Project Rio Blanco - additional production testing and reservoir analysis

Additional subsurface investigations of the Rio Blanco detonation region and adjacent reservoir have been under way since the 1975 Technical Committee meeting at the IAEA. The lowermost explosion cavity has been re-entered and a production test from it has been performed. A dry gas volume of 7.6 x 105 m3 (STP) (27 x 106ft3(STP)) was withdrawn. Chemical and radiochemical analyses of this gas show that (a) the yield of the bottom explosive was 31 +- 2 kt; (b) the cavity/chimney volume was 2.4 x 104m3 (8.4 x 105ft3); (c) about 7% of the tritium produced is associated with the gas; and (d) a slight (approximately 0.1%) gas contribution from the middle explosion region was noted. The reservoir/chimney model implies an unstimulated reservoir flow capacity of 0.15 mdarcy.m (0.50 mdarcy.ft) connected to the bottom chimney region. A cavity radius of 21 +- 3 m (70 +- 10 ft) was deduced. Unstimulated reservoir production parameters were investigated in a well offset 190 m (625 ft) from the emplacement hole. Insufficient productivity was obtained in the Mesa Verde formation (in which the bottom explosive was detonated) to evaluate reservoir properties. The productive sandstones in the Fort Union formation adjoining the top detonation region were individually evaluated. Their aggregate flow capacity was determined to be 0.14 +- 0.02 mdarcy.m (0.45 +- 0.08 mdarcy.ft). A numerical simulation model which incorporates these data is described. The lack of a high-permeability connection between the three explosion regions remains unexplained. The two chimney re-entry wells have been cemented to the surface and abandoned. The offset well has been plugged in a way which preserves the option for additional subsurface investigation in the future. Project facilities have been removed and the site restored to conditions which minimize environmental impact. (author)