Bureau of Mines Report of Investigations/l982 

Methodology for Determining Occluded 

Gas Contents in Domal Rock Salt 

By David M. Hyman 

UNITED STATES DEPARTMENT OF THE INTERIOR 
Bureau of Mines 

Report of investigations 8700 

METHODOLOGY FOR DETERMINING OCCLUDED GAS CONTENTS IN DOMAL ROCK SALT 

By David M. Hyman 

ERRATA 

On page 10, the first equation in the left-hand column should read as follows: 

(Pbi + Pvi)(Vgi) - (P)(Vi) 

Ti T ' 
UNITED STATES DEPARTMENT OF THE INTERIOR 

James G. Watt, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
This publication has been cataloged as follows: 

Hyman, 11, M. (1)ucid bl,) 

31c.rliodology tor dc~crrnining occ.1udc.d gas c.ollrc,nrs in do~ilill I.OC.~ 

sal~. 

(Iteport oT invc.stigntions / I'niretl Srarcs l)c~~artrnc~r~r oThc Inrc.- 

r ior , Rure au of Min c.s ; 8700) 

I3 ibl iogrnphy: p. I I. 

Supr. of 1)oc.s. no.: 1 28.2;5:8700. 

1. Koc:k-salt-An:~lysis. 2. Xlcti~,~ric.. 3. llinc. Saxe\. I. 'fitlr. 11. 

Scrics: lteport oT invcsrigationx (Ljnited Sriires. 13urc.a11 of Mines) ; 13700. 

'I'N23.1143 [TN900-1 622s 1622.1'82851 8%-fi00 182 t\A('It:! 

I . 
CONTENTS 

Page 

Abstract ....................................................................... Introduction ................................................................... Acknowledgments ................................................................ Background ..................................................................... Domal rock salt samples ........................................................ Dissolution method ............................................................. Experimental apparatus .................................................... Experimental procedure .................................................... Crushing method ................................................................ Experimental apparatus .................................................... Experimental procedure .................................................... Summary and conclusions ........................................................ References ..................................................................... 

1 . Schematic of dissolution method apparatus .................................. 2 . Schematic of crushing method apparatus ..................................... 
METHODOLOGY FOR DETERMINING OCCLUDED GAS CONTENTS 

IN DOMAL ROCK SALT 

By David M. Hyrnan 

ABSTRACT 

The Bureau of Mines has developed two experimental designs for determining the volumes and relative proportions of the various gas species, 

particularly methane (CH4), occluded in rock salt. To date, the rock 

salt samples analyzed have been relatively small (400- to 1,000-gram) 

grab samples from Louisiana Gulf Coast domal salt mines. The results 

obtained are comparable to results in Polish domal salt mines, in terms 

of occluded gas volumes per mass of rock salt. The notable difference 

between the Louisiana and Polish occluded gas mixtures is in the relative proportions of the individual gas species. 

The two methods used by the Bureau are ballmill crushing and dissolution. Of the two, the dissolution method appears to be more versatile 

because it can be performed in situ. The relevance and applicability of 

data on the occluded gas contents of rock salt are dependent on sample 

size, sample collection method, geologic context of the sample, and the 

correlation with gas emissions that occur as a result of mining-induced 

fractures and pressure differentials. 

