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Low permeability cap rocks retain oil by capillary forces when the pore throats of the seals are sufficiently small to prevent a flux of oil into the cap rock. In order to investigate the influence of aquifer overpressures on oil retention, water pressure was applied to a water-wet, highly permeable (1988 mD) core sample, which was oil-saturated to irreducible water saturation S wi and mounted with a low-permeability and water-wet membrane at the outlet. A water pressure difference of 0.5 MPa was applied across the core. This pressure was high enough to ensure fluid flow through the sample. The experiment was designed to see whether the water pressure would force oil through the membrane or if capillary forces at the sandstone–membrane interface would retain the oil, in which case water flow might take place in the (residual) water in the core and through the membrane. The experiment showed that oil was kept in place by capillary forces while water flowed through the core and the membrane. Accordingly, residual water can move through sandstones that are saturated to S wi . The experiments also demonstrated that the permeability associated with this residual water is high enough to prevent overpressures in the aquifer below the oil–water contact from pushing oil through a membrane seal. Thus, even for this highly permeable sandstone, the overpressure in the aquifer will not cause capillary seal failure.

All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission.Cover: Inger Sandved Anfinsen. Printed in Norway: AiT e-dit AS, Oslo, 2008. Produced in co-operation with Unipub AS. The thesis is produced by Unipub AS merely in connection with the thesis defence. Kindly direct all inquiries regarding the thesis to the copyright holder or the unit which grants the doctorate. Unipub AS is owned by The University Foundation for Student Life (SiO) III CONCLUSIONS / IMPLICATIONSThis dissertation was motivated by the realization that four concepts, which are too simplistic, are widely assumed in the petroleum industry:First, mechanical compaction and associated disequilibrium compaction are frequently assumed to be the main mechanism for overpressure formation, although data are rarely given to support this assumption. This thesis concludes that neither the North Sea nor the Haltenbanken shales compacted mechanically at moderate to deep burial. Therefore, high overpressures in these rocks were not caused by disequilibrium compaction, but more likely by diagenetic processes that were largely unaffected by fluid pressures. Traditional seismic and log-based pore pressure detection methods in these areas should be expected to result in under-prediction of fluid overpressures because the porosities are not higher in overpressured shales than in normally pressured shales.Second, observations of zods (zones of deteriorated seismic signals; at times termed gas chimneys) are often interpreted as evidence of hydrocarbon leakage. This thesis concludes that the occurrence of zods may identify hydrocarbon leakages and where pressure compartments leak. However, prior to interpreting these zones as hydrocarbon leakage, the interpreter must be aware of the various geological processes and non-geological origins that could cause such velocity variations: (a) hydrocarbon leakage, (b) leakage of water with dissolved gas (that could create an inhomogeneous gas saturation), (c) fault or fracture zones themselves, (d) fluid leakage above fault(s) or fault junction, or (e) data quality issues. As a result, applications of zods in hydrocarbon prospect evaluation should be performed more carefully than what is often seen in the industry today.Third, the consequences of high overpressures are often assumed to be hydrocarbon leakage through the caprock -either because of hydro-fracturing or because high water pressure force oil or gas through membrane seals. This thesis concludes that high overpressures are compatible with hydrocarbon preservation. Vertical water leakage from the apex of a trap may take place while oil and gas are retained by capillary forces within the structure. This result is consistent with the fact that several of the largest oil fields on IV the Norwegian Continental Shelf (Statfjord, Gullfaks, Snorre, Visund, and Kvitebjørn) are highly overpressured and leaky, and yet contain vast amounts of oil and gas.Finally, vertical leakage is often assumed to occu...

Laboratory experiments were performed to investigate if water can leak through the pore network of a water wet seal while oil stays in the reservoir below. The experimental setup consisted of a low‐permeability membrane disk attached to the outlet side of a cylindrical sandstone core plug. The core plug and the seal were water wet, and the core plug was saturated with oil to irreducible water saturation (Swi). Water was injected at the inlet side of the core, and the fluid flow through the core and the membrane was monitored. The experimental results from three samples with permeabilities ranging from 0.06 to 1900 mD and with capillary pressure ranging from 1.5 to 10 bars confirmed that water at “irreducible” water saturation moved through the core samples, while the membrane retained the oil by capillary sealing. The experiments demonstrated that the relative permeability to the residual water could be approximated by log krw @ Swi = −1.75 log(Pc √(k/ϕ)) − 1.95, where krw @ Swi is the calculated relative wetting‐phase water permeability, k is total reservoir permeability (mD), ϕ is total reservoir porosity (in fraction), and Pc is capillary pressure (bar). Application of this formula to subsurface conditions suggests that the residual water permeability is generally sufficiently small to result in overpressure gradients in the residual water across the hydrocarbon columns. The magnitudes of these overpressures are typically below 5 bars for oil columns of 200 m or less, but may exceed 50 bars in low porosity and permeability reservoirs with large oil columns. Such overpressures are not large enough to result in capillary leakage through good caprocks but may significantly reduce the sealing capacity of marginal seals.

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