The derivation of information regarding petroleum or other organic fuel deposits at an underground geological location is the subject of numerous technological approaches. The fluid flow properties of porous media have long been of interest in the oil industry. To a considerable degree this interest has been focused on nuclear magnetic resonance logging ever since A. Timur proved experimentally that NMR methods provide a rapid, non-destructive determination of porosity, movable fluid, and permeability of rock formations. See “Pulsed Nuclear Magnetic Resonance Studies of Porosity, Movable Fluid and Permeability of Sandstones,” J. of Petroleum Technology, v. 21, pp. 775-86 (1969).
NMR logging is based on the observation that when an assembly of magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field, they tend to align along the direction of the magnetic field, resulting in bulk magnetization. The rate at which equilibrium is established in such bulk magnetization is characterized by the parameter T1, known as the spin-lattice relaxation time. Another related and frequently used NMR logging parameter is the spin-spin relaxation time T2 (also known as transverse relaxation time), which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. Both relaxation times provide information about formation porosity, composition and quantity of formation fluid, and other parameters important in oil exploration.
Various techniques exist for disturbing the equilibrium of an assembly of nuclei in a static magnetic field in order to measure the relaxation parameters of interest. Usually, one applies a radio-frequency (RF) oscillating magnetic field in a direction substantially orthogonal to the static magnetic field. If the oscillating magnetic field has the proper resonant frequency (known in the art as the Larmor frequency, given by the expression ω=γBo, where Bo is the strength of the static field and γ is the gyromagnetic ratio constant), then the spinning nuclei will be tipped away from the static magnetic field direction. The nuclei precess around the static field at the Larmor frequency and generate measurable signals until they return to equilibrium according to the T1 relaxation time.
NMR measurements of parameters of a geologic formation can be done using, for example, the centralized MRIL® tool made by NUMAR, a Halliburton company, and the sidewall CMR tool made by Schlumberger. In a standard NMR measurement using these tools, the static magnetic field is provided by one or more appropriately configured magnets, and a series of RF pulses are applied to tip the spins of the nuclei in a sample volume. Signals from the precessing spins are then measured by the voltage induced in one or more receiving antennas.
In general, NMR logging devices may be separate from the drilling apparatus (in what is known as wireline logging), or they may be lowered into the borehole along with the drilling apparatus, enabling NMR measurement while drilling is taking place. The latter types of tools are known in the art as logging-while-drilling (LWD) or measurement-while-drilling (MWD) logging tools. The present invention is directed to an improvement ofLWD/MWD tools.
U.S. Pat. No. 5,280,243, to Miller, discloses an NMR apparatus and method for geophysical examination of a borehole as it is being drilled. The content of the Miller patent is expressly incorporated herein for all purposes. With reference to FIGS. 1, 2, and 3 herein (corresponding to FIGS. 1, 2, and 4 of the Miller patent) the tool is made up of several sections, including drilling section 22, NMR logging section 24, and stabilizing section 26. In operation, the tool generates a gradient static magnetic field in a region adjacent to the borehole. This static field extends radially with respect to the longitudinal axis 28 of the tool (which generally coincides with the axis of the borehole) and has a generally uniform amplitude along the azimuth with respect to axis 28. A pulsed RF magnetic field is generated by the antenna 48 to excite nuclei in a substantially cylindrical shell within the borehole, which shell defines the sensitive volume extending along the length of the tool. The antenna is also used to pick up and measure signals from the sensitive volume, which are then processed to determine petrophysical properties of the material within the sensitive volume.
More specifically, the NMR logging portion of the tool 20 disclosed in the Miller patent includes a probe section having permanent magnet 46, RF antenna 48, and control section 72 made up of electronic/electrical components. The Miller patent discloses various conditions for the proper operation of the device, including the condition that the static and RF magnetic fields produced by the magnet and the antenna must remain orthogonal to each other, and thus symmetrical about the longitudinal axis 28, while the tool 20 is being rotated.
Further, the Miller patent discloses the use of a protective sleeve to protect the magnet, the antenna, and other components forming the sensor portion of the tool from the extreme conditions in the borehole. The sleeve in Miller is made of an electrically insulating material, such as fiberglass, that does not interfere with the static magnetic field or the RF electromagnetic fields. However, although fiberglass and other polymers may be fairly hard and durable under normal conditions, when placed in a borehole that is being drilled, a sleeve made of such materials is rapidly worn away by the severe mechanical abrasion and extreme temperatures. This creates a danger of damaging the delicate and expensive tool components covered by the sleeve.
Accordingly, there is a need to provide greater strength and abrasion resistance in a probe section of a wireline or MWD logging tool, while preserving the requisite orthogonality condition between the static magnetic and RF electromagnetic fields during the measurements.