Auxiliary Sensor-Based Borehole Transient Electromagnetic System for the Nondestructive Inspection of Multipipe Strings

Abstract

Transient electromagnetic (TEM) techniques are widely used in the field of geophysical prospecting. In borehole detection, the nondestructive inspection (NDI) of a metal pipe can be performed efficiently using the properties of eddy currents. However, with increasing concern for safety in oil and gas production, more than one string of pipe is used to protect wellbores, which complicates data interpretation. In this paper, an auxiliary sensor-based borehole TEM system for the NDI of multipipe strings is presented. On the basis of the characteristics of the borehole TEM model, we investigate the principle behind the NDI of multipipe strings using multiple time slices of induced electromotive force (EMF) in a single sensor. The results show that the detection performance of NDI is strongly influenced by eddy-current diffusion in the longitudinal direction. To solve this problem, we used time slices of the induced EMF in both the main and auxiliary sensors. The performance of the proposed system was verified by applying it to an oil well with a production casing and liner. Moreover, field experiments were conducted, and the results demonstrate the effectiveness of the proposed method.

A Uniform Linear Multi-Coil Array-Based Borehole Transient Electromagnetic System for Non-Destructive Evaluations of Downhole Casings

“…The transmitter and the M receivers are located in the second layer, with their number of turns given by N T and N R , respectively, where each receiver has the same number of turns. In this paper, M receivers are utilized instead of the traditional single receiver [ 13 , 14 , 15 , 16 , 17 ] with the same total number of receiving coil turns of MN R . Thereby, with a large value of M , N R , as well as N T , can be small enough to ensure that the change in the TRD for each single receiver can be ignored and treated approximately as a single value.…”

“…In other words, the three variables in Equation (7) are coupled together in the TEM response. Of these three variables, it is the coupling between t and d , also known as the eddy-current diffusion or the time-domain property, that offers several benefits to the TEM system to achieve better performance [ 16 , 25 ]. Conversely, when a single receiver with a large number of turns or a multi-coil array is used, the TRD will strongly influence the interpretation of the NDE, where the change in the TRD cannot be ignored and must be compensated for to avoid model distortions in the borehole TEM system.…”

“…Obviously, if the receiving data of the eight receivers are directly summed (the arithmetic mean), the resolution of the casing inspection will be substantially decreased due to the shift in the receiving data along the borehole axis with respect to the different TRDs. However, comparing the early and late time data in Figure 4 and Figure 5 , we find that, even though the early time data of the TEM system have better resolution than the late time data [ 16 ], the shifting also appears more seriously, which means that the size of this type of shifting is related not only to the TRDs and the inter-element spacing but also to the eddy-current diffusion range. Therefore, it will be difficult to compensate for the shifting via an ordinary curve alignment for all the diffusion times (sampling times).…”

“…In Ref. [ 16 ], multi-pipe strings, which are required to protect the oil and gas wells from by-products for safety considerations, but which make NDEs more difficult in terms of data interpretation [ 15 , 17 ], were inspected using the eddy-current diffusion properties of a TEM system with a multi-turn coil-based auxiliary sensor in longitudinal direction. The above methods have been proven to be effective for improving the NDE performance in downhole casings.…”

“…At present, there have been minimal previous attempts to eliminate the influence of TRD on downhole measurements, which is difficult to suppress using the traditional multi-turn coil sensor with single-receiver [ 13 , 14 , 15 , 16 , 17 ], since the TRD is coupled with the electrical and geometrical parameters in only one output. On the basis of Doll’s theory in induction logging system [ 18 ], the TRD was replaced by two distances from the transmitting and receiving coils to “an elementary ring”.…”