MDHDS tool testing

Background and summary

Approximately two days of ship time during Expedition 342 will be spent testing the Motion Decoupled Hydraulic Delivery System (MDHDS). The MDHDS is an IODP-funded engineering development led by the University of Texas in conjunction with the USIO and Mohr Engineering. The MDHDS will serve as a foundation for future penetrometer and other downhole tool deployments. We will test the MDHDS at ODP Site 1073 (Leg 174A), offshore New Jersey, by deploying both the SET-P and T2P pore pressure penetrometers. Because of previous drilling at Site 1073, we know the lithology and material properties of this site. This sea trial is the culmination of multiple land-based tests, and a successful deployment will mean this tool can be reliably deployed during future expeditions. There is also significant scientific value to the test. During Leg 174A, the presence of pore overpressure near the seafloor was predicted (Dugan and Flemings, 2000). A successful test of the MDHDS and recovery of pore pressure measurements in this shallow section would illuminate at what depth pore pressure starts and the current stability of the sediments near the seafloor, which are critical questions for both hydrodynamic models of shallow sedimentary sections and our understanding of the process of slope failure. The MDHDS will be run with a real-time data link through the logging wireline to observe system behavior.

Motivation for testing and relevance to IODP goals

The ability to measure pressure and permeability in mudstone is critical to achieving the scientific goals of IODP as expressed in the Initial Science Plan (ISP) (IODP International Working Group, 2001). In the ISP, issues directly related to fluid pressure/flow include the occurrence, stability, and dissociation of gas hydrate (Hyndman and Davis, 1992; Kvenvolden, 1993; Dickens et al., 1997; Ruppel, 1997; Dillon et al., 2000; Liu and Flemings, 2009) and the geometry, structure, fluxes, and earthquake mechanics of accretionary complexes (Davis et al., 1983; Dahlen et al., 1984; Bekins and Dreiss, 1992; Saffer and Bekins, 2002; Screaton et al., 2002). Models of these systems await validation because of the lack of direct pressure measurements; IODP has started to implement short-term measurements via penetrometers (Flemings et al., 2008) and downhole formation testers (Saffer, McNeill, Byrne, Araki, Toczko, Eguchi, Takahashi, and the Expedition 319 Scientists, 2010) and long-term observatories (Fisher et al., 2005). The Engineering Development Panel prioritized measurements of in situ pressure as one of the critical technologies in need of development and supported the development of the MDHDS. The Science and Technology Panel defines the measurement of in situ pressure as a “standard measurement,” which shall, whenever practical, be carried out.

The new science plan for future scientific drilling emphasizes the need for observatory science relating to pore pressure, gas hydrate dynamics, and geohazards, including those related to fluid flow and slope failure ( Our pore pressure measurements will illuminate the role of pore fluids in continental slope geomorphology (Johnson, 1939; Rona, 1969), the cause of low-angle landslides (Terzaghi, 1950; Bombolakis, 1981), and the effect of focused flow along permeable layers on the timing and distribution of failure (Haneberg, 1995; Dugan and Flemings, 2000, 2002; Boehm and Moore, 2002; Flemings et al., 2002).

Why the MDHDS?

Previously, the T2P and the SET-P (formerly the DVTP-P), the two penetrometers used by IODP, were deployed by wireline on the Colleted Delivery System (CDS). The CDS is analogous to an old-style pointer, in which a series of cylinders slide past each other to increase or decrease the system’s length. In this configuration, the penetrometer is pushed in by the drill string, and then the drill string is raised to decouple the drill string from the formation; however, the penetrometer remains connected to the drill string through the CDS, which should expand and contract during ship heave. Analysis of previous deployments showed that when the drill string was raised, the penetrometer was pulled out of the formation >80% of the time (Fig. F23). When this occurs, the measured pressure drops rapidly and the measurement is compromised (Fig. F23E–F23G). This occurs with both the T2P and the SETP.

The MDHDS (Fig. F24) was specifically designed to overcome this problem. More broadly, it is envisioned to be the foundation for future downhole tool deployment and is designed to allow real-time communication between the tool and the rig floor via the logging wireline. It is a wireline-deployed system that uses mud pressure to advance a penetrometer into the formation (Fig. F24). After hydraulic deployment of the penetrometer, the bottom-hole assembly (BHA) is raised to completely decouple the tool from the BHA, thus eliminating the adverse effects of pipe heave. The MDHDS also uses mud pressure to insert the tool rather than lowering the BHA, which eliminates the mud plugging problem that has plagued the CDS.

The MDHDS can be deployed either on the coring wireline or on an armored conductor cable such as the logging line. If deployed on the wireline, complete decoupling will occur because of separation of the wireline from the tool. Alternatively, if deployed on the conductor cable, a continuous signal can communicate with the tool. The capability of a “hotline” to the tool opens a range of exciting possibilities for future tool developments. Unlike the CDS, which is driven into the formation by lowering the BHA to the bottom of the borehole, the MDHDS allows the BHA to remain 2 m off the bottom of the borehole (Fig. F24). This clearance greatly reduces the possibility of jamming borehole cuttings or other detritus inside the BHA, which could result in coupling between the tool and the BHA. The penetrometer is extracted from the formation by lowering the wireline through the upper latch union, allowing the RS (the RS or “Running Shoe” is designed to retrieve downhole tools by wireline) overshot to latch onto the RS fishing neck attached to the upper piston rod. In the case of a “hotline” deployment, the soft tether will recoil itself inside the upper piston rod, allowing the RS overshot to latch onto the RS fishing neck.

Operations at Site 1073

Prior to arrival at Site 1073, considerable time will be spent with Schlumberger personnel, core technicians, and the drilling crew to review the rig floor procedure for deploying the MDHDS.

Site 1073 is at 639 m water depth (Fig. F25). Sediments encountered will be soft mudstone (Fig. F26). We will wash down (or core) to ~100 m before deploying the MDHDS and associated penetrometers multiple times. We will also test the real-time data link via the logging wireline. Before each new deployment we will need to wash down ~10 m. The T2P will need to be tested with and without the logging wireline. The SET-P will be tested without the wireline. We have requested six penetrometer deployments. The core technicians will also need time to work out the field deployment of the tool and the deck-handling procedure.