Electromagnetic Propagation Logging While Drilling: Theory and Experiment

Abstract

The compensated dual resistivity (CDRSM) tool is an electromagnetic 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. IntroductionThe CDR tool 1 is a 2 ~ 106-cycles/sec electromagnetic propagation tool 2 . 3 built into a drill collar. This drill collar is fully selfcontained 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, R ps ' and attenuation is transformed into a deep measurement, Rad' The CDR tool has several new and important features. I. Rad and Rps provide two depths of investigation and are used to detect invasion while drilling. For example, in a I-a· m formation, the investigation diameters (50% response) are 30 in. for Rps and 50 in. for Rad' 2. ROil 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: I 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 verif'y 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 experimentaly 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 boreholes, 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 verif'y the predictions of the second model. Finally, CDR logs are compared to wireline logs to demonstrate the new features.

Pseudo-Analytical Modeling for Electromagnetic Well-Logging Tools in Complex Geophysical Formations

“…Both R-NMM and A-NMM approach can not be easily generalized to modeling sources inside such indentations due some enforcements of the NMM. In practice, however, we can use a tool calibration as a reference to proper approximate the real-world LWD tool response, as described in [34]. To be more clear, in geophysical prospecting using LWD tools the parameters of interest are the amplitude ratio (AR) and phase difference (PD) between the voltages at two antennas [6,34]:…”

A Numerical Mode-Matching Method Based on Multiple-Knot Cubic B-Splines Applied to the Analysis of Well-Logging Tools

“…This problem was considered before in [22,23] and is examined here to validate our formulation and illustrate the ability of the proposed algorithm based on MK cubic B-splines to efficiently model this kind of problem. The CDR sensor operates at 2MHz and is composed by two transmitters and two receivers as depicted in Fig.…”

“…11, inside a 3.25-inborehole with resistivity 0.1 Ω.m, surrounded by a soil formation with resistivity 2 Ω.m. For further details on this geometry, see [22,23].…”

Dielectric Constant Properties of Tight Sandstone

“…Most published studies have focused on the properties of the dielectric constant at high frequency, usually in the range of kHz to MHz, or even GHz. [8][9][10][11][12][13] However, the phenomenon of low-frequency dispersion(LFD), which is widely observed in many materials, was not been properly recognized until its discovery by the Chelsea Dielectrics Group 14 and the investigation of its properties with increasing precision, as described in the book Dielectric Relaxation in Solids(DRS). Earlier studies proved that most rocks and minerals show dielectric dispersion in the low-frequency region from a few Hz to several MHz.…”