FORMATION EVALUATION PETE 663 PA SS SSIV IVE E MEA MEA SU SURE REME MENT NTS S - SP
Summ Sum m er 2010 2010 Dr. Da Davi vid d Schech Schechter ter
HEADING Well loc loca atio tion n
•
•
•
Depth references •
Date of log
•
Well depth
Casing shoe depth • •
Bit size Mud data
– Type – Properties – Resistivities •
Max. Temperature
HEADING Well loc loca atio tion n
•
•
•
Depth references •
Date of log
•
Well depth
Casing shoe depth • •
Bit size Mud data
– Type – Properties – Resistivities •
Max. Temperature
•
DRILLING DISTURBS FORMATION Drilling and rock crushing
•
•
Damage zone
Mud systems and invasion
•
Oil based mud
•
Small conductivity mud
Damaged zone
Mudcake
• Shallow invasion • Thin cake •
Water based mud
• Moderate to very conductive mud
• Shallow to deep invasion • Thin to thick cake
Invading filtrate
MUD FILTRATE INVASION Uninvaded Zone (Rt ) Invaded Zone Zon e (R (Rxo )
Wellbore Mud Mu d (Rm)
Uninvaded Zone (Rt )
Mud Cake (Rmc)
COMMON TERMINOLOGY Borehole Rm : Borehole mud resistivity Rmc : Mudcake resistivity Invaded zone Rmf : Mud filtrate resistivity Rxo : Invaded zone resistivity Sxo : Invaded zone water saturation Uninvaded zone Rw : Interstitial water resistivity Rt : Uninvaded zone resistivity Sw : Uninvaded zone water saturation
PASSIVE MEASUREMENTS
• • •
Caliper Spontaneous Potential Gamma Ray
• •
Natural Spectral
CALIPERS •
Uses
• • • • •
Hole volume Tool corrections actual
Crude lithology indicator apparent
Two, three, or four arms Linked or independent
Calipers may disagree (limitations)
• •
arm
Mudcake (permeability)
Properties
• • •
Two-arm caliper
Non-circular hole Deviated wells
apparent actual
arm Three-arm caliper
CALIPER INTERPRETATION •
• •
Hole volumes • In general, more arms give better accuracy • Two arms < 100% error • Three arms < 20% error Mud cake • If d caliper < d bit • h mc = (d bit - d caliper )/2 Lithology • Shales may indicate borehole enlargement • Spikey curve may indicate fractures
SP – DEFINITION • SP is a natural occurring electrical potential relative to a surface potential measured in the borehole mud • Potentials are created by chemically induced electric current • The potential of the surface reference must remain constant
USES OF SP 1. Determine values of formation water resistivity 2. Identify permeable zones 3. Qualitative indication of shale content 4. Define bed boundaries 5. Well-to-well correlation
SPONTANEOUS POTENTIAL (SP) •
Uses • Correlation • Lithology • Shaliness indicator • Depositional environment
-12 +59 mV mV
indicator
•
Properties • Measures formation voltage • Passive measurement
-71 mV
+ + SHALE + + POROUS, - PERMEABLE BED + + + SHALE +
Ransom , PFE
THE SP TOOL
SHALE
• One electrode • Insulators on either side
SAND
SHALE
• Surface ground electrode – at a stable potential
SP PRINCIPLES • Must have water-based mud • Mud--formation water salinity difference causes battery effect
Electrochemical Effect SHALE
SAND
• Battery effect components
Flushed Zone
Virgin Zone
Less Salty Water
Salty Water
• Electrochemical • • • • • •
Liquid Junction Potential, E j In permeable region Anions more mobile than cations Membrane Effect, Em Shale acts as membrane Repels anions / passes cations
• Electrokinetic (Streaming) • Usually minor, disregarded
Membrane effect
Membrane effect + + + + +
Virgin Zone
ORIGIN OF SPONTANEOUS POTENTIAL The electrochemical potential sensed in the borehole is generated by the sum of two potentials known as the membrane potential and Em and the liquid junction potential E j. Ec = Em + E j
LIQUID JUNCTION POTENTIAL A liquid junction potential develops when a concentrated salt solution (formation water ) is in direct contact with a diluted salt solution (fresh mud filtrate) The net effect of more positive ions in formation water and more negative ions in mud filtrate creates potential difference.
