Collection and Analysis of Geothermal Gases

Lisa Shevenell' Fraser Goff Lois Gritzo

P. E. Trujillo, Jr.

LA-10482-0BES

UC-66b Issued: July 1985

·Undergraduate Student at Los Alamos. Los Alamos National Laboratory, Los Alamos, NM 87545.

Los Alamos National Laboratory Los Alamos, New Mexico 87545

r

COLLECTION AND ANALYSIS OF GEOTHERMAL GASES

by

Lisa Shevenell, Fraser Goff, Lois Gritzo, and P. E. Trujillo, Jr.

ABSTRACT

Rapid, reliable procedures are described for the collection and analysis of geothermal gases at Los Alamos National Laboratory. Gases covered are H2, He, Ar, 02, N2, CH4, C2H6, C02, and H2S. The methods outlined are suitable for geothermal exploration.

I. INTRODUCTION

Because of the rapid increase in the development and use of geothermal

resources in the 1 ast 20 years, many new geochemi cal techn iques h ave been

applied in geothermal exploration (e.g., Fournier, 1981). The geochemistry of

geothermal gases can be used to predict the subsurface temperature of

hydrothermal reservoirs if the gas sampled at the surface did not

re-equil ibrate as it traveled from a boil ing interface at depth to a surface

fumarole or hot spring (D'Amore and Panichi, 1980; Norman and Bernhardt,

1981). This exploration method is particularly useful for deep geothermal

reservoirs that do not directly supply hot water to surface springs and/or

for areas where only acid-sulfate hot springs occur (Goff and Vuataz, 1984;

Goff et al., 1985).

The object of this report is to describe rapid but reliable methods for

the collection and analysis of geothermal gases suitable for geothermal

exploration purposes. Gases covered in this report are H2, He, Ar, 02'

N2, CH 4 , C2H6 , CO2, and H2S. With modification, the analytical

methods could be refined to include more gases such as CO, S02' N20,

etc., or to detect ultraminute quantities of some gases. Noble gases other

than He and Ar are not covered by these procedures.

1

II. COLLECTING GAS SAMPLES A. Apparatu s

The basic apparatus used to collect gas samples consists of the following

items (Figs. 1 and 2): several meters of plastic tubing, a small hand pump, several sizes of funnels, SOO-m£ glass sample bottles with stopcocks at both

ends, a long collapsible wooden pole, masking tape and markers, and two bottle

clamps. One piece of tubing is attached to the suction stem of the hand pump and to one stopcock on the sample bott 1 e. Another pi ece of tubi ng connects

the other stopcock of the sampl e bottle with the stem of the funnel. Hence, tubing is attached to both openings on the sample bottle with the pump at one end and the funnel at the other end. The clamps are attached to the end of

the wooden pole in such a way that the funne 1 can be clamped to the pole. The purpose of this arrangement is to reach the funnel over the main area of

gas discharge and/or to keep the technician a safe distance away from boiling

springs or superheated fumaroles.

B. Procedure The sampling procedure is described below. 1. Locate the area where the most bubbles surface (spring) or where the

largest volume of gas issues (fumarole). 2. Place the wide end of the funnel into the bubbling water making sure

that the rim of the funnel is completely submerged. If sampling a fumarole,

dig out the area of interest, set the funnel into the earth, and pack earth

around the funnel. If the rim of the funnel is not completely covered, you

will contaminate your gas sample with air. 3. Begin pumping with the hand pump being careful not to suck water up

through the tubing into the sample bottle. As you pump while sampling a gaseous spring, you will see the water level rise in the funnel. When the water level approaches the apex of the funnel, cease pumping until gas fills

the funne 1. 4. Pump vigorously for about 2 minutes (lZS-m£ bottles) to S minutes

(SOO-m£ bottles) unti 1 all the air is out of the sample bottle and has been

replaced with geothermal gas. S. Close both stopcockS on the sample bottle before the funnel is

removed from the spring or fumarole.

2

Fig. 1. The apparatus used to collect gas samples from springs and fumaroles.

Fig. 2. The use of the apparatus is illustrated during the sampling of a bubbling spring.

3

6. Wrap masking tape around the stopcocks to avoid accidental opening of the gas bottle and contamination of the sample.

7. Wrap masking tape around the body of the bottle. With a felt marker, write the field number, sample name, or other identifier on the tape.

