On the collapse behaviour of oil reservoir chalk

Abstract

Oil exploitation in North Sea Ekofisk oilfield started in 1971, the reservoir is located in a 150 m thick layer of porous chalk (n = 40 50%) at a 3000 m depth. Enhanced oil recovery procedure by sea water injection (waterflooding) was initiated in 1987. Starting from this date, seabed subsidence due to chalk compaction evolves at a fairly constant rate (i.e. 40 cm/year). Nowadays, the decrease of the seafloor level is of about 10 m. Reservoir management and production strategies are at the origin of the growing interest of petroleum industry in disposing of a comprehensive description of the chalk mechanical behaviour. In this note the subsidence due to waterflooding is interpreted within a framework taken from the mechanics of unsaturated soils. By considering oil as the non-wetting fluid and water as the wetting fluid, chalk compaction is depicted as a collapse phenomenon due to oil-water suction decrease. A series of suction controlled tests in the osmotic oedometer cell are presented. Water weakening effects and chalk compaction (collapse) seem likely to occur through the lost of strength of the inter-granular links existing in the oil saturated sample. The nature of these links includes both capillary and physico-chemical fluids-chalk interactions, and is well characterised by the oil-water suction.