University of Nigeria Virtual Library

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Author 1

ONUOHA, K. M.

Author 2

Author 3

Title Physics as the Bedrock of Petroleum Exploration and Nigeria's National

Development

Keywords Physics as the bedrock of Petwleum Exploration

Description Physics as the Bedrock of Petroleum Exploration and Nigeria's National

Development

Category

Physical Science

Publisher

Publication Date

1996

Signature

.JIrPhysics as the bedrock of Petwleum Exploration 84 Onuoha

PHYSICS AS THE BEDROCK OF PETROLEUM EXPLORATION ANDNIGERIA'S NATIONAL DEVELOPMENT

K. MOSTO ONUOHADepartmentofGeology,Universityof Nigeria

Nsukka

Currently at: Exploration Department, S.P.D.CPort Harcourt

INTRODUCfION

It is a very well known fact that today, petroleum constitutesthe mainstay of the Nigerian economy, contributing not less than90% of the country's total yearly revenue. This trend is not likelyto change in the foreseeable future, even though efforts are beingmade by the Nigerian government to diversify the nation's rev-enue base. One often wonders what Nigeria would have lookedlike without oil. To be endowed with resources is one thing, butthe ability to harness such resources is another matter altogether.Thus, though the occurrence of large reserves of petroleum in theArctic wilderness of Alaska had been known for many years, itwas only in the last ten years that the right technology for thesuccessful implementation of exploration and. productionprogrammes in the area became" vailable in economic terms. Theuse ohery high technology in the oil industry is something that isvery well known. What is often not very obvious is thecrucial rolephysics as a basic science has to do with it all.

. In this paper, we will highlight the crucial role applied,physics plays in petroleum technology. In order to be brief, andstill achieve our set objective, we will neither attempt to cover theentire spectrum of petroleum technology, nor discuss all the fieldsof applied physics that form the bedrock of such technologies.Such an' effort would constitute a very formidable assignment.Instead of that, we will concentrate on the role of geophysics(which is just one area of applied physics) in petroleum explora-tion and development.

In Nigeria today, our entireproducing tields are located in theTertiary Niger Delta basin. Although serious exploration activi-ties have been going on in some other areas, notably, theChad andAnambra basins, and more recently in the Upper 'and MiddleBenue Valley, it is clear that exploration is now extending moreand more into very hostile, high risk environments (Onuoha,1987). This is already the case with Nigeria's deep offshore wherepetroleum prospects are now known to exist, but at much higher

cost and risk. This calls for the use of increasingly more sophis-ticated technology, and the geophysicists and petrophysicists inthe industry havc to rise to the challenge.

THE ROLE OF GEOPHYSICS IN PETROLEUM EXPLO-RATION

Since exploration is the 'cornerstone of the oil industry, andsince without it our tcnown reserves (and exploitable resourcebase) would be used up, leading to the death of the industry, itmeans that geophysics is the bedrock of the industry. Geophysicsis themajor tool used to image the interior of the Earth and unravelits contents, including the hydrocarbons that are so vital to oureconomy. Even though other professionals, notably geologistsandpetroleum engincp.rsplay important roles in petroleum explo-Iation and production (E & P), the role of the geophysiscist iscentral to itallFigure 1,illustratesdiagrammatically thiscentralisedrole when we examine the interactions between the geologist,.petrophysicist, reservoir engineer, and geophysicist in movingfrom how to understand "the known" to being able to predict "theunknown" below the surface of the Earth.

As is wellknown, severalmethods of exploration geophysics'exist. Each of these methods is based on monitoring variations ina particular physical property (e.g. changes in magnetic suscepti-bility andpolarisation-magnetic methods; changes in elastic prop-erties/changes in velocity and mode of propagation of waves-seismic method). The most common methods of geophysicalexploration are li~tedin Table 1below. For many obvious reasonsall the geophysical methods do not enjoy the same degree ofpopularity or applicability in petroleum exploration. Gravity andmagnetic methods are used extensively as reconnaissance tools inthe cady sUlge~of p(;.wleuIn ~xploration. Their main applicationis in determining the general morphology and thickness of sedi-ments above consolidated basement and in the delineation ofmajor structures and faults in the basin.

Table 1: Methods of Exploration Geophysics

In the early days of oil and gas exploration in Europe andNorth America physicists led the way by designing a suitable

. instrument for detecting variations in the Earth's gravity fieldcaused by the presence of anomalous bodies (e,g. petrolifemusrocks}~ the subsurface. The first of such instruments was a verysensitive torsion balance built iu 1895 by Baron von Eotvos inBudapest Hungary. He successfully modified the Cavendishinstrument for the determination of the universal gravitational

Nig. Joom. of Phys. Vol8S (1996)

constant G, and built a very sensitive torsion balance that wassuccessfully used in mapping hydrocarbon-bearing salt domes inthe subsurface. in the early years of oil exploration many of suchin~lrumcnt~were in high demand along with the physicists whobuilt and manned them. Many oil fields in Oklahoma and Texaswere discovered between 1900 and 1940 using this equipmentwhich was the fore-runner of today's gravimeter.

