Nature and causes of overpressuring in mudrocks of the Gullfaks Area, North Sea

Wensaas, Lars; Shaw, H. F.; Gibbons, Kathryn; Aagaard, Per; Dypvik, Henning

doi:10.1180/claymin.1994.029.4.04

Cited by 11 publications

(5 citation statements)

References 5 publications

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“…Claystones of Lower Tertiary age in the North Sea are rich in smectite, and have caused severe problems in exploration and production drilling O 1998 The Mineralogical Society (Karlsson et al, 1979;Roaldset et aL, 1984a,b;Wensaas et al, 1994). A conventional core drilled through Lower Tertiary (Eocene?)…”

Section: Starting Materialsmentioning

confidence: 99%

Smectite to illite conversion by hydrous pyrolysis

Roaldset

Wei²,

Grimstad³

1998

Clay miner.

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Experimental illitization of smectite has been simulated by means of hydrous pyrolysis, using a smectite-rich starting material from a Lower Tertiary claystone from the Oseberg field, North Sea. The <2 µm fraction was subjected to hydrous pyrolysis using KCL solutions at concentrations of 1.0 and 0.01 N, temperatures from 180-350°C and reaction times from 24-72 h. The conversion of smectite into illite has been identified by XRD analysis of the pyrolysed products. It was clearly demonslrated that the K+ concentration ([K+]) and temperature are the major factors controlling the rate and extent of illitization. The distributions of activation energies around 33 kcal/mol and frequency factors in the range of 10+8 to 10+9 S-1 have been calculated by applying a parallel reaction model. However, geological modelling and comparison with buried smectite clays indicates that both 1.0 n and 0.01 N KCL are too high in K+ content compared to the pore-water. The results suggest that dilute KCL solution close to pore-water should be used in hydrous pyrolysis to obtain proper kinetic models.

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Section: Starting Materialsmentioning

confidence: 99%

Smectite to illite conversion by hydrous pyrolysis

Roaldset

Wei²,

Grimstad³

1998

Clay miner.

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“…been questioned (Wensaas et al 1994), it forms the only available porosity data for model calibration in the Central Graben. High sonic transit times implying undercompaction have also been noted from seismic investigation of the shales in this region (Japsen 1993).…”

Section: Discussionmentioning

confidence: 99%

Central North Sea overpressures: insights into fluid flow from one- and two-dimensional basin modelling

Darby¹,

Haszeldine²,

Couples³

1998

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Jurassic and Triassic reservoirs in the Central North Sea are highly overpressured (>40 MPa above hydrostatic pressure). Simulation of the interplay between rapid Tertiary subsidence, seal permeability and fluid flow allows insight into the geological controls on the distribution and magnitude of the overpressure. One-dimensional models demonstrate that, unlike other basins, the overpressure developed in the Graben is not determined by the thickness and permeability of the shale pressure seal. A two-dimensional model simulating lateral flow beneath the pressure seal provides an accurate simulation of the overpressure distribution. Disequilibrium compaction of shale-dominated off-structure regions forms the principal overpressuring mechanism. Lateral flow in the permeable Fuhnar sandstones leads to high overpressure and focused vertical escape on an axial high, where the seal is thinner above a subcropping Fulmar Fm. A layered hydrogeological regime is suggested, with shallow Tertiary pressure cells separated from deep Cretaceous-Triassic pressure cells by normally pressured, permeable Palaeocene sandstones. The pre-Cretaceous rift-associated configuration of the Graben, in combination with 3 km of Tertiary subsidence, controls the distribution of overpressure.

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“…Pore-fluid pressures of 1.3 g/cm 3 MWE (mud weight equivalents) (12.8 kPaJm) are typically reported in the Lower Tertiary shales. Quantification of shale pore-pressures (by indirect methods) has been shown to be strongly influenced by lithological and mineralogical variations (Wensaas et al, 1994), and these variations may introduce an uncertainty in the estimated shale pressures. Calculations assuming hydrostatic pressure conditions cause the lower critical pressure line to decrease (max.…”

Section: Discussionmentioning

confidence: 99%

Mechanical properties of North Sea Tertiary mudrocks: investigations by triaxial testing of side-wall cores

et al. 1998

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In the North Sea Tertiary section, wellbore instability problems are frequently reported in Palaeocene-Early Oligocene smectite-rich mudrocks. Analysis of the mechanical properties of these Tertiary mudrocks is generally hampered by the lack of suitable core material. This study represents an attempt to study the geomechanical behaviour of mudrocks by triaxial tests of side-wall cores obtained from the borehole wall. The tests performed include measuring the changes in pore pressure during shearing and undrained shear strength in specimens initially consolidated to in situ effective stress levels. The coefficients of permeability (kf), estimated from the consolidation time behaviour range from 2.6 x 10-11 to 2.4 x 10-12 m/s. The tested cores behaved like slightly overconsolidated to normally consolidated materials with an initial near constant volume (elastic behaviour) for low deviatoric load followed by an increasingly contractant behaviour approaching failure. Compared with results from onshore analogues, the strength properties of the investigated mudrocks appear to be related to their content of expandable clay minerals. A wellbore stability chart to forecast adequate drilling fluid pressures for future wells has been developed by the use of linear (Mohr-Coulomb) failure criteria based on the peak strength data. It is demonstrated that side-walt cores can provide satisfactory test materials for rock mechanical analysis, and their use may serve to improve our knowledge of the rock mechanical behaviour of typically troublesome mudrocks for which no conventional cores are available.

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Nature and causes of overpressuring in mudrocks of the Gullfaks Area, North Sea

Cited by 11 publications

References 5 publications

Smectite to illite conversion by hydrous pyrolysis

Smectite to illite conversion by hydrous pyrolysis

Central North Sea overpressures: insights into fluid flow from one- and two-dimensional basin modelling

Mechanical properties of North Sea Tertiary mudrocks: investigations by triaxial testing of side-wall cores

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