8. Record field number, sample name, date, location (and other important observat ions) in a fi e 1 d book. Hi gh-pressure procedures can and tub ing.

gas discharges may require little be easily adapted to sampling wells by

C. Potential Difficulties

or no pumping. These using appropriate valves

1. When sampling, if air appears to be leaking into the sample bottle, start the sampling procedure from the beginning.

2. If water enters the sample bottle while you are taking a sample, try to empty the water out of the bottle and start sampl ing again. If you have an extra sample bottle, it is best to use an uncontaminated bottle to take the sample.

III. MECHANICS OF GEOTHERMAL GAS ANALYSIS

A. The _Ga~ Chrom~tograph (GC) A gas chromatograph is an analytical instrument that physically separates

two or more gases based on their differential flow rate between a stationary phase (packed column) and a gas phase (Thompson, 1977). The instrument used

in the work reported here is a Hewlett-Packard 5832A gas chromatograph with a separate Hewlett-Packard l8850A terminal unit.

The basic concept of gas chromatography is that gas samples flow through a column (tube) filled with packing through which a specific carrier gas also flows (Supelco, 1980). There are three column-carrier gas setups routinely used in this procedure to analyze the different gases found in samples: the Poropak QS column with helium carrier gas, the Molecular Sieve 5A column with

helium carrier gas, and the Molecular Sieve 5A column with argon carrier gas. Both the Poropak QS column and the Molecular Sieve 5A column are 15 feet long, 1/8 inch in diameter, and 80/100 mesh. Each column-carrier gas setup is used to quantitatively detect different gases (Table 0. The various gases in geothermal gas samples flow through the column at different rates (due to

4

TABLE I

PARAMETERS USED FOR THE THREE RUN SETUPS DESCRIBED IN TEXT

Run 1 Run 2 Run 3 Molecular Sieve Molecular Sieve Poropak with

Parameter with Helium with Argon Helium

TEMPl (DC) 0 -40 35

TIMEl 5.0 3.5 7.0

RATE (DC/m) 30.00 0 20.00

TEMP2 (DC) 295 100

TIME2 11.0 5.0

CHT SPD 0.50 0.50 0.50

ATTN 2 5 7 6

TCD SGNL A +B A

SLP SENS 0.40 0.40 1.00

AREA REJ 200 200 250

OPTN 0 0 0

VLV /EXT 2[TIME] 0.1 2[TIME] 0.1 2[TIME] 0.1

VLV/EXT -2[TIME] 0.2 -2[TIME] 0.2 -2[TIME] 0.2

SLP SENS 0.05[TIME] 12.0 (delete) 0.2[TlME] 3.0

INJ TEMP (DC) 100 50 100

TCD TEMP (DC) 250 200 200

OVEN I'1AX (DC) 300 300 225

their size, molecular weight, and polarity) and, therefore, come out of the

column at different times. The time at which an individual gas leaves the

column is called its retention time, and different gases have different

characteristic retention times. When gases exit the column, they enter the

thermal conductivity detector (TCD), and their presence in the gas mixture is

recorded on a printer in the form of peaks, followed by a retention time. The

area under these peaks is calculated by an integrator and recorded on the

printer. From these areas, one can calculate the percentage of each

constituent of the gas sample.

There are three major parts of the GC system (Fig. 3): the injection

system with vacuum pump, the gas chromatograph (GC), and the printer/

5

0\

Fig. 3. The gas chromatograph system described in this report, including the injection system, the GC, and the printer.

integrator. The injection system is the segment of the GC system through

which gas is introduced, accurately pressurized, and injected into the GC.

The gas chromatograph separates the gases in the column, detects their

relative proportions, and sends information to the printer. The printer is

attached to the GC and records its findings. Various parameters are entered

on the printer's keyboard to control operation of the GC and to enhance the

quality of analysis (Table I). The samples, carrier gases, standard gases,

and liquid nitrogen (for cooling purposes) flow to the GC through stainless

steel or copper tubing. Stainless steel is used wherever H2S is present to

avoid reaction with copper.

Each day that the GC is used, premixed gas standards are run through the

system (Table II). Two standards are routinely run through the GC: a mixed

standard that s imul ates a typi ca 1 geothermal gas for all runs and a pure

CO 2 standard for the Poropak column with helium carrier run. Air is also

analyzed as a standard to correct for gas samples that contain air

contamination. Each standard flows through tubing from a tank to a manifold

on the injection part of the system. The amount of each type of gas in the

mixed standard is known and printed on the tank. Standards are analyzed so

that the technician has the response of a known volume of gas to compare with

the responses obtained when running volumes of sample through the GC.

