Geological Space Probes – Remote Sensing in Oil Wells (“Well Logging”) Roger Samworth

Hydrocarbons:Nature Of Reservoirs (1) Hydrocarbon reservoirs are regions of porous rock where the pore spaces are filled with oil and/or gas

Hydrocarbons:Nature Of Reservoirs (2) They usually occur at depths in the ground where the pressures are many thousands of p.s.i. And temperatures normally exceed 100 deg. C

Hydrocarbons:Nature Of Reservoirs (3) The pore spaces naturally contain water. Hydrocarbons develop from the decay of organic matter, migrating upwards through the pore spaces until they encounter a geological “trap”, where they concentrate, displacing the natural water.

Hydrocarbons: Exploration And Production 1. Identify geological structures 2. Drill a borehole or “well” 3. Lower remote sensing probes into the hole & “log the well” 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges 6. Produce the hydrocarbon

Hydrocarbons: Exploration And Production 1. Identify geological structures 2. Drill a borehole or “well” 3. Lower remote sensing probes into the hole & “log the well” 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges 6. Produce the hydrocarbon

Hydrocarbons: Exploration And Production (1) 1. Identify geological structures e.g. Large scale seismic/gravity/magnetic surveys

A Seismic Survey

Hydrocarbons: Exploration And Production 1. Identify geological structures 2. Drill a borehole or “well” 3. Lower remote sensing probes into the hole & “log the well” 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges 6. Produce the hydrocarbon

Drilling A Well Wet -------- ------ Or Dry!

Hydrocarbons: Exploration And Production 1. Identify geological structures 2. Drill a borehole or “well” 3. Lower remote sensing probes into the hole & “log the well” 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges 6. Produce the hydrocarbon

Well Logging A sensor / electronic instrument package is drawn along a borehole, recording and transmitting data along an armoured cable to a surface computer as it progresses.

Well Logging - Parallels With Space Probes Remote location - difficult to fix if it goes wrong Hostile environment - Temperatures -40deg.C ambient to 175+deg.C Downhole pressures up to 15000+ p.s.i. Corrosive fluids Advanced materials required Well logging probably biggest user of titanium outside aerospace Extensive use of advanced polymers such as PEEK Digital communications over long distances Finite chance of not seeing (or hearing) the probe again !

The First Log Schlumberger Pechelbronn, France September 5th 1927

The First Logging Unit !

“Standard” Logging Equipment

“Compact” Logging Equipment

Open-hole Logs Usual to run Triple combination of Nuclear , Induction and sonic tools NMR attractive because it can estimate permeability in addition to porosity.

Hydrocarbons: Exploration And Production 1. Identify geological structures 2. Drill a borehole or “well” 3. Lower remote sensing probes into the hole & “log the well” 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges 6. Produce the hydrocarbon

Hydrocarbons: Exploration And Production 4. Cement steel pipe into hole 5. Perforate pipe with shaped explosive charges, followed possibly by pressurising the formation to fracture it (“fracing”) and make it permeable 6. Produce the hydrocarbon

Hydrocarbons: Exploration And Production Typical field Superimposed on Central London

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivity low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivity low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Rock Type Identification Sandstone, limestone and dolomite are common porous reservoir rocks, shale is non-porous compacted clay Shale has a relatively high level of natural radioactivity due to clay containing traces of potassium, uranium and thorium Sandstone is silica, limestone is calcium carbonate Calcium and silicon can be differentiated by their different abilities to absorb low-energy gamma rays (they have different “photoelectric absorbtion cross-sections” or “PE”s)

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivity low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Density Measurement Dense materials absorb gamma rays more than lighter ones. A radioactive source of gamma rays (usually Cs-137) is used to irradiate the rock formation and detectors measure the resultant scattered radiation The intensity of this radiation is related to the formation density

Basic Density Tool The presence of the borehole itself affects the measurement Variations in the hole diameter and hole fluid density have to be accounted for Additionally, the drilling process also disturbs the original formation

Drilling Induced Formation Disturbance The drilling fluid “invades” the rock pores near the well, displacing the natural fluids Solids in the invading fluid leave behind a “mudcake” on the borehole wall up to 15mm thick It is necessary to apply techniques to compensate for these effects

Well log “Compensation” The mudcake and / or invasion affect logs to a greater or lesser degree Additionally, boreholes are seldom smooth and regular To counteract these effects, logs of a similar type but having a different “measurement penetration” are combined together to compensate for the perturbations This technique is widely used in most well log measurements

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivty low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Neutron Porosity Measurement Water and oil contain Hydrogen in about the same proportions. Water = H2O, Oil = (H2C)Xn The nucleus of a hydrogen atom is a single proton Neutrons and protons are elementary particles making up an atomic nucleus. Neutrons have a similar mass to protons, and because they are un-charged, can approach, and interact with, positively charged atomic nuclei A neutron has a much better chance of interacting with a particle of a similar mass to itself than with a heavier one e.g 2 billiard balls, when colliding, interact, but a billiard ball does not interact much with a ping-pong ball A radioactive source of fast neutrons is used to irradiate the rock formation and detectors measure the resultant neutron flux The neutrons dominantly interact with the single proton of a hydrogen nucleus. The resultant neutron population therefore reflects the amount of hydrogen present which, in turn, is a measure of the rock porosity

