spe920-what is reservoir engineering_texto

8/11/2019 SPE920-What is Reservoir Engineering_texto

1/30

., .

..

.

What

Abstract

Reservoir engineering involves morethan applied reservoir tnechanics. Tfwobjective of engineering is optintization.To obtain optinumt profit jrotna field the engineer or the engineeringteam must identify and define allindividual reservoirs and their physicalproperties, deduce each reservoirsperformance, prevent drilling of utlnecessarywells, initiate operating controlsat the proper time, and considerall important econontic factors, includingincome taxes. Early and accurateidentification and definition of

the reservoir systent is essential to effectiveettgineering. Conventional geologictechniques seldom provide sufficientdata to identijy and define eachindividual reservoir; the engineer tnustsuppletnent the geologic study withengineering data and tests to provide

the necessary infortnation.

Reservoir engineering is difficult;

The most successful practitioner is

usually the engineer who, through ex

tensive efforts to understand the res

ervoir, tnanages to acquire a few more

facts and thus needs fewer assutnp

tions.

Introduction

Reservoir engineering has advanced

rapidly during the last decade. Theindustry is drilling wells on widerspacing, unitizing earlier, and recoveringa greater percentage of the oilin place. Techniques are better, tools


2/30

are better, and background knowledgeof reservoir conditions has been greatlyimproved. In spite of these general

advances, many reservoirs are being

Or[ inal manuscript recdved in Society ofPetro Yeum Engineers office Aug. 8, 1964, Re.vised mnnusdtM received Nov. 2. Paper resentedat 39th SPE Annual Fall Meeting keldin Houston Oct. 11-14, 1964,

RESERVOIRENGINEERING

is Reservoir Engineering?

P. L. ESSLEY. JR. I SINCLAIR RESEARCH, INC.iEhtEER AIME TULSA, OKIA.I

developed in an inetllcient manner,vital engineering consideratiorm oftenare neglected or ignored, and individual

engineering efforts often are inferiorto those of a decade ago. Reservoirengineers often disagree in theirinterpretation of a reservoirs performance,It is not uncommon fortwo engineers to take exactly oppositepositions before a state commission.Such disagreements understandablyconfuse and bewilder management,lawyers, state commission membersand laymen. Can they be blamed ifthey question the technical competenceof a professional group whose

members cannot agree among them

selves?

There is considerable difference betweenthe reservoir engineering practicedby different companies, The differencesbetween good engineering andineffective engineering generally involveonly minor variations in fundamentalknowledge but involve majordifferences in emphasis of what is important,

Some companies or groupsemphasize calculation procedures andreservoir mechanics, but pay little attentionto reservoir geology. Othersemphasize geology and make extensiveefforts to identify individual reservoirsand deduce their performanceduring the development period or duringthe early operating period. Theyuse reservoir engineering equations


3/30

and calculation proceduresprimarilyas tools to provide additional insightof a reservoirs performance, Thoseutilizing the latter approach generally

are the most successful.

The. differences in practice observed

indicate that many individuals, in

cluding managers, field personnel, ed

ucators, scientists and reservoir en

gineers do not understand the full

scope of reservoir engineering or how

the reservoir engineer can be used

most effectively. A better understandingof the ba~c purpose of reservoirengineering and how it can be utilizedmost effective y should result in improvedengineering.

Reservoir Engineering A GroupEffort

The Purpose of Engineering

The goal of engineering is optimization.

The purpose of reservoir engineeringis to provide the facts, informationand knowledge necessaryto control operations to obtain themaximum possible recovery from areservoir at the least possible cost.Since a maximum recovery generaliyis not obtained by a minimum expenditure,the engineer must seek someoptimum combination of recovery,cost, and other pertinent factors. How

one defines optimumwill depend

upon the policies of the various operatorsand is immaterial to the viewspresented in this paper.

From an operators point of viewany procedure or course of actionthat results in an optimum profit tothe company is effective engineering,and any that doesnt is not, There aretwo reasons why a company may not


4/30

receive effective engineering. Its engineersmay be poorly trained andfail to perform properly. However, acompany can employ competent engineersand receive good engineeringwork from them, but as a company,still do an ineffective job of engineering.For instance, an engineer mightdo an excellent. jQb of water. floodinga reservoir, However, if even greaterprofit could have been received bywater flooding five years earlier, thenobviously the reservoir was not effectivelyengineered by the operator.To provide optimum profits, all oper


5/30

xticms must be initiated at the propertime. Effective reservoir engineering,therefore, must provide the necessaryfacts sufficiently early to allowmost dfective control of a reservoir.

