矿化度界面对震电测井波场的影响

Influence of salinity interface on seismoelectric logging wave field

震电测井的现场试验中测得电磁首波幅值大于伴随电场幅值,这种情况在前人的理论模拟和模型井测量中从未出现.本文研究矿化度界面对震电测井波场的影响,基于Pride方程和边界连续条件模拟了震电测井波场,其中井外孔隙地层可以视为均匀的或者是径向分层的.计算结果表明:对于均匀孔隙地层,当井内流体矿化度大于井外地层孔隙流体矿化度时,电磁首波幅值可以比伴随斯通利波的电场幅值还大;井内流体矿化度与井外地层孔隙流体矿化度的比值越大,电磁首波相对伴随斯通利波的电场的幅值越大.对于径向分层地层,当井内流体矿化度大于内层地层的孔隙流体矿化度时,电磁首波幅值可以比伴随斯通利波的电场幅值还大;当外层地层矿化度小于内层地层矿化度时,电磁首波幅值可以比伴随斯通利波的电场幅值还大;增加内层地层厚度,电场全波幅值减小,电磁首波相对伴随电场的幅值减小.本文的模拟结果表明,震电测井中确实会出现电磁首波幅值比伴随电场幅值还大的情况,这对于解释震电测井的测量结果具有指导意义.

In the field test of seismoelectric logging,the amplitude of electromagnetic head waves(EH)is larger than that of the electric signal accompanying acoustic wave(EA),which has never occurred in the previous theoretical simulation and model well measurement.The influence of salinity contrast is studied on synthesized seismoelectric logging responses.Seismoelectric logging wave fields are calculated based on the Pride equations and continuity conditions at interfaces.The porous formation outside the well is seen as either homogeneous or radial stratified.Simulations for the homogeneous formation case show that the amplitude of EH can be larger than that of the electric signal accompanying acoustic Stoneley wave(ESt)when the salinity of borehole fluid is larger than that of pore fluid in the formation.The greater the ratio between the salinities of the borehole fluid and the pore fluid,the greater the amplitude of the EH relative to the ESt.The set of examples for stratified formation case show that the EH signal can be stronger than the ESt signal when the borehole fluid salinity is higher than the salinity in the inner layer formation.The amplitude of EH can also be larger than the amplitude of ESt when the salinity of the outer layer formation is smaller than that of the inner layer formation.However,the amplitude ratio of EH to ESt decreases with the increase of the interlayer thickness.The simulation results in this paper show that the amplitude of EH can be larger than that of ESt in seismoelectric logging,and it has guiding significance for interpreting the measurement results of seismoelectric logging.