Guidelines for Step Rate Testing This 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 pseudomatrix - a fracture may already be present but the bulk of the flo is radial! flo to fracturedominated injection" according to a change in slope of a plot of pressure versus rate# Step rate testing testing allos allos for determinati determination on of hen a ne hydraulic hydraulic fracture occurs and$or hen a preexisting fracture opens$propagates# Step Step rate rate testin testing g allos allos for determi determinin ning g hen hen a fractu fracture re ill ill propaga propagate te and hen hen a prepreexisting fracture ill reopen# %t can be run after a conventional falloff or a final falloff segment can be used in the test# Repeated falloff testing can also be used to assess if a reservoir has been altered by thermal changes in in-situ stresses or changes in kh associated ith thermal effects# %njection is carried out at a number of rates belo fracturing pressure# At each rate injection continues continues until stabili&atio stabili&ation n appears appears to occur# occur# The injection injection operations operations are continued continued after indications of fracture opening$propagation# The opening pressure is inferred from a significant change 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 possibl possible" e" have a reason reasonabl able e concep conceptio tion n of hat hat the in-situ in-situ stress stresses es are so that that the step rate test can be appropriately designed# )e certain that there ill be ade*uate data points before breakdon or reopening of the fracture#
+# ,nder any circumstances" have an approximation of the conversion beteen surface and bottomhole treating pressure# %n addition to determining in-situ stress levels" this can be useful for evaluating the completion efficiency# # ,nder certain" restricted circumstances onshore" dead string calibration may be possible if the backside has enough integrity# .# %f reasonable" obtain bottomhole pressure measurements# %f bottomhole data are being ac*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 on vacuum# Some operators ill not opt for tandem gauges if the supplier is reliable" if past history shos 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-rate analysis# 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 breakdon 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 fracture groth can occur# 8# %t has been recommended that each time step is one hour long to ensure that the radius of investigation is large enough# 9bviously" this may not alays be practical# 6ither shorter times are called for because of economic or operational limitations or substantially 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 lo permeability formations (k ; 0 md!" each injection should last for one hour< =-minute injection 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 the largest anticipated pressures# +# 'racture gradient can be dependent on the average reservoir pressure# %s there an independent measurement of the reservoir pressure (temporal! or can a measurement be made in conjunction ith the test7 # After the rate increasing segment" to approaches are possible# The first is to back don on the rates to assess if the cement has been damaged# Alternatively" some specialists prefer a long falloff after the last rate because it can be analy&ed to get fracture 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 &ones and$or a*uifers# .# 1ompile all data to determine if there is a consistent relationship beteen apparent insitu 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 $ou %e Fooled& The signature on the pressure-rate curve can be anomalous if there are reservoir variations or mechanical failure occurs during the testing# 'or example@ 'ifferent fluid loss. %f the fracture gros out of &one into a different fluid loss regime" the slope of the post fracturing curve can vary and may not be constant# %f step rate data from different tests are being compared" recogni&e that the slopes can be different if the ater *uality is different# This is shon in 'igure +# Some people only sho the fracturing behavior
changing (not behavior during radial flo - matrix injection!# Strictly" this is not true# Slight changes can also be seen in the portion of the curve before fracture opening$reopening# Recogni&e that slope changes can be associated ith different fluid characteristics#
