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Evaluation of low resistivity zones
1. EVALUATION OF LOW RESISTIVITY ZONES
USING NUCLEAR MAGNETIC RESONANCE
LOG
PRESENTED BY
SHAHNAWAZ MUSTAFA
M.Tech. PETROLEUM GEOLOGY
2nd Sem. Roll No. 02
24-04-08
Dibrugarh University
Assam – INDIA.
2. Resistivity Log
The Resistivity Log:
Resistivity logs measure
the ability of rocks to
conduct electrical current and are
scaled in units of ohm-
Meters.
The Usage:
Determine Hydrocarbon versus
Water-bearing zones,
Indicate Permeable zones,
Determine Resistivity Porosity.
LOW RESISTIVITY
When the resistivity is<1.0
ohm meter
3. Why NMR?
The combination of conventional logs such as density,
neutron and resistivity logs is proven to be very effective in
the evaluation of normal reservoirs.
For low resistivity reservoirs, however, an accurate
determination of the Petrophysical parameters with the
conventional logs is very difficult.
In case of low resistivity contrast reservoirs it is difficult to
determine oil-water contact & irreducible water saturation
with resistivity logs.
NMR log has been only available as a supplement tool to
provide additional information on the producibility of the
reservoirs, to distinguish between bound and free water
and helps accurately to determine the reservoir rock
Petrophysical properties.
The main limitations of NMR logging, however, as been the
cost and long time of acquiring data.
4. NUCLEAR MAGNETIC RESONANCE LOG
Some nuclei actually behave like magnets rotating
around themselves particularly protons i.e.
hydrogen nuclei which enter into the composition
of water as well as hydrocarbons
Under the effect of a magnetic field the nuclei
precess like gyroscopes in a field of gravity. The
frequency of this precession in a given magnetic
field characterizes the nuclei.
In the logging sondes, the magnetic field used is
the earth’s magnetic field.
NMR log does not make use of any radioactive
property of the nucleus but of its gyromagnetic
property.
5. PHYSICS OF NMR LOGGING
NMR measurement can be made on any nucleus that has odd
number of protons e.g. 1H, 23Na.
NMR log is basically a hydrogen Index measurement, which
responds to movable protons only.
Spinning proton represents a current loop that generates a
magnetic field or magnetic moment aligned with the spin axis.
When many hydrogen atoms are present and no external field
exists, the proton nuclear spins are randomly aligned and the
net magnetic field is zero.
In the presence of an external magnetic field B0, spinning
proton experiences torque and starts precessional motion
around the field B0 . The precessional frequency called
larmour frequency is proportional to the field strength.
6. The permanent magnet magnetizes the formation materials and
essentially the hydrogen nuclei present in hydrocarbons and water present
in pore spaces and bound to clay minerals.
An antenna surrounding the magnet transmits in to the formation precisely
timed radio frequency pulse sequence.
Between these pulses the antenna is used to listen for decaying echos from
those hydrogen atoms which are in resonance with the field from
permanent magnet.
As the proton resonance frequency depends upon the applied field, the
frequency of the transmitted and received energy can be tuned to
investigate cylindrical regions of formation at different diameters around an
MRIL tool just like an image of a narrow slice of any organ of a patient in
Hospital MRI.
8. APPLICATIONS OF NMR LOGGING
S. FEATURE BENEFIT VALUE
NO
1. CMR porosity Lithology Decrease in
independent coring cost
Salinity independent Determines Phie
Shale independent
2. No radioactive Safety Reduced risk
source liability
3. Free fluid Irreducible water Improved
porosity saturation analysis of
Prospective
zones
4. Pore size Permeability Decrease testing
distribution estimation cost
5. High resolution Thin bed analysis Adding to
producible
reserves
6. Combinable Save rig time Increase rig
efficiency
7. Specific tool No borehole Decrease logging
design correction cost
8. T2 distribution Facies analysis Support to
Viscosity estimation formation
Capillary pressure evaluation
Cementation Expo.
9. PARAMETERS OF NMR
For the analysis of NMR data, several aspects of NMR
technique have been used;
1) T1/T2 ratio, for fluid identification,
2)The difference between NMR derived porosity and total
porosity, to determine the types of clay minerals,
3) NMR relaxation properties, to identify fluids nature and
rock properties of low contrast / low resistivity reservoirs.
Longitudinal Relaxation Time( T1) also called spin-lattice
relaxation time is a time constant characterizing the
alignment of spins with the external static magnetic field.
Transverse Relaxation Time (T2) also called spin-spin,
relaxation time is a time constant which characterizes the
loss of phase coherence that occurs among spins oriented at
an angle to the main magnetic field.
