Oil and Natural Gas Sector: Definitions of Low Pressure Gas Well and Storage Vessel

40 CFR Part 60, Subpart OOOO

Response to Public Comments on

Proposed Rule (80 FR 15180; March 23, 2015)

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Sector Policies and Programs Division

Research Triangle Park, NC 27711

July 2015
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Table of Contents

1.0 Summary................................................................................................................................... 2

1.1 Background........................................................................................................................... 2

2.0 Response to Comments Regarding Subpart OOOO................................................................. 5

2.1 Definition of Low Pressure Gas Well................................................................................... 5

2.1.1 Support for the Alternative Definition........................................................................... 5

2.1.2 Revisions to the Alternate Definition ............................................................................ 7

2.1.3 Relationship of Low Pressure Wells with the Stages of Flowback ............................... 9

2.1.4 Accuracy of the Turner Equation................................................................................. 10

2.2 Storage Vessel Definition and Requirements for Storage Vessels in Parallel.................... 14

2.2.1 Support for the Proposed Changes to the Storage Vessel Definition and Requirements 

for Storage Vessels Installed or Connected in Parallel......................................................... 14

2.2.2 Support for Retaining the Requirements for Storage Vessels in Parallel .................... 14

2.2.3 The EPA Should Grant a Stay of Final Provisions for Storage Vessels in Parallel..... 15

2.2.4 Clarification of Whether the Storage Vessels in Parallel Provisions were in Effect ... 16

2.2.5 The EPA Must Propose New Requirements................................................................ 18

2.2.6 The EPA Should Clarify the Storage Vessel Requirements Instead of Removing Them

............................................................................................................................................... 19

2.2.7 The EPA Should Extend the Final Requirements to All Tank Batteries..................... 19

2.2.8 Add Exclusion for Storage Vessels Used Primarily in Water Recycling Processes.... 20
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Acronyms and Abbreviations

CDPHE Colorado Department of Public Health and Environment

CFR Code of Federal Regulations

EPA Environmental Protection Agency

GPA Gas Processors Association

HAP Hazardous Air Pollutant

IPAA Independent Petroleum Association of America

PTE Potential to Emit

PTE Potential to Emit

REC Reduced Emissions Completion

TPY Tons per Year

TVD True Vertical Depth

VOC Volatile Organic Compounds

1.0 Summary

1.1 Background

On July 17, 2014, the Environmental Protection Agency (EPA) published proposed 

amendments to the new source performance standards (NSPS) for the Oil and Natural Gas 

Sector. One of the issues addressed in the proposed amendments was the EPA’s proposed 

definition of “low pressure gas well.” A petitioner’s timely submitted comment on the proposed 

amendments concerning the definition was, inadvertently, not made part of the record in the 

rulemaking docket and was, therefore, not available to be considered by the EPA when the 

agency finalized the definition of “low pressure gas well” in its December 19, 2014, final 

amendments to the NSPS. To correct the above mentioned procedural defect, in a notice 

published March 23, 2015, the EPA re-proposed its definition of “low pressure gas well” for 

notice and comment (see 80 FR 15180). In that action, we also proposed to amend the NSPS to 

remove provisions concerning storage vessels connected or installed in parallel and to revise the 

definition of “storage vessel.”

This document presents a summary of the public comments received regarding specific 

issues and topics related to March 23, 2015 proposed amendment and the EPA’s responses to 

those comments. Table 1 provides the list of comments received by docket document ID, the 

commenter and their affiliation. The last 4 digits of the docket document ID are used to identify 

comments summarized and responded to in this document.
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Table 1. List of Commenters on the Oil and Natural Gas Sector: Definitions of Low Pressure Gas Well and 

Storage Vessel; Proposed Rule (80 FR 15180; March 23, 2015)

Document ID Name and Affiliation Date Received

EPA-HQ-OAR-2010-0505-4752

James Elliott, Spilman Thomas & 

Battle, PLLC, on behalf of 

Independent Petroleum Association 

of America et al.

8/18/2015

EPA-HQ-OAR-2010-0505-4755

Urban O’Brien, Vice President, 

Government Affairs, Apache 

Corporation

4/20/2015

EPA-HQ-OAR-2010-0505-4756

David Stuart, Vice President, Fluids 

Conditioning, Rockwater Energy 

Solutions, Inc.

4/22/2015

EPA-HQ-OAR-2010-0505-4757

Steven Fleming, Co-Chair, 

Regulatory Practices Committee, 

Permian Basin Petroleum 

Association

4/22/2015

EPA-HQ-OAR-2010-0505-4758 Thure Cannon, President, Texas 

Pipeline Association 4/22/2015

EPA-HQ-OAR-2010-0505-4760

Gretchen Kern, Senior Regulatory 

and Environmental Policy Advisor, 

Sustainable Development, Pioneer 

Natural Resources USA, Inc.

4/22/2015

EPA-HQ-OAR-2010-0505-4761 Cory Pomeroy, General Counsel, 

Texas Oil and Gas Association 4/22/2015

EPA-HQ-OAR-2010-0505-4762

Matthew Todd, Regulatory and 

Scientific Affairs, American 

Petroleum Institute

4/23/2015

EPA-HQ-OAR-2010-0505-4763

V. Bruce Thompson, President, 

American Exploration and 

Production Council

4/23/2015

EPA-HQ-OAR-2010-0505-4764

Kathleen Sgamma, Vice President, 

Government and Public Affairs, 

Western Energy Alliance

4/23/2015

EPA-HQ-OAR-2010-0505-4765 Darin Schroeder, Clean Air Task 

Force et al. 4/23/2015
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Document ID Name and Affiliation Date Received

EPA-HQ-OAR-2010-0505-4766

Theresa Pugh, Vice President, 

Environment and Construction, 

Interstate Natural Gas Association of 

America

4/23/2015

EPA-HQ-OAR-2010-0505-4767

James Elliott, Spilman Thomas & 

Battle, PLLC, on behalf of 

Independent Petroleum Association 

of America et al.

