A new, small-diameter ultrasonic scanning logging tool has been field tested and commercially deployed for simultaneous cement-sheath and casing evaluation. The tool is designed for use in 9-5/8-in„ heavy-weight casing down to 4-1/2-in. casing. Enhancements in downhole electronics, computing capabilities, and telemetry transmission allow the new circumferential acoustic scanning tool to operate on 7/32-in., 5/16-in., and 9/32-in. mono-conductor e-line cables, which allow for the data acquisition to be performed off a small foot print cased-hole unit with a smaller, lighter tool at the same logging speed as previous generation tools run off a seven-conductor e-line cable on an open-hole unit. The enhanced portability and evaluation capability in casings as small as 4-1/2 in. make the system ideally suited for land operations. Analysis of the cement data is accomplished with the same statistical variance process that has been widely used over the years to analyze data provided by all ultrasonic logging tools available in the industry, resulting in a robust and easy-to-interpret computed product in traditional, light-weight, and foam cement. The cement-evaluation process uses the measured acoustic impedance of the material in the casing-formation annulus. The casing's internal diameter is obtained from the twoway travel time and the measured borehole-fluid travel time. The casing-wall thickness is determined from the frequency content of the reflected waveform data. Both internal diameter and wall thickness are used to determine internal and external casing wear. Legacy ultrasonic scanning tools could not provide quality data in small-diameter casing because of tool size and tool-fishing constraints and near-field transducer ring-down effects. A new, interactive job-planner program has been developed to optimize data acquisition for casing size, weight, and borehole fluids. The planner determines the number of shots per scan and maximum logging speed required for the tool to provide 100% circumferential coverage without sector gaps or interpolated data. Examples comparing the multi-conductor circumferential acoustic scanning tool and the mono-conductor circumferential acoustic scanning tool in conventional and nonconventional cement slurries are presented.
The techniques used to process open-and cased-hole image logging and other oriented tools that are run in offshore exploration and development wells require highly accurate navigation logs. Navigation packages usually consist of triaxial accelerometers and magnetometers that require calibration of the offset (bias), gain, and tool axis alignment for each sensor. Despite pre-or postacquisition surface calibrations, the downhole environment will alter calibrations of offset and gain as a result of many factors, such as temperature and electrical noise. These factors are impractical to characterize or predict, and it is impossible to completely isolate the navigation package from these factors. Experience shows that the offsets and gains of the sensors may change appreciably over time scales much shorter than the total logging time.This paper provides four detailed examples of possible navigation logs with various problems to develop a methodology for evaluating their quality and proposing corrective action. The goals of this study are to determine which navigation sensors to adjust or which sensors are not responding to the appropriate Earth field and need reconstruction. The paper proposes several quality measures for identifying, from the navigation log, drift in sensor response, including measured total field variations and correlations of tool deviation to tool rotation.When the methods described in this paper are used to align images with respect to a geographical reference (in cases where the navigation was affected by sensor failure or external factors, e.g., permanently magnetized formation layers, that alter the sensor calibrations), they significantly enhance the navigation quality. For example, image logs that originally wobbled erratically in ferrous formations can be oriented to the north allowing dipping-bed orientation to be measured. In addition, magnetometer reconstruction and accelerometer correction can easily manage oriented navigation in metal casing. In general, the algorithms developed in this study assume that a rough surface calibration is available to obtain an initial solution, but time is no longer required for meticulous wellsite surface calibration.
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