OBSERVATIONS OF LARGE DIELECTRIC EFFECTS ON INDUCTION LOGS, OR, CAN SOURCE ROCKS BE DETECTED WITH INDUCTION MEASUREMENTS?

摘要

Wireline induction logs provide standard, deep-reading formation conductivity measurements. Formation elec elec- trical conductivity is obtained from an in-phase voltage measurement. The quadrature (out-of-phase) portion of the measured signal is usually small, and its main use is as a skin-effect correction at higher formation con con- ductivities. In Maxwell’s equations, there is a dielec dielec- tric-permittivity term present as the imaginary part of a complex-valued conductivity. At induction operating frequencies (10s of kHz), dielectric effects are negligible within the range of typical formation conductivities (0.5 to 5000 mS/m), and relative dielectric permittivites up to that of water (80). The expected dielectric contribu contribu- tion is less than 0.01 mS/m, which is much less than the measurement uncertainty. However, some peculiar rock formations have been ob ob- served that cause large dielectric effects on induction logs. Array-induction logs from Oklahoma and Texas serve as examples. The raw data show unexpectedly large negative quadrature signals in some shale zones. In adjacent sandstones and shales, the same quadrature signals are small and positive as expected from the mea mea- sured in-phase conductivity. Therefore calibration errors can immediately be excluded. The reason for the large negative quadrature signals must lie in the nature of these particular shales. If dielectric effects are the cause, the quadrature signals should be proportional to the op op- erating frequency. A systematic examination of the raw data in the strange shale zones con?rms this hypothesis. Inversion of the raw log data in the Woodford shale, a known source rock, yields apparent relative dielectric permittivities between 10,000 and 30,000. Inversion of raw data in non-hydrocarbon-bearing shales gives negli negli- gible apparent dielectric permittivities. Thus abnormally high apparent dielectric permittivities could be useful in source rock identi?cation. The petrophysical cause for such elevated dielectric permittivities lies in the microstructure of the shales. It is fairly well known that pyrites are associated with or or- ganic matter in source rocks. In minerals such as pyrites, surface redox reactions in the presence of an external ?eld generate a polarization layer as conduction changes from being ionic outside the mineral grain to being elec elec- tronic inside the grain. High dielectric permittivities are caused by an accumulation of ions outside the grain. In shaly non-source rocks, the dielectric enhancement nor nor- mally seen at lower frequencies arises from polarization of a double layer of counter-ions. A controlled laboratory study of sidewall cores is needed to correlate the quan quan- tity of organic carbon and types of minerals present with the magnitude of the X-signal logs. This may be aided by geochemical (spectroscopy) logs. Operating compa compa- nies in these formations are invited to join us in pursuing further analysis.