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2. WELL LOGGING AND INTERPRETATION TECHNIQUESThe Course For Home StudyI. Log Interpretation Fundamentals: Open Hole II. Study GuideIII. Induction LogsIV. Electrolog(st) V. Laterolog and Dual LaterologVI. Flushed Zone Resistivity DevicesVII. Spontaneous Potential Log VIII. Gamma Ray LogIX. Compensated Densilog" x. Acoustic LogsXI. Neutron Logs 3. FOREWORDLop, Interpretation Fundamentals: Open Hole is a home study course which covers the important basic elements of open hole log interpretation. The emphasis is on log interpretation, not on tool measurement theory. The first few lessons introduce relevant rock and fluid characteristics. Subse- quent lessons present progressively more complex log interpretation tech- niques. The number of interpretation techniques is kept to a minimum.This is a lesson-by-Iesson course. Participants should study each lesson and then answer the related questions. (A study guide has been provided). Supplementary reading is suggested throughout the text. The text, along with the supplementary reading, should provide a sound basis for basic open hole log interpretation.Comments or questions, regarding any of the course material, should be made to Dresser Atlas sales or log analysis personnel worldwide. 4. bottom eight hours after the Compensated Neutron- Densilog" service combination left the same point in2 ~---~at~/280~6,518ft /the borehole. The temperature recorded at the time was~ I228F./ 270 To determine the true static formation temperature //from this formation,(Table 4.2), plot the BHT datavs the corresponding dimensionless time on Figure4.3. Note that the true static formation temperature Ie 260LLo!::I~is 235 OF at the depth of 10,000 ft. CD0.ECDt-CD 235F250 (3 , Statics: IE BHT at o ,. 10.000 ftt: ,S ,, I240 , I,I ,,,I 230230u, 0 225 ~ ::I n1 0.10.51.0 Ci> a. Dimensionless time, ~U(t + LJ.t) E Q) t-FIGURE 4.4 220Extrapolation technique.exploration concepts, is obvious. In the hightemperature well (curve 1) four logs were run to 6,518ft. The first log, four hours after mud circulationstopped, recorded 218 of. Straight-line extrapolation 0.2 0.5 to infinite time (log 1.0) indicates a bottom-hole6t/(t + ~t)temperature of 281 of. FIGURE 4.3 Example calculation. In the deep Texas onshore well, (curve 2), threelogs were run to 12,548 ft. The first recorded Figure 4.4. illustrates the extrapolation techniquetemperature was 272 OF, while actual stabilized BHTfor true static formation temperature in two wellsis 284 OF.which have been drilled in different geothermalFigure 4.5 shows borehole temperature variationsregimes. Well No. 1 is a high temperature wellwith time and the true static formation temperatureslocated in the South China Sea; well No.2 is a deep in eight South China Sea wells. Note the drastically hole drilled onshore Texas. different temperature regime encountered. Such in- The drastic difference in the geothermal gradients, formation is of great importance in the search forin the two wells and the impact on completion and either oil or gas. 24 31. South China Sea Area320----------------~300250 351---_ _"""-!-_ _.....&--..............--_~u,1.012 1.41.61.82.0o Thermal Gradient. F/l00 f1eli:;(ti FIGURE 4.6QiPhysical state of oil and gas.a.E~ in regions of known or suspected overpressures, since200 recent experience indicates that steeper than normaltemperature gradients do occur. Figure 4.7 showssuch a field case from Offshore Louisiana, and 23160"__ ... 40.203 0.40.5 060.7 0.8 0.9 1.0 5 Hours After }Stopping Time. ~t/(t + 6t) 6 CirculationAGURE 4.57: 8 Mud Weight. .~Borehole temperature variations. 1b/9~1i __ 6a9 -13.5~HYDROCARBON DISTRIBUTION ~ ~~ ~ 1FI1 00 ft The wide range of geothermal gradients from area12 --- .....I--- - ""IliIl " t". " ... ... "!lIlIo..~~ ~ ~ --~~ ~ ~-+-- " "~~~ ""~ ~ ~ ~.~ !l-.!lI. ~ ~~ .....~ -- .....-~ -- - I-~-- _....... ~. -~~f-- T""~N ~~---< ~" "--..... ~~" ~~""llll-.i~ 10 , ~--~IlliIo.. !Ill.. """ .~ ~ "z~ "~ ""Iii...: " ",1IIIIIl..."" - ~""IIIlil .....11..... ..... ""lIl~"~ ~ """" ~ ~~~ ~ ""-.. I" """IIil -.~" ~ lll "-~.... ~ " ..tn0 -...:~ ~--.:ft5 " """"Q.4"."" " ""II1II~ -. ......~ ..,.; -... rllIIo.: t......... ....,;lO.:."""Ilii~IV~ ""lIIii " ~ """IIIIIl .... ~"""lIIii ..... ~ "~"~ ... " r-. rIIlhf"IIIlI..,~,~Iii..... ~"""IIilr""li-...."""Ilili " ""l1lI3" r..~I. ~ "" ~~ ~ ""-.~~......~ !."~ "" "~z JJ ~l" i"lllli~ ~l..l1B~, ~~2~"lIIIl~ _/~ ~ f~ ~ ~~la ~~~~" lot"~~~~ 4ll~m= 1 [4 ~ ~4ll~ ~~ ~~~ 1~25 10 20 50 1002005001000500010,OOC Formation Resistivity Factor, FEstimated m values Equations:Uncemented < 1.4 F::::1 0mVery slightly cemented, 1.4 to 1.6 Low +carbonates (Shell Oil)Slightly cemented, 1.6 to 1.8m= 1.87 + 0.019(21Moderately cemented, 1.8 to 2.0HumbleHighly cemented sands, carbonates~2.0 F = 0.62 0 2 . 15RGURE 5.1Formation resistivity factor, F, determination. A concept which should be helpful in understandingderived from porosity.other aspects of the interpretation problem is that ofapparent formation resistivity factor, Fa. (The nota- s,100070tion F R is often used interchangeably with Fa denoting n,R1that it is a formation factor derived from resistivity FRFfZmeasurement as opposed to a porosity measurement.) 0w0. o ta l In the case of a 100010 water-bearing interval, theR. value obtained from a deep-investigating resistivity However, if the porosity contains some hydrocarbonsdevice is essentially the same as Ro so the apparentas well as water, the resistivity measurement is affectedformation factor Fa (F R) equals the formation factor only by the water-filled porosity, since hydrocarbons34 41. arc non-conductors. In this case, the apparent forma- If both sides of this equation are squared,tion factor will be greater than the formation factorderived from porosity since the water-filled porosity 2 21 wmust be something less than the total porosity.s~. (16) s, < 100070 Ro < R. FR > R0the appropriate terms from the basic Archie Equa- {ZJw < 0totaltion relating porosity to R o and R w may besubstituted.total fiuid volume Since 0 totabulk volumeRw R 0; Rt and 0to ta1 w--Ro (17) volume formation water and !25 wbulk volume soR.IRtR oRthe relationship between the formation factor and S2 wx -- (18)water-filled porosity is, therefore:Rw/Ro R[Rw(12) The term R w cancels out of the equation, leavingThe apparent formation factor in terms ofresistivities then is as follows: (19)Rt= (13)Rw A more general form of this equation results from replacing the exponent "2" with "n", referred to as The concept of water saturation is important sincethat is one of the primary formation parameters thethe saturation exponent:log analyst wants to obtain from well logs. Thesaturation of any fluid in a porous interval is theratio of the volume of that fluid to the total pore S~(20)volume. volume formation water (14) Sw =:total pore volumeThe most genera] form of Archies saturation equation is as follows:Values of water saturation are normally calculatedthrough potential pay zones because the resistivity aRwmeasurements are determined by the amount of for-S"(21)wmation water in the interval and by its resistivity. Theremaining fluid in an interval that has less than 100070water saturation is then inferred to be hydrocarbons,but the resistivity measurements cannot normally (The saturation exponent, n t is normally equal todistinguish between oil and gas. two, but may vary slightly as local experience dic- Since the formations bulk volume is a common tates).denominator in both of the porosity definitions men-tioned previously t water saturation, Sw may be ex- pressed as follows: SAMPLE PROBLEMSOwTo best illustrate how Archies fundamental rela-(15) tionships are applied to actual practice, typical inter- 0totalpretation problems need to be solved. 