Elevated temperature-pressure flow simulator

An apparatus and process for simulation of field fluid flow conditions under elevated temperature and pressure conditions of oil and gas production, refining, cooling towers, desalinization. The process involves evaluating inhibitor action in reduction of a specified chemical reaction under simulated field aqueous flow conditions by forming first and second aqueous flow streams having a preset concentration of anions and cations, respectively, for the specified chemical reaction with at least one of these streams having a concentration of the inhibitor to inhibit the specified chemical reaction. These streams are combined to form a single process stream where the specified chemical reaction may take place. The process is repeated with successive reductions in inhibitor until detecting the specified chemical reaction in the process stream. The system of this invention is useful for study of problems of scaling, corrosion and other fluid-solid reactions and evaluation of inhibitors therefore, particularly with respect to BASO.sub.4, whose solubility decreases with low temperatures and pressures and which frequently include substituted naturally occurring radioactive materials.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an apparatus and process for simulating field 
fluid flow conditions under elevated temperature and pressure for 
evaluation of precipitation, corrosion, and other fluid-solid reactions. 
The system of this invention may be used to study problems encountered in 
gas and oil production, refining, cooling towers, desalination and to 
evaluate chemicals for their effectiveness in reduction of these problems. 
2. Description of Related Art 
Electronically controlled proportioning pumps for varied mixing of a 
plurality of liquids providing a smooth composition gradient in the output 
flow of uniform velocity and pressure are taught by U.S. Pat. No. 
4,714,545. The output flow may pass through a sample injection valve and 
combined output flow with sample delivered to an analytical instrument, 
such as a chromatograph. 
There are a wide variety of systems for sensing and adding desired chemical 
in a bypass line of a flowing liquid: U.S. Pat. No. 4,945,939 teaches a 
computerized pH control system to control the pH in a reservoir system by 
monitoring a side recirculating stream for pH changes from preset 
tolerance and injecting pH affecting liquid in the side stream downstream 
from the monitoring tap for passage to the reservoir; U.S. Pat. No. 
4,306,581 teaches a chemical concentration control system for a fluid 
circulator having an independent diverter and chemical addition line 
downstream from the circulator and returning to the fluid reservoir and 
within the diverter line a flow-through conductivity cell continuously 
monitors the chemical composition and at a preset value activates a 
solenoid valve allowing concentrated chemical to be aspirated into the 
line and passed to the solution tank until the desired concentration is 
restored and the solenoid valve deactivated and closed; A computer 
controlled system for introduction of chemicals into a water treatment 
system taught by U.S. Pat. No. 4,648,043 has a shunt line having a first 
sensor, a downstream injector for adding a second fluid, and a downstream 
second sensor, the sensors signalling a computer which controls the 
injector in accordance with preset parameters. 
U.S. Pat. No. 4,460,008 teaches a cooling water tower control system which 
senses tower water and make-up water conductivity and utilizes these 
readings to establish an indexing factor for adjusting the trip point. 
U.S. Pat. No. 4,705,503 teaches a catheter having an internal metabolite 
sensor downstream from a semipermeable region where dynamic equilibrium is 
attained between in-vivo external metabolite and higher concentration 
metabolite in the infusate. Changing metabolite concentration controls a 
chemical valve providing codelivery of a drug. 
U.S. Pat. No. 5,034,190 teaches an apparatus and process for accelerated 
corrosion testing of nickel alloys by subjecting a mechanically stressed 
sample to a high temperature mixture of steam and hydrogen. Hydrogen is 
injected into the pressurized vessel through a selective hydrogen 
permeable membrane. 
U.S. Pat. No. 4,863,571 teaches simulating and electrochemically 
determining corrosive behavior of an electrical conducting element 
embedded in a polymer by using the element as a working electrode of an 
electrochemical cell having a viscous electrolyte containing at least one 
corrosive substance. The electrode may be pretreated with a corrosion 
inhibitor to evaluate that inhibitor. 
