Tracer-gas verification of P&A barriers

Thousands of oil and gas wells are permanently plugged each year. The Norwegian Continental Shelf will need to plug approximately 3,000 wells in the coming decades. Globally, it is estimated that between a quarter and a third of these plugged wells leak. A challenge for the industry is the lack of a cost-effective method to test whether the permanent barriers that are set are completely sealed. Without an effective testing method, it is difficult for the industry to adopt new, cost-effective plugging solutions. Exedra has developed a method to address this challenge. The method involves installing a tool beneath the barrier to be tested; this tool releases a tracer gas. If there is a leak over the barrier, the tracer substance will penetrate and be circulated back to the surface. There, a highly sensitive tracer gas detector will activate if there is a leak. To fully benefit from the method, it is necessary to understand how this tracer gas is transported through the relevant barriers. Questions we seek to answer include how long it takes for the tracer gas to penetrate a given barrier and the volume of the leakage passage. This is important for both planning and interpreting such a leak test. The research will help us create a leakage estimator, which can assist the industry in adopting new cost-effective plugging solutions. This will reduce costs, emissions of greenhouse gases, and simultaneously decrease the risk of abandoned wells beginning to leak.

Exedra AS has developed a new method to verify that permanent well barriers downhole are leak tight. A downhole tool has been developed, and demonstrated offshore 4 times offshore at the Gullfaks O&G field. Using tracer gas for leak detection is a well known method, and is widely used, for instance in the process and semiconductor industry Exedra's invention is to move this technology downhole. The method is based on the principle of injecting tracer molecules in the high-pressure side of a closed system. If there is a leak in the system, the tracer molecules will follow the leak path, and leak through to the low-pressure side, where a highly sensitive tracer gas detector will detect these molecules. A major difference when moving the method downhole, is that the downhole barriers are typically long, up to 100 meters, and there are multiple leak pathways, both inside the well construction, and outside, into the formation boundary. In order to use this leak detection method downhole, it is necessary to understand how tracer molecules flow across typical leak pathways, for different types of barriers. The tracer molecules can flow across in fluid phase, gas phase or in a two-phase flow. The geometry of the leak pathway is one of the variables that determines the time until the tracers breaks through the barrier ('breakthrough time'), leak rate, and how much volume that will be required to break through. These parameters will depend on type and length of the barrier, differential pressure, fluid properties, etc. The project has selected 3 different barriers to be investigated. They all hold significant cost saving potential, but are not commonly used today, as there is no method to verify their integrity. This project will enable these methods to become available to the industry, reducing P&A costs significantly. The types of barriers to be characterized for this verification method are dual casing PWC, bismuth alloys plugs, and 'tubing left in hole' barrier.