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Well logging is an interesting method that deals with the subsurface layers providing every necessary detail required about them. The process basically involves specialized tools which are sent down through the well bore to gather the information about the subsurface properties. This data collected is represented over a series of measurements covered over the depth range of the well bore and this assimilation is collectively known as well logs.
The first well log was made in 1927 at Pechelbronn Field in Alsace, France. Invented by the Schlumberger brothers, Marcel and Conrad, the tool measured the electrical resistance of the earth’s subsurface. They recorded the data at each meter as they retrieved the sonde, a specialized tool suspended from the cable, from the borehole. This data log of the corresponding resistivity change was used to identify the location of oil.
The well logs are useful in defining a number of parameters that include physical rock characteristics, lithology, mineralogy, pore geometry, porosity and permeability. By interpretation of the well log data we can also estimate the productive zones with their thickness and depth by defining the basic parameters like fluid composition and relative saturation. The other prominent methods that involve the well data is relating them to seismic to obtain a proper correlation pattern that will help in confirming the hydrocarbon presence.
For oil and gas prospecting the well data helps in the rock type identiﬁcation of geological environment, reservoir ﬂuid contact location and fracture detection. One can also estimate the total hydrocarbon in place along with the recoverable hydrocarbon. Determination of water salinity, reservoir pressure, porosity or pore size distribution is also done with its help. The other properties that can be interpreted from the data are water ﬂood feasibility, reservoir quality mapping, interzone ﬂuid communication probability and reservoir ﬂuid movement monitoring.
Applications of well logging data differ according to the specified specializations.
For the Geologist:
1.Depths of formation tops.
2. Geological environment and hydrocarbon accumulation.
3. Presence of hydrocarbons and its quantity.4.What are the reserves?
5. Conditions for an offset well.
For the Geophysicist:
1.Predicted formation tops.
2. Potential zone analysis for properties such as porosity as assumed from seismic data.
3. Analysis of synthetic seismic sections.
For the Drilling Engineer:
1.Hole volume for cementing.
2. Presence of any Key-Seats or severe Dog-legs.
3. Location for a good packer seat for testing.
4. Location to set a Whipstock.
For the Reservoir Engineer:
1.Thickness of pay zone.
2. Homogeneity of the section.
3. Volume of hydrocarbon per cubic meter.
4. Duration of well pay-out.
For the Production Engineer:
1.Determination of well completion zones.
2. Expected production rate.
3. Chances for water production.
4. Suitable well completion methods.
5. Determination of hydraulically isolated potential pay zone.
2. Methods of Logging
- Performed on a well before the wellbore has been cased and cemented.
- Logging is done through the bare rock sides of the formation.
- Common type of logging method because the measurements are not obstructed.
- Done during or after the well has been drilled.
- Retrieve logging measurements through the well casing, or the metal piping that is inserted into the well during completion operations.
- Cased-hole logging is performed more rarely but it provides valuable information about the well.
3. Log Types
The following are the most important type of logs:
1. Electric logs – self potential, resistivity and conductivity logs. Electric logs were the first type to be employed in petroleum exploration, because it was fairly simple to make the measurements. This involved measuring the electric resistance (R) (resistivity) and the current that is set up between the drilling mud and the porewater in the rock (formation), i.e. the self-potential (SP).
2. Radioactivity logs - gamma ray and neutron logs. Gamma logs measure the natural emission of gamma rays from rocks in well. A neutron log is obtained y using neutron source which sends radiation into the rocks. The absorption, mostly by hydrogen atoms, occurring in water and hydrocarbons is then measured.
3. Acoustic (sonic) logs – measure how fast sound travels through rocks, and in particular provide information about porosity. This also indicates whether a liquid or gas phase occupies the pore spaces.
4. Dipmeter logs – a type of electric log which measures the slope of beds and lamination in rocks.
Logs which directly measure properties of the well itself:
- Caliper logsregister variations in the diameter of the well.
- Temperature logs record borehole temperature and can be used to calculate the true formation temperature.
- Image logs provide a picture of the well wall and may reveal layering, sedimentary structures and fractures
3.1. Electric Logs
- SP logs record the electrical current (in milivolts) that arises due to salinity differences between a salt water based drilling mud and the fluid in formation.
- Indicate permeability of rocks by measuring voltage difference between the drilling fluid and formation water.
