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The Role m Townhole Flow and Pressure Measurements inReservoir TesJngby J. Joseph and C.A. EhligEconomides, Schlumberger Well Services, and F. Kuchuk,Schlumberger-Doll Research

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ABS.TRACT INTRODUCTION

In practice, 8 reservoir teat that ia Succeeafully executed Dynamic downhole meaeurem enta such ae pru%mre, flow-

~ tos specikd p-k design will provide in- rate, deneity, and temperature, are acquired routinely by

terpretable remdte aatk&ng the teat objective. Uau- production logging and taating devices. Production leg-

ally, there ie an optimal teetdeeign among eeveral op- ging (or PL) surveys vemus depth under stabilized flow-

tionn which will rninimk the coet of the ted+ without ing or shut-in conditions are used to diagnoae and moni-

jeopardiaing the liieliiood that useful interpretatio~ will tor well performance.l Surveys veraua time with the tool

ertaue. Consequently, ae numerous testing con@urationa in a tied poeition in the wellbore yield the reservoir’s

have been introduced, it hae become neceeaary to develop trannient response pattern to rate perturbation. ThM

interpretation procedure whkh are M straightfonvard .responm ie used to help dellne the well/reservoir model

ae the conventional tecldquea while offering a more r~ and to quantify well performance. By production log test-

liable or retied anewer. The uae of downhole flow and ing ia understood the synergy between the ‘vemuE depth’

pmaaure measurements, or even downhole shut-in devices, and ‘vemua time’ acquisition schemes.

are caeea in point. With proper deeign, execution and in-The acquisition and interpretation procedure used in

terpretation, teste which employ theee configuration canthw paper have been extensively employed.2-* Experi-

provide reeemoir propeitiea that could be much more dt-

flcult to obtain by othe: meana.ence with wellbore hvrate and preaaure meaauremente

has meulted in the guideliiee offered herein for deeign-

One objective of this paper ie to present a methodolog~ ing teeti which maximiae the demiuble characteristic of

for reservoir t4M interpretation. Thw detiee the Me?. the downhole senaom. The purpoee of thii paper ia to

archy and sequence for employment of all analysia tech. d=uee when and why a particular teeting con@ration

niqueu, and ie intended to be general enough in ccncept ie preferable over another, and to define a sequence of

to accommodate any type of teat. Field exampke are ueed interpretation steps that accomodatee any type of teet

to illustrate the merits of speciilc acqukkion prccedurea, while ako provid~ a mechardam for consistently correct

aud ako how the methodology carI aid in acldeving the allalyeie.

●%st objective with conaiatent and coherent analyea.

Reference and illuatratiom at end of paper.

2 TNB ROLE OF ~- PLON At@ PRESSURE SPB 18379NBASUREMENTS IN RBSBRWIR TESTING

. .

Five field examples are presented in a progression at-

kmpting to provide ● rationale for why the meaeurs-

ments were conducted in s certain way and what wee

gained in the process. The tit example illuetmtes the

advantage of downhole shut-in during a buildup teett The

second example ehowe how convolved downhole flowrate

and pressure transients during ● buildup test revealed an

●arly-time he ~genous reservoir response that wee par-

tially masked by wellbore ntorage. In the thud example

the measurement of downhole tiowrate allowed straight-

forward anelyeie of data acquired during drawdown, even

though surface rate variation rendered the pressure vir-

tually uninterpretable by itself. The next example

demonstrates the added advantage of a production log

9owrate eurvey in analysis of a partially penetrated reaer-

Voir response. The fiwl example is ● layered reservoir

teat in which permeabilitiee and ekii were determined

for each of four sones in ● commingled well using produc-

tion kg testing measurements over ● total of 30 hours of

teat time.

TESTING AND iNTERPRETATION

METHODOLOGY

Formation evaluation by reservoir tasting has been an

active area of research and developinmt over the last 40

years. Scores of models, metho&, techniques end ap

plicatione that cover● wide spectrum of poaeibfitiee are

today ●vailable in the literature.* Because of new mea-

surements and interpretation techniques, it is neceesary

to erwxe that methods are used in the proper order, that

techniques are applied to jtiet those portions of data over.

which they are valid, and that checking and verification

procedures be instituted where necessary. This is clearly

a practical concern, 10 the objective being a ~yatematic

approach to overall test interpretation in such away that

the most eelf-coneietent and correct resulte would be ok

tainad. A logical sequence in interpretation m.othodo].

ogy, is presented in Figure L

The major entities identifiable in the flowchart of Fig. I

are

● data acquisition

● preprocessing for interpretation

● interpret at ion

● report of results

Thaee entities are gfo6af in the sense that they ●pply to

any reservoir test. Hence no distinction M made in prin-

ciple between eay a eurface shut-in test and ● DST, or

between a ‘single-layer’ teat and ● layered reeemoir teat

(LRT). This %on-dimensional’ or ‘generic’ feature of Fig.

1 in its totality is extremely attractive from several points

of view. Most importantly, it lays the blueprints for

streamlining an entire interpretation effort — from the

school of thought to software implementations thereof.

Data Acquisition

Acquisition procedures and hardware selection should

ideally be decided during the test design phase. The four

major types of raw data acquisition shown in Fig. 1 are

(a) downhole pressure and possibly surface flowrate

(b) downhole pressure (BHP) and downhole flowrate (BHF)

(c) liquid level monitoring for pumping wells

(d) production logging profiles

Configuration (a) has been the most popular for testing

in the traditional fashion (i.e., DST, elickline, etc.). In

(c) an acoustic device at the surface M used to moni-

tor the wellbore liquid level as a function of time during

buildup in a pumping well,~lJ2 Sonic travel-time infor-

mation thus gathered, plus wellhead pressure are con-

verted into BHP and BHF using an appropriate

transform,ll@~14 and theretiter follow the same path

through Fig. 1 M for configuration (b) above.

Although shown as a test acquisition configuration in Fig.

