Antelope shale (Monterey Formation), Buena Vista Hills field: Advanced reservoir characterization to evaluate CO2 injection for enhanced oil recovery

Very large volumes of oil in developed siliceous shale reservoirs of the so uthern San Joaquin basin, California, may be recoverable through carbon dio xide (CO2) flooding. More than 7 billion bbl of oil, trapped in the Miocene Monterey Formation, are present at relatively shallow depths in a series o f large and giant fields discovered during the early part of the 20th centu ry yet that have typical recoveries of less than 6%. Such low recoveries ar e due to the unique nature of the reservoir and to early completion practic es. Siliceous shales consist mainly of biogenic silica (original diatom fru stules) and varying amounts of clay and silt/sand. These rocks are thinly l aminated and typified by high porosities (> 30%), very low permeabilities ( < 1 md), extremely small pore throats, and variable degrees of fracturing. Early well completions, up through the 1950s, employed slotted liners and c ommingled production at high rates, thereby preventing interval evaluation and also causing rapid loss of reservoir pressure. To better understand the se reservoirs, and to establish baseline criteria for evaluating CO2 floodi ng as an option for enhanced recovery, a multiyear study sponsored by the U .S. Department of Energy and Chevron U.S.A. was performed in Buena Vista Hi lls field, focused on the Antelope shale zone (upper Monterey Formation). A combination of advanced reservoir characterization, three-dimensional eart h modeling, and now simulation was performed. Resulting data provide essent ial new insight into the detailed nature of these reservoirs, for which mos tly vintage log data (pre-1966) was previously available. Criteria establis hed for CO2 flood evaluation include current oil saturation levels, reservo ir pressure, lithologic heterogeneity, degree of fracturing, and well spaci ng. On the basis of these criteria, a site was chosen for a CO2 pilot test in Lost Hills field. This pilot is encompassed within the present Chevron-D OE study and will be covered in a future E & P Notes article.