Well Logging
Content
WELL LOGGING
A well log is the continuous recording of the characteristics of the hole drilled formation, as a function of depth. Well logs are recorded at the various stages in well under drilling. The drilling is interrupted during the log recording. The data is recorded and transmitted to the surface instantaneously. Well logs are essential tools for enhanced reservoir evaluation.
Electric Logs
Spontaneous potential The SP log is the difference in electric potential between a fixed electrode at the surface and a moving electrode in the borehole. It is measured in millivolts, and there is no absolute zero; only changes in potential are recorded. Two types of potential may contribute to the SP effect. These are the electrochemical potential (Ec) and the electro kinetic potential (Ek). The electro kinetic potential (Ek) is produced by the flow of mud filtrate through a porous and permeable formation. The electrochemical potential (Ec) results from the transfer of ions from a more concentrated electrolyte (usually the uninvaded zone in the formation) to a less concentrated electrolyte (usually mud in the bore hole).
The SP log is used in the identification of permeable beds and the location of their boundaries, and for determination of formation water resistivity in the uninvaded zone (Rw).
A deflection is observed opposite the reservoir rock compared with a “base line” of the shale. These deflections are due to different salinities of the reservoir water and the drilling mud. Resistivity log Resistivity logs measure and record the resistance offered by the rocks surrounding the bore hole to the passage of the electric current. A system of electrodes sends an electric current into the formation. The apparent resistivity of the reservoir is measured in ohms per meter. The resistivity logs may be divided into conventional or non focused devices, focused tools and induction systems.
The Laterolog
systems contain an array of electrodes to focus the survey current and
force it to flow laterally into the formations surrounding the borehole. The effective depth of laterolog investigation is controlled by the extent to which the surveying current is focused. The induction log measures the conductivity of the rocks surrounding the borehole by inducing an electric current through them. The tool consists of a transmitter and a receiver coil. A constant, high frequency alternating current is sent through the transmitter coils. This generates an alternating magnetic field which induces secondary currents (also known as eddy currents) in the rocks surrounding the borehole. These currents flow in circular paths coaxial with the transmitter coils through the surrounding
rocks. The resulting magnetic field, in turn induces signals in the receiver coils. These signals are proportional to the conductivity of the formations from which resistivity is derived and recorded on the log. The resistivity recorded is a function of the porosity and saturation
(water/hydrocarbons). The rock matrices are insulating and the hydrocarbons have high resistivity, whereas the resistivity of the water decreases with increasing salinity. The resistivity can differentiate the water from hydrocarbons.
Empirical equations
Ro a F m Rw
1 Sw Where
n
Rw Rt
Ro = resistivity of rocks 100% saturated with water of resistivity Rw. F = formation factor a = tortuosity coefficient m = cementation factor n = saturation exponent Rt = calculated resistivity of rock at water saturation Sw.
Radioactivity Logs
Gamma ray log (GR) This log records the natural radioactivity of formations. The radioactivity arises from the presence of uranium(U), thorium(Th) and potassium (K40) in the rocks. These elements continuously emit gamma rays, which are short bursts of high energy radiation similar to x-rays. Gamma rays are capable of penetrating a few inches of rock, and a fraction of those that originate close to the borehole traverse the hole and can be detected by a suitable gamma-ray sensor. The detector gives a discrete electrical pulse for each gamma ray detected, and the parameter logged is the number of pulses recorded per unit of time by the detector. The GR log is useful in detecting shale beds. Non radioactive minerals like coal may be detected by their characteristically low gamma response. This log is used for correlation of formations in cased holes.
Neutron log In neutron logging the formations surrounding the borehole are bombarded by high energy neutrons from an artificial source carried on the device. Neutrons are electrically neutral particles with a mass almost identical to that of a hydrogen atom. Upon leaving the source the neutrons enter the formations and collide with nuclei in the rocks forming the borehole wall. With each collision a neutron loses some of its energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus of practically equal mass, ie. a hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron down very much. Thus the slowing down of neutrons depends largely on the amount of hydrogen in the formation. The sonde emits fast neutrons which bombard the formation giving rise to slow neutrons, The neutron count rates increase with decreasing hydrogen content (low porosity in clean formations) and decrease with increasing hydrogen content (high porosity in clean formations).
