Well Logging Summary_Master

Abbreviations (shown on Logging Program from office)  BHC: Sonic (Bore hole compensated)

 DSI: Dipole Sonic

 AIT: Array Induction Tool (Laterlog resistivity + SP), can be replaced with HRLA if use salty mud e.g. KCl

 HRLA: High Resistivity Laterlog (Resistivity Laterlog + SP)  PEX: Platform Express (Neutron & Density porosity)  Dual TLD: Dual density (density at 90 degree)  MLTB: Microlog

 FMI: Microlog resistivity, gives color image for dip  XY-Cal: for cement volume & hole condition

 XY Cal-ECRD: can apply extra pressure on string, if tool stuck  ECRD: Electrical Control Release Device

Device uses a remotely operated emergency electrical disconnect that makes it possible to carry out a controlled release. It leaves a “fishing hook” behind so that the elogs can be retrieved after the source safely out of hole. Can apply extra pressure on string, if tool stuck.

Logging Tools vs. Mud Systems

Note: Logging tools are selected based on mud system (mud system dictates which tools to run because some tools works in one type of mud and some in other)

 Air base mud (air drilling): GR, Induction, density & neutron (side wall porosity: SNP). (Electrode Resistivity, SP & Sonic/Acoustic, CNS (neutron: compensated neutron Sonde) does not work)

Reason: none of these tools affected by air

 Oil base mud: GR, DIL, SP, Sonic/Acoustic, Density & Neutron (CNS) tools Reason: none of these tools affected by oil base mud

 Water base mud plus polymer additives: GR, SP, DIL, Sonic/Acoustic, Neutron & Density tools

Reason: none of these tools affected by water base mud, neither by polymer additive

Note: One single log can not provide all information about a formation/zone of interest; however a set of logs help to interpret the zone/formation.

Well Logging Summary

1. Correlation: well to well of formation tops, major lithologies, major unconformities

2. Locating the porous, permeable reservoir rocks

3. Locating the hydrocarbon shows using qualitive “quick look” techniques 4. Quantify reservoir engineering data

a. Thickness of net pay b. Average porosity

c. Average water saturation d. Estimate of permeability 5. Stratigraphic and structural studies 6. Well completion

7. Geophysical application

RESISTIVITY LOGS

Tool measures conductivity of the formation Formation evaluation:

 Conductivity is inversely proportional to resistivity. Resistivity is calculated from conductivity value (R = 1/C). Resistivity is used to identify hydrocarbons. Higher resistivity with clean & porous zone indicates hydrocarbon zone. Also used to identify lithology.

 3 curves (shallow, medium, and deep). You can read directly from deep curve/RILD is equal to formation true resistivity

 Can be presented either on linear or logarithmic scale

 Scale: 0.2 – 2000 (left to right) in Track 2 & 3 and 0.2 – 20 in Track 1  Units: ohm-meter

 If resistivity is high, either has hydrocarbon, water, tight/dense formation/rock (no porosity). If R is too high, might have Anhydrite, Gypsum, Salt, Chalk, could be tight LS or Dolomite, confirm with PE log.

 Fluid contact: If clean and porous zone (based on GR & SP logs), lithology (PE log) & N/D porosity (N/D Porosity log) is almost constant and resistivity drops, there is fluid contact (oil-water). Low resistivity in this zone is water. Resistivity of Gas is less than water & oil.

 Formation Changes: resistivity log is a good secondary method of picking formation tops, as many formations have fairly characteristic resistivity profiles. Sandstone and coal beds tend to be more resistive than shales. Limestone or dolomite beds are usually much more resistive than either sandstones or shales, and anhydrite is extremely resistive. Shales which contain a large amount of organic matter (such as the Nordegg and Exshaw and Fish Scales zones) show typically high resistivities on the induction log.

Environmental Effects, which affect tool’s accuracy:  Gap between hole and tools creates problems

 Best works in oil base mud and fresh mud, where conductivity of mud is zero  Reading up to 250 ohm-m accurate, above 250 ohm-m starts errors

 Bed thickness 4 ft or > 4 ft and hole diameter 8 inch, works good  Hole damage/caving (skin effect) effects it accuracy

 Tool is conductivity seeking device, higher the conductivity, better tool works. Main features/components of tool:

Receiver coil, transmitter coil, ground loop, a constant power, direct coupling (or reverse wound coil) to eliminate unwanted signals e.g. X-Signals.