'>9

Method Physical Property Changes on which method is bttsed

Gravity Prospecting Changes in the gravitational attraction at aplace caused by anomalous mass distributionMagnetic Method Changes in the magnetic susceptibilitylPolarizationElectrical! Electromagnetic Changes in electrical conductivity / PolarizationMethodsSeismic Methods Changes in elestic Properties

.Radiometric Methods Changes in the Radioactive content and intensity

Onuoha 85 Physics as the bedrock of Petroleum Exploration

.JII' /'

Understamling"The Known" ~

RESERVOIRENGINEER

~.

\

GEOPHYSICIST

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Predicting''The Unknown" v

DrillingRecommendations

Figure 1: The Centralized role of the Geophysicist

THE SEISMIC REVOLUTION

Although the tools for petroleum exploration have changedand are still changing, the role of the physicist in the industryremains central to it even today. This role is performed in variouscapacities, either as a geophysicist, or petrophysicist, or well-logging expert or in any of the numerous names that today'sphysicist goes with in the oil industry. The most reliable com-monly used tool for petroleum exploration is theseismicreflectionmethod. About seventy.five percent of the world's annual expen-diture on geophysical expioratIon is in the area of reflectionseismology applied to the search for oil and gas.Gone are the daysof analogue recording and low fold data; also gone are the dayswhen the objectives of seismic investigations were geared essen-tially towards structural features and trap delineation. Weare nowin the days of three-dimensional (3D) seismics and fast-tracking.Physics as a basic science, and the theory of wave propagation invarious media (elastic or heterogeneous) is of course the bedrockof this revolution that is so vital to our economy.

3D Surveys and Fast-Tracking

Fast-tracking is one 0f the most recently introduced oil andgas "buzzwords" and essentially refers to a vast array of activ~ties. ~.that collectively accelerate and shorten the entire process oflicensing, exploration and production,in order to bring hydrocar-bons to the marker sooner. The rust signs of fast-tracking can Detraced to the advent of 3D seismic surveys, and especially lIIulti-line marine 3D surveys. Fast-tracking activity is now dramaticallychanging the oil and gas business. Recent advances in seismictechnology have played an expanding role in improving theefficiency of devfloping proven reserves. Geophysicists areat thefore-front of fast-track!,ng.

The 3.D seismic method has ,emerged as a key tool insupporting, the field development phase of the industry. Thismethod improves the aXl:;'acy of the sul>,;urfaceinfJrmationobtained from surface measurements by comprehending the spa-tial natureofthe problem to be solved. The increased accuracy anddetail obtained from the technique allows better definition anddelineation of the structural (or stratigraphic) trap than is possible

using conventional seismic techniques. This allows reservoirmanagement to proceed with greater certainty. Fewer dry holes,better well placement, earlier production, and hence, improvedcash flow are the results. Thus, the 3D seismic method offerspositive leverage on the net present value of the oil field.

In Nigeria today, 3D seismic surveys are the order of theday,but this is a relatively recent development. Most of the biggeroperators in the country,e.g. Shell, Chevron, Elf, Mobil and othershave found the approach very useful in imaging the subsurface,thereby increasing the reserve base of the country through bettervertical and lateral resolution of events as recorded in the seismic

surveys. Special studies involving the relationship between theamplitudeof seismicreflections and corresponding offsets (AVO)within the 3D data volume have provided new tools for fluidcontent prediction from seismic data and for reservoir description.

Seismic reflection amplitudes vary with offset for manyreasons, but the only interesting one is mode conversion. Com-pressional waves incident on ,an impedance contrast at normalincidence (I.e., at small ~Ol!rce-receiveroffset) are reflected withan amplitude proportional to the impedance contrast At obliqueincidence (large'source-receiver offset), both compressional andshear waves are reflected and transmitted. Th,eamplitude of thereflected compressional wave, which is measured by the seismicacquisition system at the surface, depends therefore on the shearimpedance contrast as well as the compressional wave impedancecontrast (Onuoha, 1991). This is the basis of the AVO effect.