'~eolo~ist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 
INTRODUCTION 

The Bureau of Mines became interested 

in developing information concerning the 

occurrence and distribution of gases, 

particularly methane (CH,), in the various salt dome lithologies, as a result of 

increased concern about potentially hazardous methane occurrences in Louisiana 

Gulf Coast domal salt mines. Basic 

information concerning the types of 

gases, their relative abundances, and the 

circumstances relating to their occurrences and distribution in North American 

domal salt mines is generally lacking in 

the literature. A notable exception is a 

study (?I2 that was performed on gases 

contained in gas-enriched salt from a 

salt dome in central Louisiana. Methane 

and other gases have been reported 

to occur in the domal salt mines of 

Poland (3). Various gases, such as 

hydrogen, methane, nitrogen, and hydrogen 

sulfide have been found in the domal 

salt formations and bedded evaporite 

deposits of Louisiana, New Mexico, and 

Poland (1, 3, 5, 7). ---- 

Given the relatively impermeable nature 

of salt, the occluded gases generally 

are not released from their containment unless mining activities intercept 

(1) a gas-filled fissure zone, an area 

where the voids between the salt crystals 

are interconnected, (2) a mechanically 

unstable zone of gas-enriched salt 

that disaggregates, releasing its entrained gases (a blowout), or (3) some 

other relatively permeable geologic 

anomaly (1, 7). A siiliple illustration of 

the nature oi gas occlusion in rock salt 

is seen when a piece of gas-enriched rock 

salt is immersed in water. The rock salt 

will "pop" sporadically, releasing gas 

bubbles as it dissolves. This simple 

demonstration is the foundation for one 

method of determining the gas content of 

a rock salt sample. The sometimes rather 

energetic "pops" as gases are released 

from a gas-enriched rock salt sample 

attest to the high pressures under 

'underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 

which the gases can be occluded. The 

pressures postulated for these occluded 

gases are lithostatic but could be higher 

depending on local stresses related to 

the low creep limits of rock salt (I), - 

particularly around mine openings. 

However, a gas pressure was estimated 

by melting a gas-enriched salt sample in 

a sealed container and back-calculating 

the occluded gas. This calculated value 

was 500 to 1,000 atm (at 0' C) and 

occurred in "bubbles" within the salt 

crystals of domal rock salt that had been 

ejected from a blowout at a depth of 

about 244 m (800 feet) (5). The lithostatic load corresponding-to this sample 

was calculated to be about 50 to 60 atm. 

It is possible that this difference 

between the gas pressure and the lithostatic load was due to gross experimental 

or sampling error, locally high stress 

concentrat ions in the mine structure 

where the blowout occurred, or some other 

mechanism(s) not considered. 

As part of its program to promote mine 

safety, the Bureau of Mines has undertaken two main areas of research in order 

to assess the species of gases released 

during mining, their modes of occurrence, 

their quantities, and their characteristics of emission into mine openings. The 

first area concerns the reservoir characteristics (volume, permeability, etc.) of 

fissure and anomalous zones or other 

areas of significantly interconnected 

void spaces in domal rock salt, where any 

gases present will migrate in response to 

a pressure gradient. The second area 

concerns the quantities of the various 

gases occluded in the rock salt itself. 

In this mode of occurrence, the gases 

will be released when the rock salt mass 

is mechanically disturbed or dissolved. 

This report addresses the second area 

of research. It presents two methods 

developed by the Bureau's Pittsburgh 

Research Center for determining the gas 

contents of rock salt, as well as some 

preliminary data on small samples from 

Louisiana Gulf Coast domal salt mines. 
ACKNOWLEDGMENTS 

The author wishes to acknowledge gratefully the technical contributions of 

David C. Oyler and Patricia M. Erickson 

of the Methane Control Group and the 

Environmental Control Group, respectively, at the Pittsburgh Research Center. Sylvester Sudduth, Steven Schatzel, 

and Deborah Burse, of the Methane Control 

Group, participated in the development of 

the ballmill crushing method, and Helen 

Lang of the Fire and Explosions Group in 

the development of the dissolution 

method. The personnel of the Louisiana 

Gulf Coast Five Islands Salt Mines (Jeff erson Island, Avery Island, Weeks 

Island, Cote Blanche, and Belle Isle) 

permitted access and sampling and contributed their time, information, experiences, and cooperation toward this and 

related research efforts in understanding 

the nature an,d effects of gases in domal 

salt mines. The sum of these persons' 

efforts, along with others, has made this 

study and, it is hoped, future studies 

possible. 

A number of gases are occluded in varying proportions in rock salt. Some 

results of occluded gas contents analyses 

performed on rock salt samples from Polish salt domes are reported in reference 3. One suite of analyses using an 

undescribed dissolution technique yielded 

mean contents of 2.85 cm3 N2, 0.23 cm3 

C02, 0.10 cm3 H2, and 0.06 cm3 CH4 per 

100 grams NaC1. In one Polish mine, 

nitrogen was the dominant gas occluded 

in salt, with up to 10.65 cm3 N2 per 

100 grams NaCl reported. The maximum 

occluded methane content given for this 

mine was 0.38 cm3 per 100 grams NaC1. 

Another Polish domal salt mine reported 

maximum occluded gas contents in rock 

salt samples (dissolved in water) of 

5.17 cm3 H2, 1.18 cm3 C02, and 0.03 cm3 

CH4 per 100 grams NaC1. Data in reference 5 portray a carbon dioxide-dominant - gas mixture occluded in the "popping" 

salt from a blowout. The carbon dioxide 

and methane concentrations were 46.9 pct 

and 1.5 pct, respectively. No data for 

gas content per mass of salt were given 

for this analysis. 

