Abstract

The Spraberry trend area of west Texas, once known as the "largest uneconomical field in the world," contains as much as 10 billion bbl of original oil in place. These hydrocarbons are stratigraphically trapped in fine-grained, low-permeability, and naturally fractured siltstones and sandstones deposited in submarine fans of Permian age. Despite five decades of production, including several large-scale waterflood projects, recovery from the Spraberry rarely exceeds 8-12%. Lack of scientific attention since the 1960s has resulted in most core samples suffering deterioration and loss, hampering any application of new methods for reservoir analysis.

A significant new effort has been launched to correct this situation and to evaluate the efficiency and economics of using carbon dioxide (CO2) flooding to enhance recovery from Spraberry reservoirs. This effort has involved collection of new core samples, including the first horizontal cores taken in the Spraberry. Eight lithofacies and six major rock types are identified, with coarse siltstones and very fine grained sandstones as the primary reservoirs. These rocks exhibit porosities and permeabilities of 7-18% and 0.3-3 md (average <1 md). Reservoir quality is limited by grain type and size, clay content, and degree of secondary dissolution porosity. Rock-log modeling indicates a need to revise pay zone identification based on gamma-ray log data; more reliable pay zone cutoffs are shale volumes of less than 15% and effective porosities greater than 7%. Three sets of fractures were identified in the horizontal cores, with north-northeast, northeast, and east-northeast orientations. Comparisons with historical data imply that fracture spacing and orientation are localized both areally and in individual reservoir units. Engineering analyses of Spraberry core indicate that the oil-saturated matrix is weakly water wet. Displacement of matrix oil by water imbibition is an important process in recovery during water injection. Laboratory study of CO2 gravity drainage in Spraberry core suggests that significant additional oil can be recovered in this manner; however, the degree to which this can be achieved at the field scale, within a time frame enhancing to profitability, remains undetermined. Future work will seek to resolve this question by implementing a pilot CO2 injection program in the E. T. O'Daniel unit. Together, these new studies will provide an important base of data for future evaluations of Spraberry potential.

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