Guidelines for Step Rate Testing

8/9/2019 Guidelines for Step Rate Testing

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Guidelines for Step Rate TestingThis document has been reviewed by Sponsors, revised and is considered to be final.

If future additions or modifications are made, all parties will be notified.

Introduction

A step rate injectivity test is normally used to estimate the transition from matrix (or pseudo-matrix - a fracture may already be present but the bulk of the flo is radial! flo to fracture-dominated injection" according to a change in slope of a plot of pressure versus rate# Step ratetesting allo s for determination of hen a ne hydraulic fracture occurs and$or hen a pre-existing fracture opens$propagates#

Step rate testing allo s for determining hen a fracture ill propagate and hen a pre-existing fracture ill reopen# %t can be run after a conventional falloff or a final falloff segmentcan be used in the test# Repeated falloff testing can also be used to assess if a reservoir hasbeen altered by thermal changes in in-situ stresses or changes in kh associated ith thermaleffects#

%njection is carried out at a number of rates belo fracturing pressure# At each rate injectioncontinues until stabili&ation appears to occur# The injection operations are continued afterindications of fracture opening$propagation# The opening pressure is inferred from a significantchange in slope of a plot of bottomhole pressure versus injection rate# 'igure is an example#

Figure 1. Step rate test on Magnus C .

!eneric "rocedures

# %f possible" have a reasonable conception of hat the in-situ stresses are so that thestep rate test can be appropriately designed# )e certain that there ill be ade*uate datapoints before breakdo n or reopening of the fracture#



+# ,nder any circumstances" have an approximation of the conversion bet een surfaceand bottomhole treating pressure# %n addition to determining in-situ stress levels" thiscan be useful for evaluating the completion efficiency#

# ,nder certain" restricted circumstances onshore" dead string calibration may be possibleif the backside has enough integrity#

.# %f reasonable" obtain bottomhole pressure measurements# %f bottomhole data are beingac*uired" tandem gauges are a reasonable option# /ave continuous surface readout

under any circumstances# This is a particular issue if some of the injection is onvacuum# Some operators ill not opt for tandem gauges if the supplier is reliable" if past history sho s fe failures and if the cost of failure is minimal#

0# 1heck and calibrate all rate meters prior to testing#

2# )e certain there is enough ater on location#

3# ,se uniform rates and time steps4 This ill be demonstrated in the section on multi-rateanalysis# Record pressure and time and provide the analyst ith this information#

5# 6nsure that there is appropriate e*uipment available to fracture the ell initially# %f breakdo n is re*uired" mud or cement pumps are often used# )e reasonably certain of

hat ill happen during the fracturing operation or use the test to make an inference of this# 1an the injection interval accept the re*uired rates7 %f not" out-of-&one fracturegro th can occur#

8# %t has been recommended that each time step is one hour long to ensure that theradius of investigation is large enough# 9bviously" this may not al ays be practical#6ither shorter times are called for because of economic or operational limitations orsubstantially longer times are re*uired to allo for thermal stabili&ation# At a minimum"consider these issues before the test# 6arlougher states that :in relatively lopermeability formations (k ; 0 md!" each injection should last for one hour< =-minuteinjection times are ade*uate for formations ith permeability exceeding = md#:

=# Attempt to obtain at least three readings above and three readings belo the :parting:pressure#

# 6nsure that the pressure and rate gauges are calibrated and ill accommodate thelargest anticipated pressures#

+# 'racture gradient can be dependent on the average reservoir pressure# %s there anindependent measurement of the reservoir pressure (temporal! or can a measurementbe made in conjunction ith the test7

# After the rate increasing segment" t o approaches are possible# The first is to backdo n on the rates to assess if the cement has been damaged# Alternatively" somespecialists prefer a long falloff after the last rate because it can be analy&ed to getfracture properties (dimensions!# >eterioration of the cement bond can be a cause of altered step rate signatures or a risk to the injection process if there are multiple &onesand$or a*uifers#

.# 1ompile all data to determine if there is a consistent relationship bet een apparent in-situ stress and reservoir pressure#

0# ?here possible" combine falloff testing ith the measurements" as described in item "above#

Where Possible, Falloff Should Be An Integral Part Of Step Rate Testing.

