Operations plan/drilling strategy

To meet the scientific and engineering objectives, primary operations at proposed Sites NT2-11B and NT2-01J will be to drill and case to TD, as shown in Figures F7 and F8. At proposed Site NT2-11B, cuttings will be collected and analyzed during riser drilling from 700 mbsf (bottom of 20 inch casing) to ~1600 mbsf. Wireline coring (~90 m) will be conducted at proposed Site NT2-11B between 1510–1600 mbsf using a rotary core barrel (RCB) to obtain the highest quality and most complete core samples immediately above a potential target interval for observatory instruments between 1510 and 1600 mbsf (Fig. F7). Operations at proposed Site NT2-01J will include measurement while drilling (MWD) and LWD to TD of 525 mbsf, casing to TD, and installation of a temporary pore pressure and temperature monitoring system (Fig. F8).

Operations plan

The operations plan and time estimate (Table T1) are based on formations and depths inferred from seismic and regional geological interpretations, combined with data from previous drilling operations at nearby sites. The MWD/LWD holes drilled at each site will be referred to as Hole A in all cases. We will use these operations and data to guide the operations described below. Our expedition plans are to conduct operations in one hole at proposed Site NT2-11B (Fig. F7) and one hole at proposed Site NT2-01J (Fig. F8). An abbreviated list of primary sites, holes, drilling depths, and contingency plans can be found in "Site summaries."

Proposed Site NT2-11B

We will run and jet-in a 36 inch conductor casing to 60 mbsf. Following this, the wellhead and blowout preventer will be set at the seafloor, and a 26 inch hole will be drilled to 700 mbsf with MWD (Fig. F7). The upper 700 m will be cased with 20 inch casing, then cemented. We will then drill a 12¼ inch hole from 700 to 1510 mbsf, and then RCB core from 1510 to 1600 mbsf. After coring, we will run three sets of wireline logs from 700–1600 mbsf (described in detail in later sections of this prospectus). After wireline logging the hole will be opened to 17 inches and cased to TD with 13⅜ inch casing. After cementing the casing, a cement bond log (CBL) will be run inside the casing. Following the CBL, we will run a vertical seismic profile (VSP) experiment. This will include a zero-offset VSP, as well as a two-ship experiment, pending coordination with a second (shooting) vessel. The VSP is planned for 2 days but may include additional time depending on operational progress and drilling schedule. Following the VSP, the wellhead will be set on the surface.

As a contingency plan at proposed Site NT2-11B, installation of a temperature monitoring system is planned (Fig. F9). This consists of up to 10 stand-alone self-recording temperature loggers in a 100 m long 9⅝ inch outside diameter (OD) casing hung below the tubing hanger. The main objective is to monitor any intrahole advection that will affect the performance of midhole observatory sensors to be installed in the future. If installed, this temperature monitoring system will be recovered in ~2 y when Chikyu revisits this hole to install the borehole observatory. In either case, the cased hole will then be capped with a corrosion cap.

Proposed Site NT2-01J

We will begin by jetting-in a 20 inch conductor to 36 mbsf. The hole will then be drilled with MWD/LWD to TD at 525 m and cased to TD. The casing string will include two screened casing joints that will span the fault zone at ~410 mbsf. After cementing, a "dummy run" to evaluate future observatory installation operations will be conducted, followed by a wireline temperature log inside the casing to identify the top of cement. The hole will be suspended by installing a retrievable casing packer at ~390 mbsf that is modified to include an instrument package below, and by setting a corrosion cap.