Efficient simulation of transient electromagnetic measurements in the presence of a metal mandrel

Abstract

Summary Many wireline induction and logging while drilling (LWD) resistivity tools basically comprise transmitter and receiver coils wound over a metal mandrel/pipe. Transient electromagnetic (EM) LWD measurements have excellent potential in proactive geosteering with look-ahead capability. Additionally, borehole transient EM measurements are extremely promising for monitoring oilwater fronts in the reservoir from a single well. These applications also involve the use of wire coils in the presence of metal collars/casing. The design and analysis of such instruments involves extensive modeling and 3D simulation of transient fields because of EM diffusion in the vicinity from a step excitation of the transmitter. The simulation of EM fields is complicated by the presence of the metal mandrel, which is typically more than 10 6 times more electrically conductive than the surrounding formation. The finite element method (FEM) is a popular technique for the numerical solution of Maxwell’s equations, especially to account for minute details of the sensor design. The mesh size in any material medium is governed by the skin depth of that material for a given frequency of excitation. Analogously, for transient simulations, the required mesh size depends on the diffusion distance in the medium. The metal mandrel has extremely small skin depth compared to the size of the computational domain, owing to its extremely large conductivity. Conventional application of the finite element method in this case leads to an intractable problem because of the huge number of nodes. This paper presents a novel technique for efficient use of the finite element method for 3D simulation of the transient EM diffusion process in the presence of a metal mandrel and other highly conductive objects. The technique exploits time-harmonic solutions at a number of frequencies, with (1) coarse mesh and computation of fields both outside and inside the mandrel at low frequencies, and (2) fine mesh and computation of fields outside the mandrel only at high