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The capillary-sealing ef®ciency of intermediate-to low-permeable sedimentary rocks has been investigated by N 2 , CO 2 and CH 4 breakthrough experiments on initially fully water-saturated rocks of different lithological compositions. Differential gas pressures up to 20 MPa were imposed across samples of 10±20 mm thickness, and the decline of the differential pressures was monitored over time. Absolute (single-phase) permeability coef®cients (k abs ), determined by steady-state¯uid¯ow tests, ranged between 10 À22 and 10 À15 m 2 . Maximum effective permeabilities to the gas phase k eff (max), measured after gas breakthrough at maximum gas saturation, extended from 10 À26 to 10 À18 m 2 . Because of re-imbibition of water into the interconnected gas-conducting pore system, the effective permeability to the gas phase decreases with decreasing differential (capillary) pressure. At the end of the breakthrough experiments, a residual pressure difference persists, indicating the shut-off of the gas-conducting pore system. These pressures, referred to as the`minimum capillary displacement pressures' (P d ), ranged from 0.1 up to 6.7 MPa. Correlations were established between (i) absolute and effective permeability coef®cients and (ii) effective or absolute permeability and capillary displacement pressure. Results indicate systematic differences in gas breakthrough behaviour of N 2 , CO 2 and CH 4 , re¯ecting differences in wettability and interfacial tension. Additionally, a simple dynamic model for gas leakage through a capillary seal is presented, taking into account the variation of effective permeability as a function of buoyancy pressure exerted by a gas column underneath the seal.

Laboratory experiments have been performed to determine diffusion coef®cients of natural gas components (methane, ethane and nitrogen) and isotope fractionation effects under simulated in situ pressure (up to 45 MPa effective stress) and temperature conditions (50±2008C) in water-saturated pelitic and coarse-grained rocks. Effective diffusion coef®-cients of molecular nitrogen (0.39 Â 10 À11 to 21.6 Â 10 À11 m 2 sec À1 at 908C) are higher than those for methane (0.18 Â 10 À11 to 18.2 Â 10 À11 m 2 sec À1 at 908C). Diffusive¯ux rates expressed in mass units are generally higher for N 2 than for CH 4 . Both methane and (to a lesser extent) nitrogen diffusion coef®cients decrease with increasing total organic carbon (TOC) content of the rock samples because of sorption processes on the organic matter. This effect decreases with increasing temperature. Effective diffusion coef®cients increase upon a temperature increase from 50 to 2008C by a factor of four. Effective diffusion coef®cients and steady-state diffusive¯ux decrease with effective stress. Stationary diffusive¯uxes drop by 50±70% for methane and 45±62% for nitrogen while effective diffusion coef®cients are reduced by 38% (CH 4 ) and 32±48% (N 2 ), respectively. Isotope fractionation coef®cients of diffusive transport are higher for methane (À1.56 and À2.77%) than for ethane (À0.84 and À1.62%). Application of the experimental results to geological systems show that diffusive transport has only a low transport ef®ciency. Signi®cant depletion of natural gas reservoirs by molecular diffusion is only expected in cases of very poor caprock qualities (in terms of thickness and/ or porosity) and over extended periods of geological time. Under these circumstances, the chemical and isotopic composition of a gas reservoir will change and maturity estimates based on these parameters may be deceptive. To account for these potential effects, nomograms have been developed to estimate diffusive losses and apply maturity corrections.

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