MEMBRANE POTENTIAL, Em Created when a shale is introduced between a concentrated salt solution(formation water) and a diluted salt solution (fresh mud filtrate)
SP CURRENTS
Note: Reverse SP occurs when formation water is fresher than mud filtrate
P S D E S (+) R E V E R
5
4
TYPICAL SP RESPONSES – BASED ON THE DIFFERENCE BETWEEN Rw and Rmf. 5. Rmf << Rw - Amplitude large and positive
3
4 . Rmf < Rw - Amplitude positive but not large 2
1
P S L A M(-) R O N
3.
Rmf = Rw
- No SP deflection
2.
Rmf > Rw - Amplitude negative but not large
1.
Rmf >> Rw - Amplitude large and negative
STATIC SP (SSP) If it were possible to prevent SP currents from flowing and measure the potential of mud this would provide a value for the SSP Conditions where the SSP is recorded directly: 1. Thick zones 2. Clean (no shale) zones 3. Only water – bearing zones 4. Permeable zones
SELECTING A 100% WATER SATURATED ZONE
Low resistivity suggesting a water bearing formation
SELECTING A SHALE BASE LINE
Sandstone baseline
20 mV
- 110 mV
Shale base line is the SP response across a thick shale or several shale intervals
PSEUDO- STATIC SP (PSP) • Presence of shale in the formation will reduce the static SP • Shale lattice will slow the migration of chlorine ions and assist the flow of sodium ions, decreasing E j • This reduces SSP to a pseudo-static value, PSP • The volume of shale can be calculated: Vsh = 1- (PSP)/(SSP)
EXAMPLE PROBLEM
SP RESPONSE IN THIN BEDS
USING THE SP EQUATION FOR Rw DETERMIMATION - CLASSICAL METHOD 1. Determine formation temperature 2. Find Rmf at formation temperature 3. Convert Rmf at formation temperature to Rmfe value 4. Compute Rmfe / Rwe ratio from the SP 5. Compute the Rwe 6. Convert Rwe at formation temperature to Rw
USING THE SP EQUATION FOR Rw To determine Rw , we must know: 1. Formation Temp, Tf Original sample: R w = 0.1 ohm-m@ 150F; What is Rw at formation temperature (T f ), which is 250F?
• Actual temp reading or • BHT and geotherm gradient • Chart GEN-2 (H) GEN-6 (S)
2. Rmf at Tf • Actual measurement or • Correct surface Rmf • Chart GEN-5 (H) GEN-9 (S) • or Ar p’s equation •
R1(T1 + 7) = R2(T2 + 7) (T ºF)
•
R1(T1 + 21.5) = R2(T2 + 21.5) (T ºC)
3. Ess p •
SP log
1
0.1 ohm-m, 150
2
Rw = 0.058 ohm-m
0.58 ohm-m, 250
4 3
A
B
THE SP EQUATION - 1 -20mV+
• Define Essp = (Esp )max
Essp
• We assume:
Shale
- 80 mV
E ssp
≈
( E l
+
E m ) Clean Sand
• From electrochemical theory: E ssp
= −0.133(T f + 460) log10 ( a w
/ amf )
Shaly Sand
- 60 mV
where Tf = formation temp, deg F aw = formation water activity amf = mud filtrate activity Essp = max SP deflection, mV
Sandy Shale
S h a l e B a s e l i n e
-20 mV
Shale
C
THE SP EQUATION - 2 • Difficult to measure activities • Substitute resistivities for activities E ssp
= −0.133(T f + 460) log10 ( Rmfe
E ssp
= −0.24(T f + 273) log10 ( Rmfe
/ Rwe )
/ Rwe ) e f m
• For small salinities, a = 1/R – For fresh mud filtrate, assume – Rmf e = Rmf or
R r o
e w
R
– Rmf e = 0.85Rmf (Schlumberger)
• For high salinities – Correction needed – Use Chart SP-2 (Schlumberger) – Use Chart SP-3 (Halliburton)
Rw or Rmf
D
EXAMPLE • Determine Rmf @Tf
(Arp’s Eq.)