Submersible pump flow simulations for CaCO.sub.3 scaling and effectiveness 
of scale inhibitors therefore has been described in J. E. Oddo, J. P. 
Smith, and M. B. Tomson, Analysis of and Solutions to the CaCO.sub.3 and 
CaSO.sub.4 Scaling Problems Encountered in Wells Offshore Indonesia, SPE 
22782, Soc. Petr. Engr. 66th Ann. Tech. Conf., Dallas, Tex., (Oct. 6-9, 
1991). The simulations described were only CaCO.sub.3 scalings which were 
achieved by raising the temperature and monitored by pH change. 
Simulations were restricted to certain carbonate chemistries because in 
most instances carbonate scaling would occur in the container without 
inhibitor before the test began. Sulfate scales could not be detected by 
the apparatus shown in this article since they do not cause a pH change in 
water under most desired conditions of simulation. 
SUMMARY OF THE INVENTION 
Scale, corrosion and fouling in energy production equipment used in gas, 
oil or geothermal/geopressured water production are serious and costly 
problems which occur in the high temperature, pressure and ionic strength 
conditions of the production environment. To study and evaluate controls 
for these systems, it is important to have a laboratory flow simulator 
which accurately provides simulation of these production conditions. 
Surface discharge of brines has drawn recent attention since it has been 
found that water from some oil fields have radiation levels considerably 
higher than those allowed for discharge from nuclear power plants. 
Naturally occurring radioactive materials (NORM), particularly long lived 
radium and thorium, are common constituents of sediments of the earth's 
crust. Radium commonly substitutes into BaSO.sub.4 causing the scale to be 
radioactive and presents a potential environmental problem which can be 
extremely expensive to alleviate and incur potential long term 
liabilities. With subsurface disposal of produced water, the low 
concentration of the dissolved species or radioactive radium is of low 
concern since the water is injected back into brine bearing aquifers in 
the sedimentary column. It does become important, however, that scale 
produced in the production system, particularly BaSO.sub.4 scale 
concentrates and fixes radium in the well or on production equipment, 
which may then be classified as hazardous waste. This problem is further 
complicated by the fact that once formed, the BaSO.sub.4 scale is most 
difficult to resolubilize and in many cases tubing must be drilled out or 
pulled and the scale physically removed, posing serious problems for 
producers. BaSO.sub.4 scale also can seriously impact on field economics 
due to lost production and damaged equipment. Systematic methods to 
reliably control BaSO.sub.4 scale formation, therefore, has the potential 
of saving the gas industry considerable money in reduced production, 
clean-up costs and future liabilities. 
BaSO.sub.4 scale occurs during gas and oil production in many places 
throughout the world, including the Michigan Basin, the Gulf Coast, 
Oklahoma and Alaska, in the United States. The scale is formed due to the 
inherent chemistry of the produced brine and the production conditions by 
commingling waters from different produced zones in the same well and by 
mixing incompatible waters in waterfloods. BaSO.sub.4 has a very low 
solubility in water, which is demonstrated by the extreme likelihood of 
formation of the scale when a water containing very low concentrations of 
sulfate is mixed with a water containing relatively low concentrations of 
barium. Both pressure decreases and lowering of the temperature during 
production processes contribute to the likelihood of formation of 
BaSO.sub.4 precipitation. Unlike CaCO.sub.3 whose solubility increases 
with decreases in temperature, the solubility of BaSO.sub.4 decreases 
significantly with decreases of temperature. Generally, the solubility of 
BaSO.sub.4 is about half as much at 77.degree. F. as it is at 203.degree. 
F., regardless of salt concentration. Further, its solubility is about 
half as much at atmospheric pressure as it is at 6250 psi. 
An effective scale control should prevent the formation of solid scale 
material with NORM, such as radium, remaining in solution at relatively 
low concentrations. However, there has been much disagreement in the 
literature as to the most effective chemical scale inhibitors to use and 
the dosage to use to prevent BaSO.sub.4 scale. For accurate simulations, 
since BaSO.sub.4 is more soluble at higher temperatures and pressures, 
solutions must be heated and pressurized for mixing, as opposed to prior 
inhibitor evaluations most of which have been performed at atmospheric 
pressure under static conditions. 
