Basics of Geophysical Well Logs_Lithology&Resis7vity

Transcription

1 1

2 Spontaneous Poten7al When the well bore is filled by a water based mud and in presence of an alterna7on of permeable and impermeable layers, due to electrochemical phenomena, electrical currents are spontaneously generated at the interfaces between mud and forma7on and between impermeable shales and reservoir sands. The SP log is the measurement of the poten7al of a down hole electrode with respect to a surface reference electrode which is propor7onal to the intensity of the currents generated. 2

3 Spontaneous Poten7al + - Flushed zone Virgin zone Membrane poten7al Flushed zone + - Virgin zone Liquid-junc7on poten7al The magnitude of the SP generated is a func7on of the salinity contrast between mud and forma7on water. Two are the main SP genera7on mechanisms: membrane poten7al, liquid-junc7on poten7al. E m = Rθ I a I c ln C 1 Fn I a + I c C 2 E l = Rθ Fn ln C 1 C 2 The SP log is primarily a permeability contrast indicator as well as a fundamental lithology log especially in shaly sand sequences 3

4 Spontaneous Poten7al (fresh mud and salty forma7on water) SP sh SP sd A Vsh = A / B Vsh = (PS-PSsd)/(PSsh-PSsd) SP log B 4

5 SP log interpreta7on problems Main problems of SP log interpreta7on are mostly linked to: lack of permeability contrast lack of mud/forma7on water salinity contrast thin beds hydrocarbon occurrence Low permeability contrast Marls Low permeability limestones Marls mv 5

6 SP log interpreta7on problems SP log behavior as a func7on of mud/forma7on water salinity contrast Hydrocarbon effect on the SP log Shales Shales High permeability sandstones Gas bearing sands Shales Water bearing sands Shales mv SSP = -K log (Rmf/Rw) mv Rm > Rw Rm = Rw Rm < Rw 6

7 SP SP Rmf > Rw Rmf = Rw 7

8 Inverted SP: Rmf << Rw 8

9 SP log interpreta7on problems SP log Shales inflexion point High permeability sandstones inflexion point Shales Ver7cal resolu7on of the SP log 9 mv Boundary loca7on by means of the SP log

10 Geological applica7on of the SP log In absence of SP anomalies, when dealing with water saturated forma7ons and with muds of constant salinity in all the wells under evalua7on, the SP log may be used for geological correla7on among wells and to define the different sedimentary facies (in order to define lateral and ver7cal evolu7on of the sedimentary environments). 10

11 The natural radioac7vity of geological forma7ons is due to the presence in rock forming minerals of the radioac7ve isotopes of elements such as Uranium (U), Thorium (Th) and Potassium (K). These isotopes are mostly related to clay minerals whose content in Th and K is generally higher than associated sand and sandstones. In carbonate forma7ons the radioac7vity is mostly due to the presence of U and the Gamma Ray level is not directly related to forma7on shalyness. Gamma Ray log 11

12 Gamma Ray log Low activity Cement High activity Cement K = 4% Th = 24 ppm U = 12 ppm 200 GAPI Typical responses Of GR log Low activity Cement Gamma Ray American Institute Test Pit 12

13 Shale volume from GR log BS, CALI, SP, GR LLD, LLS, MCFL TNPH, RHOB, DT Caliper - GR Resistivity Density/Neutron GRsh GRsd GR (GR-GRsd) Vsh = (GRsh-GRsd) 13 13

14 Gamma Ray: environmental correc7ons Main factors affecting GR measurements are: hole diameter sonde position in the well mud loaded with radioactive material Sand γ γ Shale γ γ γ mud γ Sand γ γ γ γ mud Shale γ γ γ γ γ γ Sand Tool eccentered Tool centered BS Measured GR CAL Corrected GR 14

15 Gamma Ray: environmental correc7ons 15

16 Gamma Ray Spectrometry Gamma Ray emission spectra WLL Services SLB NGS SLB HNGS (PEX) BA SL 16

17 GR in cased hole GR Neutron GR correla7on Log in cased hole Forma7on Evalua7on logging in CH 17

18 Caliper log Single arm caliper Two arm caliper Three arm caliper Four arm caliper 18

19 0 6 6 GR 200 BS MCFL LLS 2000 CAL LLD PEF RHOZ NPHI -15 Mud cake resul7ng in a hole diameter restric7on Comparison among different caliper measurements 19