Formation Density Compensated (FDC) Log The Formation Density Compensated (FDC) Log records the bulk density (ρ of the b) formation surrounding the borehole. Gamma rays are beamed at the formations by the source. These enter the formations and undergo multiple collisions with the electrons in the frocks, as the result of which they energy and become scattered in all directions. This is known as Compton scattering. Some of the scattered gamma rays return to the borehole and are recorded by the detectors on the device. The intensity of the returned radiation is proportional to the number of electrons in the formation, and provides a measure of the electron density of the material. Electron density is approximately is equal to the bulk density of the rocks and this is recorded in gm/cm3.
D .D f 1 ) D m (
Where D = total density read on log Df = fluid density (filtrate) Dm = density of rock matrix
The Borehole Compensated Sonic Log (BHC) The sonic or acoustic log provides a continuous record of the time taken, in milliseconds per foot (µsec/ft), by a compressional sound wave to travel through one foot of formation. Known as the interval transit time, this is the reciprocal of the compressional wave velocity (Vp ). The velocity of sound through a given formation is a function of its lithology and porosity. Dense, low porosity rocks are characterized by high matrix velocities (Vm), while porous and less dense formations are characterized by low Vm, values.
1 1 or V Vf Vm
. f 1 ). t t ( tm
Where Δ = travel time in the transmitter/receiver interval t
Some other logs Caliper log This log system with arms furnishes the borehole diameter and helps in identifications of caving, constrictions etc. Dipmeter log This is the simultaneous recording of four microlaterolog curves along four 90 degree generating lines in a plane normal to the bore hole axis. The difference in the four curves gives the value of dip and its direction.
Cement bond log (CBL) This log system is a continuous cased hole recording of the amplitude of the acoustic signal versus depth. The analysis of signals provides information on the presence and bonding the cement to the casing and to the formation In the presence of cement, the signal is weak because cement attenuates the vibrations of the metal. In the absence of cement, the casing vibrates freely generating a strong signal.
Cement Bond Log
A well log is the continuous recording of the characteristics of the hole drilled formation, as a function of depth. Well logs are recorded at the various stages in well under drilling. The drilling is interrupted during the log recording. The data is recorded and transmitted to the surface instantaneously. Well logs are essential tools for enhanced reservoir evaluation.
Electric Logs
Spontaneous potential The SP log is the difference in electric potential between a fixed electrode at the surface and a moving electrode in the borehole. It is measured in millivolts, and there is no absolute zero; only changes in potential are recorded. Two types of potential may contribute to the SP effect. These are the electrochemical potential (Ec) and the electro kinetic potential (Ek). The electro kinetic potential (Ek) is produced by the flow of mud filtrate through a porous and permeable formation. The electrochemical potential (Ec) results from the transfer of ions from a more concentrated electrolyte (usually the uninvaded zone in the formation) to a less concentrated electrolyte (usually mud in the bore hole).
The SP log is used in the identification of permeable beds and the location of their boundaries, and for determination of formation water resistivity in the uninvaded zone (Rw).
A deflection is observed opposite the reservoir rock compared with a “base line” of the shale. These deflections are due to different salinities of the reservoir water and the drilling mud. Resistivity log Resistivity logs measure and record the resistance offered by the rocks surrounding the bore hole to the passage of the electric current. A system of electrodes sends an electric current into the formation. The apparent resistivity of the reservoir is measured in ohms per meter. The resistivity logs may be divided into conventional or non focused devices, focused tools and induction systems.
The Laterolog
systems contain an array of electrodes to focus the survey current and
force it to flow laterally into the formations surrounding the borehole. The effective depth of laterolog investigation is controlled by the extent to which the surveying current is focused. The induction log measures the conductivity of the rocks surrounding the borehole by inducing an electric current through them. The tool consists of a transmitter and a receiver coil. A constant, high frequency alternating current is sent through the transmitter coils. This generates an alternating magnetic field which induces secondary currents (also known as eddy currents) in the rocks surrounding the borehole. These currents flow in circular paths coaxial with the transmitter coils through the surrounding
rocks. The resulting magnetic field, in turn induces signals in the receiver coils. These signals are proportional to the conductivity of the formations from which resistivity is derived and recorded on the log. The resistivity recorded is a function of the porosity and saturation
(water/hydrocarbons). The rock matrices are insulating and the hydrocarbons have high resistivity, whereas the resistivity of the water decreases with increasing salinity. The resistivity can differentiate the water from hydrocarbons.
