Nuclear Magnetic Resonance While Drilling in the Southern North Sea
- Matthias Appel (Shell U.K. Exploration & Production) | Nigel J. Radcliffe (ExxonMobil) | Prabhakar Aadireddy (Halliburton Energy Services) | Ron J.M. Bonnie (Halliburton Energy Services) | Ridvan Akkurt (NMRPlus Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2003
- Document Type
- Journal Paper
- 351 - 360
- 2003. Society of Petroleum Engineers
- 5.6.1 Open hole/cased hole log analysis, 1.6.7 Geosteering / Reservoir Navigation, 1.12.6 Drilling Data Management and Standards, 1.6.1 Drilling Operation Management, 1.12.1 Measurement While Drilling, 4.1.5 Processing Equipment, 5.1.5 Geologic Modeling, 1.6.9 Coring, Fishing, 5.1 Reservoir Characterisation, 1.2.3 Rock properties, 1.6 Drilling Operations, 5.6.2 Core Analysis, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.3.4 Scale, 4.1.2 Separation and Treating, 1.12.2 Logging While Drilling, 2.4.3 Sand/Solids Control, 5.5.2 Core Analysis, 5.5 Reservoir Simulation, 4.6 Natural Gas, 1.11 Drilling Fluids and Materials, 5.8.1 Tight Gas, 5.2 Reservoir Fluid Dynamics, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.1 Well Planning
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Standard formation evaluation of an exploration well in the U.K. southern North Sea was supported by magnetic resonance while drilling (MRWD).
In this paper we show that even in tight gas sands, MRWD provides information about porosity and producible fluid fraction and allows estimation of formation permeability.
This successful introduction of MRWD technology in a known hard-rock environment illustrates a powerful addition to the logging-while-drilling (LWD) tool suite.
Magnetic-resonance (MR) logging has developed into a powerful petrophysical tool for reservoir characterization. The application of downhole MR logging tools has become widespread within the oil and gas industry. Several "answer" products (such as total porosity or bound-fluid volume) are now considered to be standard and are reliably provided by the service industry.
However, the design and completion strategy of development wells often limits or complicates data acquisition using wireline or pipe-conveyed logging. Typical examples are highly deviated, multilateral wells that are commonly required for an economical field development. Furthermore, concerns about borehole stability frequently prevent any traditional openhole wireline data acquisition.
Under these conditions, LWD might be the only method of obtaining cost-effective openhole data for formation evaluation. During the past few years, service companies have made significant efforts to add MR technology to the suite of LWD tools.1,2
The development of MRWD is complicated by the fact that any MR measurement is very sensitive to vibrations and motions of both the transmitting and receiving radio frequency (RF) antenna and the fluids in the sensed volume. As a result, the design of MRWD tools has to cope with and overcome drilling-induced noise.
Halliburton Energy Services has recently introduced its design of an MRWD tool.1 This particular tool was first tested successfully in a Gulf of Mexico well in fourth quarter 2001.3 Encouraged by this success, Halliburton included the MRIL-WD™ tool in the bottomhole assembly (BHA) for the reservoir section of an exploration well in the U.K. southern North Sea. This was the first use of this technology in a known hard-rock environment or in a gas well anywhere in the world.
The main objective of this trial was to test the feasibility of MRWD technology in highly consolidated, low-permeability gas-bearing sandstones. Furthermore, we wanted to investigate potential step changes in cost effectiveness and operational efficiency for a forthcoming development-well drilling campaign in late 2003.
The MRIL-WD™ tool was run in the 8.5-in. hole section in combination with various other measurement-while-drilling tools to acquire T1 and T2 data in drilling and sliding modes, over cored and noncored intervals. To verify the data, and to enable local calibrations, subsequent formation evaluation also included MR logging, and a full compliment of standard service, on wireline.
MR logging exploits the effect of nuclear magnetic resonance (NMR). NMR is a consequence of the intrinsic magnetic moment of protons and neutrons. For most atoms, such as 12C and 16O, the individual magnetic moments of protons and neutrons offset each other, and the effective magnetic moment of such nuclei vanishes. This makes these nuclei invisible to NMR. Protons (1H) provide the strongest NMR response and are typically targeted in NMR logging, but the measured signal also can be affected by the response of other NMR-active nuclei, like the 23Na found in brine.
As with bar magnets, when a magnetic nucleus is placed between the poles of an external magnet, it will try to align itself with respect to this externally applied magnetic field. In the macroscopic world, two magnets can be aligned in an infinite number of orientations. At the atomic level, however, these alignments (also called spin states) are quantized. There are only a finite number of alignments a nucleus can take relative to an external magnetic field. This number depends on the shape of the nucleus' magnetic field.
The alignment of magnetic moments is disturbed by RF pulses transmitted from an antenna in the tool into the formation. The return of the magnetic moments toward their equilibrium state is governed by various relaxation mechanisms, each of which can be characterized by a spectrum of relaxation times.4
In MR logging, two different relaxation mechanisms are typically exploited: the longitudinal (T1) and the transverse (T2) relaxation. T1 relaxation characterizes how fast the RF-induced energy is dispersed to the surrounding molecular lattice. T2 relaxation describes how quickly the precessing spins lose their phase coherence.
MR is generally considered to be a measurement of only the formation fluids because the relaxation of any MR-active nuclei in the rock matrix is too fast to be detected by logging tools that are commercially available today. In weak, homogeneous magnetic fields, the T1 and T2 relaxation times of formation fluids are quite similar because both are governed by the pressure/volume/temperature properties of the fluids and the structure and surface chemistry of the surrounding rock matrix.
However, if the magnetic field is not homogeneous but features a gradient over length scales that are comparable with the dephasing length of the spins, the T2 relaxation process is sensitive to the displacement of the nuclei. The larger the displacements of molecules within the gradient magnetic field, the shorter the measured T2 relaxation times.5
A variety of RF pulse sequences have been developed that are optimized for measuring T1 or T2 . By varying the timings of these pulse sequences, the measurement can be optimized to the expected MR response of the formation fluids. This is an important part of the prejob planning of MR logging, which is crucial for acquiring useful data.
Fig. 1 illustrates a typical procedure for acquiring and interpreting MR logging data.
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