Identical volumes of sample and standard gases are injected into the GC

during each run using a corrmercially available gas injection valve that will

contain a known volume of gas (1 cm3 for the apparatus described here).

Gas pressure is accurately measured before each run. When the pressure,

volume, and response spectra of samples and standards are known, the

volume-percentage of each gas component can be calculated.

The three gas detection setups are

1. Molecular Sieve SA column with argon carrier gas detects, in order,

He* and H2*.

2. Molecular Sieve SA column with helium carrier gas detects, in order,

3. H2, 02*' Ar*, N2*' CH 4*· Poropak QS column with

Ar+02' CH 4*, in samples.

helium carrier gas detects,

H2S*; C2H6

in order, H2 ,

rarely is N2,

found

The asterisks (*) indicate those constituents to be analyzed in a particular

run. See the Appendix for examples of GC print-outs for each run setup.

7

TABLE II

ANALYSES OF STANDARD GASESa AND REPRESENTATIVE ANALYSES OF TWO GEOTHERMAL GASES ON THE LOS ALAMOS GC; VALUES IN VOL%

CO 2 H S 2 H2 He

N2 O2 Ar

CH4 C2H6 CO

Total

aAnalyses b 1 . Ana YS1 s

Airb at Main Spg.

Mixed Gas "Pure" CO2 Hydrocarbon at Jemez Standard Standard Standard 2600 m Springs,

82.39 99.91 1.00 0.032 93.05

4.57 <0.0005

2.81 <0.005 0.28

0.82 <0.005 0.75

4.91 96.0 78.3 4.85

0.09 21.0 0.22

0.80 0.93

3.70 0.999 <0.0002 1.47

1.00 <0.001

1.00 <0.001

100.00 100.00 100.00 100.26 100.62

listed were obtained from suppliers of gas standards.

listed is from Evans et al., 1981.

B. Injection System

Main Spg. at Soda

NM [)am, NM

97.47

2.43

0.07

99.97

Before analyzing a sample, the injection system is first evacuated with

the vacuum pump to 0 psi absolute on the vacuum gauge. Standards or samples

are introduced, their pressure is precisely calibrated, and an exact volume of

gas is injected into the GC through a special injection valve. The system

descritJed nere was modified and built by F. Goff from a functioning system

designed by A. H. Truesdell, U.S. Geological Survey, Menlo Park, California

(see Figs. 4 and 5).

1. Valves. There are three intake valves on the far left of the system

shown in Fig. 5. One valve (A) allows the gas sample to enter the system.

The sample bottle is attacned below this valve by using a rubber hose that

fits snugly around the stem of the bottle. Air or additional standards are

8

Fig. o.

Fig. 4. The injection system.

\---- Carrier Gas, In

Carrier Gas and Sample, Out to GC

A, B, C, D, E, F, G: Values described in text H: Piston cylinder I: Coiled tubing can be immersed in dewar of dry ice·ethanol J: Rough pressure gauge to protect system from overpressure K: Precise pressure gauge, 0 to 30 psi absolute L: Valco 6·port gas injection valve with 1·cm3 coiled loop

Schematic diagram of the gas injection system: enter at the 1 eft, are pressure-adjusted in injected into the GC on the right.

samples or standards the center, and are

9

admitted into the system as standards through this valve. Valve B allows a

CO2 standard to enter the system. Valve C allows the mixed standard to

enter the system.

There are two other valves (0 and E) in Fig. 5 which control access to

the vacuum pump. These valves are always left closed if a sample that you

wish to analyze is in the system. Before running the next sample or standard,

these valves are opened, as well as two additional valves, F and G, described

below, so that the vacuum pump will purge all existing gas from the system

before you introduce a new gas sample. The system is ready for a new sample

when the large pressure gauge (K) reads 0 psia.

Two valves that control the pressure of the gas (F and G, Fig. 5) are

located in the center of the injection system. When both valves are closed,

gas will not enter the pressure-regulating part of the injection system; gas

will be trapped to the left of knob F. Opening the left valve allows gas to

enter the section of the system containing the large pressure gauge and the

piston cylinder. Opening the right valve (G) allows gas to pass into the

right (injection) part of the system. The gas is removed from this section

and injected into the GC for analysis.

