Thursday, December 13, 2007

ShalElog- an Electric Log for Wellbores

ShalElog- a Geochemical Log, made from cuttings retrieved from well drilling
Information is vital, when drilling a well for water, for hydrocarbons, or for steam. Not only should the one investing in an expensive borehole (twenty dollars or more per foot) have an estimate of the risks of failure or success, but he should have an accurate appraisal of the earth penetrated (in terms of encountered rock and fluids). He may find the fluid is obscure, even when it is present.
Water wells are usually drilled by individuals interested in finding water as shallow and as inexpensive as possible, on their properties. After all, water should be cheap, since it seems to be available wherever there is life. However, in the desert water is more elusive- for the quantity man needs, the amount found may prove to be skimpy.
The water table in desert areas (below which water saturates the rock) may be thousands of feet deep. People living near the Grand Canyon find that the gash in the earth has allowed water to percolate (drain down) into the canyon at 5000 feet depth or more. This fact makes domestic water in The Strip very dear. These expensive wells require as much information as possible from drilling, since the total cost is large, and the information found from drilling may incur only a small part of the total cost.
Landowners are reluctant to ask commercial well loggers to log shallow wells (measure rock properties continually from near the top to the bottom of the hole), since it may double the cost of the well to do so. However, they can get a log from the cuttings brought to the surface by the driller. This represents free information, providing it is analyzed or measured later.
Shale cuttings, made into a ShalElog
(patented shale Electric Log)

provide one method of analyzing the rock penetrated while drilling the hole. This may be valuable whenever the hole appears to be dry (there is no readily available fluid in it), and the owner has to decide on abandoning the hole.
The driller should wash and bag cuttings each 10 feet from the well, for use in evaluating the well whenever fluid is not found (this can be done if requested in advance, at no extra cost). These cuttings will keep indefinitely, and if they have been washed and dried they will not be contaminated when stored. The fine material, such as clay, silt, and fine sand stores both organic and inorganic chemicals and these may be measured later should the rock penetrated not be understood. Low Conductivity water (low salt content, with high Resistivity) can be found in zones with thick sands, suitable for drinking, using this log.
These chemicals may be flushed from the cuttings by means of making a mud or slurry from them. This is done by dis-aggregating the dried and heated (in a kitchen skillet) slivers of rock with a mortar and pestle (or stirring dish and mashing tool- not grinding), so that they appear to be clay-like. These then can be mixed with equal weights of distilled water to make a mud or slurry, which can be measured for several properties:

a. Electrical resistivity, R, or recipocally the conductivity of the mud, in ohm-meters or mho/centimeters (Mho is the inverse of Ohm- but not the WHO!);
b. The rate of fluid flow from the slurry, by means of a filter press, in minutes per cubic centimeter. This may vary from one to ten minutes per cubic centimeter, min/cc;
c. The color of the filtered fluid, called effluent, which can easily be graded into clear, slightly yellow, yellow, amber, and gold or brown visual colors;
d. Resistivity of the effluent or filtrate, ohm-meters;
e. Contents of the filtrate, Cion (ionic concentration) , such as Na+, K+, Ca++, or other dissolved chemicals, using an ion-sensitive membrane (measured in electrical millivolts); and
f. Trace elements, such as Boron or other constituent of interest. All of these operations may be made in ten minutes, to keep up with the driller collecting the drilled samples, and a record (Graph presentation) later can be made to present these data- which is called a Log.
Other parameters or calculated terms may be found from the above data, which can be used to evaluate the rock and fluids encountered. These include a factor F= Rmud/Rfluid, where R is resistivity, which is sensitive to the solids found in the cuttings (e.g. limey solids compared to silt). Some of these terms are shown in the following ShalElog, which was made in Turkey by me.

The Presentation of ShalElog is similar to Electric logs made for drilled wells
The left-hand curve, which presents the Na+ content (read left-ward), is a measure of the saltiness of the fluid. In oil wells, this saltiness increases with depth, and is particularly interesting around oil deposits, since the salt water and black oil seem to have an affinity for each other. It is somewhat similar to the SP log, which is conventionally made for oil wells, but the contents pertain to shales or fine sediments, and this is not necessarily the same as with sandstones which contain both salt water and oil (sometimes).
Potassium, K+, is an interesting ion, and it is not common in subsurface waters in large amounts. It is normally some one-twentieth to one-tenth of the concentration of Na+. I have found that it is present more so whenever an active fault or fracture allows fluid to rise vertically from a hotter zone in the earth. This occurs because of its small ionic size, its relatively small hydration with water (recall that physicians prescribe it in lieu of sodium for heart patients, who take on water with ordinary salt), and its increased solubility with higher temperatures. Generally, geologists do not agree that it indicates anything abnormal, so you may want to get other opinions. But I have mapped this ion on county-wide maps, and found that it occurs in linear map presentations in springs, in subsurface wells, and whenever there is abnormal temperature. It travels much more easily when the fluid is warmed, compared to Na+, and consequently may be mapped or measured in boreholes for anomalous geological circumstances. It is known to derive from weathered potash feldspars, shales with illite in them, and from granites. It may be more common whenever evaporites such as playa lakes occur, since it is more soluble than most other salts and occurs whenever the lakes completely dry (as in the desert).

Notice that K+ is highest on the Thrace log, near an amber filtrate, and that it generally increases with depth (temperature).

ShalElog in Geothermal Logs indicates abnormal K+ and anomalous Geology
Notice that K+ (following Photo) is plotted occasionally on the geothermal log shown below, and it indicates abnormal temperature or open fracturing or faulting. Again, it generally increases with depth in this hot hole in an area which produces steam and has fumaroles at the ground surface. In this case there is also an anomaly near the surface (60- 70 meters), and there is steam emission from the nearby area. Again, K+ is a small ion, which has small hydration, compared to Na+, consequently it moves easily through small fractures found at faulting or which are over-pressured.

Geothermal Well Logs shows wide Variations of Dissolved Ions
Sodium Ion variation in the Earth
Sodium ion is the most common ion in groundwater and in seawater, the reason being that it is a result of the dissolution of feldspars and hornblende- the most common soluble minerals in igneous rocks. It is also a result of HCl acid from volcanoes reacting with alkali rocks to produce a salt plus water:

HCl + Na Rocks > H20 + NaCl + anions or other minerals

For limestone areas, Ca++ and Mg++ will predominate in near-surface waters, but again the Na+ ion will increase in importance with depth and temperature, until it is dominant.
This is shown in the Thrace log, but not in the geothermal case, since K+ has supplanted Na+. Sodium and Potassium ions seem to compete, similarly as they do in the human body. Living cells generally contain K+ =10x the Na+ inside, compared with the free fluid outside having 1/10x the Na+. This is mentioned because shales act as membranes in the earth, which cause an ionic concentration contrast across their boundary- similarly to the cell wall. I conjecture that Life is involved with this chemical change, at least in taking advantage of it- notice that the highest K+ in the geothermal well occurs near the bubbling cuttings emissions.
A Model is shown below, which indicates depths where the various waters occur in Large Basins; this indicates that there are four types of Water- somewhat stratified in the Crust, according to temperature and compaction of the Rocks (Permeability or ease of water movement):
1. Meteoric Water, which is potable or drink-able;
2. Ionized Water, which may be too salty to drink;
3. Chemically-Reduced Water, containing stinking compounds; and,
4. Acid Water, which has a pH less than 7.0, depending upon abnormal temperature.

Model for a Large Basin, for vertical distribution of Water Types