Preliminary analysis has been done by 

the Bureau of Mines on Gulf Coast Louisiana domal salt. This study of the composition of gas mixtures occluded in rock 

salt ejected from blowouts has yielded a 

maximum of 8 to 10 cm3 CH4 per 100 grams 

rock salt, with lesser quantities of 

other gas species. The minimum occluded 

methane content was less than 0.1 cm3 CH4 

per 100 grams of rock salt. The occluded 

methane contents for the "popping" rock 

salt analyzed to date range from about 

0.6 to 10 cm3 CH4 per 100 grams rock 

salt. Conversion of these determinations 

to 100 grams NaCl does not significantly 

change these values, owing to a very high 

assay of NaCl for the rock salt samples 

tested. In terms of volumes of occluded 

gases per mass of rock salt, the results 

of the Bureau's work are comparable to 

the values published in reference 3. The 

apparent differences are in the rFlative 

proportions of the gas species occluded 

in the salt. 

An important consideration is that 

these occluded gas contents of rock salt 

may not represent the gas mixtures 

released during a blowout, emitted 

through boreholes penetrating interconnected void systems, or emitted through 

fissure systems intercepted by mining. 

The potential relationship of the 

occluded gas to gas released or emitted 

during mining is currently being explored 

by the Bureau, with comparisons between 

occluded gas contents of cored samples 

and corehole gas emissions. Data on 

these occluded gas contents will be useful if they can be correlated to a significant degree with mine atmospheres, 

blowout-prone zones, or gas emissions 

from drill holes in conventional mining 

operations. As the sampling process 

improves in terms of representative Sample size and representative in situ 
conditions through sophisticated core 

drilling techniques and reservoir mechanics analysis, the probability of useful 

application of these data should 

increase. 

The gas emissions during mining should 

depend on the characteristics of the 

occluded gas contents of a representative 

sample combined with the reservoir characteristics of the interconnected void 

spaces and fissure systems. To build a 

data base for use in predicting in-mine 

emissions, the Bureau has initiated a 

program to analyze systematically a variety of domal salt lithologies with 

respect to gas contents, geochemistry, 

and geologic context. 

DOMAL ROCK SALT SAMPLES 

To date, the rock salt samples have 

been gathered from mine faces, ribs, and 

salt piles of known origin. These are 

essentially grab samples with a maximum 

mass of about 3.5 kg (7.7 pounds), which 

have lost any gases not occluded within 

the salt crystals and/or confined in 

intergranular void spaces. The exposure 

of these salt samples to the mine atmosphere has allowed the rock mass to 

"loosen" over time along grain boundaries 

and fractures because of the effects of 

humidity and loss of original in situ 

confining pressure. Thus, the gas contents of this suite of samples represent "residual" occluded gas contents. 

Although this residual gas represents 

only a portion of the gases in domal 

salt, this is the current state of the 

art. This residual gas content of rock 

salt is not directly analogous to the 

residual gas in coal (4), - owing to 

different entrainment modes. Methane has 

no real affinity for or interaction with 

pure salt; however, the presence of bitumen changes this (2). - 

The state of the art will advance with 

the acquisition of larger (with respect 

to grain size) core drilled samples that 

are maintained under in situ pressure and 

sealed from atmospheric effects until 

analyzed. As these cored samples are 

recovered, gas pressure measurements and 

gas samples should be taken from the 

corehole to characterize the interconnected void spaces and their gases and 

thus maximize the information presented 

by the corehole. In short, the reliability of a gas content determination 

depends upon the size and physical 

integrity of the sample analyzed, along 

with its geologic context and in situ 

conditions. 

DISSOLUTION METHOD 

Two methods are used by the Bureau of 

Mines to determine the gas contents of 

rock salt samples. One is based on the 

dissolution of the rock salt mass in 

a sealed chamber with distilled and 

degassed water, in a helium atmosphere. 

Occluded gases are released during dissolution and collected for analysis by 

gas chromatography. This method allows 

the gas volume to be normalized to either 

the mass of the rock salt or to the mass 

of the water-soluble NaC1. There are two 

major advantages to this dissolution 

method. One is the relatively inexpensive nature of the apparatus. The other 

is that this test can be performed at any 

scale, from single crystal samples to a 

solution mining operation. 

