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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 meth ane 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 clari ty 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 importa nt 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 e missions 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 Un conventional Oil and Gas Development2

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 oi l 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 exp loitation 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 explo itation. 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 m ay 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 inc lude 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 bra denhead valve employe d as a gas management strategy when observing sustaine d casing head pressure build

- up . This is largely a methane emission most often caused by stray gas migra tion 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 s eems subject to considerable variation. Recent top down studies have suggested that methan e emissions during drilling, although of a short term nature, could be significant . Hi gh methane emissions were recent ly 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 port ion 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 provide s 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 drill ing (pre

-geophysical logging) and efforts to contain, capture or flare such largely uncontrolled releases of methane should be examined.3

**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 t hermogenic 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. **P roduction gas (methane) that becomes s** **tray gas**  – again predominately thermogenic methane gas from the production stream that origin ates 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 c ement or micro
- annular flow etc.) . T his gas is also often under moderate to high pressure depe nding 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 t hermogenic**  **origin from a non**  **- target**  **zone/ formation**  **ca pable 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
. T he Encana Schwartz 2
- 15B (02) well in Piceance Basin, Garfield Co., Colorado where a high pressure non

-target gas zone in the Williams F ork 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 escape d 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 an d condensate surface seep that appeared 3000 feet from the well location
. The seep was coincidently positioned4

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 t he top of production casing cement not being extended high enough to cover the zone so it occurs opposite an open annular interval after well construct ion (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 common ly 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 d epend on well de signs 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
– EM CPA 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 .**  Nui sance 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 problem s 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 t he 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 outsid e 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 rea ch the surface in thi s manner through this mechanism ( e.g. t he P3 well pad on the Div ide 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 (casing head 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 should5

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
. H istorically, 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 l ower 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 eq uipment or combusted on site (more recently?) at the operator’s discretion . That may in clude 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 depen ding on the extent of the non

-t a rget 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 n ear 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 vent ing 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 m ethane 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 i ts 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 constru ction 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
. S hou ld gas bubble formation occur in the annular environment, buoyancy m ay 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 b uild

- up in the annulus may migrate upward as pulses of gas or be episodic in nature (personal communication Richard Jackson, Geofirma) . If correct, this6

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 a nnular 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 cas ing 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. E scape 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 bore hole 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. Paleozoi c) 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 fa ilure or development of joint sets (surficial bedrock i n 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 ope n 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 a nnular 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 deep er 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 bene ath 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 shallow er areas yet where freshwater zo nes are present . This is one possible interpretation for the situation in Parker CO.7

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 mu ch 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
. W ater 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 st ray 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 m igration 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
. o perator’s stray gas management strategy in the context of the state’s regulatory regime for stray gas. Note: At least t hree states (New York, A rkansas, 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 h ave a policy of no open annuli in their well designs ( policy may be area/play sp ecific) , 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 engineerin g 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 limit ing 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 migrate s to groundwater or air
. Quantification of stray gas emissions as a portion of all fugitive metha ne emissions is particularly important now that meth ane venting is being more strictly regulated. Methane a ir 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 Cited8

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, Location s 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. P enoyer, 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 a t the GWPC Stray Gas Forum, Cleveland, Ohio, USA, 25 July 2012


### . http://www.gwpc.org/sites/default/files/event

sessions/Pen oyer_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