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SPE~p~ j~~~~

Acoustic Static Bottomhole Pressuresby J.N. McCoy,* Echometer Co.; A.L. Podio, U. of Texas; and K.L. Huddleston* and B. Drake,Echometer Co.qSPE Members

Copyright198S,Sociityof PetroleumEngineersThispaparwasprsssntadat theSPE 1985ProductionOperationsSymposiumeldinOkIShOMaCky,Oklahoma,Maroh10-12, 1985.Thamaterialssubjsctomrractionbythaauthor.PerrniaaiinocopyisraatrbtsdoanS&tractofnotmorethan300worda.WriteSPE,P.O.Sox833836,Richardson,. . . . . - ., lm.. . . . . -1-d... ~ananaem=ma,,exail ,-. ,. .. . . , .,.,... , . . . . .

AESTIUCT gas gun. On wells having less than 100 psi, thegas gun volume chamber ia pressurized to approxi-Acoustic instruments have been used routinely mstely 100 psi in excess of well pressure. Thefor many years as an aid in analyzing well perfor- gas ia then rapidly released into the well to

mance of normal-pressure oil producers.l create the pressure pulse. On wells having pres-sures in excess of 100 psi, the volume chamber

Recent developments in equipment and techniques in the gaa gun is bled to a pressure leas thannow permit more-accurate calculations of acoustic the well pressure. Then, a valve is rapidlystatic bottomhole pressures at surface pressures opened to permit wellhead pressure to expand intoup to 15,000 psi in corrosive (C02 and H2S) environ- the volume chamber and create a rarefractionments. Equations and charts are presented herein pressure wave.for determining static bottomhole pressures fromacoustic and well data. Also, a special technique A microphone converts the pressure pulsesis recommended for shutting-in a well which in reflected by collars, liquid, and other obstruc-most casea will yield more-accurate results. tions (or changes in area) into electricalsignals which are amplified, filtered, and re-

ThLs method has been programmed for an corded on a strip chart (Fig. 1). The liquidinexpensive, portable notebook-size computer level depth can be determined by counting thewhich can be used in the field to easily perform number of tubing collars to the liquid-levelthese calculations. reflection.INTRODUCTION Changes in cross-sectional area are also

recorded. When these changes are known, theyThe liquid level in a well may be determined can be used as depth references to determineacoustically by generating a pressure pulse at liquid-level depth. Also, the distance to the

the surface and recording the ethos from collara, liquid level can be calculated by travel timeobstructions, and liquid level. from the acoustic chart and acouatic-velocitydata. Acoustic measurements were generally ob-A blank cartridge was the conventional source tained by shooting down the casingitubing

of pressure pulse until development of the modern annulua in packerless completions (Fig. 1).References and illustrations at end of paper.


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2 ACOUSTIC STATIC BOTTOMHOLE PRESSURES SPE 13

However, equipment haa been recently developed forshooting down the inside of close-makeup (flush)tubing at big-npressures.

In most of the deeper, high-pressure gaswells, the casing/tubing annulus is isolated fromthe production string by a packer. Thus, astatic bottomhole pressure must be obtained byahooting down the inside of the tubing.

Weeka2 discusses a high-pressure gas gun(Fig. 2) which will operat= up to 15,000 psiand can be operated through needle valves alreadyinstalled on the well. This gun utilizes gas inthe well to generate the initial pressure pulse.Neither an external gas supply nor a blank isnecessary and the gun is suitable for wellshaving high concentrations of H2S and C02.PRESSURE DETERMINATION

The static bottomhole pressure ia the sumof surface pressure, gas column pressure, andliquid column preasure. The accuracy of eachof th.e~epre~~.~r=~deter~i~~e~the a~~~raey ofthe static bottomhole pressure.

The principal uncertainty in the calculatedbottomhole pressure generally arises from errorain the determination of the liquid column pres-sure. Such errors can be minimized by a shut-inprocedure suggested herein.

The surface caaing pressure can be measuredwith a calibrated pressure gauge or more accuratelywith a dead-weight teeter. In moat cases, thecasing pressure is a high percentage of thestatic bottomhole pressure and an accurate measure-ment ia desired.

The pressure due to a gas column is usuallydetermined by equations and charts using gasgravity andlor gas composition, temperaturegradients, surface pressure, and depth. Tech-niques are offered herein for determining thegas column pressure in most wells.

Liquid column gradients are given herein withcorrections for dissolved gaa, pressure, andtemperature.

