GG 450 March 6, 2008 BOREHOLE LOGGING

Drilling Technology Basics Drilling is an art that has progressed a great deal since the first wells were drilled about 150 years ago. At that time, the main product was kerosene to be burned in lamps. Gasoline didn't come in until the automobile was on its way around 1900. Drillers didn't trouble themselves with environmental concerns, and the process was dirty and wasteful.

BOP Stack: Wells now all have a blow-out-prevention stack (BOP stack) at the ground level which can cut and seal the pipe and/or any casing strings quickly if pressure in the hole indicates that a highly over pressured zone has been encountered.

Drill pipe is HEAVY. The Joides Resolution string weighs about 600 tons. The drillers must be sure that the pipe is always in tension. The only part of the pipe in compression is at the bit itself. Heavy weighted pipes (drill collar) just above the bit keep the bottom section in tension.

The pipe is kept in rotation as much as possible with fluid (drilling mud) pumped down through the pipe through the bit. The fluid keeps bits of rock (cuttings) from falling back down and clogging the pipe. The pipe can only rotate in one direction. If it were to rotate backwards, the screw joints between pipe stands would come apart.

Drilling mud is used to bring heavy cuttings back to the surface Drilling mud is used to bring heavy cuttings back to the surface. If water were used, these cuttings would tend to sink back down the hole because of the density difference. What can be measured in a borehole is limited by the characteristics of the hole. While drilling, holes are CASED with steel pipe to prevent the hole from collapsing in around the drill pipe. As the hole is deepened, more casing – with smaller diameter is inserted in the hole. The diameter of the outermost casing might be 16”, and the smallest only 5”.

Most drill holes are not cored Most drill holes are not cored. Drillers depend on cuttings brought up with the drilling mud to tell what rock they are in, and on WELL LOGGING. The pressure at the cutting head is kept high to prevent blow-outs. Water thus penetrates the formations and can confuse the picture obtained from logging.

Riser drillings in marine drilling allows deeper penetration because mud can be circulated up to the drill ship.

Down-hole logging methods In all “wireline” methods, a string of probes is lowered down into the well. Different properties are measured by the probe and plotted as a function of depth. Logging is usually done as the probe is coming back up the well. Probes are stacked together in strings to obtain as much information in as little time as possible. A string might be 30 m long.

Advantages of Logging Provides objective and quantitative data for existing wells. Requires only one or two people. Test can be repeated. Non-destructive. Fast and economical. Much cheaper than coring. Disadvantages of Logging Methods Equipment is expensive. Will not define all important contacts. Requires trained “logging analyst” for data collection and interpretation.

Array Seismic Imager The Array Seismic Imager (ASI) consists of an array of five seismic sensors. Each sensor package contains orthogonal geophones. This tool is used in Vertical Seismic Profiling, where s seismic source at the top is recorded by the sensors in the hole, providing excellent estimates of interval velocities.

Dipole Sonic This seismic tool provides excellent estimates of interval velocity that can be used in conjunction with reflection processing. Low frequency penetrates deeper into the formation and helps read beyond altered zones.

The BHTV provides an acoustic reflection image of the borehole, Particularly useful for detection of dipping layers and faulting.

Dual Induction Tool (DITE) The Phasor Dual Induction - Spherically Focused Resistivity tool (DITE-SFR) provides measurements of spontaneous potential (SP) and three different resistivity values; deep induction, medium induction, and shallow resistivity. Resistivity is controlled mainly by the conductivity of the pore fluids and by the amount and connectivity of the pore space. Applications: Porosity estimate: In sediments which 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.

Dual Laterolog (DLL) The Dual Laterolog (DLL) provides two resistivity measurements with different depths of investigation into the formation: deep and shallow. A current beam 2 ft-thick is forced horizontally into the formation by using focusing currents. For the deep measurement both measure and focusing currents return to a remote electrode on the surface; thus the depth of investigation is greatly improved, and the effect of borehole conductivity and of adjacent formations is reduced. Applications: Porosity estimate: Because of the inverse relationship between resistivity and porosity, the dual laterolog can be used to compute the porosity of the rock from Archie's equation.

Geologic High-Resolution Magnetic Tool (GHMT) The Geological High-Resolution Magnetic Tool (GHMT) provides magnetic susceptibility and total magnetic induction measurements. The main use of the GHMT is to provide a magnetic reversal sequence in sediment. The Nuclear Resonance Magnetometer Sonde (NMRS) accurately measures the total magnetic induction in the borehole. Its depth of investigation is theoretically infinite (most of the Earth's field is generated in the Earth's core) and its vertical resolution is about 45 cm.

Example of magnetostratigraphy (ODP Leg 165) The figure shows the derivation of magnetostratigraphy from the susceptibility and total induction measurements. Correlation indicates normal magnetic polarity zones, anti-correlation indicates reverse magnetic polarity zones. The interpretation column is then compared to a standard geomagnetic polarity time scale.

Formation MicroScanner (FMS) The Formation MicroScanner sonde (FMS) is a high-resolution resistivity-measuring device consisting of four orthogonal imaging pads each containing 16 microelectrodes in direct contact with the borehole wall. The button current intensity is sampled every 0.1 in (2.5 mm). The tool works by emitting a focused current from the 4 pads into the formation measured by potential electrodes. The current intensity measurements reflect microresistivity variations of the formation.

FMS Applications･Mapping of bedding planes, fractures, faults, foliations, and other formation structures and dip determination.･Detailed correlation of coring and logging depths.･Precise positioning of core sections where core recovery is less than 100%.･Analysis of depositional environments.

Multi-sensor Gamma Tool (MGT) and the Natural Gamma Tool (NGT) measure the natural gamma ray radiation of the formation. Different isotopes (mainly potassium, thorium, and uranium) emit gamma radiation at different energies. By counting gamma rays at different energy levels, by measuring the gamma-ray spectrum, one can estimate the concentration of the three different elements. Spectral gamma-ray logs provide means to investigate mineralogy of sedimentary sequences, since different minerals contain different amounts of these isotopes.

Natural Gamma Some applications are:
A. Clay typing: 
Potassium and thorium are the primary radioactive elements present in clays; 
B. Mineralogy: 
Carbonates usually display a low gamma ray signature; 
C. Ash layer detection: 
Thorium is frequently found in ash layers. The ratio of Th/U can also help detect these ash layers.

In the log to the left, depth is the vertical axis, and the gamma data are plotted. An experienced log analyst can interpret such records to provide the geologist with likely lithology of the rocks surrounding the well.

Accelerator Porosity Sonde (APS) The Accelerator Porosity Sonde (APS) uses a powerful neutron source to detect hydrogen. Five detectors provide information for porosity evaluation, gas detection, shale evaluation, and borehole corrections. Applications: Porosity: Reservoir engineering; in the study of the volcanic rocks that make up the upper oceanic crust, a good in-situ porosity measurement is important. Other properties such as density, seismic velocity, and permeability, depend on porosity. Lithologic determination: The porosity curve, often combined with the density log, can be used to detect shaly intervals, or minerals such as gypsum, anhydrite and salt layers.

Logs from nearby drill holes can be correlated to identify variations in structures and lithologies. In the oil industry, they provide data on how quickly a reservoir is being depleted and how quickly water is encroaching on producing regions. The logs to the left (red areas) show the location of the Cretaceous impact deposits in the Yucatan wells. The logs on the next slide show an area of producing wells.

In the Ocean Drilling Program, logs fill gaps where core recovery was poor and provide “regional” data away from the hole.

Correlation of various logs can provide considerable information to the geologist.

Modern data and methods allow correlations between well logs to include hole geometry and 3-D representations.