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Nuclear magnetic resonance (NMR) log data can be analyzed independently or in combination with conventional-log and core data. As an independent logging service, NMR can provide porosity, permeability index, and complete information on fluid type and saturation of the flushed zone. Some data-interpretation methods operate in the echo-decay time domain, while others operate in the T2-relaxation domain.
The three key applications of NMR logging include:
Residual oil (Sxo) calculation
Estimation of residual-oil saturation is one of the oldest applications of NMR logging. Unlike resistivity-log analysis, NMR analysis does not rely on formation-water salinity to obtain water saturation. This feature provides NMR logging with a distinct advantage over conventional resistivity analysis in mixed or unknown salinity conditions—an advantage that can be extremely useful in waterflood or steamflood projects for evaluating residual-oil saturation after the flood or to look for bypassed oil. Initially, NMR evaluation of residual-oil saturation required use of a dopant (e.g., Mn-EDTA and MnCl2) to flush the invaded zone with paramagnetic ions to shorten the bulk relaxation time of the brine. This process enabled separation of the oil and water signals, leading to direct measurement of Sxo. With the combination of modern multifrequency NMR tools and new methods of analysis, such as Enhanced Diffusion Method (EDM) and time-domain analysis (TDA), the use of borehole dopant is no longer necessary. Standalone NMR interpretation is possible in oil-based mud (OBM), but in water-base-mud (WBM), it is possible only when Swirr is known.
For oil and water, the diffusion constant, D0, can be approximated by Eq.2:
where A = 2.5 for water, A = 1.3 for oil, and TK is temperature in K.
NMR properties of gas can be obtained from published charts that relate viscosity to (1) the center of the relaxation curve, (2) to an American Petroleum Inst. (API) standard value, or (3) from published formulas. These published sources assume that methane is the dominant component of the gas. In the absence of laboratory data at in-situ conditions, reservoir NMR properties can be estimated by use of Eq.1. One note of caution:Eqs.1' and 2 are based on "dead oil" measurements. As discussed in NMR petrophysics, the relaxation-time dependence on viscosity/temperature of live crude oil may differ significantly from correlations based on hydrocarbon liquids at ambient conditions.
Anisotropy and geomechanics
- Sand control
- Hydraulic fracturing
- Wellbore stability
- Determination of formation stress
Low-permeability (tight) sandstones
Field experience indicates that invasion or imbibition in tight sands is very shallow. Depending on borehole size, the diameter of invasion, and fluid properties, a mandrel tool may have sufficient DOI to measure beyond the flushed zone. A combined interpretation using both T1 and T2 can provide positive identification of fluids in these reservoirs. In addition, in these tight formations, in which formation testers typically may not obtain a fluid sample within a reasonable time period, NMR fluid characterization can separate hydrocarbon from oil filtrate and other pore fluids.
Heavy oil, tar sands, and tar mats
The early acquisition of reliable viscosity information is essential to efficient development of heavy-oil reservoirs. NMR logs offer a viable alternative to downhole fluid sampling for determining viscosity information in heavy-oil reservoirs. The presence of tar mats in a reservoir, commonly near the bottom of the oil column, may form vertical permeability barriers and, thereby, isolate the oil leg from the water-drive aquifer. NMR logs, in conjunction with conventional logs, can provide accurate identification of tar-mat levels and viscosity estimation, from empirical relationships.
Carbonates and complex lithologies
NMR-log evaluation is relatively routine in what might be termed "conventional reservoirs," namely those of homogenous lithology and uniform pore sizes, typically sandstone and chalk reservoirs. In contrast, log evaluation of complex and heterogeneous reservoirs, with complex pore geometries, is not straightforward. These reservoirs include, in particular, the highly important Middle East carbonates, as well as other reservoirs comprised of mixed lithologies and mineralogies, or both, in which wettability may also vary. In these reservoirs, there is likely no simple relationship between petrophysical properties and porosity. Instead of a dependence on the volume of pore space, they are dependent on:
- Typically heterogeneous pore distribution
- Pore types
- Pore connectivity
- Grain sizes
This fundamental difference between siliciclastic and carbonate rocks (primarily the result of diagenetic processes) limits the applicability of routine NMR methods, especially permeability evaluation. Improving NMR evaluation of carbonates has proved challenging and is the subject of a number of recent studies and proposed techniques.
As discussed in NMR petrophysics, establishing a correlation between NMR T2 distribution and mercury-injection capillary pressure (MICP) is fundamental to NMR interpretation and the computation of Sw. Once such correlations are established for a particular reservoir or field, pseudocapillary-pressure curves can be generated directly from the NMR-log relaxation-time distributions.
