Introduction to Well Logging (The Borehole Environment)

The Borehole Environment

The Borehole Environment, Mud, Mudcake, and Invasion

After drilling through a permeable formation, generally an invasion process begins. If the pressure in the mud column exceeds formation pressure, fluid from the mud will move into the formation (provided it is porous and permeable) and deposit a mud cake on the borehole wall.

It is important to distinguish between the resistivity of the fluid within the pore space and the resistivity of the rock-fluid system itself. The terms used in Table 1 should be well known to everyone involved in well log evaluation work.

 

The flushed zone is important because it affects the readings of some logging tools and because it forms a reservoir of mud filtrate to be recovered on a drillstem test before formation fluids are recovered.

Depending on the type of mud used, oil- or water-base, and the relative values of R_mf and R_w, the invasion process may result in a radial resistivity profile that increases or decreases with distance from the borehole wall. Figure 1 illustrates what may be expected in a number of cases.

 

 

 

 

Symbol / Nomenclature

UNDER CONSTRUCTION …!

A.4. Logging Tools

General Description

Logging Tools

Logging tools are cylindrical tubes containing sensors and associated electronics that can be attached to the logging cable at the logging head. Although there are wide variations in sizes and shapes, a typical logging tool is 3 5/8 in. in diameter and from 10 to 30 ft long. They are built to withstand pressures up to 20,000 psi and temperatures of 300 to 400 F. The internal sensors and electronics are ruggedly built to withstand physical abuse. Modern tools are "modularized" to allow combination tool strings. By appropriate mixing and matching, various logging sensors can be connected with each other. Among the obvious limitations to this method are the difficulty in handling very long tools and the limited information-transmitting power of the cable conductors.

Because logging tools have multiple sensors at different points along their axes, their respective measurements have to be memorized and placed on a common depth reference. Thus, the signal from the sensor highest on the tool must be "remembered" until the signal from the lowest sensor arrives from the logging depth being memorized. Figure 1 and Figure 2 illustrate this characteristic.

 

 

The reference point for the logging tool shown in Figure 1 is the sensor A. Higher up the tool, sensors B and C record other parameters. Without memorization, sensors B and C record curves off depth that appear on the log to be deeper than sensor A by a distance equal to the spacings A-B and A-C. It is important, therefore, to ensure that all curves recorded simultaneously are on depth on the log by means of proper memorization ( Figure 2 , right).

Another associated depth problem arises when several surveys are recorded on different trips into the hole. Unless care is taken, these surveys may not be on depth with each other. The only method of ensuring good depth control is to insist on a repeat section that passes a good marker bed. Each subsequent log should be placed on depth using this repeat section as a depth reference before the main logging run is made.

Openhole logging tools currently in use are

Formation Fluid Content Indicators

• Induction
  

Laterolog

Microfocused and microresistivity devices

Dielectric

Pulsed neutron

Inelastic gamma

Porosity-Lithology Indicators

• Acoustic (sonic)
  

Density and lithologic density

Neutron

Natural gamma ray

Spectral gamma ray

Reservoir Geometry Indicators

• Dipmeter
  

Borehole gravimeter

Ultra-long-spacing electric

Formation Texture Indicators

• Electrical borehole imagers
  

Ultrasonic borehole imagers

Formation Productivity Indicators

• Wireline formation tester
  

Production logging

These are the basic tools that will answer 90% of the questions about the formation. Omitted from the list are various types of old logging tools (such as electric logs), and some standard auxiliary devices, which, although important, do not rate as separate tools since they always piggy-back along with one of the basic tools. Among those auxiliary tools are the spontaneous potential (SP), and the caliper.

A discussion of common basic tools follows.

Induction Tools Induct ion tools belong to the resistivity tool family and measure apparent formation resistivity. They work like mine detectors by inducing electrical currents in the formation. They may be run simultaneously with a spontaneous potential (SP) or gamma ray (GR) log (or both) and optionally with various combinations of porosity tools. Curves recorded on a dual-induction log include deep induction, medium induction, shallow-focused electric, and SP and/or gamma ray and caliper.

Laterolog Tools Laterolog tools also belong to the resistivity tool family. The most important is the dual laterolog. This tool can be run with SP, GR, and caliper logs. The curves recorded are laterolog deep, laterolog shallow, and shallow-focused electric.

Microresistivity Devices Microresistivity devices attempt to measure formation resistivity in the zone very close to the borehole wall where invading mud filtrate has displaced any moveable formation fluids. They are all variations of a basic microfocused electric log. When certain constraints on hole conditions are met, these devices produce a measurement of the parameter R_xo, the resistivity of the flushed zone surrounding the borehole.