3S 42. Example 1:"m" = 2. The porosity log response through a sand interval Rearranging the equation to determine porosityin a well gives an indicated porosity of 18010.gives the following: The resistivity from an induction log indicatedR, = 22 Q-m through the interval. 0m= 1:: =~5. Rw is known to be 0.025 Q-m at formationtemperature. The porosity then is The analyst wants to know the calculated water{Zj =J 0.025 = O.16or16OJo.saturation for the interval. Using the saturationequationStill, another typical interpretation problem takes advantage of the basic Archie Equation in the estima- tion of R w from the porosity and resistivity response in a water-bearing interval. Example 3:local experience in the area dictates that a = 0.81 A Compensated Densllogv-Gamma Ray survey in-and m and n = 2 for the sand evaluated. dicates 27ftJo porosity through a clean water sand. It is not necessary to solve for F as an intermediatestep in the solution since the analyst is not usually in- The Induction Electrolog" survey through theterested in F. Therefore, the determination for watercorresponding interval gives a value of R o = 0.35saturation is: Q-m. Since the interval is a sandstone, the Tixier relation-0.81 x .025ship between F and porosity is applied. Solving the sw = (0.18)2 x 22equation for R w gives the following:Ro X 02Sw = O. 17or 170100.81Another fairly typical interpretation problem is toestimate the porosity in an apparent water zone when 0.35 X (0.27)2only R w and the resistivity response from a deep-0.81reading resistivity tool are available. When thiscalculation is made, it is assumed that the resistivity Rw= 0.03 Qrn.tool response is providing a value of Ro Alsonecessary are the values for "a" and "rn." Knowing The example problem was a partial solution of thethese values, the Archie formula can then be solvedRwa method of log evaluation.relating F to porosity, R o and R wfor porosity: Example 4:aF= rzJrn R w I f the water resistivity term in the saturation equation is replaced with mud filtrate resistivity, and the for-Example 2: mation resistivity term replaced with Rxo , the resistivity of the flushed zone, the fluid saturation The induction log response in this instance is calculated would be the mud filtrate saturation in the2Q-m. flushed zone, Sxo. The formation water resistivity is 0.05 Q-m.S n The interval is a highly-cemented sandstone sowlocal experience in this case dictates that the ap-propriate values to apply for "a" = 1 and36 43. Sxo n =When a micro-resistivity curve or shallow focusedcurve is recorded in a well to provide an Rxo log, solv-ing for Sxo is beneficial for determining residualhydrocarbon saturation. Sxo = (1 - RHS).The significance of knowing the value of Sxo through a porous interval as well as Sw is that it per- mits the determination of the degree of hydrocarbon flushing by the invading filtrate. As long as Sxo is numerically greater than Sw it can be inferred thatt there are movable hydrocarbons present; if Sx_ o = Sw it can be inferred that there are no movable hydrocarbons present.BIBLIOGRAPHYArchie, G.E. The Electrical Resistivity Log as an Aidin Determining Some Reservoir Characteristics,Petroleum Technology, Vol. 5, 1942.Dresser Atlas. Log Interpretation Charts. 1980 Ed. 37 44. QUESTIONS5(1) The porosity in a water sand is 25010; R = 0.03 Q-m at T f and the resistivity from an Induction Log reads 0.4 Q-m.If a = 0.81 and m = 2, what is the apparent Formation Factor for the sand?_(2) If R mf = 1.8 Q-m at T f in question 1, what would be the value of the apparent Formation Factor in the sands flushedzone or R zone? __ __ __.xo(3) An lnduction-Electrologt survey and BHC-Acoustilog!> survey were recorded through a sand where an oil-watercontact was observed. The Acoustilog" indicated the porosity to be constant through the interval; the InductionLog indicated 0.5 Q-m below the oil-water contact and 5.0 Q-m at the top of the interval. The formation has a waterresistivity, R w = 0.02 Q-m. If m = 2 and a == 0.81 (Tixier), and n = 2, what porosity was indicated through thesand?_(4) What would the water saturation be in the top of the interval in the preceding question? _(5) A Micro-Laterolog on the well in the preceding problem indicated R xo in the top of the sand to be 16 Q-m andR mf = 0.5 Q-m @ formation temperature. What is the calculated value of Sxo? Are any movable hydrocarbons in-dicated?38 45. INDUCTION LOG6INTRODUCTIONment is one of conductivity, errors that are insignifi-cant at high values can be a problem at low values Focused induction logs have proven to be the best(conductivities). For this reason the measurement ofmethod for obtaining formation resistivity in wells resistivities greater than 200 Q-m (or less than fivedrilled with fresh mud, air or oil base mud. The logmmhos/m) is not the best application for an inductioncurves recorded on the lnduction-Electrolog" survey device. The accuracy of the measurement is limitedare the SP, 16-in. short-normal, induction conductivi-above 200 Q-m.ty and its reciprocal, the resistivity curve. (Fig. 6.1) InBed boundaries on the conductivity curve are half-boreholes containing non-conducting fluids, the Gam-way between the high and low reading that result fromrna Ray and Induction curves are recorded.changing from one bed to another. (Fig. 6.1) In thin Induction logging instruments are composed ofbeds, peak values, whether high or low need furthertransmitter-receiver coil pairs. The number of coilscorrection to read true resistivity. In thick beds whichand spacing of these coils determine the depth of vary in resistivity and are considered to be a singleinvestigation, borehole response and the resolution ofunit, the curve is often averaged. In normal interpreta-the instrument. tion, the induction curve is assumed to be measuring An alternating current is applied to the transmitter true resistivity and is input directly in the Archie equa-coils which induces eddy currents by electromagnetiction as R t induction in the formation surrounding the coilBorehole effects on the induction curve are smallsystem. These currents have a magnetic field which except when highly conductive, salty fluids are presentinduces voltages in the receiver coils. These voltages in large boreholes. Normal borehole enlargementsare related to the conductivity of the formation.have little effect on the curve. The induction log may Conductivity is the reciprocal of resistivity. Thebe run in air, oil or foam filled holes as the inductionconventional units of conductivity used in well loggingsystem does not require the transmission of electricityare mmhos/m which gives the relationship:through the drilling fluid.C = lOOO/R (I)SHORT NORMAI~where: The short normal resistivity measuring system has a C = conductivity in mmhos/mshallow depth of investigation, and is designed tomeasure the resistivity of what is normally the invaded R = resistivity in ohms-m-/m zone. When compared