It is, therefore, an object of this invention to provide an apparatus and 
process capable of reliable and accurate flow simulation utilizing 
controlled temperatures up to about 150.degree. C. and pressures up to 
about 5000 psi to enable study and simulation of production equipment, 
particularly with respect to BaSO.sub.4 scaling, but also including 
FeCO.sub.3, SrSO.sub.4, CaSO.sub.4 and CaCO.sub.3 scaling. 
Another object of this invention is to separately heat and pressurize a 
plurality of salt solutions prior to mixing for evaluation of inhibitors 
for scale and corrosion under field simulated conditions. 
These and other objects and advantages of this invention are achieved, in 
one embodiment, by a plurality of reactant high pressure pumps separately 
providing a plurality of pressurized aqueous field simulated streams, each 
containing an ionic concentration of one species required for the conduct 
of the desired simulated chemical reaction, with inhibitor to be evaluated 
initially present in at least one of the streams in an amount at least 
sufficient to effectively prevent the desired simulated chemical reaction, 
to a heater and separately passing the streams through the heater to heat 
the streams to a preset desired simulation temperature. The heated streams 
are then passed to a mixer where the streams are combined into a single 
process stream in which the simulated chemical reaction takes place. A 
pressure transducer and a backpressure valve/controller at the downstream 
end of the pressurized process stream maintains the preset desired 
pressure and flow rate of the process stream. The extent of reaction 
taking place in the process stream is measured. The formation of scale in 
the process stream causes pressure increase at the high pressure pumps due 
to constriction of flow in the process stream by the scale. Monitoring of 
the pressure differential between the high pressure pumps and the pressure 
transducer has been found to indicate scale formation very reliably. 
Another high pressure pump, capable of introducing material into the 
downstream end of the pressurized process stream before the backpressure 
valve, is used to halt or neutralize the scaling or corrosion reactions, 
by dilution and/or chemical addition to prevent scaling or precipitation 
in the back pressure valve. This high pressure pump is not necessarily 
programmable. After reduction in pressure and temperature, downstream of 
the backpressure valve, pH and chemical composition may be monitored, by 
analyses known to the art, and results may be fed to a computer for 
adjustment of the inputs to the simulation system. Each of the pressurized 
streams may be controlled by a computer analyzing desired preset and 
monitored conditions of temperature, pressure, pH, and chemical 
composition. A series of successive tests are conducted, each successively 
only reducing the concentration of the inhibitor, until the desired 
simulated chemical reaction is observed, such as scaling, from which the 
minimum effective concentration of inhibitor is ascertained. The apparatus 
and process of this invention have been found to be reliable and effective 
in the evaluation of inhibitors for prevention of BaSO.sub.4 scaling with 
the concurrent scale formation of NORMs and is suitable for simulations at 
temperatures of about -20.degree. to about 150.degree. C. and pressures up 
to about 5,000 psi. 
