1

Chapter 1◆

Comparison of Seal Capacity Determinations:Conventional Cores vs. Cuttings

Robert M. SneiderJohn S. Sneider

Robert M. Sneider Exploration, Inc.Houston, Texas, U.S.A.

George W. BolgerJohn W. Neasham1

PetroTech AssociatesHouston, Texas, U.S.A.

◆

ABSTRACT

Comparison of hydrocarbon column heights (HCHs) calculated from sealsrecovered in conventional cores with HCHs calculated by using cuttingsfrom the same interval indicates that mercury/air capillary pressure mea-surements of cuttings can be extremely useful to estimate seal capacity. Anempirical adjustment factor (EAF), expressed in psi, needs to be added to thecapillary pressure value determined on cuttings to approximate that mea-sured with mercury/air capillary pressure of conventional cores.

For top and lateral seals that are the result of lithologic changes (asopposed to fault seals), good to excellent agreement is found between thehydrocarbons actually trapped in fields and the HCH calculated from mer-cury/air capillary pressure curves of vertical plugs cut perpendicular to thesealing surface. The plugs are sealed with epoxy so that mercury can enteronly from the top and base of the plug. The mercury/air capillary pressurecurves are generated using a system that can inject mercury at pressures upto 60,000 psi [8703 kPa] (equivalent to a hydrocarbon column of >10,000 ftfor 35° API gravity oil and normal saline water).

Depending upon seal type, high-pressure mercury/air injection curves(HPMIC) of cuttings can be used to approximate those of samples from con-ventional cores. Injection pressures for cuttings are usually lower than thosefrom equivalent cores for a particular percent pore volume occupied by mer-cury. Empirical adjustment factors (EAFs), expressed in psi, for different sealtypes are derived from comparisons of HPMIC on epoxy-sealed vertical conventional core plugs with cuttings or “simulated cuttings” of the sameseal interval. The EAF values are added to the capillary pressure measure-ment of cuttings to obtain the approximate value of mercury/air capillary

1Now with Poro Technology, Houston, Texas

Sneider, R.M., J.S. Sneider, G.W. Bolger, and J.W.Neasham, 1997, Comparison of seal capacitydeterminations: conventional cores vs. cuttings, inR.C. Surdam, ed., Seals, traps, and the petroleumsystem: AAPG Memoir 67, p. 1–12.

2 Sneider et al.

INTRODUCTION

This paper addresses the question: “How useful arecuttings of seals to estimate the hydrocarbon columnheights?” This is an important question because cut-tings of seal lithologies are very common, but cores ofseals are rare.

During the past 25 yr, we have been coring seals andcollecting cuttings while coring or crushing seal coresto produce “simulated cuttings.” Figure 1 is an exam-ple of a cored seal–reservoir interface. A comparison ofhigh-pressure mercury/air injection capillary pressurecurves (HPMICs) of both the cores and associated cut-tings or simulated cuttings shows that it is possible toestimate the capillary pressure equivalent to a verticalseal lithology from HPMICs of cuttings by adding anempirically derived adjustment factor (EAF) in psi.

This paper reviews some principles of hydrocarbonentrapment, discusses sample preparation, and pre-sents examples of empirical relationships betweenHPMICs of core samples and associated cuttings.

HYDROCARBON ENTRAPMENT

Schowalter’s 1979 paper on seals reviews the princi-ples of hydrocarbon entrapment/accumulation. Abrief review of the key principles is presented toexplain the entrapment/accumulation processes andhow mercury/air measurements relate to hydrocar-bon/water capillary pressure and, in turn, to hydrocar-bon column height (HCH) trapped against seals.

Figure 2 is a schematic of a stratigraphicallytrapped reservoir with its adjacent seals. Hydrocar-bons entering the reservoir are driven by the buoyancyforce or pressure (Pb), which is the difference in den-sity of the hydrocarbons and formation water × thehydrocarbon column height (h) × 0.433, the gradientof fresh water. Hydrocarbons entering the reservoirmust enter the pores and displace the pore water. Theequation of this resistive force (Pc, a rock’s capillarypressure) is shown in Figure 2. Hydrocarbons willcontinue to fill the reservoir and be trapped against theseals until the buoyancy pressure due to the hydrocar-bon–water system in the reservoir exceeds the capil-lary entry pressure (Pe) of the weakest seal rock. Inwater-wet or mostly water-wet systems, hydrocarbonswill continue to leak into the seals until a balance orequilibrium is reached between the seal entry pressureand the pressure within the reservoir system.

