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Hydrologic experiments

Drill string packer experiments

A drill string packer was used during Expedition 327 to isolate and test the hydrological properties of the basement section in Hole U1362A. The tools and methods employed were similar to those developed and applied during ODP, especially as described for ODP Legs 139 (Shipboard Scientific Party, 1992b) and 168 (see Appendix of Becker and Fisher, 2000). In summary, the method involves

  1. Activating the inflatable element of the packer to isolate the formation to be tested;

  2. Using the rig mud pumps to pump seawater into the isolated formation in a controlled fashion, either as short (<1 min) pressure impulses for “slug tests” or at a constant rate for “injection tests” lasting tens of minutes to hours; and

  3. Recording the pressure response in the isolated zone and at the rig floor standpipe.

The pressure records can be interpreted to estimate the average or bulk permeability of the isolated zone close to the borehole.

The methods used in previous JOIDES Resolution packer operations were modified and improved during Expedition 327 as follows. A new generation of electronic pressure gauges was used to record both downhole pressures in the isolated zone and rig floor pressures at the standpipe. Downhole pressures were recorded with 0–10,000 psi Micro-Smart HT-750 gauges having a programmable sampling rate and memory capacity for up to 500,000 data points. These gauges are much smaller than those used in previous ODP/IODP packer experiments, and they were deployed in a newly designed gauge carrier attached to the go-devil that enables packer inflation. Pressures at the rig floor were recorded at comparable sampling rates by the standard Rig Instrumentation System standpipe pressure gauge. Injection test duration was also extended. Following Expedition 301 procedures, we expected to run injection tests on the order of 1–2 h, longer than the typical 20–30 min tests conducted during ODP.

24 h tracer injection/hydrologic experiment

A long-term tracer injection test unlike any previously conducted on the JOIDES Resolution was run in Hole U1362B in order to assess rates and patterns of fluid flow in basement and to quantify basement hydrogeologic properties across a range of spatial scales (Fisher, Cowen, et al.). This test was conducted after the new CORK was installed in Hole U1362A. This test included 24 h of continuous injection (at a nominal injection rate of 5–7 L/s), with the addition of multiple tracers, to produce pressure, chemical, and microbial signals to be monitored with instruments deployed in Hole U1362B, and with gauges and fluid samplers deployed earlier in nearby CORKs in Holes 1026B, 1027C, U1301A, U1301B, and U1362A. Because the inflatable drill string packer is subject to deflation on large heaves and relies on friction between the packer element and the borehole or casing wall to hold position, we ran the 24 h hydrologic experiment in Hole U1362B using a casing running tool in lieu of a drill string packer. The casing running tool was lowered into the throat of the reentry cone and then set down such that it was held mechanically by the 10¾ inch casing hanger and sealed by an O-ring between the running tool and hanger. This configuration allowed use of the same downhole pressure gauges used in the drill string packer experiments, in addition to multiple high-rate OsmoSamplers that were placed in a newly designed carrier system positioned at the end of a stinger that extended just beyond the end of the 10¾ inch casing. During Expedition 327, only data from the instruments in Hole U1362B were available because pressure data and fluid samples in the nearby CORKs will not be recovered until CORK servicing in summer 2011 or 2012.

The tracers used for the 24 h injection experiment comprised freshwater (alternated with seawater); SF6 (an inert gas); the salts CsCl, ErCl3, and HoCr3; fluorescent microspheres of several sizes and excitation wavelengths; and fluorescent-stained bacterial cells (strained from surface seawater). Different tracers were injected using different methods throughout the 24 h hydrologic experiment, as discussed in Fisher, Cowen, et al.