2. Pisto~~~~er (H, Fig. 5). The piston cylinder is used to

precisely adjust pressure in the injection system. Turning the crank on the

piston cylinder counterclockwise (out) decreases the pressure in the system,

while turning it clockwise (in) increases the pressure. By thoughtful use of

the piston cylinder and valves, the pressure of sample and standard gases can

be varied as necessary, and sample gas pressures can be increased well above

their original collection pressure. For example, when keeping the left center

knob closed (F, Fig. 5), a clockwise turning of the crank increases the

pressure from this valve toward the right. If both center valves are closed,

clockwise turning of the crank increases the pressure in the center section of

the system--the part containing the large press~re ~auge.

Before each run, the fcrar:lkijs tllllf1T!led ~!'I 1Jtqe 'C'(!)'Unterclockwise direction as

far as it will go. Most SiaII1I!l]es ifrrom fumaroles, springs, and gas seeps need

to be pressurized to get (gl!l~1I analytical results; therefore, the crcank'Shaft

should extend outward as 'far as possible in order to allow 'for nla'X'imum

compression of each sample.

10

C. The Detector

The

GC.

, " ':":- ,,1 ON-dFF switch for the detector is located in a small DOX on top of

The dete~tormust be turned on only after the carrier gas is turned , '. ' J •

the

on and a flow is established t~rough the columns. The detector requires a

half-hour warm-up period before use. The method for checking and adjusting

flow is described below.

D. The printer/KeYboard'

The parameters needed tocontrq 1 the GC operati on, stri.pc~art .recorder,

and data integration are entered and deleted by. pressingc,ertain key

combi nat ions. To enter par.ameters, press the funct ion key, th.e numeri cal . .' .' ! .' " ' ~' , .

value desired, and ,then the enter key.

Exampl e: TEMP 1 200 ENTER

If you wish to omit a command, punch out.the full command. then press the , ' . ' , ' " , .

delete key.

Exampl e: TEMP 1 200 DELETE

To pri nt an enteredpa(,ameter value, press ,the Hst key.fo 11 owed by the

parameter key.

Example: LIST -. TEMP. 1

To 1 ist all entered parameter~~ press the list key twice in succession.

Before starting to run gas through the GC, always ,check to see if the

stripchart base 1 ine . is stable and .propt;rly located. Press STGCDNL

, START .....

ZERO, CHT to st~rt the printer. Once you see where the base 1 iriels

locatea,adjust the pen position so, that it lines up, with the small black line

on the printer glass. The base line is adjusted with the knobs labeled

"coarse" and "fine," which are located in the same box as the detector ON-OFF

switch (Section D, above). When the base line is properly located,. press STOP CHART"

The red 1 i ghts on the

operational mode of the GC.

right side of tre printer indicate the current

When, th~ rep. 1 ightison .in the READY positiori,

11

the GC is ready to run a sample. If the gas sample is already loaded into the

injection system, press ~~~RT to begin analysis of the gas.

If you wish to termlnate a run before its completion or if you make some

type of mistake, press the ESCA~~ key and begin functions anew.

E. Operational Gases

Several additional gas tanks having appropriate valves and hardware are

required to make the GC operational. Large cyl inders of pure He and Ar with

filters attached to their respective gas regulators provide carrier gases to

the analytical system at 60 pSi. A second large cylinder of Ar provides gas

at 60 psi to actuate various mechanisms inside the GC and injection system.

A dewar of liquid N2 must be filled each day to provide coolant for the GC

during temperature-ramping analyses or during analyses that must be performed

cold. The liquid N2 is introduced into the GC at 8 to 10 psi using

compressed air.

Carrier Gases. The carrier gases He and Ar flow through copper tubing

that is attached to the back of the GC. The copper tubing should be

identified with either an Ar or an He label. Once the appropriate carrier gas

is attached, pressurize the system and check the connections for leaks with

"snoop" (a soapy mixture). If bubbles form around the connections, then there

is a leak. Tighten the nuts and check with "snoop" until the leak has been

eliminated. If no bubbles form, there is no leak. Be careful not to

overti ghten the nuts.

Once the carr ier gas is attached and turned on, the flow mus t be

adjusted. On the right-hand side of the GC on the top are two knobs labeled

A and B. The flow of the carr ier gas through each column, A and B, is

adjusted by turning these knobs and subsequently checking the flows by

pressing LIST ~ and LIST ~ on the keyboard. To increase the flow, turn the

knob counterclockwise. To decrease the flow, turn clockwise. Continue

adjusting and checking the flows until the flow through Column A equals the

flow through Column B.

