Reliable LWD Calliper Measurements

Abstract

Abstract While perhaps not so glamorous as other measurements, calliper measurements are nonetheless an essential component of formation evaluation. Not only do we need to know the hole size for environmental corrections to other logs, such as nuclear measurements and sonic dispersion analysis, but an accurate indication of hole shape will tell us much about wellbore stability and mud optimization. For example, an elliptical wellbore can indicate the maximum and minimum stress directions. Hole enlargement/washout can indicate that the mud is too heavy or too reactive with the formation. An under-gauge hole might indicate bit wear. We also need to know the hole size to determine how much cement should be used, which is an important environmental and safety concern. This paper reviews the physics of LWD hole-size indicators, including ultrasonic, sonic, density, and resistivity tools. Design aspects, such as transducer geometry and processing techniques to determine standoff, hole size, hole shape, and tool position, will be considered. Operational factors, such as tool centralization, mud density, formation velocity, and tool location in the borehole assembly will be discussed. We will also consider what data can be transmitted in real time vs. what is stored in memory. Methods for determining the mud velocity, a critical factor in acoustic calliper measurements, will be considered. Field examples of ultrasonic, sonic, and density-derived hole size are shown. Finally, we will conclude with a summary of conditions under which callipers can be expected to function well and where they might not as well as techniques to optimize calliper data acquisition.