with deeper measured resis-tivity, such as the induction, the existence of invasioncan be detected. Invasion indicates that the formation The measured value of conductivity is reciprocatedis permeable.by the surface processing equipment as the log is run With its 16-in. spacing, the short normal recordsand is presented as a resistivity curve, in addition togood resistivity values in relatively thin beds, down tothe conductivity curve.four feet thick.The curve shape is symmetrical around The induction system works best where the undis-the center of the bed. In thick beds, an average valueturbed formation has lower resistivity than the in-may be used if a single value of resistivity for the bed isvaded zone. This is typical of logging in a fresh muddesired, otherwise, a thick bed may be divided intosystem. A low resistivity zone between the tool and theseveral thin zones. The short normal generallyundisturbed formation, as typified by invasion by saltmeasures the resistivity of the invaded portion of themud, creates problems if the invasion is deep. This isformation, which is partially saturated with muddue to the induction device being more influenced byhigh conductivity (low resistivity) zones than by low filtrate. I f the undisturbed formation contains oil,conductivity (high resistivity) zones. The induction logthere is usually oil in the invaded zone, but to a lesserwin normally measure a good value of resistivity in extent. The short normal often reflects some of thezones of five feet or thicker.fluid properties in the undisturbed formation as well The induction device is better at measuring loweras those of the mud filtrate. The size of the borehole resistivities than higher resistivities. Since the measure-and the resistivity of the mud in the borehole influence 39 46. GR SP lFPiH RESISTIVITY CONDUCTIVITY (Ohms rn-vm) (Mllhmhos/m)/,1O, 16" NORMAL- i-I INDUCTION CONDtJCl IVI Ty40 SPACING O~ 10 I------ ~ -- ~---0 -- GAMMA RAY ~BiJ,APt 0, ..INDUCTION RESISTIVlii--- 80004000 ~o ___________ _3,E 40 SPACING 02 ~- ----- ------ - - --10;~ , 1~ I:Ji,~Ir AI~" I I I !iIV~~1J~~~-f r"-- I I I1t...~ ~ I,HIGH t-- ~[",/ BED BOUNDARY~li t- ..."""r----~~ I I~N ~lOWD I "" II~ r., < Il I! I "~I! ~ , ~ , ;II ~ ~,tII! JI .....-.II ~VI ,~ --- II~~ 1-J I II! ~ ... !I j I -I~ ~ i II..- - _IJ;j!II ~~ I j I I(~)i1Iii1 ~~ L-~ I! !t]11III I I I ~I !II I I i11: 1~ ~II:~ 1 ,I I ~- --~... III~I ~~ I~ !I lI c .... :II, I T~i~Ir: ; l---""l.--""1~ ~ t;>I !I rI I If I~~ ........... ~ - -~- I III - ""--FIGURE 7.2Laterolog/Gamma Ray. 53 60. Dual laterologSimultaneousShallow and t Salt MudDeep Measurements I ,.;~Shallow- R ~ I " ............ _~,,::"-------~f ~:::::_---- Deep - RI------:.--"WI128 Hz. . ::::::::::::: ca32 HIT~--=-=:~~~-:f - --------- ....========::::~-:.:./ 24-in 4------ . . ~,,----------.Beam Width ~-------E ~-- ::===:::1---... ---- - ~_ ~----------- .... .... t#o,.,_--_ ~.; .;----Mud Flushed Invaded UndisturbedFIGURE 7.4 Zone Zone Formation Dual Laterolog Current Paths,Ao~~A ~,Vw Am Ame s.,AI At8 3. The ratio of the borehole diameter and theFIGURE 7.3diameter of the logging tool b small.Laterolog current path. DlJAL tATEROL()GWhen the above conditions are not present simultaneously, the values calculated will generallyIn highly resistive formations drilled with salt mud,depart to some degree from values that are actually two Laterologs with different depths of investigation recorded. The formula generally used is: are often recorded simultaneously in order to compute (I) true resistivity (R l ) . The shallow Laterolog primarily measures the resistivity of the invaded Lone, and the deep Laterolog measures the resistivity of a muchThis equation simply states that the total resistivity deeper part of the formation. The true resistivity (R ,)(R a )is the sum of the contributions of the borehole can thus be calculated more precisely by taking intoresistivity (JmRn1) , the resistivity of the invaded zone account the effect of an invaded zone in the vicinity of(JiRi) and the resistivity of the undisturbed formation (JtR t). the borehole.The current pattern for the case of the DualThe pseudo geometrical factors for the mud (J rn ) . Laterolog is shown in Figure 7.4. The presentation of the invaded zone (Ji) and (he undisturbed formarlon the Dual Laterolog is usually on a logarithmic scale. (L) for the Laterolog depend upon the geometry andFigure 7.5 shows a Dual Laterolog./Gamrna Raydimensions of the logging tool. borehole diameter (dh) and diameters of invasion (dj).recorded on a four-cycle logarithmic scale from 0.2 to 2,000 Q-m. The pseudo geometrical factors will satisfy the following classic expression: PSEUDO (~":OM":TRICAI~ .ACTOR 1.0 (2)Mathematical calculations [0 correct apparent resistivity values can be made for the l.. aterolog based upon the pseudo geometrical factor concept. I f the The radial response curve for Dresser Atlas following conditions are complied with, the results Laterolog is shown in Figure 7.6 which gives the obtained from these calculations are within acceptablenurnerical values of the horizontal pseudo geometrical limits. factor in terms of diameter in inches. Also shown in the chart for the purpose of comparison is the radial 1. The formation of interest is greater in thicknessresponse curve of the Dresser Atlas Induction l.ogs.than the disc of measure current. (>2 ft)The difference in the radial response curves for the Laterolog and the Induction Log illustrates that the 2. The resistivity contrast near the logging tool is notLaterolog pseudo geometrical factor for the invadedtoo great. zone (J j) is considerably greater than that for the54 61. GRDEPTHRESISTIVITY(Ohms m 2/m)GAMMA RAY(API Units)SHALLOW LATEROLOGo120 021.0101001000 -----------~-----------------r-----------------r-----------------r---DEEP LATEROLOG 021.0101001000Ir I.! I IISII (IiIII(~SHALLOW LATEROLOG........~"~""1II~ ~ I Ifl ~~~DEEP LATEROLOGII ~IIoeIII (""I , rIcII ~4~fIGR !/II Ie, ~I!t I ~IFIGURE 7.5Dual Laterolog/Gamma Ray Presentation Format.55 62. 0.8From Dual Laterolog survey I II I I,.,.",.. ~ RLLS = 375fl 0.6 - Standard LaterologasLLg Ra = JmRm + JjR, + J,R t~ ....... ~~~----RLLD = 600Q)~~ ~E 0.4 From surface measurementoQ)V ~C)~ /o0~ 0.2j"~~V Rm = 0.22 @ Tr,/enQ."0/ RLLI)/R m = 2727~asV800 Series Induction Log~ 0 J -- ~1 = Gm + G 1 + Gt_AnI Am F RtI II - -0.2Figure 7.7 gives: o 20 40 60 80100120 Diameter in Inches RLLDcorr/Rl.I.D= 0.89 FIGURE 7.6 Borenole DiameterV5.Geometric Factor.RLl.D = (0.89)(600)=534 corrected for borehole Induction Log. However, where a low value of Ri/R. exists, the influence of the invaded zone (JjRi) on theRLLS =(1.01)(375) = 379 corrected for borehole apparent resistivity will be less for this device than for the Induction Log. This effect becomes moreTo enter Figure 7.8: pronounced with lower values of Rj/R, and increasing dj values. RLLD/RLLS -= (534) + (379) = 1.41 (534) -;- (15) = 35.6 ENVIRONMENTAL CORRECTIONSwhich gives:As with the Induction Log, it is often necessary to make corrections for borehole and invasion effects to obtain the most accurate value of RI .Rl/RLLD= 1.2; Rt/R xo = 43; dj = 22 in.For borehole effects, Figure 7.7 is entered with the ratio RLL/R 1 The ratio Rl.Lcorr/RLL is read at the nR,= (1.02) (534) = 641 intersection with the borehole diameter line.Figure 7.8 corrects the RLLD value from the Dual R xo = (641) -;- (43) = 14.9 Laterolog for invasion if R.