An Ultrasonic Device for Nondestructive Testing of Oilwell Cements at Elevated Temperatures and Pressures
Authors: Rao Prabhakar P. , Sutton David L. , Childs Jerry D. , Cunningham Willis C. 
Published: December 2007

This paper describes a measuring technique and a hardware system that automatically monitors and records the complete history of compressive strength development and initial set of oilwell cement slurries under high pressures and high temperatures (HP/HT). The design is based on the transmission characteristics of ultrasonic compressional waves through cement slurries. In principle, an analyzer measures the transit time (reciprocal of velocity) of an ultrasonic wave pulse through a slurry sample, converts it to apparent compressive strength, and records the results continuously. One of the main advantages is that testing is nondestructive, and only one sample of slurry is required to determine both the compressive-strength development history and initial set of the cement slurry. Another important advantage of this device is that initial set of a cement slurry can be determined above 100 deg. C and at pressures far above atmospheric, the limiting factors in other methodsCompressive strengths, determined from ultrasonic transit time data, have shown deviations equal to or better than those obtained from mechanical tests. Since specimens are not removed and exposed to atmospheric pressure, the results should be more representative of actual downhole conditions. Also, the process promises to be well-suited to strength retrogression studies since there are no time gaps in the test results. Further experimental and analytical work is being carried out to evaluate the potential of this technique for measuring other slurry properties.

Introduction

The times required for and oil- or gaswell cementing slurry to reach an initial set and to develop useful compressive strength are important parameters in scheduling drilling or completion operations, such as temperature logs to locate top of cement fill, bond logs to evaluate cement jobs, continuation of drilling after setting intermediate casing or liners, perforating production zones, and cleanout and stimulation treatments. Without reliable set-time and compressive-strength data, temperature and cement bond logs can be run too soon or can be delayed unnecessarily. Further drilling also can be caned out either prematurely or later than needed, which translates into lost time and money. If cleanup and stimulation treatments are carried out too soon, inter-zone communication problems may develop. A complete knowledge of the history of compressive-strength development allows determination of the optimal time for perforation. This minimizes shattering and results in better zone isolation. Thus, a precise knowledge of initial set time and strength-development data is essential to perform these operations at the optimal time to avoid unnecessary delays. In the past, the usual method of determining the time required to reach initial set or a specific compressive strength was to cure a number of cube specimens in different HP/HT autoclaves and schedule test times that bracketed the strength value specified. Such procedures often wasted time and equipment. Problems were compounded in situations where slurries had to be specially tailored for a specific job. This is especially true for deep wells, where basic variations in the cement itself often necessitated extensive testing.

JPT

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