Chapter 8 Induction logs. Lecture notes for PET 370 Spring 2011 Prepared by: Thomas W. Engler, Ph.D., P.E.

Transcription

1 Chapter 8 Induction logs Lecture notes for PET 370 Spring 2011 Prepared by: Thomas W. Engler, Ph.D., P.E.

2 Introduction Why resistivity logs? Estimate Rt Invasion Resolution (vertical, horizontal) Classification - based on depth of investigation - salt vs fresh mud

3 Classification Flushed Zone 1-6 in. Shallow ft Medium ft Deep 3+ ft Years Name Comments 16 Normal 64 Normal 18 Lateral up to 1955 ES, EL obsolete Fresh Mud R mf >2R w Microlog (ML) Minilog 16 Normal Spherically Focused Induction (6FF40) IES, IEL ISF Obsolete Obsolete Or R t <200 Proximity (PL) LL8/Short Guard Spherically Focused Medium Induction Deep Induction DIL-LL8, DIFL, DISG DIL-SFL Obsolete Current Spherically Focused Medium Phasor Induction Deep Phasor Induction 1985 Phasor Current Array Induction Tool 1990 AIT Current Salt Mud Microlaterolog (MLL) Laterolog-7 Laterolog-3 / Guard LL7, LL3, guard Obsolete R mf <2R w Or R t >200 MLL MSFL Shallow Laterolog (LLS) Deep Laterolog (LLD) 1972 DLL-MLL DLL- MSFL current

4 Induction Log History IES or IEL; short normal or spherically focused curve with a deep induction. (No correction for invasion!) Replaced in the mid-1960s with a Dual induction tool (shallow, medium, and deep curves); DIL-LL8, DIFL, DISG Improved DIL with the addition of the spherically focused shallow log; DISF Next improvement is known as the Phasor Induction Tool - better thin bed resolution and automatic corrections Latest generation induction tool known as Array Induction Tool or Imager - eliminates need for assumption of a step function for the invasion profile.

5 Example: Old ES 10 Normal 19 Lateral 32 LS Lateral Bagley Penn Field Chambers No. 2 Lea County, NM

6 Example: IEL

7 Example: High resolution or Phasor induction Conventional induction log Phasor induction log Source: Halliburton

8 Example: Array induction Ambiguous reading Correct reading DIL log AIT log Source: Halliburton

9 Selection Induction log where fresh mud or oil-base mud (or air-filled holes) is used, where the R mf /R w ratio is greater than 3, where R t is less than 200 ohm-m, and

10 Principle of Operation measures conductivity C= 1000/R (mmho/m) current is fed to a transmitter which generates a magnetic field and hence a circular current in the media. This current induces a voltage at the receiver. Array of transmitter-receiver coils; jamming the signal into the desired formation. Thus, minimizes borehole, invasion, and adjacent bed effects. Typically, six or more coil pairs, 40 spacing from main coil to obtain deep reading. Only resistivity tool to run in air drilled holes.

11 i t Theory Circumferential Current I 1 RILD CILD Gm Rm Gxo Rxo or Gt Rt Gs Rs GmCm GxoCxo GtCt GsCs Tool R m R xo R t Mud Invaded zone G m /R m Uninvaded zone I m G xo /R xo G t /R t I xo I T Induction Equivalent Circuit

12 Geometric Factors Example 1 Given Rt = 10 ohm-m Fresh mud, Rm = 1 Rxo = 20 di = 65 Solution G(ILD) = 0.2 Ra = 11 ohm-m (10% error)

13 Determination of Rt Read apparent resistivity from well log,r a Correct for borehole effect (if necessary) Correct for bed thickness effect (if necessary) Correct for invasion effect (if three curves are present) True formation resistivity, R t

14 Borehole Correction Given 1.5 standoff in a 14.6 hole Rm = 0.35 ohm-m ILD = 50 ohm-m (20 mmho/m) Solution hole signal = 5.7 mmho/m Ct (corr) = Ct - hole signal Rt = 70 ohm-m

15 Borehole Correction significant when - mud is salty - hole size is large and/or oval - formation resistivity is high Corrections greatest for - induction tool against borehole, standoff is zero. - ILM than ILD - holes > 12

16 The bed thickness effect is a f(bed thickness, vertical resolution of tool, resistivity contrast R t /R s ) Corrections necessary for: thick beds w/ R t /R s >> 1 thin beds with large R t /R s contrast Bed thickness correction

17 Bed thickness correction Deep Induction Log Bed Thickness Correction (Schlumberger)

18 Invasion Correction Assume a step profile for invasion Tornado charts to correct for invasion, determine R t, R xo, and d i Rt correction factor between 0.75 and 1.0 Depth of invasion is reflected by induction resistivity contrast: - Rim/Rid > 1.5 indicates deep invasion - Rim/Rid < 1.2 indicates shallow invasion

19 Invasion Correction Example R ILD = 10 Wm R ILM = 14 Wm R SFL = 90 Wm

20 Invasion Correction Example R ILD = 10 Wm R ILM = 14 Wm R SFL = 90 Wm

21 Invasion Correction Example of Given : Deep, medium, Solution : (1). (2). (3). (4). (5). are 10,14, 90 W - m R SFL R ID Invasion Corrections 9; R IM R ID and shallow readings 1.4 Rt 0.89 from chart RID R t Rt *RID 8.9 m R W ID Rxo 16 R xo 144W m R t d i 50"

22 Example: SFL-ILM-ILD Big Lake (San Andres)

23 Phasor Induction Improvements (1). Thinner bed resolution (2 ft.) (2). Comprehensive set of automatic corrections for: - shoulder effect and thin bed resolution - skin effect - borehole and cave effect - large boreholes - invasion effects

24 Example: High resolution or Phasor induction Conventional induction log Phasor induction log

25 Array Induction Array Induction Image Tool (AIT) or High Resolution Imager (HRI) Main features: 1. full borehole corrections over a range of R t /R m contrasts 2. the ability to use short array information to solve for effective borehole parameters 3. Five log curves are presented at median depths of investigation of 10, 20, 30, 60 and 90 inches. Three vertical resolutions of 1, 2 and 4 ft. 4. improvement in invasion profiles for both oil- and water-based muds. This includes accurate R t estimate and a quantitative description of the transition zone. 5. Capability of producing resistivity and saturation images of the formation.

26 Example: Array induction Ambiguous reading Correct reading DIL log AIT log Source: Halliburton

27 Example: Array induction Estimated invaded volumes from AIT log (Halliburton)

28 References Bassiouni, Z: Theory, Measurement, and Interpretation of Well Logs, SPE Textbook Series, Vol. 4, (1994) Chapter 5, Sec 5 and 6 Schlumberger, Log Interpretation Charts, Houston, TX (1995) Western Atlas, Log Interpretation Charts, Houston, TX (1992)