USC_Class_Section_7_GR_ppt

USC_Class_Section_7_GR_ppt - PTE-461 Formation Evaluation...

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Unformatted text preview: PTE-461 Formation Evaluation Fall Semester, 2007 Section 7 Natural (Passive) Gamma Ray Logs Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 1 What Do Gamma Ray Logs Measure? Natural gamma radiation from the materials in and surrounding a borehole A passive measurement Natural -ray sources are Potassium (40K), Uranium (238U), and Thorium (232Th) Because clay minerals contain K, -ray logs are used (almost exclusively) to estimate shale volumes, in reservoir rocks Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 2 Background First -ray logs run by Humble (reported: 1939), using ionization chamber to measure natural radioactivity in well bore Uranium Boom, following WW-II sparked interest in surface & subsurface -ray measurements -ray emission found to correlate to geology First commercial service (1944) by Well Surveys, later purchased by Layne Wells Schlumberger introduced their tool after Layne wells had established the market Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 3 K, U, & Th Decay Series Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 4 Detailed K-U-Th Spectra has only one decay, to 40Ar 238U and 232Th decay series involve many daughter products, with half-lives ranging from seconds to billions of years Not all decays in 238U and 232Th series involve -ray emission Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com 40K PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 5 Why Run Passive -Ray Logs Shales contain more naturally radioactive minerals than clean, non-arkosic (feldspathic), ion-glauconitic, nonmonazite, non-zircon rich sands, limestones, and non-potash evaporites Shales have high potassium content and collect uranium and thorium salts -ray logs often used as shale indicator -ray logs can be used for well-to-well correlation (even in cased wells) -ray logs used to estimate relative amounts of clay minerals in formations (with assumed petrophysical model) -ray logs can be used to provide rough estimates of effective porosity and permeability (with assumed petrophysical model) This requires a petrophysical model and MUCH faith Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 6 Measurement of Random Events Radioactive decay of unstable isotopes is a "Spontaneous Random Event": We cannot predict when a specific unstable isotope will decay We can only count the number of decays per unit time (cps) and predict the the probability of how many, unstable isotopes, within a large ensemble, will decay within a given time period This usually measured as the isotope's "half-life": the length of time required for half of a given assemblage of unstable isotopes to decay Because radioactive decay is a "Random Event" The standard deviation ( ) of the counts within a given time window is the square root of the mean value of the counts over time windows, of that length Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 7 Random Events, Continued A more useful statistic is the relative standard deviation, or the ratio of the standard deviation to the mean, or: x = x x Where x is the count rate for an individual time sample This means that quadrupling the count rate would halve the relative error Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 8 -ray Ray Detectors Ionization Chambers Proportional Counters Geiger-Mueller Tubes Scintillation Detectors Semiconductor Diode Detectors Germanium and doped Germanium (e.g., GeI) Detectors Other Solid-State Detectors Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 9 -Ray Log Header Tool serial number Detector type Detector size Detector sensitivity Time constant (analogue tools) or sampling rate (digital tools) Calibrator Identification Logging speed Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 10 Drilling mud will adsorb gamma rays Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 11 K-U-T Gamma Ray Spectra Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 12 Natural Gamma Ray Spectral Log Disclaimers KUT relationships to lithology are NOT as direct as once thought Utility must be established on case-bycase basis Proper usage requires slow (900 - 1500 ft/hr, or less) logging speeds Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 13 High Resolution 238U Spectra Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 14 Need for Industry Calibration Standards Detector count Rates depend Upon: Formation radioactivity Detector type Detector size Detector efficiency Detector age Initially, each vendor set their own (normalized count rate) calibration standards Eventually, industry-wide standards were adopted under sponsorship of the American Petroleum Institute (API) Slide No.: 15 Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray API -Ray Calibration Facility Located on the University of Houston campus 18' x 4' cylindrical cassion, with a 5.5" borehole extending to a 15' rathole. The cassion is filled with three materials 8' low radio activity concrete blocks (Ottawa Sand) at the top and bottom Single 8' radioactive concrete block: Ottawa Sand spked with 13 ppm Uranium (Radium), 24 ppm Thorium (Monazite sand/, and 4% potassium (Mica) By definition, the -ray log log response between the center "hot" zone and non radioactive ends is 200 API units Slide No.: 16 Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray -Ray Field Calibrator Chemical radioactive source of known strength at a given distance from the Sonde Calibration jig attached to sonde with pegs locked into the tool housing Tool reading difference (with and without calibration source) is defined as 100, or 150 API units Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 17 -Ray Calibration Record Background signal Tool reading at the surface with no calibrator in place Calibrator signal Tool reading at the surface with calibrator locked into position By definition, this will be either 100 or 150 API units above background Detector stability check Time variation of detector reading, with the tool held stationary at some depth Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 18 Poor Statistical Stability Check Low radioactivity materials result in high relative errors and large fluctuations in count rates In above example the fluctuations during the stability check were greater than the difference between sand and shale count rates Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 19 Good Statistical Stability Check More efficient and larger volume detector, greatly reduces the statistical stability check count-rate variations Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 20 Petroleum -ray logs should repeat within 2 API units Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 21 Poor -Ray Log Repeat In some cases, the difference between the primary and repeat log are greater than the differences between sands and shales Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 22 Good -Ray Log Repeat Larger and more efficient detector greatly improves repeatability Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 23 Schematic -ray and neutron log response through a sand-shale section Shales have high radioactivity and high apparent neutron porosity Sands have low radioactivity and high apparent neutron porosity Carbonates & evaporites, if present will have low radioactivity and low apparent neutron porosity Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 24 Estimating Vsh from -Ray Logs Establish sand and shale lines Scale log between these lines IGR = Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com log sh sd sd PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 25 Linear & Non-Linear -Ray Vsh Models Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 26 X-Ray Diffraction vs. -Ray Vsh Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 27 Bi-Modal Grain Size Shaly-Sand Model Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 28 Bi-Modal Sieve Analysis Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 29 Bi-Modal Model Porosities Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 30 Log(R) vs. -Ray Break in trend corresponds to broken line in Slide No 25 Display Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 31 -Ray Vsh Disclaimers This approach is most effective when shales are only rocks with significant radioactive minerals Arkosic sands contain K-Feldspars Glauconitic sands contain potassium Monazite and zircon rich sands contain thorium Some dolomites have radioactive contamination Carnallite, Langbeinite, Leonite, Kainite, Polyhalite, and Sylvite are potash evaporites, associated with commercial and non-commercial potash deposits PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 32 Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com Donald G. Hill, Ph.D., R.Gp, R. G., R.P.G., L.P.Gp. Dgh@hillpetro.com PTE-461: Fall 2007 Section 7: Gamma Ray Slide No.: 33 ...
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This note was uploaded on 02/27/2008 for the course PTE 461 taught by Professor Donhill during the Fall '07 term at USC.

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