EVALUATION OF LOW RESISTIVITY ZONESUSING NUCLEAR MAGNETIC RESONANCE LOG PRESENTED BY SHAHNAWAZ MUSTAFA M.Tech. PETROLEUM GEOLOGY 2nd Sem. Roll No. 02 24-04-08 Dibrugarh University Assam – INDIA.
Resistivity LogThe Resistivity Log: Resistivity logs measurethe ability of rocks toconduct electrical current and arescaled in units of ohm-Meters.The Usage: Determine Hydrocarbon versusWater-bearing zones, Indicate Permeable zones,Determine Resistivity Porosity.LOW RESISTIVITYWhen the resistivity is<1.0ohm meter
Why NMR?The combination of conventional logs such as density,neutron and resistivity logs is proven to be very effective inthe evaluation of normal reservoirs.For low resistivity reservoirs, however, an accuratedetermination of the Petrophysical parameters with theconventional logs is very difficult.In case of low resistivity contrast reservoirs it is difficult todetermine oil-water contact & irreducible water saturationwith resistivity logs.NMR log has been only available as a supplement tool toprovide additional information on the producibility of thereservoirs, to distinguish between bound and free waterand helps accurately to determine the reservoir rockPetrophysical properties.The main limitations of NMR logging, however, as been thecost and long time of acquiring data.
NUCLEAR MAGNETIC RESONANCE LOGSome nuclei actually behave like magnets rotatingaround themselves particularly protons i.e.hydrogen nuclei which enter into the compositionof water as well as hydrocarbonsUnder the effect of a magnetic field the nucleiprecess like gyroscopes in a field of gravity. Thefrequency of this precession in a given magneticfield characterizes the nuclei. In the logging sondes, the magnetic field used isthe earth’s magnetic field.NMR log does not make use of any radioactiveproperty of the nucleus but of its gyromagneticproperty.
PHYSICS OF NMR LOGGINGNMR measurement can be made on any nucleus that has oddnumber of protons e.g. 1H, 23Na.NMR log is basically a hydrogen Index measurement, whichresponds to movable protons only.Spinning proton represents a current loop that generates amagnetic field or magnetic moment aligned with the spin axis.When many hydrogen atoms are present and no external fieldexists, the proton nuclear spins are randomly aligned and thenet magnetic field is zero.In the presence of an external magnetic field B0, spinningproton experiences torque and starts precessional motionaround the field B0 . The precessional frequency calledlarmour frequency is proportional to the field strength.
The permanent magnet magnetizes the formation materials andessentially the hydrogen nuclei present in hydrocarbons and water presentin pore spaces and bound to clay minerals.An antenna surrounding the magnet transmits in to the formation preciselytimed radio frequency pulse sequence.Between these pulses the antenna is used to listen for decaying echos fromthose hydrogen atoms which are in resonance with the field frompermanent magnet.As the proton resonance frequency depends upon the applied field, thefrequency of the transmitted and received energy can be tuned toinvestigate cylindrical regions of formation at different diameters around anMRIL tool just like an image of a narrow slice of any organ of a patient inHospital MRI.
APPLICATIONS OF NMR LOGGINGS. FEATURE BENEFIT VALUENO1. CMR porosity Lithology Decrease in independent coring cost Salinity independent Determines Phie Shale independent2. No radioactive Safety Reduced risk source liability3. Free fluid Irreducible water Improved porosity saturation analysis of Prospective zones4. Pore size Permeability Decrease testing distribution estimation cost5. High resolution Thin bed analysis Adding to producible reserves6. Combinable Save rig time Increase rig efficiency7. Specific tool No borehole Decrease logging design correction cost8. T2 distribution Facies analysis Support to Viscosity estimation formation Capillary pressure evaluation Cementation Expo.
PARAMETERS OF NMRFor the analysis of NMR data, several aspects of NMRtechnique have been used; 1) T1/T2 ratio, for fluid identification, 2)The difference between NMR derived porosity and totalporosity, to determine the types of clay minerals, 3) NMR relaxation properties, to identify fluids nature androck properties of low contrast / low resistivity reservoirs.Longitudinal Relaxation Time( T1) also called spin-latticerelaxation time is a time constant characterizing thealignment of spins with the external static magnetic field.Transverse Relaxation Time (T2) also called spin-spin,relaxation time is a time constant which characterizes theloss of phase coherence that occurs among spins oriented atan angle to the main magnetic field.