1, production logging is not usually considered as part

of i‘ s well test unless a layered reservoir test ls- 17 is

beink conducted. LRT’s are multi-rate tests in which

stationi+ry measurements are taken above each sone, and

PL surveys are made across all sones just before changes

in the surface rate. A test conducted in this fashion will

be presented with other field examples later. Stabilized

PL information can be ●n important and essential input

to any tranaient interpretation stream, as will be shown

in another of the examples presented. The stabilised PL

survey is an effective means for directly measuring the

actual flowing thickness ht. Quite often this value is

different from th~ formation thickness h read off the well

log. IMrthermore, PL information concerning the entry

points of various fluid phases into the wellbore, or the

S,= 18339 J. A. JOSkli, C. A. I$CONOMXDBS, AND F. J. XUCHUK 3

c

identification of the vertical flow regime, can be eaeential variable-rate dataeet, which is then analyzable es a drn-

to the transient acquisition e.trategy, and to the ultimate ple drawdown. It ie for thw reaao.. that the outlet km

interpretation. the deconvolution box in Fig. 1 k towwd the ‘prCSCUM-

Preproceuing for %terpretat50nonly’ side of the flowchart, since the rate-dependency of

the acquired dataset has been removed.ReaeIvoir teats are often conducted as a series of events

[static, thing) according to a specified flow sequence.FinaI1y, open hole logs, seismic data, or whatever ad-

During interpretation it maybe desirable to analyse just aditional inputs that could subsequently be used to help

particular event individually, or all events simultaneously.in constructing the final answer should be secured and

Fhrtharmore, interpretation techniques rarely invoke theinspected before entering into the interpretation phase

below.actual acquisition meaaure (e.g., time of job, pressure,

$pinner speed, sonic travel time) but rather some trane- Interpretation

formation thereof. It is for these reasons that preprocess- Ovewiew: It is at this point in the flowchart of Fig. 1ing, or data preparation for the main interpretation, is that a methodology really comes into effect. As seen in theneeded on every joblo figure, reservoir test interpretation begins with a diag-

Preprocaaaing phase can be compactly summarised ae fol- noatic plot (and there are several types of these). Entry

10W* into the next Jevel of interpret ●tion — the epecialiaed

analyde plot● Event lkfinition Baaed On Sequence Of Events

— is pern&ted oniy if the pertinent di-

●gnoetice were eeen at the diagnostic level above. If notSegmentation and Data Editting then type-curve analycis could be attempted, but re-

Data Reduction suits may not be conclusive.

● Data ‘Ihnsformations The procedure just described is repeated for all events

in the testing eequence. Sn this manner initial estimatesGeneration of interpretation functions for parametem are aecqred, and used aa startihg values

● QuaIity Control foi the next level of interpretation — history match-

It is usually poesible to correlate the squence of eventsing — should thw degree of eophutication be required.

with a recent flow history for the wekl so that superposi- Self-consistency checke and analyses are performed at this

tion effects can be rigorously accounted for. Some formstage, and the resultant ver~cation plots produced. In

of transformation is almost univemally applied before a some cases the interpretation is carried into a compl-

diagnostic plot can be viewed or a formation parametertion analysis and/or sensitivity study before genera-

can be computed. For example, the tranefonnation ~tion of the final results.

is known as the Horner time function. 18 Another popu- Diagnostic Plots: A diagnostic plot is a plot of mea-

lar transformation is the pressure derivative.19*20 Certain sured data from which a well or reservoir condition can

types of interpretation quality control are possible at this be inferred; it is also used to eetabliih the validity of a

point if needed, including smoothing for derivativea20 and particular specialised analysie. This implies that when-

tirna-error corrections.21 Further discussion of data pre- ever a new specialized analysin technique is proposed, its

processing is beyond t% scope of the present paper, but engineer should also define the accompanying diagnostic

some of the more commonly used transformations and to ensure that the method will be applied only when, and

corrections may be found in Refs. 21 snd 22, where, it is valid. Because of their generality, diagnostic

According to the flowchart in Fig. 1, all acquired tran-plots often serve more than, one purpoee.

sient data must paM through the preprocessing box. The StabiIiied PL surveys (pressure, flowrate, temperature

data which enter thie box from the right have both BHP and density profiles) constitute an important claea of di-

●nd BIfF channels present, and so deconvolution23 ie op ●gnostic plots.1 They can be used for the determination

tionally poaeible at this stage if desired. In principle, de- of fluid entry points, fiuid type and ito location in the

convolution produces a constant-rate response from the wellbore, and for assessing the effective flowing thickneae

4 THE ROLB OF 00NNNOLB - MO PRESSURE sPq 18379NRASURBNENTS IN RESBRVOIR TESTING ●

of an intarvala’.The latter application can help identify

poeaible partial penetration or completion effects.

Density measurements can be ueed to help determine ei-

ther an optimum tlowrate change or where a PL wnde

could be stationed downhole in order that the data acqui-

sition proceed in a predominantly single-phase environ-

ment.25 For example, gas wells frequently have standing

water present in the wellbore. The water may be Iiited

past the tool in response to a rate increase (see Figs. 14

and 22 in Ref. 25). Density di~gnosie immediately signals

the problem condition, which could be corrected in real

time by a change of rate or relocation of the tool. Finally,

continuous density recordings can be used as an on-site

aid in determining the ●ppropriate drawdown praeaure

(eg. above bubble or dew point pressure), and an indicw

tion that the pm-teat cleiump period can be terminated

(when the density drope to that of the raeervoir fluid).

The log-log praaaura and preaaura derivative praaantat-

ionlg’zo haa rccestly become ● standard diagnostic tool

used in transient analysie. When plotted on quare cy-

clee the data retaintheir aspect and visual diagnoaia of

●ven rather complicated well/raaervoir behaviour is pos-

sible. If, in addition to BHP the downhole flowrate ie also

available, then the appropriate diagnostic plot is the log-

log convolution derivative. 6*26 Th~ triple plot was drat

used in Ref. 26 on sandface flowrates computed based

on the constant storage assumption (it was inadvertently

termed ‘deconvolution’ by the authore).. Itu 6rat appli-

cation in connection with msa8ured downhole %owrates

wss in Ref. 6 (removing any assumptions concerning the

nature of the storage).

