Experimental mechanical and chemical compaction of carbonate sand

Abstract

Uniaxial compression tests were conducted on bioclastic sand and crushed calcite crystals. Mechanical and chemical processes were investigated to better quantify petrophysical properties of carbonates and their evolution with burial or during fault zone processes. The grain size was in the range 63–500 μm, and the samples were saturated with water in equilibrium with carbonate, glycol, decane, or air. During loading, effective stress was increased to 32 MPa. Mechanical compaction processes (i.e., grain rearrangement, crushing) could be separated from chemical processes (i.e., pressure solution, subcritical crack growth). P and S waves monitored during the tests showed low velocity in samples saturated with reactive fluids. This suggested that chemical reactions at grain contacts reduced the grain framework stiffness. Creep tests were also carried out on bioclastic sand at effective stress of 10, 20, and 30 MPa. No creep was observed in samples saturated with nonreactive fluids. For all the samples saturated with reactive fluids, strain as a function of time was described by a power law of time with a single exponent close to 0.23. Parameters controlling creep rate were, in order of importance, grain size, effective stress, and water saturation. Microstructural observations showed that compaction of bioclastic carbonate sand occurred both mechanically and chemically. Crack propagation probably contributed to mechanical compaction and enhanced chemical compaction during creep. Experimental compaction showed that compaction of carbonates should be modeled as a function of both mechanical and chemical processes, also at relatively shallow depth and low temperature.