Materi Seminar 3 GeofisikaRev

SEMINAR

PERANAN FISIKA DALAM EKSPLORASI

MINYAK DAN GAS

METODE EKSPLORASI MINYAK & GAS

METODE GEOFISIKA

UNIVERSITAS MATARAM 23-24 SEPTEMBER 2011

OLEH:

WAHYU TRIYONO

PERTAMINA PHE ONWJ

Lingkup Kerja Geofisika

1. PENGUKURAN GRAVITY/MAGNETIK UNTUK STUDY

AWAL

2. PENGAMBILAN DATA SEISMIK

3. PENGOLAHAN DATA SEISMIK

4. INTERPRETASI DATA SEISMIK

5. PEMETAAN BAWAH PERMUKAAN

6. PENENTUAN LOKASI PENGEBORAN

7. PENENTUAN KEDALAMAN PENGEBORAN

8. PENGAMBILANG DATA KECEPATAN SUMUR

(CHECK SHOT/VSP)

Seismic Acquisition

Akuisisi data seismik terdiri dari:

1. Land Acquisition

2. Marine Acquisition

3. Transition

Seismic• sources and recek/ers On land On water

Sources:

Impact

Sledpi e harnmer

Drop-weight

Accelerated \veiailt

Impulsive Dynamite

Detonating cord

A irgun

Shotgun

Borehole sparker

Airoun Pinger

Gas gill] Boomer

Sleevo gun Sparker

Water gun Steam gun

\ ibrator Vibroseis

Vibrator plate

Rayleigh wave generator

Multipulse

Geochicp

Receivers: Kieophonc. accelerometer I lydrophones (streamers)

IL Arff PERTAMINA

PHE ONWJ bpmigas

Land Acquisition

• Sumber gelombang (source): Dinamit, vibroseis

• Receiver: Geophone

• Recording unit menggunakan truk atau mobile unit

Source

Dynamite

Land Acquisition Jenis-jenis Vibroseis

Acquisition setup

• Single channel measurements (profiling)

Only one source and receiver are used with often an equal distance between the source

and receiver. This is repeated for several positions along a line.

• Multi channel measurements

– Multi channel systems use one source and several receivers, which

measure at the same time.

Several spreads are possible to orient the sources and receiver:

Types of reflection spreads. The symbol o and + represent source and geophone-group center

locations, respectively.

Jenis-jenis Geophone

Kondisi Seismik Akuisisi Darat

Schematic Land Acquisition

Marine Acquisition

•Sumber gelombang (source): air gun

•Receiver: Hydrophone

•Recording unit: menggunakan kapal

• Reciever dibentangkan menggunakan

streamer

Marine Streamers

• ~6 km length

• Groups of hydrophones every 25

meters

“birds” receivers

Marine Acquisition

Seismic Acquisition

MARINE SEISMIC DATA ACQUISITION

Transition Zone Acq

•Sumber gelombang (source): air gun &

Dynamite

•Receiver: Hydrophone & Geophone

•Reciever dibentangkan menggunakan kabel

•Area di sekitar pantai atau laut dangkal yang

tidak bisa dilewati kapal

TEORI SEISMIK

HUKUM SNELL

KECEPATAN GELOMBANG SEISMIK

Faktor-faktor yang mempengaruhi :

Bulk density

Porositas

Perbedaan tekanan

Fluida pori

Litologi

KECEPATAN GELOMBANG & LITOLOGI

GL-5044 INTERPRETASI SEISMIK GEOLOGI

Waves

fvvelocity

kwavenumber

fperiod

.

2

21

WAVELET

Definisi :

gelombang dengan durasi waktu (t) yang pendek

yang dihasilkan oleh suatu impuls

Dalam pengolahan data seismik biasa digunakan

dua jenis wavelet


Minimum phase and zero-phase wavelet

Energy is maximally front-loaded

(Energy does not arrive before zero time) Symmetric with respect to zero time

and peaks at zero time

(Energy arrives also before zero time)

Instrument Balance

Noise

Source Strength-9 Interference and Coupling of Different Events Array Directivity

Scattering

Tran,missivity

Peg—Leg Multiples from Thin Reflectors

Spherical Divergence Absorption

Reflection Coefficient Reflection Curvature

Variation of Reflection and Rugosity

Coefficient with Incident Angle

Superimposed Geophone Sensitivity and Coupling

IllfatiVer V V

FACTORS WHICH AFFECT AMPLITUDE

PERTAMINA PHE ONWJ

Exam & OF r0.110CtiOn CO.ISrnO9rarn

Morriant of clischar9E

First arrivirlEi icripulses

0 100 200 300. 41X1 1:o4) 600 703

Vibrator Apcoming trkr.K.

