Compaction study for shearwater field

C J Kenter (Shell RTS); Gerd-Jan Schreppers (TNO DIANA BV); M N Baaijens (Shell Expro); T L Blanton (Mobil MEPTEC); G G Ramos (Arco EPT)

In order to get a realistic prediction of the in situ stress evolution, compaction and subsidence of the HP/HT Shearwater field, full 3-D finite element calculations were made with the program DIANA. An interface was made to generate 3-D meshes directly from seismic files, which led to a most realistic representation of the actual geometry. An extended and modified Cam Clay model was used with and without a Duvaut-Lions creep extension to describe the observed test behavior. Detailed calculations were made of cross sections, including main faults, secondary fault systems, slip layers and wells. It was found that stress arching was very significant in the Shearwater field geometry and led to a reduction of the compaction strains within the reservoir till below the critical values. However, the stress arching also reduces the total stresses in shales, faults and slip layers in the rock directly above it. Assuming that the high initial pore pressure is preserved in the overburden, this may lead to very low normal effective stresses and relatively high shear stresses, and as a consequence to a reactivation of faults and slip layers at the end of field life. It was found that only secondary faults close to the main fault will be reactivated and that the slip in activated slip layers is minimal above the crest of the reservoir. Therefore a drilling window could be identified that will minimize the risk of damage to the completion. Further, zero effective stress or full liquefaction of the overburden shales close to the reservoir was identified as a realistic problem in case of strong stress arching and high pore pressures; however because of the high strength of the shales it did not appear to be a problem in Shearwater.

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