- Can distinguish shale from carbonates and sandstones.
- Porous sandstones with high permeability generate more electricity than shale
- Shale positive; sand negative.
- Shale has values between 0 to -20 mV, sandstones and carbonates typically have values between -20 to -80 mV.
- Identifies permeable zones and boundaries
- Not good indicator of lithologic boundaries.
- Resistivity is the physical property of a formation which impedes the flow of electric current.
- Distinguishes type of fluid; hydrocarbon, fresh water and brine
- Measures the effectiveness of rocks in conducting electricity
- Short penetration reflects drilling mud; longer is due to formation of water
- It is base on Induction Principle.
- Low resistivity means shale/ wet sand; high resistivity means hydrocarbons
- Resistivity is measured by drilling tools like DLL, HRI, HRAI etc.
- Determines the true resistivity of formation
- Indicated presence of movable hydrocarbons
3.2. Acoustic Logs
- Petroleum applications of acoustic-wave-propagation theory and physics include both:
- Surface-geophysical methods
- Borehole-geophysical methods
- Measures a number of sonic parameters like compressional & shear velocities and travel time.
- Determines porosity by measuring the speed of sound waves in the formation.
- Identify zones with abnormally high pressures.
- Identification of gas-bearing intervals.
- Estimate rock permeability.
- Cement Evaluation.
- Improve correlation and interpretation of seismic records.
- Help in identifying lithology and fractures.
- Helps in Geophysical Interpretation like Synthetic Seismograms, VSP, AVO Analysis, etc.
- Study of rocks mechanical properties and acoustic impedance (in combination with the density log).
- Displays travel time of P-waves versus depth.
- They are recorded by pulling a tool on a wireline up the wellbore.
- Tool emits a sound wave that travels from source to the formation and back to the receiver.
- Tool measures the time it takes for a pulse of sound to travel from a transmitter to receiver. Both of which are mounted on the same tool.
- The transmitted pulse is very short and of high amplitude. The wave travels through different forms like dispersion and attenuation occurs.
- When the sound energy arrives at receiver, it captures them at different times in the form of different types of waves.
- Travel time is the difference in the arrival of compressional waves at the receivers.
3.3. Nuclear Logs
- Determines porosity by measuring the amount of hydrogen atoms (neutrons) in the pores.
- Tool has a neutron source.
- Hydrogen absorbs neutrons and emits Gamma rays.
- Hydrogen is mostly found in formation fluids like water or hydrocarbons.
- Can be run in Cased holes.
- Measures effect of bombarding a formation with a strong source of neutrons. This bombardment upsets the radioactive equilibrium of the rocks in the bore hole and induces a secondary gamma ray intensity many times greater than the natural gamma ray radiation from these rocks.
- Generally appears similar to resistivity curve of electrical log. Different neutron values due to presence of fluid will alter this similarity somewhat.
- Does not represent lithology.
- Is difficult to interpret alone.
- Cannot be always correlated because it represents primarily fluid content.
- Shale is normally the lowest curve value. This is due to the presence of hydrogen in the shale that slows the fast neutrons and reduces the incidence of capture, with resulting low secondary gamma ray radiation.
- Shales may be used as a baseline for curve.
- Is not generally affected by highly radioactive formations.
- The tool operates by bombarding the formation with high energy neutrons. These neutrons undergo scattering in the formation, losing energy and producing high energy gamma rays. The scattering reactions occur most efficiently with hydrogen atoms. The resulting low energy neutrons or gamma rays can be detected, and their count rate is related to the amount of hydrogen atoms in the formation.
- In formations with a large amount of hydrogen atoms, the neutrons are slowed down and absorbed very quickly and in a short distance. The count rate of slow neutrons or capture gamma rays is low in the tool. Hence, the count rate will be low in high porosity rocks.
- In formations with a small amount of hydrogen atoms, the neutrons are slowed down and absorbed more slowly and travel further through the rock before being absorbed. The count rate of slow neutrons or capture gamma rays in the tool is therefore higher. Hence, the count rate will be higher in low porosity rocks.
- There are mainly three types of neutron tool, which are:
- The Gamma Ray/Neutron Tool (GNT)
- The Sidewall Neutron Porosity Tool (SNP)
- The Compensated Neutron Log (CNL)
Density or Porosity Logs
- Determines porosity by measuring electron density.
