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doi:10.2204/iodp.sp.327.2010 Operations plan/drilling strategyTarget depths for Expedition 327 operations are listed in Table T2. Planned operations are summarized in Table T3. The expedition will begin with a jet-in test at proposed Site SR-2, followed by emplacement of a reentry cone and a 20 inch conductor casing. This cone and casing system will be used to establish the deep basement hole (Hole SR-2A). Should drilling problems occur in the first hole, a second attempt at a "deep" installation can be initiated, and the first attempt will become the "shallow" basement penetration (Hole SR-2B). A more traditional operations strategy would begin with sediment coring, but this approach is not planned for Expedition 327 for several reasons. First, we already have a good understanding of sediment thickness and properties on the basis of extensive site survey data and previous work at nearby Sites 1026 and U1301. Second, we would like to wait to dedicate time to sediment coring until we have greater confidence in achieving high-priority basement and observatory operations. After installation of the cone and surface conductor casing in Hole SR-2A, we will drill with a 21½ inch underreamer through ~255 m of sediment to the basement contact. We will then drill with a 21½ inch bicenter bit through another ~20 m of the uppermost basement and run and cement a 16 inch surface casing. To minimize the risk of the casing strings and reentry cone sinking below the mudline (as happened during Leg 168 and Expedition 301), the casing will be held for 8 h while the cement hardens. This will complete Stage 1 operations in Hole SR-2A. Once the casing hanger is released and the drill pipe is pulled out of the hole, the vessel will be offset 30–40 m to the planned location of Hole SR-2B, and we will repeat the emplacement of the reentry cone and 20 inch casing. This will be followed by drilling, emplacement, and cementing of the 16 inch casing string. This will complete Stage 1 operations in Hole SR-2B. Upon returning to and reentering Hole SR-2A, the cement will be drilled out, and drilling will continue with a 14¾ inch tricone bit to penetrate quickly through the most unstable zone in upper basement to ~360 mbsf. For this effort we will use a bottom-hole assembly (BHA) that consists of extra 8¼ inch drill collars to ensure that only slick pipe is exposed to the unstable formation while the top of the BHA remains above the basement contact. The 10¾ inch casing will be run and cemented into place. We will install this casing without stopping to install the cementing manifold and subsea release system in order to ensure rapid emplacement of the casing string and unimpeded landing of the 10¾ inch casing hanger. Cement with lost-circulation materials (LCM) will be pumped into the bottom of the hole and allowed to set. This will complete Stage 2 operations in Hole SR-2A. The vessel will then return to Hole SR-2B, where we will reenter the hole and drill ahead with a 14¾ inch tricone bit to ~290 mbsf. The 10¾ inch casing will be run and cemented into place. This will again be accomplished using an extended-length BHA and without the cementing manifold. This will complete Stage 2 operations in Hole SR-2B.
After dynamically positioning the ship once again over Hole SR-2A, the cement plug will be drilled out and coring will begin using a standard 9 Once total depth is achieved in Hole SR-2A, the hole will be logged with one deployment of a wireline logging string designed to identify optimal placement of straddle packers as well as basic formation properties of the oceanic crust (see "Logging/downhole measurements strategy"). Three sets of packer experiments will be conducted in the open hole. Once those tests are complete, the open hole depth will be verified before deploying a CORK-II observatory. The CORK-II will be configured to isolate two intervals of upper basement and will include wellhead fluid samples and pressure gauges, as well as downhole temperature sensors, fluid samplers, and microbiological incubation substrate (Fig. F10). A landing platform will be deployed following CORK installation. This will complete Stage 3 operations in Hole SR-2A. Upon completion of operations in Hole SR-2A, we will move to Hole 1027C and recover the CORK system currently installed in the reentry cone. We will then run in with an RCB assembly, clean up the hole, and core from 635 to ~675 mbsf. Wireline logging of the short basement interval is possible but unlikely. We will reenter the hole to conduct open-hole straddle packer tests (two sets) and then deploy a CORK-II system that will isolate two intervals of uppermost basement (Fig. F12). The CORK-II will include wellhead fluid samples and pressure gauges, as well as downhole temperature sensors, fluid samplers, and microbiological incubation substrate. A landing platform will be deployed following CORK installation.
