Previous   |    Next

doi:10.2204/iodp.proc.347.102.2015

Downhole logging

Offshore downhole logging operations for IODP Expedition 347 were provided by Weatherford Wireline Service and managed by the EPC. The EPC is primarily responsible for the planning, acquisition, QA/QC, and science support related to petrophysical measurements on MSPs. Petrophysical data collection in relation to this consortium includes downhole measurements (i.e., borehole logging) and physical properties (e.g., magnetic susceptibility, acoustic properties, resistivity, and density) measured on cores (continuous and discrete; see “Physical properties”).

The set of downhole geophysical instruments utilized during Expedition 347 was chosen based on the scientific objectives, the coring technique, and the anticipated borehole conditions at the six sites.

Weatherford’s Compact logging tools were used (Table T4). The suite of downhole geophysical methods was chosen to obtain high-resolution electrical images of the borehole wall, measure borehole size, and measure or derive petrophysical or geochemical properties of the formations such as porosity, electrical resistivity, acoustic velocity, and natural gamma radiation. No nuclear tools were deployed during Expedition 347.

The Weatherford Compact suite comprised the following tools:

• The gamma ray tool (MCG) that measures the natural gamma radiation.
• The spectral gamma ray tool (SGS), which allows for the identification of individual elements that emit gamma rays (e.g., potassium, uranium, and thorium).
• The sonic sonde (MSS) that measures formation compressional slowness (inverse velocity).
• The array induction tool (MAI), which measures electrical conductivity of the formation. The output of this tool comprises three logs of induction electrical conductivity at shallow, medium, and deep investigation depths.
• The microimager (CMI), which is a memory capable resistivity microimaging tool. The eight arms in two planes of the CMI provide four independent radii measurements, which can be used to record borehole size and identify near-borehole stress regimes.

Operations

The logging team consisted of one engineer and one operator from Weatherford, supervised and assisted by the Petrophysics Staff Scientist.

A total of nine boreholes (Holes M0059B, M0059E, M0060B, M0062D, M0063A, M0064A, M0064D, M0065A, and M0065C) were prepared for downhole logging measurements. Measurements were performed in open borehole conditions (no casing). Despite challenging borehole conditions (nonconsolidated formations, risk of collapse, etc.), the recovery and overall quality of the downhole logging data is good. The gamma ray tool was used at the top of every tool string as a communication tool and for correlation between different runs. The induction tool was usually run first, followed by the other tool strings. The choice of tool strings was based on borehole conditions and drilled depth, and because of borehole conditions, it was not possible to log with all tools in every borehole (Table T5).

Each tool was logged on a tool string with the MCG always on top. A logging run commenced with zeroing the tool to a fixed point (drillers zero) corresponding to the top of the drill pipe. Each logging run was recorded and stored digitally. During acquisition, data flow was monitored for quality and security in real time using tool-specific acquisition boxes (Weatherford Well Manager Software). Tools were raised at speeds that ranged from a minimum of 5 m/min to a maximum of 10 m/min depending on the tool string used and the resolution chosen. Recordings were taken both downhole and uphole. Because of the real-time data display, it was possible to stop uphole recovery once the drill pipe was entered. At the end of logging, the Well Manager Software was used for visualization, QA/QC, processing, interpretation, and plotting of the data. Only uphole logs were used to process data.

Logging tool descriptions and acquisition parameters

Technical schemes on individual tools are shown in Figure F22. Additional information can be found on the Weatherford Web site (www.weatherford.com). Detailed information on their geological applications is available in Schlumberger (1989), Serra (1984, 1986) and Rider (2002).

Natural gamma probe

The MCG tool is a sensitive gamma ray detector consisting of a scintillation counter and a photomultiplier. When gamma rays pass through the sodium iodide crystal they cause a flash. These flashes are collected by the photomultiplier and stored in the attached condenser over a set period of time. The energy accumulated during this time is the detector value at that depth. The MCG tool was calibrated to API gamma ray units (gAPI), a unit defined by the American Petroleum Institute for gamma ray log measurements. The vertical resolution of the MCG tool is 305 mm. The downhole measurement interval was 0.1 m as standard and 0.025 m for high resolution.

Spectral natural gamma probe

Unlike other instruments that record total gamma ray emissions, the SGS tool allows identification of certain individual elements that emit gamma rays. Naturally occurring radioactive elements such as K, U, and Th emit gamma rays with a characteristic energy. K decays into two stable isotopes (Ar and Ca), and a characteristic energy of 1.46 MeV is released. U and Th decay into unstable daughter elements that produce characteristic energy at 1.76 and 2.62 MeV, respectively.

The most prominent gamma rays in the U series originate from decay of ²¹⁴Bi and in the Th series from decay of ²⁰⁸Tl. It is thus possible to compute the quantity (concentration) of parent ²³⁸U and ²³²Th in the decay series by counting gamma rays from ²¹⁴Bi and ²⁰⁸Tl, respectively. The SGS detector for gamma rays is a sodium iodide (NaI) scintillation crystal optically coupled to a photomultiplier. The instrument was master-calibrated by the manufacturer prior to the expedition. As the probe moves up the borehole, gamma rays are sorted according to their emitted energy spectrum and the number of counts in each of the three preselected energy intervals is recorded. These intervals are centered on the peak values of ⁴⁰K, ²¹⁴Bi, and ²⁰⁸Tl. Tool output comprises Th and U in ppm and K in percent of total gamma ray counts in API. The vertical resolution of the used tool is ~305 mm. The downhole measurement interval was 0.1 m standard or 0.025 m for high resolution.

