Institute of Petroleum Exploration 9, Kaulagarh Road, Dehradun Keshav Deva Malaviya Institute of Petroleum Exploration 9, Kaulagarh Road, Dehradun Ph: 0135-2795000, 6370, 5681 Well Logging M.K.Tewari D.G.M. (wells) ONGC KDMIPE Dehradun tewari_manojkumar@ongc.co.in

ELECTROMAGNETIC EFFECTS The electromagnetic properties of a material that effect E-M tools log responses are: e (eR) = Dielectric Constant s = Conductivity m = Magnetic Permeability

The NEW Tool operates at 32 kHz and 8 kHz In the lower frequency range (around 20 kHz) the conductivity effect dominates the tool response

The (High Frequency Tool) operates at 1 GHz (1000 MHz) In the mid frequency range (around 20 MHz) both conductivity and dielectric effects are significant In the higher frequency range (around 1GHz) the dielectric effect dominates the tool response The (High Frequency Tool) operates at 1 GHz (1000 MHz)

Comparison between the HRI and the HFDT

Induction tools measure formation conductivity by inducing a current flow within the formation

Induction Tools (DIL, HRI, HRAI) Measure the resistivity of the formation by inducing current flow R V  R Uses Electromagnetic forces to induce current flow in formation Do not require conductive medium in borehole T Can be run in freshwater muds, air-filled holes, and oil-based muds Can provide measurements at different depths of investigation

Faraday's Magnetic Field Induction Experiment

Induction tool Induction tools are based on principles of electromagnetic induction. A magnetic field is generated by an AC electrical current flowing in a continuous loop. An AC electrical current is generated when a continuous loop is subjected to a magnetic flux. The magnitude of this current is proportional to the conductivity of the continuous loop

Induction tool Lets consider a simple induction tool with one transmitter coil and one receiver coil, Applying signal to the transmitter coil it create a magnetic field and induce a current directly in the receiver coil. The signal in the receiver coil is 90 degrees out of phase to the transmitter signal

Induction tool The magnetic field generates current loops in the formation because the conductivity of the formation, those loops are in the same axis of the tool and the current is 90 degrees out of face to the transmitted signal

Induction tool The current flowing through the ground loops generates a secondary magnetic field This secondary magnetic field induces a current in the receiver coil with 90 degrees out of phase with respect to the ground loops and 180 degrees out of phase with respect to the transmitted signal.

Induction tool The received current is proportional to the conductivity of the formation in the loop. The constant of proportionality varies with the radius and position of the loop along the tool axis, as well as the position and construction of the coils. This constant of proportionality is the geometric factor.

Induction Measurement Principle E = K.g.C

Real and Quadrature signals From this model it can also be deduced that the received signal has undergone two 90-degree phase shifts with respect to the current in the transmitter. It is therefore back in-phase with this current. (In fact, it is 180 degrees out-of-phase, but this is treated as in-phase with a sign change).

Real and Quadrature signals This in-phase signal is usually called the "Real Signal." This phase relationship is convenient because there is also a signal in the receiver that is coupled directly from the transmitter. This signal is large, but unaffected by the formation conductivity. This signal is called the "direct-coupled" or "Mutual Signal." This signal is 90 degrees out-of-phase with the transmitter current, so it can be separated from the desired in phase signal by using phase-sensitive detection.

Real and Quadrature signals There is also a signal which is 90 degrees out-of-phase with the current, which is affected by the formation conductivity. This is called the "Quadrature Signal."

Signal Phase with respect to IT (transmitter current) X, XE VR ,RE VX¢ 0° 90° 180° 270°

THE GENERAL INDUCTION RESPONSE is the skin effect correction

SKIN EFFECT δ = (2/μώσ)½ = skin depth σ = formation conductivity L = spacing between transmitter and receiver K=tool constant ώ= Tool frequency

SKIN EFFECT SKIN EFFECT CORRECTION DESIRED RESPONSE ACTUAL CONDUCTIVITY(mmhos/m) APPARENT CONDUCTIVITY AS SEEN BY TOOL(mmhos/m) DESIRED RESPONSE SKIN EFFECT CORRECTION §