F. Changin~_~~l~~~

Before changing the columns inside the insulated compartment of the GC,

be sure to turn off the de1:.e_ctor. The two columns should be labeled A and

B. B is attached to the back connections in this compartment, and A is

12

attached to the front connections using wrenches. The ends of each column should be engraved with either the letter I (injection side) or the letter D

(detection side). Attach the I side on the left and the D side on the

right. Tighten the connections with wrenches, pressurize the system, and

check for leaks with "snoop."

IV. PROCEDURE FOR GAS ANALYSIS

A. Run 1 - Molecular Sieve with Helium - Detection of O2, Ar, N2, and CH4

The col umns must be "baked out" (heated) overni ght at 290·C to cl eanse the

columns of any adsorbed gas impurities present. The procedure for baking out foll ows.

1. With the detector off, attach the Molecular Sieve columns inside the

GC.

2. Turn GC to ON in back and to OPERATE in the front.

3. Adjust the flow rate of carr i er gas, He, so that the flow through

Columns A and B equals 30.

4. Set the starting temperature to 295·C; press TEMP 1 295 ENTER.

5. Set the injection temperature to 100·C; press i~~;-lOO ENT~~ 6. Set the thermal conductivity detector t0250·C; press i~~p

250 ENTER.

7. Set the oven maximum to 300·C; press ~X~N 300 ENTER

8. Let the columns bake out overnight.

9. When you arrive the following morning, check the flow rates and

adjust them unt i 1 they equal 30. When the flows are equal, turn on

the detector to let it warm up (approximately 1/2 hour).

10. Enter each of the parameters for Run 1 found in Table I followed by

ENTER .

11. Fill the liquid nitrogen tank.

12. Check flows again to make sure they equal 30.

13. Check the location of the b<:sf ine and adjust, if necessary.

When the GC is ready, the following sequence is carried out to run a sample (see Figs. 3, 4, and 5):

13

14. Open the' two lever valves connecting' the vacuum pump to the gas

injection system and evacuate (thi s' can b~ done. after. Step 8i n'the

previous list) •

. 15. Open the two center knob val ves;

16. After the system is completely evacuated and the pressure gauge reads

o psia, close the two lever valves.

17. Release the sample or standard into the ~ystem with the proper valve

and stopcock •.

18. Close the left knob valve (~) and pressurize the si\mple or standard

wi th the cran.k on the pi stan cyl i nder (the. pressures uS.ed can vary,

dependi ng on the pressure and va 1 ume of the samples to be analyzed';

8 to 12 psia is commonly used by the authors).

19. Close the right knob valve (G).

20. When the red 1 i ght near READY on the keyboard 1 i ghts up, press the STAAT key RUN .

21. After you hear the val ve on the small cyl inder open and clpse, open

the right rear lever valve (E) to let the vacuum pump evacuate that

part of the system.

To repeat the run wjththe same gas:

22. Close the right rear lever valve (E, Figs. 4 and 5).

23. Open the right knob valve (G, Figs. 4 and 5).

24. Repressurize gas with crank.

25. Close right knob valve. . STAAT

26. Press RUN. when .GC is ready.

Note: Kunni n9Your standards twi ce at two different pressures is mandatory

to ensure that you have satisfactory data for the computations in Section.V.

Before running the next gas or standard, open the two center knob valves

and the right rear (E, Figs. 4 and 5) and left front (D, Figs. 4 and 5) lever

valves to evacuate the system. If you plan to run a sample, attach the bottle

to the rubber tubing and open the lever valve directly above it (make sure the

stopcock on the sample bottle is closed). When the entire system is

evacuated, repeat the outlined procedure with successive gases.

14

B. Run

1. Z. 3. 4. 5. 6. 7. 8. 9.

10. 11.

lZ. 13. 14.

15.

Z - Molecular Sieve with Argon - Detection of He and HZ

Attach the argon carrier gas in the back of the GC. Adjust the flows of A and B to equal 20. Set TEMP 1 = 300°C. Set OVEN MAX = 300°C. Set INJ TEMP = 50°C. Set TCD TEMP = 200°C. Let columns bake out overnight.

The following morning, check and adjust flows and turn on detector. Begin filling the liquid nitrogen dewar. Enter each of the parameters for Run Z found in Table I.

To delete the previous Time 2 and TEMP 2, press ~~J~ 0 ENTER. Adjust flows A and B to equal ZO at the new temperature (-40°C). Reconnect the filled nitrogen dewar. Evacuate the system following the previously outlined steps.

Run the samples and mixed standard when ready according to Steps A14 through AZ6.

