DOWNHOLE MEASUREMENTS

Introduction

Downhole logs are continuous records of physical, chemical, and structural properties of the formation penetrated by a borehole. The logs are made using a variety of probes combined into several tool strings. These strings are lowered down the hole on a heave-compensated electrical wireline and then pulled up at a constant speed to provide continuous measurements as a function of depth. Logs can be used to interpret the stratigraphy, lithology, mineralogy, and structure of the penetrated formations. Where core recovery is incomplete or disturbed, log data may provide the only way to characterize the borehole section. Where core recovery is good, log and core data complement one another and may be interpreted jointly. Finally, downhole logs are sensitive to formation properties on a scale that is intermediate between those obtained from laboratory measurements on core samples and geophysical surveys.

Three logging tool strings were used during Leg 186: the triple combination logging tool (triple combo)/temperature, the Formation MicroScanner (FMS)/sonic, and the borehole televiewer (BHTV). In addition to wireline logs, in situ temperature measurements were made with the APC temperature tool and Davis-Villinger temperature probe (DVTP).

Logging Tools

The tool strings used on Leg 186 were

  1. The triple combo tool string (resistivity, density, and porosity; see Fig. F8) consists of the accelerator porosity sonde (APS), the hostile environment lithodensity sonde (HLDS), and the phasor dual induction-spherically focused resistivity tool (DITE-SFL), depending on formation resistivity. The hostile environment spectral natural gamma-ray sonde (HNGS) was included at the top of the string, and the Lamont-Doherty Earth Observatory temperature/acceleration/pressure tool (LDEO-TAP) at the bottom.
  2. The FMS/sonic tool string (Fig. F8) consists of the FMS, the general purpose inclinometer tool (GPIT), and a sonic sonde. The dipole shear sonic imager (DSI) and the long-spaced sonic imager (LSS) were used during Leg 186. The natural gamma-ray tool (NGT) was included at the top of this tool string.
  3. The BHTV tool string (Fig. F8) consists of the BHTV and the GPIT. The NGT was included at the top of this tool string. This string was used to address the specific objectives of Leg 186: to constrain the borehole condition such as hole geometries, stress, and stabilities in the lower part of the borehole, where we expected to install downhole instruments.

Each tool string includes a telemetry cartridge for communicating through the wireline with the logging laboratory on the drillship and a natural gamma-ray sonde that is used to identify lithologic markers, providing a common reference for correlation and depth shifting between multiple logging runs. Logging runs are typically conducted at 250-275 m/hr.

The logging tools are briefly described below, and their operating principles, applications, and approximate vertical resolution are summarized in Table T6. More detailed information on individual tools and their geological applications may be found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), Serra (1984, 1986, 1989), and the LDEO-Borehole Research Group Wireline Logging Services Guide (1994).

Natural Radioactivity

Two spectral gamma-ray tools were used to measure and classify natural radioactivity in the formation: the HNGS and the NGT. The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of K (in weight percent), Th (in parts per million), and U (in parts per million), the three elements whose isotopes dominate the natural radiation spectrum. The HNGS is similar to the NGT, but it uses two bismuth germanate scintillation detectors for a significantly improved tool precision. Spectral analysis in the HNGS filters out gamma-ray energies below 500 keV, eliminating sensitivity to bentonite or KCl in the drilling mud and improving measurement accuracy.

The NGT response is sensitive to the borehole diameter and the weight of the drilling mud, which is controlled by the concentration of bentonite or KCl. These effects are routinely corrected for during postcruise processing at LDEO.

Density

Formation density was determined from the density of electrons in the formation, which was measured with the HLDS. The sonde contains a radioactive cesium (137Cs) gamma-ray source (622 keV) and far and near gamma-ray detectors mounted on a shielded skid that is pressed against the borehole wall by a hydraulically activated eccentralizing arm. Gamma rays emitted by the source experience Compton scattering, which involves the transfer of energy from gamma rays to the electrons in the formation via elastic collision. The number of scattered gamma rays that reach the detectors is directly related to the number of electrons in the formation, which is in turn related to bulk density. Porosity may also be derived from this bulk density if the grain density is known.

