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Comparison of Reservoir Management Strategies in Fractured Carbonate Versus Sandstone Reservoirs: Lessons Learned from Giant Fields

Jack Allan1, S. Qing Sun2, and Rod Sloan3
1 C&C Reservoirs Inc, Fullerton, CA
2 C&C Reservoirs Inc, Houston, TX
3 C&C Reservoirs, Maidenhead, Berks, United Kingdom

Reservoir characteristics and production histories of 65 tight, naturally fractured carbonate and sandstone oil reservoirs, many of them giants, were compared to evaluate similarities and differences in successful reservoir management strategies between the two groups. In tight solution-gas drive carbonate reservoirs, fractured sweet spots exhibit high productivities and oil recoveries. Targeting sweet spots makes good sense, since these reservoirs often produce for decades with little risk of water incursion. Pressure maintenance programs may be necessary for reservoirs close to the bubble point. In tight aquifer drive carbonate reservoirs, production rates in fractured sweet spots are also high, but water incursion may soon follow. The highest recovery factors are generally attained in less fractured parts of a reservoir, which produce for longer times at lower water cuts. When flow rates are optimized, many reservoirs achieve good recovery under primary depletion. In tight gas-cap expansion drive carbonate reservoirs, gas channeling and water coning through fractures often leave matrix blocks unswept. Early initiation of pressure-maintenance gas injection is critical for achieving acceptable recovery.

Tight fractured sandstone reservoirs tend to have labyrinth-style reservoir architectures with poor aquifer support. Most produce by solution-gas drive. Primary recovery factors are low, so secondary recovery programs are essential. Pattern waterflooding is frequently used because the close proximity of injectors to producers provides the best control over sweep efficiency in reservoirs with poor lateral connectivity. Fractured sandstones experience problems less commonly encountered in fractured carbonates, such as swelling clays, fines migration, and gouge-filled shear fractures that compartmentalize reservoirs.