VALLEY GROUNDWATER – HOW IT WORKS (page 3)
The secret to constructing a productive well is to have a clear understanding of the underlying geology
> Exploratory drilling
> Sample collection and analysis
> Geophysical logging
The first field work involves drilling a small diameter exploratory test hole. The depth of the test hole is in part selected by the depth of any existing wells in the general surrounding area. When extremely high capacity wells are being sought, the test hole should be drilled very deep, or to the bottom of the unconsolidated formations. This allows for consideration of all available water bearing intervals to be included in the final well design.
Test hole Exploratory Drilling
> Drill a small diameter test hole
– Mud rotary rig used
– 6” Borehole
> Run Electric Log
> Recover Gravel/sand samples
> Properly Abandon
A test hole gives the opportunity to compare information between different drilling locations. By comparing the electric logs between a new test hole and the log of an existing well, it is possible to determine if the new well site contains a representative amount of water bearing formation for the area. When possible, it is best to consider more than one test hole site for a final well site selection.
A good practice in cases where a test hole is not completed as a new well, is to properly abandon the test hole. The filling of the test hole with either bentonite chips or cement slurry material eliminates the possibility of surface contamination of deeper water bearing zones.
Preliminary Investigations Gravel / Sand Samples
> Formation samples are taken from the test hole at regular intervals
> Used to determine the type and size of gravel pack material necessary
The top illustration shows a formation sample recovered from the drilling fluid after reaching the top of the bore hole, as a test hole was drilled.
The material on the bottom is after the clay material is washed out. Each sample is weighed and the percentage of the weight of the sample retained at each size of the sieve analysis screen is recorded. The first step in developing a well design is selecting the samples of potential water bearing formation for sieve analysis. The necessary information for sample selection includes the electric log, the drillers descriptive log, with the length of time it took to advance each drill pipe.
Preliminary Investigation – Sieve Analysis
Work Sheet
> Line on left shows the percentage of retained fine grain material
> Line on right indicates retained coarse grain material
Data is then ready for plotting on logrimithic graph paper. The cumulative percent retained on each sieve is plotted as a point against the sieve opening in thousands of an inch or mm. The percentage of sample weight retained is given on the vertical scale of the graph and the size of opening, or particle size, is on the horizontal scale. The interpretation of the sieve analysis is the basis for the selection of gravel pack size and design. The gravel pack size and particle distribution is the basis for the selection of the slot size for casing perforation or screen size.
A critical factor in a well design is the selection of gravel pack material and the method by which it is installed. The gravel pack material is how the entrance velocities of the water are controlled. Water moving into a well with too great a speed will move sand and silt particles from the water bearing formation into the well, this can be a chronic problem with wells completed in unconsolidated formations
Preliminary Investigations – Geophysical Logging
An electric log is a geophysical survey taken from the test hole. The interpretation of these logs is key elements in developing successful water wells in unconsolidated formations. It is used to verify and supplement the descriptive log taken by the driller. The electric log consists of the apparent resistivities of the subsurface formations and the spontaneous potentials generated in the borehole. Electric logging offers several important advantages. These include, locating the top and bottom of each distinct formation, determining relative water quality, and differentiating between clean sand strata, and silty sand with clay stringers.
> Provides a preliminary review of gross water quality
– Spontaneous Potential
– Identify quality of water (salinity)
> Identify potential production intervals
– 6 Foot Lateral, 16 and 64 Normal, Point Resistivity
– Find water bearing gravel formations
Fundamentals of Electric Logging –
Conventional Resisitivity Logs
In conventional resistivity logs, currents are passed through the formations via certain electrodes, and voltages are measured between certain others. These measured voltages provide the resistivity determinations. So that there will be a current path between electrodes and formation, the sonde must be run in holes containing electrically conductive mud or water.
Spontaneous Potential (SP) Curve
The Spontaneous-Potential (or SP) curve is a recording verse depth of the difference between the potential of a movable electrode in the borehole and the fixed potential of the surface electrode.
The SP is useful to:
> Detect the permeable beds.
> Locate their boundaries and to permit correlation of such beds.
> Determine the values of formation-water resistivity
> Give qualitative indications of the bed shaliness
Survey completed immediately after total
depth of hole reached.
Modern equipment allows for many surveys to be done at once.
Technological advancements have greatly increased the abilities and reliability of digital equipment used in field conditions for geophysical logs.
Electric logging surveys are run as soon as practical after the drill pipe is removed from the bore hole. The bore hole must be keep full of drilling fluid during the entire process. New digital hardware and software are making electric logging information much more adaptable and useful.
The electric log gives a visual presentation of the depths and the electrical values calculated in ohm’s of resistance, throughout an uncased borehole while filled with drilling fluid.
Preliminary Investigation Water
Sampling
> Water quality information prior to actual well construction
> Place a small diameter water sampling well into the test hole
– Allow for recovery of discrete water samples from isolated water bearing zones
– Design is based on specific test hole drilling information
It is possible to learn the exact water quality prior to completing a new well. This knowledge is gained by the completion of a water sampling well. The sampling well is sometimes completed in multiple zones, allowing for independent analysis of water from each different water bearing zone encountered in a test hole. In the case that the static water level is different in each zone, an assumption can be made that each zone is not connected, and is fed from different sources. The multiple zone sampling well includes intermediate seals placed in the clay intervals between different water bearing zones. This insures a discreet water sample taken from an isolated area of the bore hole. Additionally the sampling well itself will not become a conduit allowing an exchange of water between the zones of differing water quality.
Valley deposits of transported unconsolidated formations are generally deposited in a manner consistent with the underlying consolidated bedrock formations. The influence of nature determines how and where water bearing material will be found. Many of the old stream channels with the highest value water bearing formation sometime appear to be laid out in a random fashion. A thoughtful test hole drilling program prior to final well installation will greatly improve a projects probability of success. A line of test holes crossing a valley is more likely to disclose important variations in the thickness of alluvial material than if drilling is done parallel to trend of the valley