We have estimated the HCHs in more than 200reservoirs where low-permeability lithologies (notfaults) are the seals. We observe that the estimatedHCHs of hydrocarbon-bearing reservoirs correspondto the mercury/air capillary pressure between ~5%and 10% nonwetting phase saturation. We have usedPc at 7.5% nonwetting phase saturation to indicate thesaturation at which the seal actually leaks hydrocar-bons through its pore network. This saturation iscalled “breakthrough” or “leakage” saturation.

Figure 3 shows capillary pressure vs. nonwettingphase curves for the mercury/air system for seals “A”and “D.” Using the assumed densities of oil, gas, andwater given and interfacial tensions (in dynes/cm) of30, 70, and 480 for oil/water, gas/water, and mer-cury/air systems, mercury capillary pressures are con-verted to oil/water and gas/water systems. If the sealcapillary pressure (Pc) for leakage is taken at 7.5% non-wetting phase saturation, the “A” seal will hold ~1200ft of oil or 520 ft of gas before leakage through the sealsoccurs. The “D” seal will hold ~120 ft of oil or ~50 ft ofgas before leakage through the seal. The equations toconvert mercury/air capillary pressure to oil/waterand gas/water capillary pressure and to HCH are out-lined in Appendix 1.

It is important to remember that when mercuryinjection capillary pressure (MICP) data are convertedto reservoir conditions, the values of oil/gas/waterdensities and interfacial tensions must be corrected forreservoir temperature and pressure.

SEAL TYPES

Sneider et al. (1991) studied several hundred seal litholo-gies and presented an arbitrary classification of seals basedon the hydrocarbon column held. The seal types andhydrocarbon columns held are shown in Table 1.

The study used mercury/air capillary pressurecurves, which were converted to an oil/water capillarysystem assuming 35° API gravity oil and normal salinewater. The oil/water capillary system was convertedinto the hydrocarbon column height (HCH) heldbefore leakage through the seal. Leakage is assumed tobe where Pc equals 7.5% nonwetting phase saturationbased on a comparison of HCH observed in the fieldand the column height calculated from themercury/air capillary pressure curve when the satura-tion is 7.5%. In Figure 3, the mercury/air capillarypressure at Pc = 7.5% nonwetting phase saturation

pressure of a vertical plug. The EAF vary from ~1900 psi (mercury/air) for type “A” seals to ~25 psi for type “D” seals, using 7.5% mercury pore vol-ume saturation as the reference saturation.

Careful sample preparation and accurate closure corrections are critical toobtaining accurate HPMIC measurements and corresponding EAF values forHCH calculations.

converts to an oil/water capillary pressure for “A” and“D” seals of 113 psi and 11.4 psi, respectively, which inturn converts to a HCH held of ~1200 and 120 ft,respectively, for the “A” and “D” seals. In other words,the “A” seal will hold a HCH of ~1200 ft if the oil is 35°API and the pore water is normal saline water. If moreoil enters the reservoir, hydrocarbons will leak throughthe seal. The hydrocarbon column held by a seal is afunction of its capillary pressure curve (i.e., pore throatsize distribution) and the density of the hydrocarbonsand pore water. For other oil, gas, and water densities,the hydrocarbon column held can be determined bythe equations in Appendix 1.

SAMPLE PREPARATION ANDCAPILLARY PRESSURE

MEASUREMENTS

Conventional core samples are cut perpendicular tobedding or perpendicular to the potential sealing sur-face (Figure 4). Core samples and cuttings areextracted to remove all hydrocarbons and are dried atapproximately 60°C for at least 24 hr; the dry weight ofthe samples is then measured. The sides of the conven-tional core plug are coated with epoxy so that mercurycan enter only at the top and bottom of the plug.

