Compensation effect analysis in DIE method for through-casing measuring formation resistivity

The measuring technique based on Double-Injection-Electrodes (DIE) and its compensation arithmetic method have been proven to be very useful for eliminating the errors caused by electrode-scale mechanical tolerances in formation resistivity measurement through metal case. In this paper, we found that even minor casing joint or casing corrosion may deteriorate the measurement accuracy. Based on theoretical analysis and self-adaptive goal oriented hp-Finite Element (FE) simulations, the compensation effects of DIE measurement technique were estimated. The calculated results from this measuring method are always close to the real formation resistivity, regardless of whether the metal casing is ideal or not. Meanwhile, large errors occur when recording measurements based on Single-Injection-Electrodes (SIE), since the calculated formation resistivity may provide negative values when casing joint or casing corrosion exists. The Double-Injection-Electrode (DIE) measurement technique is predicted to have good compensation effects in many non-ideal situations with uneven metal casing besides electrode-scale mechanical tolerances.MTM2010 1651

New post processing method for interpretation of through casing resistivity (TCR) measurements

We propose a new iterative method for post processing through casing resistivity (TCR) measurements. This method can be seen as a correction or extension of Kaufman´s theory to more complex scenarios, in which the casing is no longer assumed to be uniform along the axis of the borehole. It can also be seen as a simplified inversion method based on the main physic al principles of TCR measurements. To derive our post processing method, we first use the method of images, which enable us to introduce all key factors involved on TCR measurements, including casing length, casing radius, casing thickness, formation resistivity, location of injected current and resistivity distribution of formation layers . Then, we design our iterative Post processing Method by simplifying the complex formulation resulting from using the method of images. Numerical results using goal oriented hp FEM simulations show that our correction method provides a more accurate approximation to the actual formation resistivity than that predicted by Kaufman´s theory. In addition, the new method is pr oven to be very efficient and robust, since it is independent of spatial resistivity variations in the formation.MTM2010 1651