Gas Storage Organic Shales

7/30/2019 Gas Storage Organic Shales

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The Laboratory Measurement of the Gas St orageCapacity of Organic ShalesRichard F. Sigal, I. Yucel Akkutlu, Seung Mo Kang, Mery Diaz-Campos

(MPGE, University of Oklahoma) and Ray Ambrose (Devon Energy)

AbstractA significant part of the gas storage in organic shales is in nanometer scale pores locatedin the organic material. These pores have gas species-specific adsorption on the surface

of the pore walls and are in material that may have significant pore volume dependenceon pore pressure. In the smaller pores molecular dynamic calculations for methane show

that the storage model that consists of a single high density adsorbed gas layer and a freegas component that obeys the equation of state for bulk methane is only an

approximation.

The volume of the adsorption layer, which reduces the pore volume available for free gas

storage, is a function of gas species, temperature and pressure. This along with porevolume compressibility require that gas storage be measured with the reservoir gas under

reservoir conditions on a solid core sample. The current methods that use ground upsamples to measure adsorption with the reservoir gas but pore volume with helium do notsatisfy the requirements for an accurate gas storage determination.

To address these issues, a new methodology to measure total gas storage on a coresample at reservoir conditions is described. A method to model the measured storage as

an adsorbed component and free component is developed. For methane an equation toextract an average pore radius from the modeled adsorbed-state density is developed. The

methodology is illustrated on a measurement on an organic shale sample.

IntroductionOrganic shales have a complex gas storage system. The organic material in the shales isnow understood to contain a complex nanometer scale pore system where gas is stored

(Passey et al., 2010, Sondergeld et al., 2010). The walls of these pores are adsorptionsights for gas (Ambrose et al. 2010). As pore pressure increases the adsorbed gas volume

increases, which decreases the pore volume available for free gas in the organic pores.The adsorbed gas mass decreases with temperature. The volume taken up by adsorbed

gas can in principle close pore throats and restrict access to pores as pressure increases.This effect is gas species specific, as it depends both on the volume of adsorbed gas, and

the effective size and geometry of the molecular species. A further complication is that inthe smaller pores molecular dynamics calculations on methane show gas is confinedunder the influence of pore walls such that its density distribution does not consist exactly

of a high density adsorbed gas layer and then a constant density equal to that whichwould be calculated from the bulk equation of state.

Pore volume compressibility effects also enter into storability. As pore pressure increasesat constant confining pressure the pores become larger. Because the organic material may

not be stress supporting the concept of effective pressure may not be applicable in


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describing this process. Gas may also be stored in inorganic pores, which are not subjectto adsorbed gas effects, and will have different pore volume compressibility than the

organic pores.

The effect of adsorbed gas storage on reducing pore space for free gas storage, the

possibility of gas species specific pore access, the temperature dependence of adsorption,

and pore volume compressibility require that an accurate gas storage measurement mustbe made with the gas species of interest using a methodology that measures total storageat as close to reservoir conditions as possible. The measurement must be done on a solid

piece of core not fragmented samples. The current standard methodology measureshelium porosity on ground up samples, and an adsorption isotherm also on ground up

samples and computes total gas storage from their sum. This methodology satisfies noneof these requirements.

This manuscript develops a methodology (This methodology has been described in aUnited States patent application by Sigal and Akkutlu (2010)) for measuring the

storability curve, total storage as a function of pore pressure, on a plug sample with anyspecies of gas. The measurements are modeled to obtain pore volume at zero pore

pressure, the two Langmuir adsorption parameters, the density of the adsorbed state, andpore volume compressibility. An example of the method is given for a methane

storability measurement on an organic shale sample. The method has also been applied tothe measurement of storability of CO2 in shale (Kang et al., 2010) and methane in coals

with varying thermal maturity (Koleowo, 2010). A simpler version of the methodologyhas been used to measure the amount of methane injected into organic shale samples as

part of an NMR Study (Sigal and Odusina, 2010).

The modeling provides a density for the adsorbed state and assumes the free gas in a pore

satisfies the real gas equation of state. Possible values for this adsorbed state density arediscussed along with the assumption that the free gas density in a pore is correctly

modeled by the real gas equation of state.An equation for the effective average pore radius assuming Langmuir adsorption is

developed from the modeled storage parameters.