Tool mechanism/Principles of measurement: a constant current is applied to transmitter coil, which creates and induces magnetic field into formation, a second magnetic field is created by ground loop and induces in receiver coil and current is measured (strength of formation current depends on formation conductivity). Dual Laterlog Log (DLL)

Tool measures resistivity of the formation Formation evaluation:

 Resistivity is used to identify hydrocarbon zone and also used to identify lithology to some extent. High resistivity with clean & porous zone indicates hydrocarbon presence

 3 curves (shallow: LLS, deep: LLD, MSFL: Micro-Spherically Focused Log, used to measure Rxo)

 Presented on logarithmic scale

 Scale: 0.2 – 2000 (left to right) in Track 2 & 3 and 0.2 – 20 in Track 1  Units: ohm-meter

 Read last 3 points from Induction Log: high resistivity, Fluid Contact and Formation Changes:

Environmental Effects, which affect tool’s accuracy  Works good in salty mud, not good in oil base mud

 Works well when Rt > Ro (Commonly used for low resistivity muds and high resistivity formations)

 Hole dia 8 inch

 Bed thickness and bad hole conditions effect its accuracy

 Tool is resistivity seeking device and higher the resistivity, better the tool works Main features/components of tool:

 Tool uses electrodes to pass current into formation and measure drop/change in voltage/current between 2 other electrodes

 Deep looking device is hole centered

 Shallow looking device is mounted on a pad which pressed against bore hole wall Tool mechanism/Principles of measurement:

A constant current (I) is forced into formation and measure change in voltage, which is related to resistivity of formation

Micro-Resistivity Log

 Tool indicates permeability of formation & measures close to bore hole (Rxo & Ro). Only tells thickness of permeable zone (mud cake)

 It measures the voltage drop from electrodes and resistivities are determined from these voltages drop

 Presence of mud cake is indicated where micro-normal curve (deep) reads higher than micro-lateral curve (shallow: reads close to borehole or Rmc)

 2 curves: give 2 different depths of investigation (Normal or micro-normal: deep resistivity & Lateral or micro-inverse: shallow resistivity)

 Also called MLL (Microlaterolog) or MSFL (Micro-Spherically Focused Log) Environmental Effects, which affect tool’s accuracy

 Mud cake thickness

 Bore hole mud & fugacity on sides of hole => tool does not work (as pad’s loose contact with formation)

 Effected by bore hole e.g. caving, good for straight hole

 Only work in conductive medium e.g. good in water base mud (does not work in oil base mud), additives change Resistivity values

Main features/components of tool:

Current electrodes, guard electrodes, tool is pad mounted that pressed against bore hole wall (just like density tool, sometime used with density tool)

Tool mechanism/Principles of measurement:

 Resistivity measured based on guard/electrode method. Bottom electrode emits current that flows through mud cake, formation and borehole before it returns to tool case. It measures the voltage drop from electrodes and resistivities are determined from these voltages.

 Measurements only see mud cake and flushed zone Array Induction Tool (AIT): 5 curves

POROSITY LOGS

Tool measures bulk density of formation Formation evaluation:

 It measures electron density which is directly related to bulk density of formation. Bulk density of formation helps to calculate porosity of formation and bulk density also indicates lithology

 Dense formations absorb more Gamma rays and count rates at detectors is low  Discriminates between gas & oil when presented with Neutron porosity log  Presented in Track 2 & 3, Units: PU

 Density correction (delta rho) in Track 3, sometime in Track 2

 Limestone porosity scale presentation: 45 – (-15) (left to right) or 0.45 – (-0.15)  Sandstone porosity scale presentation: 60 – 0 (left right) or 0.6 – 0.0

 Bulk density scale: 2 – 3 (left to right, units gm/cc or c3) or 2000-3000 (kg/m3)  The rule of thumb, if the correction curve is greater than +0.15 g/cc (+150

kg/m3), the data in the main curve will not be very reliable. It is the value not the scale that matters. However, acceptable range of the density correction is from -0.05 to 0.15 gm/cc (or -50 to 150 kg/m3)

 If bulk density is too high (porosity too low), tight/dense formation (no porosity). If BD jumps out of scale or close, might have Anhydrite, Gypsum, Salt, could be Dolomite, confirm with PE log.