A comprehensive manual published by the3Canadian Energy Resources Conservation Boardalso presents useful information for determiningdownhole pressures.

Podio4 discusses a small portable computerto determine bottomhole pressures even in deep,high-temperature, high-pressure wells. Thecomputer and software also calculate the gascolumn pressure in wells which contain non-hydrocarbon gaaes.TUBING NEAR CASING PERFORATIONS

In a conventional oil well, the tubing intakela located near the casing perforationa. Whensuch a producing oil well (liquid level at the

tubing intake) is shut-in, the liquid fill-up inthe casing annulus will be the same ratio of oiland water that ia normally produced by the well(Fig. 3).

If a producing oil well haa liquid above thepump before being shut-down, the liquid abovethe pump in the caaing annulus is oil. If gasis bubbling through the oil, the actual amount ofoil present in the gaseous column can be deter-5mined by6techniques discussed by Swaim & Gipsonor McCoy .

If the caaing and tubing of this producingwell are shut-i~ additional liquid will oftenflow into the casing annulus. This additionalliquid will also be the same ratio of oil andwater that ia produced by the well.

The total oil at static conditions will bethe amount of oil originally present in thecasing annulua, plus the amount of oil thatflowed into the wellbore after shut-in. Theremainder of the liquid column will be water(Fig. 3A).

I ANNULUS GASA special shut-in procedure ia recommendedfor wells producing gas up the casing annulus.

1. Close the caaing valves and continueto pump the well.

2. The casing pressure will increase andgenerally stabilize.3. Shut-down the well and close the tubingd IiqcidalVt2 until che c~!+l?l~ ~~~SSU~6! ~fi.

stabilize.I 4. Then, run the acouatic survey.I

During casing pressure buildup (step 2),the producing bottomhole pressure will increaseas gas flow from the caaing annulus is stopped.The increase in casing preasure depresaeacasing annulus liquid into the pump, reducesthe pump capacity to formation fluids, andresults in a higher producing bottomhole pressure

Note, in a producing well which has gas andliquid flowing into the wellbore, the liquid leveis at the tubing perforations when the well isproduced with the casing valves closed, i.e.,gas collects in the casing annulua and depressesthe liquid level to the tubing perforations.

Often, Mquid doea not flow into ~he well-bore after shut-down (see Brownscombe ). Thus,the static bottomhole pressure is simply theaum of casing pressure and gas column pressure(if no liquid ia above the casing perforations).If liquid buildup occurs, the ratio of oil andwater present in the liquid column will be thesame ratio that is normally produced by thewell (Fig. 4).


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SPE 13810 JAMES N. MCCOY, AUGUSTO L. PODIOAn SBHP Calculation Sheet is enclosed tofacilitate calculating acoustic bottomhole pres-

sures in wells which have the tubing intake nearthe caaing perforations and have the liquid levelat the tubing intake when the well is shut-down..TUBING ABOVE CASING PERFORATIONS

In a producing well which haa the tubingintake located a considerable distance above the~=ei?=~Perfer=tie~&~,~ ~~~.~m_-af l~-..~a-a+- i~,.*UAU G*....the casing that ia approximately the same ratioof oil and water aa that produced by the well.

When such a well (liquid level at the tubingintake) is shut-in, a fill-up of liquid willgenerally occur which ia also the same ratio ofoil and water that is produced by the well.

The oil originally in the casing and theoil in the after-flow will migrate to the topof the liquid column.

This oil will be in the casing annulus unlessthe oil volume exceeds the capacity of the casingannulus above the tubing intake. In that caae,the oil column will extend down into the casing.Water will be below the oil to total depth (Fig. 5).

If liquid exists above a high-pump intakein a producing well, the liquid will be oil. Ifgas is vented at the surface, the column willbe gaseoua. When this well ia shut-down, theafter-flow will also be the same ratio of oil andwater that is produced by the well.

Thus, the total oil will be the originaloil in the caaing annulus above the tubing intake,plus the oil in the casing between the tubing in-take and the caaing perforationa, plus the oilin the after-flow.

This oil column will exist in the caaingannulus, and too may extend below the bottomof the tubing into the caaing.below the oil (Fig. 5A).-US GAS VENTED

If the well is venting gasannuius, a shut-in procedure is

1. Close the casing valve

Water will be

up the casingrecommended.and allow the

-..ae..-C&2Si?.g I-..GZSC3IAL~~ iECiE~SS Uiltil stabilized.If a gaseoua oil column existed above the pump,the oil will be depressed to the tubing intake.

