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Tracer monitoring

Rig floor sampling of injectate

Although quantities of tracers mixed and added to fluids pumped into the formation were known in advance, additional steps were taken to verify tracer concentrations as the injectate passed across the rig floor and through the end of the stinger before entering the formation. Collection of samples in these locations was important for verifying the extent and variability of mixing that could occur between the mud and cement pumps and the rig floor and because we could not know in advance if there might be difficulties with system leaks or maintaining consistent flow rates. In addition, we wished to evaluate the potential influence of changing injectate chemistry (salt water versus freshwater and brine concentration) on the gas, solute, and particle tracers as mixed on the ship and injected into the formation.

A sampling station was established just prior to beginning the pumping experiment on the starboard side of the rig floor for collection of injectate (Fig. F8). A stainless steel needle valve was installed on a hammer union connected to a rig floor standpipe, and a ¼ inch OD stainless steel line was run from this valve. The hammer union and needle valve were positioned just upstream from where the standpipe directs pumped water up the rig and back down through the top drive to the drill pipe. Inflow of fluid from the cement pumps was rerouted so that mixing would occur before the fluid passed the sampling point. Fluid passing down the sample line was directed through a three-way valve to a syringe needle attached to a Luer-Lok fitting or through an open-ended section of fiber-reinforced flexible tubing. Additional ball and needle valves were added to ensure that the sampling lines were sealed when not in use and that flow could be controlled by those collecting injectate samples.

A pretest evaluation of system pressures demonstrated that fluid in the standpipe was expected to remain <100 psi throughout the injection test, and the needle valve installed at the hammer union could be adjusted to reduce the pressure of fluid in the sampling tubing if needed. In general, the fluid sampling pressure remained <10 psi, but there were brief periods of higher pressures on the rig floor during the experiment.

SF6 gas sampling on the rig was accomplished using standard methods and two kinds of containers: glass vacutainers and copper tubes (Clark et al., 2004). Vacutainers were sealed with rubber septa, without additives, and were evacuated prior to the experiment. Just before each sample was collected the syringe needle was elevated, and air in the line and needle was pushed out by the flow of injectate, aided by tapping the side of the line, fittings, and needle. While injectate continued to flow, the needle was pushed quickly through the septum of a prelabeled vacutainer and then withdrawn rapidly when the vacutainer was full. Vactuainer samples were collected every ~30 min. Copper tubing samples were collected by attaching the end of the fiber-reinforced tubing to a piece of precut and prelabeled copper tubing (6.4 mm inside diameter [ID]), allowing the injectate to flow freely. The far end of the copper tubing was elevated to allow air to escape while injectate flowed out of the tube, and when fluid had flowed freely for several seconds, clamps on both ends of the copper tubing were tightened to make a cold weld (crimp). Copper tube samples were collected every 1–2 h.

Fluid samples to monitor inputs of metals salts and freshwater were collected at ~30 min intervals in prelabeled, 25 mL high-density polyethylene (HDPE) bottles that were acid washed in 2M HNO3 prior to use. Injectate was used to rinse the bottles and caps three times before a sample was collected. The periods of salt addition were relatively brief, and we generally attempted to collect two samples during the time of peak concentration for salt tracers. Collection of salt- and freshwater samples throughout the experiment will help with assessment of contamination as these materials pass through mixing systems and pipes.

Rig floor samples for fluorescent microspheres, stained bacteria, and general microbiological analyses were collected in additional prelabeled containers at specific times of interest throughout tracer injection. Background samples were collected at 30 min intervals for several hours prior to fluorescent particle injection, at intervals of 2–3 min during the brief periods of particle injection, and then at 10–30 min intervals following particle injection. These samples were collected in acid-washed and sterilized prelabeled HDPE bottles and centrifuge tubes.

Stinger sampling of injectate

A specialized downhole sampling and pressure monitoring system was designed and constructed for use during the tracer experiment (Fig. F8). The system was housed in a ~6.5 m long piece of 6⅞ inch OD pipe, which was deployed at the end of the stinger. The upper end of this pipe housed a fluid sampler carrier, which was used to collect injectate continuously as it flowed out through slots near the end of the pipe. The pipe also housed a pair of Micro-Smart pressure gauges (model HT-750SN, 0–10,000 psi absolute) that were placed in a chamber below the fluid sampler carrier (Figs. F2, F8). The pressure gauges were isolated from direct contact with fluid flowing out of the pipe and into the formation to reduce noise associated with turbulent flow, but they were exposed to formation pressure through holes in the side of the carrier.

Because we wished to collect fluid samples at a rate much greater than that possible using the continuous fluid sampling systems deployed on or in CORKs (typically 1–2 mL/week), specialized reverse-osmosis (RO) pumps were developed specifically for this application (Wheat et al.). These pumps were designed to recover 1–2 mL/h in 1.2 mm ID tubing at temperature and pressure conditions anticipated at depth during tracer injection (2°C and 26 MPa, respectively). The actual RO pump rate during tracer injection will be evaluated on the basis of samples collected. Because the concentrations of tracers in the injectate are so high (Table T1), 0.5 mL sample sizes should be sufficient for analysis after being diluted by several orders of magnitude.

The downhole sampler included four complete RO pump and sample coil systems (Fig. F8). Two of the systems had sample coils composed of polytetrafluroethylene (PTFE), and the other two were made of copper. Fluids collected in the PTFE coils will be used for analysis of trace metals and chlorinity. Copper tubing was used for fluids collected for analysis of dissolved SF6 gas because PTFE is permeable to gases. Intakes for all sample coils were located at the lower end of the RO sampler carrier, which was installed inside the 6⅞ inch pipe just before the stinger was submerged in the moonpool (Fig. F8). After completion of the pumping and tracer injection experiment, the sampler carrier was recovered and sample coils were removed. Subsamples from the PTFE tubing were extracted immediately in splits of 0.44 mL (40 cm tubing lengths), and injected into acid-cleaned 0.5 mL centrifuge tubes. The ends of the copper coils were crimped before being shipped to shore for extraction and analysis of dissolved SF6.

Tracer monitoring with downhole and wellhead sampling

Autonomous fluid sampling systems were operating in and on existing long-term CORKs in Holes U1301A, U1301B, U1362A, and 1026B (Fisher, Wheat, et al.) before tracer injection began in Hole U1362B. An additional CORK was deployed in Hole U1362B after the tracer injection period. Downhole OsmoSamplers were deployed on CORKs installed in Holes U1362A and U1362B during Expedition 327. Wellhead samplers (drawing fluids from stainless steel lines sampling from upper basement) will be attached to these CORKs in summer 2011. In addition, a “GeoMICROBE” sampling sled (Wheat et al.) was deployed in Hole U1301A during R/V Atlantis expedition AT15-66 (using remotely operated vehicle [ROV] Jason) a few weeks prior to the start of Expedition 327. This system is connected to a fluid sample line that terminates in a screen in uppermost basement (~271 meters below seafloor; ~11 meters subbasement) and will collect twice-monthly samples, alternately 500 mL (whole fluids) and 8000 mL (in situ filtrations). Samples from this system will also be recovered in summer 2011, and additional CORK servicing is planned for summer 2012 to download data and collect fluid samples.