SKIN EFFECT SKIN EFFECT µ f 1/2 SKIN EFFECT µ s1/2 SKIN EFFECT µ L THE TERM SKIN EFFECT COMES FROM THE WELL- KNOWN PHENOMENON IN METALLIC CONDUCTORS IN WHICH A HIGH FREQUENCY ALTERNATING CURRENT TENDS TO FLOW IN THE OUTER MOST PART, THE SKIN, OF THE CONDUCTOR . SKIN EFFECT µ f 1/2 SKIN EFFECT µ s1/2 SKIN EFFECT µ L

Induction logs run in very large holes Induction logs run in salt muds DEEP INDUCTION AND RT In 90% of the cases, it is permissible to assume that the deep induction reading is equal to Rt. Conditions where this assumption is not valid include: Induction logs run in very large holes Induction logs run in salt muds Places where the bed of interest is thin Where the shoulder-bed resistivity is markedly different from the resistivity of the bed under consideration Where invasion is abnormally deep

DUAL INDUCTION TOOL VERTICAL RESOLUTION DEPTH OF INVESTIGATION DEEP --- 5 FEET MEDIUM---- 4.5 FEET SHORT GUARD --- 12 INCHES DEPTH OF INVESTIGATION DEEP ---- 5.4 FEET MEDIUM --- 2.5 FEET SHORT GUARD ---- 15 INCHES

HIGH RESOLUTION INDUCTION TOOL VERTICAL RESOLUTION DEEP --- 2 FEET MEDIUM---- 2 FEET DFL--- < 17 INCHES DEPTH OF INVESTIGATION DEEP ---- 7.58 FEET MEDIUM --- 3.25 FEET DFL----17 INCHES

THE FIRST LOGGING OPERATION PECHELBRON FRANCE 1927

THE FIRST LOG (1927) RESISTIVITY DEPTH

Conclusions drawn from the first logging job were Hard formation layers appeared on the diagrams as peaks Contrasting clearly with the soft and conductive marls (sands). When the log results were confirmed by actual physical core samples, electrical coring was firmly established as a valuable tool for geological surveys

Spontaneous potential Even with no current emitted in the borehole by their tool, a potential difference was measured across a pair of monitor electrodes on the sonde. After integrating this self potential [called the Spontaneous Potential (SP)] The SP is a natural occurring potential relative to a surface potential measured in the borehole mud. Uses Of The SP 1. Determine values of formation water resistivity 2. Define bed boundaries 3. Identify permeable zones 4. Qualitative indication of shale content 5. Well to well correlation

Electrode Tools (DLLT) Measure the formation resistivity by injecting current into the formation CR Current moves from source to current return Current is focused so travels through the formation to the current return Require conductive medium in borehole (saltwater mud) Can not be run in air-filled holes, and oil-based muds Provides different depths of investigation

Unfocused Log The simplest measurement of the resistivity of the formation can be done placing a current emitting electrode downhole and a return electrode at the surface, at surface we can measure the current applied and the voltage to calculate the resistivity. This configuration measures the sum of all the resistances between the surface electrode and the downhole electrode.

Focused Log In highly resistive formations or in very low-resistivity muds, the unfocused electric log does not work very well. The mud column tends to short-circuit the current of the electric log, causing the readings to be too low and lacking in definition.

Focused Log In order to measure the resistivity of the formation the measure current must be forced to flow in the formation. This is known as focusing. The tools using this technique are called Laterolog devices. The term laterolog came about because the current is forced to flow "laterally" away from the tool.

Focused Log

Focused Log

DEPTH OF INVESTIGATION DUAL LATEROLOG TOOL VERTICAL RESOLUTION DEEP --- 2 FEET MEDIUM --- 2 FEET MICROGUARD (MGRD) --- 1 INCH MSFL --- 3 INCHES DEPTH OF INVESTIGATION DEEP --- 5 - 7 FEET MEDIUM --- 2- 3 FEET MICROGUARD (MGRD) --- 3 - 6 IN MSFL --- 1- 4 INCHES

Introduction In a laterolog-type system, the measure current is forced to flow perpendicular to the borehole (in a `lateral' direction) by the potential lines created by the focusing currents emitted by the electrodes surrounding the central current measure electrode.