The ‘derivatives’ that are used in transient analysis can

be simply described ac the sfoge of the appropriate sp-

cialised plot, plotted vemu~ test time in log-log coordi-

nates. For example, in a buildup test the derivative is the

slope of the Horner plot; if there ~e multiple rate changes

before a buildup, the derivative is the dope of the gener-

alised Horner plot. Similarly, the convolution derivative

is the slope of the sandface rate convolution23*27 (sFRC)

plot.

The main use of the log-log diagnostic plot is to identify

flow regimee or well/reeervoir models that are present in

the transient so that the specialized analysie (usually per-

formed in another coordinate system) can be performed.

As an example, straight lines on ● Horner plot are valid

only where the derivative m fiat (constant). Similarly,

straight line analysia on the SFRCplot ia valid only where

the convolution derivative ie flat.

In addition m the identification of flow ragimea on a lo-

cal basis (e.g., a period of !inear flow in the data), log-

Iog diagnostic plots can provide ● global representation

of the entire system behaviour -- considerably enhanc-

ing the so-called pattern recognition problem28 with the

introduction of the derivative. 10*20Hence, wellbore dom-

inated flow, ‘homogeneous’ systems, heterogeneous (fis-

sured) and layered systems, channels and boundad sys-

tems, etc., may all be recognised by their characteristic

fingerprints on the log-log plot. Such diagnostics ~hould

either be supported by, or used to support external infor-

mation, especially from eeiemic and geological aourcea.

A log-log diagnostic pattern look-up table and general

diecuasion ie found in Ref. 29.

Specialhed Plotw A apecialiaad analysis technique ie

one that ie valid only for a specific flow regime or well/~

servoir geometry. Itusually yielda the moat reliable esti-

mate for the parameter(s) aceociated with that particular

90W regime. As a raault the majority of specialized anal-

vsis techniques are based on straight line picks over data

plotted in the appropriate coordinate system.

The fundamental tenet of the methodology depicted in

Fig. I is that the specialised plot for a particular con-

dition shall not be constructed unlwa the accompanying

diagnostic for that condition indicate its validity. Ra-

stated, a 8peciahed t8chn@e 8hould not be u8ed a8 h

own diagno8tak Specialised analyeaa, once deemed ap

plicable, are preferable to type-curve analyses for local

parameter estimation. Common examples of speciahsed

analyeie for radial flow are Horner,18 MDH30 and SFR@7

plots. Linear, hi-linear, spherical, peeudo-steady, and nu-

merous other flow regimae each have their corresponding

specialised plcts, which can be found elsewhere2i in the

literature.

Type-Curve Analysis: Typecurves are plots of aolu-

tiona to theoretical flow probleme used to repreeant real-

ity. Most often, but not always, the solutions are plotted

in log-log coordinate. Their role in reservoir description

has been continually refined once it wae realised that they

are no panacea.zl!3i!92 A eurvey and discuuion of diffar-

ent type-curves can be found in Refs. 21, 31 and 33.

s~ 18379 J. A. JOSEPH, C. A. BCONOHIDBS, AND F. J. KUCSUK 5

Withii the framework of Fig. 1, itiawan that the PM= There are some caaaa where it may not be poaeible to

farred interpretation hierarchy for properly designed and lower and anchor ● dowrthole shut-in device into ● com-

conductad teats ia the dhgnoatic plot, followed by sp- pleted production or injection well, due to restrictions

cialiaed analyaie plot.. Subeeqaently, type-cwwe match- in the wellbore or to the lack of ● landing nipple ●t the

ing can be used to dnd ● global model of the wel@servoir tubing shoe. Many of the newer well. are quipped with

eystem that rapreaants all of the tramieut data from ●oin- the aeceaaary hudwam for anchoring a downhole shut-

gle flow period. Choice of the model chould draw fmm in tool, without having to pull the tubing. Otherwise,

and consolidate all of the observations from external data surface shut-in may be utilised, This configuration can

(the box in Fig. 1 labeled Additional Info.), the diagnosis sometimes lead to complications during interpretation,

step, and computed parametem from epecialiaed analy- Wrticularly in producing welle where multiphase flow con-

sia. Refining the match may involve the use of nonlinear ditions are Iikely in the weIlbore.

optimization aad search achemesio’i’sa’There are clearcut advantage to employing the down-

Hiatory Matching and Verification: This portion of hole shut-in technique. Of foremost importance is the

Fig. 1 i. ueed for performing condatency checkiig ●nd fact that the portion of the buildup data dominated by

global analysia (a. in LRT) for an entire dataaet. wellbore storage effects will be considerably shorter. The

duration of thee. effect. ie proportional to the fluid-filledTkre are award ●pproached to verify an interpretation. “‘ volume below the shut-in valve. For example, in ● 10,000

One of the more popular ia by eelacting just one event ft well, if the top of the interval teated ia ●t 9500 ft andfrom a multi-rate teat aaquence, and rnodelling that event the shut-in valve ie located●t the same depth, then theby type curve matching ae diacueeed dxwe. ThM type storage coetlicient and hence the wellbore storage dur~curve [and its aaaociated parametem) are then extrap tion can be reduced by 95% (in the caee of aero skin).elated baaed on the principle of superposition into the

remaining events in the teat sequence that had not been Furthermore, the duration of wellbore storage ia propor-

analyaed. If the model were correct, then its extrapola- tional to the compresaibiity of the wellbore fluid, At

tion via superposition ohould provide a match for off of depths near the flowing interval, oil produced from an

the transient data. Strictly speaking, this ia not varifica- undematurated reservoir will be in the liquid phase. As

tion of the results of interpretation but rather a verifica- liquid rise. in the wellbore, the pressure may fall below

tion that an adequate model had been choeen. the bubble point and gas originally in solution will be

liberated. When the well ie flowing, produced gas can be

Statistical principles’ have alao been used to assess the mixed with the oil. Following shut-in, gas and oil phacesuncertainty of parameter eatimatea during history match- may segregate leading to a complex pressure behaviour