4/22/2015
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2.0 Response to Comments Regarding Subpart OOOO

2.1 Definition of Low Pressure Gas Well

2.1.1 Support for the Alternative Definition

Comment: Several commenters (4752, 4761, 4762, 4764, 4767) express support for the 

alternative definition of “low pressure gas well” provided by IPAA as being more representative 

of current industry practice of defining these wells.

According to one commenter (4761) the alternative definition is based on the fresh water 

gradient, is widely used in industry and appropriately describes the well conditions where 

installation of reduced emissions completion (REC) equipment is impractical. The commenter 

stated that the fresh water gradient (i.e., 0.433 psi/ft or 8.33 lbs/gal x 0.052 x True Vertical Depth 

(TVD)) represents normally pressured wells based on the hydrostatic overhead pressure of fresh 

water that increases linearly with true vertical depth. If reservoir pressure is less than the 

hydrostatic pressure of water, the well will not flow on its own because of the overhead pressure 

of fracture fluids in the wellbore that will be higher than the reservoir pressure which may make 

REC equipment impractical. The commenter added that whether a well’s productive reservoir 

pressure is above or below the water gradient may be readily confirmed by reading offset 

reservoir pressure data in the development field or by evaluating certain wireline well logs that 

may be run after drilling a well before well completion begins.

Another commenter (4767) stated that the EPA’s current definition is does not accurately 

define what industry has historically defined and recognized as a low pressure well. According to 

the commenter, because EPA's definition does not accurately delineate low pressure wells, the

current definition of low pressure well will subject a subset of wells to RECs where the operation 

of a separator is not physically possible thereby making the wells uneconomical as a result of 

being subject to REC requirements. The commenter included a table showing the values 

calculated using the EPA’s definition for various well depths and flowline pressures. According 

to the commenter, the alternate definition would classify all of the values in the table as a low 

pressure well, while the EPA’s definition would only consider about a quarter of the wells as low 

pressure.

The commenter further stated that the permeability of the reservoir and other reservoir 

characteristics play a critical role in determining when a well is a low pressure well or underpressured. In addition to overcoming the hydrostatic pressure and sales line pressure, the 

separator necessary for the REC adds to the pressure which must be overcome for gas to flow 

from the reservoir. The commenter stated that the separator pressure is arguably the controlling 

parameter on when a REC is feasible versus the sales line pressure. Unlike the sales line 

pressure, which is easily known, the commenter contended that the separator pressure can vary

greatly depending on gas and liquid rates, liquid composition and equipment limitations. The 

commenter pointed out that the EPA's definition does not take separator pressure into account 

thereby making the definition overly conservative. The commenter admits that the alternative 

definitions does not contain an adjustment for separator pressure either, but the definition is more 

accurate and is inclusive of wells recognized by the industry as "low pressure.”
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In addition to the pressure associated with the separator, the commenter stated that in 

order for a separator to function, there must be a sufficient volume of gas (at appropriate 

pressure) to lift the associated liquids and overcome the pressure of the separator. The 

commenter added that if that gas rate is not achieved, the well will load up and a REC will not be 

possible. According to the commenter, the gas rate necessary for a REC varies based on reservoir 

pressure and casing/tubing diameter. The commenter provided a graph of Coleman curves to 

illustrate this point, which illustrates that as the pressure and casing diameter increase, so must 

the gas rate.

Response: The EPA believes that the alternative definition of low pressure well, based 

only on fresh water gradient, may not adequately account for the parameters that must be taken 

into account when determining whether a REC would be feasible for a given hydraulically 

fractured gas well. We believe that, to determine whether the flowback gas has sufficient 

pressure to flow into a flow line, it is necessary to account for reservoir pressure, well depth, 

flow line pressure, and surface (i.e. REC) equipment. In addition, it is important for any such

determination to take into account pressure losses in the surface equipment used to perform the 

REC. The EPA’s definition in the rule was developed to account for these factors.

The EPA agrees that there must be a sufficient volumetric flow of gas (caused by 

adequate reservoir pressure) to lift the associated liquids and overcome the pressure of the 

separator, enabling the gas to be collected (i.e., enter the flow line). However, the EPA disagrees 

that the current definition, which we re-proposed for notice and comment, does not take into 

account the additional backpressure caused by REC equipment. The model uses an energy 

balance to determine the pressure drop based on the calculated velocity, and then the model 

accounts for pressure losses caused by REC equipment, including the separator. The result of 

the model is a prediction of the pressure of the flowback gas immediately before it enters the 

flow line. The result can be compared to the actual flow line pressure available to the well. For 

wells with insufficient predicted pressure to produce into the flow line, as predicted using the 

EPA equation, combustion must be used to control emissions. For wells with sufficient pressure 

to produce into the flow line, gas capture in combination with combustion must be used to 

control emissions.

According to some of the commenters, the EPA’s definition of low pressure gas well 

should be revised because it does not comport with what the industry has historically considered 

to be a low pressure gas well. We are not making a determination on the similarity of the two 

definitions because we do not believe that the two must be the same for purposes of the oil and 

gas NSPS. The EPA has provided a definition of “low pressure gas well” in the NSPS in order 

to designate a class of wells where a REC is not technically feasible. Our definition of “low 

pressure gas well” in the NSPS is for a singular purpose – to identify the wells that cannot 

implement a REC because of a lack of necessary reservoir pressure to flow gas at rates 

appropriate for the transportation of solids and liquids from a hydraulically fractured gas well

during flowback against additional backpressure which would be caused by the REC equipment, 

thereby making a REC infeasible. (80 FR 15182) To the extent that the industry definition is 

different from the EPA definition, the industry likely defines a particular well as being low 
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pressure for a variety of reasons.1 As such, it is not clear that REC is not technically infeasible 

for all of the wells that the industry has historically considered to be “low pressure wells.” 

