Effects of drilling and stress release on transport properties and porewater chemistry of crystalline rocks

SummaryThe experimental verification of matrix diffusion in crystalline rocks largely relies on indirect methods performed in the laboratory. Such methods are prone to perturbations of the rock samples by collection and preparation and therefore the laboratory-derived transport properties and fluid composition might not represent in situ conditions. We investigated the effects induced by the drilling process and natural rock stress release by mass balance considerations and sensitivity analysis of analytical out-diffusion data obtained from originally saturated, large-sized drillcore material from two locations drilled using traced drilling fluid. For in situ stress-released drillcores of quartz-monzodiorite composition from the Äspö HRL, Sweden, tracer mass balance considerations and 1D and 2D diffusion modelling consistently indicated a contamination of <1% of the original pore water. This chemically disturbed zone extends to a maximum of 0.1 mm into the drillcore (61.8 mm × 180.1 mm) corresponding to about 0.66% of the total pore volume (0.77 vol.%). In contrast, the combined effects of stress release and the drilling process, which have influenced granodioritic drillcore material from 560 m below surface at Forsmark, Sweden, resulted in a maximum contamination of the derived porewater Cl− concentration of about 8%. The mechanically disturbed zone with modified diffusion properties covers the outermost ∼6 mm of the drillcore (50 mm × 189 mm), whereas the chemically disturbed zone extends to a maximum of 0.3 mm based on mass balance considerations, and to 0.15 mm to 0.2 mm into the drillcore based on fitting the observed tracer data. This corresponds to a maximum of 2.4% of the total pore volume (0.62 vol.%) being affected by the drilling-fluid contamination. The proportion of rock volume affected initially by drilling fluid or subsequently with experiment water during the laboratory diffusion and re-saturation experiments depends on the size of the drillcore material and will become larger the smaller the sample used for the experiment. The results are further in support of matrix diffusion taking place in the undisturbed matrix of crystalline rocks at least in the cm range.

► The verification of matrix diffusion in crystalline rocks largely relies on laboratory experiments. ► The effects of sample preparation, drilling and stress release are explored on samples drilled with traced drilling fluid. ► Quantification of the induced uncertainty on derived parameters occurred by mass balance, 1D and 2D transport modelling. ► The mechanically disturbed zone with modified diffusion properties extends to about 6 mm into the drillcore. ► The chemically disturbed zone with modified porewater chemistry makes less than 2.5% of the rock’s total pore volume.