Acoustic Tools The modern acoustic log commonly used is known as the borehole compensated sonic, or BHC. It may be run with a GR, SP, and caliper, or in combination with other porosity and/or resistivity-measuring devices. Long-spacing sonic tools and tools with multiple transducers are also in use for special applications.

The curves recorded are D (sonic travel time), GR, SP, and caliper (optional).

Various other acoustical parameters can also be recorded, either simultaneously or on a separate run. Sonic amplitude logs are used for fracture detection. They may be recorded by various arrangements of gates for the received wave trains. The tool may also be used to record the cement bond log (CBL), in which case the recorded curves are D (a single-receiver travel time), amplitude, and VDL (a variable-density display, or wave trains).

Density Tools Compensated formation density tools are also known as gamma-gamma tools in some parts of the globe (because their mode of operation is to send gamma rays to the formation and detect gamma rays coming back).

They record two basic curves _b (bulk density) and _ (correction) .

Natural gamma ray and caliper tools are normally run simultaneously. Additionally, an apparent porosity curve can be generated and recorded and, from those data, a formation factor (F) curve can be generated and recorded as well. A density-derived F is referred to as F_D.

A variation to the density tool is known as the lithodensity tool; in addition to measuring bulk density, it measures the photoelectric factor P_e. P_e is a direct indicator of formation lithology and, as such, is a valuable adjunct to the basic density measurement.

Neutron Tools There are several types of neutron tools. Today's standard is the compensated neutron log, which records _N, the neutron porosity index, normally recorded for a particular assumed lithology. Reading the porosity curve requires close attention to the porosity scale and the assumed matrix. Normally, a natural gamma ray curve is recorded simultaneously with the neutron log. The standard presentation is a combination density/neutron, where the caliper from the density survey is also available.

Pulsed Neutron Log The pulsed neutron tool measures the formation capture cross section for thermal neutrons. The end result is a measurement that helps distinguish oil from salt water in the formation in cased holes.

The curves appearing on the log are:

S sigma, the formation capture cross section

T tau, the thermal neutron decay time

 ratio, a porosity-type curve.

Dipmeter Dipmeters come in several versions: four-arm dip-meters, six-arm dipmeters, and eight-electrode types. High-resolution dipmeters record all the necessary curves for computing formation dip, hole drift, and azimuth.

Wireline Formation Testers (RFT) There are several types of wireline formation testers available, which are proving to be a valuable addition to the formation evaluation arsenal. These devices allow a small sample of formation fluid to be drained from the formation and brought up for analysis. They also allow multiple formation pressure tests to be conducted during one run into the hole.

Carbon/Oxygen Logging This relatively new service uses inelastic fast-neutron scattering to attempt to measure directly the relative abundance of carbon, oxygen, and other elements in a formation. Its application is in cased holes, and it is a natural candidate in those parts of the world where fresh formation waters preclude the use of a pulsed neutron-logging survey.

Gamma Ray Spectral Log This service measures the number and energy of naturally occurring gamma rays in the formation and distinguishes between elements and daughter products of three main radioactive families: uranium, thorium, and potassium. Since these elements and/or their decay products are associated with certain distinct types of mineralogy, sedimentology, and formation waters, the service has obvious appeal.

Borehole Gravimeter The borehole gravimeter measures perturbations in the gravitational acceleration constant caused by the proximity to the borehole of rock material that is denser or less dense than normal. Thus, this tool can spot higher porosities, gas, and the like. Its use requires an exacting set of prerequisites relating to depth, temperature, time, and so forth; it may not be available or applicable to all wells everywhere.

Dielectric Logging These tools send microwaves along the wall of the wellbore. The speed and attenuation of these electromagnetic waves are measured and the dielectric constant of the formation is deduced. Oil and water, having very different dielectric constants, can be distinguished. The application is in open holes where formation waters are fresh.

Nuclear Magnetic Resonance Measures the precession rate of hydrogen nuclei after the removal of an intense magnetic field. The measured quantity is related to the free fluid content of the formation. Recent advances allow determination of formation porosity, permeability, and irreducible water saturation.

In order to put these tools, surveys, and curves in perspective, the section Logging Tools: Quick Reference sets out a summary of all the common logging tools, what they measure, and their uses. Included in this catalog of common wireline logging measurements are some common interpretive presentations derived from the basic measurements.

Formation "Texture" Indicators Include both electrical and ultrasonic borehole wall imaging devices that reveal near wellbore sedimentary details as well as wellbore intersections with fractures and fault planes. These devices are particularly valuable in carbonate formations where various forms of secondary porosity are imaged.