ing. Thm is a check on the goodnem of the interpretation called wellbore phase redistribution.ss

in terms of some chosen statistic. This approach is ex-

tremely powerful, and is the basis for nonlinear estima-Downhole shut-in is a technique for isolating the pres-

tion procedures used in type curve matchingio and thesure sensor from lengthy and complex wellbore transient

analysis of layered reservoir tests. 17behaviour, Distortion due to the fluid in the volume

below the shut-in vahre are likely to be manifest es a

SELECTED FIELD EXAMPLES (small) constant storage effect, and the compressibility

DiscussioIIof the fluid there will usually be much smaller than the

effective compressibility of the two-phase mixture above

the valve.The most effective way to conduct a buildup test in gen-

eral is to shut the well in near the sandface using a down- For formations in contact with a gas cap or active wa-

hole shut-in valve (wells produced or injected at very high ter drive, the effects of the constant pressure boundary

rates may require special consideration). Newly drilled may be apparent before the end of wellbore dominated

wells are usually tested in this way, with the shut-in valve transient behaviour.3G The same is also possible in com-

i duded in the drill stem test assembly. plex fauhed reservoirs, where the effect of a near planar

6 THE ROLE OF mNNNoLB PLm AND PRBSSURB SPB 18379MEASUREMENTS IN RBSERWIR TESTING #

“

boundary may be missed because of wellbore effects.s’

Dual porosity or layered eystem response patterns can

also be maeked by lengthy wellbore storage. For these

reaeons, there are some testswhich do not offkr conck-

sive results from pressure transient data recorded during

a surface shut-in.

If it is not possible to conduct a brildup test using down-

hole shut-in, one alternative is to measure downhole ilow-

mtea simultaneously with the pressure. Methods such as

convoIutiont27 deconvolution} 2s or rate-normaliiationge

can achieve an analogous reduction in the storage effect.

These techniques work well when the flowmeter sensor

can be positioned in a predominantly single phase envi-

ronment in the wellbore, and the dynamic range of the

measured flowrate is ●bove the threshold level for the

particular tool. Otherwise, special devices’ catering to

the very low flowrate ●pplicationsso may be required.

Several of theee pointe will be demonstrated by examples

later in the paper. Well, fiuid and rock properties for

Examplee 1-4 era contained in Table 1, and for Ex. 5 in

Table 3. Interpretation results for Ex. 1-4 are in Table

2, and for Ex. S in Table 4.

Example 1: Surface/Downhole Shut-Yn

The 6rst field example provides a comparison between

buildups on the same well conducted with surfms and

downhole shut-in. The jobs were performed in West

Africa. A 185 hour surface shut-in test was fimt run

on this producing oil well, Two yeare later a battery

operated downhole shut-in tool wae run in a nipple in

a standard production completion, providing the second

buildu~ of 91 hours duration. Pattern watertlooding was

ongoing in the field during the period between the two

tests.

The log-log diagnostic plot for the pressure transient re-

sponses from the two tests is presented in Figure 2, ‘lMan-

gles and squares represent the pressure change (Ap) and

its derivative for the surface shut-in, while the stare and

plus symbols show Ap and its derivative for the down-

hole shut-in. The data from each buildup have been nor-

malised by the surface flowrate immediately preceding

shut-in. Normalisation in this fashion facilitates superim-

posing all of the data on the same plot for identification

or comparison of the gross well/reservoir behaviour from

test to test.

The drst buildup is eeverly dietorted by wellbore stor-

●ge effects, which last some 100 hours. Them is also

evidence of changing storage probably caused by well-

bore phase segregation during the Scat hour of the first

buildup. Storage becomes essentially constant thereafter,

but another 99 houre are needed for a stabilization to

be evident in the derivative (thie would indicate infinite-

acting radial flow or 2ARF, in the formation).

The downhole shut-in teat, on the other hand, has es-

tablished lARF after just 1 hour of shut-in, based on ite

derivative (plus signs). The duration of wellbore domi-

nated ROWbehaviour was reduced by about 99% in this

exampJe. Whenever IARF ie diagnoeed from the deriva-

tive the pertinent specialized plot (in this case it is the

generalized Homer plot, which accounts for the flow hb

tosy of the well) should be used for reservoir parameter’

estimation.

Itisinstructive to examine the dataeeta on a modikl

MDHgO plot (normalii pressure change versus log At),

as shown in Figure 3. This confirms that the eemilog

slopes are roughly the same (implying the same M),

However, obaeswe that the line for the downhole shut-

in (stare) istarte approximately one cycle in time ●arlier

than for the surface shut-in cask. Fig. 3 also indicates a

slight increase in the slope of the downhole shut-in data

at later time, confirmed also in the diagnostic plot (Fig.

2).

The generalized Homer plot for each test is presented

in Figure 4.The permeability and skin computed from

each are about the same, The difference in the pressures

extrapolated over the two year period is attributed to

pattern water injection.

Deviation from the semi-log line at late time in the 8ec-

ond buildup could be caused by several factom, including

mobility or phase effects associated with the water injec-

tion, or, possibly, a fault. If it were caused by a fault

(and this is a distinct pomibility in this area), the same

deviation should have been evident on the first buildup

test. Hence, the mobility change due to the water in-

jection is the favored explrtnztion, since the oil bank ie

moving toward the producer, and would have been more

distant at the time of the fimt buildup test,

sp 18379 J. A. JOSEPH, C. A. ECONOMIDBS, AND F. J. KUCHUK 7

&emple % Pressure Buildup With Downhole

Flowrate

As a logical continuation of the iirst example, a buildup

test where downhole flowmtea were measured is next pre-

sented. This job was performed on a new gas well in

the thrustbelt region of western USA; tlie well had been

producing for 39,5 hours. A flow profile run just before

shut-in indicated that 20 ft of the 26 ft net pay inter-

val was flowing. This sandstone reservoir is located in a

river bed/channel type of depositional environment; an

active tectonic history has made fissuring (natural frac-

turing) and block faulting not at all uncommon in this

region. Openhole log interpretation revealed essentially

constant formation properties over the pay sone, and so

a singlelayer analysis was justifiable. However, the re-

siativity measured by the spherically focused log wee less

than the reeietivity from the induction log — a qualitative

and empirical indicator of nstrml fruturing.’”

