Basic well Logging Analysis – The Resistivity Logs

1. Basic well Logging Analysis – The Resistivity Logs Hsieh, Bieng-Zih Fall 2009

2. General • Resistivity logs are electric logs which are used to: • (1) determine hydrocarbon versus water-bearing zones, • (2) indicate permeable zones, and • (3) determine resistivity porosity. • By far the most important use of resistivity logs is the determination of hydrocarbon versus water-bearing zones.

3. General (Cont.) • Because the rock’s matrix or grain are non-conductive, the ability of the rock to transmit a current （電流） is almost entirely a function of water in the pores. • Hydrocarbons, like the rock’s matrix, are non-conductive; • therefore, as the hydrocarbon saturation of the pores increases, the rock’s resistivity also increases.

4. Calculate water saturation • A geologist, by knowing a formation’s water resistivity (Rw), its porosity (Φ), and a value for the cementation exponent (m), can determine a formation’s water saturation (Sw) from the Archie equation: • Sw= water saturation • Rw= resistivity of formation water, Rt = true formation resistivity as measured by a deep reading resistivity log • a = tortuosity factor, m = cementation exponent, n = saturation exponent (most commonly 2.0)

5. Two basic types of resistivity logs • The two basic types of logs in use today which measure formation resistivity are induction （感應式） and electrode （電極式） logs. • The most common type of logging device is the induction tool (Dresser Atlas, 1975)

6. Principle of the induction log 接收線圈 感應電流 地層環路 渦電流 發射線圈 通以電流

7. Induction log • An induction tool consists of one or more transmitting coils that emit a high-frequency alternating current of constant intensity. • The alternating magnetic field which is created induces secondary currents in the formation. • These secondary currents flow as ground loop currents perpendicular to the axis of the borehole, and create magnetic fields that induce signals to the receiver coils. • The receiver signals are essentially proportional to conductivity, which is the reciprocal of resistivity (Schlumberger, 1972). • conductivity = 1000/resistivity

8. Electrode log • A second type of resistivity measuring device is the electrode log. • Electrodes in the borehole are connected to a power source (generator), and the current flows from the electrodes through the borehole fluid into the formation, and then to a remote reference electrode. • Examples of electrode resistivity tools include: normal, Laterolog*, Microlog*, and spherically focused logs.

9. Resistivity Log • 電阻井測是量測電極(A)與另一端電極(M及N)之間的電位差，再利用歐姆定律計算而得地層電阻。

10. Choose an appropriate log survey • Induction logs should be used in non-salt-saturated drilling muds (i.e. Rmf > 3 Rw) to obtain a more accurate value of true resistivity (Rt). • Boreholes filled with salt-saturated drilling muds(Rmf≒Rw) require electrode logs, such as the Laterolog* or Dual Laterolog* with or without a Microspherically Focused Log*, to determine accurate Rt values.

11. Use Induction log or Laterolog Determining when use of an induction log is preferred over an electrode log such as the Laterolog*.

12. DEPTH OF RESISTIVITY LOG INVESTIGATION • Flushed Zone (Rxo) Invaded Zone (Ri) • Microlog* Short Normal## • Microlaterolog* Laterolog-8*## • Proxmity* Log Spherically focused Log*## • Microspherically Focused Log* Medium Induction Log • Shallow Laterolog* • Uninvaded Zone (Rt) • Long Normal • Lateral Log • Deep Induction Log • Deep Laterolog* • Laterolog-3* • Laterolog-7* • 電阻井測可依據其偵測的範圍（由井口深入地層的半徑大小），而分為淺測徑、中測徑及深測徑電阻井測，可以分別得到浸污區、過渡帶、及未浸污區的地層電阻值，進而求得地層的真電阻值。

13. Common resistivity logs

14. Induction Electric Log

17. Dual Induction Focused Log SFL (solid line) ILM (dotted-and –dashed line) ILD (dashed line)

23. Exercise • Find the true formation resistivity (Rt) and corrected resistivity of the flushed zone (Rxo) by using the Tornado Chart at depth 13590, 13600, 13610, 13620, 13630, and 13640.

24. Microlog (ML)