Tracer-gas verification of P&A barriers

Tusenvis av olje- og gassbrønner plugges permanent hvert år. Norsk sokkel vil de neste tiårene måtte plugge totalt ca. 3000 brønner. Globalt anslås det at mellom ¼ og ? av disse pluggede brønnene lekker. En utfordring for industrien er at det ikke finnes en kostnadseffektiv testemetode for å verifisere at de permanente barrierene som settes faktisk er helt tette. I mangel av en effektiv testemetode, er det vanskelig for industrien å ta i bruk nye, kostnadseffektive plugge løsninger. Exedra har utviklet en metode for å løse denne utfordringen. Metoden innebærer å installere et verktøy under den barrieren som skal testes; dette verktøyet slipper ut en sporgass. Hvis det er lekkasje over barrieren, vil sporstoffet trenge gjennom og bli sirkulert tilbake til overflaten. Der vil en svært følsom sporgassdetektor slå ut dersom det er lekkasje. For å dra full nytte av metoden, må man forstå hvordan sporgassen transporteres gjennom de aktuelle barrierene. Spørsmål vi søker svar på er blant annet hvor lang tid det tar for sporgass å trenge gjennom en gitt barriere, og hvor stort volum lekkasjepassasjen har. Dette betyr mye for både planlegging og tolkning av en slik lekkasje-test. Forskningen vil hjelpe oss å lage en lekkasje-estimator, som kan hjelpe industrien i å ta i bruk nye kostnadseffektive plugge løsninger. Dette vil redusere kostnader, utslipp av klimagasser, og samtidig redusere risikoen for at forlatte brønner begynner å lekke.

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.