Neutron Porosity Tool Isotopic neutron source 2 He-3 proportional counters “Compensation” achieved by computing ratio of count rates at the 2 detectors N/F counts Porosity The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Interpretation And Calibration It is important to remember that: Density tools do not measure density They measure gamma radiation intensity at point(s) in space Neutron tools do not measure porosity They measure neutron fluxes at point(s) in space. Getting Density and Porosity is then a question of interpreting the measurement, assuming that it has been properly calibrated

Neutron Hydrogen Density Log What type of probe? Significant features 5 passes displayed Helium -3 detectors Source-detector spacing = 64 miles The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Neutron Hydrogen Density Log Polar aaaaa ice What’s this aaaa then ? How much money do you think an oil company would invest based on these logs? The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Response Calibration And Characterisation Using CALLISTO CALibration at Leicester & In - Situ Tool Optimisation EUROPA’s sister CALLISTO is a World-standard facility for the calibration and characterisation of (predominantly) nuclear well logging probes

CALLISTO Calibration Facility

CALLISTO Calibration Facility

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivty low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Electrical Conduction In Rock Conduction only takes place in the fluid between the rock matrix The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Induction Tool Direct induction cancelled by “bucking” coils and by phase detection Ground current induces current in receiver coil shifted by further 90 deg., it’s magnitude being proportional to the ground conductivity Currents induced in ground 90 deg. phase shifted Transmitter coil excited at 20khz Borehole fluid need not be conductive The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

“Laterolog” Current Emitting Tools Current is focussed by arrays of electrodes into different patterns Conductive borehole fluid required The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Logging The Well a) Identify Rock Type e.g. Measure level and nature of natural radioactivity b) Measure Rock Porosity e.g. Measure bulk density using gamma ray transport (Porous rocks are less dense) Measure hydrogen density using neutron transport (Both water & oil contain hydrogen) c) Identify Pore Fluids e.g. measure electrical resistivty low resistivity = water high resistivity = hydrocarbon d) Measure Rock Mechanical Properties e.g. measure acoustic velocities

Acoustic Velocity Measurement Time measured for an acoustic pulse to travel a fixed distance “Up” and “down” measurements averaged to compensate for probe tilt TRANSMITTERS RECEIVERS

Acoustic Velocity Measurement Waveform can be displayed as a “variable density” plot Rock strength moduli calculated from compressional and shear-wave velocities

Electrical Imaging Tool (1) The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Electrical Imaging Tool (2) Original image “In-filled” image Images are of “opened - out” borehole Clearly shows rock features. (Strata dipping with respect to the well appear as “sine- waves”) The spatial distribution of epithermal or thermal neutrons resulting from the interaction of fast neutrons within a formation is related to the hydrogen content. If the hydrogen is only contained within the pore space then the measurement yields porosity. At the source energy the primary interaction is elastic scattering- in lime the mfp is ~8cm and independent of water content. But at lower energies e.g. 100 keV it ranges from 4cm in pure limestone to 2cm in 40% water-filled lime. N transport depends on Ls and Diffusion at thermal energies until they are captured.It can be shown that the epithermal flux has an exponential dependence on Ls and diffusion coefficient   exp(-r / Ls) Depi.r For thermal neutrons   Ld^2 . exp(-r / Ls) - exp(-r / Ld) D(Ls^2 - Ld^2) r D=Ld^2.  expressed in capture units which is 1000 times sigma in cm^-1

Application to Space Exploration Seismology (“Echo sounding”) Seismology can potentially reveal internal structure and dynamic processes of other rocky bodies—planets, moons, and asteroids— in the solar system, if seismic sensors can be deployed Magnetometry Magnetometers are used occasionally in well-logging, but are used extensively in space probes to investigate all sorts of magnetic properties and phenomena. Gravimetry Again, used occasionally in well logging to indicate density or mass anomalies, but used extensively in space probes

Application to Space Exploration Neutron methods Remember “The resultant neutron population therefore reflects the amount of hydrogen present which, in turn, is a measure of the rock porosity” This can also indicate the presence of water, as we already discussed with respect to the Lunar orbiter albeit with a 64-mile source detector spacing! The neutron source in this case was the lunar surface itself, where neutrons were released by cosmic ray bombardment. Neutron spectrometry Neutron bombardment can also provoke the release of specific gamma-rays from which elemental composition can be determined

Application to Space Exploration Density measurements The Bepicolombo Mercury mission, launching this year, originally had a lander containing a mole with a density measurement. Cancelled due to cost. Natural Radiation Curiosity Martian rover contained a RAD detector to assess various sorts of natural radiation.

Conclusion 45-year career in Well-logging R&D (1972-2017) A large variety of techniques employed Seen dramatic changes From the perspective of 1972: By 2000 there would have been no oil left. We would now be running on a coal economy No-one worried about global warming, in fact cooling was the concern if the Gulf Stream “turned off”. The only computing power we had in 1972 was a 4-function calculator and that belonged to Accounts!