The high dip angle in formations is one of the reasons makes it difficult to drill in Tarim Basin, resulting in much longer drilling time. To counteract the high pressure from the salt layer above the targeted gas zone, 16-mm heavy casing and drilling mud as heavy as 1.9 g/cc must be used. As drill bit and pipes continue repeatedly tripping in and out the well the heavy casing may get wore thinner. Therefore it's important to know the degree of the casing wear, and to recalculate the remaining strength. Furthermore, if channel in cement behind the casing exists across the high pressure layer, it will pose another threat to corrode the casing from outside and reduce the overall strength of the casing. Hence the cement quality behind the heavy casing must be measured and assessed by reliable means. The tradition technology for well integrity evaluation has limitations in providing answers to the casing corrosion and zonal isolation in such challenge downhole condition.
A new generation of the ultrasonic imaging tool was introduced to increase the certainty of casing mechanical property measurement and cement evaluation. The technique combines the pulse-echo ultrasonic measurement with a new flexural wave measurement. The analysis from this combination allows not only a better discrimination between solid, liquid and gas for cement evaluation behind casing, but also more accurate casing information including internal diameter and casing thickness at high resolution in one trip in a well. A logging example was illustrated in this paper with high quality logging data acquired in difficult down hole conditions. As can be seen in this example, many casing grooves were found, and metal loss were up to maximum 4mm at some spots. This paper also shows a new way to look at cement quality behind the heavy casing.