Fines migration and formation damage.

Abstract

This study focuses on a reduction in the permeability of porous sandstones due to migration of in-situ colloidal fines. The problem manifests itself in the release (or detachment) of fine alumino-silicate particles from the pore walls when contacted by a low salinity or high pH solution. The detached particles are entrapped at pore constrictions reducing the area available for flow and leading to drastic reductions in the permeability (damage) of the porous medium. The current study elucidates the effect of ionic strength (salinity) and pH on permeability reduction. Solutions having high pH (greater than 10) and low salinity (10$\sp{-5}$ M) generally enhance the detachment of fines and hence are detrimental to the permeability. A 100-1000-fold reduction in permeability was observed when alkaline solutions of Na$\sp+$, K$\sp+$, NH$\sb4\sp+$ were used. However, no damage occurred when Ca(OH)$\sb2$ was used. This difference can be explained by the variation in the surface potentials of fines in the different solutions. While an abrupt change in salinity reduced the permeability drastically (factor of 1000), a gradual reduction of injection salinity caused marginal reduction in permeability (factor of 2). Until this thesis, the above disparity could not be explained by the DLVO or other existing theories. Further experimental analysis shows that ion-exchange played a key role in the release mechanism. A decrease in ionic strength induces an increase in pH which, in turn, has a synergistic effect on the release process. This idea was reinforced when damage was prevented by the addition of a buffered acid to minimize pH change during an abrupt salinity decrease experiment. The pH change can also be modulated by varying the rate of decrease in salinity and hence damage can be minimized. A convective-dispersion model incorporating mass-transfer effects and ion-exchange was developed to represent the brine chemistry at the rock/fluid interface and its influence on salinity and pH. These predictions were further combined with a comprehensive phenomenological model to predict reduction in permeability. The model was able to accurately predict the changes in brine chemistry and permeability. In addition to explaining the results of earlier researchers, this work also was able to resolve conflicting interpretations of findings existing in this area for the last two decades.