iptc 17582 from feasibility to processing: a time-lapse ... · a feasibility study which aims to...

IPTC 17582

From feasibility to processing: a time-lapse seismic experiment on Al Khalij field – offshore Qatar

Fabrice Cantin, Bruno Pagliccia, Total E&P Qatar; Patrick Charron, Emmanuelle Brechet, Eddy Brozille Total S.A.; Michael Emang, Qatar Petroleum

Copyright 2014, International Petroleum Technology Conference This paper was prepared for presentation at the International Petroleum Technology Conference held in Doha, Qatar, 20–22 January 2014. This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435

Abstract Time lapse seismic survey (aka 4D seismic) is a geophysical tool used by the industry to guide and maximize field

development. It has been proven successful mainly in clastic deposits environment (Gulf of Mexico, Gulf of Guinea, North

sea,…) but sparsely tested on carbonate fields because of seismic quality concerns.

Total E&P Qatar engaged in 2012 a “proof of concept” 4D pilot study on its operated Al Kahlij field – offshore Qatar- to

demonstrate the value of information provided by a 4D seismic in a middle-east carbonate field under development. This

study is made of:

A feasibility study which aims to quantify the expected 4D effect by using a petro-elastic model and several chosen depletion

scenarii. These input data are used to compute a synthetic time-lapse seismic dataset through a massive seismic modeling.

A 4D pilot seismic survey shot in summer 2012 (monitor) which was designed to repeat the first seismic survey shot in 1998

(base).

A dedicated seismic processing of both base and monitor in Total’s 4D processing dedicated center followed by a specific

inversion process (time warping) to extract the awaited 4D signal.

The main objective on Al Khalij field is to validate that 4D seismic is able to calibrate the reservoir model and make it more

predictive.

If successful, the time-lapse seismic survey concept can be envisaged for other carbonates field in Qatar and in the Arabic

Gulf area for field development optimization.

Introduction

The Al Khalij field is an oil producing field located in Block6, offshore Qatar (Figure 1).

Total E&P Qatar has been developing the field with Qatar Petroleum since 1996. A 3D

seismic survey was shot in 1998 and has been processed several times until 2009 to build

the reservoir model used to monitor and further develop the field.

The Al Khalij field is situated in transition zone, its water production rapidly increased to

reach around 90% water-cut. Water arrival on the field is complex and implies several

kinds of phenomena: short and long range injections, bottom and edge aquifer, large scale

conductive events bringing water influx, etc. The field yields however large remaining

reserves and it is planned to redevelop it. A better understanding of the water behavior is

needed at a larger than well scale. Time-lapse seismic is a proven technique in clastic

environments to assess fluid saturation changes both in time and space. Its efficiency in

carbonate environments is still to be demonstrated in the Arabic gulf area.

Figure 1: Al Khalij location map

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Definitions

Time-lapse (or 4D) seismic refers to repeated 3D seismic surveys in time. The principle is to repeat the initial survey

(called “base”) with a newer survey (called “monitor”) acquired after several years of oil/gas production. By

comparing the seismic signal characteristics (amplitude, phase, time shift) from the two surveys at the reservoir

levels, the detected differences should represent a spacially comprehensive view of the field behavior between the 2

seismic acquisitions.

Repeatability noise: noise measured on the difference cube between base and monitor in an area where no difference

is expected. Because the seismic experiment is not perfectly repeatable, this noise is never equals to zero.

4D detectability: a concept issued from the radar technology and the interferometry. A signal can be detectable with

a time-shift that could be smaller than the sampling rate.

NRMS: (from 0 to 200%), defined by Kragh and Christie in 2002. NRMS represents the normalized RMS difference

between base and monitor traces. If base and monitor are perfectly anti-correlated, NRMS=200%. If the amplitude

of the base trace is exactly half the amplitude of the monitor trace, NRMS=66,7%.

NRMS is routinely used to measure non repeatability but is sensitive to amplitude, phase and time-shift. It should

decrease at each step of the processing sequence.

Seismic traces of base and monitor are not two random signals. NRMS for two white Gaussian noises is 100√2 =

141%. Seismic signals acquired in different location have a lot in common.

NRMS above reservoirs will represents mainly the 4D noise in case of non compacting reservoirs. The best NRMS

correspond in lower values in this case. Inside reservoirs, NRMS represents 4D signal and will show higher values.

SWD stands for Shallow Water Demultiple: the algorithm identifies and extracts the seabed multiples from the

seismic with the help of the bathymetric data and is able to reconstruct a seismic marker for the seabed from these

identified multiples.