Once a storability curve is established for a reservoir rock type it takes the place of the

equation of state and the Langmuir isotherm. At initial reservoir conditions it provides thegas in place for that rock type. As the reservoir pressure is lowered the produced gas is

predicted from the storability curve. This is primary the information needed for input intoa reservoir simulation.

MethodologyThe basic storage measurement apparatus is essentially a high-pressure pycnometer.Figure 1 illustrates the essential features. Equipment such as ovens to control

temperature, and intensifiers to raise gas pressure, and pumps to producing confiningpressure are not shown.


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Figure 1. The essential components of storage measurement apparatus are illustrated.The high-pressure gas reservoir and the piping between valve 1 and valve 2 comprise

volume 1. Volume 2 comprises all the dead volumes contained in the pressure vessel andpipes. The sample is located in a sleeve inside the pressure vessel. The dead volume is

defined as the volume downstream of valve 2 when there is a sample with zero porevolume in the pressure vessel.

The establishment of the total gas storage curve is a multi-stage process. At the start ofthe measurement the sample is placed under a confining pressure that is larger than the

largest pore pressure that will be needed in the test. At the beginning of a stage valve 1 isclosed and valve 2 is open so the sample pressureP2 and the reservoir pressure are equal.

A measurement consists of closing valve 2, and opening valve 1 to establish a reservoirpressureP1 whereP1 is greater thanP2. Valve 1 is then closed and valve 2 opened and

the pressure is allowed to equilibrate to a final pressurePf. From this measurement themoles of gas added to the sample ns in going from pressureP2 toPfare calculated from

the moles of gas n1 lost from the reservoir volume V1 and n2, the increase of moles of

gas in the dead volume V2.

Using mass balance, the correct equations of state, and the measured values forV1, the

total volume between valve 1 and valve 2, and V2, the dead volume defined as the volumeon the downstream side of valve 2 when the sample has zero pore volume, one has

n1 = V1(P1/z1 Pf/zf)/RT(1)

n2 = V2(Pf/zfP2/z2)/RT(2)ns = n1-n2..(3)

In the gas equation of state R is the universal gas constant, and z is the gas

compressibility factor. There are numerous tables and calculators available that providezas a function of temperature and pressure. In this paper when a value ofz is used or a

bulk gas density calculated it was obtained from SUPERTRAP, a program produced anddistributed by NIST.


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The values ofV1 and V2 are established by following the same procedure except usingsteel samples of precisely known pore volume. For the calibration ns in Eq. (3) is

calculated from the known sample volume and the equation of state. The unknowns are

V1 and V2.

The total storage curve is established by starting at zero gas pressure, the chosen

confining pressure and a temperature T and iterating on the above procedure. Theiteration provides the storage n(T,Pfj) at m pressuresPf1, Pf2, , Pfm. The storage at the

first pressure point n(T,Pf1) is given by ns(T,0,Pf1). For this stage, P2 is zero andP1Pf1.

At subsequent pressure points Pfi, P2 in Eq. 2 is Pfi-1. Storage at any other pressure is

given by

n(T,Pfj) = n(T,Pfj-1) + ns(T,Pfj-1,Pf1) ...(4)

In practice most samples have significant clay content so it is probably better to start theiteration with air in the sample at atmospheric pressure, but zero methane pressure. This

is the procedure that has been followed for the measurements discussed here. Dividingthe measured storage by the plug volume gets the storage in moles of gas per unit bulk

volume.

The starting state of the sample will influence the total gas storage. It is essential to age

the samples long enough so that there is no sorbed gas associated with the organics orclays. For storage under reservoir conditions the sample needs to be at reservoir liquid

saturation. Hydrocarbon liquids in the reservoir present issues that will not be discussedhere. If the only liquid is clay bound water then as long as the clays have remained

hydrated enough that their morphology has not significantly changed the gas storageshould be mainly independent of the clay water saturation.

When using plugs that have been recovered from the subsurface the storage curve, ifpossible, should be corrected for storage in stress release cracks that are produced by the

core recovery process. Methane gas has an NMR signature. Sigal and Odusina (2010)have shown that the volume of methane in the stress release cracks can be identified by

an NMR measurement. By performing the NMR measurement at two pore pressures thecrack compressibility can be modeled which provides the crack volume as a function of

pore pressure. In measurements on horizontal organic shale plugs the stress release cracksat reservoir pore pressure accounted for roughly one-half of a porosity unit.

