Imaging technologies from azimuthal logging-whiledrilling (LWD) tools provide valuable insight into borehole conditions and address multiple drilling and formation evaluation applications, such as wellbore stability assessment and fracture and bedding plane analysis. Although high-resolution images are widely available for water-based mud applications, such as from azimuthally focused resistivity tools, their availability in oil-based mud applications is limited.
This paper presents field test results from a 4%-in. ultrasonic imaging tool that provides high-resolution borehole caliper and acoustic impedance images, independent of the mud type used. Analysis of data sets collected in oil-based mud with varying mud weights under multiple drilling conditions are provided, highlighting the suitability of the imaging technology for multiple while-drilling applications. Log data and analysis from the field test wells illustrate the deliverables from both the caliper measurement and the acoustic impedance measurement. Caliper deliverables detailed include: average hole size calculation for input into cement volume calculation, as a borehole quality indicator, and for environmental corrections for other LWD sensors; borehole ellipse and azimuthal sector image plot outputs for real-time geomechanics analysis; and high-resolution borehole images for the identification of faults and fractures. Acoustic impedance deliverables detailed include: real-time images for potential porosity steering applications; and high-resolution memory images for detailed analysis of faults and fractures, and geological and lithological analyses of bedding planes, laminations, and determination of stratigraphic dips. The caliper and acoustic impedance data sets are compared directly with corresponding wireline measurements, including a multifinger caliper and ultrasonic imaging tool.
An overview of the tool geometry and associated sensor physics is given, along with details of the laboratory setup and testing performed to evaluate the sensors and the associated measurements and images. Details of the field tests, which illustrate the steps taken to ensure the sensors were evaluated across different lithologies from vertical to horizontal, using different mud weights, logging speeds, and drillstring rotation parameters are described. The logging program was optimized to obtain direct correlation with wireline data sets and maximize image quality.
Analyses of the deliverables from the field trials illustrate the value that the ultrasonic caliper and acoustic impedance measurements provide to a variety of LWD applications in boreholes ranging from 5¾ to 6¾ in., adding high-resolution imaging capability to oil-based mud systems. The excellent comparison with wireline measurements demonstrates the potential for the LWD logs to be used as the primary imaging solution in applications where deployment of wireline technologies is either risky or costly, such as in high-angle or horizontal wells, while enabling the same high level of formation evaluation.