USE OF MODFLOW TO INCORPORATE CENEZOIC STRUCTURAL CHANGE WHILE SIMULATING UNDERPRESSURES IN THE CRETACEOUS AND PALEOZOIC GEOHYDROLOGIC UNITS OF THE GREAT PLAINS IN COLORADO AND KANSAS

Drillstem tests conducted in the Cretaceous and Paleozoic geohydrologic units located in the American Great Plains of Colorado and Kansas exhibit areas where pore fluid pressure is below normal hydrostatic pressure for the depth considered (underpressure). These underpressure conditions are attractive for the sequestration of carbon dioxide for two reasons; the lower pressures result in a reduced cost of injection and the lack of recharge in the west inferred by others suggests long return times to surface outcrops in the east.

One explanation for underpressures is that they are a transient condition driven by structural changes that occurred during the Cenozoic Era, such as exposure of outcrops in the eastern Great Plains and tilt of the land surface. To investigate this hypothesis, two series of MODFLOW models were developed. The first series represents a theoretical structural change over a period of 10 million years during the Laramide orogeny. This model series consists of 10 coarse-grid cross-sectional models with 6 layers that simulate a period of increasing west to east tilt having a maximum rise of 5,000 ft (1,524 m) in the west along the Rocky Mountain Front. The models’ western boundary is defined by the Rocky Mountain Front (Golden Fault) and is assumed to be a no-flow boundary. The models’ eastern boundary extends into central Kansas, where the confined units outcrop, and all layers are represented by constant-head boundaries. Each model in the series represents 1 million years. Sensitivity analyses were conducted with hydraulic conductivities that range from 0.001 to 0.1 m/d and storage coefficients that range from 10^˗5 to 10^˗7. The second series of models with 8 layers simulates a 10 million year period of increasing westward dip extending from the Colorado/ Kansas border to the Rocky Mountain Front during subsidence of the Denver Basin. The western boundary was modeled as both a constant-head and a no-flow boundary; a hydraulic conductivity value of 0.01 m/d and a storage coefficient of 10^˗5 were used. The transient hydraulic head changes and associated underpressure values calculated by the models compared favorably with the present-day values measured during oil exploration drillstem tests.