internship PPL

Name: Osama Bin Raees

Department: Exploration Department

Position: Internee (Geophysicist)

Institution: Bahria University Karachi Campus

Internship Duration: 18 th January 2016 – 14 th February 2016

INTERNSHIP REPORT

Submitted to: Mr. Syed Muhammad Tauqeer (Manager Exploration)

Pakistan petroleum limited 1

Acknowledgment

The internship opportunity I had with Pakistan Petroleum Limited was a great chance for learning and professional development. Therefore, I consider myself as a very lucky individual as I was provided with an opportunity to be a part of it. I am also grateful for having a chance to meet so many wonderful people and professionals who led me though this internship period.

Bearing in mind previous I am using this opportunity to express my deepest gratitude and special thanks to the Mr. Syed Muhammad Tauqeer, Manager Exploration Department, who in spite of being extraordinarily busy with his duties, took time out to hear, guide and keep me on the correct path and allowing me to carry out my project at this esteemed organization.

I express my deepest thanks to Mr. Muhammad Shoaib Bilal, Senior Geophysicist, PPL for taking part in useful decision & giving necessary advices and guidance and arranged all facilities to make life easier. I choose this moment to acknowledge his contribution gratefully.

It is my radiant sentiment to place on record my best regards, deepest sense of gratitude to Mr. Tauqeer Haider, Geophysicist PPL, Mr. Ajam Abbas, Geophysicist PPL, Mr. Haris Haleem, Geophysicist PPL for their careful and precious guidance which were extremely valuable for my study both theoretically and practically.

I perceive as this opportunity as a big milestone in my career development. I will strive to use gained skills and knowledge in the best possible way, and I will continue to work on their improvement, in order to attain desired career objectives. Hope to continue cooperation with all of you in the future,Sincerely,

Osama Bin Raees

S.no Table Of Contents Page


no.1 Seismic Fundamentals

Seismic Waves and Its types Seismic waves Attenuation and propagation Huygens’ principle Snell’s Law

4-7

2 Seismic Data Acquisition Seismic Sources Seismic receivers Seismic Arrays Seismic Acquisition Parameters Spread Geometry

8-10

3 Seismic Data Processing Seismic refraction First Break Up-Hole Logging Digital Sampling Frequency Frequency Filtering

10-12

4 Seismic Data Interpretation 13

5 Assignments Stacking Chart First break Picking Up-hole logging Geological Interpretation

14-18

6 Field Report 19-25

i. Seismic Fundamentals Pakistan petroleum limited 3

Seismic Waves:

Seismic waves are messengers that conveys the information about the information about the interior of the Earth by propagating into the surface.

Body Waves: Those seismic waves which only travel through the interior of the propagating material is known as Body Waves. Characteristics of body waves are:

Travels faster Smaller Amplitude Short Duration High Frequency

Body waves are divided into two categories:

i. P-wavesii. S-waves


Seismic waves

Body Waves

P-Wave

S-Wave

Surface waves

Love Waves

Rayliegh Waves

P-Waves: They are also called compressional or longitudinal waves because the particle motion is parallel to the direction in which waves are propagating. It can travel through any kind of material (i.e.: solid, liquid and gas)

S-Waves: They are also called transverse or shear waves because the particle motion is perpendicular to the direction in which waves are propagating. It can travel only in solid material.

Surface waves: Those seismic waves which only travel along the surface and just below the surface of propagating material are called surface waves. Characteristics of S-waves are:

Large Amplitude Low Frequency Long duration

Surface waves are divided into two types:

1. Love Waves2. Rayleigh Waves

Love Waves: Loves waves are similar to surface waves in which the particle motion is similar to S-waves but the motion is only along to the surface.

Rayleigh Waves: Rayleigh waves are also called ground role because they rolls along the ground. It exhibits a retrograde elliptical motion. Its particle motion is similar to the P-waves.

Seismic Waves Propagation:

As we bombarded the ground with explosive or vibrator, induces energy in a form of seismic waves that propagates into the surface of the Earth in the form of expending spherical shells. A surface associated with a propagating waves and passing through all


points in the wave that have the same phase and perpendicular to the direction of propagation of waves in known as wavefront.