2.1.2 Revisions to the Alternate Definition

Comment: One commenter (4761) stated that the alternative definition should also be 

clarified to state “where field reservoir pressure is less than 0.433 times the true vertical depth of 

the reservoir.” According to the commenter, referring to reservoir pressure adds clarity and true 

vertical depth is a well-known standard term in the industry to differentiate from “measured 

depth,” where measured depth is the length of the well. The commenters stated that this is an 

important distinction, especially for directional or horizontal wells, that should be clarified in the 

low pressure well definition.

Another commenter (4762) similarly suggested that instead of defining the term “low 

pressure gas well” in terms of the “vertical depth” of the deepest target reservoir, it should 

instead be defined in terms of the “true vertical depth.” The commenter cited to the 

Schlumberger online Oil Field Glossary, which defines “true vertical depth” as follows:

The vertical distance from a point in the well (usually the current or final depth) to a 

point at the surface, usually the elevation of the rotary kelly bushing (RKB). This is one 

of two primary depth measurements used by the drillers, the other being measured depth. 

TVD is important in determining bottomhole pressures, which are caused in part by the 

hydrostatic head of fluid in the wellbore. For this calculation, measured depth is 

irrelevant and TVD must be used. For most other operations, the driller is interested in 

the length of the hole or how much pipe will fit into the hole. For those measurements, 

measured depth, not TVD, is used. While the drilling crew should be careful to designate 

which measurement they are referring to, if no designation is used, they are usually 

referring to measured depth. Note that measured depth, due to intentional or unintentional 

curves in the wellbore, is always longer than true vertical depth.

The commenter stated that it would be better to use “true vertical depth” because the 

measured vertical depth can overstate actual vertical depth because well bores may not be 

absolutely vertical. Thus, measured vertical depth often exceeds the true vertical depth of a well 

bore.

One commenter (4752) stated that the IPAA’s proposed definition for "low pressure 

well" was based on the weight of fresh water (8.33 lbs/gal) which is stacked on top of itself, and 

is known as hydrostatic pressure. Converting the density of fresh water to a pressure gradient 

results in 8.33 lb/gal being equal to 0.433 psi/ft. Therefore, the pressure of fresh water in the well 

bore is 0.433 psi/ft times the vertical well depth.

 

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“USEPA's proposed low pressure well definition forces controls on a segment of the industry that have no

or minimal beneficial impact on the environment while imposing significant additional costs that

will make drilling and operating such wells uneconomical.” (James Elliott, Spilman Thomas & Battle, PLLC, on 

behalf of Independent Petroleum Association of America et al., 8/8/2014)
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The commenter added that in reality, the fluid flowing to the surface could be fresh 

water, re-used hydraulic fracturing water, re-used, produced water, or a mixture. Additionally, in 

the beginning of the operation, the commenter stated that initial fluids flowing to the surface are 

essentially the fracturing fluids put down hole. At the end of the operation, the fluids flowing to 

the surface will mainly consist of reservoir fluids, and the water will be more of a brine water 

and not fresh water. The commenter added that brine water has a greater density, and more 

reservoir pressure will be required to lift the fluid to the surface. The commenter contends that 

the use of a fresh water gradient of 0.433 psi/ft should be used to keep the definition conservative

and simple.

As an alternative, or in addition, to a fresh water gradient, the commenter recommended

that the density of brine water influenced by sand or proppant should be used to more accurately 

reflect the pressure of the water column in the well bore. The commenter pointed out that the 

EPA appears to have utilized a gradient of 0.4645 psi/ft in the "Lessons Learned from Natural 

Gas STAR Partners; Reduced Emissions Completions for Hydraulically Fractured Natural Gas 

Wells" paper developed as a part of the EPA's Natural Gas STAR Program. The commenter 

stated that this is evidenced by the gradients listed in Exhibit 5 of the paper. Additionally, to 

perform a REC, the commenter contended that the downhole reservoir pressure must be 

sufficient enough to lift the hydraulic fracturing fluid to the surface and through the separation 

equipment and piping, with the resulting gas still having enough back pressure for it to get into 

the natural gas gathering line. According to the commenter, to combust flowback emissions the 

downhole reservoir pressure must be sufficient enough to lift the hydraulic fracturing fluid to the 

surface and through the separation equipment and piping, with the resulting gas still having 

enough back pressure to flow to a flare or enclosed combustion device.

To reflect these realities, the commenter proposed that no emission control be required 

when the following scenario exists:

"A well where the reservoir pressure is less than 0.4645 times the vertical depth of the

deepest target reservoir."

At reservoir pressures below this value, the commenter contended that insufficient 

pressure exists for any gas to flow to a flare, enclosed combustion device or the process. 

Consequently, the commenter proposed that combustion through a flare or enclosed combustion 

device be required when the following scenario exists:

"A well where the reservoir pressure is less than 0.4645 times the vertical depth of the

deepest target reservoir plus the gathering or sales line pressure."

At reservoir pressures less than the sum of the water column pressure and the sales line 

pressure, the commenter contended that the recovered gas will not naturally flow into the sales 

line. The commenter stated that the proposed rule does not require compression of recovered gas 

into the sales line. The commenter further states that the EPA has recognized this type of simpler

approach in estimating the level of pressure necessary for recovered gas to flow into a gathering

or sales line in their Gas STAR document cited above. In this Gas STAR paper, a table (Exhibit 
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5) is provided that shows the pressures necessary for various well depths. For instance, the 

commenter pointed out that the document indicates that the reservoir pressure necessary to flow 

recovered gas into a sales line for a 10,000-foot well would be 4,645 psig plus the sales line 

pressure.

