Formation Evaluation: Logs and cores

Transcription

1 These powerpoint files were produced for the Earth History class at the Free University Berlin, Department of Geological Sciences The copyright for texts, graphical elements, and images lies with C. Heubeck, unless otherwise specified. Download, reproduction and redistribution of theses pages in any form is hereby permitted for private, personal, non-commercial, and class-related purposes use as long as the source is identified. Despite of my efforts, I cannot guarantee the completeness, correctness and actuality of the material. Prof. Christoph Heubeck Institut für Geologische Wissenschaften Freie Universität Berlin Malteserstr Berlin GERMANY ph: ++49-(0) fax: ++49-(0) cheubeck@zedat.fu-berlin.de Why Formation Evaluation (FE)? What data are available? Formation Evaluation: Logs and cores Logs Cores Borehole geophysics Logging tools: Gamma-Ray Resistivity Sonic Neutron Image Logs RFT NMR The log environment Why coring? Types of core and fluid samples Seismic data 1

2 Data types Why Formation Evaluation (FE)? Explore and evaluate Determine and characterize phi, k, Sw Calculate oil-in-place, producibility Cores and fluid samples Direct data Rare, isolated Resolution ~ 30µ 1-dimensional (z) Logs Indirect data Many, continuous Resolution: ~1 in - ~1 ft. 1-dimensional (z) Seismic data Indirect data Many, continuous Resolution ~ 5-40m 3-dimensional (x,y,z) 1 cm 100 m 1 m Scale problem 3-D Heterogeneous data 10 m 100 m 100 m 10 m 100 m 100 m 2

3 Well logs: Basic concepts What properties do logs measure? Measure some property of the rock surrounding the wellbore and record in a record ( log ). Major Minerals Fluids Primary parameters determined Mineralogy Porosity Fraction of pore space filled with hydrocarbons / water. Clay Quartz K-feldspar Porosity Hydrocarbon Water What properties do logs measure? Integrated Formation Evaluation Fluids Major Minerals Clay Quartz Porosity The role of the formation evaluation specialist: HC water Fluids Rock Hydrocarbon Water clay quartz feldspar porosity 3

4 What properties do logs measure? Natural Radioactivity: shale content Acoustic velocity : Porosity Electron density : Porosity Neutron scattering : Porosity Electrical Resistivity: Fluid type Nuclear magnetic resonance: Porosity, permeability The wellbore environment Which physical properties affect logging measurements? Porosity Permeability Water saturation Resistivity Natural radioactivity Some typical logging instruments Corrections for Borehole shape Mud properties Temperature Salinity Schlumberger 4

5 Gamma-Ray Passive instrument; measures natural radioactivity of rock Mostly K 40 K-rich rocks: most shales, arkoses Can use to calculate volume of shale Resistivity Resistivity to electric current A measurement of resistance against electrical flow Measured by electric logs. Rocks, Freshwater, and hydrocarbon are all insulators (non-conductive, highly resistive) Resistivity is a basic measurement of the reservoir fluid s saturation It is a function of the reservoir s porosity (type of fluid, and type of rock. Because fluids move into the reservoir, resistivity measurements need to be made at different depths (different values) Non-conductive, highly resistive rock salt water Conductive SO -- Ca + 4 Mg ++ Cl - K + Oil and gas shales Ca + Na + Na + 5

6 Resistivity to electric current Active instrument; measures resistivity of rock to electric current Resistivity to electric current Ωm Ωm Deep Medium Shallow Deep Medium Shallow How much oil is in the rock? The sonic log Low S w R t high Water Saturation (%) S w = sqrt (F * R w / R t ) With F = Formation factor R w = Resistivity of Water R t = Resistivity of formation Measures interval transit time δt of a compressional sound wave travelling through 1 ft of formation Assumes knowledge of velocity of lithology (sandstone, carbonate) Porosity retards the sound velocity R t low High S w 6

7 The neutron log The neutron log Neutrons are slowed down most when colliding with H (billardball-effect) In clean formations, measures liquidfilled porosity Created from a chemical neutronemitting source (Am, Be) Neutrons collide with nuclei of formation material Maximum energy loss occurs when colliding with H due to similar mass In gas, H concentration is low, therefore porosity abnormally low (gas effect) Need to calibrate for sandstone, limestone, dolomite Sensitive tool, company-dependent, company-calibrated Measures the density of hydrogen in the rock Image logs Image logs: Resistivity The resistivity dipmeter BORE HOLE DIP CURVES N E S W N RESISTIVE THIN BED 4 DIPMETER PADS Apparent Dip = invtan (sine wave Amplitude/Diameter) Resistivity imager employs six independently articulated pads, 24 buttons per pad, (in 2 rows) producing 144 microresistivities. 0.2 resolution. DIAMETER DIAMETER

8 Repeat Formation Tester (RFT) Low High Samples fluids and measures pressure Important: Preservation of reservoir conditions ca. 840cc per tank Amplitude / Resistivity Repeat Formation Tester Nuclear Magnetic Resonance (NMR) shale seal sandstone reservoir depth ρ heavy oil = 0.9 ρ water = 1.00 contact Amplitude 90 Pressure, arbitrary units Time, msec 8

9 Nuclear Magnetic Resonance (NMR) Nuclear Magnetic Resonance (NMR) 9

10 Core Analysis Core Analysis The only tangible part of the reservoir Calibration! Geophysicist Microbiologist Oceanographer Porosity Permeability Saturation Capillary Pressure / pore size distribution Relative Permeability to oil, gas, and water Electrical properties Acousic properties Paleontologist Core Analysis Core Storage 10

11 Petrophysicist Geologist Geophysicist Engineer 11