BRIEF DESCRIPTION OF THE DRAWING 
The above and further objects and advantages of the invention will become 
more apparent upon reading the detailed description together with 
reference to the drawing which is a simplified schematic showing of a high 
temperature/pressure dynamic inhibitor evaluation apparatus according to 
one embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the drawing as shown, high pressure pump 11 has the capability 
of mixing and supplying desired aqueous field simulated test solution 
containing an ionic concentration of one species of anions or cations 
required for conduct of the desired simulated chemical reaction and high 
pressure pump 12 has the capability of mixing and supplying desired 
aqueous field simulated test solution of an ionic concentration of the 
other species of anions or cations required for conduct of the desired 
simulated chemical reaction. In each case, the aqueous solution otherwise 
corresponds to the field composition of the aqueous system being 
simulated. At least one of these aqueous streams additionally initially 
contains at least sufficient concentration of chemical inhibitor being 
evaluated to inhibit the desired simulated chemical reaction. Pumps 11 and 
12 have the capability of pressurizing the flow system to the desired 
simulation pressure, up to about 5000 psi. Suitable high pressure mixing 
pumps are commercially available, such as Eldex Laboratories, Inc., Pump 
Model 9600. The pressurized streams from pumps 11 and 12 are then passed 
separately through a temperature adjustment means, such as oil bath 14, 
which is capable of adjusting the temperature of the streams from pumps 11 
and 12 to desired simulation temperature of -20.degree. up to about 
150.degree. C., following which the separate streams are passed to mixer 
15 for mixing of the streams at the controlled elevated temperature and 
pressure to form a single process stream which accurately simulates the 
desired field conditions of temperature, pressure and chemical composition 
to allow the desired chemical reactions of scale formation, corrosion, 
inhibitor action, and the like to take place under simulated field 
conditions. High pressure pump 13 has the capability to add water and/or 
desired chemicals to the high temperature/pressure process stream upstream 
of back pressure valve/controller 17 before the process stream is cooled 
and returned to atmospheric pressure at valve/controller 17 after desired 
exposure of the process stream to desired simulated field conditions of 
scaling, corrosion, inhibitor action and the like. After cooling and 
return to atmospheric pressure, aliquots or side streams may then be 
passed to any desired measurement instrumentation for analysis of the 
process stream for specified components as determined by the type of 
testing being conducted. Shown in the figure is pH/Ion Sensitive Electrode 
18 connected to pH/ISE meter 19 for pH measurement and flow through 
spectrophotometer 30 to which a side stream may be passed by valve 31 for 
desired chemical analysis. Cooled atmospheric pressure waste stream 32 
drains from the apparatus. 
The system is controlled by computer 20. Pressure transducer 16 monitors 
the high pressure in the process stream and transmits the information via 
communication line 26 to computer 20 which analyzes the measured pressure 
at the downstream end of the process stream together with the desired 
preset pressure and adjusts pump 11, pump 12 and valve/controller 17 
through communication lines 21, 22 and 27, respectively, to obtain and 
maintain the desired pressure and flow rate in the process stream of the 
flow simulator system. When the desired simulated chemical reaction 
involves precipitation, such as scaling, the pressure differential between 
pumps 11 and 12 and pressure transducer 16 provides a measurement of the 
scaling since the formation of scale in the pressurized process stream 
causes increased pressure at pumps 11 and 12. The elevated temperature in 
the individual streams may also be monitored at the oil bath 14 and 
transmitted to computer 20. The measured temperatures together with the 
desired preset temperature of the process stream is monitored and the 
temperature of oil bath 14 adjusted to obtain and maintain the desired 
elevated temperature in the individual streams, and thus in the process 
stream, of the flow simulator system. Any suitable means for obtaining and 
maintaining the desired temperature of up to about 150.degree. C. in the 
process stream, as will be well known in the art may be used. Computer 20 
also may, but not necessarily, adjust the flow of dilution water and/or 
neutralizing chemical into the pressurized process stream prior to back 
pressure valve/controller 17 to prevent undesired scaling in backpressure 
valve/controller 17. A wide number of suitable computer, program and 
control systems for the apparatus and process of this invention will be 
apparent to one skilled in the art upon reading the above description. 
An important feature of the flow simulator system of this invention is that 
the components of the system in contact with the chemical streams are 
entirely metal free to prevent sorption of chemicals onto the pump 
components and tubing. Suitable tubing and coating materials such as 
Teflon and polyetheretherketone (PEEK) may be used. It is important to 
eliminate sorption in the system to prevent "memory effects" from the same 
or previous evaluations from interfering with the current flow simulation. 