The dissolution chamber can range in 

size from a laboratory 10-ml test tube to 

a commercially available desiccator to an 

aircraft fuel tank bladder (in the 

extreme). All that is minimally required 

is that the opening for introducing the 

salt sample be airtight when closed and 

that there be two valve fittings (or at 

least one if the dissolution chamber is a 

flexible bladder type). 

This dissolution method can also be 

performed in situ. A borehole can be 

cased with an annular pipe arrangement 

for injecting water and extracting the 

resulting brine and/or gases. These 

released gases would represent both 

occluded and fissure-interconnected void 
space gases. The pipe set should be 

cemented and able to withstand a pressure test to at least the hydrostatic 

pressure for that depth. Provisions 

should be made for collecting gas and 

shutting off gas flows. During and after 

the solution process and cavity development, the cavity can be monitored for 

brine and/or gas flows from afiy intercepted fissure-interconnected void zones. 

A core sample from the solution cavity 

salt should be extracted and analyzed 

for occluded gas content prior to 

dissolution. 

The solution cavity, if of sufficient 

size, can possibly be used to initiate 

the disaggregation of mechanically unstable gassy salt and/or to drain 

intercepted fissure zones, provided 

the resulting flow of gases can be 

controlled at the drill hole collar. A 

Graduated 

container qdiy:-] 

disadvantage is that brine-soluble gas 

determinations can become complicated 

unless the gas solubilities in brine are 

known. When the dissolution has been 

completed, the brine can be subjected to 

geochemical analysis and the waterinsoluble residues can be f il-tered, 

weighed, and examined. Any liquid hydrocarbon can be separated and analyzed. 

Experimental Apparatus 

The dissolution method essentially captures gases released from a weighed sample of rock salt as it dissolves in a 

sealed chamber that has had degassed, 

distilled water introduced into it. Figure 1 is a schematic representation of 

the experimental apparatus. The desiccator serves as the reaction vessel and 

gas collection chamber. It must have at 

least two fittings on the top for hoses 

Tank valve Thermometer 

Desiccator pressu 

Exhaust vent or 

gas sampling por 

-- 

FIGURE 1. - Schematic of dissolution method apparatus. 
and/or valves and a provision for a temperature probe (thermometer) if the distilled water is not at room temperature. 

The intake valve has two switchable 

inputs with one outlet to the reaction 

vessel. One of the inputs is for the 

inert gas (helium) that is used to purge 

(or at least strongly dilute) the air 

atmosphere in the desiccator. The purpose of purging the reaction vessel 

atmosphere is to reduce the background 

interference of the various gases in air. 

This is essential, owing to the relatively small volumes of various gases 

released from the rock salt sample in 

comparison with the volumes of atmospheric gases in the vessel atmosphere. 

The other input on this valve is used to 

introduce the degassed, distilled water 

from a graduated container (buret or 

graduated cylinder) into the reaction 

vessel. 

The output valve on the other port of 

the desiccator top has the same type of 

valve as the input but configured as one 

inlet from the reaction vessel to two 

switchable outputs. The first output 

goes to a pressure gage for monitoring 

gas pressure in the vessel. The second 

is used to sample the gases in the vessel 

atmosphere and also as an exhaust vent to 

release displaced gases when the vessel 

atmosphere is purged. 

A sample of the vessel atmosphere 

should be taken before the degassed, distilled water is introduced, to obtain 

background or baseline measurements of 

the gases in the vessel. The occluded 

gases released from the rock salt will be 

superimposed on this background during 

the dissolution process. When the degassed, distilled water is introduced 

into the vessel, an expansion chamber of 

known volume can be attached to the exhaust vent to collect the displaced 

atmosphere and the first release of 

occluded gases. 

The rock salt sample is elevated from 

the bottom of the vessel by a sample stand so that it will be exposed to 

relatively "fresh," unreacted water. 

This is because the brine that results 

from the dissolved portion of the salt is 

denser than the fresh, unreacted water 

and settles to the bottom of the vessel. 

Without a sample stand, the rock salt 

sample will be in contact with brine, 

which will inhibit dissolution unless the 

solution in the vessel is frequently 

agitated. 

Experimental Procedure 

The procedure, with some typical values, for the Bureau prototype apparatus 

is as follows: 

1. The rock salt sample (300 to 

1,000 grams) is weighed and placed on the 

sample stand in a reaction vessel with an 

internal volume (Vv) of about 3,000 to 

6,500 cm3. 