Response: We agree with the commenters that “true vertical depth” is more accurate 

terminology that better represents our intent. Although we are not adopting the alternative 

definitions for the reasons presented above, we are amending the current definition of low 

pressure gas well to include “true vertical depth.”

2.1.3 Relationship of Low Pressure Wells with the Stages of Flowback

Comment: According to one commenter (4752), during the initial flowback stage the 

EPA has indicated that there is not enough gas to operate a separator. Gas generated during the 

initial flowback stage would not be controlled under the proposal. The commenter states that the 

EPA acknowledges that during the separation flowback stage there may not be enough gas to 

operate a separator, with the gas either combusted or recovered depending on the well type. For

certain lower pressure wells (that the commenter contends would most likely not meet the EPA’s

proposed definition of a low pressure well), the overall flowback period (all three stages) is so 

short that there is an insufficient amount of gas generated during the separation flowback stage to 

be able to operate or utilize a separator for a meaningful time period.

The commenter states that their proposed definition of a low pressure well that focused 

on the "three day" flowback period attempted to recognize this point. According to the 

commenter, the dynamics of most vertical wells and energized wells are such that RECs or 

combustion by way of a separator is not feasible. The commenter requests that the EPA address 

this issue in their final rule to acknowledge that not every well will have the three flowback 

stages clearly defined, and in certain instances, the separation flowback stage is so short that 

RECs are not feasible or required, i.e., that a well can essentially go from the initial flowback 

stage to the production stage. The commenter states that there is a subjective element to this 

evaluation, which the EPA has acknowledged, so the final rule should not prevent those drilling 

low pressure wells from continuing their operations. The commenter notes that the economic 

incentive to undertake RECs exists. The EPA's proposed low pressure well definition forces 

controls on a segment of the industry that has no or minimal beneficial impact on the 

environment while imposing significant additional costs that will make drilling and operating 

such wells uneconomical. The commenter requests that the definition of a low pressure well be 

revised or that the EPA acknowledge the separation flowback stage can be so short in duration 

that RECs are not necessary.

Response: In the proposed rule we solicited comment on the alternative definition 

suggested by the petitioners and on specific concerns or questions we have with respect to the 

alternative definition. We received no comments that provided any data or other information that 

would lead us to conclude that the alternative definition is sufficient to predict whether an REC

would be infeasible for wells meeting the alternative definition. As explained in the proposal, we

agree with the petitioners that this alternative definition is straightforward and easy to use. 

However, we are concerned that it may be too simplistic and may not adequately account for the
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parameters that must be taken into account when determining whether a REC would be feasible 

for a given hydraulically fractured gas well. According to the commenter, the 3-day flowback 

duration in the alternative definition is not an attempt to define technical feasibility of REC but 

to exempt from the REC requirement well completions where a separator would not be used for 

what the commenter considers a meaningful time period. Because the purpose of defining “low 

pressure well” is to define those wells for which REC is technically infeasible, we decline to 

adopt the 3 day flowback duration. Further, we question how an operator would know before 

flowback begins that the flowback period would be less than 3 days in duration. We believe that, 

to determine whether the flowback gas has sufficient pressure to flow into a flow line, it is 

necessary to account for reservoir pressure, well depth and flow line pressure. In addition, it is 

important for any such determination to take into account pressure losses in the surface 

equipment used to perform the REC. The EPA’s definition in the rule was developed to account 

for these factors.

The commenter also appears to be questioning the current well completion requirements, 

in particular in relation to a separator. These comments do not appear to address how to define 

wells for which REC is technically infeasible, which is the purpose of our re-proposal of the 

“low pressure well” definition. In that respect, we find the comments outside the scope of this 

rulemaking. 

 

2.1.4 Accuracy of the Turner Equation

Comment: One commenter (4752) cites a recent engineering doctoral dissertation at 

Tulsa University that compared several methods of calculating liquid loading in a well, which is

relevant as it pertains to the equations used by the EPA to develop the definition of a low 

pressure well. The dissertation was delivered by Dr. Shu Luo, a graduate student at Tulsa 

University, in 2013. [The commenter letter includes the abstract of the thesis, which is omitted in 

this summary for brevity.]

The commenter states that, in relevant part, the thesis shows that the Turner equation 

typically under-predicts the velocity necessary to unload the well. The commenter contends that 

since the Turner equation under-predicts velocity, the resulting EPA low pressure well formula 

then also under-predicts the pressure necessary for a well to flowback without assistance. For 

instance, to increase velocity in the well tubing, which has a fixed flow area, the flow rate of gas 

must be increased to carry the liquid up the tubing. To increase the flow rate within a given pipe 

size, the commenter contends that pressure must increase. The commenter states that operators 

will sometimes install smaller tubing to decrease flow area, which also increases velocity. 

According to the commenter, this is one example of where the EPA's derivation of a complex 

equation to define low pressure well is flawed. The commenter believes that a more accurate low 

pressure well definition should rely on the liquid film reversal model rather than the Turner 

equation, which would result in a higher pressure threshold than currently proposed by the EPA. 

Again, the commenter believes that the burden is on the EPA to justify its definition and not to 

simply question the alternative definition proposed by the commenter. The commenter contends 

that the EPA's reliance on an outdated equation not recognized or utilized by the industry is 

inappropriate.
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One commenter (4752) takes issues with the EPA’s statement that the commenter’s 

petition for reconsideration did not include any details on which of the EPA’s assumptions 

regarding the definition is questionable. The commenter contends that it is not their burden to 

justify the definition; that burden falls on the EPA. The commenter does not believe that the EPA 

adequately justified the basis of the definition. 