The components of storage associated with sorbed gas and free gas in the pore space canbe estimated by modeling the storage curve. For this model it will be assumed that the

sorbed gas can be modeled by a Langmuir isotherm. This is the usual assumption made inadsorption gas measurements made on organic shales. The Langmuir isotherm for the

total moles of gas stored Sa as adsorbed gas at a pressure Pis given by a two-parameter

equation. The parameterSamax gives the moles of gas stored at infinite pore pressure. TheLangmuir pressure PL is the pressure at which Sa is one-half of Samax. The Langmuirequation reads

Sa (P)= Samax (P/PL)/(1 +P/PL)..(5)

Under the Langmuir assumption the volume of the adsorbed state Va(P) is given in terms

of the maximum volume of adsorbed gas Vamax as


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Va (P) = Vamax (P/PL)/(1 +P/PL)(6)

The maximum molar storage and the maximum volumetric storage are related by the

maximum molar density of the adsorbed state mamax

.

Vamax = Samax /mamax ....(7)

The free gas stored in pore space is assumed to satisfy the real gas equation of state. Thatis the moles nsp(P,T) of gas stored in a pore volume Vp(P) is given by

nsp(P,T) =PVp/(zRT) ....(8)

Vp is a function of pressure both because of pore volume compressibility, and because of

the volume taken up by the adsorbed gas. To account for this the pore volume at zeropore pressure Vp0 and the pore volume compressibility are introduced. The pore Vp(P) is

then given by

Vp(P) = Vp0 + CpVp0PVa (P) (9)

In this model the second order effect of the pore volume compressibility Cp beingdependent on the pore pressure is ignored. Also it is assumed that CpVp0P is small. If it

turns out to be too large the integral form of the compressibility equation can be used.Although not explicitly shown the pore volume is a function of the confining pressure.

The storage equations can now be re-written in terms of this five-parameter (Vp0, Cp,Samax,PL, mamax)model. For any stage the starting sample pressure is Pfi-1 and the ending

pressure isPfiso that

ns = nsa +nsp..(10)

nsa = Samax((Pfi/PL)/(1+Pfi/PL) (Pfi-1/PL)/(1+Pfi-1/PL))..(11)

nsp = (PfiVpfi)/(zfiRT) (Pfi-1Vpfi-1)/(zfi-1RT)....(12)

Vpfi = Vp0 + CpVp0Pfi (Samax/mamax)(Pfi/PL)/(1+Pfi/PL)).....(13)

Vpfi-1 = Vp0 + CpVp0Pfi-1 (Samax/mamax)(Pfi-1/PL)/(1+Pfi-1/PL).....(14)

In Eq.10 ns is obtained from Eqs. 1-3 whereP2 is equal toPfi-1.

Once the storage model is introduced the storage curve can be developed without startingthe measurement process from a state of approximately zero pore pressure. As long as thestarting point is at an equilibrium gas saturation the storage curve can be determined by a

series of increasing or decreasing pressures.

In this model gas storage in hydrocarbon liquids or pore water has not been included. For

pores on the nanometer size scale the gas solubility is enhanced (Diaz-Campos et. al.,2009), so this may need to be included. Measurements that have been done so far have

been on samples without hydrocarbon liquids. The samples mainly contained clay boundwater. The storage curves obtained could be modeled without inclusion of solution gas.

Eqs. 10-14 are non-linear. The stability of their solution depends both on the number ofpressure points and the separation of the points. This though is limited by experimental

constraints. It is not desirable to use a confining pressure greatly in excess of the reservoir


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pressure. The pressure separation between steps needs to be large enough thatapproximate pressure stability is achieved in a reasonable time. The stability of the

solution is enhanced by a priori information such as approximate values for theLangmuir isotherm parameters, the helium porosity, or the value of the density of the

adsorbed gas state.

