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Response to EPA Request for Comments on White Papers Related to Oil and Natural Gas Sector Leaks

Comments by Pete Penoyer, Hydrogeologist 

Water Resources Division, Natural Resources Stewardship and Science Directorate 

National Park Service

Please find my responses to EPA’s request for comment and technical input/additional information on 

the five white papers on methane and VOC emissions from the oil and gas sector. These comments are

limited to the paper on leaks and largely confined to understanding how EPA looks at methane 

emissions where stray gas is the source. Comments are provided to facilitate technical clarity to highly

complex issues surrounding methane emissions associated with stray gas migration. I commend EPA for 

tackling such a complex and what can be at times, a highly controversial subject. Exploitation of the 

nation’s natural gas is a hugely important resource development to get right through the minimization 

of methane emissions and their impacts on climate change. Identifying measures that “encourage 

additional cost-effective reductions” in this potent GHG as addressed in the Climate Action Plan (2014) is 

an important first step for all applicable economic sectors. 

RE: Public Stakeholder Slide Presentations (Slides 14 and 15 on Emissions from Crude Oil Production 

and Natural Gas Production)

The crude oil slide (14) meant to identify “where the emissions are coming from” in crude oil production 

identify the venting of casinghead gas as an emission source. The following slide (15) for natural gas 

production does not indicate another form of casinghead gas such as annular gas venting as an emission

source. Historically, it has been a common practice among operators of gas wells (presumably oil wells 

too) to vent this gas as a stray gas management strategy (leave bradenhead valve open or open 

intermittently as needed to relieve annular gas pressure build-up). The amount of gas vented from this 

source may be relatively small (?) but is poorly quantified and largely unknown at present. Annular gas 

can also pose a threat to groundwater. It is unclear how EPA defines annular gas vented from a gas well,

whether it is a type of casinghead gas but vented through the bradenhead valve at the casinghead or if it 

is something else (e.g. strictly annular gas different from that in an oil well tubular which may build up 

casinghead gas but may also vent annular gas from outside the well production or other casing string). 

This becomes important for a clear discussion of the details surrounding emission sources from oil wells 

and gas wells as a function of their different well design and construction and dominant type of 

hydrocarbon produced (gas, oil or condensate). Also, many wells depending on their design may have 

one or more open annuli present.

RE: White Paper on Leaks:

General Comments:

Pg. 2 General discussions of Hydraulic Fracturing and Unconventional Oil and Gas Development
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For technical clarity, in future technical discussions, EPA should consider defining hydraulic fracturing as 

a well stimulation technology or “process” that when combined with horizontal drilling permits 

economic unconventional oil and gas development. Both technologies over the past 15 or more years

have seen significant advances and have become the two key “components” facilitating unconventional 

oil and gas development. Coupling of the two technologies have made exploitation of very low 

permeability reservoirs (shales, tight sandstones) possible (economic). In general, unconventional oil 

and gas development is a better umbrella term for all the components of shale oil and shale gas (low 

permeability) reservoir exploitation. Hydraulic fracturing is simply one component (well stimulation) of 

unconventional oil and gas resource (shales, tight sandstones) exploitation that includes many 

components/activities, albeit the requirements of the hydraulic fracturing process may place increased 

demands on other components (e.g. water supply, waste disposal, emission controls). These 

components include establishing a water source/supply, well design and construction, drilling of the 

vertical and horizontal borehole, and casing, cementing, perforating, and stimulating (hydraulic 

fracturing) the well to mention but a few. While the public may understand and use hydraulic fracturing 

as the umbrella term to include all of these activities, it would reduce misunderstandings if the 

terminology was defined in technical discussions more specifically. 

Pg. 3 Definition of leaks

It may be helpful if EPA also clarifies the definition of leaks to include (or use as a non-leak emission 

example) intentional, intermittent or continuous releases of annular methane via the bradenhead valve 

employed as a gas management strategy when observing sustained casing head pressure build-up. This

is largely a methane emission most often caused by stray gas migration from a non-target formation. 

Clarification is needed to determine what characterizes a true leak in this case. How operators in 

conjunction with state regulators have historically defined and managed stray gas has not been all that 

transparent and seems subject to considerable variation.

