2evaluation of Low Resistivity Zones Con Nmr Log

EVALUATION OF LOW RESISTIVITY ZONES

USING NUCLEAR MAGNETIC RESONANCE

LOG

PRESENTED BY

SHAHNAWAZ MUSTAFASHAHNAWAZ MUSTAFA

M.Tech. PETROLEUM GEOLOGY

2nd Sem. Roll No. 02

24-04-08

Dibrugarh University

Assam – INDIA.

The Resistivity Log:

Resistivity logs measure

the ability of rocks to

conduct electrical current and are

scaled in units of ohm-

Meters.

The Usage:

Resistivity LogResistivity Log

The Usage:

Determine Hydrocarbon versus

Water-bearing zones,

Indicate Permeable zones,

Determine Resistivity Porosity.

LOW RESISTIVITY

When the resistivity is<1.0

ohm meter

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 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.

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 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.

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 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.

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.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.

FormationFormation Borehole WallBorehole WallSensitive VolumeSensitive Volume

transceiver

24“

16”

Permanent Magnet

6”6”

16”16”

mudmud

MagnetMagnet AntennaAntenna

B0(r)B0(r)

S.NO

FEATURE BENEFIT VALUE

1. CMR porosity LithologyindependentSalinity independentShale independent

Decrease incoring costDetermines Phie

2. No radioactivesource

Safety Reduced riskliability

3. Free fluidporosity

Irreducible watersaturation

Improvedanalysis ofProspectivezones

APPLICATIONS OF NMR LOGGING

4. Pore sizedistribution

Permeabilityestimation

Decrease testingcost

5. High resolution Thin bed analysis Adding toproduciblereserves

6. Combinable Save rig time Increase rigefficiency

7. Specific tooldesign

No boreholecorrection

Decrease loggingcost

8. T2 distribution Facies analysisViscosity estimationCapillary pressureCementation Expo.

Support toformationevaluation

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. 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.

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.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.

� 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 � 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.

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 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

� The standard rock porosity model for all pore fluids

matrixmatrixdrydryclayclay

clayclay--boundboundwaterwater

mobilemobilewaterwater

capillarycapillaryboundboundwaterwater

hydrocarbonhydrocarbon

MFFIMBVI

Conductive Fluids

MCBW

MSIG total porosity, MPHI effective porosity, MFFI free fluid index, MBVI bulk volume irreducible water and MCBWclay bound water.

MFFIMBVI

MPHI ≅≅≅≅ φφφφeff

MSIG ≅≅≅≅ φφφφtotal

MCBW

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).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.

NMR properties for water, oil and gas under

typical reservoir conditions.

� 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 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.

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 T2distributions to the right correspond to an increasing pore size distribution.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.

Irreducible Water Saturation

Water Cut Analysis

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 estimate of the reserves through the identification of low resistivity reservoirs that have already been bypassed by the resistivity logging interpretation.

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