Acoustically Responsive Cement for Enhanced Well Integrity
Authors: Pollock Jacob , Veedu Vinod , Elshahawi Hani 
Published: April 2018

Abstract
Cementing is an integral part of well construction. Cement provides the seal, protection, and support for the casing to maintain the strong barriers that isolate the well. The benefits of cement are well known and a compendium of knowledge on well cement design and durability has been building since the development of engineered cement application over a century ago.
To achieve effective isolation, cement needs to fill the area around the pipe and produce a channel-free section of cement over a length of the cement column suitable to isolate zones and prevent leakage into or out of a hydrocarbon productive zone. In many published case histories of cement bond studies and several multi-well studies, logs of cement quality show channels over short zones, even where isolation has been proven by decades of production. Channels probably exist for short intervals in many cemented intervals that are still effectively isolated. Unless the channels extend through the entire length of the cemented column, the isolation potential of a cement column is still acceptable. Wells can experience integrity failure due to structural instability in cemented regions due to subsidence and compaction caused by reservoir depletion over the lifetime of the well. Unique environmental conditions present significant operational and safety risks to the operator. Increased knowledge of cement placement, integrity, and condition will help to better guide well construction and operation. In this paper, a novel solution to well integrity monitoring is presented to address these issues to improve HSSE, enhance the economics of production, mitigate costs of catastrophic failure, and support commitments to improving environmental sustainability.
We have developed a smart well cement with specific enhanced acoustic signatures that can be detected by traditional sonic logging tools. This smart acoustically responsive cement utilizes a specially engineered particulate filler that acts as an acoustic band gap filter and contrast agent at specific frequencies. The cement can be used to harness the potential of the unified digital oil field in increasing productivity and consistency. The acoustic signature of the cement can be analyzed to determine the integrity of the cement, contamination in the cement, and, importantly, mechanical loading on the cement.
Finite element modeling and simulation were used to determine the acoustic response of the novel material and guide the design of the particle additive. The material was produced on the lab scale and the acoustic band gap features were confirmed using vibrational measurements. Ultrasonic measurements were used to determine the acoustic response of subscale composite structures, including under mechanical load and in simulated environmental tests. Shallow buried pipes with cemented annuli and engineered voids were constructed at a field site. A monopole sonic logging tool was then used to map the cement location and determine the location and relative degree of mechanical loading. Stress was applied using a variety of methods and mapped along the length of the wellbore.
The results indicated improved acoustic detection using sonic bond log tools including uniquely identifiable cement placement, enhanced void discrimination, and localization of loaded regions. This provides significant value for a smart acoustically responsive cement in detecting and thereby reducing well integrity risks due to cementing and formation issues. The acoustically responsive cement allows discrimination between fluids and lightweight cement, monitoring of formation depletion and reservoir compaction, and increased knowledge of wellbore stresses in the oil field. Furthermore, the material has the potential to be continuously monitored with an acoustic interrogation system for remote real-time indication of cement stress and integrity on a zone-by-zone basis. These results provide confidence in the next steps to further mature and de-risk the acoustically responsive cement technology for well integrity evaluation.