Improvement of Non-Destructive Triaxial Strength Measurements

The mechanical strength of cement is a critical parameter for ensuring the integrity of the well. However, measuring this parameter within the wellbore is challenging, Consequently, the strength parameters are only obtainable on the surface before it is poured in place, or through simulative approaches within the laboratory. Therefore, the implementation of real-time monitoring systems to assess the cement strength would substantially enhance well safety and integrity concerns. The objective of this thesis is to improve the correlation between dynamic and static strength measurements. To that end, a precise methodology is proposed, and the execution of several testing procedures is undertaken. The results from these tests are then combined and summarized in a table. The studies methodology entails the establishment of a triaxial testing installation with integrated triaxial confining pressure recording, the adaptation of the sample preparation procedure according to the material equipment and the programming of a combination algorithm. The major objective, however, the was to find out an empirical relation between the triaxial compressive strength, the ultrasonic travel velocity, and the confining pressure, and to quantify them. Therefore, a series of cement slurries were selected and subjected to ultrasonic and triaxial testing for analysis. A total of six distinctive slurries were analyzed. These included a conventional class G cement (16 ppg), a bentonite slurry (12 ppg), a slurry containing hallow glass spheres (12 ppg), and two epoxy cement slurries (13.6 ppg), exhibiting varying rheological characteristics. Additionally, an epoxy slurry (12.9 ppg) containing ultrafine cement was examined. All samples were cast into one-time two-inch cylindrical samples and subsequently cured ambient conditions. Afterwards, ultrasonic pulse velocity analysis was conducted with piezoelectric sensors, followed by axially compressing the samples under triaxial conditions. To obtain detailed information about the deformation properties in the dimension of microstrain, samples were partially equipped with electrical strain gauges for varying confining pressures. Finally, all the acquired data was combined using a MATLAB algorithm, which resulted in a table with all important parameters of the different input slurries. The framework of this thesis includes crucial dynamic as well as static parameters like Poisson¿s ratio, Young¿s modulus, acoustic travel time, triaxial compressive strength, confining pressure as well as their deformation behavior and strain recordings. This investigation contains these parameters of 185 different cement samples from six distinctive slurries. This comprehensive dataset serves as a database and can be readily expanded for additional parameters derived from permeability tests or chemical exposure to materials such as CO2. Consequently, only some minor modifications are necessary in the combining algorithm. In essence, this research offers data-driven methodology for the assessment of the triaxial compressive strength, with the confining pressure and ultrasonic travel time serving as primary variables. This enables the assurance of structural integrity of wellbore cements, and therefore well integrity.