SRME stands for Surface Related Multiple Elimination: it was developed for deep-offshore seismic where the

seabed is visible and works by substracting a model of seabed multiple from the seismic data. In shallow water (less

than 100 m), it is used in combination with the SWD which feeds SRME with the needed seabed seismic marker.

HR Radon is broadly used in the industry to identify and remove multiple by its specific curvature at long offset.

Feasibility study The purpose of this study was to model the 4D signal obtained for different production scenarios for the reservoir layers in

Al-Khalij and analyse the sensitivity of 4D seismic attributes to changes in fluid pressure and saturations. The study is based

on a massive seismic modeling approach (internal tool Cassis1Dtm

) based on 1D well log data of the exploration well ALK1

for the static properties of the reservoir. For the dynamic changes of pressure and water saturation, the seismic modeling was

performed according to production scenarios defined by reservoir engineers.

As no full rock physics model was available at the date of the feasibility study, a simplified rock physics model has been built

in order to perform the study. As no acoustic measurements on cores were available on Al-Khalij for pressure laws; velocity

laws were taken from an internal carbonates database compiled from our carbonate fields around the globe. Therefore,

uncertainties still remain on pressure laws.

IPTC 17582 3

Figure 2: Massive seismic 1D modeling, each modeled seismic trace from the lower panel corresponds to one scenario of

pressure/water saturation change with a given reservoir layers distribution (from Layer 5 down to Layer 0) represented in the

upper panel.

The upper part of the figure 2 represent the different scenarii of the seismic modeling for saturation and pressure effects for

all the reservoir layers (from Layer 0 to Layer 5). The four colurs represent pressure changes in the reservoir from +40 to -80

bars. For each pressure scenario (for each coloured zone), water saturation changes from 0 to 100% is simulated for a given

reservoir level starting from the top of the reservoir (Layer 5) in a pyramidal way: Layer 5 isd divided in three saturation

scenarii, for each scenario, the layer below (Layer 4 in that case) is also subdivided in three saturation scenarii, and so on for

all the layers below. This pyramidal distribution lead to a set of synthetic traces shown in the lower part of figure 2, each

trace represents one scenario of saturation/pressure within one possible reservoir layers distribution.

From the massive seismic modelings performed (see Figure 2), the main conclusions of the study are the following:

Pressure effects (depletion effect):

The time shift effect is very significant and in some cases it reaches approximately 5 times (maximum: -1.8 ms for a

maximum depletion of -80 bars) the limit of expected detectability (350 µs).

At the top of the reservoir (Top Mishrif) the amplitude effects are below the expected limit of repeatability (15 %).

4D seismic is likely to bring quantitative information through the analysis of the 4D time shift.

However, monitoring pressure effects are subject to uncertainties since the 4D effect due to pressure is modelled

according to a common carbonate database.

Saturation effects:

The time shift effect predicted (~ 60 μs) is well below the limit of expected repeatability (350 μs) and will not be

detectable.

Amplitude effects are in general small (with the exception of Layer 3).

Layer 3 is likely to be detected with a streamer on streamer survey (provided the expected repeatability noise of 15

% is reached).

Layer 0 is at the limit of repeatability.

This feasibility study, considering its internal limits (no core data, rock physics model incomplete, a priori repeatability noise

level), was assessed a encouraging indicator to shoot a seismic pilot survey to confirm the existence of a 4D signal with the

4 IPTC 17582

real data.

4D pilot seismic acquisition

In order to be in position to detect a viable 4D signal produced by the field depletion, it is mandatory to repeat the same

seismic experiment as the one did with the base seismic. The monitor survey must be shot with the same seismic acquisition

parameters. Of course, between 1998 and 2012, the technology changed with a quick pace and it is often not possible to use

exactly the same seismic equipment. Nevertheless, we succeed to shoot the pilot monitor survey with similar parameters

(Figure 3). The main difference is coming from the streamer group interval of 12.5 m instead of the 6.25 m used for the base

survey as nowadays no streamers are available on the market with such a short group interval, but the same processing bin is

obtained by decimating the base trace interval to match the monitor one.

Figure 3: seismic acquisition spread of the monitor survey and comparison of acquisition parameters between the base and

the monitor surveys.

IPTC 17582 5

Because of operational constraints, it was decided to

shoot the monitor pilot survey outside of the safety area

of the existing platform while keeping the same

acquisition direction as the base survey. Two swathes

from 1998 base survey have been repeated for an

acquisition surface of 52 sq.km compared to the 390

sq.km of the original seismic survey (see Figure 4). For

such a small survey, the challenge was to find a vessel

for an affordable cost and in August 2012 a vessel

opportunity allowed us to shoot this pilot monitor

survey.

