Gulf of Mexico subsidence monitoring project with a new formation-compaction monitoring tool

Formation compaction in unconsolidated geopressured turbidite reservoirs in the Gulf of Mexico (GOM) is an issue of great uncertainty and concern. Whe n compaction occurs, it changes the porosity and permeability properties of the reservoir rock and can affect recovery efficiency and well productivit y. It can deform well tubulars, creating operational problems and shortenin g well life. If compaction is significant, especially when multiple stacked reservoirs are involved (as in the case in this field example), then compa ction can create a subsidence bowl at the ocean floor. In GOM offshore oper ations, this could cause platforms to subside deeper into the water and cre ate potentially severe safety problems; therefore, failure to properly addr ess issues of compaction and subsequent subsidence during the design and de velopment phases of these capital intensive, deepwater projects could lead to severe financial setbacks. It is crucial to have a compaction monitoring program in place because of t he broad impact of compaction and seafloor subsidence. As a result, we deve loped a new formation-compaction monitoring tool (FCMT) and new methods of measurement and interpretation. The FCMT is a wireline device that uses mul tiple gamma ray detectors to determine locations of and precise distance be tween radioactive (RA) markers planted in the formation or casing. Compacti on of the formation can be measured by changes in the distance between the markers. For precise estimation of the vertical distance between a pair of markers, the new method uses an array of three or four detectors. By examining the tool response to a marker, we developed a new method to de termine the exact vertical and lateral location of the marker by using a Lo rentzian response model; consequently, not only the vertical compaction but also lateral displacement of markers can be monitored with the new method. The accuracy of the tool was established in the test facility where gamma r ay sources were placed at precisely known intervals. The tools were run cen tralized at three logging speeds (5, 10, and 15 ft/min.), and data were col lected at 0.1-in. intervals. The vertical distances between a pair of RA ma rkers spaced 30 ft apart were measured accurately to within 0.1 in. The first baseline logs were collected successfully in the four wells in a GOM deepwater development. High consistency among measurements from differe nt logging passes proved that the FCMT can provide precise distance measure ments with newly developed methods.