Study of Sonic, Neutron, and Density Logging of Low-Permeability Gas Sands. Final Report

摘要

Gas accumulations in Lower Tertiary and Upper Cretaceous formations are the object of widespread exploration in the Tight Western Gas Sands. The complex lithology of these formations has hindered the usefulness of the sonic, density, and neutron logs. Current log evaluation practices assume a matrix density of 2.68 gm/cc and a matrix travel-time of 52.6 microseconds/ft. The neutron log is calibrated for a sandstone matrix. Conventional analysis yields inconsistent and often contradictory results. Core and petrographic studies have been made on samples from Lower Tertiary and Upper Cretaceous formations in the Uinta Basin. Results indicated that a carbonate cement has filled much of the original porosity and altered the matrix density. Lower porosity samples tend to be heavily cemented and have matrix densities that approach, and even exceed, 2.68 gm/cc. Higher porosity samples tend to be lightly cemented and have matrix densities that approach 2.65 gm/cc. Log analyses in the Uinta Basin, supplemented by core data, reveal that the higher porosity samples have matrix travel-times that approach 55.6 microseconds/ft. The presence of the carbonate cement does not decrease the matrix travel-times as expected. Laboratory measured matrix travel-times substantiate these conclusions. Log analyses also indicate the neutron log, when calibrated for a sandstone matrix, will not accurately evaluate the higher porosity, non-shaly sandstones. Core and log analyses have been made on samples from the Upper Cretaceous Mesaverde formation in the Greater Green River Basin. The resulting pressure and temperature difference caused the physical properties of the Mesaverde to vary widely within the Greater Green River Basin. Matrix density and matrix travel-time for the Mesaverde are very different for the two wells. Neutron log response also varies considerably. (ERA citation 07:045729)