1BTC - April, 1999 Natural Formation Gamma Ray Logging HGNS : Highly-integrated Gamma ray Neutron Sonde SGT-L : Scintillation Gamma ray Tool

2BTC - April, 1999 Objectives Explain why a GR log can tell shale from other rock List the three principal elements that naturally produce radiation Explain pair production Explain Compton scattering Explain photoelectric effect Explain the types of GR detectors and their advantages/disadvantages Describe the applications of GR logging Explain the GR calibration procedure

3BTC - April, 1999 Outline - GR Logging Applications Physics of Measurement – Radioactivity – GR Interaction – Detector Operation Hardware Description – Intro - PEx Electronic Architecture – HGNS – Tool Hardware Operations – Environmental Effects – Parameters – Calibrations – Limitations – FIT & TRIM – Safety LQC Log Response

4BTC - April, 1999 GR Applications GR measurement – General lithology indicator – Log correlation – Quantitative shaliness evaluation – Radioactive tracer logging – Scale build-up monitoring – Others (clay typing, density,…)

5BTC - April, 1999 HGNS Introduction  Total Gamma Ray One tool (cartridge) performing several functions:  Compensated Neutron  Tool Acceleration  Tool Telemetry

6BTC - April, 1999 GR Log

7BTC - April, 1999 Natural Formation Gamma Ray Logging Physics of Measurement

8BTC - April, 1999 Naturally Occurring Gamma Rays Rock formation

9BTC - April, 1999 Radioactivity Stable Atom: One which has equal number of protons, neutrons and electrons. Radioactivity: A property possessed by some elements of spontaneously emitting alpha, beta particles and/or gamma rays as their atomic nuclei disintegrate. Unstable Atom

10BTC - April, 1999 Types of Emissions Alpha Particles –Positively charged particle with 2 neutrons and 2 protons (nucleus of He atom) –easy to stop by a thick cloth Beta Particles –Two kinds B- : electron emitted from an unstable nucleus when one of its neutrons decays to a proton 0 n 1  1 H e 0 + B+: positron emitted from an unstable nucleus when one of its protons decays to a neutron 1 H 1  0 n e 0 + –Easily stopped by a thin sheet of metal –May cause skin burn Gamma Rays –Massless, chargeless bundles of high- frequency electromagnetic energy emitted when an atom passes from an excited state to a less excited/ground state –Travel at speed of light –Referred to as photon when it has discrete quantity of em energy. –Penetrate rocks up to 15” (8” of concrete)

11BTC - April, 1999 Natural Radioactivity The 3 main radioactive series in nature: Potassium (K 40 ) - decays to stable K 39 Thorium (Th 232 ) - decays to Pb 208 Uranium (U 238 ). - decays to Pb 208 These elements decay to their rest state through a series of intermediate steps emitting  and  particles on their way They are found in various proportions in crystalline rocks During erosion and degradation these tend to concentrate in shales Hence shales are more radioactive than sands

12BTC - April, 1999 Gamma Ray Interactions Three principal Gamma Ray interactions: l Pair Production (high energy) l Compton Scattering (medium energy) l Photoelectric Absorption (low energy) Nearly all natural gamma rays in formation come from Potassium, Uranium and Thorium series In passing through the formation, a gamma ray will experience successive Compton scattering collisions, losing energy until it is finally absorbed by an atom via the photoelectric effect.

13BTC - April, 1999 Pair Production –Phenomenon of conversion of neutron into an electron and positron –GR must have at least 1.02 MeV –Dominates the 10MeV and up interactions

14BTC - April, 1999 Compton Scattering –Scattering of GR from an orbital electron –GR loses energy & e - ejected from atom’s orbit –75keV < Energy range < 10MeV –Dominates the medium energy interactions

15BTC - April, 1999 Photoelectric Absorption –Low-energy GR disappears as it collides with an atom –Results in the ejection of e - from its orbit –Gamma ray energy < 100keV –Dominates the low energy interactions

16BTC - April, 1999 Important Interactions Three principal Gamma Ray interactions: l Pair Production l Compton Scattering l Photoelectric Absorption N atural gamma rays in formation experience successive Compton scattering collisions, losing energy until they are finally absorbed by some atoms via the photoelectric effect

17BTC - April, 1999 Gamma Ray Energies

18BTC - April, 1999 Recap There are naturally occurring radioactive elements in the formation (K, Th & U) K, Th & U are primarily concentrated in shales Gamma rays are produced by the disintegration of these elements These gamma rays may further interact with the formation losing energy on the way The detector sees these gamma rays and measures the total count rate

19BTC - April, 1999 Natural Formation Gamma Ray Logging Detector Operation

20BTC - April, 1999 GR Detectors Geiger-Müller – Detects and counts Scintillating Crystal Detectors – Sodium Iodide doped with Thallium – Gadolinium Orthosilicate doped with Cerium – Others (BGO) – Crystals need a photomultipler tube to count

21BTC - April, 1999 Geiger-Muller Detector/Counter Geiger-Müller Detects and counts “Tick-tick-tick” sound! Temperature insensitive Very inefficient (6% only!) Used in some downhole tools

22BTC - April, 1999 Formation to Crystal Interaction

23BTC - April, 1999 Environmental Effects Log affected by –Hole Size –Mud weight –Barite in Mud –Casing Correction charts and software settings exist to correct the GR reading for these effects However since GR is primarily used as a correlation tool, corrections may not be applied in some areas Output ECGR (from HGNS) is fully environmentally corrected

24BTC - April, 1999 GR Safety Radiation – Treat GSR-U/Y blanket as source Crystal – Deteriorates in humid air – Very brittle – Do not eat! Photo Multiplier Tube – Avoid exposure to light – Keep away from magnets (collar locators, CMR,…) Shock hazard – 250V cartridge power – 2000 to 3000V PMT voltage

25BTC - April, 1999 Natural Formation Gamma Ray Logging Limitations, LQC, Typical Response

26BTC - April, 1999 GR Measurement Limitations ?? 4 The biggest feature of the GR log is that it can be run in almost any logging condition including: – cased wells – open holes drilled with air – open holes drilled with water based muds – open holes drilled with oil based or fresh muds

27BTC - April, 1999 LQC Checklist 3 Hardware LQC is within tolerance 3 Proper logging speed 3 Parameters correctly selected 3 Difference in before and after survey less than 7 GAPI 3 Log follows response (in shape) with other wells in the region 3 Repeatability within specified tolerances (7%)

28BTC - April, 1999 GRNeutronAccelerometer HGNS Hardware LQC

29BTC - April, 1999 Typical Response

30BTC - April, 1999 Objectives Explain why a GR log can tell shale from other rock List the three principal elements that naturally produce radiation Explain pair production Explain Compton scattering Explain photoelectric effect Explain the types of GR detectors and their advantages/disadvantages Describe the applications of GR logging Explain the GR calibration procedure