Recent top down studies have suggested that methane emissions during drilling, although of a short 

term nature, could be significant. High methane emissions were recently reported in fly-over methane 

sensing of actively drilling Marcellus wells in SW PA in a June 2014 Energywire report . This type of 

emission may be highly site, play or basin-specific, related to the geology of a particular area (i.e. 

number of non-target gas bearing zones penetrated above target formation, and their pressure status 

relative to hydrostatic conditions and spatial/lateral extent/volume of a gas bearing “show” non-target 

zone). In addition, the drilling method (air or mud rotary) used in borehole advancement of the vertical 

hole portion could be a significant variable as well in controlling methane emissions during drilling. The 

mud column in mud rotary drilling will normally counterbalance zones with gas flow potential but it is 

unclear how drilling with air acts similarly or provides a sufficient counterbalance to limit potential gas 

flows. These factors can make a well more or less prone to gas escape during drilling and may indicate 

there is potential for a short term “super emitter” of non-target gas during the drilling phase. Longer 

term methane emission potential depends on the ability to recognize the wells stray gas migration 

potential, subsequent well construction details and the gas management strategy employed. Methods 

to identify such zones of potential flow during drilling (pre-geophysical logging) and efforts to contain, 

capture or flare such largely uncontrolled releases of methane should be examined.
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RE: Recognition of methane sources and gas streams originating in the subsurface that may lead to 

atmospheric emission or increased methane levels in groundwater/methane contamination: 

EPA may find it useful as its work on methane emission proceeds to define terminology precisely to 

foster better clarity and discussions on this topic as terminology and use varies from state to state and 

among federal agencies. Methane has at least three primary sources in the subsurface related to 

unconventional (deeper plays) in the context of types of methane gas and sources making up one or 

more gas streams and the role of well construction (more sources if one were to include discussions of 

shallow coal bed methane plays and biogenic gas). Possible examples of such gas stream types, their 

origins or related sources and associated pressures along with some terminology I have seen used 

include:

1. Production gas - predominately thermogenic methane from the target formation that is 

captured or produced and transferred to the surface as the produced gas stream, usually under 

moderate to high pressure.

2. Production gas (methane) that becomes stray gas – again predominately thermogenic methane 

gas from the production stream that originates in the target formation but escapes through 

failure of some designed barrier in well construction (e.g. leakage to the annular due to hole in 

casing or through bad or damaged joint threads, failure of a production casing cement job 

opposite the target formation/producing zone etc.). This typically results in annular gas

migration, gas channeling in cement or micro-annular flow etc.). This gas is also often under 

moderate to high pressure depending on the nature of the leak. Some of the early Cabot well 

failures around Dimock, PA appear to be caused by such a barrier failure/bad cement job across 

the target formation in vertical Marcellus wells that led to significant over pressuring of the

open annulus, with possible gas escape through fractured bedrock below surface casing.

3. Stray methane gas of thermogenic origin from a non-target zone/formation capable of flow to 

the annular - This gas may migrate upward via the annulus (micro-annular flow or gas 

channeling) from a cemented interval above the target formation but is sourced below the top 

of cement for the production casing. It results from a poor/compromised cement job (i.e. 

cement barrier failure). This gas can cover a broad range of pressure scenarios measured at the 

bradenhead. The Encana Schwartz 2-15B (02) well in Piceance Basin, Garfield Co., Colorado 

where a high pressure non-target gas zone in the Williams Fork Fm. had an insufficient cement 

column above after cement “fall back” to a lost circulation zone resulted in overpressure (600+ 

psi) of the annulus is an example (D. Andrews, personal communication). This gas/condensate

escaped up the annulus and likely across the borehole wall intersected by a fault or through the 

fractured bedrock geology should it extend beneath the surface casing seat (Penoyer, 2012). 

This loss of well control “event” gave rise to the Divide Creek methane and condensate surface 

seep that appeared 3000 feet from the well location. The seep was coincidently positioned 
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relative to the well along the azimuth of a joint trend mapped in the bedrock surface (2011, The 

Walter Environmental Group).

4. Stray methane gas of thermogenic origin from a non-target formation capable of flow that 

was missed during drilling, logging and subsequent cementing operations – This condition 

reflects a failure by the operator to identify and “isolate all zones capable of flow” to the 

wellbore to provide a barrier (usually cement in the annular space). In many situations this is 

due to the top of production casing cement not being extended high enough to cover the zone 

so it occurs opposite an open annular interval after well construction (Penoyer, 2011). King 

(2013) did not appear to address this particular case of gas migration in his comprehensive 

analysis of environmental risk of well construction barrier failures. Historically this has occurred

fairly commonly as a significant percentage of older (and some newer) production wells have 

exhibited positive bradenhead pressures in some plays or portions of basins (e.g. NE PA 

Marcellus Play and Piceance Basin, Colorado). The gas volumes involved and sustainability of 

casing pressures depend on well designs and local geologic factors that will determine the 

significance and frequency under which this condition develops. State oil and gas agencies may 

manage this often area-specific stray gas occurrence/problem and management issue in several 

ways and should be consulted in arriving at a suite of best management practices that may 

cover the variable geologic and operational conditions (e.g. Colorado Oil and Gas Conservation 

Commission designation of the East Mamm Creek Production Area –EMCPA and Notice to 

Operators Policy Areas).