>R xo - Resistivities from the deep and shallow Laterolog along with the R xo measured by the Micro Laterolog or Proximity Log are necessary. The R xo devices may be run in combina- tion with the DLL. Example:Given: From Micro Laterolog surveyRxo15dbh10 in. 56 63. Borehole Correction for Laterolog Deep (LLD)1.4 ,... ....;....;.. (Thick Beds)..1.31.2dt. 16 In.et: ~1.1 ---0ri1.00.90.8 2 3 41020 30 40 50100200 300 400 500 10005000 FLo/RmBorehole Correction for Laterolog Shallow (llS) (Thick Beds)1.5 r----------------------------.---------..1.41.00.9 . :"":J ..... " ..: . :. ~ . , ..... .. ~ 4.: ~ ~ 2 34 510 20 30 4050200 300400 500 1000RGURE 7.7Borehole correction for Laterolog. 57 64. 1009070605040302010 9 8 7 ~ 6- .. --. --. a: C 5 .. .. cr.~ 4 3---. : 2 , 09 08 .--. ... ----_ .....--.-------------~..-----... ....,.- ..r- -. . R, > A ,xo 07 Thick Beds .. _ .._ 06 S In. BoreholeStep Profile 05 04 . .: . -... : ..... .- -.,.._. r- .:i ~.. , . : --: ..: "i i: 0.3 __ --_-- __ -~ _. ;;..__ ~-i .. -+.. -:-~ .............., . 0.2 o1 0420 30FIGURE 7.8At from Dual Laterolog (for At> Axo ). . ..-.. .iiiiiiiIiioooii~_......58......... 65. QUESTIONS 7(1) Laterolog devices measure the drop in__created by thenow of current through the formation to a current return electrode. a) inductionb) resistivity c) current d} potential(2) If a Laierolog device is recorded in a 100070 saline water saturated formation, which i deeplyinvaded with fresh mud filtrate. the apparent resistivity will be a) too high b) too 10 c) the same d) not enough information(3) Determine the diameter of invasion for a Dual l.aterolog/Micro Laterolog combination given:RI. D = 17 Q-m. RI LS = 15 Q-nl. Rfli = 8 Q-m.a} 20 in. b) 40 in. c) 35 in. d) 30 in.(4) Determine R, for question three.a) 17 Q-mb) 19 Q-nlc) 14.1 Q-m0) not enough information(5) In a 10 in. borehole with R n1 0.8 the I.aterolog deep reads 80 Q-In. What is the true resistivity?ta) 76.3 Q-mb) 81 Q-mc) 100.8 Q-md) not enough information59 66. ADDmONAL RESISTIVITY CONCEPTS8MiniI ogSP l""RESISTIVITYt(~ITr-INTRODUCTION y. ,..1,., I, I" ..~1 , I, The Minilog" service provides information useful,. , -, I .... "in locating and evaluating porous and permeable---_ ... -- . -- -formations penetrated by the drill bit. The closelyf1,fCIspaced electrodes, mounted on a fluid-filled pad in .-.--contact with the borehole wall, measure the resistivity II j r;;>tof a small volume of formation adjacent to theIr lt exceeds 100.Logs. The Dresser Atlas Micro Laterolog and ProximityTHf:ORY 0." M.:ASUREMENTLog are pad-type logging devices record threemeasurements simultaneously. The curves presented Micro Laterologon the log are the gamma ray or SP for lithologicalcorrelations, a caliper for determining variations inThe pad of the Micro Laterolog comprises a beamborehole diameter and the Micro Laterolog orelectrode in the center of the pad surrounded byProximity focused resistivity.focusing electrodes. These small electrodes are embed- The presentation for both logs is similar. The ded in an insulated fluid-filled rubber pad.GAMMA RAY ~{ r" DUAL LATEROLOGCALIPER M1CRO LATEROLOG GAMMA RAY =E~ COMPENSATED DENSILOGhI V CALIPER COMPENSATED NEUTRON " ... " ....1,. ."IIIr[lIof,. PO~~lTV-,-- - u"t,lMA R"~........."".. ......-.J - I f --. -,~---- ,....1, .. "CLi-.Jn ttl,m~tneT.l,lt. PlIs:.- r IT ..r, -.f.... -- -- rTT f I r :=zICr.~flI,I"II~ ~rIiI 1 ~ ~J III I ..... _- - -~---IP1~ ji: ,I.I i ~ I Ii ~J I.i ITt ~r-rI I I -~- : - ~~~ ! ~ I:1,II:--.~ rill, IIICALn~~III I .... ~~- ......--[~;I I I III Btl.. II ! III IrtI IiI~~ I I I~ f:bl~ ~I I~ I ,I ],I II I ~I! I ,f)1I ~~ I 1 II I ..... ~~--~~--+--t--- ~--~--~ II I ~~:-- 1 III" t-~o+---. ---~.~.- - --.- - I I I~ I)t1} ~1WELL AOpen hole logs for zone of interest. tertiary sand-shale sequence. 82 88. GRACOUSTILOGGAMMA RAY SPE:CIFIC ACOUSTIC T1~r o100100 40 T ------II I I~rK~"""I II 1II ~c~hI I I1 i II I I j Pma GULF COAST SHALE DENSITIES Depth Bulk Density(tt) (g/cm 3) FIGURE 12.2 o 1.80 20002.20 Shale density effects on effective porosity. 40002.34 60002.44 at shallow depths, the shale densities tend to be much80002.52 lower and the effect on Densilog" instrument readings10000 2.57 is greater. Also the shale disseminated in the pore 12000 2.60 16000 2.63 space may have somewhat lower density than the adia-lI189 95. Gulf Coast wells. The increase with depth in bulk den-sity of shale is due to compaction. If shale bulk densityis plotted vs depth, a normal compaction trend-line 5can be established. This can be seen from Figure 12.36-=o 58.,... 7.......x 6 . . .. ~. .L:.a.... . Q)o 8. . .. 7 ~ 8 . .......09 .. .. .00,.....x9s:10 Ia.1 I...1.02.0 3.02.02.2 2.4 2.5~ 10..LinearLogarithmic 11 ." .. .,,Shale Bulk Density. P sh (g/cm 3 ) 12 ,FIGURE 12.4 Log derived shale bulk density plots on linear and logarithmicBHP @ 11.300 BHP @ 11,300scales for detection of overpressure zones.= 11.300 - 9.700 (0.535)= 11,300 - 10,500 (0.535)= 6,100 psi :;; 5.700 psi Residual hydrocarbons in the Densilog" zone of in-2.2 2.3 2.42.5 2.6 2.1 2.22.3 2.4 2.5vestigation cause a reduction in the measured bulkdensity compared to the measured bulk density when (a) Density Log (b) Cuttingsthe rock is entirely saturated with mud filtrate. TheShale Bulk Density. PS (g/cm 3)variation in bulk density is a function of the porosityFIGURE 12.3 +, the density of the hydrocarbons Ph and the residualShale bulk densities measured by density log and from cut-hydrocarbon saturation Shr in the investigated zone.tings plotted vs depth in an offshore Louisiana well.The bulk density in this case would be,where shale bulk densities from the density log and P b = (1 -+> P ma + +from cuttings analyses are plotted vs depth in an off-shore Louisiana well. However, this normal compac-tion trend is found to reverse within overpressuredzones.The presence of residual hydrocarbons affects the Abnormally high pressures in formations are caused Densilog" response resulting in a variation 6,P b , suchby sealing barriers, which prevented the escape ofthat,water during the process of compaction. Because ofthis entrapment, the fluid pressure may be nearly as (6)great as that caused by the weight of the overburden.The shales remain undercompacted due to the excessIt has been shown to a good approximation thatof water. Consequently, their densities are lower thannormal. This may be seen in Figure 12.4, where the 6, P b = 1.07+Shr[(1.11 - O.lSP)P mf(7)log-derived shale bulk density is plotted on linear andlogarithmic scales for the same well. The decrease in - 1.11 Ph - 0.03]bulk density values reflects the presence of over-pressured environments. The departure from the nor- for oil-bearing formationsmal trend depends on the degree of abnormality in theformation pressure from normal hydrostatic pressureandat a specific depth. Existence of abnormally highpressures can be predicted by several methods in-cluding studies on shale bulk densities from Den-(8)silog" instrument readings, which enable taking theproper well control precautions.for gas-bearing formations90 96. where P is equivalent NaCl concentration in ppm x 1(t6 for mud filtrate. The above relationships indicate the magnitude ofthe corrections needed in hydrocarbon-bearing forma-tions in order to obtain accurate porosity values fromthe Densilog" readings. The