IODP Proceedings    Volume contents     Search


Downhole measurements

Direct measurements of in situ pressure and temperature are critical to help define fluid, chemical and heat transport, diagenesis, and microbial activity in marine sediments. Additionally, the data are valuable for validating shipboard and shore-based estimates of pressure and stress from consolidation experiments and from porosity-depth data. The pressure and temperature fields also impact the strength behavior of sediments, so direct measurements can be used to help evaluate conditions that promote or impede fault slip.

Sediment temperature-pressure tool

The SET-P tool was deployed with the RCB BHA with the aim to measure in situ temperature and pressure within unlithified sand-dominated layers below 675 m LSF. Prior to drilling operations in Hole C0011B, the SET-P tool was connected to the CDS and placed into the RCB BHA to ensure that the landing mechanism was engaging correctly. During this test, the distance that the tip of the SET-P tool extended beyond the drill bit was measured. With the RCB BHA-CDS-SET-P configuration, it was determined that the SET-P tool tip extended ~1.1 m beyond the drill bit. This extension was limited by the outer diameter of the SET-P tool spacer and the inner diameter at the drill bit. This impeded the telescoping capability of the CDS during deployments; thus, there was no means to decouple the SET-P tool from the drill string. Therefore, weight on bit (WOB) would have to be maintained during deployments into the formation. Additional measurements of the tool string showed that the distance was 16.75 m from where the wireline connected to the sinker bar (top of tool string) and the tip of the SET-P tool.

To evaluate deployment operations and provide a sensor test on the SET-P tool, a test deployment was completed in the drill string above the seafloor at the location of Hole C0011B. The deployment consisted of lowering the SET-P tool downhole with stops when the tool tip was at 988 and 1989 meters below sea level (mbsl) for reference measurements (Table T26). The SET-P tool was then recovered on the drill floor. Drilling fluid was circulated while lowering and recovering the tool; however, no circulation occurred during the reference measurements. During the short reference holds (<3 min), the measured pressure decreased to the hydrostatic pressure, assuming seawater density of 1.024 g/cm3 (Fig. F68). By the end of the reference holds, the measured pressures were within 2% of the hydrostatic pressure. Longer holds likely would have resulted in smaller error, but noting the short reference holds and stretch in the wireline, these results confirmed the tool was functioning correctly. The measured temperature in the drill string decreased initially while lowering the tool but reached a value near 2.2°C below 1000 mbsl. Based on the pressure and temperature observations, the test deployment was successful. All raw data for the test deployment are available in SET-P in "Supplementary material."

A second SET-P tool deployment was scheduled to penetrate the formation at ~680 m CSF. This deployment targeted the top of a thick sand layer interpreted from LWD gamma ray and resistivity data in Hole C0011A. The deployment strategy was to complete a hole stability check and deploy the SET-P tool with 30 min of pressure and temperature measurement in the formation. The hole stability check would have been preceded by 10 and 20 min holds without circulation and rotation. Overpull would have been measured after each hold to establish conditions in the borehole. A 1.3 SG mud (25 m3) would have been spotted on the bottom of the hole prior to each hold and prior to the SET-P tool deployment. The WOB would have been 10–20 kN throughout the holds and the deployment into the formation.

Drilling parameters were variable while approaching the target deployment depth because of thin, hard layers interbedded with softer layers. Coring showed that the harder layers were carbonate rich. However, the thin sand layers identified in the LWD data were not recovered in cores. These hard horizons, a lack of knowledge about the depth of the sand top, and no sand recovery forced a reassessment of the target horizon for the SET-P tool deployment. This included shorter coring advancements. Even with shorter advancements, any reasonable correlation between MSCL-W and LWD data was ambiguous in this interval. For a successful SET-P tool deployment, we needed to advance the drill bit in the mudstone to ~1 m above the top of the target sand bed so the probe tip would penetrate the top of the sand without damaging the hydrologic integrity of the mudstone. This ambiguity in correlating LWD and MSCL-W data, the presence of hard layers, and the lack of recovery to confirm unlithified sand layers led to our decision to abort the SET-P tool deployment at Site C0011.