Abstract
Laboratory experiments have been performed to simulate in situ and core compaction behavior of soft reservoir sandstones. Fine-grained synthetic sandstones have been manufactured under simulated in situ stress conditions with various cement contents. A systematic study shows that in situ compaction is close to elastic only under initial conditions, and that plasticity develops gradually during compaction. Two mechanisms control rock alteration as a result of stress release during coring: Cement bond breakage, which leads to softer and more stress sensitive core material at low stresses, and grain rearrangement into a denser packing, leading to permanently reduced porosity and hence increased stiffness compared to in situ behavior. The relative importance of these mechanisms depends on the degree of cementation, on the coring stress path, and on the stress level. Ultrasonic velocities have been measured and linked to time lapse seismic response of soft reservoirs and to the feasibility of predicting it with core material. Time dependent (creep) deformation has been observed, and appears related to the evolution of plastic strain.