Empirical equations
Ro a F m Rw
1 Sw Where
n
Rw Rt
Ro = resistivity of rocks 100% saturated with water of resistivity Rw. F = formation factor a = tortuosity coefficient m = cementation factor n = saturation exponent Rt = calculated resistivity of rock at water saturation Sw.
Radioactivity Logs
Gamma ray log (GR) This log records the natural radioactivity of formations. The radioactivity arises from the presence of uranium(U), thorium(Th) and potassium (K40) in the rocks. These elements continuously emit gamma rays, which are short bursts of high energy radiation similar to x-rays. Gamma rays are capable of penetrating a few inches of rock, and a fraction of those that originate close to the borehole traverse the hole and can be detected by a suitable gamma-ray sensor. The detector gives a discrete electrical pulse for each gamma ray detected, and the parameter logged is the number of pulses recorded per unit of time by the detector. The GR log is useful in detecting shale beds. Non radioactive minerals like coal may be detected by their characteristically low gamma response. This log is used for correlation of formations in cased holes.
Neutron log In neutron logging the formations surrounding the borehole are bombarded by high energy neutrons from an artificial source carried on the device. Neutrons are electrically neutral particles with a mass almost identical to that of a hydrogen atom. Upon leaving the source the neutrons enter the formations and collide with nuclei in the rocks forming the borehole wall. With each collision a neutron loses some of its energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus of practically equal mass, ie. a hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron down very much. Thus the slowing down of neutrons depends largely on the amount of hydrogen in the formation. The sonde emits fast neutrons which bombard the formation giving rise to slow neutrons, The neutron count rates increase with decreasing hydrogen content (low porosity in clean formations) and decrease with increasing hydrogen content (high porosity in clean formations).
Formation Density Compensated (FDC) Log The Formation Density Compensated (FDC) Log records the bulk density (ρ of the b) formation surrounding the borehole. Gamma rays are beamed at the formations by the source. These enter the formations and undergo multiple collisions with the electrons in the frocks, as the result of which they energy and become scattered in all directions. This is known as Compton scattering. Some of the scattered gamma rays return to the borehole and are recorded by the detectors on the device. The intensity of the returned radiation is proportional to the number of electrons in the formation, and provides a measure of the electron density of the material. Electron density is approximately is equal to the bulk density of the rocks and this is recorded in gm/cm3.
D .D f 1 ) D m (
Where D = total density read on log Df = fluid density (filtrate) Dm = density of rock matrix
The Borehole Compensated Sonic Log (BHC) The sonic or acoustic log provides a continuous record of the time taken, in milliseconds per foot (µsec/ft), by a compressional sound wave to travel through one foot of formation. Known as the interval transit time, this is the reciprocal of the compressional wave velocity (Vp ). The velocity of sound through a given formation is a function of its lithology and porosity. Dense, low porosity rocks are characterized by high matrix velocities (Vm), while porous and less dense formations are characterized by low Vm, values.
1 1 or V Vf Vm
. f 1 ). t t ( tm
Where Δ = travel time in the transmitter/receiver interval t
Some other logs Caliper log This log system with arms furnishes the borehole diameter and helps in identifications of caving, constrictions etc. Dipmeter log This is the simultaneous recording of four microlaterolog curves along four 90 degree generating lines in a plane normal to the bore hole axis. The difference in the four curves gives the value of dip and its direction.
Cement bond log (CBL) This log system is a continuous cased hole recording of the amplitude of the acoustic signal versus depth. The analysis of signals provides information on the presence and bonding the cement to the casing and to the formation In the presence of cement, the signal is weak because cement attenuates the vibrations of the metal. In the absence of cement, the casing vibrates freely generating a strong signal.
Cement Bond Log