The Engineering System

Calhounhas described the engineeringsystem of concern to the petroleumengineer as being composedof three principal subsystems: (1) thecreation and operation of the wells;

(2) the surface processing of thefluids; nnd (3) the fluids and theirbehavior within the reservoir. Thefirst two subsystems are subordinateto the last. The nature of the reservoir(s) and the reservoir tluids determineshow many wells ore neeclcd,where they should be dril[ed, howthey should be completed and produced,

and what processing equipmentis necessary to obtain optimumprofits. For effective engineering, thevarious subsystems cannot be isolated.They must be considered as interrelatedportions of a unified system.Petroleum engineering applies to theentire engineering system whereasreservoir engineering applies only toone part of the system. However, theentire system is controlled so completelyhy the reservoirs perfornluncethat there is only minor distinction

between petroleum engineeringand reservoir engineering.The reservoir engineer is concernedwith reservoir fluids and their behavior,and with identifying the geologicalenvironment and character ofeach separate reservoir with which hcmust deal. For convenience the individualreservoirs and their fluids mayhe described as composing a reservoirsystem.

The Engineering Process

The reservoir engineer applies ageneral knowledge of reservoir behavior10 a particular reservoir systemto produce u desired result. The reservoirsystems with which the reservoirengineer must deal are generallycomplex, involving multiple reservoirs,flow barriers, faults and irregular distributionof physical properties. Obtaining


6/30

a desired result from such reservoirsystems may be exceeding ydifficult, It seems unnecesssxy tostate that we cannot engineer a particularrescrvair system until we haveobtained adequate knowledge of theparticular system to iden[ify its partsand otherwise describe it. Yet we areprone to forget this vital phase of engineering,Too often we make broad,general assumptions regarding reser

voir uniformity, continuity, thicknessiird other factors. We then apply generalequations and obtain a generalsolution pertaining to an idealizedreservoir, We delude ourselves whenwe call this engineering. If we are totruly practice engineering we mustobtain particular solutions pertainingto particular reservoir systems.

Ewlmition of the Reservoir System

The first consideration in reservoirengineering and the principal functionof the reservoir engineer is todefine and evaluate the reervoir system,To definemeans to determinethe areal extent, thickness, inclination,producing limits and the geologicalenvironment of each separa!ereservoir witi-dn the reservoir system.To evaluatemeans to determine thephysical properties of each separatereservoir and its fluids, the variation

of the physical properties throughoutthe system, and the location of inhomogeneities,barriers, fractures, etc.,that may affect flow. Only when thelimits and properties of each separatereservoir are determined adequatelywill an engineer have sufficient knowledgeof a reservoir system to accuratelydeduce its future performance.

Most engineers will agree to thenecessity of defining and evaluatingthe reservoir system. Yet surprisingly

few devote adequate effort to d>ingit. Genersslly they rely on a structuralmap and a few isopachous maps, Anisopachous map of total net pay mayprove valuable for estimating originaloil in place, or as a political toolfor unitizing a reservoir, but it offerslittle help in understanding reservoirperformance if more than one reservoiris involved, Unfortunately, in the


7/30

sand-shale series which comprise manyor our so-called common sources ofsupply, we more often than not dealwith rmdtiple reservoirs, Fig, 1 showsa typical 1st Dakota, Dsandlog from a field in the Denver-Julesburgbasin. Each sand zone in thisfield is separate, with unique initialfluid contacts and individual performance.The D-5 sand zone had thehighest initial water-oil contact and

