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Pore-throat sizes in sandstones, tight sandstones, and shales_ Discussion

DISCUSSION AND REPLY

Pore-throat sizes in sandstones,tight sandstones,and shales: Discussion

Wayne K.Camp1

INTRODUCTION

Nelson(2009)prepared a nice compilation de-scribing the wide range of pore-throat sizes that span a continuum from conventional sandstone reservoirs to unconventional tight-gas siliciclastic reservoirs and shale seals.In this article,Nelson (2009)proposes a new and rather unique defini-tion for unconventional reservoirs(including tight-gas sands)that lacks sufficient supporting evidence or references to support this definition.Nelson (2009,p.329)defines a conventional reservoir as,“…one in which evidence that buoyant force has formed and maintained the disposition of oil and gas is present,”and thus for unconventional reservoirs,it follows that Nelson concludes(p.330) that,“In these systems,evidence for buoyancy as a dominant force in the disposition of oil and gas is lacking.”

It appears that the sole argument for non-buoyant or buoyancy-subordinate unconventional reservoir systems is the perceived lack of evidence of buoyancy,which is generally not good science (absence of evidence is not evidence of absence). It would be much better to present a clear and concise hypothesis followed by objective obser-vations that support the conclusions.I appreciate that such a thesis may be beyond the scope of a Geologic Note article,but until such evidence is presented,it is difficult to have intelligent dis-cussions and design experiments to test the hy-pothesis of nonbuoyant gas systems.

Although it is not clear from the definition of Nelson(2009),the reader is left to conclude that the natural laws of buoyancy are not dominant factors during the evolution of tight-gas sand ac-cumulations from hydrocarbon migration to the present-day observed distribution of hydrocarbons and water.This conclusion is misleading and ig-nores a significant body of recent published re-search(Shanley et al.,2004;Cluff et al.,2005; Fassett and Boyce2005;Camp,2008).

Nelson(2009,p.339)further concludes that one may simply use a measured pore-throat diameter cutoff in considering the function of buoyancy in the disposition of fluids in consolidated siliciclastic reservoirs and thereby classification as conventional or unconventional petroleum accumulations.Re-viewing figure2(p.332)of Nelson(2009),the pore-throat radius cutoff separating his interpreted conventional(buoyancy-driven)from unconven-tional(nonbuoyancy-driven)reservoirs is approxi-mately1to2m m.

BUOYANCY

Buoyancy is simply the upward force that keeps things afloat,such as oil on water in a beaker.The force of buoyancy is equal to the weight of the fluid displaced as defined by Archimedes more than2000yr ago.Therefore,buoyancy is only a function of the difference in density between hy-drocarbons and water and not a function of the size of the container(or pore-throat diameter),un-less the size of the void space is insufficient to

Copyright?2011.The American Association of Petroleum Geologists.All rights reserved.

1Anadarko Petroleum Corporation,1201Lake Robbins Drive,The Woodlands,Texas 77380;wayne.camp@https://www.wendangku.net/doc/1013541398.html,

Manuscript received February17,2010;provisional acceptance March26,2010; revised manuscript received April9,2010;final acceptance December14,2010.

DOI:10.1306/12141010019

AAPG Bulletin,v.95,no.8(August2011),pp.1443–14471443

contain molecules of gas and water,which is un-likely for tight-gas reservoirs(figure2of Nelson, 2009).

Unfortunately,Nelson(2009)does not spec-ify what lack of evidence he is relying upon to conclude that buoyancy is not a controlling force in certain low-permeability gas accumulations,nor does he provide evidence of,or a model to de-scribe,the development of nonbuoyant petro-leum systems.

An AAPG Hedberg Conference was held in Vail,Colorado,in2005to specifically address tight-gas sandstones.At that time,considerable contro-versy existed with what has been called“basin-centered gas systems”(Cumella et al.,2008).The idea that buoyancy was not a factor in basin-centered and other tight-gas systems is generally attributed to the following observations in many tight-gas–producing regions:(1)lack of significant water production,(2)lack of clear gas-water contacts, and(3)lack of a well-defined conventional trap-ping mechanism.