The apparatus for evaluating inhibitor action in reduction of a specified 
chemical reaction under simulated field aqueous flow conditions, in 
accordance with this invention, has first pump means capable of forming a 
first aqueous stream corresponding to the pressure and chemical 
composition of said field aqueous flow except comprising a preset 
concentration of anions for said specified chemical reaction; second pump 
means capable of forming a second aqueous stream corresponding to the 
pressure and chemical composition of the field aqueous flow except 
comprising a preset concentration of cations for the specified chemical 
reaction; means for supplying preset decreasing concentrations of the 
inhibitor to at least one of the first and second pump means; first and 
second conduit means in fluid communication at their upstream end, 
respectively, with the aqueous stream outlet of the first and second pump 
means, the first and second conduit means passing in thermal transfer 
relation to temperature adjustment means capable of adjusting the 
temperature of the first and second aqueous streams to a preset 
temperature corresponding to the field aqueous flow conditions; mixing 
means in fluid communication with the downstream end of the first and 
second conduit means and capable of mixing the first and second aqueous 
streams to form a single process stream at pressure and temperature 
corresponding to the field aqueous flow conditions at its outlet; process 
stream conduit in fluid communication with the outlet of the mixing means 
for passage of the process stream; backpressure/controller valve means in 
fluid communication with the downstream end of the process stream conduit; 
pressure transducer means capable of measuring pressure in the process 
stream in fluid communication with the process stream conduit upstream of 
the backpressure/controller valve means, and computer means in signal 
communication for receiving signals from the pressure transducer means and 
receiving signals from, analyzing and controlling the first and second 
pump means and the backpressure/controller valve means. 
The process according to this invention for evaluating an inhibitor 
chemical action reduction of a specified chemical reaction under simulated 
field flow conditions is achieved by: forming a first aqueous stream 
corresponding to the pressure and chemical composition of the aqueous 
field stream except comprising a preset concentration of anions for the 
specified chemical reaction; forming a second aqueous stream corresponding 
to the pressure and chemical composition of the aqueous field stream 
except comprising a preset concentration of cations for the specified 
chemical reaction; at least one of the first and second aqueous streams 
initially additionally comprising at least sufficient concentration of the 
inhibitor chemical being tested to inhibit the specified chemical 
reaction; separately adjusting the temperature of the first and second 
aqueous streams to a preset temperature corresponding to field conditions; 
mixing the first and second aqueous streams at pressure and temperature 
corresponding to field conditions being simulated; conducting a plurality 
of runs successively reducing the inhibitor concentration while 
maintaining the preset anion and cation concentration until detecting the 
specified chemical reaction. This will provide the minimum effective 
inhibitor concentration for the specific inhibitor being tested for the 
specified chemical reaction. It will be apparent to one skilled in the art 
that the described process may be modified to accommodate various field 
simulations being conducted. 
With the capability of high temperature/pressure flow field simulation, 
accurate and reproducible results have been obtained with the apparatus 
and process of this invention for scale inhibitor evaluation for specific 
petroleum production fields. These results have been proven by field 
application and testing. The capability of providing separate streams of 
high pressure/temperature solutions prior to mixing make possible accurate 
and reproducible results involving BaSO.sub.4 scaling and evaluation of 
inhibitors therefore. 
Gas producers in the Michigan Basin, near Traverse City, Mich., have 
experienced scale due to CaCO.sub.3 and BaSO.sub.4 solids precipitation 
from water produced in conjunction with natural gas. Barite, BaSO.sub.4, 
scale is a serious problem to the gas producing industry and has the 
potential to contain naturally occurring radioactive materials (NORM) 
which are currently an issue within regulatory bodies and more stringent 
regulations concerning NORM scales may result. By control of the scale 
problem, NORM and other scales being deposited in wells and surface 
equipment may be reduced or eliminated, thereby reducing or eliminating 
the NORM environmental risk and increasing gas production due to decrease 
of production loss due to equipment and/or facilities clogged with scale 
material. 