2. After the increased casing pressure hasstabilized, shut-down the well. Observe thecasing pressure to determine when static con-ditions are obtained.

Determine the total volume of liquid thatia present in the caaing annulus above the pumpand the volume that is present in the caaingbetween the formation and the pump. MultiplY _.the totai volume of liquid by the ratio of oilto total liquid (from well test) to determine theamount of oil in the well.

KENETH L. HUDDLESTON, BILL DRAKE 3Determine the height of the oil column in the

casing annulus. Oil may extend below the tubinginto the caaing. The remainder of the liquidcolumn will be water.ANNULUS NOT VENTED

If gas is not vented from the caaing and aliquid column does not exist above the pump,shut-down the well. Monitor the casing pressureznii~orli~uid level to determine wnenthe fiuidahave stabilized.

Determine the total volume of liquid presentin the casing and casing annulus. The amount ofoil present in the well is equal to the totalvolume of liquid multiplied by the ratio of oilto total liquid produced by the well.

The oil will exist in the casing annulusand possibly extend below the tubing intakeinto the casing. The remainder of the liquidbelow the oil will be water.

If the well does DQ~ vent ~SS SEd li~ditiis present above the tubing intake, the liquidabove the tubing intake will be oil without gasbubbles.

The caaing (between the caaing perforationsand the tubing intake when the well ia producing)contains water and oil in approximately thesame ratio that is produced by the well. Deter-mine the volume of oil present in the caaingat producing condition.

When the well is shut-down, additionalliquid will flow into the well. Determine thevolume of liquid flow into the well after shut-down. Determine the amount of oil present inthe after-flow by multiplying the after-flowvolume by the ratio of oil to total liquid.

Determine the total volume of oil in thewell by adding (1) the original oil above thetubing intake (casing annulua), (2) oil thatwas present in the caaing between the tubingintake and the caaing perforations, and (3) the,.41 4.. -k... .S*---=I -..AA Au L,..= aLLs& I.LUW.

The oil will be located in the casing annuluaunless the oil ~xt~P.ds ?)~lQF tk t*~bip.~ i~.t~ thecaa ing. Water will extend from the oil to thecaaing perforations.LIQUID COLUMN PRESSURE

The pressure gradient of oil may be deter-mined from Table 1 or Fig. 6. Fig. 6 should beused to compensate for dissolved gaa and theeffect of higher temperatures. The liquidpressure::is the average oil column pressure.

The specific gravity of produced water canbe determined by hydrometer. The water gradientis 0.433 psilft x specific gravity x correctionfactor. The correction factors for temperature,dissolved gaa, and pressure are given (Fig. 7).


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4 ACOUSTIC STATIC B(GAS COLUMN PRESSURE

The gas column pressure as a function ofspecific gravity, surface pressure and depth iagiven in Figure 8. Assumed conditions are asurface temperature of 60F and a gradient of0.015F/ft. Figure 8 can be used with goodaccuracy for most cases unleaa pressure or depthlimitations are exceeded, temperature conditionsare abnormal, or non-hydrocarbon gases exist.

The gas column pressurethe following equation also:

P = 0.0188 (P) (sg) (L)gc ZT

can be determined by

(1:

where:P=gcP .ag =z .L=T .

preaaure due to gas column, paiaverage gas column pressure, paiaspecific gravity of gas (Air = 1.0)compressibility factorlength of gas column, ft.average gas temperature, R

Below 300 psi, assume Z to be(F. +460). From 300to 2,000 pai, use Fig. 9. hove 2000 Y?: ::the Z factor correlation by Katz, et al

Figurea 10 and 11. Refer to the example problemshown with the figures.

The pressure term and compressibility factorin the above equation should be at the mid-point depth of the gas column. The approximatepressure at the mid-point can be obtained byusing Figure 8 if depth and pressure limits arenot exceeded. If Figure 8 cannot be used, uaethe casing pressure and Z factor at surfacepressure and average gaa temperature. Calculatean approximate gas column pressure. Determinethe mid-point pressure and then a new Z factor.Calculate a new gas column pressure. Iterateuntil the desired accuracy is obtained. In deepwells having a high bottomhole temperature andhigh gas gravity, the column should be dividedinto sections. See Bender and Holden14. Thegas gradient can decreaae considerably downhole.Or, uae the computer.