Model Assumptions and Free Gas and Adsorbed Gas Densityof Methane in a Small Pore The assumptions of the storability model are that all the stored gas is either as adsorbedgas satisfying the Langmuir adsorption equation, or free gas that satisfies a gas equation

of state that is independent of pore size. Furthermore, the Langmuir adsorption model hasonly a single layer of adsorbed gas covering the adsorbing surface. For the nanometer

scale pores that have been observed in the organic material of the shales moleculardynamics calculations show neither of these assumptions are completely accurate. On the

other hand, the standard adsorption measurements as a function of pore pressure that aremade on ground up samples are reported to be accurately modeled by a Langmuir

equation.For the adsorption model being used Eqs. 5, 6, and 7 give

mamax = Samax/Vamax = Sa(P)/Va(P)...(15)

Eq. 15 can be understood by considering the adsorbing surface to be covered by potential

adsorption sites that have on average a constant aerial density. One gas molecule canadsorb at each site. As pressure increases more sites are occupied and at infinite pressure

all the sites are occupied. The probability of a site being occupied is independent of theoccupancy of adjacent sites. The total volume of the adsorbed layer is Vamax. By this

model the actual density of gas in the adsorbed layerma is given by

ma = Sa/Vamax = mamax(P/PL)/(1 +P/PL)....(16)Ambrose et al. (2010) have reviewed the literature on methane adsorbed phase density.The literature reports the density to be temperature dependent, which accounts for the

temperature dependence in Langmuir adsorption isotherms.

The possible density for the adsorbed methane layer will be examined by three different

approaches. The first will be a calculation of the density of a single layer thick sphericalpacking. This can be thought of as a high-pressure low temperature limit. For many

purposes methane molecules behave like spherical balls with a fixed radius. Sphericalpackings have two end members the cubic lattice and the face-centered lattice. The

spheres in a cubic lattice occupy 52.36% of the lattice volume. Those in a face-centeredlattice occupy 74.048% of the lattice volume. In the general case let this fraction be fs.

Taking NA as Avogadro number and rm as the radius of the methane sphere, and MMW asthe molecular weight of methane, the density a of the single adsorbed layer is given by

a = MMWfs/(4/3)rm3NA(18)

For the face centered lattice with rm equal to 0.1865 nm Eq. 18 gives an adsorbed statedensity of 0.724 g/cc. This value of rm is the Lennard-Jones radius for methane. This

density is the maximum density achievable for spherical methane molecules with this


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radius. To the extent that methane molecules behave like hard spheres this is an upperlimit to amax, the maximum Langmuir density achieved at infinite pressure, and zero

temperature. Thermal motion acts to prevent this high a density. On the other hand

methane molecules are not hard spheres so pore wall potentials could promote denserpacking. For the cubic lattice the density is 0.512 g/cc.

A second line of investigation is to examine methane density isotherms as a function ofpressure. Figure 3 shows a set of methane density isotherms calculated using

SUPERTRAP. The isotherms are for temperatures -80oC, -20

oC, 0

oC, and 20

oC. The

critical temperature of methane is -82.7oC. The line through the -80

oC isotherm is a

regression fit to the data.

Figure 2. Isotherms for super critical methane as a function of pressure calculated fromSUPERTRAP.

The isotherm data stops at 43,000 psi as this is the highest pressure the program

developers have confidence in the algorithm. At 43,000 psi the calculated densities forsupercritical methane ranges from 0.438 g/cc to 0.480 g/cc. Although extending the

regression fit to the data is dangerous, at 500,000 psi for the -80 oC isotherm the projecteddensity is 0.668 g/cc. That is an algorithm that provides very accurate values for methane

density at lower pressures predicts that super critical methane at high pressure reachesdensities comparable to those calculated for the spherical packs.


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The final approach is through molecular dynamics simulation. Ambrose et al. (2010)investigated the density of methane in nanometer scale slit pores through molecular

dynamics calculations. On revisiting this calculation a minor error was discovered. Thefollowing discussion builds on the previous work but corrects the error.

The molecular dynamics calculation was done in the canonical ensemble (NVT) for

methane confined in two slit shaped pores having widths of 2.065 nm and 3.727 nm. Thepore walls were made of graphene layers consisting sp

2-bonded carbon atoms densely

packed in a honeycomb crystal lattice. Hence the pore wall approximates an organic pore

wall. In this model the methane molecules have spherical symmetry and interact witheach other through a Lennard-Jones potential. For the Lennard-Jones potential used the

interaction energy between two methane molecules is zero when their centers of mass areseparated by 0.373 nm. This distance, the Lennard-Jones diameter can be thought of as

the diameter of the methane molecule in this model. The temperature was 80 oC (176 oF).