Low Resistivity & NMRIn case of low contrast resistivity reservoirs where there was littleresistivity contrast between water bearing formation and oilbearing formation, NMR has been able to identify the fluidnature of the two formations and then the height of the oilcolumn.If a pay zone exhibits low resistivity, conventional resistivity logsbecome incapable of identifying the producing zones and also ofindicating water mobility. Because of this limitation, manypotentially productive zones with high irreducible watersaturation are overlooked.In low resistivity beds, there is little resistivity contrast betweenwater-bearing reservoirs and oil-bearing reservoirs. The water-bearing reservoirs contain relatively fresh water, and thus showrelatively high resistivity readings. The resistivity in the oil-bearing reservoirs is variable because the reservoirs containfresh or salty water. Often, the oil-bearing reservoirs show a highlevel of irreducible water saturation that depresses further theresistivity reading, thus making the pay identification from theresistivity log extremely difficult.
Control of water production and identification of low resistivity payzones with high irreducible water saturation of two formationevaluation problems are existing in many fields in the Middle Eastand other fields around the world. The problem with these zones isthat 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 watersaturation can be high, but water - free hydrocarbons are produced.The mechanism responsible for such high water saturation is usuallydescribed as being caused by microporosity.The second group is concerned with reservoirs where the calculatedwater saturation is higher than the true water saturation. Themechanism responsible for this high water saturation is described asbeing caused by the presence of conductive minerals such as clayminerals, metal sulfides, graphite and pyrite in a clean reservoir rock.NMR log can identify water free production zones, correlate boundfluid volume with clay minerals inclusions in the reservoir, andidentify hydrocarbon type.
NMR POROSITYThe NMR porosity depends only on the fluids content ofthe formation, unlike density/neutron porosity which isinfluenced by both fluids and surrounding rocks.The strength of the NMR signal is proportional to thenumber of hydrogen atoms in NMR tool dependent rockvolume. In zones containing light hydrocarbon, where thehydrogen index is less than unity, NMR porosity willtypically underestimate true porosity in proportion to thehydrogen index.The number of hydrogen atoms in gas depends stronglyon temperature and pressure. Hence it is important toestimate accurately pressure and temperature to accountfor their effect on NMR results in natural gas reservoirs
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 ≅ φtotalMSIG total porosity, MPHI effective porosity, MFFI free fluid index, MBVI bulk volume irreducible water and MCBW clay bound water.
NMR FLUIDS AND FLUID TYPESWe are enable to separate signals from gas, oil and water withNMR log data unambiguously and, in some cases, can evenquantify.The T1 contrast separates the water and light hydrocarbon (oiland gas).Freedman et al has introduced a new method called Density-Magnetic Resonance (DMR) for evaluating gas-bearingreservoirs.Laboratory NMR data show that both T1 and T2 vary overseveral orders of magnitude depending on fluid type.
NMR properties for water, oil and gas undertypical reservoir conditions.
In cases of low resistivity reservoirs with water saturationgreater than 50% and being still able to produce waterfree hydrocarbon.Zemanek has proposed certain technique to solve thisproblem. This technique is based on the comparisonbetween irreducible water saturation (Swi) derived fromlaboratory 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 hydrocarbonwill be produced and if Sw greater than Swi, water willbe produced.
Producibility AnalysisCMR example:Utilized to determine theirreducible water saturation (Swirr). This log comes from a complexdolomite formation with varyingeffective porosity andpermeability. Increasing T2distributions to the rightcorrespond to an increasing poresize distribution. The solid purple line on the T2distribution track corresponds tothe cutoff used to calculate the freefluid index (FFI). The percentageof signal falling to the left of the100-msec cutoff is the capillary orirreducible fluid volume inporosity units, and the percentageof signal falling to the right of thecutoff is the free fluid volume.
CONCLUSIONNMR technology proves to be very essential information evaluation and more specifically in lowresistivity reservoirs. The capability of NMR todifferentiate between movable and immovable fluidshas helped the log analysts to get more accurateestimate of the reserves through the identification oflow resistivity reservoirs that have already beenbypassed by the resistivity logging interpretation.