Electromagnetic Propagation Logging While Drilling: Theory and Experiment

Abstract

Summary The compensated dual resistivity (CDR) tool is an electromagnetic propagation tool for measurement while drilling. The CDR tool provides two propagation tool for measurement while drilling. The CDR tool provides two resistivity measurements with several novel features that are verified with theoretical modeling, test-tank experiments, and log examples. Introduction The CDR tool is a 2x10(6) -cycles/sec electromagnetic propagation tool built into a drill collar. This drill collar is fully propagation tool built into a drill collar. This drill collar is fully self-contained and has rugged sensors and electronics. The CDR tool is borehole-compensated. requiring two transmitters and two receivers. The transmitters alternately broadcast electromagnetic waves, and the phase shifts and attenuations are measured between the receivers and averaged. Phase shift is transformed into a shallow measurement, Rps, and attenuation is transformed into a deep measurement, Rad. The CDR tool has several new and important features. 1.Rad and Rps provide two depths of investigation and are used to detect invasion while drilling. For example, in a 1-Ohm * m formation, the investigation diameters (50% response) are 30 in. for Rps and 50 in. for Rad. 2. Rad and Rps, detect beds as thin as 6 in., however, these measurements are affected differently by shoulder-bed resistivities and both require corrections in thin resistive beds. Rps has a better vertical response than Rad. Rad and Rps cross over at the horizontal bed boundaries, this crossover can be used to measure bed thickness. 3. Both Rad and Rps are insensitive to hole size and mud resistivity in smooth boreholes. Borehole corrections are very small even for contrasts of 100:1 between formation and mud resistivities. Rugose holes and salty muds together, however, can cause larger errors than indicated by the borehole-correction charts. In these conditions, borehole compensation is essential for an accurate measurement. An extensive research program was conducted to verify these features and to ensure that the CDR tool provides a high-quality log. To achieve wireline quality, the CDR tool's physics was studied thoroughly, and its environmental effects were modeled and experimentally measured. Two theoretical models are used for the CDR tool. The first model treats the tool geometry in detail but assumes a homogeneous medium outside the tool. This model is verified by test-tank experiments and by air measurements. The second model assumes a simplified tool geometry but treats bore-holes, caves, beds, and invasion in detail. This model is used to study environmental effects and to prepare correction charts. Experiments with artificial boreholes, caves, step-profile invasion, and horizontal bed boundaries verify the predictions of the second model. Finally, CDR logs are compared to wireline logs to demonstrate the new features. Theoretical Models The two CDR models emphasize different aspects of the sonde and its environment. The first model accounts for the effects of the antenna recesses and is called the profile-collar (PC) model. It assumes azimuthal symmetry and a homogeneous medium outside the collar (Fig. 1), The azimuthal symmetry reduces the wave equation to a ID integral equation that is solved numerically. This model also accounts for the effect of the finite collar conductivity. The drill collar's varying diameter and finite conductivity produce small but measurable effects in the phase shift (0.1 degrees) and the attenuation (0.2 dB) measured between the receivers. The PC model is used to calculate the transforms from measured phase shift and attenuation to Rps and Rad. Phase shift and attenuation are calculated for many resistivity values and for the dielectric constant. Phase shift and attenuation are monotonic functions of resistivity and are inverted to obtain the transforms. The second model is the uniform-collar (UC) model. It includes the borehole and surrounding rock formations but assumes a constant-diameter drill collar (Fig. 2). The coils are contained in an insulating layer surrounding the collar. The UC model is less accurate than the PC model. The UC model handles up to three beds with arbitrary invasion, a borehole, and washouts. It is used for environmental studies and to generate correction charts for bore-holes, bed thickness, and invasion.