Seal Capacity from Cores vs. Cuttings 3

RESERVOIR–SEAL COUPLET

LITHOLOGY SEAL TYPE(OIL)

Pc @ 7.5% HgSaturation (psi)

A >50,000

B

D – E 65 – 130

RESERVOIRROCK 10 – 80

ANHYDRITE

“CHICKEN WIRE”ANHYDRITE

TIGHT DOLOMITEPARTIALLYREPLACED BYANHYDRITE

DOLOMITE

809 – 2395

Figure 1. Core of an anhydrite top seal on adolomite reservoir, SanAndres Formation, NewMexico.

Pe

PeTOP SEAL LATERAL SEAL

BOTTOMSEALPb = h ( w - hc) 0.433, psi

Pc = dynes/cm22 cos r

Pe = seal capillary entry pressure (psi)

Pe

Figure 2. Schematic of astratigraphic trap showingthe forces (pressures) control-ling hydrocarbon entrap-ment. Pb = buoyancypressure in psi or the drivingforce; Pc = capillary pressurein psi or the resistive force; Pcat 7.5% nonwetting satura-tion is assumed to be the sealleakage saturation; h =hydrocarbon column heightin feet; σ = interfacial tensionbetween water and hydrocar-bon in dynes; θ = contact orwetting angle, in degrees; r =radius of the capillary tube(or pore throat) in cm.

4 Sneider et al.

The core plug or cuttings are sealed in a glass pen-etrometer (Figure 5a) that is placed into a “low-pressure” port of a porosimeter. A vacuum of 10–20µ isdrawn on the rock sample, and the penetrometer is filledwith mercury at a “filling pressure” of 1.5 psia. This iscalled the “low-pressure” mercury injection stage.

In the low-pressure stage, the injection pressure isincreased incrementally over a series of pressuresteps up to 25.0 psia. Equilibrium condition is estab-lished at each pressure point (step) when mercuryintrusion ceases, as indicated by the lack of any pres-sure drop at the end of a 20-sec waiting period. At theend of the low-pressure injection stage, the holderwith the sample is removed from the porosimeter,weighed, and then loaded into the high-pressure cell.The injection pressure is returned to both the samepressure point and respective amount of mercuryintrusion that existed at the last low-pressure point.The pressure is then increased incrementally to

60,000 psia. Equilibrium at each high-pressure step isdetermined as in the low-pressure stage; however, a60-sec waiting period is used. Computer hardware/software controls, monitors, and records test results.Data are corrected for any artifacts by applying cali-bration data obtained by running blanks.

More than 100 pressure steps or measurements aremade during both the low- and high-pressure stages.At the conclusion of each MICP sample analysis, thetest results are printed out for examination and evalu-ation. A pressure vs. volume of mercury plot is shownin Figure 5b. These data are placed on a disk for pro-cessing and final data reduction.

Figure 3. Mercury/air capillary pressure curves of “A” and “D” seals.Breakthrough pressure orleakage of hydrocarbonsthrough the seal is assumedto be at 7.5% nonwettingphase saturation. Theapproximate oil/water and gas/water capillarypressures and hydrocarboncolumn height for oil andgas are shown.

NONWETTING PHASE SATURATION (%)

HYDROCARBONCOLUMN HELD

A SEAL113 1,189 232 521

11.4 120 23.5 53

OIL GAS

SANDY SILTSTONECLAY-RICH SHALE

DA

60 50 40 30 20 10 010

30

50

100

300

500

1000

3000

AD

MERCURY-AIR OIL-WATERPc (psi) h (ft)

o/w = 30 dynes/cm

GAS-WATERPc (psi) h (ft)

g/w = 70 dynes/cm

APPROXIMATE

Pc = 7.5% Sat.

HYDROCARBONCOLUMN HELD

D SEAL

3/4 INCH

1 INCH

VERTICALPLUG CUT

SIDES OF VERTICALPLUG COATED AND

SEALED WITH EPOXY

Figure 4. Schematic diagram shows orientation ofvertical seal plug that is coated with epoxy. Mercuryenters the sample only at the top and bottom.

Table 1. Seal Types and Hydrocarbon Columns.

Seal-Flow Barrier 35° API Oil Column HeldType (m) (ft)

A* ≥1500 ≥5000A ≥300– <1500 ≥1000– <5000B ≥150– <300 ≥500– <1000C ≥30– <150 ≥100– <500D ≥15– <30 ≥50– <100E <15 <50F Waste Zone Rocks1

1Poor-quality, low-permeability rocks that contain appreciablehydrocarbons.