2. The top of the reaction vessel is 

greased, installed, sealed, and secured. 

The top must be secured to withstand an 

applied differential positive pressure 

(1 to 30 mm Hg) used to preclude leakage 

from outside the reaction vessel. 

3. The exhaust vent and inlet valve 

for helium are opened. The tank valve is 

opened, with tank regulator set at about 

69 kPa (10 psig), and helium is flowed 

through the reaction vessel. The time of 

flow should be experimentally determined 

for the vessel used. For a 3,310-cm3 

vessel, 6 minutes of flow at about 69 kPa 

diluted the nitrogen concentration in the 

vessel atmosphere from about 78 pct to 

about 1 to 2 pct. The purging is stopped 

by closing the helium tank valve, the 

inlet valve, and the exhaust vent. 

4. After purging the sampling circuit 

with helium and just prior to the introduction of the degassed, distilled water 

to the reaction vessel, temperature (Ti), 

barometric pressure (Pbi), and vessel 

internal pressure (Pvi) measurements are 

made, and two gas samples are taken 

through the sampling port. This establishes the "before" condition of the 

experiment. 
5. Because the solubility of NaCl in 

room-temperature water is about 37 grams 

NaCl in 100 grams H20, the volume of 

degassed, distilled water to be prepared 

can be estimated from the sample weight. 

The appropriate volume of water is introduced into the reaction vessel by opening 

the inlet valve and then the exhaust port 

as desired to run the experiment at 

either atmospheric or positive pressure 

conditions. Liberal volumes of distilled 

water are recommended to insure complete 

dissolution of the salt. The final volume of water in the vessel is Vw. 

6. When the dissolution of the rock 

salt sample is coxplete, the reaction 

vessel temperature (Tf), barometric pressure (~bf), and vessel internal pressure 

(~vf) measurements are made, and two gas 

samples are taken through the sampling 

port. This establishes the "after" condition of the experiment. 

7. The reaction vessel is opened, and 

samples of the brine are taken for geochemical analysis. The water-insoluble 

residues are washed out of the vessel, 

filtered, weighed, and preserved for 

geochemical and/or mineralogic analysis. 

8. The reaction vessel is washed in 

distilled water. 

The data reduction method is presented 

below. 

The volume that the gases occupy before 

the distilled water is added--the volume 

of gases initially (Vgi)--is 

Vgi = Vv - V salt, 

where Vv = internal volume of desiccator (and expansion chamber 

if used) with sample stand, 

and V salt = volume of salt = (sample 

weight)/(density of rock 

salt). 

To approximate the density of rock salt, 

a value of 2.16 g/cm3 was chosen because 

it represented the median of a range of 

values (2.12 to 2.204 g/cm3) presented in 

references 2, 5-6. More explicit methods - -- 

to determine the density or volume of 

rock salt can be used. 

The value for Vgi is corrected to 

standard temperature and pressure (STP) 

conditions (Vi) by 

where Pbi = initial barometric pressure, 

Pvi = initial vessel internal 

pressure, 

Ti = initial temperature, 

P = 760 mm Hg, 

and T = 273 K. 

The gases under consideration for the 

pressures and temperatures quoted in this 

report behave essentially as ideal gases. 

To obtain the background individual gas 

species volumes, the concentrations of 

each gas, as determined by analysis of 

the "before" gas samples, are multiplied 

by the Vi. 

The volume that the gases occupy after 

dissolution (Vgf) is 

where Vw = volume of water, 

and Vd = volume of salt dissolved 

= (V salt) (100 pct - percent volume decrease). 

The percent volume decrease of a saturated aqueous solution of NaCl at 0" C is 

about 25.9 pct; at 25" C, 20.4 pct; at 

50.4" C, 17.9 pct (2, 6). For less than 

saturated solutions~ tKe molar concentration must be calculated and the corresponding volume decrease value found in 

the aforementioned references. 

The volume Vgf is then adjusted to STP 

conditions (Vf), as was done in the 

"before" case, to include water vapor 

pressure correction. The "after" volume 
occluded gases released when the rock The procedure, with some typical valsalt sample is mechanically crushed to ues, for the Bureau prototype apparatus 

powdered form. The stainless steel ball- is as follows: 

mill is analogous to the desiccator 

(reaction vessel) used in the dissolution 1. The rock salt sample (600 to 

method. Figure 2 is a schematic of the 1,000 grams) is weighed and placed 

crushing method apparatus. The design of in a ballmill with a 3,340-cm3 internal 

the Bureau ballmill is described in volume (Vv). 