#ow Can $ou %e Fooled&

The signature on the pressure-rate curve can be anomalous if there are reservoir variations ormechanical failure occurs during the testing# 'or example@

'ifferent fluid loss. %f the fracture gro s out of &one into a different fluid loss regime" theslope of the post fracturing curve can vary and may not be constant# %f step rate data fromdifferent tests are being compared" recogni&e that the slopes can be different if the ater*uality is different# This is sho n in 'igure +# Some people only sho the fracturing behavior



changing (not behavior during radial flo - matrix injection!# Strictly" this is not true# Slightchanges can also be seen in the portion of the curve before fracture opening$reopening#Recogni&e that slope changes can be associated ith different fluid characteristics#

Figure (. ) schematic variation *conceptual data+ of how pressure signatures canvary for different water ualities *after Murray+. The slopes, after fracturing occurs,

is dependent on the fluid loss. -ith decreasing water uality, there is reduced fluidloss, but there is additional fracture growth.

'ifferent fracture geometry. As a note of caution" this behavior is contingent on a fracturethat is contained ithin one &one# uch more complex behavior can occur if out-of-&onegro th occurs# %f the fracture gro s out-of-&one dramatically" the excess pressure maydecrease because of the gro th more than the change in pressure due to friction in thefracture# A negative slope may appear in the post-fracturing regime# This is illustratedschematically in 'igure # The negative slope can be related to gro th into a higherpermeability &one" rapid out-of-&one gro th" etc# 6ventually" depending on the degree of particle plugging at the tip and on the surface of the fracture" the post-fracturing slope maychange strictly due to changes in efficiency#

"ossibly different perforation friction and completion efficiency. %f at all possible"bottomhole pressures should be used# 'or example" in a layered situation" one &one mayfracture and the conformance can be changed completely# 9r" the pressure drop through theperforations may be different than hat you have anticipated (refer to 'riction!#

'amaged cement bond. The inflection point can indicate failure of the cement sheath# %t hasbeen speculated that it may be possible to diagnose this if rates are reduced during the test(step-do n after the highest rate of injection!# %njectivity ill apparently remain high (alongthe second slope! even at rates belo the original inflection point# The argument against thisphilosophy is that if the original perforations covered the entire height" there should not beadditional injectivity once a fracture closes# %f the test as through a partial set of perforations"

the stepdo n should be on a slope that corresponds to a ne kh# 1onsidering thesearguments" another reason for the increased injectivity belo the original inflection point(during stepdo n - reduction in rate! may be residual fracture conductivity#



'alloff behavior - it may be a toss up bet een doing a stepdo n and a falloff test# 'alloff couldbe important# %t is very off dominated by the reservoir (short fracture closure time and givesdata about stress-dependent permeability and residual permeability enhancement#

A good compromise" if possible" may be to combine both techni*ues - step up in rates"follo ed by a step do n cycle (the step do n procedure may provide indications of hether ornot there has been fracture gro th out-of-&one" etc#!" follo ed by stepping back up and afalloff# This may be the premium procedure and modifications may be re*uired because of

operational constraints and available time#

"ac er bypass. >etermining the potential for exceeding the differential pressure limits of isolation devices (if any! depends on the configuration# %t may be possible to monitor backsidepressure or have a bomb belo the lo er packer (if used in a straddle configuration!#

/pen0pre e2isting fracture. %deally" the pressure time plot for a step-rate test ould looklike that sho n in 'igure .# /o ever" if there is a pre-existing fracture" it ill al ays haveconductivity" even at pressures belo the reopening pressure# This is particularly true if it isself-propped (jammed open! or if e are testing a converted production ell that has beenconventionally stimulated# ,nder these circumstances" inflection may not be seen or there maybe a slight curvature follo ed by a straight line linear or bilinear flo regime# To detect linear

and bilinear flo regimes" log-log pressure-time plotting may be helpful# 6xamples of behaviorfor a pre-existing" closed and a pre-existing" self-propped fracture are sho n in 'igures 0 and2#

