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NOVEL CALIBRATION BOX DESIGN FOR ARRAY LATEROLOG

IP.com Disclosure Number: IPCOM000236739D
Publication Date: 2014-May-13

Publishing Venue

The IP.com Prior Art Database

Abstract

Array Laterolog tools need calibration. Calibration is performed by using calibration boxes. A calibration box is designed to represent a homogeneous formation around a borehole. In the case of conventional design, necessary conditions for the unfocused voltages and currents are enforced. The assumption of the conventional method is that since the solution is a good approximation to the unfocused voltages and currents in the design, the focused voltages and currents from calibration box should also be in proximity of the focused voltages and currents from formation. However, this assumption does not always hold and small differences in unfocused voltages may lead to great discrepancies in the apparent resistivities of the focused modes between the calibration box and the corresponding formation it is supposed to represent. The invention disclosure herein uses the focused voltages and currents serving as the input to the conventional design system. By enforcing focused voltages and currents, the solution is guaranteed to be in the proximity of the results targeted from formations. The method is robust and reliable. Novel method is more efficient than conventional methods.

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        NOVEL CALIBRATION BOX DESIGN FOR ARRAY LATEROLOG ABSTRACT

Array Laterolog tools need calibration. Calibration is performed by using calibration boxes. A calibration box is designed to represent a homogeneous formation around a borehole. In the case of conventional design, necessary conditions for the unfocused voltages and currents are enforced. The assumption of the conventional method is that since the solution is a good approximation to the unfocused voltages and currents in the design, the focused voltages and currents from calibration box should also be in proximity of the focused voltages and currents from formation. However, this assumption does not always hold and small differences in unfocused voltages may lead to great discrepancies in the apparent resistivities of the focused modes between the calibration box and the corresponding formation it is supposed to represent. The invention disclosure herein uses the focused voltages and currents serving as the input to the conventional design system. By enforcing focused voltages and currents, the solution is guaranteed to be in the proximity of the results targeted from formations. The method is robust and reliable. Novel method is more efficient than conventional methods.


1. INTRODUCTION

Array Laterolog tools typically include a central electrode A0 around a tool body, with current excitation electrodes A1, A2, ..., AN, A1', A2', ... , AN' and voltage monitoring electrodes M1, ... ,M2*(N-1),M1', ... ,M2*(N-1)' symmetrically spaced above and below the central electrode. The tool drives auxiliary currents between the current excitation electrodes and the center electrode to "focus" the current from the center electrode, i.e., to reduce propagation of the current in the direction along the tool's axis until after the current has penetrated some distance into the formation [1].

Similar to the most of their counterparts [2-3], LAT (Array Laterolog Tool) also needs calibration. Calibration of LAT is performed as depicted in Figure 1. In the first step, LAT is connected to a calibration box. The calibration box consists of a network of resistors, which simulates electrical loading of the LAT, corresponding to various formations. The resistor electrically simulates the corresponding borehole-formation impedance between all electrode pairs. Usually several different calibration boxes for different borehole-formation conditions should be designed to implement shop calibration of the LAT, the boxes with low resistor values are used to calibrate the tool while the calibration boxes with the higher resistor values are employed to extend the accuracy range of the tool. Ideally, the calibration boxes should produce substantially similar results (apparent resistivity) with the results obtained by using formations.

Once LAT is connected to the calibration box, unfocussed voltages and currents (V(u) and I(u) ) are measured. Unfocused measurements are focused by using a software focusing cod...