Spwla-Detection de Gas

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SPWLA 34th Annual Logging Symposium, June 13-16, 1993HYDROCARBON DETECTION USING THE SHEAR W AVE TRAVEL TIMEAND NEUTRON POROSITY IN SANDSTONE RESERVOIRS

Lee Sanders: Halliburton Logging Services, Inc., Midland, Texas

ABSTRACTA successful technique is being used toevaluate sandstone reservoirs for hydrocarbons. The technique consists of crossplottingthe shear wave travel time and the neutronporosity. The crossplot will produce a hydrocarbon indicator based upon the relationship between the travel time and the neutronporosity. The sandstone reservoirs that arehydrocarbon productive will exhibit a greater shear wave travel time than would bepredicted from the neutron porosity. Thosesandstone reservoirs that are non-productiveexhibit a smaller shear wave travel time thanwould be predicted from the neutron porosity. The technique can be applied in thecased hole as well as the open hole loggingenvironment.INTRODUCTION

Most models that use acoustic data forhydrocarbon detection extract compressionaland shear wave data from an acoustic fullwaveform log. (Figure 1). The relationshipbetween the shear wave travel time andcompressional wave travel time can be usedto predict the presence of hydrocarbons. Thepresence of a compressible pore fluid, suchas gas or oil with dissolved gas, causes areduction in the acoustic velocity ratio(ATS/ ATC). There have been excellentpapers written which examine the theoreticaland practica! implications of using the velocity ratio technique (Murphy, et al., 1991;Williams, 1990). These papers considersandstones of the Cretaceous age and younger with fairly high porosities.This paper considers the detection of hydrocarbons in sandstones of the Paleozoic age

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(Figure 2) in the Permian Basin. The technique evolved from an attempt to identifygas producing intervals in cased hole wells(Kessler and Sanders, 1989). In many wellsthe cement bond was of such poor qualitythat a compressional arrival was impossibleto detect. Without a compressional traveltime, there is no velocity ratio to compute.In wells with good bond, it was observedthat the shear travel time approximated thevalues of the neutron porosity except inthose intervals of anomalously low-velocityratio values, which were gas productive.Therefore, in those cases where the cementbonding was inadequate to support a compressional wave, a dipole sonic tool wasused to generate a shear wave. A shearwave travel time could then be compared tothe neutron porosity for gas detection. Thepaper of Iverson and Walker, (1992) provides an excellent study of the empiricalrelationship between sonic and nuclear data.CONCEPf

The mechanism that activates the hydrocarbon indicator is the presumption that theneutron porosity responds to the hydrogencontent of the formation fluids and that theshear wave travel measures formation porosity regardless of the fluid composition.When gas is present in the formation fluids,the presence of the gas will reflectan apparent low neutron porosity value while theshear travel time, responding to "true"porosity, will exhibit a greater shear wavetravel time than would be predicted from theneutron porosity. In "wet" zones, the presence of hydrogen rich fluids produce anapparent high neutron porosity value whi1e


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SPWLA 34th Annual Logging Symposium, June 13-16, 1993

the shear travel time is a smaller value thanwould be predicted from the neutron porosi-ty*In order to establish the proper scales toreflect matrix changes, a least-squares re-gression analysis showed thatATS=(290) * NPHI +80defined the critical point for modifyingtravel time scales. This is to say that thedata points falling above the regression lineare plotted on a 70 to 170 microsecondscale. Those points falling below the regres-sion line are plotted on a 60 to 160 micro-second span. (Figure 4). The regression linewas calculated using a 32 well data set withthe neutron and shear travel times pickedfrom the potential pay zones.LOG EXAMPLESExample 1Figure 5 shows the relationship between theneutron porosity and the shear wave traveltime in differentiating between the produc-tive and non-productive intervals. The wellis a cased hole re-entry from Edwards Cou-nty, Texas. The objective zone is a Canyonsand The reservoir rock in offset wellsconsists of medium grained sandstones. Themineral composition is eighty percentquartz, ten percent potassium feldspars, andten percent dolomite and clays. The meanshear wave travel time and mean neutronporosity in the productive zone is 135 mi-croseconds and 14 percent porosity, respec-tively. There appear to be two other zonesof production at X032 and X070 however,the shaded zone was perforated for CAOF800 MCFPD and no water.Example 2Figure 6 shows a comparison of the neutronporosity and shear travel time in a Kingsand from a cased hole re-entry in Conch0County, Texas. This reservoir rock consists

