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Ensuring the integrity of oil and gas cased-wellbores is of crucial importance to operators around the world to maintain safe production and environmental compliance. While sonic and ultrasonic measurements have been developed for evaluating the well integrity of single casing strings, there is currently a gap in characterizing the cement and annular fill behind two or more casing strings due to the lack of sensitivity of the ultrasonic tools and the complexity of the sonic response. This abstract describes an experimental study to characterize the sonic response and help fill this gap in conjunction with a theoretical modeling effort that is reported elsewhere. Scaled laboratory experiments are conducted to acquire reference data used to verify the modeling approach developed to predict the guided modal characteristics of axially-propagating waves in concentric and non-concentric cylindrical structures immersed in fluid. These structures simulate the geometries and environment encountered downhole when conducting acoustical measurements to evaluate the cementation of oil and gas cased wells. Typically, cement is expected to fill the annular space between the outer string and rock formation. Nevertheless, cement damages such as fluid channels, partial cement coverage, and weak bonding interfaces, which can compromise well safety as well as environmental compliance, inevitably occur due to the harsh downhole environment and downhole operations. Measurements are made in a dual-string steel pipe of quarter scale in an immersion tank or in a sandstone formation with concentric as well as a variety of eccentric configurations. A set of samples are prepared with the cement in annulus B (the space beyond the outer casing) with purposefully introduced fluid channels in a controlled manner. A comparison of the experimental and numerical results indicates features in the sonic response that are indicative of channels in annulus B as distinguished from eccentricity effects. These results suggest a new well integrity evaluation work-flow to characterize cement quality and coverage in annulus B taking advantage of the rich acoustic features in the sonic response. [ABSTRACT FROM AUTHOR]

Cement evaluation in cased oil and gas wells is important to ascertain well integrity before putting the well on production. It is conducted through the use of acoustic measurements, in particular an advanced ultrasonic tool with two modalities: pulse-echo and pitch-catch with the latter relying on the excitation and detection of the steel casing-thickness flexural mode. When the annular fill behind the casing is comprised of a solid with a compressional (P) wave velocity that intersects the dispersive flexural mode phase velocity curve within the signal frequency bandwidth, phase matching to a headwave at the casing-solid occurs and we observe an additional contribution that follows closely the arrival of the casing flexural mode: a feature we refer to as a clinging P. We propose to use this occurrence exhibited in the raw waveforms to our advantage, as a basis for a quantitative diagnosis of the solid behind the casing. To leverage this effect in a non-assisted robust (online) diagnosis, we use a machine learning based workflow that automates the detection of clinging P arrivals in the flexural wave data. The automated workflow highlights axial and azimuthal regions, where the annular fill behind casing is confirmed to be a solid with additionally a tight range for its compressional wave velocity - therefore rendering the diagnosis of the tool measurement more quantitative and more autonomous than delivered with existing inversion schemes. [ABSTRACT FROM AUTHOR]

Acoustics enjoys a wide array of applicability in the Oil and Gas industry. Signals with very low-frequencies (tens of Hertz) are routinely used on surface to image the earth subsurface delineating hydrocarbon reservoirs while signals with mid-frequencies (thousands of Hertz) to high-frequencies (hundreds of kilo Hertz) are used in deep boreholes to probe rock mechanical properties and evaluate completion hardware. This article reviews a few recent advances in these applications spanning both measurement concepts and processing and inversion approaches. Three applications are covered, going from high to low frequencies. The first relates to an ultrasonic imager deployed in open boreholes to probe the laminated structure of unconventional shale rock formations at depth of more than 3000 m. The imager yields rock compressional and shear wavespeed images as a function of depth and azimuth revealing a host of geomechanical manifestations of the borehole shape and near-wellbore region at an unprecedented centimetric spatial resolution. The quantitative images have bearing on rock strength and local stresses as they relate to the hydraulic fracturing of these shale formations. The second relates to the interpretation of the complex sonic response in a well cased with double steel strings cemented to the rock formation for the purpose of evaluating the integrity of the cement placed between the outer string and formation. Here, machine learning-based approaches are employed with training on modeling and experimental datasets to develop effective and wellsite diagnosis for the condition of the cement sheath. The third relates to the seismic imaging domain and the deployment of novel accelerometers added to hydrophones on marine seismic cables to capture the subsurface-reflected pressure signals and their spatial gradients. The combination of the two sensors provides the means to deghost the signal from the sea surface reflection, and more importantly, reconstruct the subsurface seismic wavefield that is poorly sampled across cables that are spaced 75m to 150 m apart. Novel compressive-sensing schemes coupled with wave physics are employed for the wavefield reconstruction at virtual sampling rates way beyond Nyquist’s criterion. [ABSTRACT FROM AUTHOR]

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 simulate current sonic measurement tools deployed in the oilfield. Synthetic wavetrains are recorded and processed with modified matrix pencil method to isolate both the dispersive and non-dispersive propagating guided wave modes. Scaled experiments of fluid-filled double strings with dimensions mimicking the real ones encountered in the field have also been carried out to generate reference data. A comparison of the experimental and numerical results indicates that the SFFEM is capable of accurately reproducing the rich and intricate higher-order multiple wave fields observed experimentally in the fluid-filled double string geometries. [ABSTRACT FROM AUTHOR]