corrections are usually small in oil-bearingzones, but they are quite considerable in low pressuregas-bearing zones, For the application of hydrocarbon corrections onthe Densilog" readings, it is necessary to know thedensity of hydrocarbons present and the residualhydrocarbon saturation. The latter can be estimated ifan Rxo log is available. For accurate quantitativeevaluation, the Densilogv survey should be used incombination with a neutron log in gas-bearing forma-tions. Example: In a clean gas-bearing limestone forma-tion, drilled with fresh mud, the Densilog" surveyreads 2.25 g/cm-, The density of gas under reservoirconditions is 0.2 g/cm-. The value of R xo from amicroresistivity log is obtained at 68 ohm-m, while theresistivity of mud filtrate, Rmf, is determined to be 1.0ohm-m at formation temperature. The true porosity iscalculated in the following manner:Shr = 1 - Sxo = 1 -V Rmf/(+2 x Rxo)(taking m = n= 2)The simplified relation for fresh mud would be,where 6,P b = 1.07+ X Shr (1.11 - 1.24 Ph) for thegas-bearing formation.True porosity can be expressed as, += Pma-PJog+ 1.07 VRmf/Rxo (I.l1-1.24Ph)P ma - 1 + 1.07 (1.11 - 1.24 Ph)Substituting the values P ma = 2.71 g/cms, Plog= 2.25 g/cm-, Rmf = 1.0 ohm-m, R xo == 68 ohm-mand Ph = 0.2 g/cm- in the above expressions, trueporosity is obtained + = 22070, while the residual gassaturation Shr = 45010 and the correspondinghydrocarbon correction is 6, P b = 0.09 g/cm-.BIBLIOGRAPHYDresser Atlas. DensilogJ , 1980.91 97. QUESTIONS12(1) Determine porosity and water saturation and R w as requested for the following geologic sequenceswhich were drilled with a fresh mud system. (a = 1, m = n = 2) RaR".P10g Formation Porosity 010 Sw 010 2000.083 2.54 Limestone- - - - -- 2000.1 2.54 Dolomite ---100.182.54 Sand ---2.16Sand 1002.37 Limestone 100140.112.56 Limestone ---100.015 2.7Dolomite(2) Most interpretations use a P f (fluid density) of 1 gm/cc. This is good where invasion is deep andresidual hydrocarbons are minimal.A. In a gas reservoir with little or no invasion the apparent porosity calculated using P f = 1 is:a) too lowb) too highc) true porosityd) not enough informationB. If there is no invasion in a water zone filled with very salty water, the apparent porosity using P f = 1 is:a) too lowb) too highc) true porosityd) not enough information(3) Calculate porosity for the zones in Figure 12.5.Zone+234S678 92 98. BULK DENSITY GR & CALIPERDEPTHGRAMS/CC GAMMA RAYCORRECTION CORRECTION (API Umts) o 100~- -0.5 0+0.5-----r-- - ----..,.- - - - - ---,- - - - - ---HOLE SIZE(Inches) 6162.0 ~--- - --------- I2.5 I3.0 ZONES,~p logI... 1-82.227 2166 2.2~-5 2.464~o ,o ,II-~-~ J,J 2 1 12.2FIGURE 12.5L. Cretaceous Sand. in Mississippi assume all sands areclean. 93 99. ACOUSTILOG 13INTRODUCTIONWhere, The Acoustilog" survey is a recording vs depth of the~tacoustic travel time from the BHCtime (~t) in microseconds for a compressional sound Acoustilog in IJsec/ftwave to cross one foot of formation . .6.t or acoustictravel time is related to the velocity, V (ft/sec), of the~tma:::: acoustic travel time of the rock matrixcompressional sound wave by: in I1sec/ft 1066.t,-acoustic traveltime of interstitial fluids .6t :: - -- (1) V in /Jsec/ft The acoustic travel time is a function of the formation Before this equation can be used to compute poros-lithology and porosity. ity, it is necessary to have values for the rock matrixand the formation fluids. With the exception of gas,POROSITY DETERMINATIONmost fluids encountered in subsurface formations donot vary greatly in V f. Listed in Table 13.1 are some Specific acoustic time measurements have beenaverage fluid velocity and 6tf values.related to porosities of the subsurface formations. Thevelocity of a compressional wave through a rock for- TABLE 13.1mation is influenced both by the solid framework orrock matrix as well as by the fluids fining the pore Vfspace.Fluidft / sec A relationship defining a uniform intergranularporosity in terms of the total formation velocity, rock Water with 2010 NaCI5,300189Vater with 1501 NaC)0S,OOO200matrix velocity, and fluid velocity was proposed by Walcr with 100;0 NaCl 4.800208M.R.J. Wyllie, et al. This relation was experimentallyWa1 cr (pure)4.6002J8determined and is generally accepted as being substan-Oil4,200238tially accurate for a broad-range of conditions. TheMethane1,600626relationship is expressed in the following equation and Air~IOO910is usually referred to as Wyllies time average formula: -- = -+ + ------ The radius of investigation of the Acoustilog" is1(I - +> (2)quite shallow. In a porous formation where the inva- Vv, V masion is moderate or deep, mud filtrate is normally thedominating fluid that influences the Acoustilog" mea-Where,surements. For most purposes, an average velocity forthis fluid can be assumed to be approximately 5,300 + = fractional porosity of the rockft/sec. However, in areas of very high porosities, highresidual hydrocarbon saturations and extremely v = velocity of formation inft/secshallow invasion, velocity for the fluids to be used inthe relation may be lower. Vf= velocity in interstitial fluids in ft/sec The velocity of the rock matrix is the other parameter to which a value must be assigned before the time Vma = velocity of the rock matrix in ft/sec average equation can be used in the determination ofThis equation can be rewritten to express porosity inporosity. The acoustic travel time in the rock matrixterms of the recorded reciprocal of velocity,~t, or (.c6.t nul) will vary considerably depending upon the for-specific acoustic t ravel time:mation. More specifically, 6l ma changes with variations in the chemical composition of the rock, and also with its compaction which is related to depth and confiningJ.T= - ---~tma ~t --(3) pressure. The selection of the proper matrix velocity tob.tf - ~tmabe used in the determination of porosity is dependent 94 100. upon knowledge of the lithology of the section beinginvestigated. Table 13.2 lists typical matrix velocities for120 r-------,..---....-----...-------,---".,some of the more common formations encountered.TABLE 13.2 v.; 6.tma100.-.-----+----......----+-...... ~~--......... ~Formation ft/sec~sec/ftSandstone: Unconsolidated 17,00058.8 90....-----t----OOO+--~...,....,..."....~---1---........or lessormore.:::Semi-consoJidated 18.00055.6 en "0 cConsolidated19.00052.6 o oUmestone 21,00047.6e 80 CD o.-.