The role of gas injection rate on the advective movement of gas in Opalinus Clay at the field scale

Abstract. This paper presents the findings of the Gas Transport (GT) field experiment investigating how Opalinus Clay (OPA) responds to gas pressure at the Mont Terri underground research laboratory in Switzerland. The experiment aimed to determine whether advective gas movement in OPA occurs through visco‑capillary flow or through the creation of dilatant gas‑filled pathways formed by localised deformation of the clay matrix. A combined strategy was adopted: laboratory studies under controlled conditions were used to inform the design and interpretation of a large‑scale field experiment. This paper focuses on the field component, summarising results from the first gas (helium) injection test from Day 447 to 1050. The experiment used a nine‑borehole array drilled perpendicular to bedding in Gallery 08. A central borehole served as the gas injection point, while eight surrounding boreholes were instrumented to monitor porewater pressure and rock deformation using fibre‑optic sensors, extensometers, and inclinometers. Key outcomes include: 1) Gas entry occurred at 4190 kPa, close to the minimum principal stress. This was followed by a reduction in gas pressure of 960 kPa over 37 days as steady state flow was established. Deformation was mainly down-dip of the injection borehole (towards ~225°), with deformation measured predominantly as borehole-axial strain (perpendicular to bedding), suggesting that the anisotropy of the OPA controlled the propagation of gas along bedding. Overall, the observations are consistent with advective gas movement through the formation of localised dilatant pathways; 2) Increased injection rates triggered a second, more energetic event at 3720 kPa, causing rapid pressure loss and deformation perpendicular to bedding. A large amount of helium was detected at the wellhead of at least four of the monitoring boreholes, confirming gas had reached the observation boreholes and migrated upwards to the gallery. The second event is interpreted as bedding‑plane splitting and the creation of a partially open macro‑feature with limited flow capacity; 3) A clear transition from dilatant pathway growth to bedding‑plane failure demonstrates that gas transport behaviour in OPA depends strongly on injection rate under the GT field-test conditions.