INSTRUCTOR © 2017, John R. Fanchi All rights reserved. No part of this manual may be reproduced in any form without the express written permission of the author. © 2004 John R. Fanchi All rights reserved. Do not copy or distribute.
To the Instructor The set of files here are designed to help you prepare lectures for your own course using the text Introduction to Petroleum Engineering, J.R. Fanchi and R.L. Christiansen (Wiley, 2017) File format is kept simple so that you can customize the files with relative ease using your own style. You will need to supplement the files to complete the presentation topics.
WELL LOGGING © 2017, John R. Fanchi All rights reserved. No part of this manual may be reproduced in any form without the express written permission of the author. © 2004 John R. Fanchi All rights reserved. Do not copy or distribute.
Outline Well Logging Subsurface Ionic Environment Lithology Logs Porosity Logs Resistivity Logs Induction Logs Log Calibration with Formation Samples Modern Log Applications Homework: IPE Ch. 9
WELL LOGGING
Well Logging Objectives Estimate near-wellbore formation properties Depth Thickness (net and gross) Porosity Formation density Acoustic velocity Temperature and pressure Lithology (rock type) Fluid saturations Indication of hydrocarbons Indicate permeability (e.g. high, low, tight) Structural trends (e.g. formation dip) Fracture properties © 2004 John R. Fanchi All rights reserved. Do not copy or distribute.
Typical Applications of Well Logs Quality of wellbore Size of wellbore (caliper log) Integrity of cement bond (cement bond log) Provide information for Geologic mapping Prospect evaluation (where to drill) Reserve estimates Indicate presence of hydrocarbons Measure Sw; infer hydrocarbons Fluid contacts, e.g. GOC, WOC Indicate which zones to complete (perforate)
Well Log Header and Tracks Scale Scale Scale Scale Scale Track 1 Track 2 Track 1 Track 2 Track 3 Depth Track
Well Log Schematic Well Data Logging Data SP Res Track 1 Track 2 Depth Track
Types of Well Logs Lithology logs Porosity logs Resistivity logs Spontaneous Potential (SP) Now tends to be replace by Gamma Ray Gamma Ray (GR) Photoelectric Effect (PEF) Porosity logs Density Neutron Acoustic (Sonic) Resistivity logs Induction Laterolog Micro Resistivity Specialty Logs e.g. FMI (Formation Micro Image) © 2004 John R. Fanchi All rights reserved. Do not copy or distribute.
Computer Generated Logs Computer performs corrections and calculations Graphical view Easy to see analysis Estimate lithology, saturations, porosity, etc.
SUBSURFACE IONIC ENVIRONMENT
Ohm’s Law Ohm’s Law: V = IR where V = voltage (volts) I = current (amps) R = resistance (ohms) Conductance = 1/R (Siemens) 1 Siemens = 1 mho = 1/ohm Electrical current is charge in motion, e.g. Na+ cation and Cl- anion.
Alternative form of Ohm’s Law E = ρJ where E = electric field (volt/m) J = current density (amp/m2) ρ = resistivity (ohm-m) L A + − Carrier of positive charge moves in direction of E, I, J Carrier of negative charge (e-) moves in opposite direction
Resistivity and Resistance Resistivity is related to resistance. For uniform conductor with length L and area A: R = ρ L / A Resistivity ρ is the inverse of conductivity σ : ρ = 1 / σ Alternative form of Ohm’s Law: E = ρJ where E = electric field (volt/m) J = current density (amp/m2) ρ = resistivity (ohm-m) L A + −
Fluids that Affect Logging Measurements Drilling mud (resistivity Rm) Mud filtrate(resistivity Rmf) Formation water(resistivity Rw) Hydrocarbons (assumed infinite resistivity) Resistivity depends on formation temperature
Invasion Zones for Drilling Fluids Adjacent Bed Borehole Uninvaded Zone Zone of Transition Flushed Mud Cake
LITHOLOGY LOGS © 2004 John R. Fanchi All rights reserved. Do not copy or distribute.
