Approximate effects of off‐center acoustic sondes and elliptic boreholes upon full waveform logs

Abstract

Full waveform acoustic well logging has become instrumental to hydrocarbon exploration because of its ability to determine in situ velocity information for P and S waves as well as the attenuation (or absorption) of seismic energy. It is therefore important that the factors infiuencing these logs be understood. In addition to formation properities, Poisson's ratio, the borehole environment, and tool centering can affect the full waveform acoustic logs. These latter factors are evaluated through modeling of elliptic boreholes and de-centralized tools in the borehole. INTRODUCTIONA number of factors affect the observed full waveform acoustic log amplitudes. Formation properities play an important role. In addition the borehole environment can cause variations. For example, borehole rugosity impedes the propagation of head waves (Morris et aI., 1964). In areas of' cavitation and washouts, critical refraction may be prevented, resulting in low amplitude waveforms. Lebreton et al. (1978) report a strong reduction in waveform amplitude proportional to an increase in mud cake thickness.Centering of the sonde and a cylindrical borehole are crucial to waveform amplitudes and arrival times. In most full waveform logging systems the tool is held centralized in the borehole by several bow type springs which press against the formation. In other systems, rubber "fingers" keep the tool centralized. The reliability of these methods in keeping the tool centralized in holes with a true circular cross section as well as in irregular holes has not been well documented. The actual amount of "rattle" that the tool experiences is not generally known.Morris estimates that moving the source one quarter of an inch (0.64 cm) off-center decreases the P wave amplitude by one half (Morris et al., 1964). This is a significant effect for such a small displacement. In experiments with a tool placed in a trough to simulate a cased borehole it was determined that moving the centralized source completely against the casing not only decreases the amplitude of the recorded waveform but also increases its complexity and reverses the apparent polarity of the first motion (Riddle, 1962). Studies by Forristall and Ingram (1969) show that an off-centered condition for a fiuid cylinder yields two types of refiected arrivals: one associated with the source waveform, and the other with its derivative.

Simulation of wireline sonic logging measurements acquired with Borehole–Eccentered tools using a high-order adaptive finite-element method

“…Due to the complexity involved in these type of numerical simulations, previous related work is rather limited. In particular, we highlight the pioneering works of [1][2][3][4][5][6] in the last part of the twentieth century, and some numerical results for logging-while-drilling (LWD) tools [7,8], for dipole, and quadrupole source excitation [9]. This paper introduces a robust, efficient, and accurate simulation method based on a Fourier series expansion over a particular system of coordinates combined with a two-dimensional (2D) hp-adaptive finite-element method (FEM) [10,11], where h denotes the element size and p the polynomial order of approximation, both varying locally throughout the grid.…”

Influence of borehole‐eccentred tools on wireline and logging‐while‐drilling sonic logging measurements

“…Numerical studies in this area include the pioneering results obtained by (Roever, Rosenbaum and Vining 1974; Willis, Toksöz and Cheng 1983), where analytical methods were applied to investigate the influence of an off‐centred point source in an open borehole. This approach was extended to dipole sources (Leslie and Randall 1990; Schmitt 1993), cylindrical tool modelling (Zhang, Wang and Dong 1996) and quadrupole sources (Byun and Toksöz 2006).…”