If the specific gravity of the gas columnis ?=QCkmnun t,~ ~p=cific -.,74+., 4= ~e~a~e~ +,... .. . . . . , 6&-A-J ~- .acoustic velocity. Fig. 12 presenta the specificgravity as a function of acouatic velocity forhydrocarbon gasea at 75F and Dressures beIOW1,000 psi.

Fig. 13 gives the specific gravity at 75Fand pressures above 1,000 psi.

The acoustic velocity should be determinednear the top of the acoustic chart at a depth _environment of 75F. With the 11 point divideraaet on the collar kicks, determine the number

:OMHOLEPRESSURES SPEof column kicks per second. The acoustic velocitis obtained by multiplying the number of collarsper second by the average joint length andthen multiply again by 2 (round trip travel time)Enter acoustic velocity (Figs. 12 or 13) andproceed horizontally to the pressure. Thespecific gravity of the gas is indicated below.

Settling of gaa due to gravity has beenobserved from acoustic velocity data in manywells. Gas specific gravity measured at thesurface using a gas gravity meter may notyield a representative average value.COMPUTER

Huddleston12 discusses a portable computerfor calculating acouatic static bottomholepressures. Entered items include surfacepressure, liquid level depth (or liquid levelacouatic travel time), liquid production data,gas gravity (or composition including non-hydrocarbon gaaes), temperature gradient, andformation depth.

The software determines the gaa-columnpressure from gas-composition data (or acousticvelocity data), oil column pressure (correctedfor dissolved gas, pressure, and temperature),and water column pressure (corrected for dis-solved gas, pressure, and temperature). Thecomputer will also calculate bottomholepreasurea in wells producing very high concentra-tions of C02 gas.

The computer ia certainly more versatilethan the equation and charta procedure presentedherein. Software alao calculates depth fromacoustic travel time when gas propertiesare known.FIELD DATA

Field results of calculated acousticsurea vs. wireline downhole pressurea arefor several wells in Table 2. Some haveextremely high pressures and environmentscorrosive gaaea and high temperatures.

pres-givenof

Excellent results were obtained on almostall wells when gaa and liquid properties wereknown. Calculated pressures were within*1% of meaaured pressures. Expected accuracyis approximately that obtained with conven-tional wireline pressure recording devices.

U4-I.------.-..


5/13

SPE 13810 JAMES N. MCCOY, AUGUSTO L. PODIO0E!!J 12000va

1100

1000.5s .6 .63 .7 .?s .8

SPECIFIC GRAVITY

FIGURE 13

.


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SBHP CalculationWellDateShut-intime and Date1.2.3.4.5.6.7.

6.9.

fo,11.

12.13.

14.

15.

16,

Casing Pressure, PSIGStatic LiquidLevel, distancetrom surface. FT.FormationDepth (Pressure Datum). FT.SurfaceTemperature, FFormationTemperature, FOil Gravity,APIOilGradient, psi~ftuse Table 1 or Figure6Water SpecificGravityWater Gradient = psin.0.433 psi/ft, x S.G. x CorrectionFactor0.433 psilft. x Item 6 x Fig. 70.433 psilft. x x

Au-rzIna CZI. CrdI IWIm Tarnnnrd, I.- i. .~,... ,w ~= , ,,,,, , e!!,p,.a.,- Kz

AcousticVelocity = (Collarskec) x (Avg. Joint Lgt.. Ft.) x 2x x2

Gas SpecificGravity (use Fig.8 or9) =Gas ColumnPressure = psiPgc = 0.0188

= 0.0188x (j~i~g

Well Production:

x P x S.G.x x Item 12x( )xBOPD = BWPD

14aOilColumn Pressure = psiHeightof fiquidcolumn x Oil Gradient(Item 3 Item 2) x Item 7( )xWater ColumnPressure = osiHeightof liquidcolumn

(Item3 Item 2)( - ..._-

SBHP = PSIGCasing Pressure +

Item 1 +

xx

Sheet*-r

Ff/Sec

L + [~x T 1Item 2 + [ x (Item 10 + 460) ]

x . i x( )1= BFPD =14b 14C

x Fractionof 011 in LiquidColumnx Item 14a Item 14C

x Water Gradient x FractionofWater inLiquidColumnx Item 9 x Ifem 14b Item 14s

)x x .

GasColumnPressureItem 13

+

ProducingLiquidLevel fu Pump whichis af Formation.EchometerCompany 817-767-4334

+ OilColumnPressure + Water ColumnPressure+ Item 15 + Item 16+ +

5001 DittoLane Wichita Falls, Texas 76302

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