 Bulk densities of common rocks/minerals: Consult table

 Mirror image porosity, if gas effect is mirror image (e.g. cross over), calculate porosity using formula = SQRT (NP2 _{+ DP}2_{/2). However, in other cases, The rule} of thumb is = NP + DP/2

 The vast majority of office geologists refer to the density porosity curve when assessing reservoir quality, and it is best to report just the density porosity values to the client unless instructed.

 Cross plot SS vs. LS Scale: does not matter either logs porosity on SS or LS scale, but when using cross plot, for LS compare porosity on x-axis and y-axis and read accordingly. And for SS compare porosity directly on SS line and read accordingly.

 Neutron/Density Porosity: is very high for Coal and may jump out of scale (left of the scale)

 Gas Effect (Cross Over): Neutron porosity tool detects hydrogen atoms and neutrons are diffuse after collision with water, some counts come back to tool (more water, less counts back to tool). Gas has less hydrogen atoms and more neutron counts back to tool and tool reads less neutron porosity and Neutron porosity curve moves to right. Density porosity tool measures bulk density (less density => more porosity). Density of gas is less than fresh water (1.0) and oil, density tool reads more porosity and moves to left. This leads to the "gas effect" on porosity logs, and shows "cross over". True porosity in a gas-bearing (cross over) zone is average of neutron and density porosities.

 Curves separation: If neutron density porosity curves come close to each other, there is potential of oil. However, if both curves start separating from each other (move away), confirm with other logs (e.g. GR, PE or so), what is causing them to separate, may be change in lithology or shaly zone.

 Also called Compensated Density Log (CDL), Spectral Density Log (SDL), Formation Density Compensated (FDC), and Litho-Density Tool (LDT). Names from different logging companies.

Environmental Effects, which affect tool’s accuracy:

 Caving/bad borehole conditions (rugosity: borehole wall is rough, increase in density correction)

 Heavy mud e.g. Barite

 Works in all types of muds and air drilled holes  Does not work well in cased hole

Main features/components of tool: GR source, 2 detectors (near/short spacing, far/long spacing). Tool is pad device & pushes against bore hole wall

Tool mechanism/Principles of measurement: Gamma-rays are injected into

formation, rays interact with formation. After Compton scattering (effect), most are absorbed, some of scatter/counts come back and detected. Actually tool measures electron density which is directly related to bulk density of formation.

 The number of gamma rays that reach the detectors is a function of the electron density of the formation being logged.

 Presence of electron in FM slowdown the GR

 Less gamma ray we get back, more electrons in Fm & density is low. Smaller reading, more porosity

 More dense formation, more GR slows down, less return to detector. E.g. LS  Less dense formations, less GR slows down, more return to detector. E.g. Coal

Neutron Porosity Log

Tool measures water content (hydrogen index/atoms) of formation Formation evaluation:

 Used to identify porous formations (determine porosity)

 Amount of Neutrons detected related to formation porosity. Less Neutrons detected, more hydrogen content in formation & higher formation porosity  When presented with bulk density, uses to identify lithology and gas detection

(cross over/gas effect where neutron porosity reads low)

 Discriminates between gas & oil when presented with density porosity log  Presented in Track 2 & 3, Units: PU

 Limestone porosity scale presentation: 45 – (-15) (left to right) or 0.45 – (-0.15)  Sandstone porosity scale presentation: 60 – 0 (left right) or 0.6 – 0.0

 Read following points from Density Porosity log: Neutron/Density Porosity, Gas Effect (Cross Over) and Curves separation

Environmental Effects, which affect tool’s accuracy:  Borehole rugosity & caving

 Mud salinity

 CNS good in oil base mud and SNP good in air/gas base mud  Pressure & temperature

 Mud cake and mud weight

 Stand off (space between tool and borehole) Main features/components of tool:

Fast neutron source, 2 detectors (1 near, 1 far), bolt spring/skid which pushes tool against bore hole wall

Tool mechanism/Principles of measurement:

Source bombardment of fast moving neutrons with high energy into formation. As they collide with hydrogen nuclei, they loose energy and slow down through epithermic to thermic level. Detectors measure their loss of energy as they pass through. Some low energy neutrons come back and detected

Sonic/Acoustic Log

Tool measure the formation’s capacity to transmit sound waves

(Measures travel time/delta-t (reciprocal of the velocity) of sound waves into formation)

Formation evaluation:

 Delta t (travel time) helps to calculate porosity, determine lithology (matrix) and fracture identification

 Units: measured in usec/ft or usec/m

 Plotted at linear scale in porosity track (track 2 & 3)

 Scale: 500 – 100 (left to right) usec/m or 40 – 140 usec/ft

 Coal beds have low velocities and high delta-t values due to their low density, while limestone, dolomite and anhydrite are much denser and have higher velocities and lower delta-t values.

 Fracture and Gas Identification: If you see cycle skipping in the sonic curve, (i.e. a significant and sudden shift in the background delta-t profile within a uniform section of lithology) this can indicate the presence of fractures or gas.

 Cycle Skip: from bottom of spike, corner is sharp at 90 degree, see picture  Noise Spike: at top of spike, corner is rounded, see picture

Environmental Effects, which affect tool’s accuracy:  Caving: cause cycle skipping

 Hole rugasity: cause Noise triggering

 If too tilts in hole, travel time between receivers cancel out

 Hydrocarbon does effect travel time some time in homogeneous rocks, if porosity > 47.6 % then problem

 Gas in hole: gas bubbles scatter and absorb acoustic energy

Main features/components of tool: a sound body with in tool, 2 transmitters-receiver groups. Each group consist of 1 transmitter & 2 receivers (near & far)

Tool mechanism/Principles of measurement: a sound body propagates compressional (sound) waves into formation. 1st _{(A) & 2}nd _{(B) negative arrival of compressional} waves (with slower velocity) come back to tool and detected. A – B = Sonic Velocity (delta-t)

(Tool measures the time it takes for a sound pulse to travel between a transmitter and a receiver, mounted a set distance away along the logging tool).

GAMMA RAY LOG (GR)

Tool measures natural radioactivity of the formation (measures the natural gamma ray emissions from radioactive formation (K-Potassium, U-Uranium, Th -Thorium) Formation evaluation:

 Used for lithology identification and correlation  Faceis identification

 Mineral analysis

 Evaluation of shale content of formation, higher radioactive material in formation, higher GR value and higher SH %. Because it is lithology indicator and used to calculate SH volume.

 Shales tend to read higher than 80 API, siltstones from ~65-80 API, and most other lithologies (sandstone, limestone, dolomite, limestone, coal, etc.) less than 65 API. Some shale beds are highly radioactive and go off scale (higher than 150 API, e.g. Nordegg, Exshaw, Fish Scales zone).

 Presented in track 1 on linear grid and scaled in API units (increased from left to right)

 Applicable in open hole and cased hole

 Spectral Gamma Ray Log: one curve of total gamma ray in track 1, and 3 curves in track 2 & 3 (one curve for each source e.g. U, K & Th)

Environmental Effects, which affect tool’s accuracy:  Caving causes lower GR values

 Heavy mud (e.g. barite in mud => lower value in mud cake)  KCl mud, increase in background GR values

Main features/components of tool:

Simple tool consists of a gamma ray detector and associated electronics for passing the gamma ray counts or count rates to the surface. (Modern tool is a sensitive GR detector consists of a scintillation counter (sodium iodide crystal) and a photo multiplier)

Tool mechanism/Principles of measurement:

 Gamma radiation is electromagnetic waves of energy, similar to light and radio waves, having no mass. This is the radioactive energy that is measured in the borehole by the gamma ray detector.

 Radioactive source (K, U, Th => if present in formation) emits gamma rays with high energy (GR definition: emission of photons with no mass & no charge but

great energy), when they passes through material (formation and mud), their

energy is absorbed through Compton scattering/effect and due to collision

between GR & electrons, their energy is degraded. Higher the density of material they passes through, rapid loss/degradation of energy and continuous spectrum of values is observed/recorded in detector at the surface.

