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Downhole logging tools

Triple Combo Tool String

Phasor Dual Induction-Spherically Focused Resistivity Tool (DIT*)


The Phasor Dual Induction - Spherically Focused Resistivity tool (DIT) provides measurements of spontaneous potential (SP) and three different resistivity values: IDPH (deep induction), IMPH (medium induction) and SFLU (shallow spherically focused resistivity). Since the solid constituents are orders of magnitude more resistive than pore fluids in most rocks, resistivity is controlled mainly by the conductivity of the pore fluids and by the amount and connectivity of the pore space. The spontaneous potential is a measure of the streaming potential generated by differences between borehole and pore fluid electrical properties; these result in both membrane and liquid junction potentials due to differences in the mobility of ions in the pore and drilling fluids. The induction sonde consists of a series of transmitter and receiver coils mounted on the sonde axis. The high frequency, alternating current of constant intensity sent through the transmitter coil produces an alternating magnetic field which in turn induces currents in the formation around the borehole. These currents flow in circular ground loops coaxial with the sonde. Because the alternating current sent by the transmitter coil is of constant frequency and amplitude, they are directly proportional to the formation conductivity. They also produce a magnetic field which induces a voltage in the receiver coil, which is in turn proportional to the ground loop currents and therefore to the resistivity of the formation.

In homogeneous formations with resistivity higher than 100 ohm m the average radial depth of investigation is about 5 ft (1.5 m) and 2.5 ft (76 cm) for the deep and medium induction curves, respectively, and 1.25 ft (38 cm) for the SFL. This drops to 4 ft (122 cm) and 2.2 ft (66 cm) at 0.1 ohm-m resistivities.

The thin bed resolution over a full range of formation conductivities has been greatly improved, due to an enhanced signal processing technique and real time correction for the effect of adjacent formations (shoulder effect).


Porosity estimate

In sediments that do not contain clay or other conductive minerals, the relationship between resistivity and porosity has been quantified by Archie's Law. Archie's Law relates the resistivity to the inverse power of porosity. This relationship has also been used to estimate apparent porosity in oceanic basalts.

Density and velocity reconstruction

Archie's equation has been used effectively to create "pseudodensity" and/or "pseudovelocity" logs from porosity over intervals where no such logs were recorded or were totally unreliable. In some instances velocities derived from resistivity logs can be used to depth-tie seismic reflectors.

Lithologic boundary definition and textural changes

Resistivity, along with acoustic and velocity logs, is a very valuable tool in defining lithologic boundaries over intervals of poor core recovery. In a particular example, the decrease in resistivity toward the top of a carbonate unit, coupled with a decrease in velocity, allowed one to interpret this unit as a fining-upward sequence in mostly carbonatic sediments. Similar saw-tooth patterns in the resistivity response can also be observed in oceanic basalt units where they are related to porosity changes towards the top of each unit.

Environmental Effects

The Phasor Dual Induction tool provides a set of corrections for different environmental effects, which can be performed in real time during logging. These include corrections for adjacent formations, borehole signal, and invasion. In general, invasion is not a problem in the boreholes logged in the IODP, because seawater is used as drilling fluid, but it can occur in land wells. In fact, depending on the type of drilling mud used and on the permeability of the formation, invasion of the mud filtrate into the formation adjacent to the borehole can lead to differences in the response of shallow and deeper resistivity devices. On the other hand, invasion can provide useful information about formation permeability and pore fluid electrical conductivity. Differences in the temperature of drilling fluid compared to undisturbed formation temperatures can also generate this effect, as conductivity in ionic fluids such as seawater is strongly temperature dependent.

Log Presentation

Deep (ILD or IDPH) and medium (ILM or IMPH) induction, and spherically focused resistivity (SFLU), are usually plotted in ohm-m on a logarithmic scale along with gamma ray and caliper logs.

Output plot of DIT data

Tool Specifications

Temperature rating: 350° F (175° C)
Pressure rating: 20 kpsi (13.8 kPa)
Diameter: 3.375 in (9.21 cm)
Length: 31 ft (9.5 m)
Weight: 445 lbs (201.9 kg)
Sampling interval: 6 in (15.24 cm)
Max. logging speed: 10,000 ft/hr (3,048 m/hr)

Measurement Specifications

Vertical resolution: 5-6 ft (1.5-1.8 m) and 7-8 ft (2.1-2.4 m) for medium and deep induction logs; 3 ft (92 cm) for spherically focused log.
Depth of investigation: (See discussion in "Description" section)
Measurement range: 0-150 ohm-m

Major Outputs

ILD: Deep induction (ohm-m)
ILM: Medium induction (ohm-m)
IDPH: Phasor deep induction (ohm-m)
IMPH: Phasor medium induction (ohm-m)
SFLU: Spherically focused log (ohm-m)
ITEM: Internal temperature (°C)

Deployment Notes

The DIT is typically run with APS, HLDS, and HNGS on the triple combo. The DIT has an internal temperature measurement that may be useful in high temperature environments.

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