Determining the physics of gas migration in Opalinus Clay; The Gas Transport (GT) project

Gas is produced in a deep geological repository mainly by metal corrosion of the waste and repository infrastructure. Since the Opalinus Clay (OPA) is a dense clay-rich host rock and the gas cannot be easily transported away, gas formation causes an increase in pressure that can affect the safety barriers. According to the current state of knowledge, there are four basic gas transport mechanisms in claystone: (i) gas movement by solution and/or diffusion; (ii) gas flow in the original porosity of the fabric, referred to as visco-capillary (or 2-phase) flow; (iii) gas flow along localised dilatant pathways; and (iv) gas flow along macro fractures similar in form to those observed in hydrofracture activities. The diffusion of gases through clay-rich rocks is relatively well understood and can be modelled to predict how the repository system will behave after closure. Considerable effort in recent years has been placed on understanding the advective transport of gas, which can occur if diffusion through the host rock is insufficient to keep the gas pressure low. The primary aim of the GT experiment was to gain definitive evidence of whether mechanism (ii) or (iii) are the dominant advective gas transport mechanisms in OPA.

The GT experiment employed a complementary three-tier approach with (1) a series of closely constrained laboratory experiments helping to define (2) a field experiment at the Mont Terri underground research laboratory in Switzerland. (3) This were supplemented by modelling the experimental data. Both experimental programs were designed to observe the coupling of stress, strain, and pore pressure during gas movement.

Four laboratory experiments were conducted: two with the long axis of the test sample parallel with bedding and two perpendicular. The samples dilated as gas started to move, with gas flow favouring pre-existing bedding planes.

The field experiment was constructed at Mont Terri in November 2020 with a central injection borehole with three injection intervals surrounded by eight monitoring boreholes. All 8 monitoring boreholes had fibre optic cabling, with two boreholes having extensometers, one having extensometers/inclinometers, and one monitoring pore pressure. After a period of stabilisation and hydraulic testing, a gas injection test was initiated in September 2022. Coupling was seen between pore pressure, strain (extensometers & inclinometers; and fibre optics), and gas injection pressure at gas entry. Close examination of the response shows that gas movement was directional and not evenly distributed around the injection borehole.

Confidence in experimental data is increased by modelling the data. This is being achieved through a PhD study and by modelling teams in the DECOVALEX-2027 HyMAR task. Similar experiments have been used previously in DECOVALEX and found to fail to represent all features seen in the data when using 2-phase flow codes. In HyMAR, the teams are modelling the experiments considering concepts such as damage to better represent the response seen in the data. Therefore, uncertainty in our understanding of the physics of gas migration in OPA has been tackled using a combination of laboratory, field, and modelling approaches.