Based predominantly on bottom-hole pressure observations from the San Juan Basin,New Mexico, Nelson and Condon(2008)deduce that buoyancy cannot exist within underpressured gas accumu-lations in low-permeability sandstones of the Mesa-verde Group if(my emphasis)all movable water has been displaced and no bottom water is present. They envision any water that may be present as forming a very thin film on mineral grain surfaces, filling only the smallest voids,and that the irre-ducible water,therefore,acts only as a part of the solid rock container holding the gas and impart-ing no buoyant force(Nelson and Condon,2008). Unfortunately,Nelson and Condon(2008)do not include any Mesaverde water saturation or capil-lary pressure measurements in support of their conclusions.

The perceived lack of evidence of water in some tight-gas reservoirs may,therefore,lead to the er-roneous conclusion that gas-water buoyant forces cannot exist.The lack of evidence of water in some tight-gas reservoirs may be caused in part by the lack of detailed studies addressing water produc-tion,the source of produced water,and in-situ water saturation.WATER PRODUCTION

Many tight-gas reservoirs produce some water.For example,the stabilized flow rates after recovery of fracture stimulation fluids average between 500and900mcf/day with1to5bbl of water per day from the east Texas Bossier sands(Newsham and Rushing,2009).The low-salinity(40,000–60,000ppm NaCl)produced water is consistent with laboratory analyses that indicate that5to 10mol%of water vapor may be dissolved in gas at reservoir conditions within the deep(12,500–13,500ft[3810–4115m])high-pressure gradient (0.6–0.9psi/ft[13.57–20.36kPa/m])and high-temperature(280–325°F[138–163°C])reservoir conditions(Newsham and Rushing,2009).Con-nate water saturations from gas productive Bossier sandstone reservoirs measured from cores obtained with low-invasion oil-based drilling mud range from5to50%(Newsham and Rushing,2001).

Shanley et al.(2004,2005)and Cluff et al. (2005)have shown that although water production from the major Rocky Mountain tight-gas fields in the Green River,Piceance,and Uinta basins is generally low,it is too high to be explained as water derived from gas condensation.Unfortunate-ly,Nelson(2009)and Nelson and Condon(2008) did not include water production data for the underpressured Mesaverde tight-gas reservoirs.

WATER SATURATION

Warpinski and Lorenz(2008)report data from three wells covering a vertical interval of4300to 8200ft(1300–2500m)within an overpressured gas-producing region at the multiwell experiment (MWX)site at Ruleson field in the Piceance Basin, Colorado.A pressure core using oil-based drilling mud was acquired to provide the most accurate water saturations possible(Warpinski and Lorenz, 2008).These carefully obtained water satura-tion measurements in low-permeability(<0.1md in situ)reservoir-quality(>4%porosity)Mesaverde core samples range from about20to70%.Irre-ducible water saturation measured from high-speed

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centrifuge capillary pressure tests are quite high, ranging from30to50%.

Recent capillary pressure data also demon-strate that many tight-gas reservoirs are not at ir-reducible water saturation,but instead,the in-situ permeability may be below the critical water sat-uration required to flow water(Cluff et al.,2005). Note that the critical water saturation is much higher than irreducible water saturation.There-fore,one cannot simply assume that the lack of significant water production is evidence for irre-ducible water saturation or lack of buoyancy.

BURIAL HISTORY

The inferences by Nelson and Condon(2008)and Nelson(2009)regarding the function of buoyancy in forming and controlling the distribution of hy-drocarbons in unconventional tight-gas systems is based on present-day observations of structure, pressure,porosity,permeability,and water https://www.wendangku.net/doc/1013541398.html,w(2002)describes the evolution of basin-centered gas systems beginning as normal-pressured water-saturated systems that become overpressured as a result of disequilibrium compaction and later hydrocarbon generation(and migration);processes that should be governed by the natural laws of https://www.wendangku.net/doc/1013541398.html,w(2002)also recognizes that some basin-centered gas accumulations,which he des-ignates as“indirect systems,”may have originated as buoyancy-dominated conventional oil traps that later converted to gas as a result of thermal cracking of oil with increased burial.Underpressured tight-gas sand reservoirs(such as described by Nelson and Condon,2008)are thought to be caused by partial gas loss and reservoir temperature abate-ment during late-stage uplift and erosion(Law, 2002).Thus,present-day observations provide only a snapshot of an evolving petroleum accumulation.

