Reservoir Aspects Of Ekofisk Subsidence

Sulak, A.M.; Danielsen, Jens

doi:10.4043/5618-ms

Cited by 24 publications

(19 citation statements)

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“…The effective stress and porosity in the 2-300 m thick reservoir at the Ekofisk field were initially 13.8 MPa and 35% on average [Sulak, 1989]. Since oil production started in the mid 1970s, the pore pressure in the reservoir has decreased and effective stress increased to at least 30 MPa [Rhett, 1998].…”

Section: Discussionmentioning

confidence: 99%

A compaction front in North Sea chalk

et al. 2011

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[1] North Sea chalk from 18 wells shows a pronounced porosity drop, from ∼20% to less than 10% over a compaction front of less than 300 m. The position of the compaction front is independent of stratigraphic position, temperature, and actual depth, but closely tied to an effective stress (load stress minus fluid pressure) of ∼17 MPa. These observations require a strongly nonlinear rheology with a marked increase in compaction rate at a specific effective stress. Grain-scale observations demonstrate that the compaction front coincides with marked grain coarsening and recrystallization of fossils and fossil fragments. We propose that this nonlinear rheology is caused by stress-driven failure of the larger pores and the associated generation of reactive surface area by subcritical crack propagation away from these pores. Before the onset of this instability, compaction by pressure solution is slowed down by the inhibitory effect of organic compounds associated with the fossils. Although the compaction mechanism is mainly by pressure solution, the rheological response to burial may still be dominantly plastic and controlled by the (fracturing controlled) rate of exposure of reactive surface area. The nonlinear compaction of chalk has significant implications for the evolution of petroleum systems in the central North Sea, both with respect to sea-floor subsidence above hydrocarbon-producing chalk reservoirs and for the formation of low-porosity pressure seals within the chalk.

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Section: Discussionmentioning

confidence: 99%

A compaction front in North Sea chalk

et al. 2011

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“…The permeability is extremely low (10 −18 -10 −21 m 2 , 10 −3 -10 −6 mD) [16]. These rocks have a very low cohesional strength which makes them difficult to sample and therefore there is sparse and uncertain data about their rock properties [17].…”

Section: The Ekofisk Reservoirmentioning

confidence: 99%

“…The thickness of the overlying sediments is 2840 m in the central part of the field and increases toward the flanks [16]. Oil is produced from Ekofisk and the deeper Tor formation which are separated by a typically 15 m thick low porosity chalk layer, the so-called Ekofisk Tight Zone.…”

Section: The Ekofisk Reservoirmentioning

confidence: 99%

“…Even the high porosity chalk has relatively low matrix permeability (i.e., the permeability of the matrix itself) between 1 and 5 mD (1 -5 · 10 −15 m 2 ). Natural fractures give a high fracture permeability (i.e., the permeability caused by the macroscopic fractures) resulting in total chalk permeability between 10 −13 and 10 −14 m 2 (10 − 100 mD) [16].…”

Section: The Ekofisk Reservoirmentioning

confidence: 99%

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Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing Reservoir

2016

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The Ekofisk field, Norwegian North sea, is an example of a compacting chalk reservoir with considerable subsequent seafloor subsidence due to petroleum production. Previously, a number of models were created to predict the compaction using different phenomenological approaches. Here we present a different approach which includes a new creep model based on microscopic mechanisms with no fitting parameters to predict the strain rate at reservoir scale. The model is able to reproduce the magnitude of the observed subsidence making it the first microstructural model which can explain the Ekofisk compaction.

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“…Surface subsidence may also be induced by hydrocarbon withdrawal from weak formations and chalk reservoirs. While reservoir compaction itself has been widely recognized as an additional driving mechanism for increasing hydrocarbon recovery, it has undesirable side effects, such as surface or seafloor settlement, that may create environmental problems and cause damage to oilfield structures and seabed pipelines [1][2][3][4].…”

Section: Introductionmentioning

confidence: 99%

Iteratively coupled mixed and Galerkin finite element methods for poro‐elasticity

Wheeler

Gai

2007

Numerical Methods Partial

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We present a finite element formulation for coupled flow and geomechanics. We use mixed finite element spaces to approximate pressure and continuous Galerkin methods for displacements. In solving the coupled system, pressure and displacements can be solved either simultaneously in a fully coupled scheme or sequentially in a loosely coupled scheme. In this paper we formulate an iterative method where pressure and displacement solutions are staggered during a time step until a convergence tolerance is satisfied. A priori convergence results for the iterative coupling are also presented, along with a summary of the convergence results for the fully coupled scheme.

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Reservoir Aspects Of Ekofisk Subsidence

Cited by 24 publications

References 0 publications

A compaction front in North Sea chalk

A compaction front in North Sea chalk

Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing Reservoir

Iteratively coupled mixed and Galerkin finite element methods for poro‐elasticity

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