Effects of Carbon Dioxide on Sandstone Reservoir Quality

T. R. Klett, W. J. Harrison, and R. F. Wendlandt

Core-flood experiments were conducted using subarkosic Weber Sandstone (Rangely Field, Colorado) to quantify dissolution processes of carbonate cements as a function of temperature, pressure, fluid salinities, and CO₂ fugacities typical of hydrocarbon reservoirs. Experimental results are consistent with fluid chemistry and petrographic observations of secondary porosity attributed to corrosive fluids in natural sandstones. Dolomite and/or calcite are the dominant authigenic cements. Experimental temperatures were 75 and 100°C, pore pressures were to 230 bars, and confining pressure was 300 bars. The flooding fluids were 0.25 and 0.5 molal NaCl solutions containing up to 0.3 molal CO₂. Preliminary geochemical modeling indicates pH values were about 3 to 4 und r experimental conditions.

Initial effluent fluids contain high Ca²⁺ and K⁺ concentrations (up to 1000 and 60 mg/L, respectively) that decrease with experimental duration. Effluent concentrations of Mg²⁺ increase with time in dolomite-cemented samples but decrease in calcite-cemented samples. High initial concentrations of Ca²⁺ are interpreted to reflect dissolution of outer calcium-rich cement rims and smaller cement crystals. With additional fluid, magnesium-rich cement cores and larger crystals are attacked. Petrographic examination of post-experiment core plugs shows cement dissolution is more effective near the inlet end. Isolated, heterogeneously distributed carbonate cements in the more porous and permeable laminae are attacked more readily than poikilotopic ce ents in finer-grained, less porous and permeable laminae. Cement crystal morphologies indicate dissolution occurred by initial breaching of the outer zones, followed by preferential dissolution of less-resistant zones or cores.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California