had an active water drive. The D-4

reservoir is lenticular, covers only a

portion of the tield, and produces by

a sol ut ion-gas drive, After five years

of, production the D-4 zone pressure,

was 800 psi less than in the underlying

zone and. 500 psi less than. in the .over-

Iying zones. The D-2 and D-3 sand

zones were connected through corn

rnon completions and thus had simi

lar pressures. Both reservoirs had

initial gas caps, active water drives,

the south fliittk of the structure (byitccident, WCIIS on the qorth flankwere completed in other zones). Thiscaused a shift of the initial gas captowards the south. Active water cncrorichmentdisplaced most of the oilon the north flank of the reservoirsinto the initial gas cap area, It is unnecessaryto carry this story muchfurther. A large portion of the recoverableoil from the D-2 and D-3zones was lost, The operator assumed

that this field was fully developedand was being efficiently drained,When considered as one reservoir, itappeared to be: wheri considered asfive reservoirs, it obviously wx not.if the nature of the multiple reservoirshad been determined sufficientlyearly, the procedures necessary to preventloss of recoverable oil would


8/30


9/30

.


10/30

..

data and production data these toolscan impart worthwhile insight intoreservoir condhions.

The Coordinated Re~ervoh

lhlunt ion Program

When the production superintendent,geologist, and engineer cooperateduring development of a field toevaluate the reservoir system, it is oftenpossible to deduce reservoir performancequite early, A coordinatedreservoir evaluation program not onlyprovides information for better engineering,it generally costs less thana haphazard program. A few drillstem tests, judiciously placed to testindividual zones at selected depths.

often can give more reservoir inform

ation than more numerous tests ofmultiple zones indiscriminatelyplaced. An extra log, or an additionalhours time on a drill stem test, mayprovide, more usable informationthan can be obtained from muchmore costly coring and core analyses.

Occasional y an early reservoir evaluation

program will present reason

,abie proof or reservoir communicationanddrainage over wide areas.This information may be the evidence

necessarY to obtain wide spacing-Such use of engineering to reducecosts is becoming more common. Afew companies devote considerableeffort to this phase cif reservoir engineering.In a recent case early proof

of reservoir drairudge by wide spacingallowed an operator to save $1,600,000in unnecessary drilling costs duringdevelopment of a relatively smallreservoir. Early evaluation also pro-vides data for early unitization andoptimum timing of pressure maintenanceoperations.Early definition and evaluation ofthe reservoir system is the, basic requirement


11/30

for effective engineering.The engineer must be allowed to obtainthe data necessaly to evaluatethe reservoir system and should participatein operating decisions withregard to the reservoir. It should bethe engineers job to obtain, as wellas interpret, the facts necessary toevaluate the reservoir system. It is hisresponsibility to know~what data arerequired and to devise a plan to obtainthem at the minimum cost.

The Geological Study

To deftne and-evaluate the reser.voir system, the engineer must considerthe depositiona! environment,continuity, Mhology and limits of thereservoir rock, The depositional environmentProvides clues concerningboth the larger geological units, which

may cause different sand zones to

behave as separate reservoirs, and thesmaller nommiformities present withinthe larger units, which may significantlyaffect flow and reservoir performance.Hutchinson has discussednonuniformities present in reservoirsystems.zs Such reservoir inhomogeneitiesmay provide the key to interpretingreservoir performance or thesuccess of an injection project. Shaleor silt streaks, or laminations, whichrestrict or prevent fluid flow, may ormay not be continuous over a wide

area. Such nonuniformities are oftentoo thin to appear on logs and wese!dom noted in core analyses butmay be observed in outcrops and areoften described in the geologists de

scription of the cores.Elkins has commented on the effectof such inhomogeneities on reducingvertical permeability in apparentlyclean sands.He has describedcalculations indicating that

these minute barriers may cause theratio of horizontal to vertical permeabilityto be as high as 10,000:1.Such barriers effectively prevent waterand gas coning and may preventgravity drainage or gravity underrunninglHowever, identification ofinhomogeneities in cores, or deducingtheir effect from well tests, does notindicate that such barriers are continuous.


