well logs

What do we need to find out?
? Rock type?
– Sandstone? Shale? Limestone?
? Engineering
– – – – – Well trajectory (coord) Shape of hole Casing Joints Quality of cement etc
? Rock Properties
– – – – – – – – – Porosity Permeability Bedding Orientation Fractures? Temperature Type (water, oil, gas) Saturation Salinity Pressure
Well Logs
Geol472-2009
? Fluids
What for? (Log applications)
? ? ? ? ? ? ? ? Stratigraphic correlation Formation Tops Quantitative Oil, Gas , Water saturations Porosity Correlation with seismic data Sedimentological studies Reservoir modeling Structural studies
What can we measure?
? ? ? ? ? ? ? ? ? ? Electrical Properties Natural radioactivity Induced radioactivity Acoustic Properties (velocity) Shape of hole Noise Temperature Depth Orientation of hole …
? etc
Log Types
? Lithologic Logs
– Spontaneous Potential (SP) – Gamma Ray (GR)
? Other
– – – – – – Dipmeter Caliper Temperature Acoustic FMI Many more …
? Porosity Logs
– Neutron – Density – Sonic
? Resistivity Logs (Fluid Type)
– Resistivity – Induction

Spontaneous Potential (SP)
? One of the Oldest Logging Measurements
– Used Commercially in 1931
? Discovered as Noise in Resistivity ? Found to be Related to Presence of Sandstone
Surface
Downhole
The earth well works like a battery
E = -K log (aw/amf) E is in millivolts
Typical SP log
aw= water salinity aw= mud salinity Ion flow is easier in permeable sandstones
SP has Poor Limestone Response
Spontaneous Potential (SP) Drift

Gamma Ray Log
Lithology log Measures natural radioactivity Uses a scintilometer (Gaiger counter) Potassium (K), Uranim, Thorium, Phosphorous ? K → abundant in clay → shales ? Unaffected by fluids ? ? ? ?
SP and Gamma Ray
?Gamma Ray in API Units ?Shales swing right
Spectral Gamma Ray
Differentiates the different sources of gamma rays
Resistivity Log Applications
? “True” Rt Formation Resistivity ? Fluid Saturation Sw from Archie’s Equation ? Determination of Hydrocarbon-Bearing vs. WaterBearing Zones ? Geopressure Detection ? Diameter of Invasion ? Porosity ? Correlation
Resistivity Tool Background
? Three Classes
– Electrode Logs – Laterologs
? Focused Electrodes
– Induction
? Measure Resistivity in Ohms

Resistivity
Resistivity of Water - Rw
Normal Resistivity Tool
Spacing of electrodes determines penetration
Increasing Resistivity
Resistivity of Water and Formation
Resistivity of Water, Hydrocarbons, and Formation
Resistivity of Formation
Induction (Conductivity) Tool
Receiver coil measures the induced electrical field created in the rocks by the transmitter coil
Guard or Laterolog Tool
The guard electrodes focus the current in a narrow disk
Log Presentation and Scales
ID, IM, LL8 measure resistivity at different distances form the borehole. ID= deep induction IM= medium induction LL8= shallow induction
Resistivity logs : sphericallyfocussed (SFL), medium induction (ILM), and deep induction (ILD) from KGS Jones #1

The Borehole Environment Calculating Fluid Saturation
Sw=
(Rxo/Rt)5/8
(Rmf/Rw)
Rxo= Resistivity of flushed zone Rw= Resistivity of formation water Rmf= Resistivity of mud filtrate Rt= Resistivity of uninvaded zone
Header
Sonic (Acoustic) Logs
Measures of acoustic travel time in formation ?t = 1/V
?
?t is related to Porosity, Fluid Content and
Wellbore Environment Depth, Diameter, Temperature, Mud Parameters
Lithology
?
Used to generate Synthetic Seismic traces to link wells logs to seismic data
Creating synthetic seismic data:
Convolution
Wavelet
Acoustic impedance from Sonic log
Seismic Reflectivity
Synthetic Seismic
GEOL?553?Lecture?3;?Subsurface?Analysis

Density Log
? ? ? ? ?
Neutron Log (CNL)
? ? ? ? Tool has a neutron source H absorbs neutrons and emits gamma rays Tool detects the emitted gamma rays H is mostly in formation fluids (water and hydrocarbons) ? Can be run through casing ? Reads low in gas zones ? Cannot distinguish oil from water
Measures of Density - ρ Tied to Porosity, Fluid Content and Lithology Tool emits gamma rays Detects returning scattered gamma rays Gamma ray absorption is proportional to rock density
Porosity calibration
High resistivity
Neutron-density cross over indicates gas!
? If lithology is known, neutron and density logs can be calibrated for porosity
Relating log character to sedimentary facies
Applications of logs
? Stratigraphic studies
– Sedimentary facies
? Well correlation ? Reservoir models ? Structural interpretation
– Fault recognition

Building a reservoir model model
1. Define facies in core 2. Relate facies to log
3. Predict facies in wells without core, but with good logs
Log Datum Terminology
? KB - Kelly Bushing elevation. ? MD - Measured Depth along the wellbore from the Kelly bushing (usually) ? SS - Depth Relative to Sealevel ? TVD – True Vertical Depth, (important for non-vertical wells) non? SSTVD - Sub-Sea True Vertical Depth Sub-
1600 wells
4. Fill the gaps between wells
28 cores
108 Million Cells
Correlation Example
Major Sands on SP