Storage Vessel Applicability Determination and End of Flowback

Issue #1: Inclusion of the word “maximum” in the average daily throughput toward 

estimating potential to emit (PTE) in the storage vessels applicability determination is 

problematic. The term “maximum average daily” in §60.5365(e) was promulgated in the 

September 23, 2013 final rule revising NSPS, Subpart OOOO, referred to as the first 

reconsideration rule of NSPS OOOO (NSPS OOOO 1.0). This language was a significant 

departure from the proposal and it was not derived from any of the comments in response 

to the proposal. API recommended1that the storage vessel emissions should be calculated 

“Using the first thirty days of production in a model to define PTE from a storage vessel, 

rather than requiring use of maximum design capacity to calculate PTE in a model or 

requiring use of enforceable operational limitations to cap PTE.” API made this 

recommendation based on the fact that production wells decline over the life of the well 

and production flow is highest immediately after the well is initially placed in production. A 

30-day average, beginning at the start of production, is most representative (and 

conservative due to production decline) while accommodating the fluctuations in flow that 

often occur due to the three phase (produced water, liquid hydrocarbons and natural gas) 

flow in the well bore. Accurate measurement can only occur after separation of the three 

phases. 

Recommendation #1.1: Remove “maximum” from §60.5365(e).

“The potential for VOC emissions must be calculated using a generally accepted 

model or calculation methodology, based on the maximum average daily throughput 

determined for a 30-day period of production prior to the applicable emission 

determination deadline specified in this section.”

There are many generally accepted models and calculation methods to calculate a storage 

vessel’s PTE. One example of volume measurement is accomplished by:

1. strapping the liquid level at the opening and closing of the measurement period 

(i.e., beginning and end of the month),

2. calculating the storage volumes from those two level measurements,

3. subtracting the opening volume from the closing volume, and 

4. adding in any truck delivery tickets shipped from the vessel during that period. 

API envisioned dividing the first 30 days of production reported by the number of days the 

well was flowing (to correct for any well shut-ins that may have occurred due to operational 

constraints). This would allow the operator to rely on data that is currently collected and 

easy for EPA to validate.

 

1 See API comments, May 28, 2013, Docket ID No. EPA-HQ-OAR-2010-0505, page 38, first bullet. 
There are two major concerns with attempting to calculate a “maximum average daily” 

production from short-term flow measurements. First, volume measurements for shorter 

periods of time are not as accurate as the accounting methods used to report production 

volumes to state regulatory agencies. Accounting volumes are used for the payment 

calculations for royalties, taxes, etc. and the accounting methods have been adjusted over 

the years for accuracy. Operators sometimes keep track of production volume over shorter 

periods of time to track the wells health and production progress, but these are not typically 

corrected for criteria such as water content and temperature. The second major concern 

when relying on short term measurements is the occurrence of peak and low flows caused 

by a variety of operational issues such as:

1. When the well is first placed in production, the well often surges high flow for a 

variable period of time. Production rates calculated during this period will 

exaggerate monthly production. 

2. Throughout the production life, the collection of liquids in the well bore can cause 

significant drops in production for a period of time, especially during the initial 

production period. Production rates calculated during this period will underestimate 

monthly production. 

Calculating the daily average using the first 30 calendar days of production and dividing it by 

the days the well was flowing is the most accurate determination of the initial production 

rate. This production rate is significantly higher than other times during the life of the well, 

due to the rapid well depletion that occurs following initial well production. 

Recommendation #1.2: EPA should not further specify what data is required to calculate 

the potential for VOC emissions as required by 60.5365(e). The data needed to calculate 

emissions is intimately linked to the “generally accepted model or calculation methodology” 

selected to calculate the emissions. The data sampling and gathering protocols must match 

the model for the calculation to be accurate. Any attempt to specify what data must be 

collected and how that data is to be obtained would necessarily restrict which models or 

calculation methodology could be used. In the “Response to Public Comments on Proposed 

Rule August 23, 2011 (76 FR 52738)” on page 125, EPA recognizes that restricting the 

methodologies used to calculate emissions would be both difficult and limit the use of new 

methodologies: 

“Response: We disagree that the EPA should specify which methodologies must be 

used to calculate VOC emissions. Different methodologies may be more appropriate 

than others for certain situations, and it would be very difficult to determine each 

possible situation and determine the appropriate methodology for that situation. 

Additionally, such a prescriptive requirement would not allow as yet undeveloped 

methodologies to be used, or would require amending the rule whenever a new 
methodology was released. For these reasons, the final rule specifies that any 

generally accepted methodology may be used to estimate emissions.”

Specifying what data must be collected and how that data is to be collected would severely 

limit the “generally accepted methodology” that could be used. For example, specifying 

that a crude or condensate sample must be taken using a defined protocol would essentially 

prohibit the direct measurement of tank emissions.

Issue #2: API has recommended in previous discussions with EPA that the end of flowback 

can, in most cases, be visibly determined by a non-industry observer by what is happening 

at the well site surface. In our recommendations we have suggested that Subpart OOOO 

could stipulate that end of flowback occurs when a well is shut-in and disconnected from 

flowback equipment, and flowback crews have left the site after being released. Any well 

flow that begins subsequent to observations indicating the end of flowback would be 

considered production. Examples of equipment onsite during flowback that will not typically 

be observed when in production are:

 Temporary flowlines tied into the well put together by hammer unions and laid along 

the ground to flowback equipment and tankage.

 Temporary piping with hammer unions connecting separators, tankage, and 

combustion device.

 A skid mounted choke manifold used to regulate flowback and divert flow as 

needed.

 Portable separators to separate produced fluids and solids.

 A portable flowback combustion device or flare.

 Portable tankage such as a frac tank.

When a well is put into production, the temporary flowlines, with hammer unions, 

connecting flowback equipment are typically removed and replaced with permanent 

threaded, flanged, or welded piping connecting the well to the permanent production 

equipment such as heater treaters, separators, or collection system. The permanent 

flowlines are often buried, but in some cases are above ground laid on pipe racks. The 

presence of a flowback crew is the best indicator of being in flowback and not production 

mode in the rare instances where some of the above equipment continues to be used 

during initial production or where production facilities constructed previously are used for 

flowback.

Recommendation #2: Use the examples provided above in the preamble of the final rule 

amendments to clarify the equipment changes when flowback ends and production begins 

to simplify the final rule requirements.