The foregoing discussion is in line with the methodology

proposed in Fig. 1. Before even viewing the transient

diagnostic plots, external information had signalled the

poeeibtity that the dataaet may display the patterns of

● fissured-faulted ayekm. Such information is invaluable

in piecing together a reservoir model,

Following the production surveys, the PL sonde was posi-

tioned 37 ft above the sone of iutere8t before closing the

wel$ 270 ft of rathole remained below the tool, Based

purely on volumetric considerations a 97% reduction in

wellbore storage (volume above tool eliminated) should

be experienced. Fig. 5 presents the transient rate and

pres8ure acquired by a fullbore spinner and quart8 cry8tal

gauge respectively. Density measurements (not shown)

confirmed that the spinner was operating in a single-

phase wellbore environment during the test.

As seen in Fig. 6, when spinners are used during a

buildup test their readings are accurate initially, but then

decline rapidly as the afterflow decays. In this case the

8pinner reading went to zero after 6 minutes. This illus-

trates the problem with measuring transient downhole

flowrates during buildup tests. Once the spinner rota-

tion rate falls below the sensor thre8hold value, acquired

data are no longer accurate, and extrapolation is usually

necessary in practice.

The extrapolated flowrates are usually computed by ae-

suming that the wellbore fluid compressibility is constant

and of small magnitude.zs This is valid and practical as-

sumption, but the observation that often the most infor-

mative featurea in the convolved reeponee are computed

with extrapolated flowratee should be acknowledged. In

contrast, tlowrates measured during drawdown do not

●uffer from such &itations as the test time progresses,.

es will be seen in the third example,

The diagnostic plot for the test is shown in Fig. 6, Convo.

lution derivative based on measured flowrate is shown by

the circlee, and for extrapolated flowrate by the crosses,

The high quality of the crystal gauge pressure measure-

ment (triangles) is reflected in the sharpness of its deriva-

tive curve (squares), which was unemoothed in F@ 6.

The initial oscillation of the pressure derivative is due to

the slow (30 second) closing of the eurface valve, The

convolution derivative, however, is quite smooth during

the same time frame as one would expect (slow closing of

the valve is akin to ● multi-rate effect).

The dip in the pressure derivative after about one hour

is a diagnostic of heterogeneity — in this case it M in-

terpreted u ● 6eaured-system response pattern based on

the external information ●vailable (and discussed above).

The gap in the convolution derivative between 0.01 and

0.1 hours was caused by negative values (unplottable on

the logarithmic scale), brought about first by declining

flowrates and finally by the crossover from measured data

(circles) to extrapolation (crosses). Both pressure and

convolution derivative cumes eventually merge at later

times, On this and all remaining examples, convolution

derivative plots show the responses computed with ex-

trapolated flowrates in a different symbol or line type,

Log-log diagnostic analysis for fi8sured systems (pseudo-

steady state fluid tranefer from fissure to matrix) require8

two stabilisations at the same level in a derivative. The

first stabilisation represents radial flow in the fissures

only, while the second represents radial flow in the to-

t al system (fissures+ matrix). Often, as is the case in the

present example, the first stabilisation is masked on the

pressure derivative by wellbore storage, However, this

level i8 quite clearly indicated by the convolution deriva-

tive in early time. After reconciling all information the

type-curve match shown by the solid lines on the diag-

nostic plot was obtained.

The multiple type-curwe presentation shown in Fig. 6

fixes the pressure match (and hence the permeability) at

8 TNE ROLE OF DOUNSOLE PLW MD PRESSURB SP$ 183?9MEASURBMB~S IN RESERVOIR TSSTING

the lWC1indicated by the convolution derivative (circlae);

semilog analyoie of the preeeura data & not jtwtijied ba-

cauee the prasaura derivative doee not stabilise at this

level. However, ● SFRC pIot wouId be wdid for com-

putations during the time frame where the convolution

derivative ia flat. It is normal to compute negative skins

in caeee such u thw one. Thie ie ueuaIly not an indica-

tion of stimulation but rather aoaociated with the mod-

elling of fiseured systems. It is alsoseen that the final

level ●ttained by both preeaure and convolution derivm

tivee (squarea and croeaee) ia higher than the initial level

of the convolution derivative (circles). This M caused by

a nearby eealing fault, the poseible exiater.ce of which waa

indicated earlier.

The convolution derivative ie matched in Fig. 6 to a

6aeured-aystam type-curve that wee computed with all

of the parameter for the choean model, escept that the

storage coefficient corraeponda to the fluid filled volume

below the tool. In eecuriag the curve match for the praa-

mre data, ● dimenaionleea storage coefficient CD = 456

wae wed. The convolution derivative type=curva match

wae made uaiag ● C~ value of 23.5, whkh represents a

95% d~aaa in the coefficient used to match the preaeure

data alone. The storage reduction effected by measured

data (95%) checks very closely with the anticipated value

(97%) baeed on well completion/tool location considera-

tions.

Even though a rather convincing interpretation was re.

alised from convolution derivative analysis of a buildup, it

should be pointed out that a drawdown test in the same

reservoir would have produced ● response similar to the

lower dotted curve in Fig. 6 without the need for flowrate

extrapolation, The next example will further clarify this

point,

Example S: Pressure Drawdown With Downhole

Flowrate

Buildup or falloff tests are historically more popular than

flowing tests (drawdown or injection) because of practical

difficulties associated with maintaining constant surface

rates, Also, buildups are conducted to determine reser-

voir pressure, However, there are advantages to testing

with the well flowing, For example, if not for the prob-

lems associated with surface flowrate fluctuations during

drawdown, the obvious reason to flow test would be to

avoid loss of production,

The tlowing well ie performing in its n~tural condition.