SP (SPONTANEOUS POTENTIAL) LOG

Tool measures the ionic potential between borehole fluid and formation fluid (ion exchange between 2 salinities).

Formation evaluation:

 Indicates permeability (SP deflects left from Shale baseline => negative deflection)

 Discriminates between porous and non porous zone  Indicator of formation water salinity

 Used to calculate resistivity of formation water  Used to identify depositional environment

 No deflection in SP log (straight line) means drilling water and formation water has same salinity

 Bigger the difference in salinity, bigger the deflection (if drill with fresh mud, no salinity)

 SP log runs with Resistivity Tool  Presented with GR in Track 1  Units: milli volts (mV)

 Hydrocarbon: If clean and porous zone (based on GR & SP logs), lithology (PE log) & N/D porosity (N/D Porosity log) is almost constant and SP curve deflects to left from shale baseline and deflects little bit to right within negative deflection, here is hydrocarbon.

Environmental Effects, which affect tool’s accuracy:

 To get deflection need water based mud (no SP in oil base mud and air drilled hole because there is no conductive fluid in borehole)

 Following factors affect shape of SP & amplitude: Rmf/Rw ratio (difference in resistivities of 2 fluids), Salinity difference, hydrocarbon presence, KCl mud, high resistivity formations

 Following factors affect validity of SP, if anyone occurs then SP is useless: Magnetism, Bimetallic effect, Telluric currents (winter affect, check logging date) Main features/components of tool:

2 electrodes (SP electrode, ground electrode), recording voltmeter Tool mechanism/Principles of measurement:

Measured by suspending an electrode in borehole and measuring the voltage difference between electrode and a “ground” electrode that makes the electrical contact with the earth at surface. No artificial currents are applied.

More General Facts:

 Used to identify depositional environment e.g. if bell shape curve indicates Transgrasive/Alluvial point bars/shoreline deposits-finer upward, Cylinder shape indicates distributary channels/turbidities-uniform grain size, funnel shape indicates regressive/delta marine fringe/shoreline deposit/offshore bares-coarser grains upward

 3 factors to provoke an SP current: a) conductive fluid in the borehole, b) a porous & permeable bed surrounded by an impermeable formation, c) and a difference in salinity between borehole fluid and formation fluid

 SP deflection could be negative from Shale baseline if formation water is more saline than mud in borehole and here resistivity of formation water is less than Rmf

 To get full SP deflection and proper bed resolution, as a rule of thumb, a bed should be thicker than 20 times the borehole diameter. SP will not show full deflection, if:

o Bed is not thick enough o Shale in formation o Invasion is very deep o Hydrocarbons are present

 Deflection to left (-ve) => fresh mud in hole & salty water in formation

 Curve moves/deflects towards less saline water (or fresh water) or away from more saline water

 Other factors which affect shape of SP & amplitude of SP: Shaliness, Bed thickness, Bed resistivity, Invasion, Borehole diameter

 Factors which affect validity of SP, if anyone occurs then SP is useless: Magnetism (partial logging truck get magnetize), Bimetallic effect (logging cable

touches/brushes inside casing, difficult to detect), Telluric currents (natural affect caused by winter, if SP tool sits at bottom then natural earth current passes through tool, SP curve moves all over, check logging date if logged in winter) Photo-Electric Effect (PE) or Photoelectric Factor Log (Litho-Density) It is a measure of the amount of light reflected from the rock layers (actually it detects the atomic number “Z” of formation constituents)

Formation evaluation:

 Lithology indicator (types of rocks in formation)  Measured in Barns/electron

 PEF curve is plotted combined with density and neutron logs (scale: 0-10 => left to right)

 If PE value of two rocks is very close then look at bulk density log (if available)  Consult table for PE values of rocks/minerals

Environmental Effects, which affect tool’s accuracy:  Rock type but water in formation does not affect

 Heavy mud e.g. Barite in mud (PE value of Barite: 267, however common rock minerals have PE value < 6). Simply does not work in mud contains Barite Main features/components of tool:

Tool is related to density tools. A high energy gamma ray source, 2 detectors (near & far)

Tool mechanism/Principles of measurement:

The emission of electrons from matter after the absorption of energy from electromagnetic radiation such as x-rays, or visible light.