Coskey(2004)demonstrated the importance of understanding the geology at the time of hy-drocarbon expulsion and migration before making conclusions regarding the function of buoyancy. For example,at Jonah field in northwestern Green River Basin,Wyoming,the present reservoir qual-ity of the Lance sandstone reservoirs ranges from 2to12%porosity,with0.0005to0.6md perme-ability(DuBois et al.,2004).However,the mod-eled porosity(and related permeability)in the Lance reservoirs was likely significantly higher (16–23%porosity)at the time of hydrocarbon expulsion and migration(Coskey,2004).

The implications of the study by Coskey(2004) is that present-day reservoir parameters may not represent the conditions of the reservoir during the time of hydrocarbon expulsion and entrapment, indicating that buoyancy may have,in fact,been the dominant force forming the distribution of hydrocarbons in tight-gas accumulations,although no clear gas-water contacts can be easily described at present.These inferences,as shown by Coskey (2004),can easily be tested with simple modeling experiments.If it is accepted that buoyancy was a factor in forming tight-gas accumulations,then the unconventional reservoir definition of Nelson (2009)becomes moot.

EXPLORATION CONCEPTS

Coleman(2008,p.246)reviewed25yr of tight-gas exploration in a variety of basins across the United States from which he recommends,“…drill as high on structure as possible;tight-gas sand-stones were not always as tight as they are now, and fluid buoyancy worked at one time,even if it may be overcome by pore-throat friction now.”

In describing the same Upper Jurassic Bossier Formation tight-gas sandstone reservoirs in east Texas and north Louisiana that Nelson(2009) shows as occurring below the1-m m pore-throat unconventional reservoir cutoff,Blanke(2005) concludes that the Bossier reservoir quality is highly dependent on many of the same conditions gov-erning conventional gas accumulations.The higher quality Bossier sandstone reservoirs(as much as 0.2md permeability)are associated with traps present at the time of peak hydrocarbon genera-tion and migration where early formed oil accu-mulations,based on pyro-bitumen“cemented”sandstone(Rushing et al.,2004),appear to have

Camp1445

inhibited extensive authigenic quartz cementa-tion that is present in poor-quality reservoirs lo-cated in downdip parts of the trap.

Perhaps the lingering misconception that most low-permeability(e.g.,basin-centered)gas accu-mulations are not governed by the natural laws of buoyancy has impacted exploration efforts in some areas.As Shanley et al.(2005)noted at the 2005Vail Hedberg Conference,there has not been a significant tight-gas discovery in the Rocky Mountains since the discovery at Jonah field in 1992.Tight-gas sand explorations based on per-haps more conventional principles in the east Texas and north Louisiana Gulf Coast basins from 1996to2006have resulted in several large(>1tcf) gas fields including Dew-Mimms Creek,Vernon, and Amoruso fields.

CONCLUSIONS

Nelson(2009)proposed a new definition for un-conventional gas reservoirs that suggests that the natural laws of buoyancy are not(and were not) the dominant force in forming and maintaining the observed distributions of hydrocarbons and water in tight-gas(basin-centered gas)and other unconventional gas accumulations.However,no evidence is presented to support this concept,but only a rather weak statement that evidence for buoyancy is lacking.

Nelson(2009)further proposes that a natural pore-throat radius threshold of1to2m m below which buoyancy becomes the subordinate force exists,and thus the present-day measured pore-throat radius may be used to separate conventional and unconventional reservoirs.Again,no evidence is presented to show the absence,or diminished function,of buoyancy in submicron pore-throat radius gas reservoirs.

The misconception that buoyancy is not a fac-tor in basin-centered and other tight-gas systems is generally attributed to(1)lack of significant water production,(2)lack of clear gas-water con-tacts,and(3)lack of a well-defined conventional trapping mechanism.Several recent studies have shown that water is much more prevalent in many unconventional tight-gas accumulations than pre-viously thought,and traps can be described by con-ventional structural or stratigraphic trap mech-anisms.Burial history studies and experimental models show that many unconventional tight-gas accumulations have evolved with time,and thus limited present-day observations may lead to mis-conceptions regarding the function of buoyancy in forming and maintaining the distribution of wa-ter and gas.

The contention of Nelson(2009)that buoy-ancy is not a dominant factor in the formation and disposition of hydrocarbons and water in tight-gas sandstone(unconventional)accumulations,which can be defined by pore-throat threshold below1to 2m m,is inconsistent with several recent studies of tight-gas accumulations and requires further clar-ification and supporting evidence.Otherwise,it is recommended that this definition be abandoned.

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