Changea occur in the near-wellbore and wellbore during

buildup that are nonexistent, or much less extreme, when

the weIl is left flowing. in”buildup these changea can re

suit in transients that are not representative of the reser-

voir but rather of the wellbore, and can be misleading if

wrongly interpreted aa such.ss In the case of water injec-

tore, the well may perform aa if vertically fractured when

in the flowing condition, but may show damage during

falloff as the hydraulic fracture closes.’l

There is therefore ● certain benefit to testing while fiow-

ing the well. This can be fully realizable if downhole

flowrates are meaeured in addition to the praesure. Fur-

thermore, spinner flowmeter actually perform much bet-

ter when the well is flowing. There ie an initial period

where the data are not reliable, but even thie can be

minimised (or avoided altogether) by deeigning teeting

eequencee appropriately. Mechanical problems that could

occur during the initial period include

● inertial effects, in turning the *pinner from a dead stop

(as when a drawdown ia run from the static condition);

typical duration ia 1 to 2.5 eeconda, and even less if rate

changeb ok..er than from static condition are used.

. overcoming the sensor threshold; this is fixed and deter-

mined by calibration for each device.

Problems seen in the previous example (spinner going

to zero and the need for extrapolation) can be largely

avoided by designing the test to include a step-rate change

before the buildup. A steprate drawdown and buildup

sequence can be used to enhance the buildup test analy-

sis. The convolution response during the ftow test min-

imizes the distortions due to wellbore storage or surface

flowrate fluctuations revealing early-time near-wellbore

features and reservoir heterogeneities= In late time, the

characteristic drawdown response patterns to faults and

closed or no-flow boundary effects are more straightfor-

ward to distinguish than the equivalent buildup respons-

es42. By identifying the reservoir model during the tlow-

ing period, the ensuing buildup data can be correctly ex-

trapolated for determination ofp; or p*, and the duration

of the shut-in period may be reduced,

Example 3 was selected to illustrate practical problems

with drawdowns, and how downho]e flowmetering can be

used to enhance the analysis under these conditions, The

-L

SW 18379 J. A. J08EPH, C. A. ECONOl@BS, AND F. J. KUCEUK 9

data c. ,ne from ● producing gee well in Europe. Ae is of-

ten the cam, production wee manifolded directly into the

exietent gathering eystem. No special surface quipment

WM used onsite for controlling the rate during the tmt.

Ae a result, produced liquids were dumped from the ~ep

arator about e~wy 2 houre, reeulting in ● variable back

preeeure on the well during the transient acquishion.

The raw data plot for thie example, which ie Event No.

5 in a multi-rate flow echedule, is shown in Fig. 7. The

log-log diagnostic plot is ehown in Fig. 8. The effect of

liquid removal at the nmface is to cauee the downhole

preseure (squares in Fig. 7) to stairstep in a periodic

manner. Such effects are greatly exaggerated on the pree-

mre derivative, which in fact oscillates between negative

and poeitive valuee each time the Iiquid knock-out occurs.

However, since the downhole meaeured flowrate wae also

varying u ● meult of the surface rate fluctuation, the

convolution derivative in Fw. 8 ie much emoother than

ie the preaaure derivative, and ia themfom analyzable in

a straightforward manner.

Since the convolution derivative tende to etabb ●bout

the line shown in the figure, the SFRC plot of Fig. 9 wee

ueed for parameter estimation. It was not poeeible, nei-

ther wae it the objective, to reetitute a perfectIy smooth

response after convolution. Yet it ia clear that Figs. 8 and

9 do afford an interpretation even in thie example, where

preeeure by iteelf would have remained uninterpretable,

Example 4: Production Log Test

The test in this example included a production log sur-

vey in the baeic drawdown/buiIdup eequence,’s Preesure,

flowrate, temperature, and density vereus depth were ob-

tained from the PL over the entire producing internal,

and provided considerable ineight when incorporated into

the transient data anafysie. Worn openhole well loge the

interval of interest waa approximately 200 ft thick, con-

sisting of eand beds interlain with aerially discontinuous

shales. Baeed on the log interpretation it wee decided

to analyse the problem as an effectively homogeneous,

single-layered system,

The test sequence wae designed to include one drawdown,

a etabhed PL mrvey, and a buildup. Ii began after a

tw-day shut-in period; the transient, pr.sssure and spin.

ner data acquired are shown in Fig. 10. With the tlowme-

ter positioned above the top of the perforated intenal,

the well wee opened to 16,000 BPD. Real-time surface

readout of the downhole acquisition indicated “kfter 20

minutes tliat the bottomhole preaaure wae about to fall

below bubble point. The flowrate wee decreaeed imme-

diately in order to avoid two-phaee flow in the reservoir.

The test wae then continued for seven hours, and the

well remained flowing for another 10 hours before the

etabilieed flow profiles were taken. The tlowrate at this

point was 13,360 BPD, and the well waa subsequently

shut-in for a 28-hour buildup with the tool above all per-

forations.

The cut-back in the production rate was accompanied by

an increaee in the flowing bottomhole preeeure during the

first flow period, as clearly seen in the Fig. 10. A very

important diagnostic plot in production log teete ie the

stabfied flow profile, Fig. 11, It shows graphically that

just the uppar four eatsof parforatione were contributing

to the total production. PL surveys from eeveral other

welle in the saint field alao showed reduced production,

which wae attributed to ecaling probleme. Siice the lower

(non-productive) intervals were believed to be in com-

munication with the upper producing perforations, the

system wae diagnoeed as one of partial penetration type.

Hence the test could be interpreted using a partial pen-

etration model, with the penetration ratio, b, fixed at a

value of 0.5 as indicated from the flow profile analysie.