The commenter states that one of the key assumptions to the EPA's definition is its

reliance on the Turner equation to calculate the minimum gas velocity needed to lift a droplet.

This equation is used to predict the velocity needed to lift to the surface the proppant used to

hydraulically fracture the well. The commenter contends that the Turner equation is from a 1969 

article in the Oil and Gas Journal. The equation is based on a droplet reversal model. The 

commenter is not aware of the oil and gas industry using the Turner equation for any practical 

applications (unlike the commenter’s proposed definition which the commenter contends relies 

on industry accepted calculations).

Response: The EPA disagrees with the commenter’s assertions with respect to the EPA’s 

use of the Turner equation in two critical ways. First, the EPA notes that in using certain data 

sets, the thesis cited by the commenter predicts the Turner equation to be effective and superior 

to other models on the velocity calculation evaluated in the thesis, which is contrary to the way 

the commenter characterizes the data underlying the Turner research. 

Second, the EPA adapted the Turner research into the equation to define a low pressure 

well, but made specific provisions to adapt the Turner liquid loading model into a model to 

predict the ability to execute a REC. The thesis referenced by the commenter deals with the 

determination of flow velocity of the liquids in the well and does not provide any methodology 

to convert this into wellhead flowing pressure, which is required for designating wells as being 

low pressure. Unlike the EPA’s adapted model, the Turner equation used in the commenter’s 

cited thesis uses a constant natural gas density, meaning the velocity is determined throughout 

the entire length of the well at only one pressure condition. Additionally, to convert this into a 

wellhead pressure requires further steps which are not available in the thesis nor is an alternative 

method for this conversion proposed by the commenter. 

However, in the adapted equation used in EPA’s composite analysis, the well is divided 

into 2,000 segments and the natural gas density is varied by pressure over each segment. The 

pressure at the end of each segment is subsequently used as the initial pressure in the next 

segment, until the pressure at the wellhead is finally estimated. Considering the calculated 

velocity is dependent on the natural gas density, having this value reflect the condition for each 

segment accounts for the variation in pressure over the entire length of the well.

The liquid film reversal model proposed in the thesis also underperforms the Turner 

equation depending on conditions. The EPA is not aware of any composite model that 

approximates not only the liquid velocity, but also the wellhead pressure using key factors that 

influence the calculation. Therefore, in the absence of a composite model to calculate wellhead 

pressure, the EPA is retaining its model that applies to all wells across the country, and one that 

is not limited to certain well characteristics.
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The EPA notes that the commenter’s definition of a low pressure well being proposed as 

an alternative is an approximation with no basis in the liquid film reversal model. The EPA also 

notes that the Natural Gas STAR Program Lessons Learned document is meant to serve as 

guidance only from a methane emissions calculations perspective and not as a definitive 

methodology to establish low pressure wells. 

Finally, the pressure gradient charts from the United States Geological Survey (USGS), 

shown below, indicate that using the commenter’s proposed definition, the pressure gradients for 

a significant number of wells are inaccurately estimated. This is to say that the commenter 

proposed definition will overestimate the well tubing pressure necessary to overcome formation 

pressure for a large number of wells. Therefore in the absence of any complete model that 

estimates wellhead pressure, the EPA is retaining its previous approach.
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Formation Pressure and Gas-Flow Rates from Well Tests, Southwestern Wyoming 

Province, Wyoming, Colorado, and Utah2

Figure 2C. Pressure data from wells with drill-stem tests in Jurassic and older Phosphoria-sourced 

units in the Southwestern Wyoming Province, Wyoming, Colorado, and Utah. Lines of constant 

pressure gradient give ratio of pressure to depth in pounds per square inch per foot (psi/ft). Well 

locations are shown in figure 2A [figure 2A well locations not shown in EPA TSD. See a copy of the

full document in the docket].

 

2 Nelson, Philip H., and Kibler, Joyce E. (2005). Formation Pressures and Gas-Flow Rates from Well Tests, 

Southwestern Wyoming Province, Wyoming, Colorado, and Utah. In USGS Southwestern Wyoming Province 

Assessment Team, compilers, Petroleum Systems and Geologic Assessment of Oil and Gas in the Southwestern 

Wyoming Provice, Wyoming, Colorado, and Utah. U.S. Geologic Survey Digital Data Series DDS-69-D, Chapter 

17.
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2.2 Storage Vessel Definition and Requirements for Storage Vessels in Parallel

2.2.1 Support for the Proposed Changes to the Storage Vessel Definition and Requirements 

for Storage Vessels Installed or Connected in Parallel

Comment: Several commenters (4755, 4757, 4758, 4760, 4761, 4762, 4763, 4764, 4766, 

4767) expressed support for the proposed changes to the definition of storage vessel and 

removing requirements specific to storage vessels installed or connected in parallel. Most of 

these commenters agreed that the provisions related to storage vessels installed or connected in 

parallel are counter to the EPA’s often stated intent that a storage vessel affected facility is a 

single storage vessel. One commenter (4767) stated that the EPA has explained its concern and 

taken the steps necessary to forewarn industry that connecting storage vessels in parallel to 

circumvent the 6 tpy applicability threshold is not allowed under the NSPS.

One commenter (4758) stated that the requirements for storage vessels installed or 

connected in parallel for the first time treat these storage vessels as a single unit with aggregated 

PTE. According to the commenter, this was a departure from the initially promulgated subpart 

rules and the rule as proposed for public comment, whereby affected facility status for storage 

vessels was based on assessment of a storage vessel's individual PTE, with no mention of 

aggregation of vessels connected in parallel. Moreover, the commenter contends that removing 

the requirements for storage vessels installed or connected in parallel would have the benefit of 

eliminating the imposition of controls on low-emitting storage vessels, which the EPA has 

already determined not to be cost effective.

