Mechanics chalk reservoirs

Migration into the Chalk reservoirs of the Joanne Field could have occurred by one of three conceivable mechanisms ... 

Migration of petroleum within the Chalk reservoirs from its point of entry on the Judy structure is considered to be the most likely mechanism by which the Joanne Chalk reservoirs are charged. At present, there is no structural spill from Judy to Joanne at either Ekofisk or Tor horizons. However, the Top Ekofisk depth map, flattened on an intra-Oligocene event, shows that the Joanne structure developed considerably later than the Judy structure. It is therefore possible that, during growth of the... 

MECHANICAL BEHAVIOUR OF CHALK RESERVOIRS NUMERICAL MODELLING OF... 

Abstract Chalk is the constituent material of numerous oil reservoirs in North Sea. The mechanical behaviour of a saturated chalk has been largely studied. However, different aspects of its behaviour are not yet well understood material characteristics depend on the saturating fluids and chalk response is time-dependent. This paper proposes the PASACHALK numerical model an elasto-plastic constitutive law is presented, which reproduces the different plastic mechanisms of the chalk (pore collapse and shear failure) and the influence of pore fluids. The water sensitivity of this soft rock is explained by the existence of suction effects in chalk. Finally, a simulation of a hypothetical reservoir is proposed to show the response of the elasto-plastic model during depletion phase and water injection phase. 

Compaction of chalky reservoirs during oil extraction and other important problems like the "casing collapse" or the "chalk production" are related to the mechanical properties of chalk. Controlling compaction is very important because reservoir deformations imply seabed subsidence that endangers the offshore stations. 

This numerical tool may be used for simulations of oil reservoir chalk in order to reproduce the compaction during water injection. The computations presented here correspond to a hypothetical reservoir. The results show that the model is able to reproduce the compaction during production phase. These deformations are related to an increase of effective stress. During injection phase, an additional compaction is predicted. Even if the reservoir pressure increases, the suction decrease leads to compressive deformations. Suction is a pertinent mechanism to explain the mechanical behaviour of chalk. 

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