用有限差分法模拟自然电位测井的侵入带影响

Modeling Effect of Invaded Zone on SP Log with Finite Difference Method

自然电位与地层流体的矿化度有密切关系,侵入带中泥浆滤液的矿化度对自然电位测井有较大的影响。在近年来提出的侵入带中泥浆滤液矿化度是渐变而非突变的自然电场模型的基础上,用有限差分法模拟了不同泥浆矿化度及侵入深度的侵入带对自然电位的影响,得到以下结论:侵入带不影响自然电位测井曲线对地层中点对称和在砂泥岩地层界面处变化明显这2个特征;当砂岩中地层水矿化度大于泥浆矿化度时,在不同的侵入深度及泥浆滤液与地层水矿化度的比值下,自然电位曲线的幅度都随着泥浆矿化度及侵入深度的增大而变小;在侵入带深度以及泥浆矿化度和地层水矿化度都不变的情况下,侵入带中流体矿化度的变化越平缓,其自然电位的幅度越小。

Spontaneous potential （SP） of a formation has tight relationship with salinity of formation liquid, the salinity of mud filtrate in invaded zone affects SP log considerably. Based on late natural electrical field model, in which the filtrate salinity in invaded zone changes gradually, not sharply, the effects of mud salinity and invasion depth on SP log are modeled by finite difference method, and concluded is that： 2 characters that the SP log is symmetrical to midpoint of the formation and that the SP log changes evidently at sand-shale interface are not affected by invaded zone; when formation water salinity is greater than mud salinity, in different invasion depths and ratios of filtrate/formation-water salinity, the amplitude of SP log decreases with increase of mud salinity and invasion depth; in condition that all the invasion depth, salinity of mud or formation water are not changed, the change of fluid salinity in invaded zone is more smooth, the amplitude of its SP is smaller.