Fines migration and compaction in diatomaceous rocks

Reservoir rocks typically have clay and other fine material attached to pore surfaces. Fines mobilization is advantageous in that the removal of fines from pore walls exposes water-wet rock thereby aiding oil recovery. On the other hand, fines mobilization is deleterious if mobilized particles obstruct pore throats. In this work, waterfloods of diatomaceous cores up to 200 degrees C were conducted using field and outcrop samples to elucidate mechanisms of fines production at conditions potentially important to thermal oil recovery. Because the rock employed is also stress sensitive, it was necessary to also understand compaction in order to gauge any resulting changes in permeability. The effect of temperature on fines migration in situ was studied using time-lapse CT scanning and the resulting permeability reduction was measured and modeled. The temperature at which increased fines concentration is observed coincides with an increase in the range of oscillations of X-ray attenuation in an averaged area in the CT images. Permeability reduction of up to 80% was seen in some cases at high temperatures confirming the importance of temperature in the elution of fines. It was found that the permeability reduction is more sensitive to salinity and temperature in comparison to pH. Such modeling and experimentation provides guidance on appropriate mitigation strategies through completion or other methods. Effluent brine analysis confirms that silica dissolution is an important factor in hot water or steam condensate movement with all of the tests indicating that silica dissolved in increasing quantities with increasing temperatures, even producing precipitates at 150-200 degrees C. Silica in solution leads to a general decrease in the effluent pH due to silicic acid. Ion exchange was also observed that is consistent with the Hofmeister series. Cations of large valency and smaller hydrated radii displaced cations resident in the rock. The effect of confining pressure on permeability was investigated by repeated loading and unloading of outcrop cores at different intermediate pressures and temperatures. With increasing net effective stress, significant permeability degradation was observed with a noticeable acceleration at about 600 psia. Mercury porosimetly of damaged material indicates that changes may be due to pore collapse. Almost no permeability recovery was detected when the core was unloaded. Significant creep was observed with significant permeability reduction resulting from a constant applied confining pressure. Permeability reduction at a slightly smaller effective stress was observed for the high temperature cases but no significant material differences in the primary compaction curves were detected between 25 degrees C and 100 degrees C. Limited rebound suggests that the stress history of diatomites may be a significant factor in determining the degree of acceptable depletion in the field development planning for thermal operations. (C) 2014 Elsevier B.V. All rights reserved.