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Introduction
Today's tremendous and ever increasing demands for water for suburban homes, for industries, for modern farming and expanding cities have rendered yesterday's hit-and-miss methods of water seeking hopelessly inadequate. The effective investigation of underground water in any area requires; (1) locating the water-bearing
formations, (2) determining their water quality and (3) estimating their yield. Although surface exploration methods can be used to obtain general geological information on the area, only drilling, logging and testing of wells will accurately solve these problems.
Geophysical logging of water wells and test holes has, in recent years, become one of the most important methods for obtaining information on our ground water resources. This information can be utilized by both hydrogeologists responsible for evaluating the water resources of a particular area or by aiding well drillers in selecting the best zones
to develop from a test hole. In addition to selecting the optimum water producing zones, geophysical logs have been used with a high degree of success in repairing wells, especially wells finished by the “open hole method”. Information from geophysical logs has also been used by various regulatory agencies to write plugging specifications for
abandoned wells that have the potential for contaminating our ground water resources.
What is Geophysical Logging?
Geophysical logging is a method by which cylindrical probes are lowered into the borehole on a cable. In the core of the cable are wires that transmit a signal generated in the probe back to the surface as the probe is moved vertically in the well bore. At the surface the signals are processed by a computer and translated into various logs depending upon the type of probe being run in the borehole.
A wide range of borehole information can be obtained from various probes that are currently available. Some of the information that can be directly or indirectly inferred from some of the more common geophysical probes include:
Formation Lithology (sand, shale, limestone, etc)
Location of Producing Zones,
Shape of the Borehole
Water Quality within the Formation
Porosity
Temperature
The geophysical logs commonly run in water wells are similar in every respect to those run in oil and gas wells. In 1927 what would become the widest application of electrical surveying was attempted, the electrical surveying of a borehole by running a probe attached to the end of a cable in the well bore. Now, nearly all wells drilled for gas and
oil along with a goodly number drilled for water are systematically logged.
It is not surprising that, pursuant to the federal Clean Water Act,
many Florida counties now require that logs be run under
conditions where waters of different qualities may be encountered during drilling.
The purpose of this paper is to provide an explanation of the
more commonly used geophysical logging techniques, how they work and what can be done with them to enhance the search for and recovery of more water.
When is it Necessary to Log?
Obviously it is not necessary to log every hole drilled, even if it were economical. Logging is especially useful in areas where there is little information available on the hydrogeology.
A good set of logs, when used with the drill cuttings can be an extremely valuable tool in matching the best producing zones with water quality available. Geophysical logs are valuable since they represent a continuous record of the formations exposed in the wall of the borehole. Thin streaks of sand or clay may go unnoticed
during drilling and sample collecting. These zones may later provide problems in developing the well.
Figure I-1 lists the tools that are generally available to the water well industry and how they are used in solving a variety of problems. Some of these tools can be used in both open and cased hole surveys, while others are restricted to open holes only because they depend on electrical signals that are short circuited by steel casing. The tools listed are run into wells on an armored cable which surrounds from one to seven insulated copper conductors.
Figure I-2 is a block diagram of the logging equipment used to obtain the logs described in this paper. The equipment is usually truck mounted and may make use of a very short, small diameter cable, or in the case of equipment also used for logging oil wells, be mounted in a large truck with 30,000 feet of heavy cable. ABS incorporates
geophysical cable for a maximum of approximately 6000 feet. Video depth capability is somewhat less at approximately 3000 feet.
To obtain the greatest amount of information from a logging program, some pre-planning is definitely in order. It has been stated that log interpretation is more of an art than a science because different phenomena can cause similar log responses. For that reason it is imperative that the proper logs be run in any given situation, that logs
be properly calibrated and presented, that necessary associated information be obtained and tabulated and that the analysis of these logs be made by an analyst familiar with the area. In the investigation planning process, it may be helpful to contact ABS for input into the process and suggestions on the logging program. The single most important component of the logging program planning is “Asking the right questions”!
Considerations that should be recognized during the planning process include all aspects of the investigation and can include;
For optimum information from an electric log, borehole geometry and some drilling fluid control should be considered. logging tool sizes and access to well bores must be taken into account for some of the logs that might be run during the producing life of a well. Proper planning is the only way to obtain the greatest benefit from logs for the least
expense.