- Dense formation absorbs more gamma rays while low-density formations absorb fewer.
- High court rate at detector means low-density formation.
- Mineral identification.
- Gas detection (used in combination with neutron log).
- Shale gas evaluations.
- Delineate thin beds.
- The formation density is a porosity log that measures electron density of a formation.
- The gamma rays enter the formation and undergo compton scattering by interaction with the electrons in the atoms composing the formation.
- Compton scattering reduces the energy of the gamma rays in a step-wise manner, and scatters the gamma rays in all directions.
- When the energy of the gamma rays is less than 0.5 MeV they may undergo photo-electric absorption by interaction with the atomic electrons.
- The flux of gamma rays that reach each of the two detectors is therefore attenuated by the formation, and the amount of attenuation is dependent upon the density of electrons in the formation.
- Dense formations absorb many gamma rays, while low-density formations absorb fewer. Thus, high-count rates at the detectors indicate low-density formations, whereas low count rates at the detectors indicate high-density formations.
- Therefore, scattered gamma rays reaching the detector is an inclination of formations density.
- Scale and units: The most frequently used scales are a range of 2.0 to 3.0 gm/cc or 1.95 to 2.95 gm/cc across two tracks.
The tool consists of:
- A radioactive source: This is usually caesium-137 or cobalt-60, and emits gamma rays of medium energy (in the range 0.2 – 2 MeV). For example, caesium-137 emits gamma rays with an energy of 0.662 MeV.
- A short range detector. This detector is very similar to the detectors used in the natural gamma ray tools, and is placed 7 inches from the source.
- A long range detector. This detector is identical to the short range detector, and is placed 16 inches from the source.
Gamma Ray Log
- Measures radioactivity to determine the kind of rocks.
- Decay of radioactive elements produces high energy gamma ray.
- This gamma radiation originates from potassium-40 and the isotopes of the Uranium-Radium and Thorium series.
- Once the gamma rays are emitted from an isotope in the formation, they progressively reduce in energy as the result of collisions with other atoms in the rock (compton scattering).
- Compton scattering occurs until the gamma ray is of such a low energy that it is completely absorbed by the formation.
- Hence, the gamma ray intensity that the log measures is a function of:
- The initial intensity of gamma ray emission, which is a property of the elemental composition of the rock.
- The amount of compton scattering that the gamma rays encounter, which is related to the distance between the gamma emission and the detector and the density of the intervening material.
- The tool therefore has a limited depth of investigation.
- Shale show high radioactivity as radioactive elements are concentrated in Shale
- Sandstone and carbonate usually show low radioactivity.
- Can be run in both open and cased hole.
Gamma Ray Curve
- Measure the natural gamma ray radiation from the formation. No electrical properties are measured.
- Generally appears similar to self potential curve of the electrical log.
- Formations generally have a characteristic curve response according to type.
- Can be readily interpreted alone in most areas.
- Can be readily correlated with other information pertaining to the formation type.
- Shale is normally the highest curve value.
- No base line or zero. All recordings are positive.
- Is greatly affected by highly radioactive formations.
- Is not affected by changes in borehole diameter.
- Id not affected by borehole and formations fluid.
- Does not represent porosity or permeability.
3.4. Other Logs
NMR (Nuclear Magnetic Resonance) Logs
- Measures magnetic response of fluids.
- Measures both porosity and permeability.
- Help determining the type of fluid in pore spaces.
- Identify low-resistivity pay within water volumes.
- Determines orientations of sandstone and shale beds.
- Determines orientations of faults and fractures.
- Measures resistivity of rocks.
- Make detailed image of the rock around the well hole.
- High resolution images of the borehole.
- Electrical micro imaging technique.
- Used to identify a variety of geological attributes like structural dip, faults and fractures.
- Insight to the condition of the borehole, stress and rock mechanics around the borehole.
- Helps in porosity determination.
- Recent developments in LWD made it possible to acquire high resolution electrical borehole images. In real time these images can help to steer the drill bit.
- Detailed stratigraphic and sedimentological analysis.
- This bed delination.
- Fault mapping and structural analysis.
- Measure the diameter and shape of a borehole.
- Indicator of good permeability and porosity zones.
- Calculation of mudcake thickness.
- Measurement of borehole volume.