PETROPHYSICAL EVALUATION CORE COPYRIGHT. Resistivity, Archie, and Saturation Determination Part 1. By the end of this lesson, you will be able to:

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1 LEANING OBJECTIVES PETOPHYSICAL EVALUATION COE esistivity, Archie, and Saturation Determination Part 1 By the end of this lesson, you ill be able to: List to or more ays to approach ater saturation, S, determination Discuss hy formation resistivity is required to use the Archie Equations Tell about three resistivity parameters required to apply the Archie Equations to calculate ater saturation Explain ho hydrocarbon saturation is determined after ater saturation is knon Describe a method of Quick Look evaluation hen a Triple Combo log is available (esistivity/density/neutron and Gamma ay) 1

2 SW ESTIMATION POCESSES Core analysis Nuclear Magnetic esonance Imaging (MI) Archie relationships resistivity, porosity saturation model Principles of resistivity logs Derivation of required parameters ( W,, m, n ) Archie Crossplot resistivity, porosity SW ESTIMATION POCESSES Other Quick-Look approaches atio method resistivity and SP F overlays or Synthetic O excellent method using porosity Special cases shaly sands 2

3 ESISTIVITY DEFINITIONS : esistivity of formation ater A function of salinity and temperature The higher these to variables, the loer the resistivity of the ater (the ater ill be more conductive). H T o: esistivity of ater-bearing formation o > (more rock, less ater) t: esistivity of hydrocarbon-bearing formation t > o (rock, hydrocarbon are non-conductive) O PY IG HOW TO MEASUE ESISTIVITY? Induction Measurement C Dual Laterolog Measurement 3

4 CITEIA FO ESISTIVITY TOOL SELECTION: INDUCTION VESUS LATEOLOG Laterolog Induction Oil Based Mud No Yes Salt Water Mud Yes Possible 1 Fresh Mud Possible 2 Yes Air Filled Holes No Yes High t Yes No Lo t Possible 3 Yes t > xo Preferred t < xo Preferred 1. Possible if the folloing conditions are met: Hole is small and in gauge. t / m is lo. Tool position in the borehole is ell knon. 2. Possible if t / m is high. 3. Possible if tool string length correction is applied. Which resistivity tool? HYDOCABON SATUATION AND THE COMBINED ACHIE EQUATION n True resistivity, t, of HCbearing rock depends on: esistivity of formation brine t m S 1 Porosity S Water saturation m, n Lithology S Saturation, is a fraction of pore volume S hc = 1 S 4

5 COMBINED ACHIE EQUATIONS The combined Archie equation can be expressed as to equations: t m S 1 n o Archie I is for et formations: F = = -m t o Archie II is for hydrocarbon zones: I = = S -n If you solve Archie I for o, and substitute that into Archie II, the result is the combined equation. Also, mathematically, S -n = 1/S n ACHIE I Archie I: The foundation of quantitative petrophysics: F = o = m F = Formation resistivity factor (FF or F) o = esistivity of 100 % brine saturated rock = Brine resistivity (m) = Porosity m = Cementation factor F Each point represents a separate rock sample 100% saturated ith ater of m 1.0 5

6 F VESUS CHAT (FO 100% WATE SATUATED OCK ) Humble Equation Easy Square oot CEMENTATION FACTO M Formation Factor, F = m is cementation exponent m has large effect on saturation calculation m is a function of pore system tortuosity (the more complex the pore system, the higher the formation factor F. 1 m 1 F = = o m L L 6

7 M-VALUES, COMMON ESEVOI OCKS ACHIE II Lithology Sandstones Unconsolidated 1.4 Loosely consolidated 1.6 Friable 1.7 Average 1.8 Very hard 2.0 Carbonates t o I = = S -n I = esistivity index t = esistivity of partly brine saturated rock 0 = esistivity of fully brine saturated rock S = Water saturation n = Saturation exponent 100 I n m Saturation exponent All points are measured on the same core sample S 1.0 7

8 ACHIE II, SW & ESISTIVITY INDEX SUMMAIZED Water saturation (S) is our variable. Saturation Exponent n Porosity ( ) and ater resistivity () are fixed ock resistivity (t) is measured O PY IG H T n = saturation exponent esistivity Index (I) = t o S = ( m ) (1/n) x t QUICK-LOOK EVALUATION 1. Identify sands in G log and identify ater-saturated zone in sand. 2. Evaluate porosity, phi, from C density/neutron logs From deep-resistivity log, identify ater zone based on lo resistivity; calculate porosity in hydrocarbon zones. 4. In density/neutron log, calculate ater (close curves) or gas (separation of curves) in hydrocarbon zones In deep resistivity log identify contacts and hydrocarbon fluid types. 8

9 QUICK-LOOK EVALUATION UNCETAIN FLUID CONTACTS Gas zone Oil zone Wet zone SOUCES OF W catalogs a Direct measurement of a ater sample F overlays Chemical analysis and conversion to resistivity Crossplots atio technique Estimation of spontaneous potential curve (SP) 9

10 ESISTIVITY, ACHIE, AND SATUATION DETEMINATION PAT 2 USE OF ACHIE I EQUATION TO DETEMINE W esistivity of formation brine Archie I, solved for = m o Example: o = 5 Ohm.m = 0.25 m = 2 = = = Ohm.m 1