The usual problem at the well is what to do with the hole you have just drilled. In few cases can the well contractor or operator simply run a log and solve all his problems. To be of any great value, the log must distinguish between non-productive and possible productive formations. In wildcat (no local history of production) areas, aside from
indicating clear cut dry holes, all that should be expected of the initial log by itself is that it forms a basis for wisely and economically selecting the various auxiliary evaluation or testing methods that may be necessary for deciding to pass up a questionable well or completing and developing a productive well. Proper evaluation of the initial electric logs may greatly reduce the number of unknowns and save unnecessary and costly mistakes. The spending of a few hours on the study of the log and other available information at the well is certainly not out of proportion to the total investment. Electric logs supply many known values from which to work. Different wells call for different
evaluation methods, Where drill cuttings and electric log studies may be all that are needed for a particular well, additional logs and studies may be necessary for evaluation of other wells. Today it is almost universally conceded that a drill cutting study and an electric log are bare essentials for rotary holes. If the use of these two parameters answer all of your questions, you need go no further. When there are still some unanswered questions, the question then arises, "how much information can I afford?.
Usually, the most difficult logs to interpret are those on which only a few formations are logged. Nature cannot be put into equations in a straightforward manner except through the intermediate process of data collection and statistical studies. Geology is a natural science.
Not all problems encountered in log analysis are attributable to nature. The human has his responsibilities and the logging engineer bears the largest responsibility. His measurements must be correct and he must be able to recognize that fact. So should the hydrogeologist who is understanding of modern logging technologies, and this
edition is no exception.
Fundamental Fundamentals
This chapter is an introduction to some basic concepts of groundwater hydrology. This
is not meant to be a full explanation but only some background to familiarize you with
some of the terms that directly relate to geophysical logging. Some notes about logs
and their responses to various factors are included..
Geophysical logging is a form of remote sensing. Much like taking pictures from a
satellite, the probes take measurements in the borehole, transmit the data to the
surface for recording and presentation. However, as with other forms of remote data
collection, the value of this data is totally a function of its correct and applicable
interpretation.
The Measurements:
Within the probes lowered into the borehole, are sensors that respond to various
influences encountered as the probe is lowered or raised in the borehole. The table
below lists the physical parameters that are being measured and the engineering units
used to display this data on the resulting log.
Probe Type
| Property Measured
| EU used in Presentation | Calculated Units
| | Caliper | Inside Diameter | Inches/Cm | Volume total/Annular | | Resistivity | Electrical Resistivity
| Ohms Meter/Meter
| Formation, Fluid Resistivity and
Conductivity
| | Temperature | Well Bore Temperature
| Degrees F or C
| Differential Temperature
Correction for Other Logs
| | Conductivity | Formation/Fluid Conductivity
| Mmohs/Cm/M
| Formation/Fluid Resistivity-Water Quality
| | Acoustic | Acoustic Travel Time, Signal Attentuation
| Formation Velocity in Feet per Second, Signal Strength in mVolts
| Porosity, Strength, Elastic Moduli, Cement Bond***
| Gamma Ray
| Naturally Occuring Gamma Radiation
| API Gamma Ray Units or Counts per Second
| Clay Content, Lithology, Correlations
| | Spinner | Fluid Velocity up or down the Borehole
| Feet/Meters per Second
| Fluid Production in Gallons per Minute |
| | | |
Using the measurements described above, recording the data in formats that allow the
scientific evaluation of the data allows us to acquire information about the subsurface
materials, fluids and hydraulics, that are penetrated by the borehole. It must be pointed
out here that the data from the borehole is a “snapshot” of the underground and may or
may not represent any other place on the planet. It is the job of the log analysist to
make that determination. But, it is also important to note that this information is not
obtainable by any other practical method.
Included in the subsurface are:
Aquifers 
The materials that act as reservoirs for groundwater are mostly sedimentary rock. This includes unconsolidated sands and gravels; sandstone; limestone and dolomite.
Igneous and metamorphic rock though present in Florida are located very deep and are not a part of the Florida hydrologic picture. These types are less common sources of groundwater that usually store and transport water through fractures. The fractures in these crystalline rocks are hard to find and may not be connected to a source of
freshwater recharge. Fractures can produce large volumes of water but they may not remove unwanted substances during transport from source to point of use.
An aquifer is a zone or stratum that is able to produce water for a well. Depending on how a well is to be used what maybe an aquifer for a single use domestic well is not for a large agricultural or municipal well. A couple of related words are aquitard and
aquiclude. Aquitard is used to describe less permeable beds of sediment that allow a groundwater study but are not able to transmit enough water for production wells. An aquiclude is a saturated formation that does not transmit significant amounts of
water. All three of these words are not precise on purpose so they can be used in a relative sense to the others depending on conditions.
Porosity
One of the main terms to discuss is porosity. Porosity is the empty space within rock.
