Directional Drilling

jraduy

Gamma Ray Applications

Tue, 10 Jan 2012 14:52:26 | Directional Drilling

Gamma ray applications include:

• Determining shale volume - By using the relative response of the gamma curve compared with a 100% shale reading, the shale volume (Vsh) in the formation can be estimated from the following equation:

To calculate the shale volume (Vsh) of any zone, determine the denominator of the equation by subtracting the clean sand reading (GRclean) from the shale baseline reading (GRsh). Then, determine the numerator of the equation by subtracting the reading in the zone of interest (GRzone) from the shale baseline (GRsh). Calculate the shale volume (Vsh) by dividing the numerator by the denominator.

Well offset correlation - The gamma ray sensor provides excellent correlation in field appraisal and development drilling, particularly if initially used on exploratory and delineation wells for development well correlation. In many fields, the gamma ray sensor alone is suitable for real-time casing and core point selection.
Safety - The gamma ray sensor will accurately chart bed stratification. In combination with the resistivity sensor, the gamma ray sensor enables pore pressure prediction, leading to faster, safer exploratory drilling and operations in difficult fields.
Log of record - In combination with the resistivity sensor, the gamma ray sensor will provide intermediate logs of definitive quality for archive uses while providing information to improve drilling operational efficiency.
Enhanced interpretation of wireline logs - Higher data sampling rates (recorded logs) give greater definition and more exact bed delineation, which aids in identifying the smoothing/averaging effects of high wireline traverse speeds. Because at typical drilling rates the gamma ray sensor passes formations slower than typical wireline gamma ray sensors, the resulting logs have higher definition and less statistical uncertainty.
Directional control - The gamma ray sensor allows for improved trajectory monitoring. By removing one bank of detectors and replaying them with shielding, azimuthal readings may be taken. In its azimuthal configuration, the gamma ray may be used not only to differentiate between shale and reservoir rock, but to also determine whether the wellbore has exited out of the top or bottom of the reservoir.

Resistivity Sensor Theory

Physical Principles

Electromagnetic wave resistivity sensors respond to the way RF waves propagate (move) through the formation
The propagation of an RF wave is controlled by the following physical properties of the material through which the wave is moving:
Electrical Conductivity, which is the ability of a material to conduct an electrical current
Dielectric Permittivity, which is the ability of a material to store electrical charge
Magnetic Permeability, which is the ability of a material to become magnetized
At transmission frequencies below 10 MHz, the formation conductivity is the dominant factor
If reasonable assumptions are made for the dielectric permittivity and magnetic permeability, measured wave parameters can be related to the formation resistivity

Resistivity Sensor Theory

What does the Electromagnetic Resistivity sensor measure?

Phase Shift - the time difference of arrival of the RF wave between the two receivers
Attenuation - the difference in intensity of the RF wave signal at each of the receivers
Both the phase shift and attenuation data can be used to compute a formation resistivity value

Resistivity Conductivity Phase Shift Attenuation

High Low Small Low

Low High Large High

Electromagnetic waves can propagate through any medium, however, low resistivity (high conductivity) mediums cause the most signal reduction
Electromagnetic sensors can be used in any type of drilling fluid (they actually perform better in high resistivity mud)
Salinity of the drilling mud and the formation water, along with the formation temperature, have the greatest effect on the measured resistivity

Resistivity Sensor Theory

Why two measurements?

The physics behind the measurements dictate that the attenuation has a deeper depth of investigation than the phase
However, the dynamic range of the phase is much better than the attenuation
Typically the phase data is used quantitatively whereas the attenuation data is used qualitatively

Resistivity Sensor Theory

1 10 100 Resistivity (ohm-m)

The dynamic range of the phase measurement extends out to 1000 ohm-m

Resistivity (ohm-m)

100 1000

The dynamic range of the attenuation measurement is typically less than 100 ohm-m

Resistivity Sensor Theory

Breaking down the formation components

Hydrocarbons, rock matrix, and dry clay are infinitely resistive
Since formation water is the only conductive component in the formation, the amount of water present in the formation volume, its salinity, and the formation temperature drives the resistivity response

Resistivity Sensor Theory • Why Multiple Transmission Frequencies?