Reservoir Monitoring

3D seismic data are also being used to measure the movementI')f fluids (oil, gas, 'and water) through a reservoir during oilproduction. The aims of such measurements are to improve theprediction offload front breakthrough, and to provide informationto assist in maximising production rates and ultimate recoveryfrom th~field. An additional benefit would be a detailed reservoir

description, mi.l'imisingthe need for drilling new wells. Seismictechnology is therefore no longer restricted to the explorationenvironment. The last decade ~as seen many technical advancesthat allow seismic data to be truly useful in the delineation and

Nig. Journ. of Phys. .vo18S (1996)

Physics as the be4rm:k of Petroleum Exploration 86 Onuoha

1

Well Data .......................Geology

Petrophysics

Seismic Base'~...,.,.u.,', SUl'vey

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During ! SurveyProduction y.L,.

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TIME

RevisedResenoh'Model 1/

'lFluid MovementPl'edictions

Figure 2: Siesmic Reservior-Monit0l1'1g

produclion stages of hydrocarbun reservoirs. The a~vent of 3Dtechnology and improved integrity of its results enables analyststo make valid inferences regarding the spatial distribution be-tween wells of key parameters of producing reservoirs such asthickness, porosity, etc.

Figure 2 shows the role of seismic data in accurately moni-

toring the movement of reservoir fluids (an important assignmentin reservoir management). An initial reservoir description stageprovides a detailed model of the reservoir based on reservoirgeology, well data and seismic data, This model is then redefinedin a second stage to match pressure tests, production history, andseismic monitoring data, in 6rder to predict the future behaviour

Pressure Tests

Production Data

Reservoir

Model

Seismic BaseAnd

MonitoringSur~eys

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Figure 3: Reservoir F1uidmonitoring using repeated seismic data

Nig. JOUr/\. ofPhys. Vol8S (1996)

oY,"

HistoryMatching r

Reservoir--Simulation

-

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of fluids irithe reservoir later in life. The potentiallhat 3D seismicoffers in thii respect was recently treated at length by Archer et aI.(1993).

Figure 3 shows the fluid monitoring stage in further detail.The reservoir model is used to simulate the fluid flow in thereservoir and the computed response is compared to the availablepressure test measurements and production data. The model isrefmed until a matcb is achieved. H repeated seismic surveys areused, then additional mod!:1refinement is possible to match themeasured change in the seismic response due to fluid movement.Reservoir simulation for the resulting revised reservoir modelthen provides fluid movement predictions for future reserv.:>irmanagement.

CONCLUDING REMARKS

Exploration is the heart of theoil industry. Job opportunitiesin virtually all tbe other facets of lhe industry arise as a result of thework of those who find and/or produce the oil or gas. There is avery close relationship between the hydrocarbon reserves of acountry and the "finding rate". The finding rate itself is deter-

87 Physics a1'the bedrock of Petroleum Exploration

mined by several fac~rs, notably lhe amount of investment inexploration, the appropriateness of the tools/equipment in use, andcalibre of personnel employed for the work. The Federal Govern-ment of Nigelia has set a reserve base target of 25 billion barrels

of oil for the country by ctheyear 1999. This represents an increaseof about 4% over the CUFl'ent(1995) reserve base. To achieve this

target requires a considerable expansion of exploration and pro-dUCl1;Jnactivit;.:, not only in the known petroleum province of theNiger Delta, but also in frontier basins in the country. The morew: invest in exploration, the higher the likelihood of our achievingthis increased res~rve base'.

The right calibre of geophysicists, petrophysicists, and otherexperts required to suCcessfully execute E & P activities in the oilsector will not be nvailable in the long run in Nigeria if the currentsituation in our universities are allowed to persist. The teaching ofthe basic sciences, especially physics and mathematics mustreceive a big boost if we are to move forward. It is important thateverything be done by all and sundry to improve the level ofscholarship and of academic activities in our universities.

REFERENCES

ARCHER, S. 1-1.,KING, G.A., SEYMOUR, R.H., and UDEN,

R.C. Seismic reservoir monitoring - potential.First Break,Vol. 11,391-397,1993.

ONUOHA, K.M., Subtle petroleum traps-a discussion of the toolsand techniques required for their effective search in the

Niger Delta. Nig. Assoc. Pelrol. Explormionisls Bull., 92-102, Jul}', 1987.

ONUOIIA. K.M., Sej"mic amplitude versus offset (AVO) analy-

~is- :\ tutorial. Nig. Assoc. Petrol. Exploralionisls, Bi-illontlily Technical Mlg"" Lagos, 27 pp., March, 1991.

(This paper war delivered as a keynfJteaddress DIllie Opening Ceremonies oflhe 191h annual and JI/bilee celeberalion oflheNigerian IlfSlilule of Physics (N.1P)aI tile lIDnquelHall oflhe Presidenlial HOlel,P011Harcourt in Oclober 1996 )

Nig. Journ. of Phys. Vol8S (1996)