The log-log diagnostic plot for the drawdown and buildup

tests is shown in Fig. 12. Observe that the same kind of

normalization ai wee used in the first example (Fig. 2) is

once again employed, and also for the same reason — to

facilitate comparison of data from separate events, The

effect of the reduction in flowrate with consequential re-

duction in overall drawdown was to render the pressure

derivative uninterpretable, as shown by the squares in

Fig, 12. However, since the convolution derivative (cir-

cles) is a continuous superposition of the variable down-

hole flowrate with the pressure, it is seen that a imooth

data trend, comparable to that in the buildup, has been

restituted in the response.

The convohttion derivative exhibits ●n effective reduction

in the wellbore storage to about 3096 of that eeen by the

pressure data alone, corresponding mainly to the rathole

volume extending 1000” ft below the tool. The convolu.

tion derivative for the buildup (asterisks in Fig. 12) wee

computed from measured Bowrates for about 30 minutee.

397

,,

10 TNB RO~ OF DOWNBOLEFWW MO PRESSUREHMSURBMENTS IN RESERVOIR TESTING

SPB 18379. .

Then the remaining computation (shown by the dotted

curve in the Fig.) was performed using 9owrates eztrap-

ofated from the measured values.

Interpretation of this dataaet using analytical techniques

was presented in Ref. 43. Penneabilit y and skin from

the drawdown and buildup using convolution analysis

yielded similar values. A history match using a r-z sin-

gle well numerical simulator was made, and the resultant

verification plot is shown in Figure 13. A nonlinear pa-

rameter estimation scheme iG.17 was used to determine in

the least-square sense global estimates for the effective

horizontal and vertical permeabilities, and the damage

skin opposite the active (upper) perforation. The initial

guesses used in the optimisation procedure were taken

from the earlier analysia,43 and the final answem ohown

in Table 2 are consistent with those obtainedpreviously.

Example S: Lay-d Reservoir llset

Layered reservoir testslG- 17.’4 are designad to maximise

the quantity, and quality, of information obtainable from

downhole pressure and 9owrate measurements. The LRT

is a commingled production test that alternates PL pr-

ofilingimme&ately before a surface flowrate change with

a stationary measurement of the transient induced by

the rate change. The sonde is strategically located above

each of the selected perforated intervals before the new

tlowrate change is induced, and after a profile at the last

prevailing rate has been comp!eted. Permeability, skin

and average pressure can then be determined for each

. zone tested in this fashion.”

The LRT example comes from an oil welI. Seismic data

showed that the well was located in a faulted region.

Openhole logs revealed five distinct sands, each of which

wiu~fully perforated and completed for commingled pro-

duction. The objective of this LRT was to determine

permeabilities and skins in each of the completed cones.

Data needed for the analysis are provided in Table 3.

The sequence of events during the test is shown Fig.

14, with the measured pressure, flowrate, density, and

temperature transients given in Fig, 15, Zones above

which measurements were taken are labelled in both fig-

ures, It is apparent from the density data in Fig. 15 that

the transients for zone E were taken in standing water,

The necessity for measuring transient flowrates below the

standing water level in a well is comman in LRT’s, and

can pose particular difficulty in gas wells. In this well, the -

main problem was that the rates recorded ●bove sone E

appeared to contain much more noise than those recorded

elsewhere in the well, The production log flowrate nur-

veya (not shown) indicated that virtually none of the fluid

was produced from sone C, Hence, sones C and B were

grouped in the analysis.

Since the downho]e pressure was acquired at different

depths in the wellbore, correction to a datum is neces-

sary as keen in Fig. 16. Pressure transients are used as

the wellbore boundary condition for generation of a si.

multaneous match to ail of the measured flowrates, using

a layered model that is consistent with geology and geom-

etry. Permeability and skin values used by the model am

automaticallyvariedaccording to a non-linear parameter

estimation scheme,17 until the match is optimised in the

least squares sense,

Before a match with a layered model was attempted, iog-

log diagnostic plots as prescribed by Fig. 1 were exami-

ned for each flow period, Each showed behaviour similar

to that seen in Fig, 17, which is normalised in the same

fashion as used earlier in Figs. 2, 6 and 12. Pressure

change, pressure derivative and convolution derivative for

the first drawdown and final buildup periods are super-

imposed on Fig, 17. A half-slope is evident at later times

on both the pressure and convolution derivatives. This is

a diagnostic of Iiraear flow, associated with parallel seal-

ing faults,

In Fig. 17 it appears that the buildup pressure derivative

(plus signs) shows a plateau (upper dashed line in Fig,

17) at a higher level than the other pressure / convolution

derivative curves in the figure, This is not an artifact of

an extrapolated spinner, because afterflow was measura-

ble for about 1 hour, Type curve analysis of the buildup

treating the system as a single hmnogeneous layer yielded

k = 391.4 md. and s = -0,126, together with estimates

of distances to the faults of 122 ft (37.2 m) and 365 ft

(111.3 m),

Analysis of the buildup convolution derivative, or any of

the other derivatives in Fig. 17 would result in perme-

abilities roughly twice ss large as those obtained from

1

Spl! 1837.9 J. A. L7CWBPfi, C. A. BC!ONOMIDBS, AND F. J. KUCHUK 11

the buildup pressure derivative alone. (The approximate on log-log coordinates ea the single layer model used forplateau for the other derivatives is given by the lower the buildup type cu~e match, However the thicknesa-horizontal line in the figure.) Such an analysis could also weighted average permeabilitiea and skim ate different inchange the estimates of the distances to the faults, Eati- each case.mates of the permeability and skin baaed on SFRC anal-

Experience with nearly 40 such tests haa indicated thatysis are indicated in Table 4.

itisquite common for the buildup reeponee of a layered

The estimates from the convolution analysis were refined system to appear, and be interpreted, as a homogeneous

by sequential analysia7’1’.44 ueing a layered model with singhr-layer system. LRT interpretation shows that this

a single fault boundary4s. Thie type of analysis is con- approach to the analysis may yield oversimplified results.

ducted from the bottom layer up. Once the permeability However, this example shows the difficulty of attempting

and skin are determined for the lowest layer, those values to determine both layer properties and the distances to

are fixed, so that the properties of the next layer can be vertical barriers simultaneously with a single well test.

determined using a two-layer model. Each time the next An interference test involving pressure measurements in

higher layer ie analysed, another layer ie added to the observation well(s) might provide more conclusive infor-

model, mation on the locatione of vertical barriem to flow, or

Since each analysis was performed on only one flow pe-altematively, vertical seismic profile data might detect

the images of nearby faults, Then the layered reservoirriod at a time, by matching only the early time transients, test would be used to determine permeabilitiea and skinsthere was no need to model the mom distant fault. Re- only, using a model that includes the boundary effects,cults of the Sequer,tial analysis are also provided in Table

CONCLUSIONS4.

