Chemical logging of geothermal wells

The presence of geothermal aquifers can be detected while drilling in geothermal formations by maintaining a chemical log of the ratio of the concentrations of calcium to carbonate and bicarbonate ions in the return drilling fluid. A continuous increase in the ratio of the concentrations of calcium to carbonate and bicarbonate ions is indicative of the existence of a warm or hot geothermal aquifer at some increased depth.

BACKGROUND OF THE INVENTION 
This invention relates to a method of chemical logging of wells drilled in 
geothermal formations. More specifically, this invention relates to a 
method of maintaining and monitoring a chemical log during a well drilling 
operation to detect the presence of geothermal aquifers before the aquifer 
is penetrated by the drill. 
Growing concern over shortages of fossil fuel has led to increased emphasis 
on finding and developing new sources of thermal energy such as that which 
is available in certain geological formations throughout the world. These 
formations, called geothermal reservoirs, contain usable quantities of 
warm or hot water in underground aquifers reasonably near enough to the 
earth's surface to be recoverable. These geothermal aquifers can provide 
energy for driving electrical generators, for space heating and for 
performing other energy functions. The growth of the geothermal industry 
has created a need for techniques which can be used, during drilling 
operations, to assist in the determination of well depth, casing location 
and well development. 
When drilling wells in geothermal formations, it is usually desirable to be 
able to detect the presence of a warm or hot water aquifers before the 
aquifer has been penetrated by the drill bit for several reasons. It 
permits the setting of well casing in place in the borehole in time to 
prevent possible contamination of any fresh water aquifers already 
penetrated by the drill string with upwelling geothermal water which often 
contains undesirable minerals such as large quantities of sodium chloride 
or low levels of fluorine on sulfur. Wells are drilled using either a mud 
or water, as a drilling fluid. Mud is preferred since it reduces the 
amount of water which must otherwise be used, seals off porous areas in 
the earth to prevent loss of water, lubricates the drill bit and flushes 
rock cuttings from the borehole better than water. However, penetration of 
a potentially productive geothermal aquifer by the drill bit while using 
mud as the drilling fluid could result in the aquifer being wholly or 
partially sealed by contact of the mud on the warm porous rock formation. 
This could result in the loss of the aquifer as source of geothermal 
energy. Thus, knowledge of the existence of an aquifer in time to permit 
discontinuance of the use of mud, could prevent loss of a productive 
aquifer. 
A chemical log of a well is a profile of the concentration of chemical 
elements found in the various geological formation fluids relative to the 
depth at which they were found. This profile may include elements such as 
Cl.sup.31 , F.sup.-, Na.sup.+, Ca.sup.++, and SiO.sub.2 in addition to 
conductivity and pH of the water. The log is prepared by analyzing the 
drill return fluid at predetermined depths, for example every 10 to 20 
meters, and plotting the concentrations of the various analyzed chemical 
species relative to the drill depth at which the concentrations were 
present. The log is useful for obtaining information as to what type of 
aquifer the drill bit has penetrated, the relative temperature of the 
aquifer and the composition of the aquifer water. However, up until now, 
the log has not been useful for determining the existence of a geothermal 
aquifer before it has been penetrated by the drill bit. 
SUMMARY OF THE INVENTION 
A method has been developed by which the presence of a geothermal aquifer, 
can be detected during drilling operations in a geothermal formation 
before the aquifer has been penetrated by the drill bit. By the method of 
the invention, the return drilling fluid recovered from predetermined 
depths during the drilling operation is analyzed to determine the 
concentration of calcium, and bicarbonate ions in the fluid, a log is 
maintained of the ratio of the concentrations of calcium ions to 
bicarbonate ions relative to the drill depth at which these concentrations 
were present, and the log is monitored to observe any changes in the ratio 
of the concentrations of calcium to bicarbonate ions whereby a 
continuously increasing ratio of the concentrations of calcium ions to 
bicarbonate ions is indicative of the presence of a geothermal aquifer at 
some greater depth. 
As used herein, the term geothermal aquifer refers to a porous zone in the 
earth's crust which contains water which is at least about 60.degree. C. 
It is therefore one object of the invention to provide a method for 
detecting the presence of a geothermal aquifer during drilling operations 
in a geothermal formation before the aquifer is penetrated by the drill 
bit. 
It is the other object of the invention to provide a method of chemical 
logging a well being drilled in a geothermal formation in order to detect 
the presence of a geothermal aquifer before the aquifer is penetrated by 
the drill bit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
These and other objects of the invention may be met by analyzing the return 
drilling fluid obtained from predetermined depths during the drilling 
operation to determine the concentration of calcium, carbonate and 
bicarbonate ions, maintaining a log of the ratio of the concentration of 
calcium ion to the sum of the concentrations of carbonate and bicarbonate 
ions relative to the drill depth of the hole at which the concentrations 
were present, and monitoring the log to observe any changes in the ratio 
of the concentrations of calcium to carbonate and bicarbonate ions, 
whereby a continuously increasing ratio of the concentration of calcium, 
to carbonate and bicarbonate ions in the return drilling fluid is 
indicative of the presence of a geothermal aquifer at some greater depth. 
The drill return fluid is analyzed for calcium, carbonate and bicarbonate 
ions at predetermined intervals of change in drill bit depth. For example, 
samples may be made every 30 meters during the first part of the drilling 
and then increased to every 10 to 20 meters when approaching potential 
aquifer locations. It may be necessary, if drilling mud is being used, to 
separate the drilling mud and other residues from the water sample by any 
convenient method such as by centrifuging and filtering. In some cases, 
the centrifuge may not settle the gelatinous mud suspension and filtering 
through a coarse filter may be necessary. The samples may be analyzed for 
calcium, carbonate and bicarbonate concentration by any suitable 
analytical technique. Satisfactory results were obtained using the 
procedures described by Brown, Skougstad and Fishman in Techniques of 
Water-Resources Investigations of the USGS, Washington, D.C., 1970, Book 
5, Chapter A1, Part IV. 