2. The grinding media (ball bearings, 

rods, etc.) of known volume (about 

725 cm3) are added. Glass wool is placed 

inside the fittings in the top of the 

mill, and the top is secured to the body 

of the mill. The glass wool acts as a 

Exhaust vent filter to prevent salt dust from plugging 

the fittings and valves. 

3. The output valve-exhaust vent and 

Ballmill 

OI7 . ".. - media about 69 kPa (10 psig) to flow into the 188eoo~~ u ballmill to purge the air atmosphere. 

For a 3,340-cm3 ballmill, about 6 minutes 

FIGURE 2. - Schematic of crushing method apparatus. of purging is required to lower the 

Grinding 

He 

the input valve are opened; the selector 

valve is closed; and the tank valve is 

opened to allow helium at a pressure of 
nitrogen concentration in the atmosphere from about 78 pct to about 1 to 

2 pct. 

4. The output valve-exhaust vent is 

closed while the helium tank valve is 

slowly turned off. This step applies a 

slight positive pressure to the interior 

of the ballmill. The two main reasons 

for this positive pressure are to avoid 

inflowing contamination from outside the 

ballmill and to aid in gas sample extraction. The selector valve is switched to 

obtain the internal pressure (Pvi), using 

a water manometer. This differential 

pressure is in the 34- to 52-cm water 

gage (W,G,) (13- to 20-inch W.G,) range. 

A mercury manometer could be used with a 

corresponding differential pressure range 

of 25 to 38 mm Hg. A barometric pressure 

measurement is also made at this time, 

along with temperature (Ti). 

5. The selector is switched to the gas 

sampling port. After the sampling circuit input valve, selector valve, and gas 

sampling port are flushed with helium, 

two gas samples of the ballmill atmosphere are taken (using 20-1 evacuated 

containers). The ballmill internal pressure is remeasured. Two 20-1 evacuated 

containers are used for gas sample collections, and the differential pressure 

decreases by about 7.8 to 25.4 cm W.G. 

(3 to 10 inches W.G.). The second manometer (Pvi) reading added to the barometric pressure (Pbi) (with temperature correction) represents the "before" 

character of the interior of the 

ballmill. 

6. The ballmill is disconnected from 

the valve assemblies and placed on the 

roller machine to rotate for about 1 hour 

and crush the salt. 

7. After crushing, the ballmill is 

removed from the roller machine and 

allowed to equilibrate with the room 

temperature. A thermometer is set on 

top of the mill to insure that the ballmill and contents come 

temperature. 

to room 

8. When the ballmill has reached room 

temperature, the sampling circuit is 

again flushed with helium, and the valve 

assemblies are reconnected to the mill. 

A ballmill differential pressure measurement is made with the manometer (Pvf), 

along with barometric pressure (Pbf) and 

temperature (Tf) measurements. Two gas 

samples of the ballmill atmosphere are 

taken. These measurements and samples 

represent the "after" character of the 

interior of the ballmill. A postsampling 

manometer reading of the remaining differential pressure is taken to insure 

that the "after" gas samples were satisfactorily extracted. Any remaining differential pressure in the ballmill is 

then removed by opening the output valveexhaust vent. 

9. The ballmill top is removed, and 

the salt and grinding media are emptied 

into a sieve stack with pan. A typical 

sieve stack made of stainless steel is 

composed of 112-inch (12.7-mm), lo-mesh 

(2-mm), 20-mesh (841-pm), and 40-mesh 

(420-pm) sieves and pan. The sieve fractions are weighed separately. The sieve 

analysis is required to characterize 

the degree of efficiency of the crushing. 

These data are also required to determine the exact weight of crushed salt 

that releases its occluded gases. Normally the minus 20-mesh salt is used 

to normalize the gas contents to salt 

mass. 

10. A representative fraction of the 

crushed salt is submitted for geochemical 

analysis. 

The data reduction method is presented 

be low. 

The volume that the gases occupy before 

the rock salt is crushed (Vgi) is 

Vgi = Vv - (V salt + V grinding media), 
10 

where Vv = internal volume 

of ballmill 

(3,340 cm3), 

V salt = (sample weight)/ 

(density of 

rock salt), 

and V grinding media = volume of grinding media. 

