Historical Background of Waterflooding the D Sandstone
Waterflooding of D Sandstone reservoirs began in the 1960's. Production by primary depletion from the D Sandstone has been good to excellent; however, secondary recovery by waterflooding has been disappointing. In 1974, there were 37 waterflood projects in the D Sandstone in Colorado according to a U.S. Bureau of Mines Report of Investigations (Biggs and Koch 1974). Data from the 37 D Sandstone waterfloods indicates incremental recovery of only 50 stb/ac-ft by waterflooding from 65 percent of the projects. Waterflood projects in the general vicinity of the Sooner Unit had marginal to negative incremental reserves compared to primary production extrapolations. Table 3 shows data from the waterflood projects in the area of the Sooner Unit.
Bureau of Mines Report of Investigations No. 7959, which tabulated statistics in 1974 on waterflooding oil fields in Colorado, is an excellent resource for production data from D Sandstone reservoirs in the Denver Basin. However, the authors of the report did not attempt to qualify the statistics on recovery by giving reasons for good or poor recovery. Coincidentally after the report was published, interest waned for waterflooding the D Sandstone in the Colorado portion of the Denver Basin. These statistics were subsequently used by engineers on technical committees for proposed waterflood projects to demonstrate that the D Sandstone was a poor waterflood candidate. Between 1974 and 1992, only two waterflood projects were approved by the Colorado Oil and Gas Commission. Some of the popular reasons which were proposed by engineers for poor waterflooding recovery were 1) high Dykstra-Parsons coefficient of permeability variation and 2) high gas-saturation at the end of primary depletion.
The average primary recovery of D Sandstone waterflood projects in the area surrounding the Sooner Unit, shown in table 3, is 16.1 percent of OOIP. The total recovery after primary and waterflooding is 17.7 percent. An average incremental recovery of only 1.6 percent after waterflooding is demonstrated by these fields. All of these waterflood projects, except the Jackpot Field, are technical failures. The decision to risk waterflooding at the Sooner Unit was influenced to a large degree by the fact that the Sooner reservoir thickness is nearly double the average of the fields listed in table 3.
