Cased-hole Nuclear Logging Interpretation, Prudhoe Bay, Alaska

Monitoring reservoir fluid movements in the Prudhoe Bay Field on the North Slope of Alaska has been accomplished by the use of cased-hole compensated neutron logs, pulsed neutron logs, and gamma ray logs. Cased-hole nuclear logging data have been collected over time (timelapse) in a majority of the wells in the field and are utilized to make reservoir management decisions. Cased-hole compensated neutron logs are used to monitor gas contact movement, sand production across perforations, and filtrate dissipation when compared to the open-hole compensated neutron logs. Log examples of gas contact monitoring (gas underrunning shales, gas coning) as well as examples of sand production are included. Monitoring filtrate dissipation with neutron logs gives an indication of reservoir rock quality. Examples of how this was utilized to develop perforation strategies in the Sag River Sandstone are also included. The monitoring of water movement has been attempted by the use of repeat cased-hole pulsed neutron logs. The capture cross section of Prudhoe Bay formation oil and water provides limited range in measured sigma from oil and water bearing sands. This limited environment for pulsed neutron logs requires a base-monitor approach with particular attention to log repeatability and borehole changes. Log examples of attempts to monitor water movement in Prudhoe Bay are included as well as a discussion of the methods used to improve pulsed neutron log interpretation in this area of marginal applicability. Cased-hole gamma ray logs, generally run with compensated neutron logs or pulsed neutron logs, monitor radioactive scale precipitation across perforations, producing significant amounts of formation water. Log examples of gamma ray increases across perforations and their use in identifying water producing intervals are included.