Middle Valley

Dead Dog Vent Field

CORK activities at Hole 858G

Initial observation of the CORK in Hole 858G from the VIT-TV revealed no evidence that the fluid flow observed earlier, was continuing. Upon recovery of the lower half of the CORK housing, we discovered that the inside housing of the CORK body was coated with hydrothermal precipitates, predominantly anhydrite, but with abundant pyrrhotite and pyrite. Naturally occurring hydrothermal chimneys from the Dead Dog vent field are predominantly composed of anhydrite, with only minor amounts of pyrite and trace amounts of pyrrhotite. Although the seals in the CORK had clearly failed due to the high temperature, examination of the location of the precipitates and the still open flow channels showed that the fluid flow may have originated in the sediment underlying the reentry funnel and that flow out of the cased hole may not have been contributing to the precipitation of hydrothermal minerals in the CORK.

The VIT was run to near the top of the reentry cone following removal of the CORK, but again, no evidence for flow of hydrothermal fluids could be observed from the open borehole. However, a Water Sampling Temperature Probe (WSTP) run 20 m into the casing indicated fluid temperatures in excess of 220°C prior to failure of an O-ring in the tool. The fluid sample collected contained a mixture of seawater and hydrothermal fluid that extrapolates to a hydrothermal end member similar to fluids sampled by Alvin in the vent field. A temperature run with the UHTMSM dewared tool (see "Operations" section for tool description) confirmed the high temperatures at the top of the open borehole and indicated that the borehole was essentially isothermal at 272°C (near the maximum measured in the vent field) at depths below 85 mbsf. The temperature tool could only be run to 205 mbsf due to an obstruction in the cased portion of the drill hole. This obstruction probably sealed the casing thereby preventing rapid flow of hydrothermal fluid from the open borehole. A water sample was taken from the borehole at 100 mbsf within the isothermal section with the Los Alamos water sampler. Failure of the seals in this tool led to both inflow of seawater and extensive boiling of the fluid sample upon retrieval through the water column.

We were forced to circulate drilling fluid (seawater) in the borehole while cleaning out the obstruction in the casing. The drillstring was advanced to 387 mbsf, near the level where the pipe last tagged bottom on Leg 139. A wash core was recovered from an unlined core barrel and is believed to be representative of the material drilled from inside the casing at depths below 205 mbsf. The wash core was predominantly loosely aggregated, fine-grained pyrrhotite and pyrite with minor amounts of anhydrite. Fluid sampled from the wash core had a very low magnesium content consistent with a high temperature origin and minimal dilution with seawater. The presence of an extensive interval of precipitated sulfide from within the casing may be an indication that the casing is no longer hydrologically isolated from the sedimentary formation.

Although we were forced to disturb the thermal structure of Hole 858G, we were successful in reinstrumenting the hole with a 370-m-long thermistor string and pressure transducer and in setting a new CORK in the reentry funnel (Fig. 9).

CORK activities at Hole 857D

The operation plan at Hole 857D was to remove the CORK and 300-m-long thermistor string set on Leg 139, to deepen the hole for 12 to 24 hr in order to circulate fluid and cool the hole, and to reCORK with an 898-m-long thermistor string (Fig. 7). This hole was drilled to 936 m on Leg 139 and has a reentry cone with 11-3/4" casing set at 573.8 mbsf. The CORK was damaged on Leg 146 by a collision with the running tool in high heave conditions. The data logger and the upper part of the CORK were successfully recovered, but the thermistor string and sinker bar were lost in the hole. The lower part of the CORK body remained in the hole, so our operations team devised a method to extract it using fishing tools available on the JOIDES Resolution. On the second attempt (after a pipe trip to modify the fishing tool), the remainder of the CORK was successfully removed.

In preparation for a temperature log and water sampling program prior to deepening Hole 857D, we ran into the hole with a grappling tool and recovered approximately 250 m of the 300-m-long thermistor string. Our intention was to begin temperature and water sample runs, but neither the UHTMSM nor the BRGM temperature tools were completely functional during bench tests. Since we could not collect a water sample with the WSTP without knowing the borehole temperature, and since our intent was to initiate downhole flow, we decided to postpone temperature logging and move to Site 1035 to begin a 2-3 day coring operation, before returning to Hole 857D to complete the CORK operation.

We returned to Hole 857D after terminating operations at Hole 1035A. As we reentered the hole, it appeared that mud stirred up in the reentry funnel was being drawn down the borehole. In order to determine the depth of the clear hole and maximum temperature before attempting to collect a water sample, we opted to run a sinker bar equipped with temperature tabs on the coring wireline until it met an obstruction. The sinker bar grounded at 642 mbsf and when we pulled it back to the rig floor we noted that the temperature tabs indicated borehole temperatures were less than 108°C. The drill pipe was lowered to attempt a WSTP sample, but at 377 mbsf met an obstruction within the cased section of the hole that required reaming. After clearing this obstruction the pipe was lowered to 621 mbsf and we used the WSTP to collect a borehole temperature and water sample. The temperature was approximately 2°C, and the water sample was seawater, indicating a strong downhole flow to this depth. The hole was washed and reamed against light resistance to 929 mbsf (8 m above TD). As far as we know, the sinker bar from the original thermistor string as well as some prongs from the thermistor retrieval tool are still in the bottom of the hole.

Considering our questionable long term weather forecast, the high priority of reinstrumenting the hole, and the knowledge that the hole was clear and cool to a depth sufficient for deployment of our 898-m-long thermistor string (Fig. 9), we decided to forego an attempt to deepen Hole 857D and proceed with CORKing operations. The CORK, thermistor string, and datalogger were installed without incident. As with Holes 856H and 858G, we were able to achieve our high priority objectives in a technologically challenging working environment. Much credit goes to the engineers and drillers who planned and executed this difficult operation.

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