Arm PERTAMINA PHE ONWJ bpm igas

Shot geometry variables

Stacking

CMP Stack a CMP gather

Stack Repeat separately for each gather

• Improve Signal—to—Noise

• Cancel Multiples ▪ One Trace Per Reflection Point • Easy—to--interpret Times

IL Arff PERTAMINA

PHE ONWJ

Example of stacking to attenuate multiple

........ ...... •01■ ,1/4.

Primary Reflector 2 STACK --A,

S I G NALIT 0-NOISE AIIPUTUDE IMPROVEMENT

Arrival Time

1 Multiple ) liN

sec 6—CMP Input Traces Stacked after NMO Correction Trace

where NI a Immalhot off mak= tow two% %WWI

EMII, Ar‘k PERTAMINA PHE ONWJ

Definisi Fold

Sampling

By measurements using a digital system, the data is not continuously measured,

but at a specific time interval measured and transported to the AD-converter.

Analogue representation

of a sinusoidal function Digital representation of a

sinusoidal function

NOISE

Definisi : gangguan yang sering ditemui pada rekaman data seismik

Noise dapat dibagi dua :

Noise koheren

Noise tak-koheren


Noise

The S/N ratio “Signal-to-noise ratio” gives the ratio between

the amplitude of a

signal (e.g.Reflections) to background noise(“random noise”)

or noise sources

(“coherent noise”).

One aim of the dataprocessing is to increase the S/N ratio.

Coherent Noise

• Sided-scattered noise

From irregularities or the point scatters

• Multiples (repeated pattern)

Ghost, Simple multiple, Water-bottom multiple,

Peg-leg multiple, Inter-bed multiple

• Vibration noise

• Interface wave

• Ground roll (Rayleigh surface waves on land survey)

Low frequency

Strong amplitude

Low group velocity

• Guided waves (refractions and head waves on shallow marine survey)

Strong velocity contrast with the substratum

Early arrivals

Multiples

• Long-path multiples

– (occur when exceptionally large reflectioncoefficients are present):

-> Ghost reflections, where rays from a buried explosion on land

(or an airgun in water) are reflected back from the ground surface

(or sea surface) to produce a reflection event, known as a ghost

reflection, that arrives a short time after the primary.

-> Water layer reverberations, where rays from a marine source

are repeatedly reflected at the sea bed and sea surface

• Short-part multiples (“peg-leg multiple”):

– Involve only a short additional path length to arrive so soon after

the primary event that they extend the overall length of the pulse.

(Multiples between two interfaces of a layer)

Ray paths of some common multiple families

Source ghost Receiver ghost Simple multiple

Water bottom multiple Peg-leg multiple Inter-bed multiple

96 bpmigas

PERTAMINA PHE ON\NJ

Random Noise

• Instrument noise

• Machinery

• Power line

• Cable noise

A, B, C, D are guided waves

trapped (refracted waves) at the sea

floor. They have a dispersive

character with low frequencies C

arriving first, higher frequencies B

arriving next, and moderate

frequencies D arriving last

A = weak reflectors

B = strong reflectors

C suggests a fault in spread

D = ground roll

Deep water shot record

• A is direct arrival

• B is water-bottom reflection

• C is shallow reflection

• M is 1rst water-bottom multiple, note double

time of B

• PL is water-bottom and peg-leg multiples of C

Resolution of two boundaries depends on

wavelength

Decreasing image separation

Resolution

Resolution refers to the minimum separation between two features such that we

can tell that there are two features rather than only one.

Comparison between the wavelength of a 30-Hz Signal, Big Ben and a Log of a drilling.

Geological model, reflection series and seismogram

Contoh Hasil Field Record

Shot Gather

Shot gather dari

seismik data darat:

A. Direct wave

B.Head wave

C. Ground roll (surface

wave)

D. Reflection events

Mann et al. (2004)

Representation

The traveltime is in general shown with increasing time along the vertical Increasing downwards

(larger traveltime corresponds to a larger depth). There are several ways to represent seismic

traces. The sort of representation depend on the processing used, but also on the number of traces.

Reflectivity

series

Wiggle

only

Variable

area

Variable

density Variable

area+wiggle

Different representations of a seismic trace.

Seismogram

Geological

section

Accoustic

impedance

log

Refflectionc

coefficient

log

Reflectivity

coefficient

Input

pulse

seismic

trace

From a geological subsoil to a seismic trace.

Alur Pemrosesan Data Seismik

• Demultiplexing/Reformatting

• Geometry

• True Amplitude Recovery

• Surface Consistent Amplitude Correction

• Statics Correction

• Velocity Analysis

• Koreksi Normal Move Out (NMO)

• Brute Stack

• Residual Statics

• Dip Move Out

• Velocity Analysis

• DMO Stack

• Migration

Various ways to display seismograms:

Wiggle trace/

Variable trace Variable trace Wiggle trace

Top Formasi

Reef

Sesar/patahan

TWT (Two Way Time) Seismik section Variable area wiggle trace yang sudah

diinterpretasi

Penampang Seismik dengan Variable Density

Materi Seminar 3 GeofisikaRev

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