The HLDS also measures the photoelectric effect factor (PEF) caused by absorption of low-energy gamma rays. Photoelectric absorption occurs when gamma rays reach less than 150 keV after being repeatedly scattered by electrons in the formation. Because PEF depends on the atomic number of the elements in the formation, it varies according to the chemical composition and is essentially independent of porosity. For example, the PEF of pure calcite = 5.08 barn/e-; illite = 3.03 barn/e-; quartz = 1.81 barn/e-; and kaolinite = 1.49 barn/e-. Photoelectric effect factor values can be used in combination with NGT curves to identify different types of clay minerals. Coupling between the tool and borehole wall is essential for good HLDS logs. Poor contact results in underestimation of density values. Poor contact may occur when the borehole diameter is greater than the length of the eccentralizing arm (e.g., for borehole diameters >48 cm).

The depth of investigation of the lithodensity tool is of the order of tens of centimeters, depending on the density of the rock.

Porosity

Formation porosity was measured with the APS. The sonde incorporates a minitron neutron generator which produces fast (14.4 MeV) neutrons, and five neutron detectors (four epithermal and one thermal) positioned at different spacings. The tool is pressed against the borehole wall by an eccentralizing bow-spring. Emitted neutrons are slowed down by collisions. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greater energy loss occurs when the neutron strikes a nucleus nearly equal to its own mass, such as hydrogen, which mainly is present in the pore water. The neutron detectors record both the numbers of neutrons arriving at various distances from the source and neutron arrival times which act as a measure of formation porosity. However, because hydrogen bound in minerals such as clays or in hydrocarbons also contributes to the measurement, the raw porosity is often overestimated.

Electrical Resistivity

The DITE was used to measure the formation electrical resistivity. The DITE provides three measures of electrical resistivity, each with a different depth of investigation into the formation. Deep and medium penetration measurements are made inductively using transmitter coils that are energized with high-frequency alternating currents, creating time-varying magnetic fields that induce secondary Foucault currents in the formation. The strength of these induced ground currents is inversely proportional to the resistivity of the formation through which they circulate, as are the secondary inductive fields that they create. The amplitude and phase of the secondary magnetic fields, measured with receiving coils, are used as a proxy for the formation resistivity. Shallow penetration measurements with a high vertical resolution are made with a spherically focused laterolog. This measures the current necessary to maintain a constant voltage drop across a small fixed interval. Because of the inductive nature of the deep and medium penetration measurements, DITE logs are accurate only for formations with resistivities less than about 100 m, such as sediments. In more resistive formations, measurement error becomes significant (>20%), and it is more suitable to use the dual laterolog (Schlumberger, 1989).

For the deep measurement, both focusing and measurement currents return to a remote electrode on the surface; thus, the depth of investigation is considerable, and the effect of borehole conductivity and of adjacent formations is reduced. In the shallow laterolog, the return electrodes that measure the bucking currents are located on the sonde; therefore, the current sheet retains focus over a shorter distance than the deep laterolog. Fracture porosity can be estimated from the separation between the deep and shallow measurements based on the observation that the former is sensitive to the presence of horizontal conductive fractures only, whereas the latter responds to both horizontal and vertical conductive structures.

The depth of investigation of the DITE depends on the resistivity of the rock and on the resistivity contrast between the zone invaded by drilling fluid and the uninvaded zone. In formations with resistivity higher than 100 m, the average radial depth of investigation of the DITE is about 1.5 m for the deep resistivity phasor induction, 76 cm for the medium resistivity phasor induction, and 38 cm for the shallow spherically focused resistivity. These values drop by about 20% for a 0.1-m formation resistivity.

The DITE also measures spontaneous potential (SP) fields. Spontaneous potentials can originate from a variety of causes: electrochemical, electrothermal, electrokinetic streaming potentials, and membrane potentials due to differences in the mobility of ions in the pore and drilling fluids. The interpretation of SP logs remains problematic due to this multiplicity of sources.

Temperature, Acceleration, and Pressure

Downhole temperature, acceleration, and pressure were measured with the LDEO-TAP tool. When attached to the bottom of the triple combo string, the LDEO-TAP is run in an autonomous mode, with data stored in built-in memory. Two thermistors are mounted near the bottom of the tool to detect borehole fluid temperatures at different rates. A thin fast-response thermistor is able to detect small abrupt changes in temperature. A thicker slow-response thermistor is used to estimate temperature gradients and thermal regimes more accurately. The pressure transducer is included to activate the tool at a specified depth. A three-axis accelerometer measures tool movement downhole, providing data for analyzing the effects of heave on a deployed tool string, which should eventually lead to the fine tuning of the wireline heave compensator (WHC).

