitas fluida

b. Log neutron Bacaan log neutron merupakan nilai hasil perhitungan konsentrasi ion hidrogen pada suatu formasi. Konsentrasi ion hidrogen pada suatu formasi sebanding dengan jumlah fluida yang mengisi pori batuan. Karenanya, bacaan log neutron dapat digunakan secara langsung untuk menentukan porositas suatu formasi/batuan/reservoir.

c. Log sonic Ф = (∆t–∆t ma) / (∆tf –∆t ma) ∆t

= bacaan log sonic pada zona interest

∆t ma

= nilai ∆t matriks

∆t f

= nilai ∆t fluida

Calculating Porosity Density Log (RHOB)

Density Log PRINCIPLE OF MEASUREMENT (Glover, 2011) • The gamma rays enter the formation and undergo compton scattering by interaction with the electrons in the atoms composing the formation. Compton scattering reduces the energy of the gamma rays in a step-wise manner, and scatters the gamma rays in all directions. • When the energy of the gamma rays is less than 0.5 MeV they may undergo photo-electric absorption by interaction with the atomic electrons. The flux of gamma rays that reach each of the two detectors is therefore attenuated by the formation, and the amount of attenuation is dependent upon the density of electrons in the formation.





The formation density log is a porosity log that measures electron density of a formation. It can assist the geologist to:  identify evaporite minerals,  detect gas-bearing zones,  determine hydrocarbon density, and  evaluate shaly sand reservoirs and complex lithologies.

• The tool consists of: – A radioactive source. This is usually caesium-137 or cobalt-60, and emits gamma rays of medium energy (in the range 0.2 – 2 MeV). For example, caesium-137 emits gamma rays with an energy of 0.662 MeV. – A short range detector. This detector is very similar to the detectors used in the natural gamma ray tools, and is placed 7 inches from the source. – A long range detector. This detector is identical to the short range detector, and is placed 16 inches from the source.

• A formation with a high bulk density, has a high number density of electrons. It attenuates the gamma rays significantly, and hence a low gamma ray count rate is recorded at the sensors. • A formation with a low bulk density, has a low number density of electrons. It attenuates the gamma rays less than a high density formation, and hence a higher gamma ray count rate is recorded at the sensors.

Calculating Porosity Neutron Log (NPHI)

• A neutron source bombards the formation with high energy Neutrons. • Most collisions of the neutrons with heavy atoms of the formation are near elastic. As a result hardly any energy is lost. • A collision with a hydrogen atom (H) lowers the speed (energy level) of the neutron significantly, as both have the same mass. • The distance over which the neutrons travel before they reach a lower (thermal) energy level, is therefore related to the amount of hydrogen atoms present in the formation,

• A source and two detectors are mounted in a tool, which is pressed against the bore hole wall. The detectors only count returning neutrons which have a thermal energy level. • From the ratio of thermal neutrons detected by the far- and the neardetector, the amount of hydrogen (H) atoms in the formation is empirically determined. • The tool assumes all H atoms to be present in the porespace (water or hydrocarbons). The tool is calibrated to read the true porosity in water filled Limestone. These limestone-porosities are computed and plotted against depth in porosity units (p.u.). • The matrix type has a small influence on the Neutron response. Across other lithologies the readings must therefore be corrected using an empirically derived chart.

Summary • Neutron logs are porosity logs that measure the hydrogen ion concentration in a formation. • In clean formations (i.e. shale-free) where the porosity is filled with water or oil, the neutron log measures liquid-filled porosity. • Whenever pores are filled with gas rather than oil or water, neutron porosity will be lowered. This occurs because there is less concentration of hydrogen in gas compared to oil or water. • A lowering of neutron porosity by gas is called gas effect. • Neutron log responses vary, depending on: – differences in detector types, – spacing between source and detector, and – lithology-i.e. sandstone, limeston, and dolomite.