Invasion Profiling With a Multiple-Depth-of-Investigation Electromagnetic-Wave Resistivity Sensor

Abstract

Summary The EWR-Phase 4SM sensor is the first measurement-while-drilling (MWD) electromagnetic resistivity tool to offer multiple depths of investigation by use of measurements made from signals originating at different distances from the receivers. As a propagating-wave resistivity device, it measures four phase shifts and four attenuations. This yields eight different apparent depths of investigation, allowing more meaningful information about the invasion profile. With the phase-shift measurements alone, the traditional three-parameter step invasion model can be used to obtain the virgin formation resistivity, Rt; the flushed zone resistivity, RXO; and the diameter of invasion, Di. These same four measurements can also be interpreted with a ramp invasion model, a sloped-transition invasion model, and a continuous-monotonic invasion model. For more complex invasion profiles, such as an annular invasion profile, all eight measurements can be used to describe the invasion profile accurately. Invasion profiling starts with an examination of the trends among measurements. These trends are used to select one or more invasion model(s) to use in the interpretation (step, ramp, sloped transition, continuous monotonic, or annulus). Once the form of a model has been selected, the model parameters are computed by use of a modified Levenberg-Marquardt algorithm, which minimizes the sum of the squares of the differences between the measured and reconstructed resistivities. Examples are presented that compare the results of interpreting invasion following one of the previously stated invasion profiles with one of the others as a reference model. Guidelines on the use of these models are given. Finally, some field examples are analyzed and presented.