Figure (. ) schematic variation *conceptual data+ of how pressure signatures can vary for different water ualities *after Murray+. The slopes, after fracturing occurs, is dependent on the fluid loss. -ith decreasing water uality, there is reduced fluid loss, but there is additional fracture growth. 'ifferent fracture geometry. As a note of caution" this behavior is contingent on a fracture that is contained ithin one &one# uch more complex behavior can occur if out-of-&one groth occurs# %f the fracture gros out-of-&one dramatically" the excess pressure may decrease because of the groth more than the change in pressure due to friction in the fracture# A negative slope may appear in the post-fracturing regime# This is illustrated schematically in 'igure # The negative slope can be related to groth into a higher permeability &one" rapid out-of-&one groth" etc# 6ventually" depending on the degree of particle plugging at the tip and on the surface of the fracture" the post-fracturing slope may change 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 may fracture and the conformance can be changed completely# 9r" the pressure drop through the perforations 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 has been speculated that it may be possible to diagnose this if rates are reduced during the test (step-don after the highest rate of injection!# %njectivity ill apparently remain high (along the second slope! even at rates belo the original inflection point# The argument against this philosophy is that if the original perforations covered the entire height" there should not be additional injectivity once a fracture closes# %f the test as through a partial set of perforations" the stepdon should be on a slope that corresponds to a ne kh# 1onsidering these arguments" another reason for the increased injectivity belo the original inflection point (during stepdon - reduction in rate! may be residual fracture conductivity#
'alloff behavior - it may be a toss up beteen doing a stepdon and a falloff test# 'alloff could be important# %t is very off dominated by the reservoir (short fracture closure time and gives data about stress-dependent permeability and residual permeability enhancement# A good compromise" if possible" may be to combine both techni*ues - step up in rates" folloed by a step don cycle (the step don procedure may provide indications of hether or not there has been fracture groth out-of-&one" etc#!" folloed by stepping back up and a falloff# This may be the premium procedure and modifications may be re*uired because of operational constraints and available time# "acer 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 backside pressure or have a bomb belo the loer packer (if used in a straddle configuration!# /pen0pree2isting fracture. %deally" the pressure time plot for a step-rate test ould look like that shon in 'igure .# /oever" if there is a pre-existing fracture" it ill alays have conductivity" even at pressures belo the reopening pressure# This is particularly true if it is self-propped (jammed open! or if e are testing a converted production ell that has been conventionally stimulated# ,nder these circumstances" inflection may not be seen or there may be a slight curvature folloed 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 behavior for a pre-existing" closed and a pre-existing" self-propped fracture are shon in 'igures 0 and 2# Settari and ?arren (6urorock" 88.! schematically summari&ed the influence of a pre-existing fracture (or at the opposite extreme" positive skin! on the step rate test signature# This is shon 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 is extremely important to incorporate any changes in reservoir pressure if you are comparing SRT data taken at different times in the injection life cycle# 'ifferent Stress 4evels. %n comparing consecutive step-rate test programs" be certain that you are aare of any stress field alterations that have occurred due to poroelastic and$or thermoelastic effects# easured differences can in fact be diagnostic of the stress changes associated ith temperature fluctuations# 'igure 5 is an example# %t is not a step rate test per se# Rather it is a compilation of rate versus injection data for an actual field situation#
Figure 5. ) schematic *conceptual data+ indicating how pressure and rate may be affected by outof6one growth.
Figure 7. )n ideali6ed, readilyinterpretable steprate test. This is conceptual data.
Figure 8. fracture.
)n ideali6ation of a closed *but still conductive+ pree2isting hydraulic
Figure 9. )n ideali6ation *conceptual data+ of a selfpropped, pree2isting hydraulic fracture.
Figure . )n ideali6ation of the influence of fracture conductivity on step rate test signatures *presuming propped, unpropped and damaged fractures+.
Figure :. This figure shows step rates for Forties )lpha F)85. The "- points lie on a steeper line than the S- points. If water uality effects were small then mobility effects would be e2pected to mae the "- line shallower. ) "rudhoe %ay performance plot shows the reverse trend to Forties. It therefore appears that the high oil and solids content in Forties "- reduces in;ectivity dramatically. The effect appears to be larger than in "rudhoe %ay and this may be the result of higher contaminant concentrations and larger particle si6es.
