Induced Diagenetic Alteration of Carbonate and Aluminosilicate Minerals Caused by CO₂ Enhanced Oil Recovery

Leta K. Smith

Induced diagenetic alteration of reservoir minerals occurs when CO₂ is injected into a reservoir to enhance oil recovery. The induced diagenesis affects both carbonate and aluminosilicate minerals and potentially may cause production problems. The amounts, rates, and nature of the reactions occurring in the reservoir were studied by monitoring produced water chemistry and examining the mineralogy of seven Wyoming oil fields that were subjected to CO₂ treatments.

Feldspars undergo dissolution and/or alteration to intermediate clay phases. This is evidenced by increases in silica and aluminum concentrations that are as much as four to ten times that of pre-CO₂ concentrations. The increase in aluminum concentration lags behind that of silica by several days. This suggests that an aluminum-rich phase such as kaolinite is also dissolving from the reservoir but more slowly than the less aluminum-rich feldspars.

Carbonate minerals undergo extensive dissolution, contributing abundant calcium, magnesium, and bicarbonate to the post-CO₂-injection produced water. Although the amount is Insufficient to improve permeability, it does increase the potential for wellbore scale when dissolved components reprecipitate. The relative amount of carbonate mineral dissolution, as well as the potential for scale, are predictable from modeled post-CO₂ calcite saturation indices. Calcite dissolution procedes until equilibrium is attained with the injected CO₂, about six and eight months. Calcite dissolution rates are higher than dolomite except where PCO2 is very high. Under those circumstances, dolomite dissolution rates approach those of calcite. Rates for both are dependent u on the volumetric ratio of carbonate minerals in the reservoir to water-filled porosity.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995