of medium to fine grained sandstone with amineral composition of ninety percent quartzand ten percent dolomite. The mean neutronporosity is 20 percent and the mean shearwave travel time is 153 microseconds. Thewell was perforated from X030 to X060.The initial production for the well was 145BOPD, 5 BWPD, and GOR of 500: 1.Example 3Figure 7 is an example of an open hole welldrilled in Eddy County, New Mexico. Theexample shows the relationship among theporosity crossplot, water saturation calcula-tions, and the Gain curve. The Gain curvedisplays the response of the acousticwaveform tool to formation attentuation inthe shear and compressional window. Thehigher values on the Gain curve correspondto higher formation attentuation, and lowerwater saturation calculations. With a de-crease in formation attenuation, the watersaturations increase. The Cherry Canyonsandstone in this well consists of fifty per-cent quartz, forty percent potassium feld-spars and five percent dolomite. The meanshear wave travel time is 161 microsecondsand the mean porosity is 22 percent. Thewell was perforated in the interval X025 toX040 for an initial production of 150BOPD; no water was reported.Example 4Figure 8 shows the productive interval of anopen hole well in Eddy County, New Mexi-co. The reservoir rock is a medium to finegrained sandstone with a composition offorty percent quartz, forty percent potassi-um feldspars, ten percent clays, and tenpercent dolomite. The mean shear wavetravel time is 108 microseconds and themean neutron porosity is 9.7%. Theregression line for this data set isY =225X + 86, placing the regression linewithin the area for the matrix scale using 60to 160 microseconds. The well was perforat-

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ed in the shaded intervals for an initial rateof 400 BOPD, 100 MCFPD, and 125BWPD.Example 5Figure 9 shows the productive interval in theBone Spring formation in Eddy County,New Mexico. The reservoir rock is a silt-stone composed of fifty percent quartz, tenpercent potassium feldspars, fifteen percentdolomite, and fifteen percent clays (illite).The permeability is less the .05 millidarcies.The mean shear wave travel time is 112microseconds and the mean neutron porosityis 10 percent. The well was perforated inthe shaded areas for an initial production of2.5 M*CFD.CONCLUSIONSThe crossplot of the neutron porosity andshear wave travel time can be applied tosandstone reservoirs in both the open holeand cased hole environments. In this studythe sandstone reservoirs varied in composi-tion from an almost one hundred percentquartz composition to a quartz concentrationof less than fifty percent. With the changesin the mineral composition and porosityvalues, a modification should be made toreflect the varying matrix travel time. Theappropriate matrix travel time is defined bycomparing the mean neutron porosity andmean shear wave travel time in the zone ofinterest to the least-squares regression lineestablished for the Permian Basin.Although many zones will calculate non-productive using standard water saturationcalculations, if the hydrocarbon indicator ispresent in this technique, the zones haveproven productive. On the other hand, manyzones that calculated productive using stan-dard water saturation calculations have beennon productive when the hydrocarbon indi-cator is absent.

ACKNOWLEDGEMENTSI wish to thank Jan Brewer, Randy Thorn-ton, and Charles Conley for their patience inthe preparation of this paper.REFERENCESIverson, W., and Walker, J., 1992, Shearand compressional logs derived from nuclearlogs, l?re Log Analyst, v. 33, no. 1, Janu-ary-February, 9 p.Kessler, C., and Sanders, L., 1989, Casedhole hydrocarbon detection in the PermianBasin using the full wave sonic tool, SPE -19610: Society of Petroleum Engineers,presented at 64th Annual Technical Confer-ence and Exhibition, 8 p.Murphy, W.F., Schwartz, L.M., and Hornby, Brian, 1991, Interpretation physics ofVp and Vs in sedimentary rocks, paper FF,in 32nd Annual Logging Symposium Trans-actions: Society of Professional Well LogAnalysts, 24 p.Williams, D.M., 1990, The acoustic loghydrocarbon indicator, paper v, in 31stAnnual Logging Symposium Transactions:Society of Professional Well Log Analysts,22 p.