The borehole temperature record provides information on the thermal regime of the surrounding formation. The vertical heat flow can be estimated from the vertical temperature gradient combined with measurements of the thermal conductivity from core samples.

The temperature record must be interpreted with caution, as the amount of time elapsed between the end of drilling and the logging operation is generally not sufficient to allow the borehole to recover thermally from the influence of drilling fluid circulations. The data recorded under such circumstances may differ significantly from the thermal equilibrium of that environment. Nevertheless, from the spatial temperature gradient it is possible to identify abrupt temperature changes that may represent localized fluid flow into the borehole, indicative of fluid pathways and fracturing and/or breaks in the temperature gradient that may correspond to contrasts in permeability at lithological boundaries.

Acoustic Velocity

The LSS was used to measure elastic compressional wave velocity in the formation. The LSS provides long spacing measurements through the "depth-derived" borehole compensation principle. Acoustic traveltime readings between two sources and two receivers are memorized at one depth and combined with a second set of readings made after the sonde has been pulled the appropriate distance along the borehole. The LSS records the full waveform for each source-receiver pair, in addition to its automatic determination of arrival time. The depth of investigation for sonic tools depends on the spacing of the detectors and on the petrophysical characteristics of the rock, such as rock type, porosity, and alteration, but is of the order of tens of centimeters.

The DSI tool employs a combination of monopole and dipole transducers to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1994). In addition to a robust and high-quality measurement of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to determine shear-wave velocity in all types of formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at the shear-wave velocity and is the most reliable way to estimate a shear velocity log. The configuration of the DSI also allows recording of cross-line dipole waveforms. These modes can be used to estimate shear-wave splitting caused by preferred mineral and/or structural orientation in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well. The tool configuration and data processing are described in the Leg 174B Initial Reports volume (Shipboard Scientific Party, 1998).

Formation MicroScanner

The FMS provides high-resolution, electrical resistivity-based images of borehole walls. The tool has four orthogonal arms (pads); each pad is 50 mm wide and contains 16 microelectrodes, or "buttons," which are pressed against the borehole wall during the recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced about 2.5 mm apart. A focused current is emitted from the four pads into the formation, with a return electrode near the top of the tool. Array buttons on each of the pads measure the current intensity variations. Processing transforms these measurements of the microresistivity variations of the formation into continuous, spatially oriented, and high-resolution images that mimic geologic structures behind the borehole walls. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation. FMS images are particularly useful for mapping structural features, dip determination, detailed core-log correlation, positioning of core sections with poor recovery, and analysis of depositional environments.

The FMS image is sensitive to structures within about 25 cm beyond the borehole wall and has a maximum vertical resolution of 5 mm with a coverage of 25% of the borehole wall for a borehole diameter of 9 7/8 in (i.e., RCB bit size). FMS logging commonly includes two passes, the images of which are merged to improve the borehole-wall coverage. To produce reliable FMS images, however, the pads must be firmly pressed against the borehole wall. The maximum extension of the caliper arms is 15 in. In holes with a diameter larger than 15 in, the pad contact will be inconsistent and the FMS images can be blurred. The maximum borehole deviation where good data can be recorded with this tool is 10°. Irregular borehole walls will also adversely affect the images because contact with the wall is poor.

Borehole Televiewer

The Schlumberger digital BHTV recorded ultrasonic acoustic images during Leg 186. A transducer emits 256 ultrasonic pulses at a frequency of 400 kHz each rotation; these pulses are reflected off the borehole wall and then received by the same transducer. The amplitude and traveltime of the reflected signal are determined and stored in the Schlumberger MAXIS computer. A continuous rotation of the transducer and the upward motion of the tool produce a coverage of 100% of the borehole wall.

