2evaluation of Low Resistivity Zones Con NMR Log
2evaluation of Low Resistivity Zones Con NMR Log
2evaluation of Low Resistivity Zones Con NMR Log
PRESENTED BY
SHAHNAWAZ MUSTAFA
M.Tech. PETROLEUM GEOLOGY 2nd Sem. Roll No. 02 24-04-08 Dibrugarh University Assam INDIA.
Resistivity Log
The Resistivity Log: Resistivity logs measure the ability of rocks to conduct electrical current and are scaled in units of ohmMeters. The Usage: Determine Hydrocarbon versus Water-bearing zones, Indicate Permeable zones, Determine Resistivity Porosity. LOW RESISTIVITY
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.
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.
24
Permane nt Magnet
transceiv er
16
Formation
Borehole Wall
Sensitive Volume
Magnet
Antenna
mud
B0(r)
B0(r)
6 16
FEATURE
CMR porosity
BENEFIT
Lithology independent Salinity independent Shale independent Safety Irreducible water saturation
VALUE
Decrease in coring cost Determines Phie Reduced risk liability Improved analysis of Prospective zones Decrease testing cost Adding to producible reserves Increase rig efficiency Decrease logging cost Support to formation evaluation
2. 3.
4. 5.
6. 7. 8.
Save rig time No borehole correction Facies analysis Viscosity estimation Capillary pressure Cementation Expo.
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.
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.
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
matrix
dry clay
clayclaybound water
mobile water
hydrocarbon
MCBW MBVI
MSIG
total
MSIG total porosity, MPHI effective porosity, MFFI free fluid index, MBVI bulk volume irreducible water and MCBW clay bound water.
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 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 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.
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.
THANK YOU