5. Nuisance (stray) gas is often of biogenic origin but may be thermogenic or a mixture of the 

two. Nuisance stray gas is most often relatively shallow, of relatively low pressure (found under 

low hydrostatic conditions because of its shallow depth) and attributed to the occurrence of gas 

in intervals generally opposite or somewhat below the surface casing interval that normally 

extends only to shallow depths. It can cause problems in obtaining a good cement job across 

the surface casing interval and lead to gas channeling or micro-annular flow in surface casing 

cement. Nuisance gas can migrate vertically to the surface via gas channels in the cement

exiting immediately adjacent the wellhead. Alternatively, when a fractured surface geology is 

present, this type of stray gas may reach the surface from its shallow depth to areas 

immediately surrounding the well pad by following pathways that combine gas channels in the 

cement outside the surface casing with fractures/joint systems in the bedrock that intersect the 

borehole wall. Thus, gas migration of short distances from the wellbore itself may reach the 

surface in this manner through this mechanism (e.g. the P3 well pad on the Divide Creek 

anticline, Piceance Basin, Garfield Co. Colorado may be an example of nuisance gas emissions 

from the Wasatch Fm. that surfaced around the well pad during drilling operations, (personal 

communication Anthony Gorody).

Stray gas examples 2, 3, 4 and sometimes 5 will likely result in a positive gauge pressure at the 

bradenhead valve (casinghead or other annuli) and depending on the depth of the source bed, 

corresponding hydrostatic pressure for the gas bearing zone or it’s degree of over-pressuring should 
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that be the case, can result in a bradenhead valve gauge pressure build-up of from a few psi to several 

hundred psi or more. Higher pressures yet may be indicative of an underground blowout/loss of well 

control related to barrier failure and the escape of gas from the target formation. Historically, 

depending on a particular state’s regulations, this condition will normally require well remediation or 

plugging and abandonment of the well if well control or remediation measures such as performing a 

squeeze job are unsuccessful. Casinghead gas in oil wells and lower sustained casing pressure (SCP) gas

of around +/- 100 psi from annular intervals in gas wells, historically has been vented, captured for 

compression and sale, used on site to power equipment or combusted on site (more recently?) at the 

operator’s discretion. That may include consultation with state regulators as stray gas management in 

the light of emissions and climate change concerns receives increased attention. 

Extremely limited non-target reservoirs may be depleted of gas during drilling (through gas kicks), upon 

well completion with a few openings of the bradenhead valve (bleed off) or through continuous venting 

over a short period depending on the extent of the non-target reservoir bleeding off the gas. It is 

unknown to what degree the various options to manage stray gas pressures in the past has subjected 

groundwater resources to a gas migration risk/impact when conditions of both an open annular well 

design is coupled with a near surface fractured geology that extends below the surface/intermediate 

casing seat (i.e. results in completed pathway for annular gas to migrate to a freshwater resource). 

While barrier failure (casing and cement) for surface casing in modern onshore oil/gas wells is 

considered rare (King, 2013), migration of stray gas to lower portions of freshwater intervals that may 

be layered and preclude further upward gas migration to the shallower depths penetrated by most 

domestic wells, has not generally been evaluated nor monitored. Presumably, the ability to and 

acceptance of venting stray gas (continuously or intermittently) as a management strategy in the past

has not been an overriding concern of most states (rarely a focus or on the radar) until recent methane 

emission concerns and their regulation became a focus (e.g. Colorado). There may also be great 

variability among operators in how stray gas is managed and this also is likely a cause for its poor 

quantification as a methane emission source.