12/30

Several reservoirs are knownwhere thin impermeable streaks, randomlylocated within a large sandbody, prevent coning but have littleeffect in preventing vertical segregationof reservoir fluids.Knowledge of the extent and kindof nonuniformities present may helpthe engineer interpret reservoir dataor design special reservoir tests toevaluate reservoir performance. Yetthe effect of nonuniformities on theperformance of a reservoir system isusually ignored by engineers.In reservoir engineering the geologicstudy must precede the engineeringstudy. However, conventional geologicaltechniques rarely provide sufficientdata to define the reservoir system,The engineer must supplementthe geology with engineering data andtests to provide the necessary information,Production ddta, formationpressure, pressure gradients, interference

tests, and build-up tests nlaY bCused to prove communication betweenwells or. zones, prove the .uistenceof faults or other barriers, andotherwise define the reservoir. Inpractice, this interrelationship betweengeology and engineering is seldom obtained.Only rarely do we find an extensivegeologic study of a reservoir,

Even less often do we find a systematicengineering effort to prove geologicalinterpretation and further define

the reservoir system. Yet suchstudies provide the base upon whichwe must build our engineering, Theability to communicate and workclosely with geologists, or to performthe functions of the geologist,is vital to reservoir engineering.

Application of Reservoir Mechunim

Reservoir mechanics generally re

ceives the most retention from reservoirengineers. In fact, many engineersspecialize in this apparentlyworthy endeavor and limit their practice(either by their own decision orby that of others) to evaluating reservoirperformance curves and predictingfuture performance. Superficially,such practice appears to be a valid engineeringspecialty. Actually, it is not.


13/30


14/30

its performance to obtain optimumprofits. This usually requires operatingdecisions before the behavior of thereservoir is apparent. Engineers, geologists,and superintendents are notinfallible. They will make mistakes.However, if operating decisions arepreceded by a systematic attempt todefine and evaluate the reservoir system,the chances of successfully deducinga reservoirs future performanceand controlling operations to obtainan optimum profit will be greatlyimproved. Calhoun has pointed to theanalogy between effective engineeringand preventive medicine.It is notsufficient for the engineer to determinethe state of a reservoirs healthand then attempt to improve it. Tobe most effective, the engineer mustmaintain the reservoirs health fromthe start.

The Importnncc of Timing

Optimization requires considerationof the time element, Often, when todo something may be nearly as importanta consideration as whaf to do,Most engineers are becoming increasinglyaware that proper timingis a vital consideration in engineering.Generalizations as to the proper timeto initiate a particular oil field operationare not possible, However, onegeneralization concerning engineering

is valid: the best time to applyreservoir engineering principles andstudy a reservoir system is as earlyas possible.

Economic Considerations

Optimization requires comparison.

For logical comparison, things which

are distinctly different must be reducedto a common basis. Thus, the

engineer must become acquainted

with certain techniques of the econo

mist and the banker. The details of

economic calculations are important

to the engineer but will not be dis


15/30

cussed here,

In such economic calculations allimportant cost items must be consid~ered, It is somewhat ironic that in-come tax, which may represent thelargest single cost item in an evalna,tion, is often ignored, The rate ofreturn calculated after income taxesare considered may be higher thanwhen , calculated before taxes, Economiccomparisons may not be validunleis income taxes are considered.Tax consequences may occasionallyrepresent the major consideration-inan operating decision. An apparentlysound secondary recovery plan mayresult in several million dollarsgreatertax liability than would an equallyattractive alternate plan. In one large

Oklahoma water flood the increased

tax liability amounted to more thana few million. In a relatively smallIllinois water flood, the operatingpractice increased tax liability by approximately$500,000, Irr both casesdevelopment drilling in stages resultedin 10SS of depletion allowance forseveral years. In both cases alternateplans could have been devised to reducethe tax liability. Several yearsago a well-known water flood engineeroutlined a stage development programfor living with prorated water

floods. The program he outlinedcould result in w loss in depletion allowanceand increased taxes.

The reservoir engineer should consultwith a tax attorney on any developmentprogram involving largeexpenditures for development drilling,or for injection of propane, butane orother materials. The engineer cannotjustify ignoring an item that may havesuch serious economic consequences.

Responsibility of the Group Effort

From a company point of viewsuccessful engineering requires optimizingan entire system. This generaiiy requires a group effort. A companysengineering may be ineffectivedue to its faiiure to recognize the aimosttotai dependence of the groupeffort upon accurately defining and


16/30

evacuating the reservoir system andcorrectiy deducing future performance.