The commenter added that the EPA is correct to recognize that the terms “connected in 

parallel” and “installed in parallel” inadvertently included storage vessels beyond those that it 

intended to address in the December 2014 Rule. The commenter adds that the EPA indicated that 

it did not intend for the final rule to have the widespread impact currently in effect. The 

commenter noted that, in the preamble to the current reconsideration proposal, the EPA stated 

that the breadth of the 2014 final rule's impact on storage vessels was “inadvertent.” The 

commenter stated that it can be expensive and time consuming to bring a storage vessel into 

compliance with subpart OOOO requirements, and those requirements should not be imposed on 

a storage vessel based on inadvertent rulemaking actions by the EPA. 

Response: The EPA thanks the commenters for their support. 

 

2.2.2 Support for Retaining the Requirements for Storage Vessels in Parallel

Comment: One commenter (4765) acknowledged the EPA’s proposal to remove 

provisions relating to storage vessels “installed in parallel” or “connected in parallel” because 

these provisions “inadvertently” encompassed storage vessels the Agency did not intend to 

address. However, the commenter contends that the EPA does not identify those vessels that it 

believes were inadvertently covered in the December 2014 rule, nor does it propose alternative 

regulatory language that would ensure adequate control measures for vessels connected or 

installed in parallel that were intended to be covered under the December 2014 rule.
15

Given that storage vessels, including those installed or connected in parallel, can be 

significant sources of emissions, the commenter opposed the EPA’s proposal to simply remove 

any provisions addressing these vessels. Instead of removing all provisions regarding vessels 

installed or connected in parallel, as the Agency proposed, the commenter urged the EPA to 

instead clarify its existing requirements for such vessels. The commenter suggested that the EPA 

could, for instance, clarify that pollution control measures apply to storage vessels operated in 

parallel in the relevant regulatory provisions addressing storage vessel affected facilities and the 

definitions of “returned to service” and “storage vessel.”

Response: The change to the definition of storage vessel was intended to preserve the 

original basis of the individual storage vessels to determine affected facility status, while 

addressing the potential situation where crude oil, condensate, intermediate hydrocarbon liquids 

or produced water is divided into two or more storage vessels operated in parallel (i.e. sharing

the emissions at the correlated fraction of what a single tank would emit). Through comments 

submitted on the March 2015 proposed rule, the public has informed us that many storage 

vessels that are configured in parallel may not be operated or constructed to divide their potential 

to emit continuously, if ever. The EPA has now reconsidered our attempt to include storage 

vessels connected in parallel to address the specific situation resulting in circumvention. We 

believe that we do not have sufficient data to evaluate the scope of storage vessels that would fall 

under the definition and for which we did not intend to cover.

We believe that we have sufficient provisions under the General Provisions at 40 CFR 

60.12 “Circumvention” to address the specific situation where storage vessels are divided into 

smaller tanks to avoid applicability of the rule and which was our intent with the previous 

amended definition. Therefore, we do not believe that our reverting to the prior definition of 

“storage vessel” will affect our ability to ensure control of these storage vessels. Consequently, 

as proposed, we are finalizing the removal of provisions made in the 2014 amendment relating to 

the storage vessels “installed in parallel” or “connected in parallel.”

2.2.3 The EPA Should Grant a Stay of Final Provisions for Storage Vessels in Parallel

Comment: Several commenters (4758, 4760, 4761, 4762, 4763) believe that the EPA 

should stay the storage vessel requirements finalized in the December 31, 2014 Federal Register 

notice. 

Two commenters (4758, 4761) recommend that the EPA stay the effectiveness of the 

revised definition of “storage vessel” at 40 CFR §60.5430 and all other newly added language 

pertaining to vessels connected or installed “in parallel” while this reconsideration process goes 

forward. The commenters contend that the Clean Air Act provides that that the EPA may grant a 

stay if the Agency has decided to reconsider a rule. 42 USC § 7607(d)(7)(B). According to one 

of the commenters (4758), Congress imposed no additional criteria, standards, or conditions 

concerning the EPA's authority to issue a stay under §7607(d)(7)(B). The commenter adds that as 

long as a reconsideration proceeding is underway, granting of a stay is within the discretion of 

the Agency.
16

One commenter (4758) contends that a stay is warranted in the present case, because 

regulated entities are now confronted with “inadvertent” but important new regulatory provisions 

regarding vessels connected or operated “in parallel,” including a revised definition of “storage 

vessel” that has a major impact on determining whether a vessel is an affected facility covered by 

subpart OOOO. While some state agencies may act to alleviate the problem, for example by 

exercising enforcement discretion while the EPA acts to remedy the current confusion, the 

commenter contends that other state agencies may not do so. The commenter stated that the stay 

should take effect immediately and should continue until this reconsideration process has been 

completed and corrected rule language has been finalized.

Response: In this action, the EPA has finalized the removal of the revision made in the 

2014 amendment to the definition of “storage vessel.” The definition at issue no longer exists

and thus no longer in effect such that a stay would have any effect. 

2.2.4 Clarification of Whether the Storage Vessels in Parallel Provisions were in Effect

Comment: Several commenters (4758, 4760, 4761, 4762, 4763) believe that the EPA 

should provide guidance on precisely which storage vessels would be subject to the requirements 

and when they would be subject.