Common Reservoir Rock Types and an Illustrative Stratigraphic Column
Gamma Ray Log or Natural Gamma Ray Log Gamma rays (GR) from NORM Measure in API units Relative unit NORM Potassium GR energy 1.46 MeV Thorium series GR energy 2.62 MeV Uranium-Radium series GR energy 1.76 MeV
Light-Matter Interaction Low-energy phenomenon Photoelectric effect Mid-energy phenomena Thomson scattering (elastic) Compton scattering (inelastic) High-energy phenomenon Pair production Photoelectric Effect Space Time Compton Scattering Photon wavelength changes
Gamma Ray Log Response
NORM in West Texas Barnett Shale Mississippian Barnett Shale above MD = 9606 ft Mississippian Limestone below MD = 9606 ft SGR – Spectral Gamma Ray CGR – Total GR minus URAN POTA – Potassium 40, wt % URAN – Uranium, ppm THOR – Thorium, ppm Barnett Shale Limestone Source: Asquith and Krygowski, Fig. 3.3, Basic Well Log Analysis, 2nd Ed (2004)
Lithology Log: Gamma Ray VARIABLE RESPONSE Gamma Ray Rock Type Detects shale from in situ radioactivity. High GR shales Low GR clean sands and carbonates In most cases, shale formations are most radioactive Most reservoir rocks exhibit low radioactivity GR log is shale indicator
SP (Spontaneous Potential) SP = Potential difference (voltage) between 2 fluids with different salinities SP electrode Grounded on surface Connected to logging tool SP affected by shale content Can calculate formation water resistivity RW from SP Need RW to calculate saturation
SP (aka Self Potential) Log Measures potential difference between drilling fluids and formation waters Distinguish permeable beds from shale Small SP response impermeable shale Large SP response permeable beds SP log hard to interpret when formation waters are fresh (not salty)
Lithology Log: Spontaneous Potential VARIABLE RESPONSE Spontaneous Potential Permeable Beds Measures electrical potential (voltage) associated with movement of ions. Low response impermeable shale Large response permeable beds
Lithology Log: Photoelectric Effect VARIABLE RESPONSE Photoelectric Effect Rock Type Measure absorption of low energy gamma rays by atoms in formation. High GR shales Low GR clean sands and carbonates (absorb GR) Photoelectric effect log is shale indicator Photoelectric Effect
POROSITY LOGS Formation Density Neutron Porosity Sonic
Density Log Gives rock density reading in gm/cc Water = 1 gm/cc (62.4 lb/cu ft or 8.33 ppg) Sandstone ~ 2.65 gm/cc Limestone ~ 2.71 gm/cc Salt ~ 1.6 – 2.0 gm/cc Calculate porosity % from log reading and rock type
Estimate Porosity from Density given Lithology
Porosity Log: Density LOG VARIABLE RESPONSE Density Porosity* Measures electron density by detecting Compton scattered gamma rays. Electron density is related to formation density. Good for detecting hydrocarbon gas with low density compared to rock or liquid. Low response low HC gas content Large response high HC gas content * The combination of density log and neutron log provides the most reliable porosity estimate and can be used to indicate gas. Shale reduces apparent porosity measured by density log Gas increases apparent porosity measured by density log
Porosity Log: Neutron LOG VARIABLE RESPONSE Neutron Hydrogen Content Fast neutrons are slowed by collisions to thermal energies. Thermal neutrons are captured by nuclei, which then emit detectable gamma rays. Note: hydrogen has a large capture cross-section for thermal neutrons. Good for detecting gas. Large response high H content Small response low H content Shale appears as high apparent porosity measured by neutron log Dry gas appears as low apparent porosity measured by neutron log
Neutron – Density Crossplot plot porosity from neutron log vs porosity from density log 1.0 0.0 density neutron Clean sand line density = neutron Gas sand density > neutron Shaly sand density < neutron
Neutron Log – Density Log Comparisons Gas indicator Crossplot can identify gas-bearing sands in sand-shale sequences Lithology indicator Apparent limestone porosity will appear high in density log if limestone contains anhydrite
Gas Effect Density-Neutron Crossover How do logs respond when gas is present? Density log reads porosity correctly Neutron log treats gas as rock so it reads low porosity Therefore curves separate when gas is present Gas probably present when density log and neutron log separate
Typical Sonic Log Velocities Velocity (ft/sec) t (second/ft) Shale 7,000 – 17,000 144 – 59 Sandstone 11,500 – 16,000 87 – 62 Limestone 13,000 – 18,500 77 – 54 Dolomite 15,000 – 20,000 67 – 50 Natural Gas 1,500 667 Water 5,000 200
Porosity Log: Sonic LOG VARIABLE RESPONSE Acoustic (sonic) Porosity Measures speed of sound in medium. Speed of sound faster in rock than in fluid. Long travel time slow speed large pore space Short travel time high speed small pore space Porous rock slows down sound waves Porosities calculated from sonic log measurements are generally high in hydrocarbon-bearing unconsolidated sands