Reservoir engineering does not startat some time after a fieid is deveioped.For maximum effectiveness itmust start simultaneously with discovery,Weii iocations, driii stem tests,seiection of iogging toois, and determinationof completion intervais areali reservoir engineering probiems. A 11development and operating decisionsshould be tnade by an individual whoreco~nizes the dependence of the en-tire systetn upon the nature and behaviorof the reflervoir. It is not necessarythat such an individual be areservoir engineer. Any manager,superintendent or foreman who considersthe entire reservoir system duringoperations, and not just the indi.vidual weli, and who deveiops andoperates the tleid as a system in a

manner which can obtain the maximumamount of reservoir information,is practicing one of the mostimportant phases of reservoir engineering.It heips if the individual hasa background knowiedge of reservoirmechanics and geoiogy. However,many nontechnical personnei deveiopan intuitive feei for the reservoirsystem and know when to seek and

accept technicai advice with regardto individual components of the system.

On the other hand, many tech.nicai personnei, with extensive trainingand background knowiedge in certaindisciplines, are so obsessed withtheir calculation procedures and balancesthat they often forget they dredeaiing with a particular system whichcannot be engineered untii it is de

fined.

Reservoir Engineerhrg -Individual

Practice

An Art, or a .%ienw?

Reservoir engineering is more of anart than an exact science, aithough ithas a broad scientific base, Most observedreservoir facts, phenomena,or symptomsare subject to more


17/30

than one iogical interpretation. Wyl-Iie discussed this peculiarity of reservoirengineering with regard to interpretingpiiot fieid tests, but extendedhis remarks to cover aii of reservoirengineering.It is anaiogous to themathematical condition of havingmore unknowns than equations andobtaining multipie solutions. Eikinshas aiso emphasized the necessity ofinvestigating aii possibie interpretationsof reservoir performance.Whenthe complexities of reservoir geometry,muitiphase fluid flow, potentiai gradientsand reservoir mechanics arcconsidered, muitipie interpretationsshouid not prove start Iing to any reservoirengineer. Yet too often we areprone to accept the first interpretationthat appears to fit most of the data.That some pieces of information dontfit into piace never seem to bother usor cause us to question our interpretation.

The most obvious interpretation ofdata often is incorrect, An exampieof this is illustrated by the reservoirperformance curves shown as Fig, 2,Generaiiy an increase in reservoirpressure foiiowing a reduction in thereservoir withdrawal rate suggests waterencroachment. However, no waterwas being produced and the reservoirwas apparently seaied at thewater-oii contact by a iow graVity,tar-iike oii. The data were questioned

but were proven to be reiiabie. Theengineering committee conciuded thatthe pressure increase couid not reflecta true reservoir condition and wascaused by the method used to obtaina weighted average field pressure. .Actuaiiy,pressure increases were observedin individual weiis in aii partsof the reservoir and in Jater pressuresurveys, confirming a field-wide pressureincrease. The present interpretation,and the oniy one that satisfies

Jo URKt\L OF PETROLJXM, TECHNOLOGY.-.

.?2.


18/30

.

all known facts, is that the apparentlyanomalous pressure increasewas due to a redistribution of fluidswithin the reservoir, resulting fromgravity segregation. The anomalouspressure effect is simi!ar to the onediscussed by Matthews and Stegemeier,At high producing rates mostof the gas released from solution inthe reservoir wasproduced at nearbywells, Following the drastic allowable~llt, gas-oi[ ratios decreased, and the

high-pressure downdip gas migratedupstructure to the low-pressure gascap, Theoretical circulations weremade to determine the effect of thefluid redistribution on the tleld pressureand inr.iiuated a good agreementwith actual field performance. Theresults are shown on Fig, 2.

Gas injection was started in this reservoirshortly after the pressurepeaked (67 million bbl cumulativeproduction), For several years priorto gas injection considerable fluids

were being withdrawn from the reservoir;yet the reservoir pressure wasincreasing. For several years afterthe start of gas injection reservoirwithdrawals were replaced, but theaverage field pressure declined. An

extensive geological and engineeringstudy revealed that the field consistedof a large number of individualreservoirs, resulting from Ienticularzones and extensive faulting. The ai%parently anomalous pressure declinewas due to the fact that many wellswere producing from ,reservoirs otherthan the ones receiving the injectedgas.