Introduction The thickness of this measure current beam (and the extent to which it maintains this beam shape) represents an equivalent resistor. The voltage drop across this resistor and the current through are the actual measurements from the tool.

Introduction Knowing the voltage (E) and current (I), the resistor can be easily calculated (R=E/I). Once the apparent resistor (Rapp) is known, apparent resistivity (rapp) can be determined since r=R .A/L where A represents the “area” and L represents the “length” of the “resistor.” The factor A/L is referred to as the tool “k factor.”

Log Presentation

Microlaterolog The microlaterolog tool is a focused device designed to measuring flushed zone resistivity. The standard MLL pad is a 9 button fluid filled pad. The center button is the measure electrode and the remaining buttons form the guard and are all connected together.

Microlaterolog MSFL/MLL GUARD ELECTRODES CENTER ELECTRODE MSFL/MLL PAD GUARD ELECTRODES CENTER ELECTRODE MSFL/MLL PAD

Shallow Micro resistivity log devices

Nuclear Tools

GAMMA RAY ORIGIN Gamma rays originate from the decay of radioactive elements. In most formations, these elements are isotopes of Potassium, Uranium, and Thorium.

PARENT (P) TO DAUGHTER (D) DECAY SEQUENCE NATURAL DECAY MODES EMITS ALPHA EMITS ELECTRON EMITS POSITRON CAPTURES ELECTRON PARENT DAUGHTER PARENT (P) TO DAUGHTER (D) DECAY SEQUENCE

ALPHA DECAY BETA - ELECTRON DECAY BETA - POSITRON DECAY BETA - ELECTRON CAPTURE

GAMMA EMMISION g = GAMMA RAY

POTASSIUM DECAY Eg = EU-EL

THALLIUM DECAY Tl

The commonly used multiples of this unit are kiloelectron volts, keV, and Megaelectron volts, MeV. These represent multiples of 1,000 and 1,000,000 electron volts, respectively. Therefore, a gamma ray with an energy of 1 MeV would have the same striking power as an electron accelerated through a 1,000,000 volt potential. ELECTRON VOLT

BASIC GR BLOCK DIAGRAM SCINTILLATIONCRYSTAL PM TUBE DISCRIMINATOR 12 BIT COUNTER BGO NaI CsI

SCINTILLATION DETECTORS GAMMA RAYS INTERACT WITH SCINTILLATION CRYSTAL (COMPTON, PHOTOELECTRIC, PAIR PRODUCTION) PRODUCING ELECTRONS. ELECTRONS EXCITE PHOSPHOR ATOMS, WHICH IN TURN, DECAY BY EMISSION OF LIGHT. THESE PHOTONS INTERACT WITH THE PHOTOCATHODE OF THE P.M. TUBE. PRODUCING ELECTRONS. EJECTED ELECTRONS ARE FOCUSSED INTO PHOTOMULTIPLIER STRING. ELECTRONS ARE ACCELERATED THROUGH SUCCESSIVE DYNODES, PRODUCING MULTIPLICATION AT ANODE (1 e- = 106 e-)

APPLICATIONS OF THE NATURAL GAMMA RAY TOOL CORRELATION BED BOUNDARIES VOLUME OF SHALE LITHOLOGY INDICATOR DEPTH CONTROL

API UNITS Shaly sand Shale Very shaly sand Clean limestone Dolomite GAMMA RAY RESPONSE FOR TYPICAL FORMATIONS 50 100 API UNITS Shaly sand Shale Very shaly sand Clean limestone Dolomite Clean sand Coal Anhydrite Salt Volcanic ash Gypsum

150 API GR shale GAMMA VOLUME OF SHALE PARAMETERS GR log GR clean

VOLUME OF SHALE FROM GAMMA RAY LOGS RELATIONSHIP EQUATION Linear Clavier Steiber

VOLUME OF SHALE CALCULATIONS SP Method Here,fN and fD are the neutron and density porosities in the shaly zone, fNsh and fDsh are the values in a nearby shale. GR Method ND Method

Introduction The Density measures: formation bulk density (rb) and photoelectric absorption index (Pe) of the lithologic column cut by the borehole.

