HOW WELLBORE DYNAMICS AFFECT PRESSURETRANSIENT ANALYSIS

L. MATTAR M.S. SANTO

this article begins on the next page FFPETROLEUM SOCIETY OF CIM and AOSTRA PAPER NO. CIM / AOSTRA 91-41 ATM-17 THIS IS A PREPRINT - SUBJECT TO CORRECTION HOW WELLBORE DYNAMICS AFFECT PRESSURE TRANSIENT ANALYSIS BY Louis Mattar Fekete Associates Inc. M. S. Santo Fekete Associates Inc. PUBLICATION RIGHTS RESERVED THIS PAPER IS TO BE PRESENTED AT THE CIWAOSTRA 1991 TECHNICAL CONFERENCE IN BANFF, APRIL 21-24,1991. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THE TECHNICAL MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN CIM JOURNALS IF FILED INWRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING. ABSTRACT There are numerous technical papers on pressure transient analysis which address both- simple and complex RESERVOIR-related phenomena. It is a fact of life, however, that the pressure recorder is located in a WELLBORE, not in The RESERVOIR, but connected to a reservoir. 'ne WELLBORE is an intrinsic link between the RESERVOIR and the RECORDERS. The WELLBORE can, and often does, interact dynamically with the RESERVOIR, and this effect is readily observed in well testing. 'ne many WELLBORE relatedphenomena that can have a significant effect on the measured pressure have not been addressed in the literature, except for wellbore storage and the classical phase redistribution hump. This paper presents several examples of tests that have been affected by wellbore dynamics, and shows that these could easily have been misinterpreted as complex RESERVOIR phenomena (dual porosity, etc ... ) instead of WELLBORE ffe e cts. These effects are often highlighted by the (semilog) derivative which is traditionally used to diagnose. RESERVOIR characteristics. The fact that these ar; WELLBOREand FM RESERVOIR effects must be established to prevent misdiagnosis. Often this can only be done from examination of The test data other than the recorded pressure-time trace. 41-1 INTRODUCTION _ There have been more than 1,000 papers published on Pressure Transient Analysis. Tlese publications address in great detail such fundamental topics as homogeneous infinite reservoirs, fractures, convolution, dual porosity, multi-layers..., or such esoteric concepts as super-bilinea-r- equivalent-pseudo-time or Integral @ecurves.' With the progress made in both analytical and numerical solutionsto reservoir problems, and with readily available powerful computers, virtually every imaginable reservoir description can be modelled and the solution presented in graphical form (Type Curve). In contrast, there are very few papers that deal with wellbore effects. The only two classical wellbore topics that have been studied in imv detail, are the concevfs of Wellbore Storage(') and Phase Redistribution(2). leven then, the treatment has been more theoretical than practical. In our presentation we will address some of the many issues under the overal . I umbrella of 'WellboreDynamics". - These reflect experience with "practical" wellt -st interpretatio , which our e n stems from years of specialization in this ffeld, (more than 10,000 tests have been analyzed in detail by the authors and their colleagues).

PETROLEUM SOCIETY OF CIM and AOSTRA PAPER NO. CIM I AOSTRA 91·41

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THIS IS A PREPRINT • SUBJECT TO CORRECTION

HOW WELLBORE DYNAMICS AFFECTPRESSURE TRANSIENT ANALYSIS

BYLouIs Mauar

FckcID Assoclate9 Inc.

M.S. SanloFekele ABollocialeB Inc:.

PUBLICATION RIGHTS RESERVEDTHIS PAPER IS TO BE PRESENTED AT THE CIMIAOSTRA 1991 TECHNICAL CONFERENCE IN BANFF, APRIL21-24,1991. DISCUSSION OF THIS PAPER IS INVITED. SUCH DISCUSSION MAY BE PRESENTED AT THETECHNICAL MEETING AND WILL BE CONSIDERED FOR PUBLICATION IN CIM JOURNALS IF FILED INWRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

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ABSTRACf

There are numerous technical papers on pressure transientanalysis which address both simple and complexRESERVOIR·related phenomena. It is a fact of life,however, that the pressure recorder is located in aWELLBORE, not in the RESERVOIR, but connected to areservoir. The WELLBORE is an intrinsic link berween theRESERVOIR and the RECORDERS. The WELLBOREcan, and often does, interact dynamically with theRESERVOIR, and this effect is readily observed in welltesting.