When Gamma rays pass through matter, after collision they loose energy. Photo-Electric absorption occurs, when gamma rays loose enough energy to be captured by electrons. Capturing electron gets energy, leaves its orbit and ionized. Degree of absorption depends on atomic number (Z) and electron density of the atoms. This is related to chemical composition and indirectly to lithology.

High energy gamma rays are slow down by collision or Compton scattering/effect (until they are absorbed through photoelectric effect). Phenomenon is known as Photo-Electric absorption, when gamma rays loose enough energy to be captured by electrons. Capturing electron gets energy, leaves its orbit and ionized. Related to average atomic number (Z) of the constituents of the formation.

PE measurement: photoelectric absorption occur when a gamma ray colliding with an electron, transfer its all energy to electron. This reaction depends on the energy of incident gamma ray and the type of atom. The photoelectric absorption index of an atom is approximately related to atomic number, Z. Measurement of the formation PE will give an indication of Z of the formation minerals.

DIP METER

 Tells dip of beds (at what angles by calculation)

 Tool works just like caliper tool => opens up & pushes against bore hole wall  Gives color photograph of bore hole, which shows bed strikes

FMI (Formation Image)

Gives color photograph of bore hole, which shows bed strikes. Same as Dip Meter Higher Archie for Porosity Determination from Logs

1. N – D (Neutron & Bulk Density) very powerful to use for porosity determination

2. N – A (Neutron & Acoustic/Sonic) 3. Bulk Density

4. Sonic/Acoustic: Delta-t => do not use 5. N (Neutron)

6. Core – only covers certain points

7. D – A (Bulk Density & Acoustic/Sonic) => not good to use, even we do not use

Log Response to Various Minerals

Mineral PE

Value DensityBulk (gm/cc)

Neutron Porosity

(LS)

Sonic

Delta-t Resistivity ResistivityRange Ohm-m Sandstone 1.81 2.65 -2.1 181 Mod - low

Limestone 5.08 2.71 0 157 Gen. high Variable Dolomite 3.14 2.86 0.5 138 Gen. high Variable Anhydrite 5.06 2.98 -1.2 177 Very high 10,000

-infinity Salt/Halite 4.6 2.03 -1.8 380 Very high 10,000

-infinity Gypsum 3.42 2.32 60 High 1000 Coal ~ 0.2 ~ 1.5 40-60 400-600 High(variable) 1,000,00010 – Fresh Water 0.36 1.00 100 620 Salt Water 0.81 1.146

Pyrite -3 Very low 0.0001–0.1

Shale Moderate Extremely

variable (0.5-1000) Oil 0.13 0.85 Gas 0.095 0.00067 Glauconite 5.32 Siderite 14.69 12 Barite 267 Shaly SS 2.7

Rock Type/Mineral PE Value

SS (@ 0% porosity) (@ 35% porosity) Shaly SS

Sandstone and siltstone beds, depending on their degree of shaliness

1.81 1.54 2.7 1.8 - 3.0 LS (@ 0% porosity) (@ 35% porosity)

LS (Dolomitic), depending on the degree of dolomitization 5.08 4.23 3.2 - 5.0 Dolomite (Pure Dolomite) 3.143.2

Shale (Usually ranges between 2.5 and 3.5, and can fall outside of this range)

3.24

Glauconite 5.32

Salt/Halite 4.65

Gypsum 3.42

Anhydrite 5.06

Coal (Pure coal) (impure coal) 1.0 0.16-0.18 Pyrite 16.97 Siderite 14.69 Fresh water 0.36 Salt water 0.81 Oil (CH4) 0.13 (variable) Gas (CH4) 0.095

Log Response to Various Minerals

Mineral PE Value Bulk

Density (gm/cc) Neutron Porosity (LS) Sonic: Delta-t

Quartz 1.81 2.65 Calcite 5.05 2.71 Dolomite 3.14 2.87 Anhydrite 5.08 2.96 Salt (NaCl)/Halite 4.65 2.165 Coal ~ 0.2 ~ 1.5 Fresh water 0.35 1.00 Salt water 1.2 1.146

Hydrocarbon Less or equal

RESISTIVITY TOOLS Laterlog & Induction Tools

POROSITY TOOLS Density Porosity Tool

GAMMA RAY TOOL