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doi:10.2204/iodp.proc.327.108.2011

Summary of Expedition 327 tracer injection operations

The sequence of pumping and tracer injection operations is summarized with plots that show the timing of pumping and tracer addition operations (Fig. F9). The 24 h pumping experiment began after a 1 h period of monitoring to establish a pressure baseline following landing of the casing running tool, which positioned the end of the stinger just at the end of 10¾ inch casing, near the top of the open basement interval. The mud pumps were set to inject fluid at 20 SPM (6.7 L/s), and this flow rate was held virtually constant throughout the next 24 h. The total pumping rate into the formation reached 11.3–13.4 L/s for four brief periods of 6–8 min each, when salt and fluorescent tracers were added to the injectate using the cement pumps (Fig. F9A).

The primary injectate fluid was surface seawater, but there were two 1 h periods during which fresh (drill) water was injected. We pumped freshwater during these times to establish easily identifiable changes in bulk water composition that can be used to determine the rate of downhole injectate collection with the RO fluid samplers (Fig. F9B). In addition, the switch from salt- to freshwater and back again might be useful as a single- or cross-hole tracer, although sensitivity to this change is likely to be less than that provided by gas, metal salt, and particle tracers because of dilution during transport (Table T1). The actual salinity of the freshwater injected during pumping hours 0100–0200 and 2000–2100 will be determined by postcruise analysis of injectate samples collected at the rig floor and downhole, as will the elevated salinity during brief periods of salt tracer injection.

The SF₆ gas injection rate was held relatively constant during the injection experiment, so the gas concentration should vary mainly during brief periods when the overall pumping rate increased because the cement pumps were turned on (Fig. F8B). During these brief periods, the dissolved SF₆ concentration in the injectate should have been reduced. During the first period of salt tracer injection, we attempted to compensate for the increase in fluid pumping rate by raising the differential pressure on the SF₆ gas regulator, but this was impractical because of difficulties in determining and maintaining the total fluid flow rate in real time. In fact, the 24 h injection period is likely to appear much like a single pulse injection when tracer is detected at other boreholes weeks to years after Expedition 327, so there is likely to be little practical benefit in making microadjustments to tracer injection rates based on short-term changes in fluid injection rates.

The total quantities and concentrations of fluorescent microspheres and microbes injected during the experiment are known to within an order of magnitude, but we will analyze injectate samples collected both at the rig floor and downhole to improve our understanding of how these tracers were introduced to the subseafloor environment. Particulate samples could have clumped during mixing and injection, and concentrations and total masses could have been reduced during passage of the tracers down the drill pipe because of dilution and interactions with pipe grease. One goal of the Expedition 327 experiment was to learn how the addition of tracers to water pumped into the formation influences shipboard operations and to assess how operations and tracer injection systems might be modified in the future to improve similar experiments. A preliminary assessment of project success may be available following CORK servicing activities in summer 2011, when the first subseafloor samples are recovered and analyzed and the first estimates of solute and particle transport rates based on tracer injection are completed.

As of the end of Expedition 327, there is a network of six long-term CORKs monitoring pressure or fluid compositional responses to both the 24 h pumping experiment run during Expedition 327 and a long-term cross-hole experiment that will begin in summer 2011 (Fig. F10) (Fisher, Wheat, et al.). The long-term experiment will be run by installing a flowmeter and opening of a large-diameter ball valve on the wellhead of the CORK in Hole U1362B, the same hole that was used for tracer injection during Expedition 327. This will allow formation fluids to discharge at 5–10 L/s from Hole U1362B, which could cause tracers that previously migrated away from Hole U1362B to flow back toward this observatory. Fluid samplers installed on the wellheads and at depth in Holes 1026B, U1301A, U1301B, U1362A, and U1362B will collect samples for evaluation of tracer concentrations in borehole fluids from these holes. These holes are aligned in a direction (N20°E) that is parallel to the structural strike of basement in this area.

Hole 1027C is oriented at an azimuth of N95°E and located 2.3 km from Hole U1362B. Because Expedition 327 researchers were not able to recover the first-generation CORK in Hole 1027C, deepen the hole, or install a new observatory system (see the “Site 1027” chapter), there is presently no fluid sampling in Hole 1027C. This is unfortunate, but if the dominant flow direction in basement in this area is oriented along structural strike (Fisher et al., 2003, 2008; Hutnak et al., 2006; Wheat et al., 2000), then it is unlikely that tracers injected in Hole U1362B would be detected in Hole 1027C. Researchers are preparing to retrofit the Hole 1027C CORK wellhead for continued monitoring of pressure during ROV operations in summer 2011, and it may be possible to install a fitting that will also permit borehole fluid sampling in this hole in the future. This would allow monitoring during a later period of the current tracer experiment (or as part of a future tracer experiment) to test the hypothesis that there is relatively little advection of solutes or particles in a direction oblique to that favored by the structural trend of regional basement.

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