Introduction The term rb is applied to the overall density of a unit volume of rock. In the case of porous rocks, it includes the density of the fluid in the pore spaces as well as the grain density of the rock.

Introduction Pe is dependent on Z, the atomic number, of the material being measured and is used to identify lithology.

Density Tool The Density utilizes a Cesium 137 (Cs137) gamma ray source two Sodium Iodide scintillation detectors and a small Cesium 137 gamma ray source beside the ls detector all of which are mounted on an articulated pad.

Density Tool To measure rb and Pe, a beam of gamma rays is directed into the rock. The detectors at fixed distances from the source measure changes in the intensity of the gamma ray flux resulting from the scattering and absorption effects of the formation. The higher the formation density, the lower the intensity of gamma radiation at the detectors.

Density Tool The tools primary applications are: Measure bulk density, Lithology determination using Pe, Correlation. The tools main limitations are: Primarily open hole application, Limited cased hole applications, Maximum hole diameter 22 inches, Minimum hole diameter 6 inches.

Eg >1.02 MeV e- e+ PAIR PRODUCTION ELECTRON POSITRON INCIDENT HIGH ENERGY PHOTON e- e+

e- COMPTON SCATTERING 10 MeV > Eg >100KeV SCATTERED PHOTON INCIDENT PHOTON SCATTERED PHOTON COMPTON RECOIL ELECTRON e- 10 MeV > Eg >100KeV

PHOTOELECTRIC EFFECT INCIDENT PHOTON ELECTRON e- Eg 100KeV

Density Tool Interactions due to Compton scattering are used in the measurement of bulk density, and those due to Photoelectric absorption used to determine Pe.

Density Tool The Density tool has two detectors which are set at different distances from the source, with the one nearest designated the short spaced detector (SS) and the other the long spaced detector (LS).

Density Tool The long spaced detector is configured to measure gamma rays associated with both: Compton scattering and Photoelectric absorption effects. Therefore the LS detector count rates are used in the determination of both bulk density and Pe.

Density Tool As with any other nuclear logging instrument, the depth of investigation of the Density is shallow. For this reason, mud cake and borehole rugosity can have an appreciable effect on the total measurement, despite the fact that the Density is a contact device.

Density Tool To compensate for these effects which occur when any material other than formation intervenes between the pad containing the detectors and source and the formation itself, a two–detector arrangement is used.

· MUD MUD CAKE FORMATION THE DENSITY TOOL

+ ¥ (662) Radioactive cesium as a logging source is used. Cesium emits beta particles(which do not escape the stainless steel source case) and gamma rays. These gamma rays have an initial energy of 662 KeV and, due to the fact they have zero charge, can penetrate deeply into the formation. The cesium source strength is 1.5 Curies and it has a half-life of 33 years.

Ne= Electron Density

COUNT RATE AT A DETECTOR This equation assumes the gamma rays can follow a straight line path from source to detector.

therefore we can write the equation as: taking the log of both sides, and considering a constant :

Density Tool Electron density is the number of electrons per unit volume. There is a direct relationship between electron and bulk density. For practical purposes a normalized “electronic” density is defined by the equation:

Density Tool

Density Tool The idealization that Z/A=0.5 for all elements of interest during logging operations was optimized by use of test pit calibrations.

Z/A FOR COMMON DOWNHOLE ELEMENTS

HYDROGEN CALIBRATION For water (H2O) For Limestone( CaCO3 ) Tool must be calibrated for rhoe in water=1.1101 Rhoe in limestone=2.7076 Linear conversion rhoeA=G+H rhoe

FOR FRESH WATER FILLED LIMESTONES Solving the two equations and two unknowns we obtain the accepted industry standard for calibrated density tools Since all density tools are calibrated to match the response of some standard tool calibrated to read the true bulk density values of pure, fresh water filled limestone formation of varying porosities (usually from 0 to 40%), the response of all density tools is given by equation above

Porosity Porosity is one of the most important reservoir parameters. This is the fractional volume of the rock representing the pore spaces between the rock grains which contain reservoir fluids. Its accurate determination enables a more precise calculation of reserves and production potential of a reservoir.