While it is possible to accurately detect the presence of geothermal water 
by monitoring the ratio of the concentrations of calcium to bicarbonate 
ion alone as is shown by the logs in FIGS. 1 and 2, it has been found that 
the presence of drilling mud during the drilling operation has a tendency 
to distort this ratio. The physical characteristics of the drill mud, such 
as viscosity, weight, density and thickness, are partially controlled by 
pH. The pH of the drill fluid when drill mud is used generally ranges 
between 9 and 11.5. When water alone is used as the drill fluid, the pH is 
normally about 7.5 to 8.5 and bicarbonate ions are the dominant species. 
Bicarbonate shifts to carbonate ion at about a pH of 11 to 12. Therefore, 
it has been found more accurate, at least when drill mud is present, to 
determine the calcium to carbonate plus bicarbonate plus bicarbonate 
ratio. 
Chemical composition changes from geothermal water intrusion are not 
definable except for the increase in Ca--HCO.sub.3 ratio and decrease in 
alkalinity. Fresh-water intrusion into the drill water would generally 
increase hardness and alkalinity and decrease F.sup.-, Cl.sup.-, 
conductivity and the calcium-bicarbonate ratio. A geothermal and 
fresh-water mixture intrusion into the drill water would result in 
increases in the calcium-bicarbonate ratio. The changes in the other 
chemical species would not be definable. 
While we do not wish to be bound by this explanation, it appears that the 
displacement uphole of the calcium to carbonate and bicarbonate ratio is 
due to the leakage or diffusion of the hot water from the aquifer into the 
less porous rock strata above the aquifer. This uphole displacement has 
varied between 25 and 120 meters for the test so far. The amount of 
displacement appears to be a function of the permeability of the material 
above the aquifer, so that a more porous material would provide a greater 
uphole displacement. 
Although the concentration of calcium carbonate and bicarbonate ions may 
vary widely between various geothermal formations, the ratio of the 
concentrations of calcium to bicarbonate or to bicarbonate plus carbonate 
ions seems to vary substantially less between geological formations. 
Nevertheless, it is the trend of the increasing ratio of the 
concentrations of calcium to bicarbonate and carbonate ions which 
indicates the presence of the geothermal aquifer. 
The following Examples are given to illustrate the invention, but are not 
to be taken as limiting the scope of the invention which is defined by the 
appended claims. 
EXAMPLE I 
A well, designated as RRGI-6, was drilled at the Raft River Geothermal Test 
Site in South Central Idaho. As the well was drilled samples were 
collected from the drill return fluid at specified intervals of change in 
the drill string length. The drill fluid was pumped from the mud pit 
through the drill string and returned up borehole between the borehole 
walls and the drill stem. Drill fluid samples were collected at the point 
where the drill return fluid flowed into the mud pit. Samples of the drill 
fluid returns were taken at 120 m intervals of drilling depth to a depth 
of 910 meters and then even 60 meters to total depth. Samples of 4-5 
liters were collected during the period of the drilling operation when 
drilling mud was used to assure an adequate sample. Once the borehole was 
cased and only water was used for a circulating medium, one-liter samples 
were collected. The drilling mud and other residues were separated from 
the water sample by centrifuging and filtering. In cases where the 
centrifuge would not settle the gelatinous mud suspension, a coarse filter 
was used. The samples were analyzed for conductivity, pH, carbonate, 
bicarbonate, Cl.sup.-, F.sup.-, Ca.sup.++, and SiO.sub.2 by the procedures 
described by Brown, Skougstad and Fishman. 
A chemical log was then prepared by plotting graphically the ratio of the 
concentration of calcium to bicarbonate ion to the drill string depth. The 
resulting log is a profile of the chemical changes taking place in the 
drill fluid during the drilling operation. 
FIG. 1 is a profile of the calcium to bicarbonate ratio of Well RRGl-6 
compared with a temperature profile of the same well. As shown in FIG. 1, 
a comparison of the calcium to bicarbonate ratio to the temperature log 
reveals that they are similar except that the calcium-bicarbonate log is 
displaced uphole about 60 m. This means that the chemical log anticipated 
the geothermal aquifer 60 m before the drill penetrated it. 
EXAMPLE II 
A second well, designated RRGP-5, was drilled at the Raft River Geothermal 
Site. Sampling intervals were 120 m until changes were observed in the 
chemical log to indicate increasing temperature. When a temperature 
increase was detected, the sampling interval was decreased to 60 m. The 
resulting calcium to bicarbonate chemical log is shown in FIG. 2. 
Evaluation of the calcium-bicarbonate log reveals a sharp increase in 
temperature at 1220 m of depth. This increase in the calcium to 
bicarbonate ratio continued until the drill string reached 1280 m of 
depth, when a flow of hot water was detected. The flow rate was estimated 
to be approximately 4100 l/minute at about 275.degree. F. Note that the 
chemical log indicated the presence of this geothermal aquifer about 100 m 
before the drill penetrated the geothermal aquifer. Note also that the 
chemical log indicated the existence of the fresh water aquifer at 488 m. 
As can be seen, the chemical log of the ratio of the concentration of 
calcium to the sum of the concentration of carbonate plus bicarbonate ion 
shows excellent results in the ability to anticipate the existence of 
geothermal aquifers in sufficient time to set casings and to take other 
such protective measures as are necessary to protect the productivity of 
the aquifer.