C. Run 3 - Poropak with Helium - Detection of CH4, COZ' C2H6, and H2S

1. Attach He carrier gas and Poropak Columns A and B as described above.

Z. Adjust flows to equal 30. 3. Bake out overnight with TEMP 1 = ZOO°C, OVEN MAX = ZZSoc, INJ TEMP =

100°C, and TCD TEMP = 200°C. 4. Check and adjust flows the following morning. 5. Turn on GC detector. 6. Fill liquid nitrogen dewar while evacuating the system. 7. Enter each of the parameters for Run 3 found in Table I. B. Reconnect the filled liquid nitrogen dewar. 9. Introduce gases into system and pressurize as described above.

10. lfttiln the samples, the mixed standard, and the COZ standard at two different pressures.

'1'). :Peitential Difficulties ll~ ,If titre rpl:ressure gauge does not pump down to 0 ps i, there is a 1 eak

:S'emewhere in the system. Close off different sections of the system 15

16

with the various valves to see which sections do pump down to ° psi.

Once you have isolated the leaky section, pressurize the system, check

each junction and connection for leaks with "snoop," and fix the leak.

2. If air is in the CO 2 line, an early peak will occur at approxi­

mately 1.6 minutes when running the CO2

standard through the

Poropak setup. With the vacuum pump on, let a small volume of CO 2 flow through the system for about 1 minute to flush out air.

3. If air is in your sample, you will see a peak after Ar at about 5.4

minutes when using the hel ium-Molecular Sieve setup.

for leaks. Try tightening a clamp around the hose that

First, check

fits over the

sample bottle stem. See if the system pumps down to O. Finally, run

air through the system as a standard. Use this air analysis to

remove the effects of air contamination from the samples.

4. If water has entered the sample bottle, it will be detected as a peak

at about 14 minutes when us ing the hel ium-Poropak setup. Make a

mixture of ethanol and dry ice in an insulated container (dewar).

Place the ethanol/dry ice mixture around the coil of tubing (I, Figs.

4 and 5). When you let gases into the system, water will be frozen

out of the gas in this section of tubing. Before running a different

sample, heat the coil with a blow dryer and evacuate the system to

° psi using the vacuum pump. Sometimes the ethanol and dry ice

freeze out H2

S also; therefore, be careful to avoid this problem,

if possible.

5. When using the argon-Molecular Sieve setup, you may get a very shaky

base line. First, try heating the system to 250°C for 1/2 to 1

hour. If heating does not help, you will have to adjust the flows

through A and B manually using a bubble manometer so that they are

equa 1 .

6. If a sample is run and

detectable in the gas,

properly into the GC.

a straight line results, there may be nothing

or it may be that the gas was not injected

Reintroduce gas to the right half of the

injection system and repeat the run. Notice if the gas injection

valve opens and closes (L, Fig. 5). If the valve does not open and

close, be sure that the VLV/EXT commands have been entered. If a

straight 1 ine is sti 11 the result of a run, check to see that the

detector is on and run the sample at a higher pressure. Check to see

that the detector is working by running a standard through the GC.

7. After pressing START CHART, the base line should be vertically

straight. If the base 1 ine slopes, repeatedly and slowly adjust the

position with the fine and coarse knobs. If this procedure fails to

stabil ize the base 1 ine, decrease the slope sensitivity. Do not

decrease the slope sensitivity after running the standards or samples

unless you plan to run the standard or samples a second time using

the new slope sensitivity.

8. When running a standard, if significantly different retention times

result between runs on different days, the flow rates must not be the

same. The flows may not be what the GC indicates that they are. It

is not imperative that the retention times be the same on subsequent

days. However, if you wi sh the flows to be equal to those used on

previous days, adjust the flows manually with a flow meter or bubble

manometer.

9. Many other problems may be encountered during operation of a GC. If

the above 1 ist does not help solve your problem, consult a trouble­

shooter's guide written for your GC or.contact your repairperson.

V. COMPUTATIONS

The purpose of performing the following calculations is to determine the

"true" volume percent of each of the gas cons tituents. The GC response

(integrated area or counts) has a nearly linear relationship with pressure.

The slope of the line between the two pressures at which the gas stan­

dard was run must be determined. The y-intercept of the line, corre­

sponding to the integrated area intersecting the y-axis, must also be

determined to correct the obtained responses to true percents. The slope and

y-intercept must be computed for each gas constituent, H2, He, Ar, N2,

CO2

, CH4

, and H2S, using the data obtained by running the standard through

the GC. Each gas will have a different slope and y-intercept. Use the

tabulated slopes and intercepts calculated from the standard data to correct

your sample gases to their true percent compositions. Values used in the

following equations are in reference to the standard. The slope of the

standard line, m, for a single gas component is

17

In Eq.

