G. P. Grove and G. N. Minerbo
Schlumberger
Abstract
Multiarray induction tools, as well as all other induction conductivity measuring devices, are affected by the environment. Correcting for the borehole effects on induction measurements requires accurate knowledge of the borehole signal or conductivity contribution. Available external measurements of borehole geometry do not have sufficient accuracy to adequately correct for borehole effect in the presence of moderate-to-large borehole-formation conductivity contrasts. The AIT Array Induction Imager Tool measures in-phase and quadrature signals from multiple ax-rays operating at several frequencies. This multitude of information allows the formulation of a self-consistent borehole correction scheme that can determine the borehole geometry and correct for the borehole signal even in difficult cases, such as irregular-shaped boreholes with caves.
The borehole signal is primarily a function of mud conductivity, borehole size and shape, tool standoff from the borehole wall and formation conductivity. To develop the borehole corrections, a large number of model calculations of the AIT response were performed over a wide range of each of the input variables. These calculations were validated against laboratory measurements taken in cylindrical plastic tanks of different diameters filled with salt water.
An algorithm that rapidly computes the borehole response of each of the raw measurements was developed by fitting the results of the model calculations to closed form mathematical expressions. The borehole correction algorithm includes a nonlinear least-squares optimization technique that matches the measured data from the shortest spacings arrays to the model calculations. Any of the variables of the borehole signal may be combined and varied to achieve this match. Results obtained using the adaptive borehole correction scheme for modeled cases and field test wells have validated the accuracy and reliability of the borehole signal determination and correction.