APPLICATION OF 3D SEISMIC DATA TO THE DEVELOPMENT OF GAS STORAGE RESERVOIRS IN THE MIST GAS FIELD, COLUMBIA CO., OREGON

Four depleted natural gas pools in the Mist Gas Field, Columbia Co., Oregon have been converted for use as underground gas storage reservoirs since 1988. 3D seismic and horizontal well technology were recently used to convert two of these. The deltaic late Eocene Clark and Wilson Sandstone of the Cowlitz Fm. comprises the reservoir rock in the gas field. Cowlitz Fm. mudstones, siltstones and sandstones of turbidite origin make up the cap rock. Normal faulting is the trapping mechanism in the gas field. Fault throws range from less than 4 m (at or below seismic resolution) to greater than 150 m. The faults are not sealing where sandstone is in contact with sandstone. To image several depleted gas reservoirs in the Calvin Creek Storage Area, a high frequency, small bin (12m) 3D seismic survey was conducted. Well data was tied to the seismic data and the data set was interpreted. The velocity gradient, used for depth conversion, was best approximated by integrating processing stacking velocity with velocities calculated at wells. The resulting depth map coupled with accurate location of the pool boundaries (including depth of new gas water contact) enabled precision horizontal well placement. Intra-reservoir faults, some with throws as small as 4 m, were mapped and evaluated for impact on reservoir drainage. Reservoir rock core data and well bore image data were qualitatively tied to seismic attributes during the horizontal well design phase to guide the well trajectory within the reservoir.