-----+---~~,..,.....,...,~~----+---- ...Dolomite 23,00043.5167~v, = 5300 ft/sec.Shale 6.0000> Cto 16.000to62.5.:r have been used. In unconsolidated sands, this should be modified byintroducing the compaction correction factor, C p : Clay Point 1.0 (16)or,0.8+ = +AC- Vsh (17) CompactedThe fraction of the total porosity occupied by~ o 0.6t ShalesLLdisseminated clay is the q factor: &~ Undercompacted +q =AC - +effective - --(18)0.4 +ACThe q factor is indicative of the producibility of shaly 0.2reservoir roc ks. Irrespective of the type of shale distribution, it ispossible to derive a pseudo q value based upon theSanddensity log and Acoustilog" survey. The effect of Point 0 0shaliness on the density log is independent of the typeof shale distribution and can be similarly expressed as: +=+D -V sh X +Dsh (19) FIGURE 13.4Reservoir producibility in shafy sands.where, "_ P ma - P sh ~D- (20)sh Prna - P f99 105. EFFECT OF HYDROCARBONS Invasion is, generally, at a minimum in highly porousformations. Occasionally in these formations, thefiltrate invasion is so shallow that the measured acoustictravel time is influenced by the original formation fluids.If this occurs in a hydrocarbon-bearing section, the in-creased acoustic travel time in the fluid phase will causea porosity calculation greater than that actually present. In uncompacted light hydrocarbon-bearing forma-tions, the porosity calculated from the Acoustilog"survey, even a fter applying correction for lack of com-paction, will remain too high. Corrections made forthe effect of hydrocarbons are purely arbitrary at thepresent time. These are based on limited empiricaldata by a comparison of the acoustic response inclean, water-bearing formations and in identical for-mations containing oil or gas. For an oil-bearing sec-tion, the calculated porosity should be multiplied by afactor of 0.9. To correct for the presence of gas, themultiplicative factor is 0.7. These are the most com-monly used average values, although entirely differentfactors may be required based on experience in a par-ticular area.BIBLIOGRAPHYDresser ALias. Acoustilog" , 1980.100 106. QUESTIONS 13(I) Given: 8t ma 47.S IJsec/ft ~tf = 189 IJsec/ft +AC = 14.5f/o Vsh = 0010 What percentage would + be if the zone was a dolomite?AC a) 12f/o b) 15.20/0 c) 170/0 d) 18010(2) Given: ~tlog = 76lJsec/ft 8tr = 189 usec/ft Vsh = 20070 ~tsh= 100 IAsec/ft ~tma= 49 IAsec/ft + Determine AC corrected for shale content. a) 8.8010 b) 12070 c) 19.39ctJo d) not enough information(3) Calculate +AC and Vsh for the zones in Figure 13.2. Assume ~tma = 43.5 #Jsec/ft, ~tf = 185>lASec/ft, GRCL = 20 API units, GRsh = 100 API units. Zone ~t Vsh+AC A B C D E F G HI J 101 107. COMPENSATED NEUTRON LOG14INTRODUCTIONTABLE 14.1 Neutron logs are primarily used for the identifica-tion of porous formations and for the estimation ofNEUTRON ENERGY LOSSEStheir porosities. Often, it is possible to distinguish gaszones from oil or water zones by the comparison of a Maximum AverageEnergyneutron log with another porosity log or with informa-Number LosslAtomic Atomiction from core analysis. Combination of the neutron ElementCollisionsCollisionWeight Numberlog with Densilog" and/or Acoustilog" surveys pro-Calcium 371 8% 40.120vides accurate porosity values, shale content andChlorine31610% 35.517lithological information.12%Silicon 26128.114 The three types of neutron logging instruments inOxygen15021% 16.08current use are the conventional neutron-gamma, the Carbon11528% 12.06sidewall epithermal neutron and the compensated Hydrogen 18 100%1.0 1neutron. The basic principle in each of these devices isrelated to the slowing down of the neutrons by theHydrogen - Average loss due to angular collision is 63%.nuclei of the substances encountered in the forma-tions. The slow neutron density in the vicinity of thedetermination of porosities in carbonate formations.detector is determined basically by the composite This is because carbonate rocks are traditionally less hydrogen index of the medium between the source andcontaminated by clay minerals and the neutron log has the detector.better resolution in the lower range of porosities Water and liquid hydrocarbons have almost theencountered in carbonate formations. same concentrations by volume of hydrogen. If the rock matrix contains only minute quantities of ENVIRONMENTAL EJFECTS hydrogen and the pore space is filled only with liquid hydrocarbons and/or water, the neutron log providesSidewall epithermal neutron and the compensated porosity with reasonable accuracy. When hydrocar-neutron logs are directly calibrated in terms of bons are present in a single vapor phase or in a two-apparent limestone porosity units. Several additional phase system, gas-liquid, neutron derived porosities considerations may have to be made, once a value of are too low and indicate only the liquid-filled pore limestone porosity has been obtained. These involve volume fraction. The neutron log by itself is, there-the correction of log data for the effects of environ- fore, unable to resolve porosities in gas saturatedmental conditions, such as borehole size, mud weight, intervals. mud cake thickness, mud salinity formation watertsalinity or temperature. These corrections are less Neutron log response is primarily a function ofmeaningful for the conventional count-rate neutron,hydrogen nuclei concentration, regardless of the nature and are usually not made. For the sidewall neutronof the molecules containing the hydrogen atoms. A and the compensated neutron, corrections should becomparison of neutron energy loss and nuclei collisionmade when there exists a difference between the actualstatistics for important oil well elements are shown in logging conditions and the standard conditions underTable 14.1. Accordingly, the chemically bound waterswhich the instrument was calibrated. The overall cor-of gypsum or clay affect the neutron log as if they rections are usually small for the open hole logs.existed as free liquids in the pore space. Neutron logporosities are, therefore, too high when hydrogen occursMUD CAKE EFFECT (SIDEWAI..L Nf:UTRON)in either the rock matrix or in the dispersed solids in thepore space. This limits the usefulness of the neutron logMost of the corrections for the environmentalin shaly sands and formations containing gypsum.effects are automatically performed in the sidewallHowever, it may be used advantageously in conjunction epithermal neutron logging instrument. However, thewith other porosity logs, like the Compensatedpresence of mud cake against the formation increasesDensilog" and the Acoustilog, to determine porosity the apparent porosity because of the higher hydrogenand to identify mixed lithology.concentration in the mud cake. Figures 14.1 and 14.2The neutron log finds a wide application in the provide corrections for mud cakes of two different102 108. Sidewall Neutron Mud Cake Effectao510 1520253035Porosity from Sidewall Neutron I +SWN (0/0) Equation: +corr = (O.00088h mc + 0.00326)+2 SWN+ (- 0.26126mc + 0.901255) 0sWN+ (- 5.27296hmc + 0.47882)FIGURE 14.1Sidewall Neutron Mud Cake Effects For 1.4 g/cm3 Mud Cake. o 510 3510 1520253035Porosity from Sidewall Neutron.+SWN (%)Equation:+corr=(0.00853h mc - 0.OOO41)+2SWNFIGURE 14.2 + (- 0.245hmc + 1.016)+sWNSidewall Neutron Mud Cake Effects for 2.5 g/cm3 Mud Cake. - (3.385hmc + 0.1105)densities. The chart in Figure 14.1 can be used for light The effect of borehole size as determined in the testmud cakes and that in Figure 14.2 for heavy mudpits of diameters varying between 4-3/4 in. and 14 in.cakes. is shown in Figure 14.3. For diameters larger than the 7-7/Sin. standard, the apparent porosity is too high;COMPENSATED NEUTRON CORRECTIONSwhile in smaller diameter boreholes, it is too low. Approximately one porosity unit change per inchBorehole Size Effect diameter change is observed. The correction becomes significant for large diameter boreholes. The change inThe compensated neutron log has been designed so hydrogen content seen by the instrument because ofthat the environmental effects on the log are greatlythe borehole fluid in the vicinity of the tool accountsreduced. The standard conditions under which the for the porosity discrepancy. Log response in an oil-compensated neutron is calibrated are an eccenteredfilled hole is essentially identical to that in a water-tool, 7-7/8 in. borehole, fresh water in the borehole as filled hole because of similar hydrogen index.well as formation, no mud cake or standoff, 75 oftemperature and atmospheric pressure. Deviations Mud Weight Effectfrom these conditions during actual logging requirethe application of corrections, which are usually quiteMud weight correction shown in Figure 14.4 issmall. minute and is due mainly to the change in hydrogen in- 103 109. 