The many WELLBORE related phenomena that can have asignificant effect on the measured pressure have not beenaddressed in the literature, except for wellbore storage andthe classical phase redistribution hump.

This paper presents several examples of tests that have beenaffected by wellbore dynamics, and shows that these couldeasily have been misinterpreted as complex RESERVOIRphenomena (dual porosity, etc...) instead of WELLBOREeffects, These effects are often highlighted by the (semilog)derivative which is traditionally used to diagnoseRESERVOIR characteristics. The fact thal these areWELLBORE and ~ RESERVOIR effects must beestablished to prevent misdiagnosis. Often this can only bedone from examination of the tesr data Olher than therecorded pressure-time trace_

41-1

INTRODUCfION

There have been more than 1,000 papers published 00Pressure Transient Analysis. These publications address in~reat detail such fundamental topics as homogeneousInfinite reservoirs, fractures, convolution, dual porosity,multi-Iayers.__, or such esoteric concepts as super-bilinear­equivalent-pseudo-time or Integral typecurves. With theprogress made in both analytical and numerical solutions toreservoir problems, and with readily available powerfulcomputers, virtually every imaginable reservoir descriptioncan be modelled and the solution presented in graphicalform (Type Curve).

In contrast, there are very few papers that deal with wellboreeffects. The only two classical wellbore topics that havebeen studied in any detail, are the concepts of WellboreStorage(l) and Phase Redistrihution(". Even then, thetreatment has been more theoretical than practical. 10 ourpresentation we will address some of the many issues underthe overall umbrella of "Wellhore Dynamics". These reflectour experience with "practical" well test interpretation, whichstems from years of specialization in this treld. (more than10,000 tests have been analyzed in detail by the authors andtheir colleagues).

.'

Pressure (MPP) = Pressure (RRD) +L:>P (Hydrostatic)

In this situation., as seen in Figure 4, the pressure response alRRD is parallel to that at MPP,

In the simplest case of a single pbase nuid and a wellboreschematic, as in Figure 1, the correction from RRD La MPPis:

Is this "ivory tower" thinking, or is the phenomenon real?From the thousands of well tests that we have analyzed, wehave observed these effects on approximately 75% of welltests where more than one phase IS flowing. Here are a fewexamples of very typical situations:

Oil wells flowing oil and ~as to surface: Al the end ofa 2-week shut in for a bUildup test, a static gradient isconducted and shows .!!9 liquid in lhe wellbore.Where did all the oil in the wellbore go?

Gas wells producing at hi~ water-gas ratio: Onceagain, at the end of a builaup test, the static gradientshows not a drop of water above the perforations.

a)

b)

L:>P (Hydrostatic) = H x gradienl

where H is the venical distance between RRD and MPP

gradient is I kPa/m for gas

5 kPa/m for oil (live)

10 kPa/m for water

'When there is mare than one phase present in the wellbore,and the liquid interface is moving up or down, Lhere can besignificant differences in the pressure trends observed on therecorder (RRD) as compared to what is really happening atthe sandface (MPP). We shall illustrate Ihis using Ihe simplecase of a "Changing Liquid Level" in a gas well with water inthe wellbore. The well is producing gas and waler LOsurface, and is shut-in for a buildup lesl. Assume (for thesake of illustration) that the pressure at the sand face (MPP)is increasing in a straight line, as shown by the line labelledMPP in Figures 5, 6 and 7. The liquid (water) level is aboveRRD and is falling, which will occur at differenL raLesdepending on permeability and damage ratio. At Lime L" itreaches RRD, at time t, it is balfway between RRD andMPP (Figure 6) and at time t, lhe liquid level has reached,or is below MPP. In Figure 5, the pressure differencebetween MPP and RRD is a cons tam, equal La thehydrostatic head of water. It stays constanL until the liquidlevel reaches the RRD. From then on. the differencebetween MPP and RRD varies and equals the sum of anincreasin~ gas column plus a decreasing water column.Figure 6 Illustrates when the liquid is halfway between MPPand RRD. When the liquid level is at, or below MPP, thecorrection from MPP to RRD is the hydrosLatic head due loa column of gas.

