FORMATION EVALUATION PETE 663

Transcription

1 FORMATION EVALUATION PETE 663 PASSIVE MEASUREMENTS NATURAL GAMMA Summer 2010 Dr. David Schechter

2 PASSIVE MEASUREMENTS Caliper Spontaneous Potential Gamma Ray Natural Spectral

3 GAMMA RAY LOGS Uses Correlation Lithology indicator; exploration for radioactive materials Mineral identification Open or cased hole; any fluids Evaluation of shale content Paleoenvironmental indicator Fracture detection Properties Measures natural gamma radiation Random fluctuations Rock Formations GR Tool

4 GAMMA RAY TOOLS 1. The gamma ray tool records the natural radioactivity of the formation without regard to the source 2. The spectral gamma ray tool identifies the source and gives the contribution of each elements (potassium, uranium, and thorium ) to the overall spectrum. Also, it is useful in identifying fractures

5 API: (1/200) OF THE DIFFERENCE IN LOG READING BETWEEN A HOT AND A COLD ZONE

6 GAMMA CALIBRATION HOT AND COLD ZONES The Gamma tool is placed in the hot zone (200 API) and the gamma counts are recorded. It is then placed in the cold zone and the gamma counts are recorded. The difference in counts is converted by a gain factor to represent 200 API. API UNIT: (1/200) OF THE DIFFERENCE IN LOG READING BETWEEN A HOT ZONE AND A COLD ZONE

7 NATURAL GR PRINCIPLE Cause Unstable isotopes in formation Isotopes decay Emit GR s (various energies) Three main contributors K 40 with half-life 1.3x10 9 yrs Th 232 with half-life 1.4x10 10 yrs U 238 with half-life 4.4x10 9 yrs Sources K 40 feldspar, mica, illite Th 232 heavy minerals, clays U 238 organic material Probability of Emission per Disintegration 1.46 Thorium Series Potassium 2.62 Uranium-Radium Series Gamma Ray Energy (MeV)

8 SOURCES OF PASSIVE GAMMA RAYS 1. Clays Kaolinite (very little K [potassium]) Illite (4-8% K) Montmorillonite (<1% K) 2. Sand and Silt Potassium (K) feldspar Heavy minerals Volcanoclastics 3. Natural Cements Fracture-filling 4. Uranium Ores

9 SCINTILLATION DETECTORS 1. Gamma rays interact with scintillation crystal 2. Electrons excite phosphor atoms, which in turn decay by emission of light 3. These photons interact with the photocathode of the p.m tube producing electrons 4. Ejected electrons are focused into photomultiplier string 5. Electrons are accelerated through successive dynodes producing multiplication at anode (1e = 10 6 e)

10 SCINTILLATION DETECTOR

11 SHALE WASHOUT From Dresser Atlas, 1982

12 CORRECTED AND UNCORRECTED GAMMA RAY CURVES IN WASHOUT From Dresser Atlas, 1982

13 STATISTICAL ISSUES Measurement problem GR emissions random Tool moving Results Imprecise measurement Details smeared out Procedures New tools better detectors Limit logging speed Old tools 1800 fph New tools 3600 fph Exercise care interpreting boundaries API ,400 ft/hr 1,800 ft/hr 600 ft/hr Shale 4ft sand Shale

14 GR 2.25 FILTER GR 2.25 FILTER GR UNFILTERED FPM FPM FPM Are these reversed? EFFECTS OF LOGGING SPEED AND FILTER LENGTH ON GAMMA RAY LOG High-resolution logging for thin bed,.i.e. coal, is usually done at low speed to better define bed boundaries and partings

15 GR RESPONSE IN COMMON FORMATIONS Shales often radioactive Clays Trace and heavy minerals Sandstones may be radioactive Non-clay minerals, e.g., mica, feldspar Clays See Appendix B, Chart Book Units GR calibrated to standard Response in mid-continent shale equals 200 API units Calibration pits API units Shaly sand Clean limestone Dolomite Clean sand Coal Shaly sand Anhydrite Shale Very shaly sand Shale Salt Volcanic ash Gypsum

16 WHAT IS Vshale? Fraction of rock made up of shale Why calculate Vsh in Sandstone? Delimit reservoir quality rock Shale = clays in FE Clays reduce perm and porosity Estimates of Sw too large Shales reduce net pay Vsh definition matrix (silt + dry clay) + fluid (bound water) φ t Vsh φ e sand silt dry clay bound water free water HC Unit volume of rock

17 VOLUME OF SHALE Gamma Ray Index I SH = GR GR MAX GR GR MIN MIN RELATIONSHIP Linear EQUATION V sh = I sh Clavier V sh = 1.7-(3.38-(I sh +.7) 2 ) 1/2 Steiber V sh = 0.5*(I sh /(1.5-I sh )) Bateman V sh = I sh (Ish +GRFactor) GRFactor =

18 CALCULATING CLAY CONTENT Shale Index I sh GR GR = GR GR max min min (VSHALE) 0 GR (API) GAPI GR (max) Shale Calculating V sh Numerous models Always have V sh < I sh May only apply locally Some Models: V V V V sh sh sh sh = = = = I I I sh sh sh 0.33(2 /(2 I /(4 3I 2 sh I sh ) sh ) 1) GR GR (min) 15 GAPI 48 GAPI 90 GAPI GR Tool Shaly sand Clean sand Shale

19 EXAMPLE PROBLEM Choose value for GR max and GR min and compute V sh in sand C using linear, Clavier, and Steiber methods

20 SOLUTION GR min = 10API Gr log =50 API GR max =132

21 Example from Slide 22 I sh I sh I sh GR GR = GR GR max = = 0.44 min min V SH RELATIONSHIPS Example from Slide 24 I sh I sh I sh GR GR = GR GR max = = min min 26% 20% %

22 SOLUTION GR min = 10 API GR max = 132 API Choosing a depth in SAND C, say GR =50 API Linear Vsh = Clavier Vsh = Steiber Vsh = 0.139

23 SPECTRAL GR ANALYSIS Gives the individual quantities of uranium, potassium, and thorium Good fracture detector, because uranium tends to precipitate with fracture-filling minerals A sharp uranium peak may indicate fractures Good for mineral identification

24 SPECTRAL GR Th, U, and K different energies Tool measures counts energies Output K, Th, U contents. Th + K gives CGR no-uranium GR curve better measure for V sh CGR Th K SGR U

25 SPECTRAL ANALYSIS PRINCIPLE The radioactivities of the 3 elements differ, based on the energy level peaks

26 SPECTRAL GAMMA RAY LOG URANIUM THORIUM POTASSIUM

27 SPECTRAL GAMMA RESPONSE IN MESOZOIC CARBONATES AND SHALES, EAST-CENTRAL TEXAS From Dresser Atlas, 1982

28 From Halliburton

29 From Halliburton

30 SOME GR APPLICATIONS - VERSATILE TOOL Lithology indicator Reservoir descrimination Vsh cutoff Correlation Well-to-well Open hole to cased hole Core-to-log Depth control Depositional Environment Uses curve shape, log responses, and characteristis of bedding contacts to infer grain sizes and sedimentary processes and environments Exploration for radioactive rocks Uranium, potassium chloride Fracture detection Some fracture-filling mineral deposits are hot