11 CALCULATE W AT 13000' Archie I = m o At 13,000 : WA EXECISE Level Porosity % t a = t m = in 100% ater saturated formation Let m = 2 to start a

12 WA EXECISE Level Porosity % t a a = t m = in 100% ater saturated formation DIECT MEASUEMENT OF W Drill stem test Let m = 2 to start Wireline formation test tool Produced sample at the separator What are the limitations to each? 3

13 W FOM CHEMICAL ANALYSIS OF A WATE SAMPLE Ion Symbol Concentration ppm Multiplier* Effective Concentration** Sodium & Chloride Na + & Cl - 25, ,000 Carbonate CO , Sulfate SO , ,850 Bicarbonate HCO , ,000 Magnesium Mg ++ 4, ,800 Total 46,000 33,450** **Effective Concentration in NaCl equivalents.. * Multiplier is a function of both type of ion and temperature CONVESION TO ESISTIVITY Function of concentration and temperature Chart for conversion Example: 33,450 ppm TDS (the red line), assume the formation temperature is 200 deg F (94 degc) in the objective formation. Then the at 200 degf is ohm.m = Chart can be approximated by Arps equation 2 = 1 [(T )/(T )] o F 2 = 1 [(T )/(T )] o C constant salinity line of 33,450 ppm 4

14 SATUATION DETEMINATION ACHIE II EQUATION S = { ( ) / ( m t ) } (1/n) S (1 - S ) Water Oil Matrix Bulk Volume Water (BVW) = S 1 - S W DETEMINATION IN HYDOCABON BEAING SAND esistivity: S hc (red) S irr (blue) ater saturation coating the grains 5

15 S W DETEMINATION ACHIE EQUATIONS COMBINED S W DETEMINATION ACHIE EQUATIONS Archie I Archie II 6

16 ACHIE SATUATION EQUATIONS S EXECISE 0 n Calculate S at 12,440' S t n S a m n t F t 7

17 A QUICK-LOOK SW CHAT Nomogram for clean sandstones SW DETEMINATION POOSITY ESISTIVITY COSSPLOTS Provide overall picture of relative potential of all zones. Errors or suspect data become obvious as a pattern Pickett Plot Widely used Available in most petrophysical softare Hingle Plot Somehat antiquated Pickett more common 8

18 SW DETEMINATION POOSITY ESISTIVITY COSSPLOTS Pickett Plot Made on log-log paper t versus Demands little prior knoledge of fixed parameters and m and may allo their determination A bird s-eye vie of all zones on one display A better perspective than looking at a long log print. An easy, comprehensive comparison of multiple zones versus S W DETEMINATION PICKETT PLOT Slope = -1/m Hingle Plot equires special graph paper eciprocal root of t versus linear Somehat antiquated Pickett more common = esistivity at 100% porosity! 9

19 S W DETEMINATION PICKETT PLOT S n a m t log t = - m log + log + log a - n log S hen S = 1, log S = 0 and y = mx + b, PICKETT PLOT SUMMAY log t = - m log + log Provides estimation of S ith minimum of pre-determined information Can help determine m and by reriting Archie equation Can plot porosity on the y axis and resistivity on the x axis 10

20 S W DETEMINATION PICKETT PLOT % slope = -1/m F OVELAY QUICK-LOOK TECHNIQUE: O VESUS T Formation Factor or F Curve is calculated from a porosity log, usually the density log F curve is readily converted to a synthetic o curve by multiplying it by (from et sand data or as assumed) F overlay quick-look is similar to a technique 1 F = = m o P W 11

21 F OVELAY QUICK-LOOK TECHNIQUE: O VESUS T 1. From the density log, B, create a porosity log, D 2. From the density porosity log, D, compute F curve (here F = 1/ Dm ) and let m = 1.8 (sands) or 2.0 (carbonates) 3. On the logarithmic resistivity scale, overlay the F curve to match the resistivity of any available et zone. At this point, the F overlay curve becomes a synthetic o curve, since o = F. 4. Or, if no et zones available in a sand-shale environment, overlay the F curve on the shale resistivity. 5. o t separation S Finally, shade in all intervals here the deep resistivity curve reads higher than the synthetic o curve. This shaded area represents probable hydrocarbon saturation. F OVELAY QUICK-LOOK : O VESUS T If is unknon, the F curve is normalized to o by overlaying it on et sand, or a barren (nonsource rock) shale. o is calculated from the F overlay curve. The separation beteen o and t on this log scale can be scaled into S. S In ater-bearing zones, o and t are approximately the same. 12

22 EVALUATION CHECKLIST Discriminate reservoir from non-reservoir G, SP, D/N logs Discriminate beteen ater-bearing and hydrocarbon-bearing reservoir intervals resistivity Discriminate beteen gas-bearing and oilbearing reservoir intervals Evaluate reservoir porosity Calculate Determine reservoir ater saturation LEANING OBJECTIVES D/N separation, sonic D/N, sonic Archie I or Pickett plot Archie 2, Pickett plot, quick-look You should are no: able to: List to or more ays to approach ater saturation, S, determination Discuss hy formation resistivity is required to use the Archie Equations Tell about three resistivity parameters required to apply the Archie Equations to calculate ater saturation Explain ho hydrocarbon saturation is determined after ater saturation is knon Describe a method of Quick Look evaluation hen a Triple Combo log is available (esistivity/density/neutron and Gamma ay) 13