Unfortunately, the weather window was totally different

from the one of the base survey as the 1998 seismic was

shot in winter time, so the sea state and the currents

were different between the base and the monitor leading

to different level of ambient noise and different streamer

feathering.

To mitigate this issue, a dedicating 4D navigation team

was placed onboard the vessel to optimize the

repeatability of the source/streamer positioning to match

the base survey. At the end of the acquisition, roughly

10% of the sail lines were not repeatable within

acceptable limits but the 4D binning of the processing

sequence was able to correct this effect.

Figure 4: extent of the base survey (amplitude map in orange-grey

scale) and of the monitor survey (amplitude map in red-blue scale).

Arrows indicate acquisition directions and black dots the surface

obstructions.

4D pilot seismic processing Seismic processing was performed by the dedicated 4D processing

team operated by CGG and located inside the Total technical centre in

Pau, south-west of France. From the beginning, the complexity of the

project, specifically the demultiple process, was identified and it was

decided to consider this project at an R&D level rather than a

production job with a well-known processing sequence. A huge effort

was performed by an extensive testing of each algorithm of the

processing sequence (Figure 5).

As for the seismic acquisition, seismic processing was designed to

minimize all the artifacts not linked to any geological events: base and

monitor survey were reprocessed at the same time, with the same

contractor, with the same version of the processing sopftware and by

the same geophysicist.

Along the reprocessing work, it was found that the base survey has

much more random noise than the monitor. This is due to the fact that

the base was shot in winter time with a significant weather stand-by

(around 25 %) whereas the monitor shot in summer time with a very

low weather stand-by (around 2 %).

Figure 5: processing sequence used for base and

monitor surveys

Multiple is the most challenging issue in this reprocessing, the processing centre spent months in testing all possible

combinations of the three algorithms (SWD, SRME and Radon) and their impact on a 4D signal. Indeed, distinguishing a 4D

6 IPTC 17582

signal from seismic noise is not a trivial task.

Figure 6 represents a stack section of the time-lapse seismic experiment at the SRME stage. On the left, the seismic sections

of the base, the monitor and the difference before application of the SRME, on the right the same sections after SRME

application.

Figure 6: seismic sections of the base, the monitor and the difference before and after SRME application

The difference between base and monitor could be considered as a rough 4D experiment and is used for QC only. More

sophisticated tools are needed for a proper 4D signal extraction from these two datasets. Anyway, figure 6 shows that SRME

generates more “signal” in the difference section than before its application. It could have two significations:

SRME is very efficient on both base and monitor survey and a meaningfull 4D signal related to fluid

movement or field depletion appears in the data.

SRME removes the multiples model from the data by proceeding to an adaptative substraction. For a

classical 3D processing, this adaptative substraction is very powerfull because for each shot point, the

multiples model is adapted to match the energy of the shot so the model is very close to real multiples. For

a 4D processing, as the base and the monitor surveys are not perfectely repeated, the SRME can introduce,

by its adaptative substraction, a 4D effect which is not related to any fluid or pressure changes in the

reservoir. In other words, the multiples model substracted from the seismic data could be different in the

base and monitor datasets.

On this dataset, SRME behavior is very suspicious as differences between base and monitor seismic sections are also visible

outside of the reservoir levels, phenomena you do not expect in a time-lapse seismic experiment. It means that in this

experiment, the multiples are not repeatable between the base and the monitor. This could come from the fact that the base

survey was shot in December-January whereas the monitor was shot in August with different sea-state. It could be also due to

seismic sources which are not perfectely similar (in terms of type of airgun and source configuration) for the two seismic

vessels used 15 years apart.

Another way to look at the data is to monitor the progress of the seismic processing through assessment of seismic

repeatability with maps of the difference between the base and monitor.

Figure 7 is a concertina of maps along the seismic sequence. The attribute used is the Normalised RMS amplitude (NRMS)

computed in a window above the reservoir levels (called the overburden), area where no 4D signal is expected. Red color

indicates strong NRMS amplitudes in the difference between base and monitor, blue color indicates weak NRMS values in

the difference between base and monitor. From a general point of view, the NRMS value decreases while the processing

sequence is moving towards the final stage. At the last step, a FK filter applied to denoise the final cubes, the NRMS values

of the overburden is close to zero showing the processing repeatability for the base and monitor is of enough quality to be in

position to see a 4D signal in the reservoir levels.

IPTC 17582 7

Figure 7: monitoring of the NRMS value of the overburden along the processing sequence computed on the difference cube

(monitor – base)

Looking at these maps in details shows that the SRME application boosts the NRMS amplitude of the difference between

base and monitor where a diminution is expected.

From the above considerations, it has been decided to not apply the 3D SRME in the processing sequence.