Seismic Waves Attenuation:

Seismic waves are attenuated with the depth because the loss in frequency and increase in wavelength due to which amplitudes become smaller. As seismic waves propagate away from the source the wave front becomes larger and larger due to which energy gets over spread over an even large area. As a result energy per unit become smaller. Seismic wave amplitude is proportional to the square root of energy per unit area so amplitude get smaller even at larger rate. This type of attenuation is called Geometrical spreading or spherical spreading. Another type of attenuation is due to the rock, which are not the best conductor of seismic energy. As the waves strike with the particles of the rock, some seismic energy converted into heat energy. Higher the frequency, higher the loss in energy later they arrive on seismic detectors. This type of amplitude attenuation is called In-elastic attenuation.

Huygens’s Principle:

This principle tells a mechanism by which a wave propagating seismic pulse lose energy with depth, stating that:

“Every point on an advancing wave front is a new source of spherical wave”


Snell’s Law:

When light travels from one medium to another medium, it generally bends or refract.Snell’s law of reflection states that, “The angle at which the ray is reflected is equal the angle of incidence.”< i = < rSnell’s Law of refraction tells us about the relationship between angle of incidence and and angle of refraction when the rays are passing from one medium to another medium. The portion of incident energy that is transmitted through the boundary and into the second layer with the change in direction of propagation is known as Refracted Ray.

Sin <i/sin<r = V1/V2

Condition for refraction: If the ray passes from more dense to less dense material then it bents

towards the normal If the ray passes from less dense to more dense medium then it bents

away from the normal. The angle at which the refraction becomes 90 is called critical angle.


ii. Seismic Data Acquisition Seismic data acquisition consists of gathering and recording continuous seismic signals from seismic stations.

Energy Sources:There are two basic types of energy sources, Impulsive and vibratory

Source Land Marine CommentsIMPULSIVE Explosives - Dynamite Ammonium Nitrate

Usually shot in drilled holes on land but rarely used as marine source.

Geoflex Shot very near at the surface

Airgun Most popular marine source

Vibratory Vibroseis Most popular land

source

Seismic Receivers:The instrument which detects the seismic signals are called seismic receivers. There are two types of receivers1) Geophones2) Hydrophones

1. Geophones:The instrument which converts ground vibrations (mechanical signals) into electrical signals is known as Geophone. It works on the principle of electromagnetic induction. Geophone

2. Hydrophones:A hydrophone is a microphone designed to be used underwater for recording or listening to underwater sound. Piezoelectric materials,


or transducers, can convert a sound signal into an electrical signal since sound is a pressure wave.

Seismic Arrays It is the geometrical pattern in which the group of receivers and sources are arranged. The pattern may be one or two dimensional (Linear or areal)

Acquisition Parameters:

Following parameters values must be determined before an acquisition program can start:

Line Parameters (No of lines, Orientation Of Lines, Line spacing and line length)

Source Parameterso For Explosives (No. of holes, Hole depth, Pattern)o For Vibrators (Sweep type, No of sweeps, sweep length)o For Airguns (Number and size of guns, Array design and no of Arrays)

Spread Parameterso Spread Types

Off end Source pulling or pushing spread

Split Spread Symmetric or Asymmetric

o Number of Groupso Group Intervalo Maximum and minimum Offset

Fold ( Number of times CDP is imaged)


Spread GeometryWhen it comes to the arrangement of geophones and receivers, we have to arrange in a pattern called spread or spread geometry. Spread geometry is of two types:1) End On Spread2) Split Spread

End On Spread: The geometrical relation is usually required is called end on spread. Here all receivers are on one side of the source.

End On Spread

Split Spread In split spread, if there is an equal numbers of receivers on each side of the source is symmetric split spread, and if there are more receivers on one side of source than the other is asymmetric split spread.

Split Spread


iii. Seismic Data Processing The purpose of seismic processing is to manipulate the acquired data into an image that can be used to infer the sub-surface structure.

Seismic Refraction: Seismic refraction is a geophysical method used for investigating subsurface ground conditions utilizing surface-sourced seismic waves. The acquired data is computer processed and interpreted to produce models of the seismic velocity and layer thickness of the subsurface ground structure.

We do seismic refraction to remove the topographic variation and to determine the velocity of low velocity layer.

Low Velocity Layer (LVL) is a weathered sub surface layer which delays time in signal and produce error in the data.

