SP and GR

Caliper logCALIPER LOGS- Applications:Measure borehole diameter (borehole geometry if multi-arm caliper tools with 2 or 3 hole diameters measurements 90 or 60 relative to each other).Important measurement for drillers: hole geometry, hole/cement volume. Hole diameters are an import input parameter for the environmental correction of petrophysical logs.Oriented multi-arm caliper logs are used to identify principle stress directions - breakout log - Basic Quality Control:Perform casing check - should read nominal casing ID.CALI, C1, C2 Washout: Shale zone? Mud cake: Permeable zone?

The SP Measurement

Liquid Junction Effects #1

Liquid Junction Effects #2

Liquid Junction Effects #3

Total SP

SP

SPMembrane potential is about 5 x Liquid Junction potentialElectrokinetic potential SP is negligible

Static SP (SSP)

SP as a permeability or shale indicatorSince invasion can only occur in permeable formations, deflections of SP can be used to identify permeable formations.

The vertical resolution of SP is poor, and often the permeable bed must be 30 ft or more to achieve a static (flat baseline) SP

Rw from SSPUnder certain circumstances Rw can be estimated from SP.The SP value remains constant for at least 30 feet. The area where the SP is constant must correspond to a very clean sandstone.The value of Rmf must remain constant across this same interval.

These conditions are rare, and large errors in the Rw estimate may occur. Use this technique with care!

Rmfeq from Rmf or Rw from RweqIf Rmf @ 75degF > 0.1 Ohmm then Rmfeq=0.85 Rmf @ BHT

If Rmf @ 75 degF < 0.1 Ohmm then Rmfeq from chart sp2

Same with RwChart SP-2

SP as Rw indicatorSP is more often used qualitatively to predict whether Rw > Rmf or not.

SP for correlation-ve SP deflection+ve SP deflection

SP for correlationKeep in mind that SP deflection is Rmf dependent and never an absolute value

SP logRmf > Rw?Where is Sand?Where is Shale?What is Vsh?

Typical SP Log Presentation

Natural Gamma RayThe Gamma Ray log is a measurement of the formation's natural radioactivity.Gamma ray emission is produced by three radioactive series found in the Earth's crust.Potassium (K40) series.Uranium series.Thorium series.Gamma rays passing through rocks are slowed and absorbed at a rate which depends on the formation density.Less dense formations exhibit more radioactivity than dense formations even though there may be the same quantities of radioactive material per unit volume.

Total GR measurementBed definition:The tool reacts if the shale is radioactive (usually the case), hence show the sands and shales, the permeable zones and the non-permeable zones. Used for inter-well correlation similar to SP.Also used for correlation between logging runs, especially cased and open hole logs

Computation of the amount of shale:The minimum value gives the clean (100%) shale free zone, the maximum 100% shale zone. All other points can then be calibrated in the amount of shale.

Vsh=(GRlog-GRsand)/(GRshale-GRsand)GR shaleGR clean

Borehole correctionChart GR-1 (wireline)

GR Log ExampleWhich has better vertical resolution, SP or GR?

GR log example

Spectral GR NGT, HNGS

NGT log exampleSGR = total GRCGR = SGR-UraniumTHOR = Thorium concentrationURAN = Uranium concentrationPOTA = Potassium %TPRA = THOR/POTAUPRA = URAN/POTA

CGR is often a better shale indicator in carbonates.

THOR is often a better shale indicator in sandstone

Mineral Identification from Spectral Gamma Ray

Chart CP-19

Clay TypingPEF vs K and PEF vs Th/K are also often used.

PEF=Photoelectric Effect of the formation (Covered under density logging)

Clay typing example

Petrolog Asymmetric Equation