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Planning for NMR log data collection
In some complex reservoirs, low-resistivity/low-contrast pay, low-porosity/low-permeability, and medium-to-heavy oil, nuclear magnetic resonance (NMR) log data—independently or in combination with other log data—provide the best and/or only means of accurate formation and fluid evaluation. Because NMR-log data acquisition is complex, job preplanning is essential to ensure optimal selection of acquisition parameters that will result in reliable and accurate data and in the maximum information possible in any given reservoir and logging environment. A clear understanding of the logging job objectives is necessary for optimizing the NMR acquisition parameters to best achieve these objectives. This process must take place before the actual logging.
Planning the NMR log job
Typical preplanning consists of three steps:
- Define the need for NMR measurements
- Collect all available borehole (e.g., diameter, mud, salinity, and temperature) and reservoir (e.g., formation and fluid properties) information needed to assess the expected NMR responses in the zone of interest, and understand what can and cannot be resolved with NMR
- Select the appropriate tool (on the basis of operational considerations, borehole size, and condition) and acquisition type (i.e., determining the appropriate acquisition parameters, data resolution, and logging speed) that will provide maximum answers for a given job
Although the actual in-situ reservoir characteristics may be unknown, estimates of the anticipated fluid properties, based on available information such as reports for nearby wells or fields, are used to define and optimize an acquisition sequence that will provide the data needed to meet the job objectives.
In addition to job objectives, determination of the appropriate NMR-acquisition parameters is also influenced by operational considerations and the anticipated in-situ reservoir properties (Fig.1).
Although reservoir lithology generally plays a minor role in NMR-data acquisition, it does play a significant role in data analysis and interpretation. Aspects of reservoir lithology that influence reservoir T2 values include the following.
Different T2cutoff values are required because surface relaxivity in carbonates is weaker than in sandstones, resulting in slower relaxation rates (longer T2). Longer T1 in carbonates than in sandstones may require longer TW during acquisition. (See NMR applications).
The presence of relatively isolated pores (e.g., vugs) will not affect NMR porosity, but it will cause the standard permeability equations (Coates and the Schlumberger-Doll-Research (SDR) model) to overestimate permeability. (See Permeability estimation with NMR logging).
Ferromagnetic and paramagnetic minerals
The presence of these minerals may enhance surface relaxation significantly, shifting the T2 spectrum to very short relaxation times. Depending on the amount of paramagnetic material, relaxation may become too fast to be detected, and the NMR measurement will underestimate porosity. In these cases, standard cutoff values do not apply.
Heavy oil and tar sands
Intervals containing these types of hydrocarbons have very fast relaxation components and may not be detected using conventional acquisition methods. Special methods have been developed for detection and accurate evaluation of these reservoirs. (See NMR applications)
Wettability can have a significant impact on NMR-log response. The use of NMR for determination of wettability has been extensively studied both in laboratory and in the field. In general, petrophysical-NMR studies and NMR-logging applications assume reservoir rocks are water wet; however, some mixed-wettability reservoirs do exist:
- Some carbonates
- Black shales
- Heavy-oil reservoirs
Assuming rocks are water wet may lead to incorrect reserve estimates and to unexpected dynamic behavior during waterflood. When a pore is water wet, oil relaxes at its bulk rate. In mixed-wettability reservoirs, the oil and water each relax through a combination of bulk relaxation and surface interaction and depend on the ratio of water-wet surface area to water volume and oil-wet surface area to oil volume. The oil relaxation spectra will be shifted from bulk relaxation into the irreducible-water part, resulting in complex spectra that are difficult to interpret. Nevertheless, these shifts provide qualitative wettability indicators that allow NMR logs to provide an early indication of reservoir wetting behavior.
The invasion of oil-based mud (OBM) or synthetic oil-based mud (SOBM) can alter formation wettability and is a significant concern in NMR logging, which typically measures fluid and formation properties in the flushed zone. Invasion by these muds can alter strongly water-wet sandstones and carbonates to intermediate-wet or oil-wet rocks. In water-wet reservoirs that have undergone OBM or SOBM invasion, the T2cutoff model may significantly underestimate Swirr because wettability alteration changes the water and oil relaxation-time distributions. The magnitude of underestimation depends on the type of OBM surfactants, their concentration in the flushing fluid, and the flushing volume. Controlling the volume of OBM invasion and the concentration of OBM surfactants should minimize the effects of OBM invasion on estimation of Swirr.
NMR-logging tools have a relatively shallow depth of investigation (DOI). Pad-type tools (e.g., the combinable magnetic resonance (CMR) tool) are run eccentered and require good contact with the borehole wall for accurate measurements. Measurements made by pad-type tools can be significantly affected by severe borehole rugosity and washouts, resulting in overestimation of porosity.
