Logging-While-Drilling in Cased Hole to Validate Top of Cement Saving Days of Rig Time: Deepwater Gulf of Mexico Case Studies

Abstract

Evaluation of cement placement is an important part of the majority of deepwater wells. Cement placement confirmation is an important step following a cementing operation. More than one technique can be used to provide information about the top of cement (TOC) and about the depth interval of a good bond between the formation and the casing. Determining the length of annular cement coverage, which is an indication of correct cement placement, is useful knowledge before drilling and/or completion operations can proceed. The requirement for additional and improved cement evaluation techniques is greater now than ever before. A variety of methods can be used to evaluate cement placement. The routine approach after a casing or liner cement job uses a job chart to calculate lift pressure and actual vs. predicted system pressure. These data enable an estimate of cement height in the annulus to be made, but they do not confirm the TOC. These methods vary in accuracy and difficulty, depending on well conditions. Common TOC evaluation methods in specific wellbore casing/liner sections typically require running a temperature survey or cement bond log (CBL) sensors/systems on a wireline. These operations use the rig's critical path time for each wireline run, which can add risk or difficulties, depending on the well trajectory. In addition, cement bond evaluation for large diameter casing can be technically challenging because it can reach the upper threshold measurement limitations for conventional wireline-conveyed CBL tools. Many operators now use logging-while-drilling (LWD) sonic sensors for compressional and shear data acquisition in openhole environments. Using the same sonic systems, with minimal additional rig time, logging data acquired through the casing/liner strings while running a drilling or clean out assembly can be evaluated to confirm the TOC. This paper demonstrates how LWD sonic technology can provide confirmation of the TOC, saving a considerable amount of rig time, as compared to performing a dedicated wireline evaluation run or potentially unnecessary cement squeeze operations. The paper presents and discusses Gulf of Mexico (GOM) case studies. Based on various specific challenges, through correct data analysis, TOC evaluation best practices are implemented to optimize the LWD acoustic data acquisition inside the casing/liner. New data examination techniques are reviewed that can be applied to different scenarios, such as TOC evaluation behind dual pipes and real-time assessment for quick data analysis turn-around. In conjunction with the case studies, the paper also provides information about the LWD cased-hole logging techniques, analysis, and results of the data application.