A predetermined condition in a fluid-filled wellbore system can be detected by generating at least one sound in the wellbore system in response to the condition, such that a detectable change is created in some characteristic of the emitted sound, and detecting the at least one sound and the change, the detection being indicative that the predetermined condition has occurred. Equipment for facilitating detection of the condition can include a trigger operable in response to the condition; a generator operable to emit sound in the borehole and to create a detectable change in some characteristic of the emitted sound in response to the trigger; and at least one sensor operable to monitor the sound and detect the change, the detection being indicative that the predetermined condition has occurred. It is also possible to estimate a value of a property of a fluid-filled wellbore system. This can be accomplished by recording data including at least one of pressure and rate of flow at one or more locations in the wellbore system, and then estimating the value of the property by employing a model for predicting at least one of pressure and rate of flow dependent upon parameters detailing at least one of wellbore system geometry, viscoacoustic properties of the fluid and entrained solids contained in the wellbore system, locations of boundaries and entrained solids, and characteristics and locations of disturbances to pressure and flow in the wellbore system, in order to determine a best prediction of some attribute of the recorded data.

Wireless logging of fluid filled boreholes

Tubewaves are used to transmit an indication of the depth at which a condition is detected in a well. In particular, the depth is calculated based on the difference in arrival time at the surface of a first tubewave which propagates directly upward in the borehole and a second tubewave which initially travels downward and is then reflected upward. The tubewaves may be generated by a canister designed to implode at a certain pressure. After being introduced into the flowline at an above ground inlet, the canister is carried downhole by gravity and the fluid being pumped. When the canister reaches a depth at which its pressure tolerance is exceeded, it implodes and generates the tubewaves. An analyzer at the surface detects the tubewaves with a hydrophone array and generates a pressure versus depth profile of the well, i.e., one data point for each implosion. Canisters may be acoustically tagged by controlling volume and orifice size in order to generate tubewaves having particular frequency and amplitude characteristics. Canisters may also be configured to produce multiple implosions, e.g., one implosion at each of a selection of different pressures. Canisters may also be equipped with triggering and arming mechanisms, and may generate tubewaves in response to conditions other than a particular pressure.

Determination of downhole pressure while pumping

Submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy at the
Massachusetts Institute of Technology and the
Woods Hole Oceanographic Institution February 2005

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Inversion for subbottom sound velocity profiles in the deep and shallow ocean

In this paper, a fast approach for estimating sediment sound speed in a shallow-water environment is developed. Under certain assumptions, this algorithm recovers the seabed sound-speed profile using pressure field measurements at low frequencies. The geometry of the problem involves measuring the pressure at horizontally placed hydrophones in the water column. The Deift–Trubowitz integral equation is then solved. This work introduces two methods for this task. The first is a modified Born approximation that builds upon a standard first-order approximation; the second is based on interpolating the integrand. It is shown with synthetic data that these methods work well with successful sound-speed estimation and identification of sharp discontinuities in sound speed. Although the methods are stable and effective with noise-free data, problems arise when noise contaminates the acoustic field. Regularization approaches, reducing the disruptive effect of singular points and smoothing a measured reflection coefficient, are developed to remedy this problem, leading to improved results in noisy environments. In addition to providing sound-speed estimates, the method also computes sediment thickness. This feature is of particular interest, since it makes the method suitable as a preprocessing step providing useful information to other inversion methods. Sensitivity analyses demonstrate that some assumptions required for the approach implementation are not restrictive.

A Direct Method for the Estimation of Sediment Sound Speed With a Horizontal Array in Shallow Water

Reservoir property estimation incorporating prestack inversion and rock-physics theory

A shallow-water ocean acoustics experiment is proposed from which it is possible to determine in principle the scattering data necessary to recover the sound-speed profile in the ocean bottom. These data are a “reflection coefficient,” which is not the usual one. The reflection coefficient is amplitude of the outgoing wave at infinite depth due to a unit amplitude plane wave incident from infinite depth. The unusual fact is that this reflection coefficient can be recovered from a measurement in the ocean. The essential assumptions are that the sound-speed profile is known in the ocean but unknown in the bottom, and that the measurements are made in the ocean layer.

A shallow-water ocean acoustics inverse problem

An imploding cavity in a fluid‐filled borehole is potentially attractive as a powerful source of acoustic energy for borehole seismic experiments. It is reasonable to model the fluid pressure response due to the implosion as purely one‐dimensional. A simple analysis based on the assumption that the fluid behaves elastically, with a linear pressure/strain relation, predicts that the fluid pressure response becomes negative at depths less than that of the source. This is an untenable prediction since a fluid cavitates under negative pressure. It is more realistic to assume that the fluid behaves in a linear elastic fashion for negative strains (and positive pressures), and that the fluid pressure is zero for positive strains. This leads to a nonlinear, free‐boundary problem in acoustic wave propagation, which surprisingly has a closed‐form analytical solution. The analysis predicts the existence of several unexpected phenomena which could complicate the interpretation of data taken in borehole seismic experiments utilizing this type of source.

One‐dimensional implosions under gravity‐induced hydrostatic pressure

The effects of temporal variations of the sound velocity on acoustic propagation from a time periodic point source in a layer of uniform depth h are studied. It is assumed that the sound speed is a slowly varying function of time, and that the upper and lower surfaces of the layer are free and rigid, respectively. An asymptotic expansion of the acoustic pressure field is obtained by using the methods of normal modes, two times and stationary phase evaluations of multiple integrals. The small parameter e that is used in the analysis is the ratio of the time scale of variation of the sound velocity to the frequency of the source. To lowest order in e, the acoustic pressure is given by the sum of a primary wave and a secondary wave. The primary wave consists of a finite modal sum of propagating waves and an infinite modal sum of evanescent waves. If the dimensionless sound speed oscillates about the value 1/am, where am is the propagation number of the mth mode, then the mth mode alternates between propagati...

Sound propagation in a temporally varying waveguide

A shallow water ocean acoustics experiment is described from which it is possible to determine in principle the scattering data necessary to recover the sound speed in the ocean bottom. This data is a ‘‘reflection coefficient.’’ The essential assumptions are that the speed in the ocean is known but unknown in the bottom and that the measurements are made in the ocean layer.

David C. Stickler

Papers

A shallow-water ocean acoustics inverse problem

One‐dimensional implosions under gravity‐induced hydrostatic pressure

Sound propagation in a temporally varying waveguide

A shallow water ocean acoustics inverse problem