First Break Picking: Events that first reach at geophones. The first prominent wave amplitude on seismogram Also called First arrival

Red line showing First break


Up-hole Logging: Up-hole logging is also used to get Low velocity layer information for static correction. This methods are costly due to drilling but provides better result as compared to surface shooting method.

Digital Sampling:1. Sampling rate: The rate at which frequency has been sampled is known as sampling rate.2. Sampling frequency : The frequency obtained after the sampling is known as sampling

frequency. Fs= 1000/sampling rate

3. Nyquist Frequency: The maximum frequency obtained from the sampling frequency is known as Nyquist frequency. It is the half of sampling frequency. Sampling Frequency Fn= Fs/2

4. Frequency Aliasing : The frequency which appears as a low frequency greater than the Nyquist frequency and cause error in our data is known as frequency aliasing. It distorts the sampling frequency.

Frequency Filtering: To remove the effect of aliasing frequency we used some filters:

1. High Cut Filter: In this we give particular frequency to the recorder, the frequency above that given frequency will not be recorded.

2. Low Cut Filter: In this we give particular frequency to the recorder, the frequency below that given frequency will not be recorded.

3. Notch Filter: The removal of a particular frequency i.e; causes by the presence of electric wires (55Hz) only that particular frequency will not be recorded.

4. Band-Pass Filter: In this we give a range of frequency to the recorder, the frequency above and below than that given range will not be recorded.


iv. Seismic Data Interpretation Seismic Interpretation is the extraction of subsurface geologic information from seismic data.

Reflection seismic data comprise:

Continuity of reflections indicating geologic structure. Variability of reflections indicating stratigraphy, fluids and reservoir fabric. The seismic wavelet. Noise of various kinds and data defects.Seismic interpretation is the thoughtful procedure of separating these effects.

Critical to the interpretation process is comparing how horizons and faults tie at line intersections. Significant effort is expended correcting miss-ties of faults, horizons, and sequence boundaries at every line intersection. In this regard, closing the interpretation in loops around the seismic grid is a particularly effective technique. On a workstation, a quick way to check for miss-ties is a contour map. Miss-ties will be evident by groups of unreasonable contours. In addition, workstations can be very helpful for working out the miss-ties among varying vintages of two-dimensional data by applying time and phase shifts automatically.

Tying all lines in both 2-D and 3-D data sets is the only way to reliably construct a three-dimensional model of the subsurface using two-dimensional images. Tying around data loops is also the best way to correlate from fault block to fault block. Otherwise, faults must be jumped using reflection character, sequence analysis, or additional well control.

After all lines are picked and tied, the results of the interpretation are then summarized and presented as maps. Basically, any observation that can be made using seismic data can be posted on a base map and mapped. Maps that are routinely made include

Time structure maps with faults Depth structure maps Seismic facies maps for reservoir, source, or seal analysis Thickness maps inferred from seismic tuning analysis Fault plane maps Isochor or isopach maps showing growth or thinning in a stratigraphic intervalSeismic velocity maps for lithology determination or depth conversion


v. Assignments:

1. STACKING CHART

OBJECTIVE:

To determine the full fold and trapper zone.

FOLD

The number of times common depth point is imaged.

Methodology:

The general idea of the method is to acquire a series of traces which reflect from the same common subsurface mid-point. In the adjacent figure source points are shown in red and receiver points in green. The traces are then summed (stacked) so that superior signal-to-noise ratio to that of the single-fold stack results. The fold of the stack is determined by the number of traces in the CMP gather.

Result:

I coincide a recorder with 6 channels and by considering the receiver interval 10m source interval 10m then got the value of full fold equal to 3.

NOTE

The assignment is attached as an Annexure. 1


2. First break Picking:

Objectiv e : To pick the first break and find the thickness and velocity of weathered layer. Methodology: I was given the chart of traces detected on the seismograph/geophone. I had to pick the first break and plotted it on the T-x graph. After plotting the first break, I passed the best fit lines across the plotted points. The lines where intersected is known as Cross over distance and the time at which they intercept is known as intercepted time. Then I find the slope of those best fit lines by using slope formula (y2-y1/x2-x1) then by inversing the slopes I got the velocity of weathered layer.

Forward Shooting Reverse Shooting

T-x Graph

Result: I got the velocity of weathered layer 250m/s and thickness is 6.21m.