Yl

Y2

Xl

x2

m =

( 1 )

=

=

=

=

( 1 )

the GC response of the standard, integrated area, obtained at the

lower of the two pressures used to run the standard.

the GC response of the standard obtained at the higher of the two

pressures used to run the standard.

the true volume percent of the particular gas constituent in the

standard.

the absolute pressure used to obtain Y2 divided by the absolute

pressure used to obtain Yl multiplied by the true percent, Xl.

This value may exceed 100%.

The y-intercept

b = Yl - m (xl) , ( 2)

where other symbols retain the same meanings.

To obtain the true volume percent of a particular gas constituent in the

sample (Xi)' we let

y. - b , m

( 3)

where

Yi = the GC response, integrated area, of the particular gas

constituent in the sample. This area must correspond to the same

pressure as the lower pressure used to run the standard. If this

pressure and the pressure used to run the sample are different,

multiply or divide the sample's integrated area by the proportion

of these two pressures, as the case warrants.

Example Calculations

See Figs. A-5 and A-6 for GC data from Men's Bathhouse mudpot,

Valles Caldera, New Mexico.

18

1. Average the He and H responses of the two 8-ps i standard resu1 ts and the

two 4-psi standard results.

He at 8 ps i

6648

6600

13248 -;- 2 = 6624 = Y2(He)

He at 4 psi

3286

3268

6554 -;- 2 = 3277 = Y1(He)

H2 at 8 psi

35920

35630 ---.-71550 -;- 2 = 35775 = Y2(H 2)

H2 at 4 psi

17510

17700

35210 -;- 2 = 17605 = Y1(H2)

2. Calculate the slope and intercept for he1 ium. The true percentage of He

in the standard is 0.80%.

m = (6624 - 3277) = 4183.75 (1.60 - 0.80)

b = 3277 - 4183.75(0.80) = -70

where X2 = 8(O.80L = 1.60 4

3. Calculate the slope and intercept for hydrogen. The true percentage of H2

in the standard is 2.81%.

m = 1l~U~_l7605'- = 6466.9 (5.62 - 2.81)

b = 17605 - 6466.19(2.81) = -565

where X2 = 8(2.81) = 5.62 4

4. Using the data obtained from the mudpot. Men's Bathhouse. Fig. A-6.

calculate the true volume percentage of H2 and He in the sample.

Yi for He = 480

Yi for H2 = 2792

xi(He) = 480_-___ (~70) = 0.131% 4183.75

xi (H2) = 279L~~C~56U = 0.519% 6466.19

19

Hence, there is 0.131% He and 0.519% H2 by volume in the geothermal gas from Men's Bathhouse mudpot.

5. Follow the same procedure as outlined in the example to determine the

percentages of any other gas constituents that may be found in the sample.

Sometimes water may contaminate a sample, and an extra peak will occur on the print-out when using the Poropak column. The extra peak, which occurs

after H2S, must be normalized out before doing the pr"evious calculations. The procedure used to el iminate the effects of water are described by us ing an example for clarification (see Fig. A-7).

1. Add all GC percentages, except the water percentage. Let us assume we

obtain three peaks having the following percentage areas.

0.103% 98.232 0.744 ---

99.079%

2. Divide the previous sum by 100.

99.079% = 0.99079 100

3. Divide one by the value from Step 2 to obtain the normalizing factor.

1 = 1 .0092956 0.99079

4. Multiply the integrated area of each of the remaining constituents by the

factor obtained in Step 3.

20

2826 x 1.0092956 = 2852 2704000 x 1.0092956 = 2729135 20490 x 1.0092956 = 20680

5. Use these corrected (normalized) integrated areas to calculate the true

percentages, us ing the slopes and intercepts cal cul ated from the standard

run on the same day.

ACKNOWLEDGMENTS

The second author wishes to thank Cathy Janik and William Evans of the

U.S. Geological Survey, Menlo Park, California, for teaching him their own

special ized methods of geothermal gas analysis. The Los Alamos system is a

hybrid design of their two vastly different procedures. Jamie N. Gardner

kindly reviewed the manuscript.

REFERENCES

D'Amore, F., and Panichi, C., 1980, "Evaluation of deep temperatures of hydrotherma 1 sys terns by a new gas geothermometer," Geochim. Cosmochim. Acta 44, 549-556.