A comprehensive methodology for reservoir test interpre-Flnally, veri6cation of the position of the faults detes- tation was presented with field examples, The follow-mined by the buildup type curve match was attempted ing observations concerning the practicalityy and choiceusing a model for the commingled-layered syetem which of .reeemoir testing procedures were made in this study:

includes the parallel fault boundary conditions4s.4G. In

this case, the verification step proved problematic, as ex- 1. In general, the interpretation of a buildup portion of anyplained in the following paragraphs. teat will be more straightforward and leee error-prone if

The history match shown in Fig. 18 was computed with the data is acquired after downhole shut-in. When com-

the layer permeabilities and skins shown in Table 4, and pletion or other constraints exclude the downhole shut-

with parallel fault boundaries at the dietancee indicated in option, or when the test sequence includes acquiai-

by the buildup analysis. However, it was necessary to in- tion while flowing, then the measurement of downhole

elude an ongoing preesure trend of 3.2 psi/hour to achieve flowrates offers considerable advantage.

this match. Since this was an exploration well and there2. The inclueion in the test sequence of a flowing period

were no nearby wells thought to be interfering, the diffi- with measured downhole fiowrates provides data whichculty in finding a simultaneous match for all of the data may improve the interpretation of a subsequent shut.inis an indication that additional features are needed in period, and possibly shorten its duration, Measurablethe mode). Perhaps there are one or more additonal downhole flowrate and pressure transients can be inducedfaults which are too distant to influence any one tran- by a etep change in the eurface rate. This strategy cansient flow period, but which are evident in production be particularly attractive for reservoir limit testing, andover the nearly 30 hours involved for the total test. motivates installation of a fiowmeter sensor in the DST

The comparison between the model used to match the string.

buildup pressure and derivative and the model that pro-3.Production logging surveys during stabilised flow and

duced the simultaneous match in Fig, 18 is shown induring the buildup, when combined with downhole pree.

Fig. 19. Fig, 19 shows that the layered mo~el used forsure and flowrate transients in the teet sequence, ●dd

the simultaneous match has the same general appearance

1.

information about the location and phase of ffuid entries

that is important and can be essential to the tranaient

data interpretation. The ultimate application for this

measurement combination is the layered reservoir test.

NOMENCLATURE

B=

c=

CD =

Cf =

h =

k =

k,, =

P =

Ap =

q =

Q=ru, =

8 =

W =

t-=P

At =

formation volume factor

storage coefficient

dimensionless storage coefficient

total compressibility

formation thickness

formation permeability

vertical permeability

pressure

pressure change

downhole flowrate

surface 9owrate

wellbore radius

skin factor

damage skin factor

producing time skin

test time

Greek Symbols

7!1 = gas gravity (air=l)

P = viscosity

+ = porosity

ACKNOWLEDGEMENTS

The authors greatfully acknowledge the four anonymous

oil companies for allowing us the use of their data, We

are thankful to the management. of Schlumberger for per-

mission to present this paper. Special thanks are due T.

Bratton, G. Clark, O. Myhrer, C, Ovens, and M, Pearson

for their assistance with the field examples.

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with a Gaa Cap,’ JPT, April 1981 (743-754).

Chen, H.K and Brigham, W.E.: ‘Preeeure Buiidup for a

Well With Storage and Skin in a Closed Square,’ JPT,

January 1978 (141-146).

Fetkovitch, M.J. and Vlenot, M.E,: ‘Rate Normaiiaation

of Buildup Preaaura By Using After!low Data,’ JPT, h

cember1984 (2211-2224).

Piers, G. E., Perkins, J. and Eacott, D.: ‘A New Fbwme-

ter for Production Logging and Well Taating,’ paper SPE

16819 presented at the 62nd Annual Technical Confer-

ence and Exhibition of the SPE~ Daliaa, TX., September

27-30, 1987.

Boyeldieu, C, and Martin, C.: ‘Ihcture Detection and

Evaluation,’ paper No. 21, 9th European International

Formation Evaluation ‘hansactions of the SPWLA,

Faris, 1984,

Koning, E.J.L. and Niko, H.: ‘i%actured ‘Water-Injection

Wells: A Pressure Falloff Test for Determining Ikacture

Dimensions,’ paper SPE 14458presented at the 60th An-

nual Fall Technical Conference and Exhibition of the

SPE, Las Vegas, Nevada, September 22-25, 1985.

Proano, E, A,, and Lilley, I. J.: ‘Derivative of Pressure:

Application to Bounded Reservoir Interpretation,’ paper

SPE 15861 presented at the SPE European Petroleum

Conference, London, (jctober 20-22, 1986.

Kuchuk, F.: ‘New Method for Estimating Parameters

of Low Permeability ReseNok,’ paper SPE 16394 pre-

sented at the SPE/DOE Low Permeability Symposium,

Denver, CO., April 18-19, 1987.

44. Ehiig-Economidaa, C. A.: ‘Taating and Interpretationin

Layered Reservoirs,’ JPT, September 1987 (1087-1OW).

45. Chen, T.: ‘Preaaure Drawdown in a Layered Raaervoir

With Linear Boundaries,’ paper SPE 16767 presented at

the 62nd Armuai Technical Conference and Exhibition of

the SPE, Khllaa, TX., September 27-30, 1987.

46. EHig-Economides, C,A. and Economidee, M. J.: ‘Pres-

sure fiansient Analysie in an Elongated Linear Flow Sys-

tem,’ SPEJ, December 1985 (839-847).

.

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