A key aspect for each HPMIC test is the determina-tion of “closure pressure.” Closure pressure is thatpressure at which mercury first enters the sample porespace (i.e., initial pore entry pressure) as opposed tomercury closing or conforming around the sample tofill sample surface irregularities that are not part of thesample pore volume. The more irregular the samplesurface(s) (i.e., cuttings), the higher the closure. Theclosure or apparent mercury intrusion must be deter-mined from examination of the plots of pressure vs.mercury volume injected. The closure must be sub-tracted prior to the calculation of capillary pressure vs.percent of pore space (pore volume) occupied. Thesecalculations were made for all the HPMIC curves mea-sured in our studies.

Figure 6 is a portion of two capillary pressure curvesthat have been corrected for closure. One curve is of anepoxy-sealed vertical plug, and the other is of simu-lated cuttings prepared from the rock surroundingwhere the plug was cut. The capillary entry pressure(Pe) is 26 psi for the simulated cuttings and 91 psi forthe plug. The pressure vs. mercury saturation valuesfor the simulated cuttings and epoxy-coated vertical

plug approach one another as the pressure increases.The difference in capillary pressure (in psia) at Pc =7.5% mercury (nonwetting phase) saturation is definedas the empirical adjustment factor (EAF). In this exam-ple, 179 psi – 170 psi = 9 psi; 9 psi would be added tothe capillary pressure value of the cuttings to approxi-mate the capillary pressure of the epoxy-sealed verticalplug. The EAF is 9 psi.

Figure 7 illustrates the mercury saturation in a ver-tical plug in which mercury can enter only at the ends(Figure 7A). At the entry capillary pressure of Pe = 0%mercury (Figure 7B), the nonwetting phase begins toenter or surround the pore system at the end of theplug. As pressure increases, mercury enters thepores. At a mercury saturation of about 7.5%, wevisualize that one or more continuous filaments orpathways of mercury connect from one end of theplug to the other (Figure 7C). We believe that this isat or near the breakthrough saturation (when thenonwetting phase leaks through the seal). It is possi-ble that the breakthrough saturation may vary fordifferent seals, but 7.5% mercury saturation is consis-tent with our empirical observations.

Seal Capacity from Cores vs. Cuttings 5

PRESSUREGENERATOR

0.010.10.051 0.510 5100 50

0.0

0.2

0.4

0.6

0.8

1.0

DIAMETER ( m)

AUTOPORE POROSIMETER(PRESSURE VS. VOLUME MEASURED)

a b

(

)

Figure 5. (a) Penetrometer with sample surrounded with mercury in the porosimeter pressure chamber. (b) Plotof cumulative pressure vs. mercury intrusion volume. (After Micromeritics, 1995.)

6 Sneider et al.

TYPICAL HIGH-PRESSUREMERCURY/AIR CAPILLARY

INJECTION CURVES

Figure 8 shows high-pressure mercury/air injec-tion capillary pressure curves (HPMIC) for four typi-cal seals. In each graph, the curve with black squaresis a vertical plug epoxy sealed on its sides, and theother curve (open squares) is of simulated cuttingsfrom the same piece of rock from which the core plugwas cut. At lower values of capillary pressure and

pore space occupied by the nonwetting phase mer-cury, the cuttings curves are below the plug curves.The cuttings have all sides of the sample fragmentsexposed, and mercury will first enter the larger porethroat sizes and the partial pores exposed on the sur-face of the sample. Although not illustrated in thispaper, nonsealed horizontal plugs of seals have cap-illary pressure curves very similar to simulated cut-tings. The HPMIC curves show the entry pressureand the capillary pressure at 5% and 10% nonwettingphase saturation, expressed as pore space occupiedby mercury. A 7.5% nonwetting phase saturation

200

MERCURY SATURATION (% PORE VOLUME)

180

160

140

120

100

80

60

40

20

030 25 20 15 10 5 0

SIMULATED CUTTING

SEALED VERTICAL PLUG

Pe10% = 187 psi

Pe7.5% core = 179 psi

Pe5% =

161 psi

Pe7.5% cuttings = 170 psi

= 7.9%, ka = 0.02 md

Pe10% = 191 psi

Pe5% = 130 psi

Pe0% = 91 psi

Pe0% = 26 psi

EAF

EmpiricalAdjustment Factor (EAF) = 9 psi

(psi

)

Figure 6. A portion of high-pressure capillary pressure curves of a sealed vertical plug and simulated cuttings of the rock from which the plug was cut.Note the capillary pressures at Pe = 0% (initial mercury entry) and at 5%, 7.5%,and 10% nonwetting phase saturation. Ka = air permeability.