An explicit determination of the density of rock salt or the volume is preferable to using an assumed density of 

2.16 g/cm3 as discussed in the section 

"Dissolution Method." 

The volume Vgi is corrected to standard 

temperature and pressure (STP) conditions 

(Vi) : 

(Pbi + Vgi)(Vgi) - (P)(Vi) 

Ti T' 

where Pbi = initial barometric pressure, 

Pvi = initial ballmill internal 

pressure, 

Ti = initial temperature, 

P = 760 mm Hg, 

and T = 273 K. 

As in the dissolution method, the STP 

volumes of the individual gas concentration and Vi are calculated. 

The volume that the gases occupy after 

crushing (Vgf) is 

(Pbf + ~vf)(Vgf) - (P)(Vf) - Tf T 

where Pbf and Pvf = final barometric and 

ballmill pressures, 

respectively, 

Tf = final temperature, 

and Vf = STP adjusted final 

gas volume. 

The volumes of gases are then calculated using Vf and gas concentrations 

determined from the "after" gas Samples. After subtracting background 

(the "before" conditions), the STP volumes of individual or groups of gases 

(for example, hydrocarbons) are normalized to the mass of the appropriate 

sieve fraction (for example, minus 

20 mesh) of rock salt or NaCl under 

consideration. 

SUMMARY AND CONCLUSIONS 

The Bureau of Mines has developed two 

methodologies to determine the occluded 

gas contents of domal rock salt, although 

these methods are not necessarily 

restricted to domal rock salt samples. 

The usefulness of the occluded gas content data from either the dissolution 

method or the crushing method is determined by the size and collection method 

of the subject sample and the correlation 

of the occluded gas content data with gas 

emissions into the mine atmosphere as a 

result of mining activities. Of the two 

methods, the dissolution method appears 

to be more versatile because it can 

be performed either in situ, through an 

exploratory drill hole, or on a sample 

the size of which is limited only by the 

size of the dissolution chamber. 

The results of occluded gas determinations can be applied to further studies 

on the correlation of occluded gas with 

gas emissions into the mine atmosphere 

during the mining of rock salt for which 

the occluded gas contents and drill hole 

gas emission characteristics have been 

determined. This correlation will allow 

the mapping of gas emission characteristics as a function of geology and thus 

provide a prediction model of gas occurrences in a salt dome through use of the 

results of these laboratory procedures. 
REFERENCES 

1. Baar, C. A. Applied Salt-Rock Mechanics. Elsevier Scientific Publishing 

Co., Amsterdam, v. 1, 1977, 294 pp. 

2. Baxter, G. P., and C. C. Wallace. 

Changes in Volume Upon Solution in 

Water of the Halogen Salts of the 

Alkali Metals 11. J. ACS, v. 38, 1916, 

pp. 70-105. 

3. Cybulski, We, C. Gorol, T. Gotkowski, and J. Sobala. Charakterystyka 

Zagrozenia Metanowego w Polskich Kopalniach Soli (Characteristic Features of 

Methane Hazard in Polish Salt Mines). 

Pr. G1. Inst. Gorn., Komun., No. 567, 

1972, 20 pp. (Safety in Mines Research 

Establishment Transl. No. 6344.) 

4. Diamond, We P., and J. Re Levine. 

Direct Method Determination of the Gas 

Content of Coal: Procedures and Results. 

BuMines RI 8515, 1981, 36 pp. 

5. Hoy, Re Be, Re Me Foose, and Be J. 

O'Neill, Jr. Structure of Winnfield Salt 

Dome, Winn Parish, Louisiana. Am. Assoc. 

Pet. Geol. Bull., v. 46, NO. 8, 1962, 

pp. 1444-1459. 

6. Kaufmann, D. We Sodium Chloride. 

Reinhold Publishing Corp., New York, 

1960, pp. 592, 623. 

7. Kupfer, D. H. Problems Associated 

With Anomalous Zones in Louisiana Salt 

Stocks, USA. Paper in Fifth Symposium on 

Salt, Hamburg, Germany, 1979. Northern 

Ohio Geological Society, Inc., Case 

Western Reserve Univ., Cleveland, Ohio, 

v. 1, 1980, pp. 119-134. 

*U.S. GOVERNMENT PRINTING OFFICE: 1982 - 505 - 002/69 INT.-BU.OF MIN ES,PGH.,PA. 26288