Figures 3a and 3b, which are similar to figures in Ambrose et al. (2010) show the

calculated layer-averaged mass density values for methane for zones 0.38 nm in widthstarting at the pore wall. For the Langmuir model the densities would have a high value

for a block one methane molecular diameter thick and then reduce to the bulk gas densityappropriate for the temperature and gas pressure. This is clearly not the exact behavior

calculated from the molecular dynamics simulation. There is a high-density zoneapproximately the effective diameter of the methane molecule with successive layers

each having lower density. To compare these densities to the bulk gas density requiresthe pressure of the gas in the slit.

The molecular dynamics calculations were done in the conical ensemble. For thisensemble the temperature is fixed but no particle exchange is allowed with an infinite

volume constant pressure reservoir, so the gas pressure is not a model parameter.Pressures can be calculated in these models. Molecular dynamics calculations seem to

have good agreement with SUPERTRAP when done for bulk gas. For example at 80o

Cand 4413 psi pressure the molecular dynamics methane density has a value of 0.165 g/cc.

The SUPERTRAP density is 0.167 g/cc.

The density profile is not constant and some of the gas is adsorbed so there is an

ambiguity in how to calculate pressure. It would be expected that the gas at the center ofthe slit best approximates a bulk gas.


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Figure 3a . The methane density profile calculated for a 2.065 nm diameter carbonwalled pore at 80 oC.

Figure 3b. The methane density profile calculated for a 3.727 nm diameter carbonwalled pore filled with methane 80

oC.

The pressure in a zone at the center of the pore has been calculated for each case. Themethod of calculation was to run multiple molecular dynamics simulations and calculate

the average pressure and its standard deviation. Because the pores only have a fewhundred molecules statistical fluctuations produce a variation that is significant relative to

the average, unlike systems that have an Avogadros number of molecules where thedeviation from the average in an ensemble of simulations is many orders of magnitude

smaller than the average. In a laboratory size sample there are roughly 1015

nanometersize pores, so the pressure in each pore averaged over the total number of pores should be

closely approximated by the simulation ensemble average. This average pressure is whatis measured in the laboratory.

For the smaller pore 475 simulations were computed. The average pressure in a zone atthe center of the pore was 2206 psi with a standard deviation of 241 psi. The density in

that center zone was 0.0903 g/cc. From the bulk equation of state at this average pressure

at 80o

C the density is 0.0895 g/cc. For the calculated density the pressure from themethane bulk equation of state is 2220 psi.

For the larger slit 495 simulations were computed. The average pressure in the zone at the

center of the pore was 3018 psi with a standard deviation of 384 psi. At 80 oC the bulkmethane density at that pressure is calculated from SUPERTRAP as 0.121 g/cc. For the

bulk equation of state a pressure of 3100 psi corresponds to the molecular dynamicssimulation density of 0.124 g/cc.


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There is only a few percent difference between the molecular dynamics calculations ofpressure and density at the center of the pore and the bulk methane relationship. It will

take additional studies to resolve if the small increase in density for a given pressurerepresents a significant deviation from the bulk equation of state.

The division of a density profile such as shown in Figures 3a and 3b into the adsorbed

and free gas component is ambiguous, since it does not agree with a simple Langmuirmodel. One approach to deal with this would be to define the adsorbed and free gascomponents in a different way.

For the quantifying of gas storage the essential difference between the components ishow they change as pore pressure is increased. To first order the adsorbed component

should have a Langmuir pressure dependence, and the free gas the pressure dependenceof the methane gas equation of state. To examine this using the molecular dynamics

simulation requires doing a series of calculations at different pore pressures. A grandconical ensemble allows pressure to be a model parameter. This data will then have to be

modeled as an adsorbed gas component and a free gas component. With the currentstudies it is not possible to determine if the higher methane density in zones closer to the

wall represent the presence of adsorbed gas in them or just deviation from the bulkequation of state.

The simplest assumption is to assume the first layer represents only adsorbed methane,and that the pressure at the center of the pore is the free gas pressure. The density in this

layer should then obey Equation 16, wherePis the pressure at the center of the pore, anda is the layer density. To calculate amax requiresPL which is not known.

To investigate the possible range ofamax we calculate it for various values ofPL Table

1 presents amax for variousPL values.