This would include gas pockets formed in the volcanic material called pumice. Pumice
is rare or non existant in our area but is unusual because it can float on water because
of all these air pockets and the pockets give it a high porosity. It will not transmit water
through the void spaces though because the pockets are not interconnected. Porosity is
symbolized in equations by the small Greek letter (phi)f and has no units; it is a volume
ratio that is expressed in percentages or decimal form. Porosity is the void volume
divided by the total volume.The ratio of void space to the bulk volume of rock containing
that void space. Porosity can be expressed as a fraction or percentage of pore volume
in a volume of rock.
f = void volume/total volume
When it is said that a sand has 40% porosity it means that a given volume will have
60% rock materials and 40% void space that could be filled with water. Table 1 - 1 below
shows porosity values for various geologic materials.
Effective porosity is the amount of void space with interconnected pores that can transmit fluid or an electrical current. The various electrical logs like the Normals and Guard respond to effective porosity. The
Neutron, Gamma-Gamma (Density) and Sonic Logs are often referred to as the “Porosity Logs.” None of these directly measures porosity but each measures a different aspect of porosity. None of the Porosity Logs distinguish the type of porosity.
Primary porosity refers to the void spaces between fragments of rock that make up sediments, and is relatively uniformly distributed. There are four main factors that affect primary porosity. They are packing, sorting, roundness and cementing. Changes in packing caused by compression can reduce porosity by forcing the particles closer
together. Uniform spheres can go from a maximum porosity of 47.6 percent with loose
packing to a minimum porosity of 25.9 percent with full compaction. See Figures 1 - 1, 1
- 2, and 1 - 3.
The second factor that can affect porosity is sorting as is shown in Figure 1 - 4.
When there is a mixture of particle sizes, smaller particles fill in the spaces between the larger particles and reduce the total amount of void space. Another closely related factor is roundness. Angular particles can be pressed closer together and interlock when
compacted but rounded particles will have more interconnected pore space between
grains. The last of the factors is cementing. These are mineral deposits that form
between grains that will reduce the pore space and can even close pore connections.
Reduced porosity from all these will cause increases in the resistivity values and lower
porosity values.
Permeability
Permeability is the ability of fluids to move through connected pore space under
unequal pressure. The more uniform the particles, larger the pore spaces and the more
interconnected the greater the permeability. The unit of measurement for permeability is
the Darcy. One Darcy is when 1 sq. cm. of rock releases 1 cc of unit viscosity in 1
second under 1 atm/cm of pressure.
As the clay content in a formation increases, the permeability decreases. The small clay
particles take up space between the larger grains; some clays swell when in contact
with water and some clays attract water particles to their surface. The relationship
between clay content and permeability is not linear. Small percentage increases in clay
content can dramatically reduce permeability.
Hydraulic Conductivity
Another related term is hydraulic conductivity. Hydraulic conductivity is the term used
when the properties of the water are also considered along with the materials through
which the water moves. Viscosity of the water increases as the water temperature
decreases, making it thicker and slower moving through formations. Hydraulic
conductivity is a flow rate in gallons per day (gpd) that moves through a cross section of
an aquifer one foot thick and one mile wide under a hydraulic gradient of 1 foot per
mile, with units of gpd/ft2.
Formation Factor
Formation Factor (F) is the ratio of the electrical resistivity of a rock 100% saturated
with water (Ro) to the resistivity of the water with which it is saturated (Rw), Archie
(1942).
F = Ro/Rw
As the porosity and permeability of a formation increase, Ro will decrease because
more space is taken up by the formation water and less space by rock particles that
resist an electrical current. Formation Factor is also related to porosity by the formula
below where f is porosity and m is the cementation exponent.
F = 1/fm
Another modified version of this formula replaces the value 1, used for clean, shale free
rocks with the coefficient a, that can vary with clay content. So the new formula would
be as shown below.
F = a/fm
Unsaturated Zone, Capillary Fringe, Water Table and Saturated Zone
As a well is drilling down through sediments, it can pass through various zones. First is
the unsaturated zone, the area above the water table that has bound or absorbed water in or on the rock, but the void space between the rock contains air. This area may also have areas of perched water. The next area down is the capillary fringe. Water in this area has moved above the water table due to a wicking effect that pulls the water upward. The fringe can vary in thickness from a few inches to tens of feet depending on the formation materials.