Operators in the Michigan Basin reported that wells and production 
equipment were scaling with CaCO.sub.3 and BaSO.sub.4 and that the scale 
had coprecipitated NORM. Additionally, very high sulfate concentrations 
had been observed in some wells to be in excess of 4000 ppm, even though 
typical concentrations were undetectable by conventional methods. Water 
sampling taken at 133 Michigan Basin locations, including 108 well sites 
with the remainder being duplications, surface facilities and disposal 
wells, showed total dissolved solids in these wells ranged from about 
25,000 to over 180,000 mg/l. Barium concentrations ranged from less than 1 
mg/l to 185 mg/l with an average of 43 mg/l. Sulfate concentrations were 
measured at between less than about 3 mg/l, the detection limit, to 3233 
mg/l with an average of 284 mg/l, in cases where sulfate was detected. The 
average values, however, can be misleading since that when sulfate was 
detected the barium concentration was generally low, and vice versa. 
Measurement of BaSO.sub.4 solubilities and ionic strength of solutions 
from wells of the Michigan Basin studied indicate that the wells sampled 
were at equilibrium with barite in the reservoir. Some wells had produced 
NORM scale in the tubing due to higher sulfate levels in the produced 
brine. Scaling problems were most severe in wells where both sulfate and 
barium levels were detected. 
Saturation index calculations, explained more fully in J. E. Oddo and M. B. 
Tomson, Why Scale Forms in the Oil Field and Methods to Predict It (Title 
changed for publication to Improvement on the Oddo-Tomson Saturation 
Indices for the Prediction of Common Oil Field Scales), J. Petr. Eng., in 
press (1993) which is incorporated herein by reference in its entirety, 
with respect to CaCO.sub.3 indicated that all the wells considered had a 
tendency to produce CaCO.sub.3 scale and while undersaturated with respect 
to gypsum, CaSO.sub.4 scale would be expected under production conditions. 
The high sulfate wells were found to be much less undersaturated with 
respect to gypsum than wells with low sulfate which indicates that waters 
in these wells may be migrating from another formation having higher 
gypsum and/or other sulfate minerals. Consideration of normalized sulfate 
divided by calcium versus normalized chloride suggests that while some of 
the waters are approaching equilibrium with calcium sulfate most of the 
waters are very much undersaturated with calcium sulfate. Radiation 
readings taken at the sampling sites indicate that NORM scaling is 
primarily taking place in the handling facilities after commingling of the 
waters, but some wellheads exhibited small amounts of radioactivity which 
correlated well with BaSO.sub.4 predictions from water chemistry. 
Radiation was also detected in the input flow lines to the separators 
indicating NORM scale forming upstream of the main surface facilities. 
Scale inhibitor evaluations for the above described Michigan Basin wells 
was undertaken in the laboratory using the high temperature/pressure 
simulation apparatus as described above. The Michigan reservoirs are very 
shallow, about 1000 to 1600 feet, and are at about 70.degree. F. and 100 
psi. The composition of the brine used in the laboratory evaluations, 
having 45,140 mg/l Total Dissolved Solids and Ionic Strength of 0.85M, was 
as shown in Table 1. 
TABLE 1 
______________________________________ 
Chemical Species 
Concentration (mg/l) 
______________________________________ 
Ba 30 
Ca 880 
Mg 680 
HCO.sub.3 150 
Cl 27,000 
SO.sub.4 1,100 
______________________________________ 
Scale inhibitors evaluated for sulfate scale inhibition in the above brine 
were tested in the elevated temperature/pressure flow simulator as shown 
in the figure. For each scale inhibitor, the temperature and pressure of 
the apparatus were adjusted to the desired points for field simulation. 