Table 1. The maximum density of the adsorbed gas layer for various values of theLangmuir pressure.Pore Diameter

(nm)

Free Gas

Pressure (psi)

PL (psi) Adsorbed Layer

Density (g/cc)

amax (g/cc)

2.065 2206 1500 0.268 0.450

2.065 2206 1721 0.268 0.477

2.065 2206 2009 0.268 0.512

2.065 2206 3342 0.268 0.674

2.065 2206 3754 0.268 0.724

3.727 3018 1500 0.268 0.448

3.727 3018 1797 0.299 0.477

3.727 3018 2150 0.299 0.512

3.727 3018 3785 0.299 0.674

3.727 3018 4289 0.299 0.724


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For organic shalesPL values measured on crushed samples have been reported from less

than 1000 psi to greater than 3000 psi. A value of 1500 psi is somewhat typical. For thisvalue both samples have a amax of 0.45 g/cc. The other values ofPL in Table 1

correspond to densities for the adsorbed layer based on spherical pack calculations. A

cubic lattice with a spherical radius of 0.191 nm, the kinetic radius of methane, has adensity of 0.477 g/cc, and for a radius of 0.187 nm, the Lennard-Jones radius, the density

is 0.512 g/cc. For the face-centered lattice the corresponding densities are 0.674 g/cc and0.724 g/cc.

The densities corresponding to a cubic lattice seem to be in a reasonable agreement withthe amax values calculated from the molecular dynamics densities for the layer next to the

wall in that thePL values that give these densities are within the typical range measuredfor mature organic shale samples. The PL values necessary to give adsorbed state

densities consistent with a face-centered package are somewhat higher than typical formeasurements on mature organic shales. A definitive conclusion though requires a

calculation at multiple pressures.The molecular dynamics calculations are reasonably interpreted as small pores have a

layer 0.38 nm thick of adsorbed gas. The center of the pore is populated by gas that hasan equation of state close to the bulk equation of state. It is not clear to what extent

adsorbed gas is contained in zones outside the layer closest to the wall. The density of theadsorbed gas in the layer next to the wall is consistent with densities calculated from a

cubic packing of spheres with the radius of a methane molecule.

Langmuir Pore RadiusThe parameters in the model fit of methane storage can be used to estimate an effective

average pore radius for the organic pores. This calculation is similar to the pore radius

calculation in Kang et al. (2010) The estimate will be referred to as the Langmuir poreradiusRLp. In Eq. 16fsis defined in terms ofVsm, the total volume occupied by a mole ofmethane spheres to the total volume of a mole of adsorbed methane Vam.

fs = Vsm/Vam = (4/3)rm3NA/Vam........(19)

The surface area of a mole of the adsorbed layerAam is under the Langmuir assumption

related to the volume of the adsorbed gas by

Vam = 2rmAam....(20)

So that

2rmAam = (1/fs)((4/3)rm3NA) ..(21)

Solving forfs in Eq. 18 and substituting into Eq. 21 and converting to molar density givesthe surface area per mole of adsorbed methane as

Ama = 1/(2rmmamax).....(22)

Aa, the maximum surface area that can be adsorbed on by methane based on the storagemeasurement is given by

Aa =AmaSamax....(23)


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If one lets fo stand for the fraction of the gas filled pore space contained in the organicmaterial then the pore volume associated with this surface area is foVp0. The ratio of

adsorbed surface area to the gas filled pore volume defines the Langmuir pore radius by

/RLp =Aa/foVp0...(24)

This givesRLp = 2formmamaxVp0/Samax...(25)For a spherical pore is 3 and for a cylindrical pore is 2. The maximum calculated

Langmuir pore radius occurs if all the gas storage is in the organics. The smaller the

molar density of adsorbed phase the smaller the Langmuir radius. Based on the moleculardynamic results the density would be suppressed if the adsorbed gas occupied more than

one layer.

Methane Storage ExampleThe previous methodology has been applied to measuring and modeling the storage curve

for an organic shale sample. This example was also been discussed in Kang et al. (2010).For this plug routine petrophysical measurements have also been done on companion

samples. Table 1 presents a summary of these measurements.

Table 1. Standardpetrophysical measurements made on companion samplesLECO TOC 3.82% Weight percentage

Helium Porosity 2.02% Volume percentage

FTIR Mineralogy Weight percentage after organics removed

Quartz 31%

Total Carbonates 12%

Total Clays 41%

Other 17%

The measurements in Table 1 were all made at University of Oklahoma labs except for

the total organic carbon TOC measurement. In Table 1 the helium porosity was done onground up samples that had been vacuum dried at 100 oC. Illite was the major claymineral.