Induction resistivity probe

The MAI tool measures electrical conductivity of the geological formation. Variations in electrical conductivity correspond primarily to variations in lithology (composition and texture), formation porosity, saturation, and interstitial fluid properties (salinity). An oscillator sends an alternating current of constant amplitude and frequency through an emitting coil. This current generates an alternating electromagnetic field that induces Foucault currents in the formation. These Foucault currents are proportional to the formation conductivity and generate their own electromagnetic fields. When passing through a receiving coil (solenoid), these secondary magnetic fields induce electromotive forces that are proportional to the flow running through the coil. The output of the MAI tool comprises three conductivity logs at varying intervals of investigation: shallow, medium, and deep induction (ranging from 0.3 to 1.2 m). Measured conductivity can then be converted into electrical resistivity. The MAI tool has a vertical resolution of 0.610 m. The downhole measurement interval was 0.1 m standard and 0.025 m for high resolution.

Full waveform sonic probe

The MSS tool measures formation compressional slowness (inverse velocity) and when bulk density and elastic properties are known (from core measurements) an estimate of porosity can be derived from sonic measurements.

This downhole instrument is composed of an acoustic transmitter and receivers. The transmitter emits an acoustic signal that propagates through the borehole fluid to the rock or sediment interface, where some of the energy is critically refracted along the borehole wall. As a result of wavefront spreading (Huygens principle), some of the refracted energy is transmitted back into the borehole adjacent to a receiver. Each receiver picks up the signal, amplifies it, and digitizes it. Recorded waveforms are then examined, and compressional (P-) wave arrival times are manually or automatically selected (picked). Arrival times are the transit times of the acoustic energy, and by measuring the acoustic transit time, knowing the distance between the two receivers, the velocity of the fluid, and the borehole diameter, the sonic velocity of the rock or sediment can be calculated. Consequently, the interval velocity value is calculated at each sampling point. The MSS tool measures formation compressional waves at five spacings with 0.3 and 0.6 m vertical resolution. Unlike traditional 3 to 5 ft (0.9 to 1.5 m) sonic tools, the MSS tool uses a single-sided array with depth-derived cave compensation and tilt correction for improved response in poor borehole conditions.The downhole measurement interval was 0.1 m standard resolution or 0.025 m high resolution.

Microimager

The CMI tool is a memory capable resistivity microimaging tool that can be deployed with or without a wireline. The design and conveyance flexibility of the tool facilitate access into highly deviated wells and past bad hole conditions without compromising borehole coverage. The CMI tool has eight arms in two planes (Fig. F22). The upper four arms have pads that contain 20 buttons in two rows. The upper caliper arms are cross-linked, helping to centralize the tool and provide two borehole diameter measurements. The bottom four pads have 24 buttons in two rows. The lower four caliper arms are independently articulated to maintain good borehole contact, which is crucial for high-image quality. They also provide four independent radii measurements, which can be used to identify near-borehole stress regimes. The vertical resolution is 5.1 mm and the depth of investigation is 25.4 mm.

Data processing

Logging data were processed using the Well Manager Software package (Weatherford) offshore and Interactive Petrophysics (Senergy) onshore.

Depth adjustments

The main processing task involved evaluating the depth of each logging run and referencing the data to the rig floor and seafloor. While deploying all the tools separately in the same section, a fixed zero depth position (loggers zero) was maintained at the top of the drill pipe. Typical reasons for depth corrections include ship heave and tide. Using Interactive Petrophysics (Senergy), the original logs were depth adjusted to the seafloor (in meters wireline log depth below seafloor [m WSF]) using the drillers depth to seafloor. Slight discrepancies (<0.5 m) may exist between the seafloor depths seen in the downhole logs (gamma ray) and those determined by the drillers as clay was present on the seabed frame.

When necessary, logs have been matched manually by the log analyst to a reference log using distinctive peaks. In such cases, gamma ray logs are taken as reference logs (continuous). Generally the depth discrepancies between logs are <1.5 m.

Invalid data

Invalid log values have been replaced by a null value of –999.25. The cause of the removed invalid log values can include the following:

• Log data in pipe and
• The effect when approaching the metallic pipe in open-hole condition.

Environmental corrections

Environmental corrections are designed to remove any effect from the borehole (size, roughness, temperature, and tool standoff) or the drilling fluids that may partially mask or disrupt the log response from the formation. Here, no postacquisition corrections of this type were applied.

Log merges

Where applicable for the processed logs, overlapping log runs have been merged to give one continuous log. In the overlapping regions, data have been checked by the log analyst to make sure distinctive peaks and troughs correlate.

Quality control

Data quality is assessed in terms of reasonable values for the logged formation, repeatability between different passes of the same tool, and correspondence between logs affected by the same formation property. Considering the challenging borehole conditions, the overall quality of the downhole logging data is very good.

Data delivery

The downhole data are available in the USIO online log database. Processed standard data are available in ASCII and LAS format. Processed depths are referred to as wireline depth below seafloor scales. CMI image files are also available in PDF format (and scales).

Top of page   |    Previous   |    Next