Accurate determination of bulk-volume-irreducible (BVI) water enables evaluation of reservoir-fluid (e.g., gas, oil, and water) contacts, production characteristics (producibilty), and the determination of net and recoverable reserves. Furthermore, NMR-derived permeability can be used to generate idealized flow profiles across a completion interval. These profiles provide a diagnostic tool for identifying nonflowing portions of the zone and the need for remedial work.
Combined NMR applications
NMR tools have shallow depths of investigation and provide results only for the invaded zone. NMR-log data can be integrated with core data and conventional log data (i.e., neutron, density, acoustic, and resistivity) in post-acquisition processing to provide improved determinations of reservoir rock properties, hydrocarbon storage capacity, and reservoir productivity in a variety of environments including gas-bearing and low-resistivity reservoirs. Interpretation models that include NMR data can provide more reliable results than those using only conventional logs.
The combination of NMR and deep-resistivity data provides a complete analysis of the fluids in the uninvaded zone. Resistivity measurements alone cannot distinguish between capillary-bound and movable water, but they do represent deep investigations of fluid saturation. Furthermore, resistivity-based methods are often inadequate or unreliable in reservoirs in which salinity and lithology vary. The addition of NMR-derived BVI and clay-bound-water (CBW) from the flushed or invaded zone can significantly enhance the estimation of resistivity-based fluid saturation, both in clastics and carbonates. The addition of NMR data allows the identification and evaluation of water-free production in low-resistivity formations (Fig.1).
Fig.1 – Integrated-NMR-resistivity shaly sand analysis. Whereas resistivity logs (Track 2) read very low values and indicate that the zone below XX200 is water wet, NMR measurements show that BVI (Track 4, gray) increases with depth. However, T2 measurements (Tracks 3 and 4) suggested the presence of hydrocarbon, which testing confirmed in what turned out to be a water-free zone.
The combination of conventional deep-resistivity data with NMR-derived CBW, BVI, FFI, and MPHI can greatly enhance petrophysical estimations of effective pore volume, water cut, and permeability. It is the preferred technique for identifying low-resistivity pay zones. Fig.1 presents the results of Halliburton’s MRI analysis (MRIAN) service in a turbidite sequence. The sand below XX200 depth has an average resistivity of approximately 0.5 ohm-m (Track 2) and average neutron-density porosity of approximately 38% (Track 4). A quick look, or preliminary analysis, using only the conventional data presented in Tracks 1, 2, and 4, would label this a wet zone. MRIL-NMR data are also presented in Track 1 (T2-distribution bin data), Track 2 [NMR-derived permeability (Coates)], and Track 3 (VDL presentation of T2 distribution). The MRIAN results, BVI, and FFI are presented in Track 4. BVI gradually increases with depth, suggesting that the sand is fining downward (i.e., as sand grains become finer, the volume of capillary-bound water that they hold increases). Comparison of the BVI with the resistivity profile (Track 2) shows that the resistivity decreases where the bound water increases. The MRIAN analysis clearly shows that the zone does not contain any movable water and will produce only oil. The interval below XX200 was tested and produced oil with no water.
The combination of NMR fluid identification with resistivity-derived saturation provides a better understanding of hydrocarbon movement in gas monitoring, as seen in the following field example (Fig.2).
NMR and logging while drilling (LWD) logs were run in a deviated Middle East light-oil carbonate reservoir to monitor gas injection. NMR logs were well suited for gas monitoring by use of the TDA technique and were not affected by formation-water salinity. NMR gas-corrected porosity was in close agreement with core porosity; conventional log-porosity was low because of salinity effects. Combined interpretation of water-saturation analyses from tools with different depths of investigation identified the movable fluid present in the reservoir as WBM filtrate. Track 1 contains T2 bin data and conventional SP and gamma ray curves; Track 2 shows NMR-permeability derived from the standard Coates model and LWD resistivity; Track 3 contains a T2 distribution in a variable-density format; Track 4 contains T2 distributions of data acquired with long TW and short TW; Track 5 is the differential spectrum; Track 6 contains TDA results; and Track 7 contains MRIAN results.