– 5.6(11+21.5)/(33+21.5) = 3.3 m
• Determine Essp
10mV -|↔|+
– Shale base line – Maximum deflection line – Calculate deflection -50mV
• Apply SP equation – -50 = -0.24(33+273)log(3.3/Rwe) – Rwe = 0.68 – Chart SP-2 gives Rw = 1.3 ohm-m
(See next page) Rarely known Usually use charts, instead
Rmf = 5.6
m @ 11º C
Tf = 33º C
F
e f m
R r o
e w
R
Rwe= 0.68
Rw or Rmf
Rw = 1.3
PROBLEM The SP deflection is –60 mV across a thick, waterbearing, clean zone. The value of Rmf at that temperature of 100 F is 0.5 ohm-m. Determine Rw at the same temperature (100 F)
Rw from SP: Classical Method First, we determine the Rmfe (effective Rmf), since the resistivity is not an accurate determination of the ion activity that produces the SP.
Rw ESTIMATION FROM Rwe
1. Determine Rmfe
Rmf, 0.5 ohm-m 0.5,100F
Figure 9-13 in the manual. Rmfe = 0.45 ohm-m at 100 F
Rw ESTIMATION FROM SSP
7
2. Determine Rwe from Rmfe
60, 100
Figure 9-14 of your manual
SSP
Rmfe/Rwe = 7. Therefore, Rwe=0.45 ohm-m/7=0.064 ohm-m at 100 F
Rw ESTIMATION FROM Rwe (Rwe=0.064 ohm-m at 100 °F)
3. Finally, determine Rw • Using Figure 9-13 of your text again, we determine Rw=0.10 ohm-m at 100° F
0.064, 100F
0.064 mV
• Here, Rw
THE SILVA-BASSIOUNI METHOD Rw ESTIMATION FROM Rwe Figure 916 of your text.
For the same problem as before, ie Rmf=0.5 ohmm at 100° F, determine Rw if the SP deflection is –60 mV.
145 mV – 60 mV = 85mV
We see Rw=0.1 ohm-m, as shown with the classical method. Figure 9-16 of
COMPARISON OF THE CLASSICAL AND SILVABASSIOUNI METHOD
•
The classical method requires 3 steps for the determination of Rw.
•
The Silva Bassiouni method combines Fig 9-13 and 9-14 into one chart ( Fig 9-16 ) and gives you the same value of Rw. Hence it is easier to use.
FACTORS AFFECTING THE SP RESPONSE
• • • •
Hydrocarbons: reduce the SP deflection Shaliness: reduces the SP deflection Bed thickness: thin beds do not develop a full SP deflection Permeability: low permeability zones will have a very high invasion diameter, so it may be impossible to read the Junction Potential, hence SP readings may be low
OTHER SP ISSUES • Shaliness V sh = 1 − E sp / E ssp
SSP = -K lo g
Rmf e Rwe
• Environmental – Uses curve shape
• Permeability indication – POOR perm predictor
• SP value & polarity depend on salinity contrast
Rmf
= Rw
Rmf
Rmf >Rw FRESH MUD
ZONATION • • •
•
•
Zonation - Defines intervals of similar properties Purpose • Well-to-well correlation • Evaluation of specific intervals Criteria • Lithology • Fluids • Porosity and permeability Begin with coarse zonation • Typically • Well-to-well correlation 20 - 100 ft • Detail evaluation 10 ft thic k or more • Easy lithologies first, e.g., shales Refine • More subtle lithology changes • Fluids in por ous, perm intervals • Depends on measurements available
PASSIVE LOG CORRELATION •
•
GR, SP, and CAL
• •
Often correlate
•
Different reasons
Different measurements
Correlation helps
•
GR instead of SP in oil base mud
•
Easier detection of shales
•
Facilitates “ zonation”
SUMMARY • •
Drilling process affects formation • Alters rock near wellbore • Invasion Passive logs respond to borehole, formation, and fluids • Caliper • Simple measurement • Care needed when interpreting and comparing caliper • SP • Needs water based mud • Estimates R
EXTRA SLIDES FOLLOW
MEMBRANE POTENTIAL
LIQUID JUNCTION SP