Settari and ?arren (6urorock" 88.! schematically summari&ed the influence of a pre-existingfracture (or at the opposite extreme" positive skin! on the step rate test signature# This issho n in 'igure 3#

Transient 3ffects. >epending on the volumes injected" thermal effects can come into play"either due to viscosity changes or in-situ stress changes# %n conjunction ith this" it isextremely important to incorporate any changes in reservoir pressure if you are comparing SRTdata taken at different times in the injection life cycle#

'ifferent Stress 4evels. %n comparing consecutive step-rate test programs" be certain thatyou are a are of any stress field alterations that have occurred due to poroelastic and$orthermoelastic effects# easured differences can in fact be diagnostic of the stress changesassociated ith temperature fluctuations# 'igure 5 is an example# %t is not a step rate test perse# Rather it is a compilation of rate versus injection data for an actual field situation#



Figure 5. ) schematic *conceptual data+ indicating how pressure and rate may beaffected by out of 6one growth.

Figure 7. )n ideali6ed, readily interpretable step rate test. This is conceptual data.



Figure 8. )n ideali6ation of a closed *but still conductive+ pre e2isting hydraulicfracture.

Figure 9. )n ideali6ation *conceptual data+ of a self propped, pre e2isting hydraulicfracture.



Figure . )n ideali6ation of the influence of fracture conductivity on step rate testsignatures *presuming propped, unpropped and damaged fractures+.

Figure :. This figure shows step rates for Forties )lpha F)8 5. The "- points lie ona steeper line than the S- points. If water uality effects were small then mobilityeffects would be e2pected to ma e the "- line shallower. ) "rudhoe %ayperformance plot shows the reverse trend to Forties. It therefore appears that thehigh oil and solids content in Forties "- reduces in;ectivity dramatically. The effectappears to be larger than in "rudhoe %ay and this may be the result of highercontaminant concentrations and larger particle si6es.





Figure <. These data, from a well controlled and monitored pilot in the Shell 3iderfield, show the variation of measured stress levels with temperature. The lines are allappro2imately parallel, indicating consistency in the uality of the in;ected water *orinsensitivity+. The inferred local in situ total stresses, as a function of temperature,are shown in Figure 1=.

Figure 1=. 32cess pressure at the inflection for each temperature regime shown inFigure <. This clearly shows the thermoelastic elevation in the local total in situstress as a function of temperature.



Figure 11. 4ong term variation of wellhead pressure and in;ection rate, showing theinfluence of temperature and uality. These are actual field data.

'ata )nalysis

# The simplest" and least desirable" method of analysis is to plot surface pressures at theend of each step versus injection rate ('igures + and !# 'igure highlights thedifficulties in ready interpretation of surface data alone#

+# %t is preferable to use measured or inferred bottomhole pressure data in these plots('igure .!# The bottomhole pressure in 'igure . as calculated# Bresuming that thecalculations are reasonable" delineation of the fracture opening$reopening pressure isimproved# easured bottomhole pressure and temperature are even more desirable"depending on the particular situation (operational and economic considerations!#

# %t is also desirable to consider this as a multi-rate test and to process the dataaccordingly# The procedures for this are described belo #

Multi >ate Testing )nalysis

# 'igure 0 sho s a generic representation of flo and time behavior during a step ratetest#

+# A step- ise approximation is used#



Figure 1(. "lot of surface pressure versus rate for an actual step rate test. Thebottomhole pressure was estimated, accounting for the hydrostatic head andfrictional effects.

Figure 15. "lot of surface pressure versus rate for an actual step rate test,emphasi6ing the difficulties in pic ing inflection points, or multiple mechanismsoccurring *such as reopening a pre e2isting fracture, more than one fractureopening, etc.+.



