Simulation and modeling of ultrasonic pitch-catch through-tubing logging

Abstract

Cased petroleum wells must be logged to determine the bonding and hydraulic isolation properties of sealing material and to determine the structural integrity status. While ultrasonic pitch-catch logging in single-casing geometries has been widely studied and is commercially available, this is not the case for logging in doublecasing geometries despite its increasing importance in plug and abandonment operations. It is therefore important to investigate whether existing logging tools can be used in such geometries. Using a finite element model of a double-casing geometry with a two-receiver pitch-catch setup, we simulated through-tubing logging, with fluid between the two casings. We found that there appears a cascade of leaky Lamb wave packets on both casings, linked by leaked wavefronts. By varying the geometry and materials in the model, we examined the effect on the pulse received from the second wave packet on the inner casing, sometimes known as the third interface echo. The amplitude of this pulse was found to contain information on the bonded material in the outer annulus. Much stronger amplitude variations were found with two equally thick casings than with a significant thickness difference; relative thickness differences of up to 1/3 were simulated. Finally, we developed a simple mathematical model of the wave packets' time evolution to encapsulate and validate our understanding of the wave packet cascade. This model shows a more complex time evolution in the later wave packets than the exponentially attenuated primary packet which is currently used for single-casing logging. This indicates that tools with more than two receivers, which could measure wave packets' amplitude at more than two points along their time evolution, would be able to draw more information from these later packets. The model was validated against simulations, finding good agreement when the underlying assumptions of the model were satisfied.

Analysis of outer-casing echoes in simulations of ultrasonic pulse-echo through-tubing logging

“…The time step dt was chosen from Courant number considerations so that c P,steel /(dx max /dt) = 0.4, c P,steel being the speed of sound in steel. We validated that this resolution is sufficient in a similar study, Viggen et al (2016). SIMULATION RESULTS For our baseline case, the received signal S(t) is shown in Figure 3 along with the FIE signal S FIE (t) from the auxiliary model and the isolated TIE signal S TIE (t) = S(t) − S FIE (t).…”

“…Furthermore, the FEM simulations do not require a cylindrically symmetric centered geometry unlike the analytically based models (though Zeroug (2004) indicates that it is possible to approximate eccentering in such models as well). The motivation of using this particular software and FEM over other simulation methods is the same as in Viggen et al (2016), with the additional benefit that the FEM can fit curved geometries well using unstructured meshes, thus avoiding the "staircase approximation" of finite difference methods. The simulated cross-section is shown in Figure 1.…”

“…Zeroug and Froelich (2003) and Viggen et al (2016). The pitch-catch technique has the advantage of the payload signal typically being strong, distinct, and not obscured by another signal; see also Klieber et al (2015).…”

“…It also compares the difference in TIE arrival times to the predictions of Equation 5 for the eccentered cases and the centered case. Zeroug and Froelich (2003), Klieber et al (2015), and Viggen et al (2016) that there is typically not a significant overlap between the components corresponding to interaction with the inner casing and those corresponding to interaction with the third interface and beyond. This is in contrast to the pulse-echo technique studied here, where the strong FIE and the weaker TIE components overlap, making them more difficult to separate.…”

“…We therefore wish to evaluate the possibility of determining information on cement bonding and hydraulic isolation behind multiple casings using current logging technologies. As described in more detail elsewhere (Viggen et al (2016)), we are particularly interested in ultrasonic and sonic technologies as they can provide single-casing logs with high azimuthal resolution. We wish to determine the potential of these technologies, or their lack of it, for multiple-casing logging.…”