RE: 5.0 CHARGE QUESTIONS FOR REVIEWERS

Question #5. For the upstream drilling and production activities there are combinations of well 

construction and local geologic conditions that make a well more prone to methane leaks to ambient 

atmosphere or alternatively, if the annulus is shut in at the bradenhead valve, subsurface stray gas 

migration to groundwater. They include:

1. A well design and construction that allows a long, cross-stratigraphic, open (un-cemented) annular 

interval in the vertical section. There is the potential that missed, gas bearing, non-target (show) 

intervals left un-isolated by cement may subsequently bleed methane to the well annulus and allow 

upward migration under a differential pressure (gradient) or gas buoyancy (density) drive. Should gas 

bubble formation occur in the annular environment, buoyancy may assist the normal pressure gradient 

(higher hydrostatic pressures at bottom less hydrostatic pressure at top) to drive the gas upward. 

There is some evidence to suggest that methane build-up in the annulus may migrate upward as pulses 

of gas or be episodic in nature (personal communication Richard Jackson, Geofirma). If correct, this 
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would complicate monitoring of such emissions when the bradenhead valve is left open and gas escape 

is intermittent from such pulses. Also there appears to be little understanding of how borehole annular 

fluids may change over time (e.g. decrease in density from precipitation and/or settling out of heavier 

fluid additives barite etc.) allowing increased annular gas flow from associated greater buoyancy drive.

2. Surface bedrock geology characterized by any of the following conditions (a – f below) that would 

increase the likelihood of deeper bedrock fracturing extending below the surface/intermediate casing 

seat/shoe depth or increase the likelihood of gas flow. Should gas under higher hydrostatic (or greater) 

pressures escape to the annulus at greater depth, this gas may develop a positive gauge pressure at the 

casing head (bradenhead valve) and over-pressure the shallower annulus. Escape of this gas to 

freshwater zones or ground surface via fractures or to the atmosphere through annular venting is 

possible. Completion of an open fracture pathway from the borehole annular and through the fracture 

in the borehole wall could even then circumvent (upward to fresh groundwater zones) a

surface/intermediate casing string of good integrity (i.e. leakage pathway would not be made apparent 

by mechanical/casing integrity testing).

a. older rocks (e.g. Paleozoic) or those of a particular rock type (well-indurated limestone, 

sandstone) occurring at the surface. In general rocks at the surface that are more brittle/indurated 

would be more prone to fracture failure or development of joint sets (surficial bedrock in NE PA may be 

a good example).

b. areas where rocks are more severely tectonically deformed or subjected to stresses (past or 

present) and thus likely fractured (e.g. mountain fronts, crests of folds/anticlines where tensional 

stresses could maintain open fractures to greater depths, basin and range horst and graben areas such 

as Nevada etc.). 

c. areas where there has been a significant unloading/removal of overburden or change in stress 

regime (e.g. burial beneath thick ice sheet/removal of overlying geologic units by erosive forces resulting 

in rebound or relaxation of earlier stress field leading to jointing)

d. areas where the geologic section is characterized by multiple gas bearing units/show 

intervals, particularly those capable of flow should annular borehole fluid density change over time. 

Drilling in such areas may require over-balanced mud systems leading to the failure to identify gas zones 

while drilling/lithologic logging or in subsequent geophysical logging. When potential flow zones to the 

annulus are identified, normally they would be isolated (zonal isolation) behind casing or cement to 

avoid development of sustained casing pressure.

e. basin flank areas where shallow strata dip into deeper areas of the basin where the same 

strata are gas generative/productive. Open annuli can foster cross-strata gas communication between 

permeable deeper zones under higher pressures and shallower zones under lower pressure. Gas 

migration into the shallower permeable zones but still beneath the surface casing interval may occur

under the pressure gradient caused by the deeper gas bearing unit. Should this gas then migrate further 

up dip by a permeable stratigraphic pathway to shallower depths, it may reach shallower areas yet 

where freshwater zones are present. This is one possible interpretation for the situation in Parker CO. 
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TX. but more related to gas sourcing from down dip wells than horizontal Barnett wells of Range 

Resources whose laterals passed closest to complainant’s wells but were much deeper. Geological cross 

sections indicate that this area of Parker County may also reflect the natural occurrence of gas from a 

shallow, legacy gas accumulation related to a possible truncation-type stratigraphic trap in or proximal 

to the freshwater bearing zone. Water wells in this area have historically flared methane gas even 

before Barnett Shale development (Pope, 2012). Whether methane gas is actually increasing (or has 

increased in the past) in the up dip groundwater aquifer intervals from stray gas migration between 

permeable zones in the Strawn Fm. facilitated by down dip wells with open annular construction is not 

possible to determine based on the current data set. The number of variables that must be controlled 

to understand subsurface migration of gas when geologic setting is complex coupled with historic 

occurrence of methane in the area groundwater makes determining a unique cause and effect 

problematic (Pope 2014). 

