Basic Well Log Analysis

1. Basic Well Log Analysis Reading Rocks from Wireline Logs

2. Wire Line Logging Tool strings used in wireline logging operations

3. Core-log Integration

4. Important Principles You Will Need To KnowTo Make Any Sense of the Wiggle Traces onWireline Log Strip Charts • Porosity = pore volume/total volume of a rock • Porosity can range from 0% to in excess of 40% • Saturation = volume of the porosity occupied by some fluid. • The possible fluids are almost always water or hydrocarbons; either liquid or gas. • SW = water saturation in percent, • 1 - SW is hydrocarbon saturation in percent. • Lithology = rock type, including fluid filled pores, with physical characteristics of: • Resistivity • spontaneous potential; SP • natural radioactivity; e.g. Gamma Ray emissions • bulk density • hydrogen content of rock and fluid filled pores • interval transit time (sonic velocity)

5. Basic Well Log Analysis • Logs Help Define • physical rock characteristics • Lithology/mineralogy, • porosity, • pore geometry, and • permeability. • Logging data are used to: • identify productive zones, • determine depth and thickness of zones, • distinguish between oil, gas, or water in a reservoir, and • to estimate hydrocarbon reserves

6. Log Properties of Interest • The most frequently used logs are open hole logs • Logs are recorded in the uncased portion of the wellbore. • The two primary parameters determined from well log measurements are • Porosity, fluid composition and relative saturation • Log interpretations are determined by one of three general types of logs: • Electrical • Nuclear • Acoustic or sonic logs

7. Bore Hole Environment • Where a hole is drilled into a formation, the rock plus the fluids in it are altered in the vicinity of the borehole

8. Borehole Environment • The formations encountered in the bore hole during drilling are invaded to some extent by drilling fluids ("mud") • Mud is used to • lubricate the bit, • circulate the broken rock fragments produced during drilling and most significantly to • maintain pressure in the hole to prevent blow out. • The mud invades the formation to at least some degree • in order to make useful physical measurements of the insitu rock properties the measurement's must be made well into the rock (if possible) or • mud infiltration must be accounted for.

9. Cased Holes • Steel pipe "casing" is set in bore holes to prevent damage and caving • Only certain down hole tools can make useful measurements through pipe, ie. • gamma ray, • neutron porosity

10. LITHOLOGY LOGS • Natural Gamma Ray (γ-ray) Logs • Decay of radioactive elements produces high energy gamma ray emissions • Radioactive elements (K, U, Th) are normally concentrated in shaley rocks while most sandstones are very weakly radioactive. • Because radioactive material is concentrated in shale, shale has high gamma ray log readings • Clay-free sandstone and carbonate rocks have low gamma ray log readings

11. Determination of Lithology from γ -Ray Logging Tools

12. Neutron Logs • Neutron logs (NL or GRN) measure the hydrogen ion concentration in a formation. • In clay-free formations where porosity is filled with water or hydrocarbons the neutron log measures liquid filled pores (the only significant occurrence of hydrogen). • The neutron log measures energy loss when neutrons emitted from the tool collide with other particles in the formation. • The maximum energy loss during a neutron collision occurs when • A neutron collides with a particle of equal mass, that is a hydrogen atom.

13. Neutron Logs • A lower neutron log reading (fewer energetic back scattered neutrons) indicates abundant formation hydrogen. • Clay rich formations contain hydrogen in the crystal structure ofthe clay minerals and give anomalous values for liquid filled pore volume. • Neutron log excursions (decreasing in value from right to left) indicate higher proportions of hydrogen in the Formation • either increased liquid filled porosity or • higher shale content. • Neutron log excursions increasing from left to right indicate • less porosity and/or • less shale

14. Gamma Ray – Neutron Log

15. Compensated Neutron Logs • Newer “radiation” logs called CNL (for compensated neutron logs) are calibrated so that the scale is in porosity units, or neutron porosity units • The CNL (sometimes called the NPHI, for Neutron porosity {φ}) is almost always displayed with • The formation density log and these logs, in combination, can be used to infer lithology

16. Bulk Density • Formation density (compensated; FDC) logs measure the density (grams/cm3) of the formation based on the density of electrons in the formation • Electron density is a function of the absolute amount of matter comprising the formation • measured by the back scatter of gamma rays emitted from a gamma ray source in the logging tool

17. Bulk Density • The absolute amount of matter in the formation is • inversely proportional to the degree of gamma ray penetration into the formation without back scatter to the detector • Since the tool averages the electron density • porous formations composed of dense minerals will appear similar to low porosity formations with lower density rock matrix • Bulk density is read on a log increasing from left to right.

18. Mineral Densities

19. FDC-CNL Log

20. FDC-CNL Log(showing density φ, DPHI)

21. Lithology interpretation from FDC-CNL logs • An industry standard "quick-look" overlay methodology can be used with CNL-FDC wire-line logs • When Neutron porosity (CNL dashed curve) and Bulk Density (FDC, solid curve) logs are overlain on a common, limestone equivalent porosity scale changes in lithology can be inferred with depth

22. Hypothetical neutron-density overlay patterns for simple log-based lithofacies. The overlay uses a common calibration to an equivalent limestone porosity scale.(From Doveton, 1986).

23. The Photoelectric Index (PE or PEF) • The photoelectric index (Pe or PEF) is a supplementary measurement by the latest generation of density logging tools • PEF records the absorption of low-energy gamma rays by the formation in units of barns m() per electron • The logged value is a direct function of the aggregate atomic number (Z) of the elements in the formation, and so is a sensitive indicator of mineralogy. • The common reservoir mineral reference values are : quartz 1.81 ; dolomite 3.14 ; calcite 5.08 barns/electron.

24. Digitally Enhanced Log Displays

25. E- LOGS • Electric logs, resistivity and spontaneous potential, were the first wireline logging tools. • Instruments were (and still are) lowered down bore holes and physical measurements were made regarding the electrical properties of the rocks encountered.

26. Resistivity • Resistance of rock R = rA/L (ohm-meter2/meter, contracted to ohm-meter or ohm-m) • r is the resistance (ohms) • A is the cross-sectional area • L is the length of the resistor

27. Resistivity • The resistivities of sedimentary rocks are determined by the rock component types and their geometry. • hydrocarbons, rock, and fresh water are all insulators (nonconductive, or at least very highly resistive) to electric current flow. • Salt water is a conductor and has a low resistivity • The measurement of resistivity is a measurement of the amount (and salinity) of the formation (connate) water.

28. Spontaneous Potential • Electrical current generated across • the boundaries between formation fluids and drilling fluids (if these fluids are of different salinity) and • the boundary between interbedded shale and sandstone. • The spontaneous potential associated with shale and sandstones is the result of higher permeability in sandstone relative to lower permeability in shale.

29. Typical e-log Response toVariable Lithology and Fluid Content

30. Wireline Logging Traces and Geophysical Logging Tools

31. Trends in γ -Ray Traces andInterpretation of Depositional Environment

32. γ -Ray Log Cross Section

33. The Oz Machine