Advanced Reservoir Characterization and Evaluation of CO2 Gravity Drainage in the Naturally Fractured Spraberry Reservoir

Abstract

Natural fractures exert a strong influence over oil production in Spraberry Trend Area reservoirs in the Permian Basin of west Texas. The importance of the fracture network has been known since the 1950s, but until recently, there has been very little detailed study of the fractures themselves. In 1996, a horizontal Spraberry well was cored as part of a DOE Class III Field Demonstration Project. Fractures from the horizontal core as well as other fractures encountered in vertical Spraberry cores were analyzed in detail for information on both large scale features including orientation and spacing and small-scale features such as the relationships between fracture mineralization and matrix rock composition. At least three sets of fractures are found within the upper and middle Spraberry cores. These sets have distinct orientations, spacing, mineralization, distribution with respect to lithology, and surface characteristics (Lorenz, 1997). Fractures found in the 1U zone of the upper Spraberry have a NE strike, and tend to be partly mineralized with barite, quartz, and dolomite. Distribution of these mineral phases can greatly affect conductivity between the fractures and the rock matrix. The 5U zone of the upper Spraberry contains fractures with NNE and ENE orientations. The NNE set of fractures has stepped fracture surfaces indicating a shear origin, and minor amounts of quartz and dolomite mineralization. The ENE fracture set has smooth planar surfaces of tension origin with some calcite mineralization present. Natural fractures in black shales overlying both the 1U and the 5U have an ENE orientation similar to unmineralized fractures in the 5U. No fractures were encountered in similar shales underlying reservoir zones. A set of hairline fractures, most completely healed with calcite cement was also found in some Middle Spraberry cores. The unique nature of each of these fracture sets implies that fracturing probably occurred as several separate events and indicates that the Spraberry Formation has undergone a more complex stress history than might be construed from its fairly flat-lying nature.