Deployment of Highly Sensitive Passive Acoustic Sensors for Well Integrity Diagnostics in Conventional and Unconventional Wells Before P&A: A Case Study from Australia

Abstract This paper highlights the successful deployment of a highly sensitive passive acoustic sensor to investigate gas migration behind casing in both conventional and unconventional onshore wells with the primary objective is to assess downhole well integrity before plug and abandonment (P&A). Ensuring the prevention of unplanned fluid movement within the wellbore is crucial throughout the well's lifespan. Therefore, identifying and rectifying any issues is mandatory to maintain extended production and environmental integrity. Conversely, detecting fluid movement provides operators with valuable insights into potential new hydrocarbon sources for future development and optimization, especially during the well's late life. A conventional oil well and an unconventional coal seam gas well, which had ceased production and were secured for abandonment, exhibited undesired sustained annulus pressure in their outermost casing annuli. To address these challenges, a highly sensitive passive acoustic tool, coupled with a quartz pressure and temperature sensor, was deployed to cover the targeted formation interval in two different well states. The tool conducted continuous logging at a rate of 10 meters per minute, with 2-meter station intervals across the targeted interval. In the first well state, the tubing and annuli were shut-in for 24 hours to establish a baseline for reservoir activity by monitoring temperature and listening for acoustic signals. In the second well state, the surface casing annulus (SCP) was continuously vented, while the tubing and production casing annulus remained shut-in. Fluid movement across a failed barrier generates turbulence, producing sound waves detected by the tool's sensor, which has a broad frequency spectrum. The tool's exceptional sensitivity, with a detection capability ten times higher for sound frequencies below 15 kHz, made it highly suitable for this application. The captured data was then analyzed across the targeted interval. Effective planning and deployment of this technology enabled operators to accurately detect fluid movement in the wellbore, crucial for planning cement plug placement during the plug and abandonment (P&A) phase. The well-organized data acquisition process not only enhanced operational efficiency but also reduced carbon emissions during conveyance. The acquired data played a pivotal role in client decision-making for P&A job planning.