ODP Technical Note 10


B. ODP Drill-string Straddle Packer

1. Packer elements
The packer used in ODP is inflatable and seals the borehole using hybrid rubber/steel elements that are pressurized with seawater pumped down the drill string from the rig floor. Each of these elements consists of an internal rubber bladder, which is bonded to an expandable steel strength member, which in turn is bonded to an outer rubber cover. The outer rubber and the steel serve to 'grip' and seal against the borehole wall and to protect the inner bladder, which must remain unpunctured for the packer to remain inflated.

The proprietary TAM design uses a tight weave of stainless steel wire rope as the internal strength member of the element (Fig. IV-2). This has the advantage that good bonds can be maintained between the woven cable and both the inner and outer rubber during expansion and contraction of the element. However, this type of element may be weak at very high degrees of inflation, if the cables separate to the extent that the rubber in the gaps must bear too much of the inflation pressure.

2. Inflation of packer elements using go-devils
The TAM packer elements can safely be inflated to as much as twice their uninflated diameters. However, a greater degree of inflation requires more time for full deflation and produces a weaker hydraulic seal that can withstand a lesser range of test pressures (Fig. IV-3). Thus, effective formation testing requires that the element(s) be positioned in zones where the borehole is in good condition and in gauge, where the degree of packer inflation can be kept reasonable. Such zones are usually apparent in logs that should be run before packer tests are begun, such as caliper, resistivity, density, and sonic.

After the drill-string packer is positioned in the desired zone, inflation of the element(s) is accomplished by pumping seawater from the ship down the pipe. Before pumping, the inflation mechanism is enabled by free-falling a retrievable go-devil, which keys into the packer plumbing system. A properly seated go-devil will perform several functions, including (1) forming a hydraulic seal within the packer to direct the fluids pumped from the rig either into the elements for inflation or into the formation for testing, (2) carrying the pressure recorders that monitor the data needed to determine pore pressure and permeability, and possibly (3) carrying a sample chamber. Because the go-devils are retrievable with the coring line, multiple sets of the ODP drill-string packers can be made with a single pipe trip as long as the elements hold pressure and seal the borehole.

3. Temperature limitations of the ODP drill-string packers
The temperatures to which the drill-string packers can be used are limited by the temperature ratings of the rubber compounds that make up the packer elements and inner seals. Standard nitrile o-rings and elements are rated to about 100°C. Commercially available o-rings made of special elastomers are effective to temperatures as high as 300°C. However, the combination of elasticity, strength, and resetability required of the rubber in the inflation elements is difficult to achieve at high temperatures. Packer elements made with high-temperature rubber compounds will probably be effective to 200°-250°C, but only at limited test pressures and limited degrees of expansion (<10%-20%), and such elements may last only for single settings. During Leg 111, two special elements rated to 120°C were successfully used at temperatures of 120°-145°C in Hole 504B, but lasted for only one setting each. During Leg 139 in active hydrothermal fields at Middle Valley, northern Juan de Fuca Ridge, packer work was confined to casing, so that high-temperature components were not needed. TAM International is continuously refining their packer-element materials and construction techniques; they now claim that their moderate and high-temperature elements are nearly as expandable and strong as their lower temperature elements.

4. Compatibility of ODP drill-string packer with logging
The non-rotatable drill-string packer should be kept in tension, like the drill pipe above the BHA, and should not be included as part of a coring, rotating BHA. Thus, formation testing with the drill-string packer will require separate pipe trips into reentry holes that have already been cored. As described below, the packers are compatible with logging, and it may be possible to accomplish logging and packer measurements during a single pipe trip.

All the standard logging tools will pass through the drill-string straddle packer, unless a packer go devil is in place. (A newly designed go-devil may also be compatible with one or more logging tools; see Chapter VIII-B.4.) Packer measurements are usually run after the completion of wireline logging, because of the chances of a go-devil getting stuck in the pipe or of rubber from the packer elements stripping off in the hole. Also, the logs are usually important in locating formations in which to set the packer.

In some situations, it may be better for operational reasons to run a separate pipe trip for packer measurements. For example, the packer BHA may require more drill collars than an optimal logging BHA to balance the upward force on pressurizing the zone isolated below the packer. It may be necessary that most or all of the BHA be in the hole below seafloor during logging, and a heavier, longer packer BHA would limit the shallowest depth to which logs could be run. Thus, if it is essential to log the upper 50 m of a reentry hole, separate pipe trips might be required for logging and packer measurements.

5. TAM straddle Packer design The design of the TAM straddle packer is quite simple, as it has no rotational capability and is intended primarily for measurements in reentry holes that penetrate stable formations. By assembling it in the BHA with or without a few essential parts, this packer can be used in four resetable modes, all of which are fully compatible with logging:

The minimum I.D. is 3.94-in. for a single packer configuration, or 3.84" for a straddle packer configuration.

When a straddle configuration is used, the straddle interval can be as small as 1 m and can be changed by spacing sections of drill pipe between the elements. Hydraulic continuity is maintained between the elements by connecting them with 3/8-in.-diameter stainless-steel tubing external to the spacing drill pipe. Thus, if either element or the tubing fails to hold pressure, neither element can be inflated. This is a good safety feature in that once the packers are inflated, a hydraulic failure anywhere in the system will result in the deflation of both elements, not permitting an element to remain locked in an undeflatable position.

Depending respectively on whether a single or a straddle configuration is deployed, the first or both of two go-devils assembled from common parts will be used:

Self-contained pressure recorders are attached to the upper go-devil to record pressures in the isolated section, whether it be between a single packer and the bottom of the hole or between the two inflated elements of the straddle configuration. During operations with the straddle packer, a single pressure recorder can also be hung below the lower go-devil to test for leakage from the straddled interval. ODP maintains several mechanical Kuster pressure gauges which can record pressures up to 15,200 psi. At present neither go-devil can sample formation fluids, but it should be feasible to develop this capability.

Each go-devil must be retrieved with a standard overshot on the coring line. The use of the single- packer configuration is fairly straightforward, requiring just one free fall/retrieval round-trip of a single go-devil, whereas use of the straddle configuration requires a more complex and time consuming sequence of go-devil operations, as follows. First, the upper go-devil must be dropped to allow inflation of both packers and possibly testing of the zone below the lower packer. Second, to test the straddled interval, the upper go-devil must be retrieved, the lower go-devil dropped into place, and the upper go-devil dropped a second time. Third, at the end of the experiment, two trips of the coring line are required to retrieve both go-devils.

To enable inflation, the upper go-devil must seal properly in the packer control sub. This assembly incorporates one principal moving part that can shift vertically by about 20 cm, controlling whether or not the inflate/deflate port to the elements is open. This control tube is directly connected to the drill string, and its position is controlled from the rig floor using the drill-string heave compensator. When the drill string is freely suspended, both the flow paths into the elements and down the pipe up the annulus are open, so that pressure in the elements will remain the same as in the borehole. Thus the elements will remain uninflated until the upper go-devil is in place and the rig pumps are used to pressurize the elements. When the elements are fully inflated against the borehole wall, the heave compensator is adjusted to put some weight on the packer, shifting the control tube so that the inflate/deflate ports are closed and the packer elements are locked in the inflated position. Then, after the completion of formation testing, the packers can be deflated by simply pulling up on the drill string at the rig floor, opening the inflate/deflate port whether or not the go-devil is in place. Technical Note 14 contains full schematics and gives detailed instructions for:

To Drill-string Packers, Section C.
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