Geophysical Petroleum Techniques Flashcards
Name different types of logging tools geophysical and direct, how the resolution is effected and why we use them?
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))
What quantities are helpful to understand reservoirs andwhy?
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
What is well log interpretation?
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
What is the gamma ray tool, how does it work and how does it benefit exploration of petroleum systems?
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
What is the calliper tool?
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
What are sonic logs, explain what they do and how the data interpreted?
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
What is the bulk density tool and how is it used to understand porosity?
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
What are resistivity logs?
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