It is obvious, as is illustrated in Figure 7, that whereas theresponse recorded on the pressure recorder at RRD shows asignificant deviation from linearity, and could be(mis)interpreted as a reservoir phenomenon, the truth is,that the pressure trend at MPP was perlecLly straighL. andthe recorded effect was a wellbore phenomenon, pure andsimple, A rising liquid level would exhibil the opposite trendto that shown in Figure 7.

The topics which will he addressed in this paper willobviously deal with weUbore phenomena However, not allof these wellbore phenomena will be studied. For example,we will not be reviewing the effects of temperature onwellbore fluids or pressure recorders; Dor will we addresssuch topics as gas/oil solution/liberation or retrogradecondensation. Moreover, some o( the ideas discussed do nottake place "in" the wellbore, but in the immediate vicinity ofthe wellbore, and often, their effects are inseparable fromwellbore effects. The "Wellbore Dynamics" that we shallillustrate will be grouped into the following topics:

WELLBQRE DYNAMICS

Liquid Influx/EffluxPhase RedistributionWellbore (and near-wellbore) Clean-upDifferences between Drawdown and BuildupPluggingRecorderMysterious Effects

Many (noL all) of these "wellbore" transients occur aL veryearly time. TheiT detection and documentation has beenfacilitated by the advent of electronic pressure recorders.These recorders are so accurate that they will detect thesewellbore effects very clearly. These effects can often bemisinterpreted a..r;;; "ReservOir" effects. Indeed, the wholethrust of our field of engineering is to interpret the"Reservoir", by using pressures recorded in a "Wellbore".This ofLen places the Reservoir and the Wellbore in a tug-of­war, and we find that, very often., "Wellbore" Transients willdiswn, or even, dominate our "Reservoir" Transients. Thisis not surprising in view of the fact that in this tug-of-war, theWellbore has a distinct advantage over the Reservoirbecause this highly sensitive recorder is sitting in theWellbore and not in the Reservoir.

For the reservoir engineering equatioI15 to be applicable, the"recorded" pressures must obviously be converted to"sandface" (reservoir) pressures when the recorders are seteither~ or .l!rlm!' the producing interval. Thiscorrection is performed by adding or subtracting theappropriate hydrostatic bead of fluid (gas, oil or water), Inthe case of tubing below the perforations, Figure 3, it is veryimportant to recognize that the fluids inside and ou tside thetubing may be different, and separate corrections must bedone inside and outside the tubing. In performing thepressure correction from RRD to MPP, the effects offriction have been ignored to simplify the presentation,thougb in high rate wells, this may cause a significant error.

LIQUID INFLUX/EFFLUX

In lhis section, we shall illustrate that the pressure trendobserved wilh the recorders does nOl necessarily reflect thepressure in the reservoir. All the welltest theory treatSpressure as measured at the sandface. In many cases, it isImpossible to land recorders at the sandface or even at themid-point nf perforations (MPP). In some cases therecorder run depth (RRD) is above MPP as shown onFigure 1, or at MPP - Figure 2, In the wellbore schematic ofFigure 3, tbe RRD is at MPP, but because of the "V-tube"effecl caused by the tubing being below tbe perforations, the~ sandface pressure trend could appear very difTerentlyon the recorders of Figure 2 and Figure 3. This will happenevery time that there is more than one phase in the wellbore,which, in the testing of oil and gas wells, is often the case.

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