Next is the water table, the surface where fluid pressure is at atmospheric. Below the water table is the zone of saturation where all the pore space is filled with water. Some
resistivity, neutron and sonic logs can show large changes moving from the saturated to
the unsaturated zone. Resistivity values increase coming out of the capillary fringe
when infiltration of the drilling fluid is controlled. The air in the formation above the
fringe area causes higher resistivity values. Neutron counts increases coming out of the
capillary fringe because there are fewer hydrogen atoms to slow down the neutrons
from the radioactive source. This effect can also occur when the tool comes out of fluid
in the well casing or water is outside the casing. On the sonic log the travel time will
increase and the VDL waveform will move out further in time as the tool leaves the
capillary fringe. The air in the formation slows down the sound signal moving from the
tool transmitter to the receiver.
Care should be used picking the water table from any of these logs. The actual water
table could be several feet below these changes on the logs depending on the
thickness of the capillary fringe. Another factor is if some aquifers are under pressure
then the water level in a well could rise above the water table area indicated on
the logs.
Resistivity and Conductivity
Resistivity refers to the degree a substance resists the flow of an electrical current. The
units used to Ohm/meter2 often shortened to ohm-m. The inverse of resistivity is
conductivity and it is measured in millimhos per meter (mmho/m). Both of these
properties are volume measurements.
There are a variety of resistivity and conductivity types from which to choose. Each type
measures different volumes of formation and borehole fluid. The standard Electric Log
is often run to see the contrast between the 16-inch and 64-inch Normals for
information about water quality. The 6-foot Lateral was designed to measure a
shell of formation that is less influenced by the borehole fluid. Today the Guard
Resistivity has mostly replaced the 6-foot Lateral because it reads deeper into the
formation and the curve is symmetrical making it easier read. The Induction Log is able
to measure formation conductivity in an air filled borehole, through PVC casing
and in fluid filled holes.
Formations have lower resistivity values (higher conductivity values) as the salinity of
the water increases or clay content increases. Resistivity values also change with
temperature; lower resistivity values in some wells may be caused by increased
temperature with depth. A temperature log may need to be run to correct for this
effect on the resistivity measurements.
Borehole Conditions
There following are some borehole factors that can affect logs and some notes about
log responses.
Salinity of the drilling fluid can determine which type of resistivity tool to run. The
Standard Electric Log is designed for freshwater, or slightly saline fluids. If drilling with
salty mud or drilling into saline formation water, the Guard Log or Induction Log should
be used or added to the log suite for better bed definition.
Mud Cake is a layer of mud particles from the drilling fluid that forms a barrier between
the drilling fluid and formation water reducing invasion into the formation by the water in
the mud. This is important for logs like the Normals to show the contrast between the
zone invaded by the drilling fluid and uninvaded area in the Saturated Zone. In the
Unsaturated Zone a good mud cake reduces invasion into the air filled pores.
By sealing this area it is often possible to show the change from the Saturated Zone to
the Unsaturated Zone using the Electric, Guard, Induction and Sonic Logs.
Permeability and pressure are other factors that can affect invasion. The higher the
permeability the harder it can be to control invasion. High pressure from the mud
column weight can also force the water in the drilling mud, mud filtrate, into the
formation.
Time can become a factor because the longer the contact time between the drilling fluid
and formation the more invasion can occur. Time also allows the drilling fluid and
formation water ions to mix and equalize. If invasion or equalization occurs, when the
log is run there may be less contrast between the curves reading various depths into
the formation.
Temperature of the borehole fluid will be affected by mud circulation and removal of
the drill stem for logging. The fluid needs time to become thermally stable and
influenced by the formation temperatures. Some shallow wells may have recharge
sources nearby with contrasting temperatures and it may be necessary to go down
several hundred feet before being able to see a geothermal gradient.
Hole Size will influence the measurements of most logs with smaller holes giving better
logs. Sometimes though, it is not cost effective to drill and log a test hole before drilling
the production hole. With the Normals if the drilling fluid is fresh, then as the hole size
increases resistivity values will decrease and the curves will become more rounded.
Many 28 inch diameter wells are logged and perforated intervals chosen from the log.
The Guard Log is often used for thin bed resolution in large diameter holes because it
reads a nine foot radius from the tool and hole size has less effect on it.
The Gamma Ray Logs will have reduced count rates as the hole size increases. When
run in large wells, the gravel pack materials can greatly influence the tool readings. All
the acoustic logs work better in smaller holes with a 15 to 20 inch maximum hole size
depending on the tool.
These explanations are intended to give you a general understand of the concepts with
the hope that you will be able to use them when learning about the various logging tools
to follow.
The thoughts and images included are the result of a collection of information over a career in this business. If you recognize something that is yours, please let me know and you will receive the recognition due.
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