Figure 17. "lot of inferred bottomhole pressure versus rate for an actual step ratetest. Some of the difficulties in pic ing an inflection point may be overcome by thesemethods. The difficulty *as evident from the one outlier+ is the assumptions used forcalculating frictional pressure drops.

The premise is that multiple-rate transient data should appear as a straight line hen plottedas@

These plots ill not be done properly unless the analyst understands the meaning of thevariables# The rate corresponding to each plotted pressure point is * n# This is the last rate

hen that pressure point as measured# As time increases" the number of rates may increaseand the last rate may change< but each pressure point is identified ith the rate occurring

hen that pressure as measured# There may be several pressure points associated ith agiven rate# This techni*ue can help clarify inflection points# 'urthermore" skin and permeabilitycan be estimated# To understand the importance of using this techni*ue" it can be envisionedthat it accounts for changes in pressure conditions due to the injection that has preceded anyparticular stage#



The units in all of the e*uations used for these analyses are@

k absolute permeability (md!"

) formation volume factor (R)$ST)!"

µ viscosity (cB!"

m slope of multi-rate plot (psi$ST)B>$cycle!"

h formation thickness (feet!"

b intercept of multi-rate plot (psi$ST)>!"

φ p porosity (fractional!"

c t total system compressibility (psi - !" and"

r ellbore radius (feet!#

Figure 18. ) schematic representation of a step rate test. The nomenclatureindicates the parameters to be used in multi rate test analyses.

32amples

'igure 2 sho s an example data set# There is an inflection point at a surface pressure of "=== psi# 'or illustrative purposes" assuming no friction" a depth of 3+2= feet and a fluid

pressure gradient of =#. psi$ft" === psi gives an estimated fracture opening$reopeningpressure gradient of =#03 psi$ft# The data can be further analysed for formation propertiesusing multi-rate analysis methods# This is sho n in 'igure 3# The first four points fall on onecurve" indicating pseudo-radial flo #



%n 'igure 3" the higher rate points do not fall on a straight line because the assumptions of radial" infinite acting flo are no longer satisfied" since fracturing has occurred#

Figure 19. )n e2ample step rate analysis *reported in 3arlougher, 1< , andoriginally from Felsenthal, 1< 7+. The multi rate plot for these data is shown inFigure 1 .

An 6xcel file (for the foregoing example! is attached (click button belo !" indicating ho multi-rate analyses are carried out#



Figure 1 . Multi rate analysis for the data set shown in Figure 19.

'igures 5 and 8 sho rate and processed step-rate data from B1B Rosemary 2-=3-++-2?.# 9nly surface data ere used in this evaluation# Ho information as available on the

times for each injection stage# A parametric variation in times as implemented# Three casesare sho n# The first is for arbitrary e*ual times# There is no consistent or interpretable

behavior# The pressure data are not clarified at all and it is still difficult to infer a fractureopening pressure# This may have been the real situation and there could have been a gradualtransition from radial to linear or bilinear (fractured! behavior# 9r" possibly the time stepsbecame shorter for each subse*uent injection stage# An arbitrary example is sho n by the redcircles in 'igure 8# T o straight lines can be delineated" indicating a surface fracture openingpressure of 2#= Ba# 'inally" another situation is arbitrarily assumed# The first three stages

ere taken to be at relatively long injection times and all follo ing stages ere short# Thisgives a dramatically different signature that is relatively meaningless# %f injection times ereavailable" some of the uncertainty in this data set could be removed (i#e#" in 'igure 5" can adiscreet fracture opening$reopening pressure be determined by processing the data andaccounting for previous response in the reservoir7!# 'igure 8 sho s that" incorporating timeeffects and being consistent in the performance of each injection cycle could make a significantdifference in interpretation of the data# Time information is not available for this case" so noimproved interpretation is possible# The example is sho n strictly to indicate the importance of accounting for duration of each injection stage#

