Bikash K. Sinha

Doubly rotated contoured quartz resonators

Doubly rotated contoured quartz resonators are used in the design of temperature-compensated stable clocks and dual-mode sensors for simultaneous measurements of pressure and temperature. The design of these devices is facilitated by models that can predict frequency spectra associated with the three thickness modes and temperature and stress-induced frequency changes as a function of crystalline orientation. The Stevens-Tiersten technique for the analysis of the C-mode of a doubly rotated contoured quartz resonator is extended to include the other two thickness modes. Computational results for harmonic and anharmonic overtones of all three thickness modes of such resonators help in optimizing the radius of curvature of the contour and electrode shape for suppression of unwanted modes and prevention of activity dips. The temperature and stress-induced changes in thickness-mode resonator frequencies are calculated from a perturbation technique for small dynamic fields superposed on a static bias. The static bias refers to either a temperature or stress-induced static deformation of the resonator plate. Phenomenological models are also used for calculating the temperature and stress-induced changes in resonant frequencies as a function of crystalline orientation. Results for the SBTC-cut quartz plate with a spherical convex contour of 260 mm indicate that normal trapping occurs for the third (n=3) and fifth (n=5) harmonic of the A-mode, the fundamental (n=1) and third (n=3) harmonic of the B-mode, and the fundamental (n=1) and fifth (n=5) harmonic of the C-mode.

Rounded wall surface acoustic wave sensors

Disclosed is an externally loaded pressure sensor using a pair of SAW devices formed on the wall of an interior cavity. The difference frequency of these devices is used as a temperature-compensated measure of hydrostatic pressure on the sensor exterior. The cavity wall immediately adjacent the long sides of at least one of the SAW devices is rounded, to improve pressure sensitivity without unacceptable increase in stresses or other adverse effects.

Long-term stability and performance characteristics of crystal quartz gauge at high pressures and temperatures

Good long-term stability of high precision quartz pressure sensors is necessary for various applications ranging from pressure transient analysis to permanent monitoring systems for optimal reservoir management in the petroleum industry. A crystal quartz gauge (CQG/sup 1/) is a dual-mode, thickness-shear, quartz pressure sensor that has been used in oil field services for the past 8 years. High accuracy, resolution, and fast response time of this sensor enable a reliable estimate of formation permeability and oil/water interfaces in reservoirs that help reduce the overall cost of oil and gas production. The sensing resonator characteristics can be described in terms of equivalent circuit parameters (motional resistance and capacitance), resonator-Q and the short-term frequency stability of both the B- and C-modes of vibrations at various temperatures. The pressure reading errors of manufactured gauges are less than 8.89 kPa (1 psi) (plus 0.01% of the reading because of the uncertainty of the dead-weight tester). The pressure resolution is better than 20.7 Pa (0.003 psi) over a 1-s gate time. An extremely effective dynamic compensation algorithm yields corrected pressure readings with a very fast response time as short as a strain-gauge-based pressure transducer while retaining the high performance of a quartz gauge. Recent long-term stability tests of CBGs show a negligibly small drift of the order of a few tenths of 1 psi (0.1 psi=689 Pa) at 103 MPa (15 kpsi) and 175/spl deg/C for a period of more than 1 yr. These results confirm that the CQG characteristics exceed the demanding specifications for both the well tests and permanent monitoring systems.

