Ralph M. D'Angelo

Ultrasonic measurements on hydrating cement slurries: Onset of shear wave propagation

Abstract The ultrasonic properties of three oil field cement slurries are studied during the early stages of the hydration process. As a percolating solid framework is established, the slurries develop mechanical integrity and shear waves begin to propagate. This transition is also apparent in the behavior of the compressional wave, but is considerably more clear-cut in the shear signals. For the three samples examined here, the ratios of the shear wave onset times are in good agreement with the corresponding ratios of the American Petroleum Institute (API) thickening times.

Stress‐induced dipole anisotropy in a dry Berea sandstone

Tectonic stress-induced dipole anisotropy exhibits a crossover n the two principal flexural wave slowness dispersions oriented parallel and normal to the far-field uniaxial compressive stress direction. This crossover phenomenon is a result of borehole stress concentrations, and can be employed in a new technique for distinguishing stress-induced anisotropy from intrinsic anisotropy. Theoretical modeling and laboratory measurements have been made on a large block of dry Berea sandstone subject to a uniaxial stress of up to 5 MPa. The two flexural dispersions are obtained by Prony’s processing of an array of waveforms for dipole orientations parallel and normal to the stress direction. The theoretical dispersions in the presence of biasing stresses are obtained from the solution of equations of motion for small dynamic fields superposed on a static bias. Good agreement has been obtained between the theoretical predictions and measured dispersions including the crossover phenomenon a feature that is exclusively due to the stress-induced anisotropy.

Theoretical and experimental investigations of acoustic waves in embedded fluid-solid multi-string structures

Current acoustic measurements provide viable inspection for single cased wells, yet their interpretation for complicated multi-string wellbores where, for instance, two or more nested steel strings are deployed, is largely hampered by a lack of knowledge of the measured acoustic wave fields. This letter reports on theoretical and experimental investigations of the acoustic wave propagation in fluid-filled double string systems. Experimental measurements show excellent agreement with the theoretical predictions by a Sweeping Frequency Finite Element Method. The results lead to the identification of acoustic signatures that are crucial for an effective diagnosis of cement conditions in double-string cased wellbores.

Three wave mixing test of hyperelasticity in sedimentary rocks

We report measurements of 3‐wave mixing amplitudes on solids whose 3rd order elastic constants have also been measured by means of the elasto‐acoustic effect. Because attenuation and diffraction are important aspects of our measurement technique we analyze our results using a frequency domain version of the KZK equation, modified to accommodate an arbitrary frequency dependence to the attenuation. We find that the value of β so deduced for polymethylmethacrylate (PMMA) agrees quite well with that predicted from the stress‐dependent sound speed measurements, establishing that PMMA may be considered a hyperelastic solid, in this context. The β values of sedimentary rocks, though they are typically two orders of magnitude larger than e.g. PMMAs, are still a factor 3‐10 less than those predicted from the elasto‐acoustic effect. Moreover, these samples exhibit significant heterogeneity on a centimeter scale, which heterogeneity is not apparent from a measurement of the position dependent sound speed.

Ultrasonic borehole velocity imaging

We describe a new technique for making high-resolution velocity images of rocks using refracted ultrasonic waves. The use of refracted waves makes this technique potentially suitable for imaging borehole walls. In the laboratory, we use a single-transmitter, two-receiver, first-arrival method for making velocity measurements, with a spatial resolution on the order of 1 cm. Our acoustic pulses are centered near 200 kHz. Scans of a borehole wall reveal dipping thin layers and fractures. When external stress is applied perpendicular to the borehole, stress concentrations appear on our images as axial bands of high and low velocities. Breakouts created by high stress also can be imaged. On a planar sample, a velocity image reveals shale laminations and carbonate stringers. For field applications, this technique offers the potential for imaging in both conductive and nonconductive muds and provides images based on a physical property velocity that currently is not used for fine-scale borehole imaging.

Slow waves, boundary conditions, and reflection and transmission coefficients

Biots theory of acoustics in fluid‐saturated porous media predicts a “fast” compressional wave, a unique “slow” compressional wave, and a shear wave. Experimental work by the authors demonstrated that Biot theory correctly predicts the measured speeds of all three waves. The boundary conditions, however, which must include an additional condition beyond those postulated by elasticity theory to describe the “slow” wave, have not been adequately studied experimentally. Measurements have been made of reflection and transmission coefficients (including velocities and attenuations) at the plane interfaces between water and a water‐filled porous slab at ultrasonic frequencies. At these frequencies, the slow wave is a propagating mode and is readily measurable, but only if the porous medium has a carefully chosen pore structure. By using either an open pore surface or a sealed (thin layer of epoxy) surface, the excitation of the slow wave can be controlled. Slow waves are generated in the solid even when the in...