Nuclear magnetic resonance technique for determining gas effect with borehole logging tools

摘要

An NMR pulse sequence technique for use in the borehole environment is provided which uses CPMG pulses according to [Wi-90-(tcp-180-tcp-echo)j]i where j is the index of the CPMG echoes gathered, i is the index of the wait times in the pulse sequence, Wi are the varying wait times before the CPMG pulses, and tcp is the Carr-Purcell spacing. Measurements are made of the signals induced in the formation as a result of the magnetic fields to find the spin-echo relaxation time T1. Using T1, the gas effect on formation parameters may be derived. Also, gas saturation and gas chemical composition can be derived. In addition, the diffusion coefficient of gas under reservoir conditions can be more than ten times larger than that of water, making the T2 relaxation time of gas more sensitive to static magnetic field gradients. In formations where gas occupies pores larger than about 10 mu m, this T2 effect (effect of diffusion on the measured T2 relaxation times of gas) can be used to identify gas even though the tool gradients are not uniform.