Two commenters (4758, 4760) believe that the EPA should state in its upcoming final 

rule that the “in parallel” provisions in the December 2014 rule were never effective because 

they were promulgated without notice and comment, in violation of the rulemaking procedures 

under the Clean Air Act. Two commenters (4758, 4761) believe that such a statement by the 

EPA would be appropriate, in light of federal case law holding that a rule promulgated without 

notice and comment is void ab initio. It would have the benefit, according to one of the 

commenters (4758), of removing the uncertainty, and the possibility of enforcement actions, 

created by the EPA's improper finalization of provisions in the December 2014 rule that 

suddenly and without notice altered the manner in which a storage vessel's affected facility status 

was determined.

One commenter (4761) added that because there are multiple configurations for 

installation and because any given installation may be operated in different configurations, the 

rule language has created significant confusion, potentially exposing companies to enforcement 

and leading to the need for case-by-case interpretations from the EPA and delegated state 

permitting authorities. The commenter states that their members have already encountered state 

regulators seeking to enforce the improperly promulgated new language for vessels not 

previously regulated under subpart OOOO, demonstrating that the rule language is having 

immediate impacts on regulated entities.

Even if the EPA grants a stay of the effectiveness of those provisions going forward as 

requested, one commenter (4758) points out that there would remain a period of time, beginning 

on December 31, 2014 and ending on the date such a stay begins, during which low-emitting 

storage vessels connected to other storage vessels could be deemed by enforcement personnel to 
17

have been subject to subpart OOOO requirements, by virtue of the new PTE aggregation 

requirements for vessels connected in parallel. A clear statement by the EPA that those “in 

parallel” provisions were never effective would, according to the commenter, remove this 

possibility and would properly reflect federal case law holding that a rule promulgated without 

notice and comment is void from the outset.

One commenter (4761) added that should these provisions remain effective from 

December 31, 2014 through the date these proposed changes are finalized, which they should 

not, the Agency needs, at least, to clarify that to the extent that these provisions were originally 

intended, at most they were only applicable to storage vessels returned to service and operated in 

parallel. The commenter believes that this would assist in clarifying that the EPA intended the 

added language to mean given that the Agency did not provide a definition of “in parallel.” The 

commenter contends that the preamble to the December 31, 2014 rule provides indication that 

this is the situation that the final action was trying to address—storage vessels removed from 

and, subsequently, returned to service—but it would be helpful to be clear on that point.

Another commenter (4762) contends that the December 31, 2014 rule is not sufficiently 

clear about when its amendments to subpart OOOO are effective. The commenter noted that the 

EPA plainly states that the revisions are intended to be effective as of December 31, 2014, 

meaning that none of the revisions should be applied to conduct occurring prior to December 31, 

2014. However, the commenter contends that none of the specific rule text implementing the 

revisions includes an applicability date. Once the revisions are published in the U.S. Code, the 

commenter believes that will create the possibility that some of the revisions may be interpreted 

to apply to conduct occurring prior to December 31, 2014. For example, the commenter stated

that §60.5420(c)(1)(A) includes new reporting requirements for gas well affected facilities. 

According to the commenter, nothing in the code will indicate that these new reporting 

requirements apply beginning on December 31, 2014. Thus, the commenter believes that for 

someone who looks just to the code and not to the underlying Federal Register final rule notice, 

that person easily could mistakenly interpret these new reporting requirements to apply prior to 

December 31, 2014.

Further, the commenter suggested that the effective date of the December 31, 2014 rule 

revisions that are unaffected by the March 23, 2015 proposed changes should be set at least 60-

days after publication of the final rule and not on the date of publication. The commenter 

believes that many of the new provisions could not be implemented immediately (such as the 

recordkeeping requirement referenced in § 60.5420(c)(1)(iii)(A), and a reasonable period of time 

is needed for such provisions to be implemented.

Response: The EPA disagrees with the comment that the provisions in the December 

2014 rule were never effective. Further, with respect to the commenter’s allegation of inadequate 

notice and comment, the proper recourse is to petition the EPA for reconsideration. (See 

Mexichem Specialty Resins, Inc. v. EPA, 2015 U.S. App. LEXIS 8903 (D.C. Cir. May 29, 

2015)) . The commenter also could have sought judicial stay of the rule. In any event, the EPA 

has appropriately addressed the issue through rulemaking. 
18

With respect to the comment suggesting that the effective date of the December 31, 2014 

rule revisions that are unaffected by the March 23, 2015 proposed changes should be 60 days 

after publication, the comment is outside the scope of this rulemaking.

With respect to the comment that multiple configurations for installation may result in 

case-by-case interpretations, we agree with the commenter. Determining applicability for any 

specific source is always case-by-case determination by definition; it will involve many factors, 

one of which may be storage tank configuration. As we discussed in the preamble to the 

December 2014 rule (79 FR 79027), we expect the situation where two or more storage vessels 

receive liquids in parallel such that the total throughput is shared between or among the parallel 

vessels and which causes the PTE of each vessel to be a fraction of the total PTE, to be a rare 

occurrence. However, these types of scenarios warrant case-by-case analysis of applicability, 

especially under the circumvention requirements of the General Provisions. Regarding the 

comment about efforts by states to “enforce the improperly promulgated new language,” for the 

reasons previously discussed, we do not believe the language was “improperly promulgated.” 

Determinations of applicability made by a delegated state, which may or may not result in 

enforcement, should be taken up with the appropriate delegated state.

2.2.5 The EPA Must Propose New Requirements

Comment: In response to the EPA’s request for suggestions on other approaches to 

discourage deliberate reductions in the potential to emit be installing multiple storage vessels, 

one commenter (4761) does not believe additional changes are necessary because they are not 

aware of an industry practice to install multiple smaller storage vessels in place of one larger 

vessel to unnecessarily reduce the PTE below the applicability threshold. Rather, according to 

the commenter, storage vessels are generally sized and installed based on the anticipated 

production of the gas well.

