- Industry: Oil & gas
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In a nuclear magnetic resonance measurement, the decay, or relaxation, caused by dephasing in an inhomogeneous magnetic field. Since this relaxation is not related to formation properties, it is unwanted and corrected by using the CPMG pulse sequence.
Industry:Oil & gas
In a nuclear magnetic resonance (NMR) measurement, the loss of synchronization of hydrogen atoms precessing at different speeds about the static magnetic field. When the signals from individual atoms are not synchronized, they are out of phase and the total signal is reduced. The dephasing occurs either because of inhomogeneities in the static magnetic field or through molecular processes. Dephasing due to inhomogeneities is known as the free-induction decay and is corrected by the CPMG sequence. Molecular dephasing is known as transverse relaxation.
Industry:Oil & gas
In a nuclear magnetic resonance (NMR) measurement, the loss of coherent energy by hydrogen atoms as they move within the pore space. Hydrogen atoms that move significantly within the pores during a NMR measurement will encounter different magnetic fields and hence will precess at different rates, or dephase. Dephasing contributes only to T<sub>2</sub> and is most significant in gas or light oils. The magnitude depends on the field gradient, the echo spacing and the diffusion coefficient of the fluid. Diffusion relaxation can be induced in water by using long echo spacings. This is the basis of the enhanced diffusion technique.
Industry:Oil & gas
In a nuclear magnetic resonance (NMR) measurement, the characteristic time for a loss of coherent energy, or relaxation, by protons in rocks. There are two types of relaxation: longitudinal relaxation, which is the time (T<sub>1</sub>) needed to align protons in a static magnetic field; and transverse relaxation, which is the time (T<sub>2</sub>) needed for protons to lose their coherent energy in an NMR measurement. Relaxations are exponential decays, for which T<sub>1</sub> and T<sub>2</sub> are the time constants. Different mechanisms contribute to T<sub>1</sub> and T<sub>2</sub>. Surface relaxation and bulk relaxation contribute to both T<sub>1</sub> and T<sub>2</sub>. Surface, bulk and diffusion relaxation contribute to T<sub>2</sub>.
Industry:Oil & gas
In a nuclear magnetic resonance (NMR) measurement, referring to the cycle of radio frequency pulses designed by Carr, Purcell, Meiboom and Gill to produce pulse echoes and counteract dephasing due to magnetic field inhomogeneities. In the CPMG sequence, an initial radio frequency pulse is applied long enough to tip the protons into a plane perpendicular to the static magnetic field (the 90<sup>o</sup> pulse). Initially the protons precess in unison, producing a large signal in the antenna, but then quickly dephase due to the inhomogeneities. Another pulse is applied, long enough to reverse their direction of precession (the 180<sup>o</sup> pulse), and causing them to come back in phase again after a short time. Being in phase, they produce another strong signal called an echo. They quickly dephase again but can be rephased by another 180<sup>o</sup> pulse. Rephasing is repeated many times, while measuring the magnitude of each echo. This magnitude decreases with time due to molecular relaxation mechanisms surface, bulk and diffusion. One measurement typically may comprise many hundreds of echoes, while the time between each echo (the echo spacing) is of the order of 1 ms or less. <br><br>Carr HY and Purcell EM: ?Effects of Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments,? Physical Review 94, no. 3 (1954): 630-638. <br><br>Meiboom S and Gill D: ?Modified Spin-Echo Method for Measuring Nuclear Relaxation Times,? The Review of Scientific Instruments 29, no. 8 (1958): 688-691.
Industry:Oil & gas
In a gradiomanometer tool, the pressure difference observed when the fluid velocity opposite the upper pressure sensor differs from that across the lower pressure sensor. This difference usually occurs opposite points of fluid entry or exit, and at sudden changes in diameter, such as at the tubing shoe. The result is a sharp deflection on the log that may be misinterpreted as a local change in fluid density.
Industry:Oil & gas
In a gradiomanometer tool or pressure derivative calculation, the apparent increased fluid density observed due to frictional pressure losses along the tool and casing in a fast-flowing fluid. The magnitude of the correction depends on the flow rate, tool geometry and the casing size, and is negligible in most casings below about 2000 B/D (318 m<sup>3</sup>/d). The fluid density will appear erroneously high unless this effect is corrected for.
Industry:Oil & gas
In a glycol dehydrator, glycol that contains water released by wet gas while percolating upward in the absorber.
Industry:Oil & gas
In a glycol dehydrator, glycol that has been boiled and no longer contains any water. When the glycol is lean, it can be pumped back to the absorber for reuse.
Industry:Oil & gas
Hydrocarbons with low molecular weight such as methane, ethane, propane and butane.
Industry:Oil & gas