Thin-film induced effects on the stability of SAW devices

Measurements show an upward shift on the order of 50 ppm in the resonant frequency of a surface acoustic wave (SAW) resonator, as taken before and after the device is hermetically sealed in vacuum following a certain glass-frit sealing process. The authors analyze some of the thin-film phenomena that are potential sources of the observed frequency shift and that may affect the long-term stability of such devices. Various factors contributing to the shifts include: 1) intrinsic or structural stresses in the bonding layers as well as in the interdigital transducer (IDT) fingers; 2) thermal stresses due to the differences in thermal expansion coefficients of the metallic IDT fingers and the bonding agent (glass frits) from those of quartz; 3) partial oxidation of the IDT fingers and transmission lines during the frit glazing process; and 4) possible metal diffusion into quartz. Quantitative estimates of the contribution of two factors to the total observed frequency shift after a certain glass-frit sealing process are provided. Rough estimates of the frequency shifts due to the oxidized film are made from the dispersion curves for a uniform thin aluminum film and for its oxide film as fully plated on a quartz substrate. It is concluded that the results may provide a way of estimating the magnitude of the intrinsic stress for a given long-term stability of the SAW device.<<ETX>>

Near-Wellbore Alteration and Formation Stress Parameters Using Borehole Sonic Data

Highly depleted reservoirs exhibit sharply lower po re pressures and horizontal stress magnitudes than in the overlying shaly formation. Drilling through such de pleted reservoirs can cause severe fluid loss and drilling -induced wellbore instability. Accurate and reliable estima tes of horizontal stresses can provide early warning of im pending drilling problems that may be mitigated by appropri ate drilling fluid design and drilling practices. We have develo ped a new multi-frequency inversion algorithm for the estimat ion of maximum and minimum horizontal stress magnitudes using cross-dipole dispersions. Borehole sonic data for t he case study presented in this paper was acquired by a cro ss-dipole sonic tool in a deep-water well, offshore Louisiana in the Gulf of Mexico (GOM). The logged interval spans 1000 ft below the casing shoe. In addition, the Modular Dynamic T ester (MDT) 1 mini-frac tests were performed at three depths in shale, yielding two minimum horizontal stress magnitudes. The borehole sonic data was suitable for inversion of crossdipole dispersions at three depths in shale and als o at a depth in a highly depleted sand reservoir. There was one depth in shale above the depleted sand where we could estimate the minimum horizontal stress magnitude using both the MDT mini-frac tests and inversion of borehole sonic dat a. The results of the two techniques are consistent, provi ding encouragement for further validation of the multi-f requency inversion of cross-dipole dispersions to estimate h orizontal stresses. Even though the overburden stress is expe cted to increase with depth, both the maximum (SHmax) and minimum (Shmin) horizontal stresses obtained from the inversion of borehole sonic data are significantly smaller in the depleted sand than in the overburden shale. How ever, both the horizontal stress magnitudes increase again in the shale below the depleted sand. Such rapid variations in h orizontal stress magnitudes cause large fluctuations in the s afe mud weight window. This challenge in drilling through t he depleted sand was successfully handled by using spe cial drilling fluid to mitigate seepage losses and diffe rential sticking in the depleted sand and overlying shale. We have also performed Dipole Radial Profiling (DRP) of for mation shear slownesses using the measured cross-dipole di spersions at three depths in shale and one in the highly depl eted sand. Analysis of radial profiles in the two orthogonal d irections indicates plastic yielding or stiffening of rock in the nearwellbore region. While plastic yielding increases t he shear slowness, stiffening would reduce the shear slownes s.

Acoustic guided waves in cylindrical solid-fluid structures: Modeling with a sweeping frequency finite element method and experimental validation

Modeling and understanding the complex elastic-wave physics prevalent in solid-fluid cylindrically-layered structures is of importance in many NDE fields, and most pertinently in the domain of well integrity evaluation of cased holes in the oil and gas industry. Current sonic measurements provide viable techniques for well integrity evaluation yet their practical effectiveness is hampered by the current lack of knowledge of acoustic wave fields particularly in complicated cased-hole geometry where for instance two or more nested steel strings are present in the borehole. In this article, we propose and implement a Sweeping Frequency Finite Element Method (SFFEM) for acoustic guided waves simulation in complex geometries that include double steel strings cemented to each other and to the formation and where the strings may be non-concentric. Transient dynamic finite element models are constructed with sweeping frequency signals being applied as the excitation sources. The sources and receivers disposition ...