Figure 3 shows the total gas storability curve measured for this sample. The measurementwas done at 5014 psi confining stress at room temperature with methane on a vertical

cylindrical sample approximately one inch in diameter by two inches long. The samplecame from core that had sat in storage under variable temperature and humidity

conditions for several years.

Measurements on other organic shale samples stored in the same way mostly display one

to two percent water porosity on NMR measurements (Sigal and Odusina, 2010). Sigaland Odusina (2010) found the moles of methane that could be injected into organic shale

samples stored in this way did not seem to have been increased over samples that hadbeen stored so as to preserve their reservoir water content. In general for preserved


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samples the sum of the NMR water porosity and the NMR methane porosity was roughlyequal to helium porosity measured on ground up samples that had been vacuum dried.

The storage measurements were started at atmospheric pressure with air in the system.Methane storage was measured at five pore pressures (1316 psi, 2138 psi, 2600 psi, 2965

psi, 3136 psi). The model curve was fit by an iterative procedure where parameters were

constrained to be in the same range as other reported measurements on similar samples.Because of noise in the data, the number of pressure points measured, and the dynamicrange of the pressure points the model parameters should not be considered unique. No

correction was made for gas in stress release cracks.

Figure 4. Example of a storability curve for an organic shale sample measured at roomtemperature along with a predicted curve calculated from a model fit to the data. Thehighest pressure point does not fit the trend of the lower pressures. This is probably due

to a problem in temperature control.

Table 2. Parameters determined from the model fit to the storability curve.

Sample Bulk Volume 22.67 cc (measured value)

Vp0 0.3914 cc

Porosity at zero pore pressure 1.73%


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Samax .00491 moles

Samax/cc .0002164 moles/cc

PL 1800 psi

Cp 5.42 x 10-6

(1/psi)

mamax .02504 moles/cc

amax .4006 g/cc

Although the parameters are not unique they show that an excellent fit to theexperimental data can be achieved with parameters that roughly agree with other

measurements made on similar samples. For organic shale from this area a typicalLangmuir isotherm curve hasPL = 1500 psi and Samax = 0.00017 moles/cc. The porosity

at zero pore pressure is in reasonable agreement with the measured helium porosity onthe ground up companion sample. The density of the adsorbed state while smaller than

the values estimated in the section of the manuscript on methane density is certainly

within the range that has been reported for adsorbed state density.

The model parameters can be used to approximate the storage curve that would beobtained by assuming the storage is a linear sum of the gas that can be stored in the pore

volume Vp0 assuming the standard equation of state for methane, and the adsorbed gascalculated from a Langmuir isotherm curve. Figure 5 compares this storage curve to the

measured storage curve. At 3136 psi, the highest pressure measured, the linear sum is19% larger than the actual measured curve. At a typical shale gas reservoir pressure the

deviation would be larger.


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Figure 5. For the shale example the Measured curve is the storage curve from Figure 4.The Sum curve is calculated using Vp0 and the estimated Langmuir adsorption curve. It

can be seen that the sum considerably overestimates the actual storage curve.

The model parameters can be used to calculate thefs andRLp, the Langmuir pore radius.

Using Eq. 19 and Eq. 22, fs = 0.413.From Eq. 25 and assuming all the methane porosity is located in the organics, using the

Lennard-Jones radius and assuming that the pores are sphericalRLp = 2.24 nm.

The filling factorfs is only 79% of the filling factor of a cubic lattice. The two most

reasonable hypothesizes for this low value are thermal motion prevents a dense packingand/or the actual adsorbed gas occupies more than a single layer.

The value of the Langmuir radius is smaller than many of the pores observed on ionmilled SEM images. The Langmuir radius is essentially an effective radius. Pores too

small to be resolved on the SEM images would be included in it. This could explain someof the discrepancy. Some of the discrepancy could also be due to the adsorbed gas density

including gas not located in the layer next to the pore wall. The molecular dynamicssimulations also suggest this possibility. Despite these ambiguities, the storage

measurement, which is a totally macroscopic measurement, confirms the observationsmade in the ion milled SEM images that the pores holding gas are nanometer scale.

