Ultrasonic-Log Response in Lightweight-Cement Conditions

Abstract

Summary Lightweight cement can present unique challenges to efficient and accurate evaluation of cement-sheath integrity. Standard bond-log tools require high compressional bonding to casing to observe an attenuation of the sonic signal that is set up inside the casing. Lightweight cement exhibits high amplitude and little alteration in the log waveform on conventional bond logs because of lower fluid density when pumped and lower compressive strength after being set up. These properties can make the log appear as if no cement is present. One must still determine the definition of the cement sheath with a method that is both valid and repeatable. The industry has demonstrated the ability to accomplish this task with ultrasonic logging tools. These devices allow very precise definition of the cement sheath with a 360 ° examination of the data, but still require innovative interpretation techniques to derive useful solutions regarding the quality of the cement. Additional complication is introduced when settable efficiency-fluid material is the material used instead of cement. This fluid is not designed to be an isolating agent, but is formulated to be used as a lead-in fluid to facilitate the placement of lightweight or conventional cement. This fluid is lighter weight and has lower compressive strength when set up than even the lightest cements. The cement design for this well was to lead in with the settable efficiency fluid and then to follow up with lightweight cement. The job did not proceed as planned, and the only fluid introduced to the well was settable efficiency fluid. One must still determine segregation of the productive reservoirs from surface water and the ability to fracture treat wells with restrained fluid mobility to complete and produce the well. This paper demonstrates the application of ultrasonic cement-evaluation tools under these difficult conditions. Examples of conventional cement, lightweight cement, and settable efficiency fluids are considered. The processed logs highlight the confidence in the reservoir segregation indicated with these tools and evaluation techniques.