AUGUST 1982 casing deformation and have identified casing corrosion and wear.

Summary A new generation laterolog tool, the Azimuthal Resistivity Imager (ARI) is described. The tool makes deep azimuthal resistivity measurements around the borehole with higher vertical resolution than the Dual Laterolog (DLL) tool. An array of twelve azimuthal electrodes is incorporated into the dual laterolog array so as to provide twelve deep, oriented resistivity measurements while retaining the standard deep and shallow laterolog measurements. To allow full correction of the azimuthal resistivities for borehole effect, a very shallow auxiliary measurement is incorporated on the azimuthal array. Though the full-coverage azimuthal resistivity image has much lower spatial resolution than borehole micro-electrical images, it complements these because of its lower sensitivity to shallow features. Fracture evaluation and computation of structural dip are applications of the tool's imaging capabilities which are discussed and illustrated with log examples. Other log examples cover thin-bed response, Groningen effect and borehole corrections, including that for eccentering of the tool in the borehole. Introduction The Laterolog technique was introduced in 1951, with the Dual Laterolog tool following some twenty years later. Though instrumentation has been upgraded as technology has developed, the Dual Laterolog deep and shallow measurements, LLd and LLs, have remained essentially unchanged since their introduction. Together with induction tools, the laterolog provides the key input for basic formation evaluation. While important advances have been made in the design of induction devices in the past ten years, few comparable developments have been made in the laterolog domain, despite known limitations to the laterolog measurements. Reference electrode effects have plagued deep laterolog measurements since their early days. Though effects such as Delaware and anti-Delaware effect have been overcome by repositioning the measure and current returns, Groningen effect remains a particularly complex problem which has yet to be satisfactorily resolved. It manifests itself as an increase in the LLd reading in conductive beds overlain by thick, highly resistive beds. The vertical resolution of the deep and shallow laterologs is two to three feet, with a typical beam width of around 28 inches. Thin beds are assuming increasing importance as potential reservoirs, and the vertical resolution of the deep and shallow laterologs is increasingly recognised to be insufficient for adequate evaluation of these beds. Development of a pad-mounted laterolog-3 has reportedly improved vertical resolution to two inches, though a consequence is reduced depth of investigation. Paradoxically, pad or skid devices suffer from a larger borehole effect than cylindrical tools. Though the effect of dip is much less severe than for induction devices, whose responses are perturbed drastically, dual laterolog response is affected significantly across dipping bed boundaries. A directional resistivity measurement around the borehole axis would provide a means of correction for the effects of dip. In one sense such measurements are already available in the form of high-resolution electrical borehole imaging tools, which have been shown to be very effective in evaluation of complex reservoirs.

*SPE Member Abstract Acoustical coupling is proposed as a new measurement to complement the attenuation rate measurement now available with multispacing cement bond logging tools. Coupling responds principally to the mechanical impedance and distribution of material outside the casing, but is largely insensitive to bonding. Estimates of cement strength may be obtained even in the presence of microannulus. Introduction Acoustic techniques have been used for many years in cased wells for the purpose of determining cement quality. The principle consisted of interpreting the amplitude of the acoustic signal received at some axial spacing from a transmitter in the borehole fluid.' Improvements on this technique included using an array of transducers to determine the actual spatial attenuation rate of the signal. This attenuation rate is primarily a function of the mechanical properties of the cement that is bonded to the casing. It must be recognized, however, that amplitude reduction between transmitter and receiver depends not only on attenuation from axial propagation along the casing but also on the efficiency of acoustical coupling between transducers and the casing wave. For single-spacing measurement systems it is not possible to separate effectively these independent contributions to amplitude decrement. Additionally, amplitude itself depends on borehole fluid properties and the temperature or pressure characteristics of the measuring apparatus. Gollwitzer and Masson have disclosed a means of computing a compensated spatial attenuation rate from multispacing data that is relatively independent of these environmental factors. Still, a shortcoming of standard cement bond logging systems arises from microannuli, which frequently appear between the casing and the solid cement column after cementing and drilling operations. Often, these microseparations are small enough that the hydraulic seal offered by the cement column to fluids is not impaired. Microannuli do, however, severely reduce the acoustic attenuation rate, increasing the received amplitudes. Those measurements are thus not reliable measurements for use in estimating the hydraulic seal. If, however, in interpretation the amplitude change between transmitter and receiver is separated into a change dependent on coupling, as well as a change related to attenuation rate, it is observed that coupling is an important amplitude-controlling mechanism that depends on properties of the material outside of the casing in functionally different ways than the attenuation. In particular, coupling is not dominated by mechanical bonding between the casing and the cement sheath. Measurement of both coupling and attenuation rate allows determination of the presence of microannulus and estimation of average mechanical properties of material behind casing at each depth. P. 243^

Horizontal well gravel-packing operations are known to exhibit a higher rate of premature screenout. The suggested causes of such failure range from extreme reservoir conditions (e.g., low in-situ stresses), to suboptimal drilling practices (e.g., extensive washouts), or even to limitations of the gravel placement process (i.e., accelerated carrier fluid leakoff). Premature screenouts are postulated with theoretical studies or with laboratory experiments. These approaches rely on idealized assumptions and therefore fail to explain gravel placement behavior observed in field practice. Further, currently available gravel-pack logging tools can provide only borehole-averaged measurements. These are inadequate in horizontal wells where the gravel-pack quality may not be radially uniform. In horizontal wells, an azimuthal, or localized, gravel-pack measurement is necessary for a proper understanding of the gravel deposition over the reservoir section. This paper describes the development and application of a new gravel-pack logging procedure. The associated logging tool is based on a radically different concept of fullbore gravel-pack imaging. It focuses on discrete sectors around the wellbore, providing the ability to azimuthally measure the gravel-pack quality. Further, its combinability with other production-logging sensors ensures that all parameters pertinent to gravel-pack evaluation can be obtained using a short, compact toolstring with minimal logging time. The gravel-pack tool can also be operated in memory mode and deployed with certain downhole completion components (i.e., the washpipe) before gravel injection. These components are normally retrieved upon completion of the pumping operation, bringing the logging tool through the gravel pack and back to surface. One can therefore obtain a gravel-pack log concurrently with the gravel placement procedure. This "logging while completing" approach will provide an enhanced gravel-pack quality measurement at a fraction of the cost of conventional electric line logging operations. Multiple field examples that illustrate the field application of this logging tool are provided. The paper also provides field evidence of the gravel placement failure mechanisms that have been previously postulated by theoretical studies. Introduction An increasing number of highly deviated and horizontal wells are being completed as open holes. This choice is driven by lower costs, increased productivity, and higher tolerance to formation damage. Gravel packs are the preferred completion option in these cases. This is particularly true in offshore environments where high operational expenses necessitate increased completion reliability. A desirable gravel pack lacks void spaces between the screen and formation annulus. Pack voidage results from accelerated fluid loss from the gravel slurry, leading to a premature screenout during the gravel-pack pumping operation. Gravel-pack voids cause annular fluid flow, leading to sand migration and development of hot spots, which lead to progressive erosion of the screen and its eventual failure. Horizontal wells with shale breaks are known to exhibit a higher rate of premature screenout.1 The presence of annular voids in the gravel pack can be identified using electric logging tools. A gravel-pack log provides a measure of the pack quality, or the fill percentage within the screen and casing or openhole annulus. This information can be used in two ways. Firstly, an incomplete gravel pack can potentially be remediated by subsequent interventions.2 Secondly, the gravel-pack log provides data so that problems may be suitably addressed during subsequent gravel-pack operations.3

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