40 Equations: 35For +a > 6% Borehole SizeEquation 30 4-3/4 in. +corr = 1.062286 + 2.9986 +a i 6-1/4-in.6-1/4 in. +corr = 1.0446 + 1.1305+8+con +8 ;; 25 ... 84-3I4"n.7-7/8 in.9-7/8 in. 12-1/4 in. = = 0.958+corr1.3467+corr:ll: 0.896861.821 a +a - -14-in. ~ Ui 2014 In.+corr = 0.8683 +a - 2.6781 e 0 c,BorehOle SIZe(;Ofr,ClIO" ~or s.,... 2.13 ~nc2 2.20 For +a ~ 6% "C1536.1 In Diameter Cr-.,OO ~ Borehole Size Equation ! ~ 104-3/4 in.+eorr= - 0.0705+a2 +1.784 +a + 1.12474 8 6-1/4 in.+corr = - 0.03+a2 + 1.313+a + 0.6707 7-7/8 in.+corr = +a 9-7/8 In.+corr 0.038 +a2 + 0.60777 +a - 0.51089 K12-1/4 in.corr = 0.0246 +a2 + 0.60431 +a - 0.9482 o 14 In. +corr 0.0229 +a2 + 0.53345 +a - 1.5386 :z:o51520 25 3035 404550 Apparent Porosity,+a(0/0)FIGURE 14.3Compensated Neutron borehole size correction.Compensated NeutronMud Weight Correction40 ...- ....- -........-..........- -..............- -....- - -........3530 Mud Weight Correction lNo correction when +8 For Sib= +a+corr +8< 200/0. > 20% MW > 8 Ib/gal ..~ 25 !; j... i - +corr = (0.386 log MW + 0.651) x :-:-:-:-:-5 : ~~ .. ~~ - ..~ I:~ t-~ L~t- -~ U ~~ :i:~ -: :~ t;l ~ ~~t : ~ : :. L ,~~. t-- - . t O.... --4-_.. . ._-.. . . ..- t ..- t...-- 1 ~ ~...-.-~-~~-~~-~ 40FIGURE 14.4Compensated Neutron mud weight correction.104 110. dex of the mud. Increasing the mud weight by adding Salinity Effectsbarite would displace hydrogen from the fluid. Onlyabout one porosity unit decrease is observed for 16Borehole and formation salinity effects on the com-lb/gal mud at 300/0 formation porosity. Mud cake cor- pensated neutron log response are shown in Figuresrections shown in Figure 14.5 are also quite small. 14.7 and 14.8. Salinity effects are twofold: hydrogenThrough the use of the dual detector system in thedisplacement by NaCl and thermal neutron absorp-compensated neutron, the influence of the tion by chlorine. Borehole salinity results in too highhydrogenous mud cake between the tool and the for-an apparent porosity because of increased absorptionmation has been minimized. Even a thick mud cake, of thermal neutrons by chlorine. Formation salinityone inch, causes less than a two porosity unit shift in a has the opposite effect. However, for open hole ap-formation of 300,10 porosity. As shown in Figure 14.5, plications, invasion tends to equalize borehole anda crossover point occurs at about 22 % porosity. Belowformation salinities which cancels the two effects onthis value, increasing mud cake thickness causes tooporosity response. The net correction is usually lesshigh an apparent porosity. Above this value, increas- than one porosity unit.ing mud cake thickness reduces the apparent porositynecessitating an additive correction.Stand-off EffectTemperature/Pressure .:ffect When the logging instrument is not completely Figure 14.9 shows the temperature and pressure ef-decentralized, porosity readings are too high. Thefect On the CNLog. Increasing temperature tends tostand-off effect on the compensated neutron responsedecrease hydrogen concentration, while increasingis shown in Figure 14.6. At intermediate porosities, apressure augments it. Measurements made in lime-one inch stand-off causes a porosity increase of aboutstone test pits of 1.7UJo to 34070 porosities forthree percent. The increase in apparent porositytemperatures up to 350F, assuming a 0.52 psi/ft (10because of displacement from the borehole wall is due lb/gal mud) pressure gradient, indicate the net effectto the presence of hydrogenous material between the to be a slight decrease in apparent porosity asformation and the detectors.temperature and pressure increase. The magnitude of Compensated Neutron Mud Cake Correction 40. . !t. .: : : ~ : : :: :::::~:: .: :::::::1: ~~ : : : : j ;h~tl; ;.;t= +corr -+8 -4~20 5-1/2-ln. - 17* casing in 8-3/4io. boreholeQ. +corr -+. -5i 15 ~--+--+-+-~-+--!85 1015 2025303540 Apparent Porosity. +. (%) FIGURE 14.10 Casingand cementeffect for CNLog.108 114. GRDepth GR tltP"t.LIMESTONE POROSITY (%) POROSITY GAMMA RAYGAMMA RAY(APIUnu) NEUTRON OPEN HOLE iAPI Un.s).......................................................... CORE POROSITYo100 NEUTRON CASED HOLEo100 COMPENSATED NEUTRON ~---.-- - --r- - --r-- - - -30200 45 30150~~NeUlron . 0.25)(5)phenomenon. The response depends primarily on thehydrogen index of the formation, which is propor-The hydrogen index of mud filtrateHmfcan be ex-tional to the quantity of hydrogen per unit volume ofpressed as:formation near the borehole with the hydrogen indexof fresh water at surface conditions taken as one. In (6) H mf = Pm f ( l - P)clean water-bearing formations, therefore, theneutron reading is directly related to porosity. Forwhere P is NaCI concentration (ppm) x 10.6 .most oils, the hydrogen index will be practically theSubstituting these values in the equation for neutronsame as in water, but gas and light oils have substan-response in hydrocarbon bearing formations,tially lower values of hydrogen index which also varywith temperature and pressure. Therefore, a neutronlog reads too low a porosity when gas or light oil ispresent within its zone of investigation. This charac- X (Ph+ 0.3)/ Pmf (1 - P)](7)teristic can often be used to detect gas zones. If a for-mation is known to have fairly uniform porosity, where Ph=0.25 to 0.9gas/liquid contact may be picked up from the neutronlog alone. A neutron and Densilog" combination,andhowever, provides a clear indication of the gas/liquidinterface as well as more accurate porosity values. Thequantitative response of a neutron log to ahydrocarbon-bearing formation depends on itshydrogen index, such that, x (2.2 Ph)/ Pmf (1 - P)] (8) where Ph < 0.25(I)111 117. 35....- - - - - - - - - - - - - - - -.......- - - - - - - - - - - - - - - - . . - , . . . . - - - - .. Ii .-. - . - . . .. . . . ....,.... ..~ .. . . . . ..: ...... . . . . ..,301 -. i .t . t - . - i .25. 1 ....# 4 f . . ~---.-- ----+- ------_..--+------~~----------#----~~....-...-..------I------.... . . . . , .. I 10 .......-------..~----..-.-----#----.-,.-.....#-~---------+---.-----""t--------~--------1 o...----.....-..--......- -.....------.....-.......- - -....o51015 20 -----.a.-----.-.....-..----..3525 30 . Corrected Porosity. +corr (0/0) FIGURE 1 15 Neutron lithology effect.112 118. Excavation Effect The presence of gas in the zone of investigation ofthe neutron log further affects the neutron response by (12)what is known as "excavation effect." Gas molecules Pmf(l - P) - 1.67Ph + 0.17x .