Seal Capacity from Cores vs. Cuttings 7

SEALED SAMPLEWITH EPOXY COATING

ON SIDES

MERCURY INJECTION AT TOP

X Y

MERCURY INJECTION AT BOTTOMA

MERCURY INJECTION AT TOP

X Y

MERCURY INJECTIONAT BOTTOM

Pc @ 0% Hg SaturationB

MERCURY INJECTION

X Y

Pc @ 7.5% Hg SaturationC

Figure 7. Schematic diagram illustrating the distribution of mercury at Pe = 0% and Pc = 7.5% mer-cury saturation. Mercury is black. Note the continuous filaments of mercury through the sample. (A) The mercury injection sample. (B) Pc at ±0% mercury (Hg) saturation. Mercury fills and conformswith the outer grain surfaces. (C) Pc at 7.5% mercury (Hg) saturation. Mercury fills many pores, andnumerous mercury-filled pathways are continuous from the top to the bottom of the plug. Pc at 7.5%mercury saturation is assumed to be the breakthrough or leakage pressure of the sample. (AfterMicromeritics, 1995.)

8 Sneider et al.

Entry5%10%

Pc@ Plug

13016003600

Cuttings

100700

2500

100 90 80 70 60 50 40 30 20 10 0Pore Space Occupied (%)

1

10

100

1000

100000

10000

Mercury Injection Capillary Pressure (Pore Volume)

Cuttings

Plug

Por. = 3.9%Perm. = 0.005 md.

Plug

40010001300

Cuttings

160450800

100 90 80 70 60 50 40 30 20 10 0Pore Space Occupied (%)

1

10

100

1000

100000

10000

Mercury Injection Capillary Pressure (Pore Volume)

Cuttings

Plug

Por. = 9.4%Perm. = 0.031 md.

"A" SEAL "B" SEAL

"C" SEAL "D" SEAL

Plug

60265300

Cuttings

25122225

100 90 80 70 60 50 40 30 20 10 0Pore Space Occupied (%)

1

10

100

1000

100000

10000

Mercury Injection Capillary Pressure (Pore Volume)

Por. = 13.1%Perm. = 0.43 md.

Cuttings

Plug

Plug

30105135

Cuttings

2285125

100 90 80 70 60 50 40 30 20 10 0Pore Space Occupied (%)

1

10

100

1000

100000

10000

Mercury Injection Capillary Pressure (Pore Volume)

Por. = 19.0%Perm. = 0.43 md.

Cuttings

Plug

Entry5%10%

Pc@

Entry5%10%

Pc@

Entry5%10%

Pc@

Figure 8. Examples of high-pressure mercury/air injection curves for seal types “A”, “B”, “C”, and “D”. Curvesfor the vertical plug are designated with black squares; the cuttings curves (open squares) are of simulated cut-tings from rock adjacent to the vertical plug.

10 µ1000 X

"A" SEAL "B" SEAL

"C" SEAL "D" SEAL

A B

DC

Figure 9. SEM photomicrograph of seal types “A,” “B,” “C,” and “D” illustrated in Figure 8.Note the 10µ scale. “A” Seal—Predominant clay fabric with limited grain support. Clay par-ticles are compacted and have only a slight recrystallized diagenetic appearance.Intercrystalline pore volume is low. “B” Seal—Rock fabric shows some grain support andhas common intergranular detrital clay. Clay particles are generally compacted and exhibitlimited diagenetic character. Minor intercrystalline pore space, largely concentrated in inter-granular areas where clay is less compacted. “C” Seal—Rock fabric shows grain support.Intergranular clay particles have a more random orientation, which corresponds to anincrease in intercrystalline pore volume. Clay morphology shows more pronounced diage-netic character. “D” Seal—Grain-supported fabric with development of quartz overgrowthcement. Clay minerals consist of diagenetic grain-coating/pore-filling chlorite and kaolinitewith common intercrystalline pore space. Partial preservation of intergranular pores, withapertures restricted by diagenetic phases.

