This paper presents a lens model, based on a physical concept of the individual pay members, that is the first model designed to calculate reserves from infill drilling and pattern changes. The model conceptualizes the reservoir as being made up of a large number of randomly located circular disks or lenses sealed at the edge. Calculations of drain ability, floodability and secondary/primary ratio are presented for two calibrated lens-size distributions. The results of these calculations are compared with the performance of some Wasson Clearfork trend waterfloods.
Determining the optimum pattern and well spacing is one of the major challenges associated with the design and implementation of a waterflood or any other drive process. The best choice depends on the characteristics of the reservoir (including permeability, porosity, and continuity) as well as the process (including relative permeability, injectivity, productivity). Different operators, working in different reservoirs with different processes, have come up with different optimums. To arrive at the best choice, quantification of the ultimate recovery for each option and comparison of the economics are necessary. In discontinuous reservoirs, the recovery can be quite sensitive to the well spacing and choice of pattern.
In several studies, comparisons of well-established decline curves on two or more patterns established that unique oil reserves can be obtained from infill drilling. Barber et al. evaluated the performance of nine fields in the midcontinent U.S. and concluded that 56% to 100% of infill-well production could be considered unique reserves. Reviere and Wu evaluated nine projects in various formations in west Texas and found that all produced incremental oil profitably.