Enhanced Mechanical Integrity Characterization of Oilwell Annular Sealants Under In-Situ Downhole Conditions: A Path to Optimized Wellbore Economics
Authors: Jimenez Walmy Cuello , Pang Xueyu , Benge Margaret , Darbe Robert , Meadows David L. 
Published: April 2017

Long-term wellbore integrity is an increasingly important factor that directly affects hydrocarbon production and optimized-wellbore economics. The industry shift toward drilling in harsher conditions, including high-pressure/high-temperature (HP/HT), deepwater, unconventionals, mature fields/high-temperature (HT) oil-recovery techniques, etc., as well as increasing regulatory scrutiny, has made operations even more challenging. Zonal isolation is typically achieved by placing annular sealants between the casing and formation. Sealants serve as annular barricades to help protect/support the casing while preventing unwanted fluid communication. Ideally, the sealant placement occurs in an optimum manner and initially presents a good bond-log result. However, zonal isolation can deteriorate over time, resulting in poor wellbore economics attributed to costly remedial treatments, potential environmental issues, and sometimes premature well abandonment.

As a result, oil companies have shifted focus toward preventing wellbore integrity failures using predictive modeling to optimally design sealants to withstand pressure and temperature cycles throughout the life of the well. These models require critical input data typically obtained by performing destructive and nondestructive tests to characterize the thermomechanical properties of sealants. Albeit multiple studies have been performed to characterize sealant properties, the vast majority do not result in the measurement of key properties under in-situ simulated downhole conditions. Testing equipment limitations require the samples to be transferred from curing chambers to loading cells, thereby exposing tested specimens to unrealistic curing and loading histories. Testing conditions are often different from those actually experienced downhole because of equipment constraints, resulting in inaccurate exemplification of the sealant’s performance in the wellbore. Consequently, the need for in-situ characterization of sealants under simulated downhole conditions becomes evident.

This study describes an innovative and novel methodology comprising an HP/HT in-situ triaxial testing apparatus for measurement of sealant mechanical properties (i.e., compressive strength, Young’s modulus, and tensile strength) under simulated downhole conditions. The equipment can be used to perform both curing and testing using the same apparatus, thus eliminating depressurization and cooling of test specimens. Additionally, at minimum, three samples can be tested sequentially for statistical analysis and uncertainty mitigation, along with performing real-time monitoring of total HP/HT shrinkage. The testing apparatus is rated to 30,000 psi for axial loading, 20,000 psi for confining loading, and 400°F. Preliminary validation of Young’s modulus was performed with five different plastic samples, yielding error percentages of less than 5% compared to measurements performed using a standardized loading frame. Compressive strength validation was performed using a 16-lbm/gal cement design, and error percentages of less than 2% were obtained compared to standardized testing procedures. Moreover, a 16-lbm/gal cement system was also used to help assess the functionality of the testing apparatus under simulated wellbore conditions with temperature and pressure ranging from 80 to 350°F and 3,000 to 8,000 psi, respectively.