20 Borehole Geometry Tool (BGT) 20

21 RHOB TNPH CALIPER DRHO Noisy logs due to poor borehole wall quality. 21

22 Classifica7on of Resis7vity logs In rela7on to depth of inves7ga7on Macro-devices to measure Rt Micro-devices to measure Rxo In rela7on to tool physics Non focused, galvanic devices (WLL) Focused, galvanic devices (WLL e LWD) Low frequency induc7on devices (WLL) EM wave propaga7on devices (LWD) 22

23 Environmental effects affec7ng resis7vity measurements Rt Rxo 1 Rt 1 Rs Rs Rt Rxo Rxo 2 Rt 2 Rs Rt Rs Rm Rt Rxo 3 Rt 3 Borehole 1D VERTICAL (shoulder bed) 1D RADIAL (invasion effect) 2D Rt 1 Rxo 1 Rs Rs Rt 2v Rm Rv Rxo 2 Rt 2hmin Rt 2HMAX Rs Rt Ro Rh Rxo 3 Rt 3 Rs 2D +dip (dipping beds) 3D Anisotropy 23

24 Invasion effects and formation parameters 24

25 Sw = Sxo = 100% Ro= F Rw = (1/Φ^m ) Rw Rxo= F Rmf = (1/Φ^m ) Rmf Sw < 100%, Sxo <100% Rt =(1/Φ^m)(Rw)/Sw^n ) Rxo=(1/Φ^m)(Rmf)/Sxo^n ) 25

26 Invasion and related resis7vity profiles 26

27 Fresh mud and salty formation water 27

28 Caliper - GR Resistivity Density/Neutron Rmf very close to Rw 28

29 Non focused electrical resis7vity tools 29

30 Old E (electrical) logs Conventional Electrical Log (ES) Depth Track 1 Track 2 Track 3 SP (mv) SN (ohmm) Ampl. SN (ohmm) LN (ohmm) IN (ohmm) Linear scale Linear scale Linear scale SN = Short Normal (spacing 16 ) Ampl. SN = Amplified Short Normal LN = Normal (spacing 64 ) IN = Inverse or Lateral (spacing 18 8 ) 30

31 Old E (electrical) logs Induction Electrical Log (IES) Depth Track 1 Track 2 Track 3 SP (mv) SN (ohmm) Ampl. SN (ohmm) 6FF40 R (ohmm) 6FF40 C (ohmm) Linear scale Linear scale Linear scale SN = Short Normal (spacing 16 ) Ampl. SN = Amplified Short Normal 6FF40 R = Induction log deep (40 ) resistivity 6FF40 C = Induction log deep (40 ) conductivity 31

32 Normal and lateral resis7vity logs 32

33 Normal electrical log responses: normal 33

34 Lateral electrical log responses: lateral 34

35 Well Secala 1 SP/ILD/SN/MLL FDC/CNL/BHC MLL (Rxo) SN (Ri) ILD (Rt) 35

36 Resis7vity tools: Wire Line (WLL) & While Drilling (LWD) 36

37 Dual Laterolog GR 200 BS MCFL LLS 2000 CAL LLD

38 Sferically Focused Log 38

39 Environmental effects on focused galvanic tools 39

40 Induc7on logging Induction log principle 40

41 ILD/SFL example black curve ILD red curve SFLU North Adria7c fresh WBM 41

42 Rxo logging: the MSFL tool GR 200 BS MCFL LLS 2000 CAL LLD

43 Caliper - GR Resis7vity Density/Neutron 43

44 While Drilling resis7vity logging While Drilling resis7vity logging are of two types: galvanic; EM wave propaga7on. While Drilling Galvanic logs (Anadrill RAB only) can be used only in presence of Water Based conduc7ve Muds. EM wave propaga7on logs, due to the presence of metallic body of the system, can be obtained only using higher frequencies with respect to the Wire Line induc7on ones with advantages and disadvantages. 44

45 LWD Propaga7on Resis7vity Ampiezza A B T R 1 PS θ t Segnale a R1 Segnale a R2 R 2 AT(dB) = K Log (B/A) = f (σ) PS (deg.)= f (ε) 45

46 LWD resis7vi7es: Real 7me vs memory Real Time Memory 46