一种改进的核磁共振测井测量脉冲序列

提出了一种可以提高核磁共振测井测量速度的新型测量脉冲序列,它基于对原有CPMG测量脉冲序列的改进.在原有的测量序列中,在一个CPMG脉冲序列结束之后,直接进行等待(T₁延时),以使磁化强度矢量能够完全恢复到平衡位置,然后再进行下一个CPMG脉冲序列的测量.由于测井仪器的连续运动,在测量含有长T₁成分的地层时,等待时间需要取得较长,使得测井速度很低.但在实际地层中,恰恰又是油气的T₁较长,这无疑给核磁共振测井带来了麻烦.作者发现,如果在一个CPMG脉冲序列结束之后在-x方向先施加一个90°脉冲(或在x方向施加一个270°脉冲),再进行等待,就能更好地使长T₁成分得到极化,从而缩短等待时间、提高测量速度.文中对这种改进后的脉冲序列进行了计算和分析,证明它确实具有一定的优越性.

AN IMPROVED MEASURING PULSE SEQUENCE FOR NMR LOGGING

A new pulse sequence for the measurement of NMR logging is proposed in this article, based on the improvement of original CPMG pulse sequence. In the original pulse sequence, after a CPMG pulse sequence, wait time (or T1 relay) is needed to allow the magnetization vector return to equilibrium, then the next measuring CPMG pulse sequence can be applied. The wait time is kept long and the logging speed is low in formations that contain long T1 components, since the logging apparatus is moving continuously along the bore hole. This is unfavorable because the spin-lattice relaxation time (T1) of oil and gas is always long . However, it's noticed that if a 90° pulse from the -x direction (or a 270° pulse from the x direction) is applied after the former CMPG pulse sequence, the long T1 components can be polarized more completely, then the wait time can be shortened and logging speed can be improved. The advantage of the improved pulse sequence is proved by calculations and analyses.