ODP Technical Note 10


B. Pore-Fluid Sampling with the WSTP

Barnes (1979) describes the first generation of WSTP, which was modified several times during DSDP, and was often called the "Barnes sampler" or the "Barnes-Uyeda probe". Barnes (1988) describes the redesigned WSTP, and the excellent results obtained with the prototype during Leg 110. Figure II-1 is a cartoon drawing of the latest WSTP design. The WSTP is a self-contained, battery-powered tool adapted to an inner core-barrel assembly that is deployed in the BHA. The sample chamber and electronics (timer and temperature/pressure data recorder) are contained within the inner core barrel; only a strong shaft and probe tip (containing the sampler filter and temperature sensors) protrude beyond the bit.

ODP staff have written a detailed shipboard operating manual for the new generation of WSTPs, copies of which are available at ODP/TAMU and on the drillship. Table II-1 shows a list of topics covered in the WSTP "cookbook". The cookbook is available in hard and electronic form so that it can be updated as tool specifications and protocols change. Table II-2 shows the specifications for the WSTP, as configured during Leg 146. These specifications are subject to change on a leg by leg basis; interested scientists are advised to check with technical support staff in advance of their cruise for updated specifications.

The WSTP has always been a tool of compromise, remaining relatively unchanged in basic design since its introduction. The tool has never returned geochemically useful samples better than about 50% of the time on a leg, even in ideal lithologies and hole conditions. Successful in-situ fluid sampling with the WSTP requires that 1) the probe penetrate well into virgin formation, 2) the formation "pack off" around the sample filter, 3) the water content of the sediment be high enough to allow rapid drainage when the formation is exposed to a pressure differential (as the tool begins sampling), 4) the formation is not so hard as to crack on tool penetration, and 5) the formation is not so fine grained as to clog the filter assembly. These requirements are likely to remain until the WSTP undergoes a radical redesign or is replaced, neither of which is presently budgeted or planned.

The following overview of the methods to deploy the WSTP for sampling pore fluids from sediments is excerpted from sections of the most recent WSTP cookbook.

1. General tool design and preparation
In fluid sampling mode, the WSTP consists of a probe tip and filter assembly; a probe shaft; a lower bulkhead with hydraulic and electrical feed-troughs; a stainless steel cylindrical shell comprising several compartments containing an electrical pass-through, a data logger (for temperature data), batteries, and a fluid sampling timer; an upper bulkhead; and sampling coils surrounded by an overflow chamber (Fig. II-1). The main body of the WSTP is contained within a long cylindrical shell, split longitudinally. The lower half of the shell is constructed from a single piece of metal, while the upper half is divided into two or more pieces, each of which covers separate compartments.

Most of the WSTP is stainless steel, but some components are titanium. Titanium parts presently include the probe filter assembly (including inner and outer perforated sleeves, Fig. II-2), 1/16-in. and 1/8-in. fluid sampling tubing, and some fittings and valves. Fluid filters are composed of titanium and stainless screen or titanium and polyester screen, and are held together with high temperature (Philly Clad) epoxy (Fig. II-2). The filter is housed below a protected titanium sheath, which is perforated with 1/4-in. holes. The filter covers a section of 1/16-in. titanium tubing, connected at one end to the hydraulics of the WSTP via a plug in the lower bulkhead, and open at the other end near the middle of the filter assembly. The opening in this tubing is the fluid-intake port.

A single fluid sample can be collected during each run of the tool. The fluid-sampling chamber is separated into three distinct parts. About 10-15 mL of fluid is trapped within reusable 1/8-in. (stainless or titanium) tubing wrapped in a coil; up to 50 mL is trapped in a copper coil, which can be crimped for later gas analyses; up to 1000 mL of sample (mixed with approximately 75 mL of distilled water and drill water) may be trapped in a stainless steel overflow chamber which surrounds the smaller coils.

Running the WSTP to collect a fluid sample may require the following operations (depending on run parameters and tool status): partial assembly and disassembly of the inner case, pressure case, and probe; flushing and back pressuring the water lines before a run, and extracting the fluid sample after a run; setting the timer for the appropriate delay and sample intervals; checking and/or changing and/or charging the batteries; keeping the driller advised of scientific and operational considerations during the station; recording events on a Downhole Tools data sheet during the station; and monitoring the condition of all sampler parts and repairing or replacing them before they cause problems.

