LEAKAGE OF GAS THROUGH CAPROCKS: HEADSPACE GAS EVIDENCE FROM THE NORWEGIAN AND UK NORTH SEA. Selegha Abrakasa Andy Aplin Newcastle University

Potential Leakage Mechanisms Along faults Pore system after capillary failure Along fractures resulting from tectonic stress or high pore pressures

STUDY SITES 48 wells from 39 fields representing: Reservoir depth range ~ 2700m – 3800m Variety of pore pressure regimes: hydrostatic to fracture pressure Variety of structural settings: diapiric, tilted/rotated fault blocks Oil, gas and oil + gas accumulations

Headspace Gas Cuttings from shale shaker in cans (canned cuttings) Gas sample in headspace collected and analysed by gas chromatography a) ppm of C 1 -C 5 b) δ 13 C c) iC 4 /nC 4 Commonly available Variable quality Can readily differentiate thermogenic and biogenic gas Do profiles suggest flowpaths and leakage mechanisms?

SUMMARY STATISTICS: THERMOGENIC GAS ANOMALIES No. OF WELLS HEIGHT OF WET GAS ANOMALY ABOVE RESERVOIR (m)

WET GAS ANOMALY > 500m: NO RELATION TO STRUCTURE

Pore Pressure & Fracturing λ = Pore pressure / Lithostatic pressure Risk of mechanical failure increases at ~ 0.8 Pressure Depth Hydrostatic Lithostatic Fracture Pp Lp

PRESENCE OF THERMOGENIC GAS IN CAPROCK NOT RELATED TO PORE PRESSURE

GAS PROFILES: VIDGIS (34/7-29S ) Draupne Fm. Sst.

SST/ CLST SHETLAND CROMER KNOLL GP VIKING FANGST BAT GP LI/CLY/SST LI/SSTONE SST CLST/ SLST/ SST COAL UNIT GAS PROFILE: TYRIHANS 6407 / ,300 m HC shows in early Cret.

SUMMARY & CONCLUSIONS 39 N Sea fields: wide range of structural and pore pressure regimes Almost all reservoir caprocks contain thermogenic gas; in this context they are leaking on geological timescales 70% of reservoirs show thermogenic gas anomalies over 500m; 40% 1000m 10% percent of leaking reservoirs had high potential for pressure induced fracture (λ = 0.8) Are the majority of reservoirs leaking via capillary failure through pores? Or: are we observing water flow? Next: numerical analysis of profiles: timing, rates, mechanisms