10. Low Resistivity & NMR
In case of low contrast resistivity reservoirs where there was little
resistivity contrast between water bearing formation and oil
bearing formation, NMR has been able to identify the fluid
nature of the two formations and then the height of the oil
column.
If a pay zone exhibits low resistivity, conventional resistivity logs
become incapable of identifying the producing zones and also of
indicating water mobility. Because of this limitation, many
potentially productive zones with high irreducible water
saturation are overlooked.
In low resistivity beds, there is little resistivity contrast between
water-bearing reservoirs and oil-bearing reservoirs. The water-
bearing reservoirs contain relatively fresh water, and thus show
relatively high resistivity readings. The resistivity in the oil-
bearing reservoirs is variable because the reservoirs contain
fresh or salty water. Often, the oil-bearing reservoirs show a high
level of irreducible water saturation that depresses further the
resistivity reading, thus making the pay identification from the
resistivity log extremely difficult.
11. Control of water production and identification of low resistivity pay
zones with high irreducible water saturation of two formation
evaluation problems are existing in many fields in the Middle East
and other fields around the world. The problem with these zones is
that the resistivity data interpretation indicates high water saturation,
but oil or even dry oil will be produced. There are two reasons:
The first group is concerned with reservoirs where the actual water
saturation can be high, but water - free hydrocarbons are produced.
The mechanism responsible for such high water saturation is usually
described as being caused by microporosity.
The second group is concerned with reservoirs where the calculated
water saturation is higher than the true water saturation. The
mechanism responsible for this high water saturation is described as
being caused by the presence of conductive minerals such as clay
minerals, metal sulfides, graphite and pyrite in a clean reservoir rock.
NMR log can identify water free production zones, correlate bound
fluid volume with clay minerals inclusions in the reservoir, and
identify hydrocarbon type.
12. NMR POROSITY
The NMR porosity depends only on the fluids content of
the formation, unlike density/neutron porosity which is
influenced by both fluids and surrounding rocks.
The strength of the NMR signal is proportional to the
number of hydrogen atoms in NMR tool dependent rock
volume. In zones containing light hydrocarbon, where the
hydrogen index is less than unity, NMR porosity will
typically underestimate true porosity in proportion to the
hydrogen index.
The number of hydrogen atoms in gas depends strongly
on temperature and pressure. Hence it is important to
estimate accurately pressure and temperature to account
for their effect on NMR results in natural gas reservoirs
13. The standard rock porosity model for all pore fluids
Conductive Fluids
clay-
clay- capillary
dry mobile
matrix bound bound hydrocarbon
clay water
water water
MCBW MBVI MFFI
MPHI ≅ φeff
MSIG ≅ φtotal
MSIG total porosity, MPHI effective porosity, MFFI free fluid
index, MBVI bulk volume irreducible water and MCBW
clay bound water.
14. NMR FLUIDS AND FLUID TYPES
We are enable to separate signals from gas, oil and water with
NMR log data unambiguously and, in some cases, can even
quantify.
The T1 contrast separates the water and light hydrocarbon (oil
and gas).
Freedman et al has introduced a new method called Density-
Magnetic Resonance (DMR) for evaluating gas-bearing
reservoirs.
Laboratory NMR data show that both T1 and T2 vary over
several orders of magnitude depending on fluid type.
15. NMR properties for water, oil and gas under
typical reservoir conditions.
16. In cases of low resistivity reservoirs with water saturation
greater than 50% and being still able to produce water
free hydrocarbon.
Zemanek has proposed certain technique to solve this
problem. This technique is based on the comparison
between irreducible water saturation (Swi) derived from
laboratory NMR surface area to Swi and water saturation
(Sw) deduced from conventional log analysis.
If Sw is less than or equal to Swi, free water hydrocarbon
will be produced and if Sw greater than Swi, water will
be produced.
17.
18. Producibility Analysis
CMR example:
Utilized to determine the
irreducible water saturation (Swirr).
This log comes from a complex
dolomite formation with varying
effective porosity and
permeability. Increasing T2
distributions to the right
correspond to an increasing pore
size distribution.
The solid purple line on the T2
distribution track corresponds to
the cutoff used to calculate the free
fluid index (FFI). The percentage
of signal falling to the left of the
100-msec cutoff is the capillary or
irreducible fluid volume in
porosity units, and the percentage
of signal falling to the right of the
cutoff is the free fluid volume.
21. CONCLUSION
NMR technology proves to be very essential in
formation evaluation and more specifically in low
resistivity reservoirs. The capability of NMR to
differentiate between movable and immovable fluids
has helped the log analysts to get more accurate
estimate of the reserves through the identification of
low resistivity reservoirs that have already been
bypassed by the resistivity logging interpretation.