The choice of transmission frequency is dictated by two physical phenomena:
The measured phase shift and attenuation values are more dependent on the formations dielectric permittivity than its resistivity at frequencies greater than 10 MHz
At frequencies below 100 KHz electrical eddy currents are induced in the steel drill collar, essentially "short circuiting" the measurement between the transmitters and receivers
Lower frequencies allow for creating higher amplitude signals, which allows for development of sensors with longer transmitter to receiver spacing
The more frequencies, the more measurements that can be made

(50% Pseudo Geometric Factor, 1 ohmm)

1-1-

Computalog (MFR-46) 1

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Industry Average ^

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0 50 100 150 200

Diameter of InvMilgatlQn In Inches

Computalog's Multi-Frequency Resistivity (MFR) sensor has a deeper depth of investigation from its 46" spacing than the industry average deepest reading spacing

Resistivity Sensor Theory

Why Longer and Multiple Transmitter to Receiver Spacings?

The depth of investigation of the sensor increases with increasing transmitter to receiver spacing
Having multiple spacings allows the sensor to "see" at different distances into the formation
Typical sensor design allows for determination of the flushed zone, invaded zone, and virgin zone resistivities
The virgin zone resistivity (true formation resistivity) is the most challenging to obtain because the measurement is affected by all the zones in between the sensor and the formation

Resistivity Sensor Theory

• Shallow, Medium, and Deep spacings will provide 2 MHz and 400 KHz transmission frequencies and yield phase and attenuation data (12 curves total)

Shallow -16"/ 24" Medium -26"/ 34"

Deep -42"/ 50"
MegaDeep spacing will utilize 100 KHz transmission frequency (2 curves only)
MegaDeep - 76"/ 84"
Highly accurate, even at high resistivities

: The MFR accuracy is better than the Anadrill and Sperry-Sun resistivity sensors

Resistivity Sensor Applications

Qualitative Hydrocarbon Zone Indentification
Determine R in Invaded Zones
Quantitative Petrophysical Evaluation (fluid saturations, formation porosity)
Identify Movable Fluids (permeability indicator)
Determine Casing and Coring points
Predict Abnormal Formation Pore Pressure
Geosteering
Indicate Bed Boundaries in Horizontal Wells (Polarization Horns)
Indicate the presence of anisotropic formations

Resistivity Sensor Applications

Qualitative Hydrocarbon Zone Identification

In general, when the resistivity response is higher than the shale baseline it is an indication of the presence of hydrocarbons
In general, when the resistivity response is lower than the shale baseline it is an indication of the presence of salt water

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Resistivity Sensor Applications

MWD data is less affected by mud invasion than wireline data • Typical MWD exposure time is less than one hour, whereas wireline exposure time is generally from one to seven days

Resistivity Sensor Applications

Quantitative Petrophysical Evaluation to calculate formation porosity, water saturation, and in-situ reserves

Archie's equations provide a quick-look estimation Other calculation methods are much more rigorous and take into account many more parameters

Resistivity Sensor Applications

Time-Lapse Logging aids in identifying movable fluids

Re-logging a potential pay zone and comparing the resistivity values from each pass can qualitatively indicated formation permeability • Multiple spacing resistivity sensors can provide similar information in a single pass

Resistivity Sensor Applications
Casing Point Selection	\ .	......
Casing Point Selection
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Resistivity Sensor Applications

Coring Point Selection
Resistivity data can be used to determine coring intervals on subsequent wells drilled after the pilot hole
Coring is very time consuming and expensive, therefore we would only want to core the hydrocarbon zone and not the salt water zone

Resistivity Sensor Applications

Pore pressure estimation in under-compacted shales

By monitoring shale resistivity values, the presence of an overpressure transition zone can be seen
Drilling into formation pressure that is higher than borehole pressure can cause a "kick" and if uncontrolled can result in a "blowout"

Resistivity Sensor Applications
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• Deeper depth of investigation is very useful for many real-time
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	The deeper the depth of investigation the better chance of
	staying in the zone of interest

Resistivity Sensor Applications

When logging at high relative dip angles, the resistivity sensor may yield some unusual responses

