ENHANCEMENTS TO GPS BASED SUBSIDENCE MONITORING AT THE WILMINGTON OIL FIELD

Subsidence was first identified in the Wilmington Oil Field in the 1940s. The City of Long Beach, Gas and Oil Department (LBGO) has been conducting surface elevation surveying using spirit leveling and, since 2002, the Global Positioning System (GPS). Recently, software for the LBGO Deformation Network of 12 real‑time GPS stations was upgraded with new state‑of‑the‑art technologies and solution to provide more reliable and accurate continuous monitoring to support elevation survey campaigns. Current oil field waterflood management practices as they relate to surface elevation changes and benefits from the software upgrade for Wilmington Oil Field subsidence monitoring and control are discussed. Waterflood operations in the Wilmington Oil Field are managed for effective oil recovery and injection/production balancing while maintaining stable surface elevations for continued regional economic growth. For cost‑effective and timelier surface elevation monitoring, a real‑time GPS network provides control for all LBGO subsidence monitoring activities. Using mobile GPS survey equipment, quarterly and semi‑annual elevations of over 240 bench marks are calculated from the 12 permanent GPS stations' elevations. The LBGO GPS network and software allow for real‑time elevation monitoring of key locations and automated data processing, with the regional picture updated semiannually. Surface elevation contour maps and individual survey bench mark elevation trends as a function of time allow for timely adjustments to the waterflood operations to maintain effective oil recovery. Real‑time GPS monitoring of surface elevations provides the capability for early detection of surface deformation resulting from production/injection imbalances. The necessary operational adjustments can then be identified and implemented to mitigate the situation. The advantage of early detection is in minimizing the extent of irreversible reservoir rock compaction, resulting in more stable future surface elevations, and preventing elevation changes detrimental to surface infrastructure.