artins et al#" 88." discussed thermoelastic effects at Brudhoe )ay@ :The pressure re*uired to open an induced fracture depends both on the initial stresses in the rock and on stress changes induced by injection at different temperature and pressure# %t has been knon since early step-rate tests that produced ater has a higher fracture gradient (=#03 - =#2= psi$ft! than seaater (=#0 - =#0. psi$ft! Cgradient values are for Brudhoe )ayD# This as linked to the higher ell-head temperature of produced ater (0=E' versus 5=E'!# ost Brudhoe )ay injectors have alternated periods of seaater and produced ater injection over the subse*uent = years# %t is found almost ithout exception that injectivity is poorer for produced ater than for seaater" typically by =-0=F#: :C%njection data for ?ell /-=8% are available on a day-to-day basis#D The ell has sitched 2 times beteen S?% and B?R% over a 3 year period# The points corresponding to S?% lie approximately on a straight line hich intersects the pressure axis at about == psi ?/B# The intersection of the straight line ith the pressure axis is a good measure of the fractureopening pressure" since step-rate tests indicate very lo rates (typically less than === bbl$day! beneath fracture pressure# The points corresponding to produced ater injection lie on a different straight line" intersecting the pressure axis at about 8== psi# Broduced ater typically re*uires 0== psi greater pressure than seaater" in order to inject at the same rate#: :These general features are reflected in data collected from across the field# There are variations of up to a fe hundred psi in the values of fracture pressure (and substantial variations in rate" depending on permeability" intervals of perforation and other factors!" but the comparison beteen seaater and produced ater follos a common pattern#: :The factors hich can cause a difference in per formance are ater temperature (3=E' armer for produced ater!" viscosity (about a factor of + loer for produced ater" hich ould therefore double the rate if all other factors ere e*ual! and ater *uality# The dominant factor e believe to be temperature" because of thermoelastic stress# %t appears that fractures are often likely to be shorter in length for produced ater injection" in spite of the higher pressure#: There is a substantial amount of other data available that indicates the influence of temperature on the injection pressure# 9ne particularly good example is from the 6ider pilot program ('igures 8 and =!# 'igure 8 plot raises certain unansered issues# These include is there a methodology for fitting both sections of the curve# %f measurements are accurately performed" the value of each point is significant and reflects specific occurrences in the reservoir# %f friction is reliably considered" the curves should be forced through the origin on a plot of this nature here the y-axis is the bottomhole pressure (measured in this case! minus the reservoir pressure# This is one of the difficulties of the plot shon in 'igure # %deally" pressure differential plots should pass through the origin if the reservoir pressure and friction are knon appropriately# 'or more information on changes in stress due to thermal and pore pressure changes@
'inally" 'igure demonstrates that the concepts of step rate testing can be used for evaluating long-term injection data (day by day plots of pressure versus rate! to evaluate changes in stress levels and conformance#
Figure <. These data, from a wellcontrolled and monitored pilot in the Shell 3ider field, show the variation of measured stress levels with temperature. The lines are all appro2imately parallel, indicating consistency in the uality of the in;ected water *or insensitivity+. The inferred local insitu total stresses, as a function of temperature, are shown in Figure 1=.
Figure 1=. 32cess pressure at the inflection for each temperature regime shown in Figure <. This clearly shows the thermoelastic elevation in the local total insitu stress as a function of temperature.
Figure 11. 4ongterm variation of wellhead pressure and in;ection rate, showing the influence 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 the end of each step versus injection rate ('igures + and !# 'igure highlights the difficulties 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 the calculations are reasonable" delineation of the fracture opening$reopening pressure is improved# 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 data accordingly# The procedures for this are described belo#
Multi>ate Testing )nalysis # 'igure 0 shos a generic representation of flo and time behavior during a step rate test# +# A step-ise approximation is used#
Figure 1(. "lot of surface pressure versus rate for an actual step rate test. The bottomhole pressure was estimated, accounting for the hydrostatic head and frictional effects.
Figure 15. "lot of surface pressure versus rate for an actual step rate test, emphasi6ing the difficulties in picing inflection points, or multiple mechanisms occurring *such as reopening a pree2isting fracture, more than one fracture opening, etc.+.
Figure 17. "lot of inferred bottomhole pressure versus rate for an actual step rate test. Some of the difficulties in picing an inflection point may be overcome by these methods. The difficulty *as evident from the one outlier+ is the assumptions used for calculating frictional pressure drops.
The premise is that multiple-rate transient data should appear as a straight line hen plotted as@
These plots ill not be done properly unless the analyst understands the meaning of the variables# 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 increase and 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 a given rate# This techni*ue can help clarify inflection points# 'urthermore" skin and permeability can be estimated# To understand the importance of using this techni*ue" it can be envisioned that it accounts for changes in pressure conditions due to the injection that has preceded any particular 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$ST)>!"
φ p porosity (fractional!"
ct total system compressibility (psi-!" and" r ellbore radius (feet!#
Figure 18. ) schematic representation of a steprate test. The nomenclature indicates the parameters to be used in multirate test analyses.
