Determining Resistivity and Low-Frequency Dielectric Constant Using Induction Data in the Presence of Strong Induced Polarization

ABSTRACT

Laboratory and field data have shown that sedimentary formations can exhibit a surprising amount of induced polarization (IP) effect in the presence of clay, pyrite, or graphitic carbon. The effect can be so strong that the quadrature signal of induction data can be pulled toward the negative direction in an appreciable manner, causing an adverse effect in standard data processing and interpretation of induction data. On the other hand, the strong IP effect makes it possible to determine the dielectric constant at induction frequencies neglected in standard data processing techniques.

A pixel inversion-based processing technique enables simultaneous determination of resistivity and dielectric constant in dipping formations using both the in-phase and quadrature signals of induction data. The resistivity log created by the processing can be used in the same way as a standard induction resistivity. The dielectric constant log, a separate output from induction data, provides a different perspective into reservoirs. It introduces an opportunity for novel petrophysical applications, for example, approximating a continuous maturity index of kerogen in unconventional reservoirs, and estimating cation exchange capacity (CEC) of shaly sand formations.

The technique is a combination of the maximum entropy and the Occam inversions, which makes the iterative process converge rapidly over a wide range of initial models for resistivity and dielectric constant. The processing considers layering and dipping of the formation systematically by means of a planar-layered model that can dip at a relative dip angle of up to 75°. With this approach, the resistivity and dielectric constant logs obtained are free of layering and dip effects, which often appear in the form of polarization horns or overshoots at large relative dip on standard logs.

Using the quadrature signal overcomes the ambiguity of resistivity estimation caused by the strong skin effect in conductive formations. This enables the unique determination of resistivity and dielectric constant over a broad range of formation resistivities with only singlearray data. The nonuniqueness cannot be resolved without joint use of both shallow and deep arrays in standard data processing. In addition, the large depth of investigation of the quadrature signal makes the inverted resistivity and dielectric constant more representative of the undisturbed zone than the resistivity obtained using only the in-phase signal.

The new processing technique has been applied to more than 20 wells for which induction logs were available. Results suggest that the strong IP effect is present in many well-known formations, with the dielectric constant ranging from thousands to hundreds of thousands. Results also confirm the superior quality of the resistivity obtained with the new processing over that of a standard processing technique in a variety of situations. The results from several field cases are analyzed in conjunction with elemental spectroscopy data to shed light on the correlation between a large dielectric constant and clay, pyrite, and kerogen containing graphitic-like carbon structures.