The commenter contends that even if the EPA were to consider additional regulations to 

address this issue, it must re-propose this, and provide the exact regulatory language it is 

considering to avoid unintended consequences. Where multiple storage vessels are installed at a 

single location, the commenter states that industry may use several configurations to connect 

these vessels and may operate these vessels in several manners, including in-series, in-parallel, or 

with common vent lines. Due to the highly technical nature of these operations, the commenter 

believes that the EPA must solicit comment from industry to determine how to best achieve its 

stated goals without dramatically expanding the scope and costs of the regulation. According to 

the commenter any proposal by the Agency must carefully consider the impacts of the rule on all 

possible vessel connections and operating scenarios and be carefully crafted to avoid expansion 

of the affected facility to storage vessels with PTE below the intended threshold. 

Response: The EPA thanks the commenter for its information on industry practice with 

regard to installation of storage vessels. The EPA further agrees that should the Agency

consider additional regulations to address the issue, the EPA will do so through notice and 

comment rulemaking.
19

2.2.6 The EPA Should Clarify the Storage Vessel Requirements Instead of Removing Them

Comment: One commenter (4765) stated that the EPA now proposes to remove 

provisions relating to storage vessels “installed in parallel” or “connected in parallel” in response 

to a petition for reconsideration filed by the Gas Processors Association (GPA). According to the 

commenter, the Agency explains that the terms “installed in parallel” and “connected in parallel” 

as they appeared in the regulatory text “inadvertently” encompassed storage vessels the Agency 

did not intend to address. However, the commenter contends that the EPA does not identify those 

vessels that it believes were inadvertently covered in the December 2014 rule, nor does it 

propose alternative regulatory language that would ensure adequate control measures for vessels 

connected or installed in parallel that were intended to be covered under the December 2014 rule.

Given that storage vessels, including those installed or connected in parallel, can be 

significant sources of emissions, the commenter opposes the EPA’s proposal to simply remove 

any provisions addressing these vessels. Instead of removing all provisions regarding vessels 

installed or connected in parallel, as the Agency proposes, the commenter urges the EPA to 

instead clarify its existing requirements for such vessels. The commenter suggested that the EPA 

could, for instance, clarify that pollution control measures apply to storage vessels operated in 

parallel in the relevant regulatory provisions addressing storage vessel affected facilities and the 

definitions of “returned to service” and “storage vessel.”

Response: The change to the definition of storage vessel was intended to preserve the 

original basis on individual storage vessels to determine affected facility status, while addressing 

the potential situation where crude oil, condensate, intermediate hydrocarbon liquids or produced 

water is divided into two or more tanks operated in parallel (i.e., sharing the emissions at the 

correlated fraction of what a single tank would emit). Through comments submitted on the 

March proposed rule, the public has informed us that many storage vessels that are configured in 

parallel may not be operated or constructed to divide their potential to emit continuously, if ever. 

The EPA has now reconsidered our attempt to include storage vessels connected in parallel to 

address the specific situation resulting in circumvention. We believe that we do not have 

sufficient data to evaluate the scope of storage vessels that would fall under the amended 

definition and for which we did not intend to cover. 

We believe that we have sufficient provisions under the General Provisions at §60.12 

“Circumvention” to address the specific situation where storage vessels are divided into smaller 

tanks to avoid applicability of the rule and which was our intent with the previous amended 

definition. Therefore, we do not believe that our reverting to the prior definition of storage 

vessels will affect our ability to ensure control of these storage vessels. 

2.2.7 The EPA Should Extend the Final Requirements to All Tank Batteries

Comment: In light of the increasingly common practice of manifolding storage vessels, 

one commenter (4765) believes that the EPA should ensure that emission controls apply to all 

connected storage vessels that have aggregate throughput exceeding the applicability thresholds, 
20

regardless of whether they are currently being operated in parallel. Without these or similar 

protections, the commenter believes that loopholes for strategic parallel installation or 

connection would undermine the protective and comprehensive regulation of these significant 

sources of emissions.

Response: The change to the definition of storage vessel was intended to preserve the 

original basis on individual storage vessels to determine affected facility status, while addressing 

the potential situation where crude oil, condensate, intermediate hydrocarbon liquids or produced 

water is divided into two or more tanks operated in parallel (i.e., sharing the emissions at the 

correlated fraction of what a single tank would emit). Through comments submitted on the 

March proposed rule, the public has informed us that many storage vessels that are configured in 

parallel may not be operated or constructed to divide their potential to emit continuously, if ever. 

The EPA has now reconsidered our attempt to include storage vessels connected in parallel to 

address the specific situation resulting in circumvention. We believe that we do not have 

sufficient data to evaluate the scope of storage vessels that would fall under the amended 

definition and for which we did not intend to cover. 

We believe that we have sufficient provisions under the General Provisions at §60.12 

“Circumvention” to address the specific situation where storage vessels are divided into smaller 

tanks to avoid applicability of the rule and which was our intent with the previous amended 

definition. Therefore, we do not believe that our reverting to the prior definition of storage 

vessels will affect our ability to ensure control of these storage vessels . 

2.2.8 Add Exclusion for Storage Vessels Used Primarily in Water Recycling Processes

Comment: Several commenters (4755, 4756, 4757) propose that the EPA further amend 

the definition of “storage vessel” in §60.5430 to exclude vessels used primarily in water 

recycling processes. 

Response: The EPA thanks the commenter for its information on industry practice with 

regard storage vessels. Consideration of this issue is outside the scope of this rulemaking. 

Should the Agency consider additional regulations to address the issue, EPA will do so through 

notice and comment rulemaking.