Evans, W. C., Banks, N. G., and White, L. D., 1981, "Analysis of gas samples from the summit crater," in P. W. Lipman and D. R. Mullineux (eds.), The 1980 Er1Jtions of Mount-S-t. Helens, Washington, US Geol. Surv. Prof. Paper 125 , pp. 221-231.

Fournier, R. 0., 1981, "Application of water geochemistry to geothermal exploration and reservoir engineering," in L. Rybach and L. J. P. Muffler (eds.), Geothermal Systems: Principles-and Case Histories, John Wiley and Sons, New York, pp. 109-143.

Goff, F., and Vuataz, F., 1984, "Hydrogeochemistry of the Qualibou Caldera geothermal system, St. Lucia, West Indies," Trans. Geotherm. Res. Council §.' 377-382.

Goff, F., Gardner, J., Vidale, R., and Charles, R., 1985, "Geochemistry and isotopes of fluids from Sulphur Springs, Valles Caldera, New Mexico." J. Volcano Geotherm. Res. (in press).

Norman, D., and Bernhardt, C., 1981, "Assessment of geothermal reservoirs by analysis of gases in thermal waters," Final Technical report, New Mexico Energy Institute, New Mexico State University, Las Cruces, 130 pp.

Supelco, 1980, "Column selection for gas and light hydrocarbon analysis," Bulletin 786A, Supelco, Inc., Supelco Park, Bellefonte, Pennsylvania, 12 pp.

Thompson, B., 1977, "Fundamentals of gas analysis by gas chromatography," Technical report, Varian Corp., 611 Hansen Way, Palo Alto, California, 94 pp.

21

APPENDIX

GC EXAMPLE PRINT-OUTS

Figures A-l through A-7 are examples of the GC print-outs for each run setup.

~179 2.67

RT AREA AREA" 1.79 3650 4.092 2.67 85540 95.908

Fig. A-l. Example response of the Mol ecul ar Sieve 5A with argon carrier gas using a mixed standard.

~===-- Ar 5.06

__ --------------------'J CO2

21.51

RT AREA AREA" 1.88 672 0.035 5.06 12900 0.671 9.39 94140 4.896

10.46 70040 3.643 21.51 1745000 90.755

Fig. A-2. Example response of the Molecular Sieve 5A with helium carrier gas using a mixed standard.

22

Fig. A-3.

~;;H;2~1;.3;4~~~~~~~~~~~~~=IllC=: N2 1.59 ~ CH4 2.53

_---------------------' CO2 4.61

~===================-- H2S 12.42

RT AREA AREA %

1.34 1941 0.048 1.59 206600 5.096 1.72 35820 0.883 2.53 157100 3.875 4.61 3479000 85.807

12.42 174000 4.292

Example response of the Poropak mixed standard.

QS with hel ium carrier gas

Air 1.61

__ ---------------------' CO2 4.41

RT 1.61 4.41

AREA 4058

5686000

AREA %

0.071 99.929

using a

Fig. A-4. Example response of the Poropak QS with helium using CO2 standard.

23

8 psia

t He 1.14

H2 1.65

RT AREA AREA %

1.14 6648 15.617 1.65 35920 84.383

\ He 1.14

H2 1.65

RT AREA AREA %

1.14 6600 15.629 1.65 35630 84.371

4 psia

FHe 1.14

H2 1.63

RT AREA AREA %

1.14 3286 15.801 1.63 17510 84.199

f== He 1.13 H2 1.63

RT AREA AREA %

1.13 3268 15.586 1.63 17700 84.414

Fig. A-S. Argon with Molecular Sieve SA example response at (from top) 8 pSi, 8 psi, 4 pSi, and 4 psi.

to a mixed standard

24

Fig. A-6.

Men's Bathhouse Mudpot, 8 pSia

~ 1.13 I H2 1.61

RT

1.13 1.61

AREA

480 2792

AREA %

14.670 85.330

Argon with Molecular Sieve 5A example response to a sample (Men's Bathhouse mudpot) at 8 psi.

Air 1.63

~==========================================:=JC02 4S4

H2S 12.65

H2O 14.79

RT AREA AREA %

1.63 2826 0.103 4.84 2704000 98.232

12.65 20490 0.744 14.79 25340 0.921

Fig. A-7. The Poropak QS with helium response to water contamination,

'* u.s. GOVERNMENT PRINTING OFFICE 1965-57&034120109 25