Nano Sealant for CO2 Seal Integrity and Overcoring at Mont Terri

Abstract

Wellbore integrity is an important consideration for the viability of subsurface CO2 sequestration. Wellbores may leak due to micro annuli that develop between annular cement and steel casing and/or the host formation, as well as fractures and flaws in the cement itself. Leakage pathways may be problematic for the long-term containment of CO2. To address the need for a repair material that can effectively seal and repair wellbores, a new nanotechnology-based sealant has been developed at the University of New Mexico, USA. The sealant, referred to TS-Nano 20™, has superior flowability, excellent bond strength to steel and rock, is very ductile, and is thermally stable. Moreover, the sealant has been demonstrated to readily flow into microcracks below 20 µm under modest pressure gradients. The sealant showed excellent bond strength and superior fatigue resistance to debonding under stress cycles than microfine cement and other sealants. TS-Nano 20™ was used in a field test at the Mont Terri Underground Research Laboratory (MT-URL) located in NW Switzerland. This experimental work at MT-URL was performed under the auspices of the CO2 Capture Project (CCP), a three-company collaboration comprising Chevron, BP, and Petrobras. The CCP effort is directed to develop more cost-effective and efficient processes for CO2 concentration, capture and sequestration applied to the energy, refining, chemical, and power generation industries in order to reduce greenhouse gas emissions. A wellbore with a cemented annulus was located beneath the floor of the underground laboratory. The wellbore was intentionally damaged by thermal cycling inside the casing to produce leakage pathways in the wellbore system. The wellbore system included injection ports at several locations along its length to allow pressure and flow tests to be conducted to document the leakage along the length of the wellbore system. Subsequently, various sealants were injected into the injection ports to seal the leakage pathways within the wellbore system. The sealant was then successfully injected into two of the intervals on site. After injection of sealants, the entire wellbore system needed to be stabilized for eventual overcoring and forensic analyses. To this end, a large open hole test interval (uncemented) at the bottom of the wellbore was first filled with a very flowable cement slurry to chase out water and reduce the dead volume, and then the sealant was injected on top of the cement and squeezed into leakage pathways that remained near the bottom of the wellbore system and cracks in the clay formation. Additionally, the sealant was used to further stabilize portions of the wellbore by injecting into all the previously used injection ports. We report on the ability of the nano sealant to maintain its flowability and mechanical characteristics under relatively low temperature and meet all field injection requirements, including having a relatively long-time to gel while hardening after injection. Pressure and permeability measurements from the field tests are reported documenting the ability of the sealant to seal relatively very thin microcracks and create appropriate conditions for overcoring.