10 Sneider et al.

correlates best with the hydrocarbon column heights(HCH) found in reservoirs.

The air permeabilities of the seals measured understress are low to very low. Scanning electron micro-scope (SEM) photomicrographs (Figure 9) show thepoor interconnection of pores in the “A” and “B” seals.For the “C” and “D” seals, pore size and interconnec-tion increase.

EMPIRICAL ADJUSTMENT FACTORS

The empirical adjustment factor (EAF) is the differ-ence expressed in pressure (psi) between the capillarypressure at a specific mercury saturation measured on avertical plug of a seal cut perpendicular to the sealingsurface (i.e., usually a vertical plug cut perpendicular toa bedding surface) and that measured on simulated

Table 2. Empirical Adjustment Factors (EAFs) to Estimate Air/Mercury (Air/Hg) Capillary Pressures ofVertical, Epoxy-Coated Plugs from Air/Mercury Capillary Pressure of Cuttings.

EAF Values Added to Cuttings Air/Hg CapillaryPressure (psia) at 7.5% Mercury Saturation

NumberSeal Type of Samples Average Min. Max.

A* 6 2315 1402 3120A 72 1810 923 4009B 79 455 423 1040C 48 140 22 363D 27 30 27 91

100 80 60 40 20 017041700

1694

1684

1674

1664

1654

1644

1634

1624

1614

1604

1554

1594

1584

1574

156414

12

10

8

6

4

2

0

140

120

100

80

60

40

20

0

N = 905

FIELD OOWC

MERCURY AND OIL SATURATION (% PORE VOLUME)

"D"SEALS

7.5%

IC, ID-II

IB

IA-B

IA

IAA

(psi

)

(psi

)

Figure 10. Mercury/air andoil/water capillary pressurecurves of reservoir and sealrocks from the Tar SpringsFormation, Benton Field,Illinois. The field columnheight is about 90 ft, whichcorresponds to the oil column held by the weakest“D” seal. Leakage isassumed to be at 7.5% oilsaturation. IAA =>1000–5200 md, well-sorted,lower medium-grainedsandstone; IA = 200–1000md, very well sorted, uppervery fine grained sand-stone; IA-B = 50–200 md,well-sorted, lower andupper very fine grainedsandstone; IB = 10–50 md,moderate to well sorted,lower very fine grainedsandstone and siltstone;and IC, ID-II = 0.1–10 md,moderately sorted siltstone.OOWC = original oil/watercontact.

cuttings of the same seal rock type. The saturation val-ues chosen for EAF usually are determined at 5%, 7.5%,or 10% nonwetting phase saturation. In this paper, 7.5%nonwetting phase (mercury) saturation is used.

From hundreds of pairs of HPMIC curves like thosein Figure 8, EAFs are derived by averaging pressure val-ues between the plug and cuttings values determined at7.5% nonwetting phase saturation. The EAFs, in psi, arethe average values that need to be added to an HPMICcurve of cuttings to approximate the capillary pressurethat would be measured on the vertical plug in whichthe sides are coated and sealed with epoxy. We havestandardized on deriving the EAFs at 7.5% mercury sat-uration. This is based on empirical data that show anequivalence between column heights held in reservoirsand the estimated seal capacity derived from HPMICdata of the capping seals.

The most up-to-date data sets on EAFs are shown inTable 2. These data are based on more than 230 seals.As we continually add additional pairs of verticalplugs and cuttings or simulated cuttings, the EAFsmight be modified, but we expect that future valueswill not be significantly changed.

Figure 10 shows mercury/air capillary pressurecurves based on 905 samples of reservoirs and sealsfrom the Lower Carboniferous sandstone in the BentonField in Illinois. The trap is a simple, four-way closedanticline. The 24 seals measured are “D” type. The mer-cury/air capillary pressure curves of the reservoir andseals are converted to the oil/water system, and then theheight of hydrocarbon column held by the weakest “D”seal is calculated. The field HCH of ~90 ft agrees closelywith the hydrocarbon column predicted for the sealcapacity using the capillary pressure value at 7.5% non-wetting phase saturation.

CONCLUSIONS

1. Vertical plugs cut perpendicular to sealing sur-faces and epoxy-coated on the sides are the mostreliable sample type to obtain high-pressure mer-cury/air capillary pressure curves on seals.

