The Paradox of Anisotropy Revisited

Stan Gianzero
 

ABSTRACT
The apparent resistivity measured by a system of electrodes (whether operating in direct or alternating current mode) aligned parallel to the vertical resistivity component in an anisotropic medium is equal to the horizontal resistivity component. Historically, this phenomenon is described as the "paradox of anisotropy"; its existence thwarts efforts to detect the vertical resistivity component of an anisotropic medium. As mentioned, in the case of alternating current, even when the transmitters and the receivers are both vertical electric dipoles, the paradox of anisotropy is observed. However, the paradox can be circumvented by detecting the magnetic field of a vertical electric dipole transmitter using a toroidal receiver. The formulae which convert the resistive and reactive voltage signals into apparent horizontal and vertical conductivities are simple. In addition, when skin effect is small, an apparent anisotropy coefficient is estimated that is within a few percent of the actual anisotropy coefficient.

Medhat W. Mickael
 

ABSTRACT
Pulsed neutron capture (PNC) logs have been in routine use for decades for the measurement of formation capture cross section. The measurement is robust, accurate, and statistically precise. However, borehole and diffusion effects on the measurement are difficult to characterize and use since they require accurate knowledge of not only borehole size but also borehole capture cross section.
This paper presents an innovative approach to correct pulsed neutron logs for borehole and diffusion effects without the knowledge of borehole capture cross section or water salinity. This is accomplished through the utilization of all the information available in the time decay spectra of both the near and far detectors. The correction to the apparent formation capture cross section of one of the detectors is described as a mathematical model of the relative counts in different gates of the time decay spectra of both detectors. The coefficients of the model are obtained from a nonlinear least-squares fit of the model to data from different borehole and formation conditions. The model was optimized using over 5000 data points generated from accurate Monte Carlo simulations covering a very large range of down-hole conditions. The accuracy of the new algorithm when tested on modeling data was found to be on the order of 0.5-1.0 cu over the entire range of formation and borehole capture cross sections.

Log Interpretation With Fast Induction Log Inversion

Qiang Zhou
 

ABSTRACT
By now it should be well known that apparent resistivity (Ra) is quite different from the true formation resistivity (Rt) in complex formation environments. Efforts have been made to apply inversion techniques to derive Rt from Ra. The advantages of using inversion are that the method automatically derives an Rt model and that the inverted model is consistent with the logs. Inversion improves bed boundary definition and the water saturation calculation. However, there are two bottlenecks to the method, i.e. the processing speed and solution uniqueness. Because of these problems, inversion still has not been routinely used in log interpretation.
The major part of the processing time in a rigorous inversion algorithm is spent on calculating the Jacobian matrix that sets the direction of model adjustment. In this 1-D fast algorithm, the Jacobian matrix calculation is avoided. The fast algorithm first applies a shaping filter to the logs. Equivalently, to a first order approximation, the shaping filter maximizes the diagonal elements of the Jacobian matrix as well as symmetrizes the Jacobian matrix. The shaping filter differs from the conventional focusing filters in that it has no resolution enhancement. It only ensures that for each log point of the reshaped log the largest sensitivity is from the formation at the same depth point. The inversion solution is then updated iteratively according to the difference between the filtered log and the calculated tool response (with the same shaping filter applied) of the predicted model.
The stability of the inverse algorithm is achieved by using the fact that induction measurements have little sensitivity to a resistive layer with thickness smaller than the main coil spacing. A simple automatic adjustment in correction step length is built into the algorithm to avoid resistivity over-correction and consequently the instability in updating the model. The algorithm converges to a stable Rt model typically after three to five iteration steps. Since there is no Jacobian matrix calculation, the total computation time is roughly equal to the cost of a single forward run multiplied by the number of iterations.

Log ASCII Standard (LAS) Version 3.0

Kenneth Heslop, James Karst, Dennis Schmitt, and Stephen Prensky
 

ABSTRACT
An updated and enhanced version of LAS (Log ASCII Standard) is proposed. LAS 3.0 is a consistent approach that provides a more robust standard for digital data exchange and storage.  This revision increases the applicability of LAS files by accommodating all types of digital log data, as well as non-log borehole data.
We anticipate that the final specifications will achieve wide acceptance, and we are therefore requesting feedback and input on the proposed revisions from industry to ensure that LAS 3.0 meets the needs of users and data managers.