When the total depth of a hole scheduled for logging has been reached, a series of activities is initiated to prepare the hole and rig floor for the logging operation. The borehole is conditioned by pumping a viscous mud into the hole to flush remnant cuttings from the borehole, running the bit up and down to break through any bridges or swelling clays and finally filling the hole with a drilling mud such as sepiolite to stabilize the hole. Based on data collected from the capillary suction test (CST), the hole may be filled with fluids containing potassium chloride to inhibit the swelling of re-hydrated clays. The next step is determined by the type of bottom hole assembly (BHA) used. If the APC/XCB BHA and core barrel assembly is deployed, logging can commence directly through the bit with the use of a go-devil and the lockable flapper valve (LFV). If the RCB BHA and core barrel are deployed, the drill bit must be removed using one of three methods listed below:
- The drill bit may be dropped at the bottom of the hole (if hole deepening will not occur)
- The drill bit may be dropped at the seafloor but a reentry cone or Free Fall Funnel (FFF) is required to reenter the hole
- A pipe trip may be used to remove the bit at the rig floor and the hole reentered assuming a reentry cone or FFF is deployed.
Next, the base of drill pipe is placed at a depth of 50 - 80m below the sea floor to provide confining pressure to the upper regions of the hole and to prevent the pipe from pulling out of the hole. Once the pipe is set, the rig floor is converted from drilling operations to logging operations.
To prepare the rig floor for logging, the top drive is pushed back and the wireline is threaded through the derrick, winch and wireline heave compensator (WHC). The first logging string, typically the Triple Combo, is now prepared for rig up. The tools are assembled from the bottom up; therefore the DIT-E is rigged up first, then the Lamont TAP tool is attached to the bottom of the DIT-E. These two tools are placed in the pipe and successive tools are added to the top. After the tool string is assembled, the cablehead is attached and the tool zero point is established by pulling the bottom of the DIT-E to the level of the rig floor. The tool string is then lowered, the fluid seal is fastened and the tool is run into the hole. The tool descent speed is typically 10,000 ft/hr. Once the sea floor is reached, the tool string is held stationary for 2-3 minutes to allow the TAP tool to equilibrate. The tool is run down to the bottom of the hole and then pulled up at a constant rate to complete the first pass. A second pass can be completed if desired. As the first toolstring is being pulled back to the surface, the Schlumberger engineer may slow the ascent speed as the tool string crosses the mud line to measure the depth of the sea floor accurately.
The first toolstring is rigged down and the TAP tool is cleaned by the logger and returned to the Downhole Measurements Lab for the retrieval of the data. The next toolstring (typically the FMS/Sonic) is then prepared. The same logging procedure is followed for running the FMS/Sonic toolstring. The third toolstring deployed may be a specialty tool such as the GHMT, WST or other.
During the logging process, the Logging Staff Scientist is involved in many steps including:
- Taking detailed operations notes on the Logging Event forms and including all listed depths and times.
- Discussing toolstring configurations with the Schlumberger Engineer.
- Initializing the Lamont TAP tool and operating the data acquisition system.
- Connecting the TAP tool to the bottom of the DIT-E.
- Disconnecting, cleaning and downloading the data from the TAP tool.
- Remaining with the Schlumberger engineer to monitor tool progress and to inspect the data in real-time.