The amplitude depends on the reflection coefficient of the borehole fluid/rock interface, the position of the BHTV in the borehole, the shape of the borehole, and the roughness of the borehole wall. To decrease the eccentric motions of the tool, a centralizer was mounted immediately above the head of the BHTV and above the NGT (Fig. F8; Table T7). Unfortunately, the design of the BHTV does not allow mounting of a centralizer below the DHTV; thus, eccentric motion of the tool could not be completely eliminated. The change of the borehole wall's roughness (e.g., at fractures intersecting the borehole or in washed-out sections) is responsible for the modulation of the reflected signal; therefore, fractures or changes in character of the drilled rocks can easily be recognized in the amplitude image. On the other hand, the recorded traveltime image gives detailed information about the shape of the borehole, which allows calculation of one caliper value of the borehole from each recorded traveltime (in total, 256 readings per rotation).

Amplitude and traveltime are recorded together with a reference to magnetic north by means of a magnetometer, permitting orientation of images. If features (e.g., fractures) recognized in the core are observed in the BHTV images, orientation of the core is possible. The BHTV can also be used to measure stress in the borehole through identification of borehole breakouts and slip along fault surfaces penetrated by the borehole. In an isotropic, linearly elastic rock subjected to an anisotropic stress field, breakouts take place in the direction of the axis of minimum principal horizontal stress for subvertical boreholes.

Magnetic Field Measurement

Downhole magnetic field measurements were made with the GPIT. The primary purpose of this sonde, which incorporates a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the FMS/sonic and BHTV tool strings during logging. The acceleration data allow more precise determination of log depths than is possible on the basis of cable length alone, as the wireline is subject to both stretching and ship heave. Acceleration data are also used in processing of FMS and BHTV data to correct the images for irregular tool motion.

Log Data Quality

The quality of log data may be seriously degraded by excessively wide sections of the borehole or by rapid changes in the hole diameter. Resistivity and velocity measurements are the least sensitive to borehole effects, whereas the nuclear measurements (density, neutron porosity, and both natural and induced spectral gamma rays), FMS, and BHTV are more sensitive because of the large attenuation by borehole fluid or because of poor pad contact. Corrections can be applied to the original nuclear data to reduce the effects of these conditions and, generally, any departure from the conditions under which the tool was calibrated.

Logs from different tool strings may have depth mismatches, caused by either cable stretch or ship heave during recording. Small errors in depth matching can distort the logging results in zones of rapidly changing lithology. To minimize the effects of ship heave, the hydraulic WHC adjusts for rig motion during logging operations. Distinctive features recorded by the NGT run on every log tool string provide correlation and relative depth offsets among the logging runs and can be calibrated to distinctive lithologic contacts observed in the core recovery or drilling penetration (e.g., basement contacts). Precise core-log depth matching is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section within the cored interval.

In Situ Temperature Measurements

In situ thermal measurements were made using the APC temperature shoe and the DVTP. Techniques using the APC tool are similar to those used since Leg 137 (Shipboard Scientific Party, 1992) and are described in Fisher and Becker (1993).

The DVTP was deployed for the first time during Leg 164 (Shipboard Scientific Party, 1996). The probe has a nearly cylindrical casing that terminates in a pointed tip. In situ temperatures are logged by two thermistors, one located 1 cm from the tip of the probe and the other 12 cm above the tip. A third thermistor, referred to as the internal thermistor, is located in the electronics package. Thermistor sensitivity is 1 mK in an operating range of -5° to 20°C, and the total operation range is -5° to 100°C. In addition to the thermistors, the probe contains an accelerometer sensitive to 0.98 m/s2. Both peak and mean acceleration are recorded by the logger. The accelerometer data are used to track disturbances to the instrument package during the equilibration interval. Detailed techniques are described in the Leg 164 Initial Reports volume (Shipboard Scientific Party, 1996).

Synthetic Seismograms

Synthetic seismograms can be generated using known density-depth and velocity-depth profiles, the product of which is called acoustic impedance. The impedance in soft sediments is lower, resulting from a lower density and velocity than found in stiffer formations. Any impedance difference between two formations, usually called impedance contrast, could cause a possible seismic reflection. However, the strength and form of a reflection event observed on a field seismic record depends upon the distance of the reflector from the seismic source and the frequency content, in addition to many other factors. Nevertheless, comparison of a synthetic seismogram with a field seismic record are instructive to determine which seismic reflections observed on the record were generated by the drilled strata or geological boundaries.

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