--------- --- -have significant spatial separation compared to liquids Pmf (1 - P)(water or oil). Neutrons travel longer distances beforeinteracting with the gas molecules. This neutron(for Ph ~ 0.7)energy reduction is slower in a gas zone giving lowerporosity log readings. The effect of this inter-Since the hydrocarbon effect on the neutron logmolecular "space" has been named the "excavation"depends upon the residual hydrocarbon saturation ineffect.the zone of investigation, the effect is likely to be The neutron log response in hydrocarbon-bearing much greater on the Compensated Neutron Logformations may be expressed as,because of its deeper investigation. An approximate relation, which is recommended in the case of Com- (9) pensated Neutron Log. including the excavation effect and for both oil as well as gas-bearing formations, is:where,6+NH is the hydrogen index effect.(13)and Pmf (1 - P) - 2.67 Ph + 0.87 x -- - - - - - - --- Prof (1 - P)6+NEX is the excavation effect. (for Ph ~ 0.7)The expressions for 6+NH are,Example: In a clean gas-bearing limestone forma- tion, eN reads 100/0 limestone porosity. The density 0 f gas under reservoir conditions is 0.2 g/cm-, The valuefor oil-bearing formations and of R xo from the microresistivity log is 40 ohm-m and the well is drilled with fresh mud, Rmf at formation temperature being 0.8 ohm-meThe true porosity is calculated as follows:for gas-bearing formations. 1Shr = 1 - Sxo = 1 - [(a x Rmf)/(t m x Rxo)]n An approximate relationship for the excavationeffect is,In case of fresh mud, Prof = 1 and P =0, therefore:K (2 +2 x SWH + 0.4 +) (1 - SWH)where: S""HTaking a = 1 and m = n = 2,and -VRmr/Rxo (1.87 - 2.67 Ph) -- -- ------K = (Pma/2.65)22.67 Ph - 0.87The value of K is 1 for sandstone, 1.046 for Substituting the values +NCN = 0.1, Rmf = 0.8 Q-m,limestone and 1.173 for dolomite.Rxo = 40 Q-m, Ph = 0.2 g/cm- in the above expres- The total correction for the hydrocarbon effect onsions, true porosity + = 26.5010, the residual gasthe sidewall epithermal neutron log response for oil orsaturation Shr = 46.6010 and the correspondinggas, including the excavation effect, is approximatedhydrocarbon correction on the neutron log ~+NCNby the relation: = 16.5010. 113 119. SHALE EFFECTSUMMARY The presence of shale within a formation will cause Fast neutrons emitted by a neutron source arethe calculated apparent neutron porosities to be tooslowed down in the formation. The response of thehigh. The effect of shale is such that it may completelyneutron instrument depends primarily on themask the presence of light hydrocarbons, whichhydrogen content of the formation as a whole.diminish the neutron porosity values. Porosity derived Neutron logs are affected to a large extent by shalefrom neutron logs will be greater than the effectivecontent, by variations of lithology and by presence ofporosity because the neutron instrument responds to hydrocarbons, particularly gas and light oils.the hydrogen index of the formation as a whole, evenAfter converting the conventional neutron logif a part of it is due to the bound water in clay whichreadings from neutron API Units into limestonedoes not correspond to effective porosity. porosity values, these values should be corrected for Assuming that the neutron log response to shale appropriate lithology. The conventional neutron log iswithin a reservoir is the same as that to adjacent shale used in either open or cased holes for the delineationbeds, one can write for a water-bearing shaly forma- of porous sections, hard formations and shales intion,combination with a Gamma Ray Log.The Sidewall Epithermal Neutron Log readings,(14) usually corrected automatically for borehole size, need small mud cake corrections. The sidewall neutron instrument is designed for use only in open holes. It provides porosity readings with minimum boreholewhere Vsh is the bulk volume shale fraction and +Nsh is effects. It may be run in gas-filled holes.the neutron reading in adjacent thick shale beds. TheIf run in combination with a density log, the Com-value of +Nsh depends upon the type of clay minerals pensated Neutron Log can also be automatically cor-present and changes with locality and depth. Its value rected for borehole diameter. However, whenusually decreases in a gradual manner with increasing automatic correction is not made, the readings shoulddepth and ranges between 0.15 and 0.40. The be corrected for borehole size and for other environ-nomograph in Figure 14.15 may be used to obtain mental detractors. The Compensated Neutron Logporosity corrected for lithology. may be run in cased or open holes and it has an ap- In a hydrocarbon-bearing shaly formation, the preciably larger depth of investigation. neutron log response is:The log readings are subject to a number of correc.. tions for the determination of true porosity. Deter- mination of certain effects like lithology, hydrocarbon +N = +- ~+NH - ~+NEX + Vsh X+Nsh(IS) and shale require more information from data like a density log, an acoustic log or one of the microresis-where ~+NH and ~+NEX are the terms for hydrogentivity logs. The uncertainties involved in the deter-index effect and excavation effect as discussed before.mination of porosity from only a Neutron Log shouldIt is assumed that the neutron reading has been cor- be recognized.rected for lithology and environmental conditions.BmLIOGRAPHY Example: The Compensated Neutron Log in a shalysand reads 20070 (apparent limestone porosity) at a cer-Dresser Atlas, Neutron Logs, 1980.tain depth where the shale content is estimated to be20010. The neutron porosity opposite an adjacent thickshale bed is 30010 (apparent limestone porosity). The nomograph shown in Figure 14.16 is used fordetermining the corrected porosity of the sand. A ver-tical line is drawn through the point of V sh :::: 20010and +Nsh = 30070 in the bottom grid up to thediagonal line which corresponds to +LMS == 20070.Through this point of intersection, a horizontal line,which gives a shale corrected porosity of 140/0(apparent limestone porosity) is extended to the sand-stone line for Compensated Neutron Log to obtainlithology corrected value of 18010 neutron porosity atthe particular depth on the log.114 120. Sidewall Compensated Neutron Neutron Log Porosity (0/0)Porosity (0/0)Q)G>gQ)c:oUJ"0 "e 0Cii"0 c~I+~8 tUen. . . .,-. ~ -. -. -- --. ~ 30 "t ...- - -~3535 252530.-t-~30 201202525 1520 !A.---------~- . . . . . - --+-.....20152020 10101515 5510105 oo53 Vsh(010 Shale)FIGURE 14.16Compensated Neutron Log and Sidewall Neutron Log:lithology and shaliness corrected porosity" 115 121. QUESTIONS14(I) The neutron logs see dry gas as___1a) high porosityb) liquid c) low porosityd) no influence(2) A sidewall neutron log indicates the apparent limestone porosity as 13010. If the zone is a sandstone,the real porosity is: a) 13070b) 16.5070 c) 10010d) unknown(3) A compensated neutron log indicated porosity is 20070 in apparent sandstone units. The zone is asand. If the zone is a limestone, the real porosity is: a) 16070 b) 23.5070 c) 14.2010 d) 24.1070(4) What is the actual porosity for the dolomite zones in Ex. 14.1.Zone + A) B) C)D)116 122. GR DEPTH POROSITY%CALIPER GAMMA RAYIAPI Untl0~ I LIMESTONE POROSITYt10Ll SllF.In r.e1;16 6 30 201C o--------T- -- ----- III I ;i ,:)XXIIII..J~iI t8 iqr--r--., II~ I! I ~ Iti ~~ iI ~iII r:~~ ~Gamma ~ay II I I~ ~ I I II ~~ ~II~~~ ,A-"Caliper I A , " ,...........r-, r-- ; . ;,Ir-.... ./I"" -,....1I ~ I.l ,I ~"""-~~I I I v; """""I ......~........Xx~ ~ P--. .... / ...~-- ~ C~~ - I-~ } If ~ -- -- -- -- -...~: I-C:: -- -- --;- ~lll""""" ~ I ..1--,- F=:.--D :~ ~ r---- ---- ~ -.. Ir--.--:,......J,, -- - --1 r-- r--- I ~ - r--- :::>"j... -:.~r> -- -- -- -- ......... ~00-- ~~~ .. "" ~f..~ ,, , ~ ~~~ J.r:> Ie:>, ~ ? , (,/.,,...:?c::-