This example illustrates the difficulty of interpreting field performance

curves and the complexity ofsome reservoir engineering problems.Theoretical calculations in this fieldhave little meaning except as clues toaid interpretation of observed phenomena.Due to the complexity ofthe field, large volumes of oil couldeasily be trapped and not be drained.Reservoir engineering in this fieldconsists almost entirely of identifying


19/30

and defining the numerous reservoirs.Engineering tests are beingconducted to confirm or disprovethe geological interpretation, to lo-

cate flow barriers and determine communicatingzones. It will not be aneasy task. Nature hides her ,secretswell.

Irr theory, reservoir engineering isbased on broad scientitlc principles.In practice, however, it is not rigorouslyscientific. To start with, . wedeal witha system which may be unbelievablecomplex and impossible todefine completely. To arrive at a di

:JANUASSY, 1.{J65.. . ..

agnosis of our system we generallyrely on: (1) a few physical facts;

(2) production statistics (often ofdoubtful reliability); (3) samples representingapproximately one bilIionth of the reservoir; (4) statisticalaveraging techniques (often misapplied);and (5) stylized mathematicalequations derived from assumptionswhich may only remoteiy representreservoir conditions.Is it any wonder, then, that thereservoir engineer has been describedas an individual who takes a limited

number of facts, adds numerous assumptionsand arrives at an uniimitednumber of conclusions? Such astatement may have been made injest; nevertheless, it provides an intrinsicdescription of reservoir engineeringas it is often practiced. Unfortunately,due to the complexity ofthe reservoir system, reservoir engineeringwiil aiways remain this way.

The fact that we must rely on insufficientfacts, data of poor quality,

and an imperfect knowledge of thereservoir does not mean that we cannotdo a good job of engineering. Itdoes mean that we cannot expect per-~~fection and that we should continuallystrive to obtain better data andiearn more about the reservoir. Occasionalfaiiures are inevitable. Whatwe must strive for is the highest possiblebatting average. The most successful


20/30

practitioner of the art isusually the engineer who, throughextensive studies to define and evaluatethe reservoir system, manages toobtain more facts and thus requiresfewer assumptions, Additional factscan be obtained only by hard workand imaginative thinking. Assumptionsare easiiy conceived. This nodoubt explains our innate tendencyto substitute assumptions for factswhen the facts are not readiiy ob

tainable.

The Hypnotic Effect of theCsdculated Solution

As a profession grows it iogicallytries to reduce concepts to mathe

matical expressions, Reservoir engineeringis no exception, even thoughdue to the complexity of reservoirsystems it is iii-suited for exact mathematical soiutions. As a consequencewe have a generation of engineersschooled in the mechanics of themathematical solution. A few apparentlybeiieve that engineering invoivesno more than obtaining soiutions withequations and balances. While suchengineers are a small minority, their

fanaticism illustrates the hypnotic effectwhich a calculated solution OC-casionaliy has on ali engineers. Thissirens cali has iured many an engineerto a rocky concision in the past andno doubt wili continue to do so in thefuture.

A ciassic example was given re

centiy by an engineering committee

report and iater testimony of the

chairman of the committee before a

state commission. Pressure data in

the reservoir in question were sparse

but ciearly indicated a severai-thou


21/30


22/30

fact that water had invaded an ap

preciable portion of the reservoir,

i!i!w that numerous wells had wateredout, and that actual water productionexceeded 1,500 B/D didnt seem

go s.

..,CWW16D FaEww

-... ~, to bother them, They were so hypno

~~ I

tized by their calculations that the

i .

i,,..

chairman later testitied under oath

f+.\ .1

:;, .-;-R,

that oniy a minor volume of water

b ~~~

$,~, q %,, :I had moved. ..into the reservoir. In:Pf?awculoRATE\> stead of questioning their own re-

MO\ . . . . . ..~...-. =--sults, they went to considerable

1!. ......

trouble to concoct a theory to make

JWwrnt samsatiw ion

CW,M.AT,VE PROCWTION -MILLION OBLS the apparently anomalous facts fitFig. WProduction history undctdctrlatcd and observed. pressure.the resultslation.indicated by their calcu23. . . ,,


23/30

All equations we use m reservoir

engineers are bused on certain SSS.sumed conditions, which may ormay not represent the conditions inu ptirti.cular reservoir, An equation

which M valid in one situation maynot apply in another. Often ourequrrtions must be modified to fitparticular reservoir conditions. Usedwisely and cautiously our equationsare valuable engineering tools. However,they are by no means our onlytools and on occasions may be ourleast important ones.