The vessel will return to Hole SR-2B, and we will reenter the hole and drill out cement with a 9 Following completion of operations in Hole SR-2B, we will position the ship over Hole U1301B in order to reenter the hole and retrieve the existing thermistor string and replace it with a new one. Afterward, we plan to position the vessel over Hole U1301A and attempt remedial cementing operations at the reentry cone/casing hanger interface. Note that operations in Holes U1301A and U1301B may be completed during calm conditions earlier in the expedition based on the schedule and success of earlier operations. If time permits, we will address secondary objectives involving sediment coring. This could include complete or spot coring by APC/XCB at proposed Sites GRB-1, GRB-2, GRB-3, or FR-1 or RCB coring at proposed Sites DR-1 or DR-2. Work at secondary sites will occur only if we have completed all primary objectives or are unable to complete primary objectives and additional time remains. Sediment coring will be accompanied by measurements of sediment temperatures using the Sediment Temperature Tool (SET) or the third-generation advanced piston corer temperature tool (APCT3). Logging/downhole measurements strategyThe principal objectives of the Expedition 327 wireline logging program are to (1) identify suitable depth intervals for setting the inflatable and swellable packer elements for use during hydrogeologic testing and CORK installation, and (2) expand on the Leg 168 and Expedition 301 work in quantifying crustal lithostratigraphy, alteration, and hydrogeologic and petrophysical properties. Downhole logging data can help define structural and lithologic boundaries, delineate fracture densities and orientations, identify water flow pathways, assess variations in alteration, and be compared to results of laboratory core analyses. Logging data will also complement core measurements when recovery is poor. We will also use the logging line to deploy CORK instrument strings by way of an electronic release in lieu of the hammer release system used during Leg 168 and Expedition 301. Hydrogeologic tests in basement will be run to assess ease of fluid flow through basement and the nature of connections between different parts of the volcanic crust at a scale of meters to kilometers. In situ measurements at secondary sedimentary sites will be used to determine the thermal state of sediments and underlying volcanic crust and to estimate rates of fluid seepage in sediments and lateral flow within basement. Wireline logging
A single tool string deployment is planned for the 9 The tool string we will use will consist of caliper, image, and density measurements. The Hostile Environment Litho-Density Sonde (HLDS) can provide a single-arm long-axis caliper in addition to standard formation density measurements. Additional caliper data will be acquired by the Environmental Measurement Sonde (EMS), which also measures mud temperature, and the Ultrasonic Borehole Imager (UBI). When run at a speed that ensures high vertical resolution, the UBI's ultrasonic image can deliver a borehole interpretation comparable to or better than that acquired by standard IODP resistivity imaging tools. Spectral gamma ray measurements (K, U, and Th) can be acquired by the Hostile Environment Gamma Ray Sonde (HNGS), and we may include a spontaneous potential (SP) tool. The Logging Equipment Head-Q Tension (LEH-QT) cablehead can transmit downhole tension measurements, and the General Purpose Inclinometry Tool (GPIT) can provide downhole acceleration and orient the UBI images. Detailed descriptions of wireline tools and applications are provided at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html. The estimated time for the logging string deployment, from rig-up to rig-down, is <9.5 h. An additional 4.5–6.5 h will be needed for hole conditioning, RCB bit release, reentry, and so on. Deployment of CORK instrument stringsCORK instrument strings will deployed using the wireline logging cable, winch, and cablehead along with a MultiFunction Telemetry Module (MFTM) being developed by Lamont-Doherty Earth Observatory (LDEO) and an Electronic Release System (ERS) being developed by Stress Engineering. This new deployment technique should be an improvement over previous methods and offer further control and constraint because downhole cable tension will be read and interpreted at surface in real time. Hydrogeologic experimentsWe will run short-term drill string packer experiments in the deeper of the two new basement holes, Hole SR-2A, to determine near-borehole hydrogeologic properties. This will provide a useful comparison to similar measurements made in nearby Hole U1301B and at the few other upper basement sites worldwide where such experiments have been completed. The packer will be inflated at one or more locations at depth to test hydrogeologic properties between the packer setting depth and the bottom of the hole, and the packer will be set in casing above the open hole to test the complete open interval. Each of these tests will last 1 h, with an additional hour of recovery time between tests, and multiple pumping rates will be used at each packer setting depth. In addition, we will run a longer hydrogeologic experiment in the shallower of the new basement holes, Hole SR-2B. There will be a short interval of open basement in this hole, which is expected to be cavernous where it is not cased, so we will use a new approach for these experiments. Instead of using the drill string packer, we will use a casing running tool that will be landed in the casing hanger at the top of the 10¾ inch casing. The contact between the casing running tool and the hanger will provide a hydraulic seal. A "stinger" of drill pipe will extend below the casing running tool and penetrate just past the shoe for the 10¾ inch casing so that fluid pumped into the open hole will be in immediate contact with the surrounding formation. This pumping experiment will last 24 h, more than 20× as long as any other packer tests run to date during scientific ocean drilling. In addition, during this pumping experiment we will pump a mixture of hydrologic tracers along with the surface seawater normally used as a drilling fluid. These tracers, including SF6, rare earth elements, and fluorescent spheres and stained cells, will be pumped into the formation in Hole SR-2B, and the fluid chemistry in surrounding CORK observatories will be monitored for the following 3–4 y, allowing assessment of the rates and patterns of fluid circulation in basement. Rig Instrumentation System (RIS) data will be acquired and used in real time to facilitate drilling, packer test, and tracer injection operations. Surface data—including but not limited to drilling rate of penetration (ROP), surface weight-on-bit, and surface torque—will be viewed on monitors while drilling and made available for download immediately following operations. These data will be used to help determine the depth of competent basement rock and may be used to identify bit trips, packer intervals, logging deployments, and hole total depth. During packer tests and tracer injection experiments, pump rate and standpipe pressure will be monitored to determine and control flow rates and volumes. These data will also be downloaded and evaluated in relation to pressure responses from collocated and nearby instruments. In situ sediment thermal measurementsIf secondary priority sediment coring is completed at GRB, FR, or DR sites, we will also collect in situ sediment temperatures. We will use the APCT3 tool in portions of hole cored with the APC. The SET will be used in portions of hole (just above basement) cored with the APC/XCB system and in places cored with the RCB. | |
inch bit and the RCB system. We anticipate 160 m of penetration to ~520 mbsf, but we may core somewhat more or less basement depending on rates of penetration, drilling conditions, and the nature of the rocks recovered. We anticipate a bit replacement trip to reach total depth.