Geophysical Petroleum Techniques Flashcards

1
Q

Name different types of logging tools geophysical and direct, how the resolution is effected and why we use them?

A

Gamma Ray, resistivity, Neutron porosity etc. - measuring the response of the formation by physical processes

Direct measurement that allow collaboration of log interpretation - mudlogs, core etc.

Resolution depends on the depth of investigation. Vertical resolution - also dependant upon sample rate and logging speed

Use - to stratigraphically correlate the unit, to create reservour models (logs can estimate; porosity, permeability, net to gross ratio (shale content))

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2
Q

What quantities are helpful to understand reservoirs andwhy?

A

Porosity - empty space
Water saturation
Irreducible water saturation - fraction of water saturation that can be removed without using pressure

To understand the quantity,recoverable amount and the amount that contains hydrocarbons

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3
Q

What is well log interpretation?

A

A record of the response of the formation to a tool lowered into the borehole and pulled slowly back up through through the formation to the surface - data transmitted to the surface electronically
WIRELINE - sonde about 4 inches so can go down a small ball hole. To go downwards it relies on gravity and the weight of itself

MWD - measure while drilling - positioning information
LWD - logging while drilling - formation properties
Pulses in pressure are converted to electrical currents by piezoelectric crystal (filtering noise) to then be sent to the surface

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4
Q

What is the gamma ray tool, how does it work and how does it benefit exploration of petroleum systems?

A

Source of radioactive decay - K (mica), Th (Shales), U (Shales)
Measured using scintillation counter - gammas rays interact with a sodium iodide crystal to produce flashes of light which is callibrated to API Standard.

Indicators - V shale determined by establishing sand and shale lines with typical GR values OR using a sample GR cut off value to distinguish potential reservoir from non-reservoir (diagram)

N/G ratio - understanding the amount of potential reservoir

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5
Q

What is the calliper tool?

A

Measures variation in a holes diameter from changes in the position of a spring loaded arm attached to a variable resister (electrical current). These variations in electrical current show variations in diameter by; potential for cave ins (poorly consolidates) and/or slouching (squeezing shakes) and mud cakes (overlying SSTs)
CALLIPER = QUALITY CONTROL

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6
Q

What are sonic logs, explain what they do and how the data interpreted?

A

They measure the time taken for an acoustic to travel through a given thickness
Waves - P (compressional), S (shear waves), Rayleigh (motion on the surface)
Refraction - the wave bends when it encounters an interface. If the waves speed is slower than the mud, then the critical refraction can not take place
ACCURACY - borehole compensated - 2 sources and 4 receivers = averaged readings
HEADER - for given lithology higher porosity (more fluid) means a lower velocity and therefore a higher interval travel time
CYCLE SKIPPING - occurs when the return signal does not trigger the receiver meaning the next will activate it
TYPICAL RESONANCE - poor lithology discriminator as ITT (interval travel times) are not distinguishable. Porosity tool.
CALCULATING POROSITY - Wyllie time average equation - velocity is an average of the time spent in the pore space and time spent in the matrix (DOES NOT see fracture porosity)
POROSITY DETERMINATION - ITT values against matrix velocity

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7
Q

What is the bulk density tool and how is it used to understand porosity?

A

Tool emits gamma radiation (CO60)
Compton scattering - interaction with electrons causes attenuation (loosing energy)
Density of electrons closely related to real density of the formation - low gamma ray count (as gamma rays lost energy) = high density
TYPICAL RESPONSE - poor lithology discriminator - density related to porosity more than lithology.
POROSITY CALCULATION - bulk density is a function of the matrix (higher density - lower porosity - lower gamma count). Matrix density will vary in impure formation, single matrix density taken therefore giving misleading results

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8
Q

What are resistivity logs?

A

Measures resistance to flow of electrical current. A function of; porosity, presence of clay. Expressed by; R (original resisitivity) = F (formation factor) * Rw (resistivity of the formation water)

BASIC PRINCIPLE - current passed between two electrodes, potential drop measured between other electrodes
Normal logs - one current electrode on the tool and the other grounded
Focussed tool - aims the current flow

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