f. operator’s stray gas management strategy in the context of the state’s regulatory regime for 

stray gas. Note: At least three states (New York, Arkansas, West Virginia) have changed regulations in 

response to high volume hydraulic fracturing/unconventional oil and gas development to include 

circulating cement to surface for all casing strings. That would eliminate any open annular interval in the 

well construction that might allow cross stratigraphic stray methane gas migration to occur 

uninterrupted up the annulus notwithstanding other forms of gas migration through cemented annuli 

(micro-annular flow and gas channeling). Some companies (tend to be larger ones) also appear to have 

a policy of no open annuli in their well designs (policy may be area/play specific), possibly in part for 

liability reasons, and circulate cement for all casing strings to surface. Other companies cement casing 

strings some minimal distance (e.g. 100 to 200 feet) into the next casing string above. 

However, some engineers argue against these latter two well casing cementing designs as offering little

or insufficient capability to monitor annular pressures for leaks (at joints/connections/tubulars or in the 

case of various barrier failures) during the hydraulic fracturing process and/or during subsequent 

production of the well. Some view this annular pressure monitoring data as very critical information to 

have and should not be precluded by the well design or construction. EPA is encouraged to incorporate 

the range of engineering perspectives on designing and maintaining wellbore integrity. It is also 

important to understand monitoring options that may be sacrificed in well designs that appear to be 

more protective in limiting annular gas migration by circulating all cement for casing strings to surface. 

The positives and negatives of these and other well design considerations are necessary to determine if 

a single but somewhat flexible well design is preferred or all decisions in this area are best site-specific 

as local geology certainly does play a role in the overall risk as states generally maintain. There are 

many challenges to quantifying and monitoring stray gas that migrates to groundwater or air. 

Quantification of stray gas emissions as a portion of all fugitive methane emissions is particularly 

important now that methane venting is being more strictly regulated. Methane air emissions are likely 

to be the primary focus in the future as these impacts (primarily climate change risk but also as ozone 

precursor)appear to significantly outweigh impacts/risks to groundwater from subsurface methane 

migration. 

References Cited 
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Energywire Article June 2014

King, G.E. and King, D. E. (2013). Environmental Risk Arising From Well-Construction Failure –

Differences Between Barrier and Well Failure, and Estimates of Failure Frequency Across Common Well 

Types, Locations and Well Age. Publication from Presentation made at Society of Petroleum Engineers 

ACE, SPE Production and Operations, Nov. 3013, p. 323 - 344

New York State Department of Environmental Conservation, 2011. Revised draft Supplemental Generic 

Environmental Impact Statement on the Oil, Gas and Solution Mining Regulatory Program, prepared by 

NYSDEC with Assistance from Alpha Environmental Inc., Ecology and Environment Engineering P.C., ICF 

International, URS Corp, NTC Consultants and Sammons/Dutton LLC. 

Penoyer, P. E. 2011. The Hydraulic Fracturing Process (HF): Real Concern or Misdirected Focus 

Concerning Threats to Drinking Water Supplies (DWS). Poster Presentation at the 2011 Marcellus Shale 

Summit, Pennsylvania State University, State College, PA.; U.S. Department of Interior National Park 

Service, Natural Resource Stewardship and Science Directorate, Fort Collins, Colorado.

Penoyer, P. E. (2012). Mamm Creek Field Case History Piceance Basin Colorado, An Unconventional 

Resource Development Example of Stray Gas Migration. Presented at the GWPC Stray Gas Forum, 

Cleveland, Ohio, USA, 25 July 2012. http://www.gwpc.org/sites/default/files/eventsessions/Penoyer_Pete2.pdf

Pope, P. G. 2012. Parker County Study. Presented at the GWPC Stray Gas Forum, Cleveland, Ohio, USA 

July 25, 2012. http://www.gwpc.org/sites/default/files/event-sessions/Pope_Peter.pdf

Pope, P. G. 2014. Water Well Complaint Investigation Report Silverado on the Brazos Neighborhood 

Parker County, Texas. Produced for the Railroad Commission of Texas. 12 p.

Walter Environmental Group, 2011. Divide Creek Area Joint Study, Summary Report, 20 p.

Attachments are provided that may assist in a further visual understanding of the comments made here.

For any follow-up questions related to these comments, please contact Pete Penoyer at 970-225-3535

or pete_penoyer@nps.gov .

Thank you,

Pete Penoyer, Hydrogeologist

National Park Service, Water Resource Division, Natural Resource Stewardship and Science Directorate