Reservoir Characterization of a Reefal Carbonate for Crestal CO₂ Flood, Reinecke Field, West Texas

Abstract

Reinecke field is a carbonate buildup in the southern part of the Horseshoe Atoll. Since discovery in 1950, it has produced more than 82 million barrels of oil. The south dome of Reinecke field has been characterized with core, wireline logs, 3D seismic, crosswell tomography, and 3D cellular models of porosity, permeability, and fluid saturations. Four main depositional sequences, approximately 60-80 ft (18-24 m) thick, were identified in the Upper Pennsylvanian reservoir interval. Nine depositional facies were recognized, including mudstones, wackestones, packstones, grainstones, and boundstones. The reservoir is approximately 70 percent limestone and 30 percent dolomite. Porosity is widespread in both lithologies. Most depositional facies have average porosities of 9-13 percent where still limestone. Important pore types in limestones include intercrystalline microporosity, molds, intergranular pores, fractures, and vugs. Limestones dominated by microporosity have low permeability, commonly 1-30 mD. Limestones with fractures and vuggy pores commonly have permeability greater than 100 md. Lime mudstones are rare but have distinctly lower porosity (average of 1.4 percent) and permeability (average of <1 mD). Average limestone porosity is 11.2 percent, and average limestone permeability is 165 mD. In contrast, dolomite has generally lower porosity (average of 8.3 percent) but much higher permeability (average horizontal of 894 mD). Discontinuous shales compose less than 1 percent of the gross reservoir. Discontinuous lime mudstones and shales in the lower part of sequences form low-permeability baffles. Therefore, the south dome of Reinecke is characterized by relatively continuous vertical and horizontal porosity and permeability with high-permeability streaks and discontinuous low-permeability baffles. Excellent reservoir continuity and water injection into the underlying aquifer have allowed a good bottom water-drive and excellent primary and secondary recovery (55 percent of the original oil in place). Pore systems that are well connected throughout the reservoir have allowed a crestal CO₂ flood to be designed for Reinecke field. CO₂ is being injected into the top of the structure to mobilize residual oil and push an oil bank down through the reservoir to recover residual and bypassed mobile oil.