Well construction typically involves cementing a casing placed in the well in order to support the casing and isolate it from the surrounding formation and fluids. For oil and gas wells, well integrity is a critical issue that can affect safety and productivity throughout the life of the wells. Hydraulic isolation primarily enabled by cement reduces unintended fluid communication within and between wells and prevents contamination of the aquifers. Well integrity monitoring can significantly improve the reliability, efficiency, and safety of oil and gas production and related operations. However, current pulse-echo ultrasonic and other bond log evaluation techniques for determining well integrity rely largely on effects at the casing-cement interface and provide limited information about the actual cement properties. An improved method for construction and inspection of cemented wells would greatly enhance well integrity diagnostics.
Laboratory and field trial results indicate that well integrity diagnostics can be improved by the incorporation of a specialized cement additive along with slightly modified detection techniques based on industry standard acoustic logging methods. The additive is composed of acoustic metamaterial particles that impart to the cement a frequency-dependent response, that is highly sensitive and indicative of cement contamination and mechanical stress. Traditional cement bond log tools can be used with the smart material to investigate the cement and create a log of cement presence and mechanical stress. Measurements of cement integrity and load conditions can help predict and prevent failure mechanisms and optimize production operations.
Two shallow wells were drilled, cased, and cemented at a test site in South Texas. Engineered voids with different lengths and circumferences were secured to the casings at specific locations before installation. Along with the void configurations, the lightweight cement formulations and completion and pumping operations for each well were identical, except that the composite cement contained acoustic metamaterial particles. The cements in the wells were then interrogated using several downhole acoustic tools. Traditional CBL/VDL, multi-sector bond log, full waveform triple sonic, and ultrasonic radial scanning tools were used to investigate the cement properties under different well pressures.
The results of the study illustrate the utility of the composite cement for improving well cement integrity diagnostics using standard acoustic tools. The cement bond logs indicated an increase in the resolution and accuracy of contamination profiles. The acoustic frequency band gap response, a unique feature of the added acoustic metamaterial particles, was clear and consistent allowing detection of cement presence and assessment of mechanical loading. This enables reliable time-lapse monitoring of not only cement bond, but also cement integrity and identification of failure mechanisms.