NOTE: The assignment is attached as an Annexure. 2


3. Up-Hole Logging:

Objective: To find out the thickness of weathered layer. Methodology: I was given the up-hole log data in which offset is 2m. The data is plotted on the graph is which depth is on y-axis and time on x-axis. Pass the best fit lines and then find out the thickness of weathered layer.

Up-hole Data Time-Depth Graph

Result: I got the thickness of weathered layer 5.5m.

NOTE

The assignment is attached as an Annexure. 3


4. Seismic Data Interpretation

OBJECTIVESThe analysis of seismic data interpretation includes five steps namely as follows:

1. Reflectors identification2. Picking & correlation of reflectors3. Fault location4. Time to depth conversion5. Constructing Geo-seismic cross-sections

Method1. Identification of reflectors

It is usually better to start picking reflectors by inspecting seismic sections passing through boreholes. Reflectors are identified through tying the seismic sections to the well data. Composite logs are used to determine the depth to tops of different formations.

2. Picking and correlation of reflectorsStudied horizon was picked up across seismic lines after the reflector identification.

3. Fault location detectionFaults of large vertical displacements are easily recognized, especially from the sudden stepping-out of reflections across their planes.

Evaluation of Seismic Section Sirani 1. IntroductionA display of seismic data along a line, such a 2D seismic profile. A seismic section consists of numerous traces with location given along the x-axis and two-way travel time or depth along the y-axis.

2. DataThe data provided to me for interpretation was of Sirani field. Seismic Sections of Lines:

Sirani-02, Sirani-04, Sirani-06, Sirani-08, Sirani-03


3. ProcedureIdentification of Horizons

Interpretation starts from bringing the control from well data known as seismic well tie. Used wells for seismic tie. After identification of horizons tie the line where they are intersecting each other. Pick horizons by joining the same reflector. Dip line and strike line interpreted in a loop. After tying all lines note down the two way times on each Shot Point of each line. Put the time values of all Shot Point showing in the Base Map. Then Time-Structure map is contoured by following the same time values.

NOTEThe assignment is attached as an Annexure. 4


Field Report ( HUB BLOCK - 2D)

INTRODUCTION:

Exploration department of PPL arranged a field visit of 05 days for me on 2D seismic data acquisition. As per plan I arrived in BGP base camp on 31st January 2016. This crew was working in the HUB block of PPL which covers an area of 418 km2 and contains 3 strike lines and 12 dip lines

Mr. Sohrab Baig set my field plan in the crew as follows:

1. Day 1- Cable Crew2. Day 2- Drilling Crew3. Day 3- Recorder4. Day 4- Survey Crew

HSE induction was given by the HSE team.

CABLE CREW Cable crew is divided into different smaller groups because of their separate work responsibility of job nature.

a) Front Crewb) Back Crewc) Trouble Shooter

Front crew

Front crew is used to layout, equipment (geophones, cables). Front crew use pattern of layout which is given by client.


Responsibilities of front crew

Spacing of geophones should be constant and strings proper connect with each other and with cables.

Spikes of geophones should plant properly (900). All cables and receiver stations which cross the roadways or access routes should be buried

under the ground. All geophone strings should be tested immediately before connecting to cable. Geophone pattern must be kept according to the client’s demand. The center of the pattern must be on the receiver point.

More emphasis was given to normal Geophones layout geometry.

Geophone Type 30DX-10Hz Geophone array Rectangular Geophones per array 12, 1string

b) BACK CREW After recording completion the geophone strings, LAUL, LAUX, batteries and cables are picked up. These cables sent to cable shop for testing and again transfer to line for further use in data acquisition.

c) TROUBLE SHOOTING Active persons of the cable crew make the seismic line clear for recording and overcome the noise factor by removing the faulty and disconnected cables, geophone strings, batteries, LAUL, LAUX etc. Also try to stop people, vehicles and animals movement in the cable spread etc.

Trans-cable is connected with LAUX and then from right, left both side of recorder. Actually Tran’s cable is the


LAUX Line Acquisition Cross Unit is connected with all the receiver lines. It is used to connect all the receiver lines to the recorder. One LAUX is connected on each receiver line and all LAUX are connected to the recorder by the help of Tran’s cable. A 12V battery is connected on each LAUX which gives power to it.