Porosity The bulk density measured by the instrument is the volumetric average of the densities of all the rock components:

Examples

NEUTRON TOOLS EPITHERMAL THERMAL PULSED NEUTRON TMD TMD-L PSGT RMT Spectral Flow

CLASSIFICATIONS OF NEUTRONS HIGH ENERGY FAST INTERMEDIATE EPITHERMAL THERMAL

NEUTRON SOURCE

KE BEFORE ¹ KE AFTER (RECOIL) + KE AFTER (SCATT) INELASTIC SCATTERING INELASTIC GAMMA RAYS KE BEFORE ¹ KE AFTER (RECOIL) + KE AFTER (SCATT)

ELASTIC SCATTERING q = SCATTERING ANGLE e = RECOIL ANGLE INCIDENT NEUTRON SCATTERED NEUTRON RECOIL NUCLEUS q = SCATTERING ANGLE e = RECOIL ANGLE KE BEFORE = KE AFTER (RECOIL) + KE AFTER (SCATT)

ELASTIC HEAD-ON COLLISION M = Mass of struck nucleus in AMU M = 1, for protons or neutrons (FE)max loss = Maximum fractional energy loss of neutron

MAXIMUM FRACTIONAL ENERGY LOSS

CROSS SECTION The term "Cross Section" can be thought of as a measure of the probability that a nuclear event will occur between a particle and a target. Cross section is usually expressed in terms of the effective area which a single target presents to the incoming particle

CROSS SECTIONS SLOWING DOWN CROSS SECTION CAPTURE CROSS SECTION

HYDROGEN INDEX It is more correct to refer to the Neutron derived porosity as a Hydrogen Index, since the porosity is indirectly measured through the evaluation of hydrogen content.

HYDROGEN EFFECT ON NEUTRON TOOLS In a formation with large amount of hydrogen, source neutrons will be slowed more quickly than in formation with little hydrogen. HIGH HYDROGEN CONTENT = LOW COUNTS

CHARACTERISTIC LENGTHS * Total Migration Length (M): (crudely speaking) the square of the three characteristic lengths squared: M 2 = (Ls )2 + (Le )2 + (Lt )2 This length can be considered (crudely again) the total average distance the neutron travels from source to capture. * Thermalizing Length (Le) : (crudely speaking) the average distance travelled by a neutron in going from an energy value of 1.46 eV to 0.025 eV. * Slowing Down Length (Ls ) : (crudely speaking) the average distance traveled by a fast neutron in going from the source energy at 4.6MeV to the energy level of 1.46eV. Ls is mainly a function of hydrogen concentration. * Thermal Diffusion Length (Lt ) : (crudely speaking) the average distance the neutron travels from the point it first reaches the thermal level at 0.025 eV to its capture still at the thermal level (0.025 eV). Lt is a function, not only of the hydrogen content, but of the concentration of elements with high capture cross section, (e.g. chlorine).

DEPTH OF INVESTIGATION 8 TO 12 INCHES VERTICAL RESOLUTION 24 INCHES DEPTH OF INVESTIGATION 8 TO 12 INCHES

FORMATION ENVIRONMENTAL EFFECTS ON THE NEUTRON POROSITY LITHOLOGY EFFECT SHALE EFFECT GAS EFFECT

LITHOLOGY EFFECT Different formation types have different abilities to slow down and capture neutrons. This makes our neutron porosity lithology dependent.

NEUTRON SLOWING DOWN LENGTHS §

SHALE EFFECT The neutron tool, responding to the total amount of hydrogen in the form of water, will consider the volume occupied by both bound and free water as porosity in shales. Since the bound water is structurally and chemically bonded to the shale, the neutron porosity reads too high. Shales can also have unusually large thermal neutron capture cross sections ( e.g. Boron). This makes thermal neutron tools read even higher for the porosity.

NEUTRON SHALE CORRECTION Here fneutron-shale is the neutron porosity in a nearby shale

GAS EFFECT Because of the low amount of hydrogen in gas, the neutron tool sees gas as fresh water occupying a smaller volume than the actual volume present. The porosity therefore reads too low. Also the excavation effect - which states simply that gas in the rock will cause less rock material to be present to slow down, scatter, or absorb the neutrons - will cause the porosity to appear even lower.

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