TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract.Compaction Drive consists in the expulsion of oil due to the reduction of pore volume. Generally, unconsolidated sands display an elasto-plastic behaviour during depletion. The limit between the elastic and the plastic behaviour is called the preconsolidation stress and referred to as P c0 . As long as the effective stress is less than P c0 , the rock behaves elastically and the pore compressibility is low. If ever the effective stress exceeds P c0 , the compressibility increases significantly. In heavy oil fields of unconsolidated sand, the additional recovery due to compaction drive may be estimated between 0.5 and 1 % if the rock remains elastic during the field life and to more than 3% if the plasticity threshold is reached early. Experience has shown that it is almost impossible to determine P c0 from laboratory tests on sand samples, mainly because of cores disturbance. TOTAL has been investigating other methods to determine this parameter. The method presented in this paper is based on : • Laboratory tests on preserved intra-reservoir shales • A shale compaction model derived from the critical state theory of Cam-Clay and calibrated on a normal compaction trend for shales • Geological description of the deposition-erosion history on a basin scale.

TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract.Compaction Drive consists in the expulsion of oil due to the reduction of pore volume. Generally, unconsolidated sands display an elasto-plastic behaviour during depletion. The limit between the elastic and the plastic behaviour is called the preconsolidation stress and referred to as P c0 . As long as the effective stress is less than P c0 , the rock behaves elastically and the pore compressibility is low. If ever the effective stress exceeds P c0 , the compressibility increases significantly. In heavy oil fields of unconsolidated sand, the additional recovery due to compaction drive may be estimated between 0.5 and 1 % if the rock remains elastic during the field life and to more than 3% if the plasticity threshold is reached early. Experience has shown that it is almost impossible to determine P c0 from laboratory tests on sand samples, mainly because of cores disturbance. TOTAL has been investigating other methods to determine this parameter. The method presented in this paper is based on : • Laboratory tests on preserved intra-reservoir shales • A shale compaction model derived from the critical state theory of Cam-Clay and calibrated on a normal compaction trend for shales • Geological description of the deposition-erosion history on a basin scale.