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What are logs used for?
correlate zones, assist with structure and isopach mapping, and help define physical rock characteristics such as lithology, porosity, pore geometry, permeability, identify productive zones, determine depth and thickness of zones, distinguish between oil, gas and water in a reservoir, estimate HC reserves, determine facies relationships and drilling locations
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Two types of logs?
Open hole logs and cased hole logs
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open hole logs
run in newly drilled, uncased wells, used to determine whether or not to run casing
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cased hole logs
used to perforate the well
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two primary parameters determined from well log analysis are:
porosity and fraction of pore space filled with HC (HC saturation)
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Three general types of logs
Electrical, nuclear, acoustic or sonic log
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word log may refer to:
a particular curve, a group of curves, the physical record of the measurements, a logging tool (sonde), or the process of logging
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Rock properties or characteristics that affect logging measurements are:
porosity, lithology, mineralogy, permeability, water saturation, resistivity of the rock
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porosity
ratio of voids to the total volume of the rock
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effective porosity
the amount of void space that is interconnected.
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lithology
used to describe the solid portion of the rock
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mineralogy
describes the minerals in a particular rock
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lithology effect
shale in sandstone causes log readings to change
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some measurements, primarily nuclear are...
made as a result of molecular level interactions between the formation and the logging tool
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acoustic measurements interact...
with the formation on a bulk level so it is more affected by lithology
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parameter that has been used to account for the lithology effects
shale volume
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permeability
ability of a rock to transmit fluids, controlled by the size of the connecting passages between pores
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absolute permeability
ability of a rock to transmit a single fluid, when completely saturated with that fluid
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effective permeability
the ability of the rock to transmit one fluid in the presence of another fluid when the two fluids are immiscible
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Water saturation
amount of pore volume in a rock that is occupied by formation water
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Irreducible water saturation
the water saturation at which all the water is absorbed on the grains in a rock or is held in the capillaries by capillary pressure
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Hydrocarbons, the rock, and the fresh water of the formation are all assumed to have a _____ resistivity
high
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_______ is the rock property on which the entire science of logging first developed
Resistivity
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______ is the inherent property of all materials, regardless of their shape and size, to resist the flow of an electric current
Resistivity
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Salt water has a _____ resistivity
Low
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The measurement of resistivity is a measure of the...
amount of salinity of the formation water
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Resistivity is a basic measurement of a reservoir's...
fluid saturation, and is a function of porosity, type of fluid, and type of rock.
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Laterologs
resistivity logs run in salty mud
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Induction Logs
resisitivity logs run in fresh mud
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First electric log was run in...
France by the Schlumberger brothers in 1927
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Archie's Equations:
Ro=F*Rw, F=a/Phi^m, where a=tortuousity factor, and m=cementation factor
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What is Rxo, Rmf, Sxo, Rt, Rw, and Sw?
Rxo=resistivity of flushed zone, Rmf= resistivity of the mud filtrate, Sxo=water saturation of the flushed zone, Rt=resistivity of the uninvaded zone, Rw=resistivity of formation water, Sw=water saturation of uninvaded zone
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Invaded zone
zone in which much of the original formation fluid is replaced by mud filtrate
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Ri
resistivity of the transition zone
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The bulk resistivities are...
Rxo, Ri, Rt
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Diameter of invasion
depth of mud filtrate invasion into the invaded zone
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The amount of invasion that takes place is dependent on...
the permeability of the mud cake, not the porosity of the formation.
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The diameter of invasion is greatest in _____ porosity rocks
low
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dj/dh for high, intermediate, and low porosity rocks.
high=2, intermediate=5, low=10
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Ri, Rz, Si
Ri=resistivity of transition zone, Rz=resistivity of the fluid in the transition zone, Si=water saturation of the transition zone
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If invasion is deep or moderate, the flushed zone is...
most likely completely cleared of its formation water by mud filtrate.
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When oil is present in the flushed zone, the degree of flushing by mud filtrate can be determined from the...
difference between water saturation in the flushed zone(Sxo) and the water saturation in the uninvaded zone(Sw)
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Usually about ___ to ___ percent of the oil is flushed out
70, 95
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The ratio of the uninvaded zone's water saturation to the flushed zone's water saturation is an indicator of
hydrocarbon moveability (MHI)
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There are 3 commonly recognized invasion profiles:
Step, transition, annulus
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The transition profile has a _____ geometry, with two parameters:
cylindrical, di (diameter of flushed zone) and dj (diameter of transition zone)
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The transition profile needs at least ____ resistivity measurements, each a different diameter away from the wellbore
3
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The 3 resisitivity measurements of the transition zone are:
Rxo, Ri, Rt
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The ability to estimate the invasion in a formation was
made possible with the introduction of the...
dual induction log and dual laterolog
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“dual” means that each tool made
tow induction or two laterolog measurements, one at medium and one at deep distance from the wellbore
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The dual induction and laterolog made a third measurement which was...
shallower than the medium and deep measurements
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Distances for Rxo, Ri, and Rt
Rxo=.5-1.5ft, Ri=1.5-3ft, Rt=3ft+
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Annulus profiles...
only present in HC bearing zones, indicate high relative permeability to HC, will dissipate with time, and can only be observed on fresh mud resistivity logs
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An annulus profile is detected only by an _____ run soon after a well is drilled
induction log
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Annulus profile can only occur....
in HC bearing zones
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Annulus effect
is detected by a higher resistivity reading on the deep induction log than the one on the medium induction log, Rt>Ri
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How many resistivity measurements does a step profile and transition profile have?
Step-2, transition-3
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3 facts about an annulus profile-
Rt>Ri, only in induction logs, and indicates HC productivity
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Fresh water based mud has resistivity profile of:
Rmf>>Rw
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Salt water based mud has resistivity profile for:
Rmf=Rw
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A saltwater mud results in a wet profile where
shallow, medium, and deep resistivity measurements all read low resistivity
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2 reasons why it is important to know lithology
- -porosity logs require a lithology constant before the porosity of the zone can be calculated
- -the formation factor also varies with lithology
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3 parameters that vary with temperature are
resistivities of the drilling mud, mud filtrate, and formation water
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The formation temperature is determined by knowing:
formation depth, bottom hole temperature, total depth of a well, and surface temperature
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formation temperature is calculated by using
linear regression y=mx+c where x=depth, y=temperature, m=geothermal gradient, c= mean annual surface temperature
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Arp's Formula can be used to
correct fluid resistivities to formation temperature
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four most fundamental rock properties use in well logging are
porosity, permeability, lithology, and water saturation
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Lithology of a formation must be known because
it is needed to determine porosity, the formation factor varies with lithology, and water saturation varies with the formation factor
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The four fluids that affect logging measurements are
drilling mud, mud filtrate, formation water, and hydrocarbons
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Resistivity profiles for Wet Zones:
For fresh mud where (Rmf>>Rw)
For salt mud where (Rmf=Rw)
-Rxo>Ri>Rt
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Resistivity profiles for HC zones:
For fresh mud where (Rmf>>Rw)
For salt mud where (Rmf=Rw)
- Rxo>Ri<Rt, annulus profile: Rt>Ri, all read high
- Rxo<Ri<Rt
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The SP log is used for
determining gross lithology, to correlate zones
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SP log is used to:
detect permeable beds, detect boundaries of permeable beds, determine formation water resistivity(Rw), determine the volume of shale in permeable beds
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