The Magnetic-Resonance While-Drilling Tool: Theory and Operation
- M.G. Prammer (NUMAR, HES) | E. Drack (NUMAR, HES) | G. Goodman (NUMAR, HES) | P. Masak (NUMAR, HES) | S. Menger (NUMAR, HES) | M. Morys (NUMAR, HES) | S. Zannoni (NUMAR, HES) | B. Suddarth (Shell Offshore Inc.) | J. Dudley (Halliburton Energy Services)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- August 2001
- Document Type
- Journal Paper
- 270 - 275
- 2001. Society of Petroleum Engineers
- 1.12.2 Logging While Drilling, 5.1 Reservoir Characterisation, 2.4.3 Sand/Solids Control, 1.10 Drilling Equipment, 1.6 Drilling Operations, 5.2 Reservoir Fluid Dynamics, 1.5 Drill Bits, 5.6.1 Open hole/cased hole log analysis, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.14 Casing and Cementing, 1.12.1 Measurement While Drilling, 1.11 Drilling Fluids and Materials, 1.6.1 Drilling Operation Management
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The first Magnetic-Resonance-Image Logging-While-Drilling (MRI-LWD*) tools have been built and field tested. The hardware was designed to provide data that are compatible with the Magnetic Resonance Imaging Log (MRIL®) wireline tool in terms of processing and interpretation. This paper discusses the theory of robust MRIL measurements in the drilling environment and reviews the theory of T1 relaxation time measurements. We report on the results of a field test in the Gulf of Mexico. The MRI-LWD device logged the entire total depth (TD) run from casing to TD and provided additional hydrocarbon-typing data in wiping mode over the target zones. Wireline MRIL data and conventional logs were used to verify the MRI-LWD measurements.
We previously reported on our initial field experiences with the MRI-LWD tool in a recent transaction paper.1 The present paper focuses on the petrophysical deliverables of the MRI-LWD measurement and their robustness in the drilling environment. Furthermore, we report on a field test conducted in the Gulf of Mexico in March 2000. The test was run with an experimental prototype version (EX), and we were able to compare the results with wireline data from the MRIL-Prime tool.2 The comparison indicates satisfactory performance during both drilling and wiping operations.
The primary application areas of MRI-LWD will be high-cost offshore exploration and development wells, potentially in deep water. Typical bit sizes for the final TD run are 81/2 to 105/8 in. High rates of penetration (ROP) and long bit runs are common, aided by poorly cemented sediments, oil-based mud systems, and advanced bit technologies. In this context, openhole logging operations are expensive in terms of added rig time and risky in terms of hole stability. Highly laminated reservoirs (e.g., turbidites) are additional problems that frustrate traditional log interpretation because electrical and nuclear measurements may show little or no contrast when identifying reservoir rocks. In these situations, breakthroughs in reservoir characterization have been achieved by computing hydrocarbon volumes directly from wireline MRIL data, circumventing the problems with conductivity/ resistivity.3 Real-time magnetic resonance data from an LWD device will result in faster, better, and cheaper evaluation and exploitation of these reserves.
To be commercially successful, logging-while-drilling (LWD) tools must be technically and financially attractive compared to the wireline alternatives. In particular, the technical expectations for the LWD version of an MRIL device consist of the following:
Provide a measure of rock porosity that is lithology-independent and that does not require radioactive sources.
Collect a spectrum of nuclear magnetic resonance (NMR) relaxation times suitable for input to lithology models that estimate bound-fluid volumes, free-fluid volumes, and rock permeability.
Enable fluid typing by exploiting the inherent differences in T1 and/or T2 (longitudinal and transversal relaxation times) between the water, oil, and gas phases.
To withstand the shock, vibration, and erosion associated with drilling.
Provide noninterference with the drilling process.
Provide noninterference with any other LWD or measurement-while-drilling (MWD) measurements.
Tool Hardware and Sensor Physics
Fig. 1 shows the basic tool layout conforming to these requirements. The tool has two main sections: the sensor and the electronics package. The total length of the EX version (42 ft) is driven by the inclusion of extra electronics and will be reduced in the forthcoming commercial tool. The tool diameter is undergauge at about 7 1/4 in. All compressional, torsional and tensional stress ratings exceed those of standard 4 1/2 IF API connections. To meet the sensor-to-sensor noninterference requirement, a distance of 45 ft to the MWD directional package is recommended.
The sensor section consists of the high-strength stainless steel drill collar, a flow tube, the permanent magnet, and a radiofrequency antenna. The magnet is dimensioned to produce a circumferencially symmetric field around the tool and the borehole. The magnetic field strength diminishes as the square of the radial distance. At a diameter of 14 in., the field strength is 120 gauss, which corresponds to the operating frequency of 500 kHz. Therefore, the sensitive volume resembles one or more concentric and thin-walled cylinders of approximately 14 in. diameter. The height of the sensitive volume is defined by the length of the antenna and is 24 in. The antenna is encased in a combination of fiberglass and rubber. The antenna section is stood off from the borehole wall by two spiral wearbands located above and below the antenna.
The electronics section contains lithium battery packs, power supplies, several digital signal processors (DSP's), nonvolatile random access memory (NVRAM), a high-efficiency, 10-kW, 500-kHz transmitter, and sensitive receiver circuitry. The EX version does not have the capability of sending data in real time to the surface. Rather, all recorded waveforms are committed to the NVRAM and are downloaded through the sidewall readout (SWRO) port after each run.
To render the measurement robust against drillstring motions, it is essential to establish two main data acquisition modes.
Reconnaissance logging (RL), which is practically insensitive to lateral, rotational, and vertical motion and is suited for while-drilling operations.
Evaluation logging (EL), which closely matches the MRIL-Prime wireline measurement and which is suited for wiping and tripping, but not drilling operations.
Mode switches between RL and EL can be triggered by elapsed time or by counting measurements, or they can be tied to the outputs of accelerometers/magnetometers that differentiate between drilling and nondrilling conditions. For simplicity, the EX tools are programmed to continuously toggle back and forth between reconnaissance and evaluation logging. In post-processing, drilling and nondrilling periods are identified, and the invalid EL data recorded during drilling are discarded.
Table 1 summarizes the data acquisition parameters for the wireline tool (using the MRIL-Prime as base comparison) for both RL mode (while drilling) and EL mode (measurement after drilling, or MAD). EL parameters closely resemble those for MRIL wireline, except for slightly lower frequencies, field gradients, and lower logging speeds. EL speeds should be limited to 300 ft/hr if hydrocarbon typing (i.e., gas detection) is a logging objective.
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