Mandrel devices are run centered, and DOI can range from 1 to 4 in., depending on borehole size. The sensitive area is normally beyond minor borehole rugosity. When borehole conditions result in an elliptical borehole (e.g., breakouts or erosion) or otherwise inhibit or prevent centralization (e.g., in highly deviated or horizontal boreholes), contact tools may be a better choice if pad alignment and contact with the borehole wall can be maintained.
The quality of NMR data acquired in OBM is generally superior to that acquired in water-based mud (WBM). The conductivity of OBM is lower; lower conductivity reduces loading effects on the transmitter/receiver system, resulting in higher S/N ratio. This issue is of greater concern for mandrel tools (e.g., magnetic resonance imaging tool (MRIL)) because, in conductive muds, the power of the radio frequency (RF) pulse is reduced as the pulse is transmitted across the borehole. The use of fluid-excluding sleeves can minimize this problem.
Signal dissipation in conductive mud is not a serious concern for pad-type tools (CMR) that maintain contact with the borehole wall. Because NMR tools read the flushed zone, OBM filtrate invasion produces an additional hydrocarbon signal that may significantly complicate log interpretation. Careful prejob planning can reduce interference of the OBM-filtrate signal and the response from the native fluids. The relatively long T1 relaxation times and diffusivity of OBM make it difficult to differentiate its signal using the shifted-spectrum or differential-spectrum approaches.
Metal drilling debris in the borehole fluid may affect NMR-measurement quality by distorting and altering the logging tool’s magnetic field. The pad-type tool is more susceptible to field distortion. Metal debris should be removed from the mud, either through the use of the prepolarizing magnets (included in the latest tool designs) or by using magnets at the shale shaker. A new wireline tool uses an autotuning feature to correct the operating frequency for changes in the static magnetic field caused by metallic debris in the borehole.
Logging speed and running average
The logging speed of an NMR tool is influenced by a number of factors, primarily by tool type (e.g., centered or eccentered, number of operating frequencies, or length of antenna), logging objectives (e.g., acquisition type—TW, TE, or NE—sequence repetitions and vertical resolution), and borehole properties (e.g., diameter and mud resistivity). S/N ratio is primarily controlled by borehole size and mud resistivity. As S/N ratio decreases, the running average (RA) needed to maintain a specified error in porosity increases. The general practice is to require a porosity standard deviation of ≤ 1 p.u. The value of the running average (RA) and the antenna aperture (i.e., length), combined with the logging speed, determines the vertical resolution. Even so, there is always a complex tradeoff in logging speed, accuracy (e.g., S/N ratio or NE), and job objectives. High accuracy and precision require reduced logging speeds. A method for increasing the overall logging speed is to reduce the vertical resolution in zones of secondary or no interest and also to reduce the vertical resolution in homogeneous intervals.
The vertical resolution of NMR-logging tools is primarily a function of antenna length (i.e., tool design) and logging speed. The maximum vertical resolution, usually obtained when the tool is at rest (e.g., in stationary mode), is the length of the antenna. During continuous logging, vertical resolution decreases at a rate proportional to logging speed. Contact-logging tools, in general, use smaller sensors and antennae and, thus, have better vertical resolution than centered tools. The contact-NMR tool (i.e., the CMR tool) has a resolution advantage when bed thickness is in the range between 0.5 and 5 ft. Outside of this range, both designs deliver similar results. Prejob planning includes selecting a logging speed to obtain the optimum resolution.
Post-job data reprocessing to enhance bed resolution may result in a loss of repeatability. Vertical resolution can also be improved by optimizing the NMR signal through the removal of signal noise during data processing.
|NE||=||number of echoes|
|Swirr||=||irreducible water saturation, %|
|T1||=||longitudinal relaxation time, seconds|
|T2||=||transverse relaxation time, seconds|
|T2cutoff||=||T2 cutoff value, seconds|
|TE||=||CMPG interecho spacing, seconds|
|TW||=||polarization (wait) time, seconds|
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Noteworthy papers in OnePetro
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
Minh, C. C., Heaton, N., Ramamoorthy, R., Decoster, E., White, J., Junk, E., … McLendon, D. (2003, January 1). Planning and Interpreting NMR Fluid-Characterization Logs. Society of Petroleum Engineers. doi:10.2118/84478-MS
Chen, S., Georgi, D., Fang, S., Salyer, J., & Shorey, D. (1999, January 1). Optimization of NMR Logging Acquisition and Processing. Society of Petroleum Engineers. doi:10.2118/56766-MS
Edwards, C. M. (1997, January 1). Effects Of Tool Design And Logging Speed On T2 Nmr Log Data. Society of Petrophysicists and Well-Log Analysts.
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