2. High-pressure mercury injection curves (HPMIC)of cuttings from seals at low mercury saturationvalues give capillary pressure curves whose pres-sure values for seal capacity (i.e., hydrocarbon col-umn heights held) are lower than those of thevertical plugs.

3. Empirical adjustment factors (EAFs) added to thecapillary pressure curves of the cuttings can beused to approximate the capillary pressure ofvertical plugs.

4. The EAF values are picked at a nonwetting phasesaturation of 7.5%. The capillary pressure at 7.5%saturation corresponds best with the heights ofhydrocarbon columns measured in many fields.

ACKNOWLEDGMENTS

The authors are grateful to Dr. C.L. Vavra of ARCOE & P Technology, Plano, Texas, and an unknown

reviewer for their editorial comments. We owe specialthanks to Florence Rollins for drafting assistance andto Ramona Sneider for preparing the manuscript.

APPENDIX 1

Equations to Convert Mercury/Air CapillaryPressure to Oil/Water and Gas/Water Capillary

Pressure and HCH

The hydrocarbon column held is a function of thebuoyancy pressure, generated by the differencebetween the hydrocarbon and water density, necessaryto overcome the capillary pressure of the seal rock at agiven nonwetting phase saturation (e.g., 0%, 5%, 7.5%).The following equation illustrates this relationship:

(1)

The following values are commonly used for theabove calculation: water density (ρw) = 1.110 g/cc; oildensity (ρh) = 0.8498 g/cc (35° API); and gas density(ρh) = 0.050 g/cc.

The calculation of seal capacity and hydrocarboncolumn held for a given rock type is based on theair/mercury (a/Hg) capillary pressure data and itsconversion to an equivalent hydrocarbon/water(h/w) capillary pressure system using the followingequations:

and

where a/Hg = air/mercury/solid/system, h/w =hydrocarbon/water/solid/system, o/w = oil/water/solid system, and g/w = gas/water/solid system.

The following values are commonly used air/mer-cury contact angle (θ ) = 140°; oil/water contact angle(θ ) = 0°; gas/water contact angle (θ ) = 0°; air/mer-cury interfacial tension (σ) = 480 dynes/cm;oil/water interfacial tension (σ) = 30 dynes/cm; andgas/water surface tension (σ) = 70 dynes/cm. Thesevalues are for surface or near-surface conditions.When using equations 1 and 2 for reservoir condi-tions, the values of fluid densities and interfacial ten-sions must be corrected for reservoir temperatureand pressure.

P

P

c h/w

c a/Hg

h/w h/w

a/Hg a/Hg

h/w h/w

a/Hg a/Hg

= =

cos

cos cos

cos

2

2

σ θ

σ θσ θ

σ θr

r

P h( – )0.433

h (ft) P

( – )0.433

c h/w (psi) w h

c h/w

w h

=

=

ρ ρ

ρ ρ

Seal Capacity from Cores vs. Cuttings 11

P Pc h/w c a/Hgh/w h/w

a/Hg a/Hg

=

σ θσ θ

cos

cos

(2)

12 Sneider et al.

Incorporating the values into equation 2 yields thefollowing conversions:

(3)

By incorporating the values for water, oil, and gasdensity into equations 1 and 3, the hydrocarbon col-umn heights (h) for oil and gas yields:

(4)

These equations are used to calculate the HCH bysubstituting the appropriate air/mercury capillarypressures at 0%, 5%, 7.5%, or 10% mercury saturation.

REFERENCES CITED

Micromeritics, 1995, Operators manual AutoPore III:Norcross, Georgia, 258 p.

Schowalter, T.T., 1979, Mechanics of secondary hydro-carbon migration and entrapment: AAPG Bulletin,v. 63, no. 5, p. 723–760.

Sneider, R.M., K.K. Stolper, and J.S. Sneider, 1991,Petrophysical properties of seals (abs.): AAPG Bul-letin, v. 75, no. 3, p. 673–674.

For oil : h(ft) 0.728 P

For gas : h(ft) 0.414 P

ca/Hg

ca/Hg

=

=

P . P

P . P

co/w ca/Hg

cg/w ca/Hg

=

=

0 082

0 190