Aj. De Kock et al., Gulf of Mexico subsidence monitoring project with a new formation-compaction monitoring tool, SPE DRILL C, 13(4), 1998, pp. 223-230
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.