NMR acoustic/density combination
Because NMR tools are affected by the lower HI in gas-bearing reservoirs, a moving tool may not fully polarize the gas, which has a long T1. Conventional acoustic and density logs are not influenced by these factors and, when used jointly with NMR logs, they can provide robust porosity evaluation. The basic assumptions of these techniques are that the reference porosity is measured correctly (e.g., appropriate matrix density is used) and that the NMR TW is long enough to recover the water but not all of the gas. If the TW is too short, a false gas signal may be seen. One way to check whether the TW is appropriate is to log with a longer wait time (e.g., TW = 2 seconds rather than 1 second) and look for a significant difference in porosity (i.e., the porosity may be slightly higher because of increased polarization of the gas).
|A||=||pore-fluid-specific value used to approximate the diffusion constant|
|D0||=||molecular diffusion coefficient, gauss/cm|
|Sw||=||water saturation, %|
|Swirr||=||irreducible water saturation, %|
|Sxo||=||flushed-zone saturation, %|
|T1||=||longitudinal relaxation time, seconds|
|T2||=||transverse relaxation time, seconds|
|TK||=||absolute temperature, K|
|TW||=||polarization (wait) time, seconds|
|η||=||fluid viscosity, cp|
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Noteworthy papers in OnePetro
Cao Minh, C., Crary, S. F., Zielinski, L., Liu, C., Jones, S., & Jacobsen, S. J. (2012, January 1). 2D-NMR Applications in Unconventional Reservoirs. Society of Petroleum Engineers. doi:10.2118/161578-MS
Altunbay, M., Ismail, S. B., aula, nurul, & Das, S. (2012, January 1). Translating Petrophysics to Engineering Economics. Society of Petroleum Engineers. doi:10.2118/159191-MS
Minh, C. C., Jaffuel, F., Poirier, Y., Haq, S. A., Baig, M. H., & Jacob, C. (2011, May 14). Quantitative Estimation Of Formation Damage From Multi-Depth Of Investigation Nmr Logs. Society of Petrophysicists and Well-Log Analysts.
Toumelin, E., Sun, B., Manzoor, A., Keele, D., Wasson, M., & Sagnak, A. (2011, May 14). Revisiting Log-Inject-Log Nmr For Remaining Oil Determination: A Field Application Of T2-D Nmr In The Permian Basin. Society of Petrophysicists and Well-Log Analysts.
Chen, S., Di Rosa, D. E., Gyllensten, A., Georgi, D., & Tauk, R. S. (2008, April 1). Use of the NMR Diffusivity Log To Identify and Quantify Oil and Water in Carbonate Formations. Society of Petroleum Engineers. doi:10.2118/101396-PA
Gladkikh, M., Chen, J., & Chen, S. (2008, January 1). Method Of Determining Formation Grain Size Distribution From Acoustic Velocities And Nmr Relaxation Time Spectrum. Society of Petrophysicists and Well-Log Analysts.
Nicot, B., Fleury, M., & Leblond, J. (2007, January 1). Improvement Of Viscosity Prediction Using Nmr Relaxation. Society of Petrophysicists and Well-Log Analysts.
Galarza, T., Giordano, S., Fontanarosa, M. B., Saubidet, M. E., Altunbay, M., Saavedra, B. E., & Romero, P. A. (2007, January 1). Pore-Scale Characterization and Productivity Analysis by Integration of NMR and Openhole Logs: A Verification Study. Society of Petroleum Engineers. doi:10.2118/108068-MS
Akkurt, R., Kersey, D. G., & Zainalabedin, K. A. (2006, January 1). Challenges for Everyday-NMR: An Operator's Perspective. Society of Petroleum Engineers. doi:10.2118/102247-MS
Looyestijn, W. J., & Hofman, J. (2005, January 1). Wettability Index Determination from NMR Logs. Society of Petroleum Engineers. doi:10.2118/93624-MS
Altunbay, M., Sy, R., & Martain, R. (2003, January 1). Formation Damage Assessment and Remedial Economics from Integration of NMR and Resistivity Log data. Society of Petroleum Engineers. doi:10.2118/84384-MS
Altunbay, M., Martain, R., & Robinson, M. (2001, January 1). Capillary Pressure Data From NMR Logs And Its Implications On Field Economics. Society of Petroleum Engineers. doi:10.2118/71703-MS
Romero, P., & Quintairos, M. (2001, January 1). New Applications of NMR in Understanding Heavy-Oil Behavior. Society of Petroleum Engineers. doi:10.2118/69696-MS
Crary, S., Pellegrin, F., & Simon, B. (1997, January 1). Nmr Applications In The Gulf Of Mexico. Society of Petrophysicists and Well-Log Analysts
Chang, D., Vinegar, H. J., Morriss, C., & Straley, C. (1994, January 1). Effective Porosity, Producible Fluid And Permeability In Carbonates From Nmr Logging. Society of Petrophysicists and Well-Log Analysts.
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