This paper illustrates the surveillance methods used in Shell's 1-1/4 and 5/8 acre (0.5 and 0.25 ha) waterfloods in the South Belridge Diatomite field: (1) computer-assisted monitoring of injection pressures and rates, (2) online databases with well tests and allocated production and injection data, (3) step pressure tests in numerous water injectors, (4) a geological database with cycle markers and directional surveys, (5) sonologs, and (6) salinity tests. Methods (1) – (4) require use of custom software to be practical.
The geometry of waterflood patterns in the Diatomite (well spacing compared with length of hydrofractures, injectors in-line with or offset from producers, and pattern orientation relative to the direction of maximum in-situ stress) influences the rate and frequency of coupling between the injectors and producers. It is shown that wells in the "direct" 1-1/4 acre patterns are less prone to coupling than the "staggered" ones because a "linkage potential" (defined in the text) is higher.
The proximity of hydrofractures in the 5/8 acre staggered patterns makes the injector-producer coupling unavoidable if the patterns do not follow the direction of maximum in-situ stress. The coupling develops in the N20°±5°E direction, probably along the cycle tops. The step pressure tests of many injectors in waterflood Phases I through III have shown that hydrofracture extensions are common and we are currently unable to predict the "correct" injection pressures for individual wells. It is concluded that to avoid reservoir damage, each injector must be controlled individually. Injection pressures can be increased with time by trial-and-error, but the injection rate must be kept below a safe limit to preclude excessive damage if the hydrofracture is extended.