Velocity Evolution during Controlled CaCO₃ Precipitation and Dissolution

Carbonate rocks undergo changes when in contact with interstitial water. The occurring dissolution and/or precipitation alters the rocks pore structure, porosity, permeability, and the rocks acoustic properties. Depending on the rate of flow, these changes can occur extremely fast in time spans of months or days. As a result, the comparison of different vintages of acoustic data from different generations of log data or from time-laps seismic data might be compromised. We present laboratory experiments that help assess these diagenetic and petrophysical changes during precipitation and dissolution.

Precipitation experiments where performed in which loose ooid sediment was initially pressurized to 20MPa to ensure full compaction, and then filtrated with supersaturated CaCO₃ solution for ~90 hrs. Substantial precipitation occurred within days causing >10% of porosity reduction, and up to 20% of velocity increase. The morphology of precipitated crystals varied. In some samples, variable amounts of small crystals form on ooid surfaces but many of these crystals form directly at the grain to grain contact. These contact cements fuse the ooids together into a more coherent unit, creating a stiffer rock frame. In some samples, needle like crystals were precipitated, creating a finer grained, more complex, and softer framework.

Dissolution experiments where performed on cemented Pleistocene ooid grainstone samples. They were subjected to fluids under-saturated with regards to CaCO₃. Substantial dissolution within days caused a porosity increase of more than 10% and 3-5% velocity decrease. Acoustic velocity decreased (60m/sec in ~120hrs), but the observed decrease in velocity much smaller then model predictions based on the large increase in porosity.

All experiments produced permanent alteration of the rocks within days. The observed changes in velocity are inconsistent with theoretical model predictions. Velocity decreases proportionally less than the increase of porosity would indicate. Likewise velocity increases less than what can be explained by the porosity reduction. A possible explanation for this discrepancy is that both dissolution and precipitation drastically change the internal pore geometry, resulting in disproportional softening of stiffening of the rock.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California