Long Term Annular Sealing of CO2 Injection and Storage Wells Using Shale Barriers
Authors: van Oort E. , Lucas A. , Kverneland J. , Godøy R. , Reitan H. , Aldin M. , Thombare A. 
Published: September 2024

As part of the energy transition and the aim to reduce greenhouse gas (GHG) emissions, more carbon in the form of CO2 will be captured and stored underground in wells intersecting suitable reservoirs for storage. The long-term integrity of such wells is a considerable concern, given that CO2 is a fluid that reacts with Portland cement and steel, which can erode well barriers over time. Moreover, low temperatures and temperature cycling in injection and storage wells can lead to cement cracking and debonding from casing, creating annular flow paths for CO2 to surface and allowing for CO2 to attack cement more severely. This paper reports on an investigation into using shale formations as alternative annular barrier that can guarantee integrity during CO2 injection and long-term storage.

Building upon previous work done as part of our ongoing Shale-as-a-Barrier (SAAB) investigation, rock mechanical laboratory tests were conducted into the behavior of shale creep in wells experiencing CO2 injection. A special experimental setup was constructed to be able to establish an annular shale barrier at simulated field conditions (using either in-situ formation temperature or thermal stimulation) and then testing this barrier during simulated CO2 injection conditions. During CO2 injection, the well will experience a very significant reduction in temperature, which in conventional wells can lead to the debonding of cement from the casing and the formation of a micro-annulus that compromises the annular barrier. Note that temperature cycling in wells is a lead cause of the loss of annular isolation and flow to surface in oil and gas wells.

In the experiments, shale barriers were first generated and verified at a variety of in-situ and elevated temperatures (which affect shale creep rate). The barriers were than subjected to a significant temperature reduction and temperature cycling with wellbore temperatures reaching a low value of -14 °C. In all cases, the shale barrier continued to function and maintain annular pressure integrity, indicating that well temperature reduction and cycling associated with CO2 injection will not negatively affect it. This is a very significant result and insight, because the same cannot be guaranteed for a Portland cement barrier. In addition, shale barriers are impervious to any chemical attack by CO2 and are expected to last for an indefinite time period, given that we are dealing with actual caprock material.

Carbon storage wells pose new challenges to well construction. These include the low absolute temperatures and large cyclic temperature cycles during CO2 injection which could lead to cement debonding and micro-annulus formation providing a pathway for CO2 migration to surface, as well as chemical attack of cement and casing by CO2 during long-term storage. This work shows that creeping shale formations can deal with both challenges, and provide a superior annular isolation solution when compared to conventional Portland cement. The work could have large positive implications for how (barriers in) carbon storage wells will be constructed in future, and how permanent storage of CO2 underground can be guaranteed.