RESISTIVITY LOGS
Gamma Ray and Resistivity Logs Short Long Gamma Ray Log Resistivity Log
INDUCTION LOGS
What is induction logging? Based on Faraday’s law of electromagnetic induction Oscillating magnetic field induces electric field Transmitter coil in tool creates primary magnetic field Primary magnetic field induces toroidal electric field Toroid = doughnut shape
What is induction logging? (cont.) Toroidal electric field creates electrical “eddy current” Eddy current is induced in conductor by changing magnetic field Strength of eddy currents depends on conductivity Eddy currents create secondary magnetic field Measure secondary magnetic field with receiver coil
What is induction logging? (cont.) Transmitter coil Primary magnetic field Eddy current in conductor (e.g. ionic environment) Secondary magnetic field Receiver coil
SI Unit of Conductivity Conductivity is inverse of resistivity Conductivity unit is siemens/meter (S/m) or millisiemens/meter (mS/m) where 1 Siemen = 1 mho = 1/ohm Common conductivity unit is micromho/cm 1 micromho/cm = 1 μS/cm. Convert to logged units using 10 μS/cm = 1 mS/m Example suppose resistivity is 10 ohm-m conductivity = 1/(10 ohm-m) = 0.1 mho/m = 0.1 S/m
Electrode Log or Dual Laterolog VARIABLE RESPONSE Electrode or Dual Laterolog Fluid Type Measures resistivity of formation water. High resistivity hydrocarbons Low resistivity brine
Resistivity Logs and Borehole Fluids Need Conductive Borehole Fluid? Comment(s) Induction No Work with oil-based mud and air-filled boreholes. Unreliable in boreholes filled with very conductive mud. Dual laterolog* Yes Will not work with oil-based mud and air-filled boreholes *Laterolog tools use electrodes to measure formation resistivity (shallow and deep) through saline borehole fluids
Distinguish between Water-Bearing Zone and Hydrocarbon-Bearing Zone Dual Laterolog Curves Distinguish between Water-Bearing Zone and Hydrocarbon-Bearing Zone Log Measures GR Gamma Ray CALI Caliper LLD Deep Laterolog True formation resistivity (Rt) LLS Shallow Laterolog Resistivity of invaded zone (Ri) RXO Microresistivity Resistivity of flushed zone MSFL Microspherically focused Source: Asquith and Krygowski, Figs. 1.7 & 1.9, Basic Well Log Analysis, 2nd Ed (2004)
Activity Well Log Responses – 1 Place the correct answer in the left hand column. Log Response Gamma Ray 1 Measures electrical potential (voltage) associated with movement of ions. Density 2 Detects shale from in situ radioactivity. Photoelectric Effect 3 Measures speed of sound in medium. Speed of sound faster in rock than in fluid. Electrode (dual laterolog) 4 Fast neutrons are slowed by collisions to thermal energies. Thermal neutrons are captured by nuclei, which then emit detectable gamma rays. Acoustic (sonic) 5 Measures resistivity of formation water. SP 6 Measure absorption of low energy gamma rays by atoms in formation. Neutron 7 Measures electron density by detecting Compton scattered gamma rays. 8 None of the above
Activity Well Log Responses – 2 Place the correct answer in the left hand column. There may be some duplication. Log Identifies Neutron 1 Porosity SP 2 Fluid Type Density 3 Rock Type Photoelectric Effect 4 Hydrogen content Acoustic (sonic) 5 Permeable beds Electrode (dual laterolog) 6 None of the Above Gamma Ray
LOG CALIBRATION WITH FORMATION SAMPLES
Mud Log ROP Rate of Penetration Gamma rays (GR) from NORM Measure in API units Relative unit Potassium GR energy 1.46 MeV Thorium series GR energy 2.62 MeV Uranium-Radium series GR energy 1.76 MeV
NORM in West Texas Barnett Shale Mississippian Barnett Shale above MD = 9606 ft Mississippian Limestone below MD = 9606 ft SGR – Spectral Gamma Ray CGR – Total GR minus URAN POTA – Potassium 40, wt % URAN – Uranium, ppm THOR – Thorium, ppm Barnett Shale Limestone Source: Asquith and Krygowski, Fig. 3.3, Basic Well Log Analysis, 2nd Ed (2004)
Evaluate Well Cuttings Well Site Geologist Examines Well Cuttings Makes note of cores Full-Diameter Cores Side Cores
MODERN LOG APPLICATIONS
Principal Applications of Common Well Logs (after Selley and Sonnenberg [2015, page 86]) Log Type Lithology Hydrocarbons Porosity Pressure Dip ELECTRIC SP X Resistivity RADIOACTIVE Gamma Ray Neutron Density SONIC DIPMETER
Illustration of a Fence Diagram (A) A clean sand interval indicated by gamma ray (GR) logs. (B) Fence diagram displaying clean sand correlation.
Interpret Depositional Environment Using Well Logs Typical electrical log shapes… S.P. Resistivity Well Shale Sandstone Interbedded SS & Shales S.P. Resistivity Well Shale Sandstone for beach or barrier island marine SS for fluvial SS *Fig. 158, P.K. Link, Basic Petroleum Geology, 3rd Edition (2001), Tulsa: OGCI
QUESTIONS?
SUPPLEMENT