LAUL Line Acquisition Unit is connected with FDU cables from high to low side with 12V battery which gives power to it. LAUL is installed after every 40 channels. LAUL provides power to the FDU’s and also passes on the data from 40 channels to the next LAUL.

LAUL

FDU CONNECTIONFDU (Field Digitizer Unit) digitizes the data in the field and passes it to the next FDU. FDU is installed on each channel and 6 geophones are connected to each FDU. The advantages of FDU are that it digitizes data in the field.


DRILLING & LOADING CREW:There are different techniques used in seismic industry for source points, every project have different requirements regarding drilling, in current project the technique used for drilling shot point ( SP’s) is jackrob due to hard surface.

Drilling pipes Generator

Jackrob

LOADING CREW:After the drilling loading personal load drilled hole with the explosive. Loading is the last step of completion of a hole. Loading crew makes sure that dynamite is properly damped in the hole and the hole is again closed with mud. Labeled wooden strip is mounted above the hole so that at shooting time hole can easily be identified.

Following parameters are used for source points.

Source DynamiteSource parameters for Shot Hole:

1- Charge size: 5kg/hole 2- Charge depth: 20m


RECORDING:Parameter of Recording Crew:

Recording Parameters/Receive & source parameters of HUB 2D Project:

Receiver Station Spacing 20 metres Receiver lines are nominally oriented in an E-W

direction.

Fold 600

Geophone Type/Frequency frequency 10Hz. Recorder Type 428XL

Geophone Array 12 geophones per station. Rectangular array, in-line with receiver line over 20m group, at least

0.667m between geophones.

Record Length 6 seconds

No. Of Active Channels 600

Receiver Interval 20m

Source Interval 10m

Near Off Set 20m

Far Off Set 12000m

Source Dynamite

Source parameters for Shot Hole:

Charge size: 5kg

Charge depth: 20m


Survey Crew:

In GPS there are three modes of survey. RTK (real time kinematics) First Static. Static.

In RTK base station, repeater and GPS (rover & TSC) are used. In first static no base station is used. It directly takes signals from satellite. Rover, TSC & tripod are used. Where as in static technique, no need of base station. Rover card & tripod are used. It takes one value in 15 minutes.

Survey Operation:In performing navigation survey base station, Rover & GPS (TSC & Rover) are the main features.

Base StationIn starting survey in an area first of all base station is deployed. It transmits signals to GPS & guide to the exact location of the point. Tripod, TSC, Rover, Radio & Radio antenna are used. Base antenna receives signals from satellite. It transfers it to rover. Rover further transmits the signals to radio. Radio transfer signals to repeater. In the end repeater transmits the signals to the GPS where operator marks the points.

Repeater:It consists of radio, battery & radio antenna. Repeater enhances the signals in the area where base is too far from GPS operator or any hindrance between base and GPS operator.

GPS (Rover & TSC)It comprise of TSC & rover. TSC is picking by the GPS operator & rover is in the bag which is suspended in his back as shown in the figure 9. In TSC co-ordinates of points are stored by chief surveyor by attaching TSC with computer. These co-ordinates can be given in WGS & in grid system.

Trimble Survey Controller:

TSC receives signals from base station that guides it to reach the desired point. There are 24 satellites in geodetic survey. Minimum six satellites are necessary to record the point. TSC can only record any position if & only if the TSC is fixed. Fixed is the condition in which satellites, base station & Rover is communicating with each other properly. If satellite signal are weak, then TSC floats and in this way we can’t be able to record any location or point.


Rover:Rover is the part that part of the GPS that helps TSC to record any position or point. It has two antennas one in GPS & other in radio. GPS antenna receives the signals from satellites whereas radio antenna receives signals from base station.

CONCLUSION I bow in gratitude against ALMIGHTY ALLAH for giving me such a great opportunity. I would like to show my heartiest gratitude to Mr. Shoaib Bilal, under whose supervision I have worked. Words cannot express my sense of gratitude and profound reverence for the general encouragement, constructive criticism and valuable guidance of all those who helped me during my internship program. My struggle would have been futile if I had not received their co-operation and guidance. The work enhanced my knowledge of the practical aspects of seismic field activities. I thank ALMIGHTY ALLAH who blessed me with such knowledgeable, kind and outstanding instructors, such amazing facilities and an opportunity to get training from such a great company.


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