The Effects of Steam Injection In a Sandstone Reservoir (Etchegoin Formation), Buena Vista Field, California

Abstract

At Buena Vista field, California, 120 ft of post-steamflood core, spanning the middle Pliocene Wilhelm Member of the Etchegoin Formation, was taken to assess the influence of stratigraphy on light-oil steamflood (LOSF) processes and to determine what steam-rock reactions occurred and how these affected reservoir properties. High-quality steam (600F (300C)) had been injected ({approximately}1,700 psi) into mixed tidal flat and estuarine facies in an injector well located 55 ft from the cored well. Over a period of 20 months, steam rapidly channeled through a thin ({approximately}7 ft), relatively permeable (1-1,000 md), flaser-bedded sandstone unit. Conductive heating above this permeable unit produced, in the vicinity of the cored well, a 35-ft steam-swept zone (oil saturation = 0), overlain by a 29-ft steam-affected zone in which oil saturation had been reduced to 13%, far below the presteam saturation of 30%. Steam-induced alteration ('artificial diagenesis') of the clay-rich reservoir rock was recognized using SEM, petrography, and X-ray diffraction. Salient dissolution effects were the complete to partial removal of siliceous microfossils, Fe-dolomite, volcanic rock fragments, and labile heavy minerals. The artificial diagenetic effects are first encountered in the basal 6 ft of the 29-ft steam-affected zone. Based on the distribution of the authigenic phases,more » the authors conclude that the reactions took place, or were at least initiated, in the steam condensate bank ahead of the advancing steam front. Although these changes presumably reduced permeability, the steamflood process was effective in reducing oil saturation to zero in the steam-contacted portion of the reservoir.« less