ABOUTTHEAUTHORLee Sanders is a Log Analyst for Hallibur-ton Logging Services in Midland, Texas. Heholds a BA degree from Texas Tech. Hejoined Welex (now Halliburton LoggingServices) in 1971 in Houston, Texas work-ing as a dipmeter analyst. He is a memberof SPWLA, SPE, and West Texas Geologi-cal Society.

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ACOUSTIC FULL WAVE-FORM LOG

COMPRESSIGNAL

Figure 1: Comprdonrl, ahear, mnd ube W8V8 8rdvrl8wlth retpect to tlm8 on the full wrvs-form log.

PERMIAN BASlN - CORRELATlON CHART

SYSTEM SERlES FORMATION EXAMPLE

f _Y CANYONEXAMPLE 1QuADMuPMNf MU8HY CANYON EXANPLE 4

BLEONARDIAN BOESPMNG ULE 5

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CISCO EXAMPLE 2KINGsAm3%!! I CANYON EXAWLEla

Figun 2: Strrtionphic cornblion chrrt of thePd8ll BttiIl fOr th8 OX8mpk Wd18 in thit 8tUdy.

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INDEX MAP - TEXAS AND NEW MEXICO

Figure 3: Index map of Texas and New Mexico showing wall locations forexamples used in the study .

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SPWLA 34th Annual Logging Symposium, June 13-16.1993

LINEAR REGRESSION FOR MATRIX TRAVEL TIME

18i

16C

14(

12(

lO(

80

60

170 - 70 ,J01

10cB8,+Q9, ,,L4

YATMX SCAL1

1

I

NEUTRON POROSITY ( % )LIMESTONE MATRIXFigure 4: The crosrplot of the shear wave travel time and theneutron porosity data set establishiig a regression line. Thengrersion line definer the matrix travel times for an interval.

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SPWIA 34th Annual Logging Symposium, June 13-16, 1993

GAMMAb 150

Canyon SandsEdwards County, TsxasCased Hole

DTSl ?O MS/F1 70

DTS 9 135 msF NPHI b 14 %

Flgun 5: A crrod holo well in Edwrrd8 County comp8nt th8 rha8r w8v8 travel tlm88nd n8utron porority. The 8hrd8d 8~88 rhowr the hydrocrrbon hdlcator wlth thenONrOnpOKb8ity dU8 rerrding 888 th8n th8 8h88r WtV8 tt3V.i tlm8.

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SPWLA 34th Annual Logging Symposium, June 13-16.1993

King SandstoneConch0 County, Texas

Cased Hole

GAMMA0 API 150

KOW

Figure 6: A cared holo n-entry ahowing hydrocarbon indicator in tbashaded ngion.

w1~1~1~1~1~~~~ -1-11-111-11.,3 LIMEsTONE MATRIX -.lODTS

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SPWLA 34th Annual Logging Symposium. June 13-16, 1993

GAMMA1 API 100GAINo___~~~-~

SW *-60 ff

Cherry CanyonEddy County, New MexicoT21-S

NPHI,111~~11~~~~11~~~~11~~~~~~~.3 LIMESTONE MATRIX -.lIDTS70 MSlFT 70

DTS : 161 msNPHI = 22% I

coo0

4-tFire 7: The producti ve interval shows an increase in thevalue of the Gain curve with decnrrmg water saturations.

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Brushy CanyonEddy County, New Mexico

KlOO

T23S

Figum 8: This inted roquirod matrixhange on theshoe wavetmvoiimeo to wtivstohe hydrocarbonindiutor. Thengnrsioninoor tiis intomi sy * 225x 86, y * dts and x 31tlpk

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Bone SpringEddy County, New Mexico

DTS

Spwla-Detection de Gas

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