The model parameters and the measured TOC can be used to estimate the intrinsicorganic porosity ki. From Sigal (2010)

k= ki /(1 + (k/ib)(1-ki)((1-Fo )/Fo )) ..(26)In Eq. 26 k is the contribution to the measured porosity from gas filled pores, k is the

kerogen matrix density, ib the bulk density of the inorganic material, andFo the mass

fraction of organic material. Using TOC as an estimate forFo, taking kas 1.2 g/cc, ib as

2.7g/cc, and using Vp0 as the methane pore volume gives ki equal to 18%. This is

somewhat less than the intrinsic organic porosity calculated for samples from a different

well that was examined in Sigal (2010). This is clearly due to the low porosity of thissample despite it not having a low TOC.

ConclusionsThe traditional approach to measuring the total gas storage capacity of a organic shale gascore sample is to measure the adsorbed gas contribution and the free gas contribution

separately. For the adsorbed gas contribution, an adsorption isotherm on a ground upsample is measured. The measurement is done at reservoir temperature and with the

reservoir gas. The free gas contribution is measured using helium to measure a grainvolume and calculating the helium porosity. The measurement is done at low pressure on

ground up samples at room temperature. The total storage is then taken as the sum of theadsorbed gas at reservoir conditions plus a free gas contribution calculated from the

helium porosity using the bulk gas equation of state.

This procedure has several serious issues. SEM imaging of ion milled shale gas reservoir

rocks shows that pore space located in the organic material is the dominate component of


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storage for many organic shales. Both the images and surface to volume ratio calculationsshow that the organic pores are on the nanometer scale.

Organic pores strongly adsorb gases such as methane and carbon dioxide, but essentiallydo not adsorb helium, specifically at the low pressures where helium is used for grain

density. For nanometer scale pores the adsorbed gas significantly reduces pore volume

available for the free gas. This reduction is a function of pore pressure. Also since thepores are in organic material pore volume compressibility may be a significant effect.This effect is difficult to estimate both because the elastic properties of the organic

material are not well known and the organic material may not be stress supporting. Afinal issue is that in the smaller pores in the organic material molecular dynamics

calculations for methane have shown that the gas density profile deviates from the idealLangmuir adsorption plus free gas model. All of these issues imply that taking total

storage as the sum of an adsorbed state contribution and a helium porosity contributionhas serious potential errors.

This manuscript presents a multi-stage modified pycnometer methodology to establishthe total storage curve as a function of pore pressure on a whole plug at reservoir

conditions using the reservoir gas. The measurement can be corrected for the mostserious systematic source of error, stress release cracks, by use of an NMR measurement.

This methodology eliminates the issues with the standard measurement that uses groundup samples.

The multi-stage measurement process can be used to estimate a five-parameter set thatdefines the storage curve. The parameters are the porosity at zero pore volume, the pore

volume compressibility, the two Langmuir parameters (the maximum amount ofadsorbed gas, and the Langmuir pressure), and the density of the adsorbed state. With

reasonable assumptions these parameters can be used to estimate an average radius forthe organic pores, and the intrinsic organic porosity.

The procedures for establishing the storage curve and its parameterization have beenapplied to storage curve measurements of carbon dioxide and methane in shales, and

methane in coal. An example of a measurement of methane storage on an organic shalesample has been presented. On this sample computing storage in the traditional manner

would significantly overestimate the measured value.

AcknowledgementsMost of the work reported here was supported by very generous grants from Devon

Energy Corporation as part of a large multi-component study of organic shale gasreservoirs. Discussions with Jerry Youngblood and other Devon personal have proved

very helpful in understanding issues involved in organic shale gas reservoirs.

Gary Stowe and Bruce Spears provided valuable support in building the measurement

systems and keeping the measurement systems functioning.

ReferencesAmbrose, R. J., Hartman, R. C., Diaz-Campos, M., Akkutlu, I. Y., and Sondergeld, C. H.,

2010, New Pore-scale Considerations for Shale Gas in Place Calculations, SPEUnconventional Gas Conference, Pittsburgh, PA, SPE 131772


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Campos, M. D., Akkutlu, I. Y., and Sigal, R. F., 2009, A Molecular Dynamics Study onNatural Gas Solubility Enhancement in Water Confined to Small Pores, SPE 124491,

SPE Annual Technical Conference and Exhibition, New Orleans

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Gas Storage Organic Shales