The process stream flow was started with anions and cations being 
individually introduced by the pumps at a rate such that the combined 
stream had the simulated concentration of the actual oil or gas field 
water under consideration. The cation aqueous solution had sufficient 
scale inhibitor so that no scale formed in the system. When the apparatus 
reached equilibrium, the inhibitor in the cation stream was incrementally 
decreased in successive tests in a manner such that the combined stream 
had the identical concentrations of cations and anions, but only the 
amount of scale inhibitor was incrementally decreased. At some point, 
scale formed in the system due to the decreasing concentration of scale 
inhibitor and was detected by the change in pressure at the pumps as 
described above. At this point, the minimum effective dose or 
concentration required to inhibit scale for the scale inhibitor being 
tested under the test conditions is known. This results in a field 
applicable minimum concentration to inhibit scale and a ranking of the 
inhibitors being tested. Since the concentration of inhibitors being 
tested is a minimum effective dose, significant cost savings are realized 
in chemical treatment using correct concentrations and not treating with 
an excessive amount of chemical. 
Eight generic threshold inhibitors were evaluated by this method and the 
results are shown in Table 2. Active concentrations refer to active 
concentrations of the inhibitor chemical in an aqueous solution. Minimum 
concentration refers to the lowest concentration that was effective for 
inhibition under conditions of the laboratory evaluations at 70.degree. F. 
and 100 psi. Two values separated by a backslash indicate two runs to 
verify reproducibility. 
TABLE 2 
______________________________________ 
Min. 
Product Active Active 
Conc. Conc. Conc. 
Scale Inhibitor (mg/l) (%) (mg/l) 
______________________________________ 
Phosphinopolycarboxylate A 
1.0/1.0 50 0.56/0.51 
Phosphinopolycarboxylate B 
1.3/1.2 42.5 0.56/0.51 
TEA Phosphate Ester 
3.8 32.7 1.2 
Polyacrylate 6.8 34 2.3 
Phosphonate/Polymer Blend 
7.0 45 3.2 
Diethylenetriaminepenta 
10.0 41.2 4.1 
(methylene phosphonic) Acid 
Phosphonate A &gt;20.0/ 38.5 &gt;7.7/ 
&gt;20.0 &gt;7.7 
Aminotrimethylene phosphonate 
&gt;20.0 42 &gt;8.4 
______________________________________ 
The most effective scale inhibitor against BaSO.sub.4 tested was 
phosphinopolycarboxylate (PPPC). The above tests have been described in J. 
E. Oddo, C. Stiz, I. Ortiz, D. Linz, A. Lawrence, A. T. Kan and M. B. 
Tomson, Naturally Occurring Radioactive Materials (NORM) Scale 
Formation--A Case Study of the Chemistry, Prediction, Remediation and 
Treatment in Wells of the Antrim Gas Fields Near Gaylord, Mich., SPE/EPA 
Exploration and Production Environmental Conference, San Antonio, Tex., 
Mar. 7-10, 1993, which is incorporated herein by reference in its 
entirety. The effectiveness of this inhibitor at the about the above 
minimum active concentration has been confirmed in the field by squeezing 
ten wells with no further scale detected in these wells. 
Scale inhibitor evaluations were conducted in the same manner as described 
above at 194.degree. F. and 250 psi for FeCO.sub.3 scaling using aqueous 
solutions of the composition shown in Table 3. 
TABLE 3 
______________________________________ 
Chemical Species 
Concentration (mg/l) 
______________________________________ 
Cl 12,000 
HCO.sub.3 16,000 
SO.sub.4 1,150 
Fe.sup.+2 600 
______________________________________ 
The scale inhibitors evaluated by this method and the results are shown in 
Table 4. 
TABLE 4 
______________________________________ 
Min. Active 
Scale Inhibitor Conc. (mg/l) 
______________________________________ 
Aminotrimethylene phosphoric acid 
2.5 
Phosphinopolycarboxylic acid 
15 
______________________________________ 
The measurement of FeCO.sub.3 scaling could not have been accomplished 
using prior methods of detection by pH change since FeCO.sub.3 formation 
does not change the pH of the aqueous solution used. 
While in the foregoing specification this invention has been described in 
relation to certain preferred embodiments thereof, and many details have 
been set forth for the purpose of illustration, it will be apparent to 
those skilled in the art that the invention is susceptible to additional 
embodiments and that certain of the details described herein can be varied 
considerably without departing from the basic principles of the invention.