MINERALOGICAL RESPONSES OF SILICICLASTIC CARBONATE-CEMENTED RESERVOIRS TO STEAMFLOOD ENHANCED OIL-RECOVERY

Rock-fluid interactions induced by steamflood enhanced oil recovery we re investigated in laboratory simulations to determine the geochemical reactions and the effects of these reactions on reservoir permeabilit y. Flow-through laboratory experiments using mixtures of quartz, kaoli nite, and siderite were performed in a high temperature/high pressure permeameter at a confining pressure of 1200 psi and temperatures betwe en 150-250 degrees C. Fluid compositions used in the experiments simul ated the vapor and residual liquid phases encountered in steamflood op erations as well as an intermediate fluid composition. Effects of flui d pH, fluid salinity and flow rate were systematically investigated in the experiments. The most extensive fluid-rock interactions were obse rved in the vapor phase simulations and high temperature/high pH condi tion simulations. Smectite, chlorite, illite, mixed-layer clays, green alite, analcime, and K-feldspar were all identified as products of roc k fluid interaction in the experiments. Smectite was the dominant auth igenic phase to reduce permeability in the experiments. The experiment s showed that the formation of smectite in Fe-rich environments does n ot require a clay precursor. Smectite is likely the most damaging neof ormed mineral to reservoir permeability under different hydrogeochemic al conditions for several reasons including: (1) its relatively high s urface area (including microporosity in the ''honeycomb texture''), (2 ) its propensity to migrate and block pore throats during fluid flow i n porous media because of its small particle size, pore-lining texture , and electrochemical surface properties, and (3) the wide range of st ability of smectites in the physical and chemical conditions that exis t in reservoirs undergoing steamflood EOR. The rapid precipitation of authigenic minerals in these experiments suggests that the period requ ired for fluids and rock to reach equilibrium in diagenetic environmen ts are extremely short when considering geologic time scales. The armo ring of pre-existing minerals by grain-coating authigenic minerals app ears to result in the attainment of local equilibrium conditions prior to when one would predict assuming a continuous supply of reactant mi nerals was present. (C) 1998 Elsevier Science Ltd. All rights reserved .