Downhole Ultrasonic Thickness Measurements for Inspection of Casing: A Comparison of Log Derived Modeled Casing Wear, Surface Measurements, and Calculations for Casing Integrity

Abstract

Abstract Drillpipe wear is a concern of operators when the tubular and/or the main bore of the well will be reused, for example in a sidetrack, or when the casing will be pressure tested. Drilling of long reach wells also creates a surface or intermediate casing wear problem. The fear of possible casing wear dictates the location of gas lift mandrel, which greatly affects well production and productivity. The objective of this paper is to show how downhole measurements have been compared with surface measurements of casing thickness to validate their use on casing wear models. Downhole tubular thickness measurement with ultrasonic tools is a well-known technique that has been used for many years in the oil and gas industry. The use of such downhole measurements varies from calculating burst/collapse rating to validating tubular integrity for reuse. These ultrasonic thickness measurements are done by computing the resonant frequency of the casing azimuthally in several samples sized around 1 in., then converting the resonant frequency to a thickness using known art and/or proprietary algorithms. The casing thickness can also be measured in a surface laboratory. The thickness of newer casing is measured in the laboratory to confirm its specifications and create a baseline. Similar measurement can also be done on retrieved casing to inspect their well and verify wear prediction models. The surface measurements are of much higher resolution and azimuthal sampling than a downhole measurement tool. The results of downhole thickness measurements and those of surface thickness measurements performed in a laboratory facility are compared and will be presented. The study highlights the match between the two types of measurements, the impact of a-priori knowledge of steel velocity, and comparing the log calculated burst pressure ratings to an actual laboratory measurement on joints. These joints have both downhole and surface measurements. When the tubulars are manufactured with corrosion resistant alloys, the steel acoustic properties deviate from the standard steel which the downhole measurements are based upon. The difference on acoustic velocities of the materials will be reflected on the thickness measurements of both downhole ultrasonic tools and surface laboratory measurements. Whereas surface laboratory measurements can be calibrated in-situ, downhole measurements need a priori knowledge of the acoustic properties. This paper describes the range of acoustic properties and how they affect the log derived thickness. Also, the accuracy of the downhole measurement is compared with the surface measurement when the steel properties are known