The Use of Models

Due to the complexity of mostreservoirs, it is impossible to duplicatea reservoir or build a true prototypemodel. All models with

which we deal are greatly sinlplitfedsystems. Such. models providewduirble information concerning thegsneral nztLlre of reservoir systemsand the mdurc of fluid flow in suchsys[cnls. Indeed. it is from the study

of sLlch IUOL!eISthat we obtain muchntoLtr knowledge concerning reservoirmcchdrsics,

The advent of the high-spcecl digilalconlpLlter hiis Wowed conNruc[

imr of mathematical models for thestudy of mLl[tiphase, nlultidinlentionaltluicl flow. These modelscome close to duplicating simple reservoirsystems and provide additionalinsight concerning reservoirbehavior. As a scientific tool, theyarc supcrh. However. in our cxLlbernncewi[h our new toy, let LIS notforget two significant facts: themat hernat icrd reservoir models arestill greatly simplified comptrred tomany reservoirs, and until we can

dctine o reservoir system, it is impossibleto duplicate it in a model.No model. however rigorous, canprovide an exact answer if the inputinformation is wrong,

My remarks concerning the calcu-Itited solution also apply to themathematical model. With the glamourof the cotnpu[er and the intriguing


24/30

sophistication of the mathematicrr[model it will he doubly ditlicultto treat such calculated solutions objectively, These remarks are not in-tended to discourage use of suchmodels. Rather, they are intended aswords of caution to the engineerswho would use these scientitlc toolsto obtain engineering solufions. Usedwisely, [hey will provide valuablecluesabout a-reservoirs performance.Used unwisely, -they can leadone blindly astray,

lw{,.~iillellsittllttlI{cpmentution of Dkita

It surprises most engineers to

$,.

learn that we use two-climensionnltechniques less today than oLw predecessorsdid 20 years ago. Any map

or cross section is a graphical twodimensionalrepresenttttion of information,Data plotted on such rmspsand cross sections will indicate reservoirperformance trends far morequickly mtd accurately than a fieldperformance curve. Yet we are relyingincreasingly on field performancecurves and less on two-dinlensionalplots of the data and individualwell. performance curves. This isa step backward, as a fkld performancequrve cannot show variation of

data throughout the reservoir. Variationprovides the key to early interpretationof reservoir. performitnce.

Field performance curves have littlevalue for early assessment of reservoirperformance, They have ledmany engineers astray. The techniqueof plotting, or visLlalizing, alldata in two dimensions, used extensivelyby our predecessors, will allowmuch earlier evaluation of reservoirperformance, The technique is simple,

quick and effective. It shoLlki heused more widely,

The I)ifliculty ofReservoir Engineering

Reservoir engineers deal with systemswhich cannot be examined physically,A complete knowledge of thereservoir system is not possible. The


25/30

engineers joh is further complicatedhy the lack of exactness of mostdata. Water nnd gas production dataare often unreliable, memured pressuresmay not represent stabilizedpressures, and results obtained fromfluid samples may not represent thereservoir fluids, Consequently, weshould not expect exttct sol L~tionsfrom our calculations even on therare occasions when we use rigorousequations. This does not mean thatOllr equations are worthless. Itmerely means that we should regardour calculations as providing clues toreservoir behavior and not as exrrctindicators of reservoir behavior. Further,we should always question theresults of our calculations, If we areto obtain the right answer, wemust continually seek answers to thefollowing questions (I) what doesthe answer mean: (2) does the answerf-l! all the facts: (3) why

doesnt it; (4) are there other pos

sible interpretations of the data;

(5) were the asstlmptionscorrect:(6) are the data reliable; (7) are additionaldata necessary: (8) hasthere hcen an ideqtiiit~ geologicttlstudy: and (9) has the reservoirbeen adequatcl y defined?To be successful wc must bc innatelycurious and scientifically honest.