32amples 'igure 2 shos an example data set# There is an inflection point at a surface pressure of "=== psi# 'or illustrative purposes" assuming no friction" a depth of 3+2= feet and a fluid pressure gradient of =#. psi$ft" === psi gives an estimated fracture opening$reopening pressure gradient of =#03 psi$ft# The data can be further analysed for formation properties using multi-rate analysis methods# This is shon in 'igure 3# The first four points fall on one curve" 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 steprate analysis *reported in 3arlougher, 1<, and originally from Felsenthal, 1<7+. The multirate plot for these data is shown in Figure 1. An 6xcel file (for the foregoing example! is attached (click button belo!" indicating ho multirate analyses are carried out#
Figure 1. Multirate 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 cases are shon# 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 fracture opening pressure# This may have been the real situation and there could have been a gradual transition from radial to linear or bilinear (fractured! behavior# 9r" possibly the time steps became shorter for each subse*uent injection stage# An arbitrary example is shon by the red circles in 'igure 8# To straight lines can be delineated" indicating a surface fracture opening pressure of 2#= Ba# 'inally" another situation is arbitrarily assumed# The first three stages ere taken to be at relatively long injection times and all folloing stages ere short# This gives a dramatically different signature that is relatively meaningless# %f injection times ere available" some of the uncertainty in this data set could be removed (i#e#" in 'igure 5" can a discreet fracture opening$reopening pressure be determined by processing the data and accounting for previous response in the reservoir7!# 'igure 8 shos that" incorporating time effects and being consistent in the performance of each injection cycle could make a significant difference in interpretation of the data# Time information is not available for this case" so no improved interpretation is possible# The example is shon strictly to indicate the importance of accounting for duration of each injection stage# This is not an academic exercise# %t is intended to demonstrate@ # %t is desirable to use e*ual injection time periods# +# %f this is not done" at least record the time at hich pressure stabili&ation occurred and preferably record" pressure-time data (at the very least at the surface! in detail# # 9therise" meaningful and *uality-controlled interpretation may not be possible#
Figure 1:. >aw, surface steprate data from "C" >osemary 19=((19-7. It is difficult to determine a distinct reopening pressure either because of frictional effects, variable in;ection time effects, reopening of a conductive fracture or even more complicated fracture growth behavior. Multirate analysis can help to remove some of the uncertainty.
Figure 1<. )rbitrary processing of surface steprate data from "C" >osemary 19= ((19-7, showing the influence of different in;ection stage times. The recommendation is that uniform time steps should be used and that multirate evaluations are important for discriminating behavior.
More >igorous 3valuation of S>Ts
SRT data actually contains much more information than hat is used in the techni*ues described above" both about reservoir and fracture properties# /oever" to access this information it is necessary to@ # Record data continuously at a reasonably high fre*uency" +# %nclude a long falloff after the last stage" and" # Analy&e the data by simulation rather than by graphical means#
)nalysis %elow Fracture /pening "ressure 6ach step belo the fracture initiation$opening$reopening pressure is a transient event# There is a large slope initially and a small slope at the end of a constant rate# 9nly the end point is used in graphical analyses# Simulation of the early stages can be carried out ith conventional models and early and late slopes can be matched# These matches ill yield both kh and system compressibility# The simulation can incorporate any knon 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 using fracture mechanics principles" partially coupled (e#g#" G69S%" ith fracture coupling! or fully coupled ()Bs 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-dependent fracture representation!# Simultaneous matching of the rate steps and the falloff (hile keeping the reservoir properties from the match belo fracturing pressure! allos estimation of@ # fracture groth (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 graphical method result and can be loer for a multitude of reasons# The pre-existing fracture can be incorporated in the simulation analysis and ill generally result in more gradual change of the slope" as seen on the Rosemary example# %n general" simulation analysis can give clear ansers in some complex cases that cannot be interpreted by the graphical methods# /oever" 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 rates makes sense# The first step is graphical# The second is to match the lo and high rate ends ith analytical models to look at the to limiting situations" ith and ithout a fracture# %f there is a deviation from lo rate prediction and 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 and forecast to higher rates# The point of departure beteen this theoretical curve and the measured data is an independent measure of the value of p foc# Above pfoc" it is necessary to iterate ith a fracture model to match the measured data# Some operators evaluate SRT ith spreadsheet models that have a radial flo and a fracture flo component#