This is not an academic exercise# %t is intended to demonstrate@

# %t is desirable to use e*ual injection time periods#

+# %f this is not done" at least record the time at hich pressure stabili&ation occurred andpreferably record" pressure-time data (at the very least at the surface! in detail#

# 9ther ise" meaningful and *uality-controlled interpretation may not be possible#



Figure 1:. >aw, surface step rate data from "C" >osemary 19 = (( 19-7. It isdifficult to determine a distinct reopening pressure either because of frictionaleffects, variable in;ection time effects, reopening of a conductive fracture or evenmore complicated fracture growth behavior. Multi rate analysis can help to removesome of the uncertainty.

Figure 1<. )rbitrary processing of surface step rate data from "C" >osemary 19 =(( 19-7, showing the influence of different in;ection stage times. Therecommendation is that uniform time steps should be used and that multi rateevaluations are important for discriminating behavior.

More >igorous 3valuation of S>Ts



SRT data actually contains much more information than hat is used in the techni*uesdescribed above" both about reservoir and fracture properties# /o ever" to access thisinformation it is necessary to@

# Record data continuously at a reasonably high fre*uency"

+# %nclude a long falloff after the last stage" and"

# Analy&e the data by simulation rather than by graphical means#

)nalysis %elow Fracture /pening "ressure

6ach step belo the fracture initiation$opening$reopening pressure is a transient event# Thereis a large slope initially and a small slope at the end of a constant rate# 9nly the end point isused in graphical analyses# Simulation of the early stages can be carried out ith conventionalmodels and early and late slopes can be matched# These matches ill yield both kh andsystem compressibility# The simulation can incorporate any kno n ellbore skin components#

?hen the simulation starts to deviate from the data" this is an indication of induced fracturing"or increasing conductivity of a pre-existing fracture# At this point" the conventional model

cannot give any more detailed information" beyond indicating the approximate position of departure#

)nalysis )bove Fracturing "ressure

1oupled fracture-reservoir modeling is re*uired at this point# These can be models usingfracture mechanics principles" partially coupled (e#g#" G69S% " ith fracture coupling! or fullycoupled ()B s model" after 1lifford" et al#!# The process can be also modeled as re-opening of

joints or creating a high permeability channel (Iisage" or G69S% ith stress-dependentfracture representation!#

Simultaneous matching of the rate steps and the falloff ( hile keeping the reservoir propertiesfrom the match belo fracturing pressure! allo s estimation of@

# fracture gro th (geometry! ith time"

+# fracture conductivity (adjustments to theoretical predictions!"

# rock mechanics parameters (controlling net propagation pressure!"

.# the initial minimum stress" and"

0# possibly thermoporoelastic effects#

The estimate of the undisturbed minimum stress ill generally differ from the graphicalmethod result and can be lo er for a multitude of reasons#

The pre-existing fracture can be incorporated in the simulation analysis and ill generallyresult in more gradual change of the slope" as seen on the Rosemary example#

%n general" simulation analysis can give clear ans ers in some complex cases that cannot beinterpreted by the graphical methods# /o ever" this is achieved at a considerably higher effort#

The first step is to look at basic radial flo relationships and see if radial flo at the lo ratesmakes sense# The first step is graphical#

The second is to match the lo and high rate ends ith analytical models to look at the t olimiting situations" ith and ithout a fracture# %f there is a deviation from lo rate predictionand the actual data (in the right direction! it indicates a good test#

The final step" if arranted" is numerical analysis to make all parts fit together#



%t is important to take into account pre-existing fractures# 'or matching pressures belo p foc "(closure pressure! determine the reservoir permeability and the fracture conductivity andforecast to higher rates# The point of departure bet een this theoretical curve and themeasured data is an independent measure of the value of p foc # Above p foc " it is necessary toiterate ith a fracture model to match the measured data#

Some operators evaluate SRT ith spreadsheet models that have a radial flo and a fractureflo component#

Documents

Guidelines for Step Rate Testing