We must continually questionour own results and search for additionalfacts. Elkins stated this quiteaptly: Since nearly all basic featuresof reservoir performance must he inferred,periodic re-evaluation of specificcases is imperative,

From an individLial cngineerspoint of view, successful engineeringis limited to optimizing a systemfrom the time he first becomes acquaintedwith it. Even from this

more limited viewpoint effective engineeringis dependent upon recognizingthe nature of the reservoir andits performance. Most examples ofpoor individual engineering resultfrom an unwarranted reliance uponfield performance curves and calculationprocedures as tools to evuhmtereservoir perfornumce, An increasedeffort to define and evaluate the reservoir


26/30

system and greater use of twodimensionalplots of datu should intproveindividual engineering etior(s.

1lw Ihwkgrwmd Reqnirrd forIbsf,rmir Engintwring

The diversity of the turrctions thereservoir engineer is expected 10 performalso compounds the difficultyof his job, He may be required toplan a reservoir evaluation progriimduring drilling wtd cfeveiopmcnt, determineproper well spacing. evaluatelogs, calculate reserves, evaluateopen flow tests or drawdown andl-lL1ikf-up tests, invest igate the economicsof the proposed expend iIurcs(including income taxes in th~ evaluation),participate in engineeringc.ommittec studies and unitizationmeetings, recommend procedures forpressure mainteriance. dig throughaccounting data to determine costs

or to determine the p~st productionof depleted reservoirs, evaluate pilotfloods and plan secondary recoveryprojects, explain why a particular projectfailed, or undertake any number ofother duties.

To succeed, the engineer must developthe geologist% knowledge ofsediments and environmental conditions;the physical chemists knowledgeof reservoir fluid properties,phase behavior, electrical conductivity,

and fluid flow in porous systems;and the mathematicians knowledgeof numerical analysis and theuse of. high-speed digital computers,. .Inaddition, hemust be completelyfamiliar with past production andcompletion practices in the reservoir,including a knowledge of whichzones are perforated in every welland each wells pertornumcc. He also


27/30

must be art eoonomist, an ac~ountarttof sorts, an expert negotiator, itrufhave a working knowledge of prorationlaw, unitization law and taxes.Few engineers develop a backgroundin depth that is this extensive, However,the engineer must develop aworking knowledge in each area andknow when to consult with specialistsfor additional information. The experiencedreservoir engineer is a generalist,not a specialist.

The purpose of reservoir engineeringis to control each separate reservoirsperformance to obtain an optimumprofit, To accomplish this purposegenerally requires operating decisionsbefore the performance ofeach reservoir can be determined. Tocorrectly deduce the performance ofeach reservoir requires that the reservoirsystem be identified and defined

by geologic techniques and specialengineering tests to provide a basisfor deduct[on. Efleclive engineeringrequires that the reservoir engineer,or someone jal niliar with reservoirenginteriqg principles, participate iitdevelopment and operating decisions.

Companies which do not consider thedevelopment and operation of an oilfield as, parts of an engineering system,and do not utilize reservoir engineeringprinciples as a basis for development

and operating decisions,

generally do not-obtain optimumpmffts from their operations, They

may employ numerous engineersbut their engineering is too little andtoo later and is usually inadequate,h such cases company policiesgreatly handicap the efforts of the individualengineers, They have difficultyobtaining necessary data, andgenerally must attempt to salvage

lost profits rather than to create newprofits,

Reservoir engineers often disagreein interpreting a fields performance.Cienerally incorrect interpretationsresult from ignoring signicant factsor from a failure to dig deep enoughto uncover all the facts. Incorrecf interpretationsalso result from an unwarranted


28/30


29/30

Trws., A131E(1949) 186, 30Fi,P, L, ESSLtiY,JR.. is a senior resenrchen fiineerwitlt Sinclair Re

w., search, !nc. in Ttd.$k. so. He worked jiveyears with MarathonOil Co, andseven years withkg Skeily prior to joiningSinclair in 1962. He received anMS degree in pe(rrtlemn engineeringfrom the U. of Tul.ra in 1950.

.~,s

JAiWuARY, 196s.... .

. .-

-.

.,

.

..


30/30

spe920-what is reservoir engineering_texto

Documents