Before each use the sampler plumbing is flushed with distilled water using a high-pressure pump and assorted rigging in the Downhole Tools lab. Sample coils of an appropriate volume are available on the ship or can be constructed from spare tubing. There is a check valve installed in front of the coils which holds some of the in-situ pressure in the sample coils upon recovery (generally up to 500-1000 psi). A timer is set so that a motor will be activated to open and close the valve connecting the sampling tube behind the filter to the sample coils and chamber, which are kept at surface atmospheric pressure. The timer is set with a delay time (after which the valve is opened) and a sampling time interval (after which the valve is closed). The timer is activated by hooking it up to a charged battery pack and depressing a reset switch.

2. Tool deployment
After the WSTP is dressed for deployment, a landing sub is screwed in place around the base of the probe shaft, and the tool is carried out to the rig floor and made up to its pressure case. The landing sub provides protection during deployment and acts as an upset to limit the extension of the tool through the bit. The WSTP pressure case is a modified core barrel which screws onto the lower bulkhead, covering everything from the bulkhead to the back of the tool (Fig. II-1). The tool is lowered down the drill pipe on the coring line. Downhole conditions vary, and the procedures used to collect a useful sample will vary accordingly. Fill at the bottom of a hole is a normal occurrence, however, so it is important to remove as much fill as possible and permit the probe to penetrate undisturbed sediment. The driller can keep fill washed out of the hole by pumping continuously as the tool descends. The operator may wish to stop pumping and tool descent just above mud line for 5-10 minutes to record the ambient bottom water temperature, if temperature data are being collected. Pumping then resumes, and the tool is lowered into the BHA and landed. Immediately before the tool is latched in, the driller can lower the bit and "clean up" the bottom of the hole. The bit should then be raised several feet above hole bottom for long enough to latch the sampler into place, with only the probe shaft and filter tip extending beyond the drill bit. The drill string is then lowered (while still pumping) until the bit reaches the hole bottom, as indicated by a loss of hook weight. The probe is usually pressed in with about 10,000-12,000 lb. The probe should be lowered into the bottom at least several minutes before the timer causes the motor to open the sampling valve, and held in bottom until several minutes after the sampling time ends (when the sampling valve is closed).

Several deployment methods have been devised for the WSTP over the years, with (a) latched-in operation (RCB)and (b) colleted delivery (APC/XCB) being the safest and most reliable. Free-fall deployment, which has been used in the past, may damage the probe tip and filter assembly of the WSTP and probably should not be attempted.

a. Latched-in operation

b. Colleted delivery operation When it is equally important to measure in-situ temperature as to collect a fluid sample, it may be necessary to modify these methods, as described in Chapter III. For example, while only one water sample can be taken during a single deployment, it is possible to make several measurements of temperatures at different depths during the same deployment. It is critical to carefully plan the testing sequence so that the probe is at the desired depth at the proper time for fluid sampling.

Both latched-in operation and colleted delivery allow the WSTP to retract back inside the bit if the formation proves too hard for penetration, so ODP scientists should not be overly concerned with damaging the tool or breaking off a probe because the formation is lithified. Some surprisingly hard formations have yielded good fluid samples with the WSTP, although it is likely that the formation will fracture during probe penetration before it becomes too hard for WSTP insertion. A fractured formation will generally allow borehole water to be drawn in through the WSTP filter, compromising the fluid sample. A reliable temperature measurement (as described in the next section) is not necessarily an indication that a good fluid sample has been collected, nor is the reverse true. Each fluid sample and set of temperature measurements must be judged on its own merits.

Fluid circulation can continue while the WSTP is in bottom throughout the sampling period, provided that the formation is reasonably competent. Often it is desirable to continue circulation throughout a station to keep the tool (1) free of fill which might bury and trap it in an unstable hole, or (2) cool in a hydrothermal setting. It is difficult to sample pore fluids from very shallow sediments, as the sediments generally are not competent enough to support the weight of the bit (or even the tool) and may be washed away by circulating fluids. Also, the WSTP is usually not deployed until enough hole has been drilled so that a significant portion of the BHA is below mud line, often after the third or fourth core. In practice, the WSTP has been effective at depths of about twenty to several hundred meters below seafloor.

When the tool is brought back on deck, the downhole tools scientist or marine specialist should remind the rig crew to clean off the filter tip with a high-pressure ("car-wash") water hose. Mud that is packed around the filter assembly is very difficult to clean when dry, and could potentially clog the filter during later deployments. After the probe tip is clean, the WSTP is brought up to the Downhole Tools Lab and placed on a bench. Downhole-tools marine specialists will assist with recovery of the fluid sample, clean out (or exchange) the filter if necessary, and prepare the tool for its next deployment.

To In-Situ Fluid Sampling, Section C.
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