Logging While Drilling Operations

The downhole logging while drilling (LWD) operations in the Gulf of Mexico Gas Hydrate JIP Drilling Program (GOM-JIP) was designed in part to obtain data needed to assess the occurrence and concentration of gas hydrates in several key sites within the Gulf of Mexico. LWD operations are planned for two different offshore lease areas, Atwater Valley 13/14 and Keathley Canyon 151, in water depths in the range of 1280 to 1330 m. Proposed drilling and LWD logging depths range from 307 to 553 m beneath the sea floor.

During the GOM-JIP program, five Anadrill LWD and measurement-while-drilling (MWD) tools will be deployed at all four of the deep drill sites to be cored in Atwater Valley 13/14 and Keathley Canyon 151. These tools are being provided by Schlumberger-Anadrill services.

LWD and MWD tools measure different parameters. LWD tools measure in-situ formation properties with instruments that are located in the drill collars immediately above the drill bit. MWD tools are also located in the drill collars and measure downhole drilling parameters (e.g., weight on bit, torque, etc.). The difference between LWD and MWD tools is that LWD data are recorded into downhole computer memory and retrieved when the tools reach the surface, whereas MWD data are transmitted through the drilling fluid within the drill pipe by means of a modulated pressure wave, or “mud pulsing”, and monitored in real time. MWD tools enable both LWD and MWD data to be transmitted uphole when the tools are used in conjunction. The term LWD is often used more generically to cover both LWD and MWD type measurements.

The LWD and MWD tools used during the GOM-JIP drilling program will include the resistivity-at-the-bit GeoVision tool (GVR), the EcoScope tool (DVD with APWD), the power pulse MWD tool, a magnetic resonance while drilling tool (MWD-ProVision), and the azimuthal density neutron (VDN) tool. Figure-1 shows the configuration of the LWD/MWD bottom hole assembly (BHA).

LWD measurements are made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. Fluid invasion into the borehole wall is also reduced relative to wireline logging because of the shorter elapsed time between drilling and taking measurements. The LWD equipment is battery powered and uses erasable/programmable read-only memory chips to store logging data until they are downloaded. The LWD tools take measurements at evenly spaced time intervals and are synchronized with a system on the rig that monitors time and drilling depth. After drilling, the LWD tools are retrieved and the data downloaded from each tool through an RS232 serial link to a laptop computer. Synchronization of the uphole and downhole clocks allows merging of the time-depth data (from the surface system) and the downhole time-measurement data (from the tools) into depth-measurement data files. The resulting depth-measurement data are transferred to the processing systems in the Schlumberger-Anadrill logging unit onboard the Uncle John for reduction and interpretation.

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Configuration of the drill string used for LWD-MWD operations