Conclusion

4D Al Khalij pilot survey is one of the first time-lapse seismic experiments in offshore Qatar. It was also one of the only

surveys shot in carbonate environments within Total group worldwide.

This lack of previous experience was a strong limitation for the feasibility study and some arbitrairy hypothesis (like awaited

repeatability noise level) were introduced.

However, the feasibility study results were good enough to take the decision to acquire a 4D pilot survey to validate the

concept in real life. The seismic acquisition ran smoothly and occurred in a favourable weather window with a very low

ambient noise. It is an important aspect to pinpoint because the first reaction of people when a time-lapse seismic survey is

envisaged is that you should whatever possible to repeat the base survey, even shooting in the same weather windows.

Unfortunately the ambient noise is not repeatable and shooting in summer for the monitor (base was shot in winter) was

proven effective as the processing centre had to deal with ambient noise on the base only.

Seismic processing highlights the following points:

Multiples elimination without affecting a possible 4D signal was the main challenge. From more than 10

4D processing projects performed within Total in the past 5 years, AL Khalij was by far the most difficult

one. A lot of time and efforts were spent to monitor the effectiveness of the demultiple algorithm and the

processing in general

At the end of the processing sequence, the repeatability noise (around 10 %) is of good level and in the low

range of all the previous 4D shot within Total group. Geophysicists are now confident enough in the

seismic dataset of the base and monitor to start the 4D signal extraction if any.

At the time of this paper, the 4D signal extraction and interpretation are on-going and, if successful, will help Al Khalij field

redevelopement for the next decade.

Paper No.17582 From feasibility to processing: a time-lapse seismic experiment on Al Khalij

field – offshore Qatar Michael Emang, Bruno Pagliccia*, Fabrice Cantin, Patrick Charron, Emmanuelle Brechet, Benoit Blanco, Eddy Brosille, Michel Radigon

Al Khalij Field Location

Slide 2

Paper 17582 • From feasibility to processing: a time-lapse seismic experiment on Al Khalij field – offshore Qatar • Bruno Pagliccia

• Geography – Water depth about 60 m – Burial about 1100 m

• Discovered in May 1991, first oil in March 1997

• Limestone reservoir of Cenomanian age (mid-Cretaceous)

• 3D seismic survey shot in winter 1998 • 4D seismic pilot survey shot in summer

2012

TIME-LAPSE EXPERIMENT

Slide 3


4D PILOT on Al Khalij : baseline shot in 1998 – monitor shot in 2012

TIME-LAPSE EXPERIMENT - NOISE CONCEPT

Slide 4

Base amplitude (Env(Base))

4D a

mpl

itude

(Env

(Mon

itor –

Bas

e))

0 5000 10000 0

500

1000

Ambient noise level Ambient noise : weather,…

Rep. Noise : acq. parameters,…


PRESENTATION OUTLINE

Slide 5


• FEASIBILITY STUDY – Seismic modeling – Time & Amplitude difference

• SEISMIC ACQUISITION

– Survey Definition – Acquisition Parameters

• SEISMIC PROCESSING

– Processing sequence – Demultiple strategy – Quality Control

Base amplitude (Env(Base)) Base amplitude (Env(Base))

4D a

mpl

itude

(Env

(Mon

itor –

Bas

e))

0 5000 10000 0

500

1000

Ambient noise level

4D FEASIBILITY STUDY : and scenarii massive 1D modeling

Slide 6


ΔP ΔSw

4D FEASIBILITY STUDY : RESULTS

Slide 7


4D PILOT : SEISMIC ACQUISITION – DATA EXTENT

Slide 8


Baseline 1998 Monitor 2012

4D PILOT : SEISMIC ACQUISITION – main parameters

Slide 9


4D PILOT : SEISMIC ACQUISITION – RAW SHOTS

Slide 10


4D PILOT : SEISMIC PROCESSING - SEQUENCE

Slide 11


4D PILOT : SEISMIC PROCESSING – IMPACT OF DEMULTIPLE

Slide 12


4D PILOT : SEISMIC PROCESSING – NOISE MONITORING

Slide 13


CONCLUSION

Slide 14


• Feasibility said : up to 1.7 ms time-shift and amplitude difference below 12 % of repeatability noise • 4D pilot data said : up to 1 ms of time-shift, repeatibility noise of 12 % but amplitude difference above that threshold… • Feasibility study is a … study with limited data input

• A real field test is a must (4D pilot)

• Acquisition design to maximise as much as possible the experiment repeatability

• Seismic processing must be identical between base & monitor to reduce noise

Acknowledgements / Thank You / Questions

The authors gratefully acknowledge Qatar Petroleum and Total E&P Qatar for permission to publish this paper

Slide 15

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