Resistivity Sensor Applications

Polarization Horns

This phenomenon is caused by a discontinuity in the propagating electrical field as the tool crosses the boundary between beds having different resistivities
The size of the horn depends on the contrast between the resistivities of the adjacent beds and the relative dip angle between the borehole and the formation; the higher the relative dip angle and greater the resistivity contrast, the larger the horn
The magnitude of the horn diminishes with decreasing transmitter to receiver spacing

Anisotropic Formation Indicator
Anisotropic means that the resistivity measured parallel to the bedding planes is different from the resistivity measured perpendicular to the bedding plane
Shale, with its flat clay structure, is a classic example of an electrically anisotropic rock
The effect increases as the relative dip angle increases and is more pronounced on the shallow reading spacings

Environmental Effects on the Measurement

Borehole Enlargement (washout)
Drilling Fluid (depth of investigation)
Formation Shale Content ("bound" water)
Formation Resistivity (depth of investigation)
Bed Thickness (vertical resolution)
quot;Shoulder Bed" Resistivity (masks Rt)
Anisotropic Effects (curve separation in shale)
Formation Temperature (fluid resistivity)
Metallic Shielding (signal attenuation in casing)
Formation Dielectric Effects (assumed value)

Resistivity Sensor Data Interpretation

General Resistivity Response

Shale response is typically low due to the high amount of associated water with clays
The hydrocarbon response (gas and oil) is generally the same, but very different from the salt water
Salt has no fluid associated with it therefore its' response is infinite (off scale)

Formation Exposure Time

Data logged in a high salinity water sand with fresh mud
No appreciable invasion seen on the MWD data (1 hour)
Significant invasion seen on the MAD data (7.5 days, 23")
Wireline data shows even more invasion effect (12 days, 63")
Shallowest MWD spacing equivalent to wireline shallow guard measurement
Notice the superior vertical resolution of the MWD data versus the wireline

Resistivity Sensor Data Interpretation • Step Invasion Profile

Rxo>Rt)
High salinity water sands logged with fresh mud will show an invasion profile with the deepest reading spacing showing the lowest resistivity and the shallowest spacing showing the highest resistivity

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Resistivity Sensor Data Interpretation

Phase vs. Attenuation

Depth of Investigation phase Attenuation

Attenuation data has deeper depth of investigation than the phase data
After 1.4 hours of exposure time the phase shows more effect from invasion than the attenuation data (more separation between the curves)
Phase data has better vertical resolution than the attenuation data

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Zone #1 Evidence

Resistivity decreases, indicating either a decrease in the resistivity of the pore fluids or an increase in porosity
Bulk density decreases and the neutron porosity increases, both indicating an increase in porosity
Gamma ray moves from the shale baseline towards a less shaley formation
Qualitative interpretation of this zone based on these curves is a porous, wet zone

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Resistivity Sensor Data Interpretation

Zone #2 Evidence

Resistivity increases, indicating either an increase in the resistivity of the pore fluids or a decrease in porosity
Bulk density increases and the neutron porosity decreases, both indicating a decrease in porosity
Gamma ray moves from the shale baseline towards a less shaley formation
Qualitative interpretation of this zone based on these curves is a low porosity (tight) zone

Gamma

Resistivity

Resistivity Sensor Data Interpretation

Zone #3 Evidence

Resistivity increases, indicating either an increase in the resistivity of the pore fluids or a decrease in porosity
Bulk density decreases indicating an increase in porosity; however, the neutron porosity is showing a decrease in porosity (the two curves crossover each other)
Gamma ray moves from the shale baseline towards a less shaley formation
Qualitative interpretation of this zone based on these curves is a porous, gas zone

Gamma

Resistivity

Gamma

Resistivity

Geosteering Objective:

Keep wellbore in oil zone (avoid shale, gas, and water)

Sensors Required for Geosteering:

Gamma Ray - to differentiate between shale and sandstone

Resistivity - to differentiate between oil and water zones

Neutron porosity and Formation Density - to differentiate between oil and gas zones

Resistivity - to differentiate between oil and water zones

Neutron porosity and Formation Density - to differentiate between oil and gas zones

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