A Comparison of Air Emission Estimation Methods for Drilling Rig Emissions

Ever since the Texas Commission on Environmental Quality determined that the Eagle Ford and its supporting industry will be included in future air emission inventories, it has become crucial to identify the most accurate and cost effective methods for determining air emissions of drilling operations. Estimation is the preferred method for creating regional emission inventories since direct measurement of diesel engine exhaust is often cost prohibitive. These estimations are commonly calculated using engine load, conservatively estimated at 100%. This introduces considerable error in the emissions inventory since electric rigs are rarely run at full load and drilling engine activity dramatically varies from job to job. Conducting an air emission inventory of drilling rigs requires a novel way to estimate emissions without relying on engine load as a primary variable. With this in mind the research team employed an estimation method based on fuel consumption rather than horsepower. Fuel use data is readily available on drilling sites and so more accurately reflects the engine activity of electric rigs in drilling operations. This study finds that calculated emissions can vary from 9 to 106 pounds per hour of NOx depending on the estimation method used. Given the deviation that can occur in estimation, the fuel consumption method offers an opportunity for more accurate, cost-effective assessment of regional emission inventories.


Introduction
There are three main methods for collecting air emissions data.It can be directly measured at the engine tailpipe, it can be measured through ambient, downwind monitoring, or it can be estimated through a collection of engine data, fuel data and emission factors for the family of engines being studied.Since direct emission measurement of all emission sources in an air emission inventory can be limited by funding, time, or staff size, estimation is typically used to determine the air quality impact from a particular industry (United States Environmental Protection Agency [USEPA], 2014).
The Texas Commission on Environmental Quality has determined that the Eagle Ford and its supporting industry will be included in future air emission inventories.However, the current methods for estimating emissions impose significant error in the inventory thus compounding the variance between regional air shed models.These high levels of variance result from the quality of the data being entered into the equations as well as the equations themselves.
In this study, we attempt to determine the best way to estimate emissions from drilling operations.Researchers from the Institute of Renewable Natural Resources (IRNR) teamed with industry partners to determine the most accurate method for estimating emissions from drilling rigs via data collection directly from energy producers with active operations in the Eagle Ford Shale Play.
Since drilling engines have high variability in engine load, conducting an air emission inventory of drilling rigs requires a novel way to estimate emissions without relying on engine load as a primary variable.The team therefore employed an alternative calculation that used fuel consumption data rather than total horsepower and engine load data.This method appeared to minimize the error significantly, giving a more accurate picture of drilling engine activity.This project was conducted by Texas A&M IRNR which is part of the Environmentally Friendly Drilling Systems Technology Integration Program (managed by the Houston Advanced Research Center -HARC and the Texas A&M Global Petroleum Research Institute -GPRI).

Planning
Planning for the drilling rig study began with the formation of the Eagle Ford Air Emission Inventory Group, which consisted of Texas A&M Agrilife Research, Alamo Area Council of Governments (AACOG), ConocoPhillips, Chesapeake Energy, Marathon Oil, Carrizo, EOG Resources, HOLTCAT, Pioneer Natural Resources, Energy Transfer, Plains Exploration and Production, Shell Oil and the Texas Oil and Gas Association (TxOGA).
Initial meetings consisted of introductory presentations and a description of how emissions inventories are typically calculated.Operators became increasingly concerned about the accuracy of emission inventory methods; specifically that emissions inventories multiply total potential engine load by total available horse power.According to drilling engineers participating in the group, generator engines for electrical rigs rarely run at full engine load and there may be several engines located on site as back-up that are not running at all.Furthermore, since engine load can fluctuate dramatically during a drilling operation, standardization poses a sizable challenge with risk of significant error.Therefore, it was agreed that using fuel consumption as an alternative method to using total potential horsepower and engine load would yield a clearer picture of actual emissions.What resulted was a refined equation for estimating emissions from drilling rigs based on fuel consumption.

Data Collection
Fuel consumption data was simpler to obtain than engine load data and could be acquired directly from the operators without site visits or the acquisition of highly sensitive engine controller data from the service providers.This is appreciable since most air emission inventories are survey driven and do not include site visits or nondisclosure agreements.
Data was collected by submitting a survey to nine participating companies within the Eagle Ford Air Emission Inventory Group operating within the Eagle Ford Shale Play.
The following data was gathered on the survey for both mechanical and electric drilling rigs: Field data from the surveys were compared with default data from literature using the fuel consumption method as explained further in the materials and methods section.Additionally, emission results from the fuel consumption method were compared with emission results from the horsepower method.

Understanding Emission Factors
In order to understand how emissions are estimated, it is first necessary to understand emission factors.Emission factors are often averages of available data and assumed to be representative of all emissions within a certain source category.They are representative values which relate the quantity of pollutants released into the atmosphere to the activity releasing the pollutants and are expressed as the weight of the pollutant divided by unit weight, volume, distance, or duration of the activity emitting the pollutant (USEPA, 2014).The emission factor is used to calculate the total emission from a source as an input for an emission inventory (USEPA, 2014).
The general USEPA (2014) equation for emission factor development is: ) where E = emissions; A = activity rate; EF = emission factor; ER = overall percentage emission reduction efficiency.
General emission factors are available to the public.However, variations in engine conditions can significantly affect the emissions at an individual location depending on temperature of combustion or emission controls; the development of local emission factors is highly advantageous and will provide more accurate estimations (USEPA, 2014).
Emissions of criteria pollutants are usually given as mass of pollutant emitted per mechanical energy produced by the engine, (i.e.g/kWh).The energy developers participating in the study reported using Caterpillar 3512C diesel generator sets that were rated Tier 2. Emission values that were most representative of 3512C engines were found on the California Air Resources Board (CARB) certificate (CARB, 2007).These criteria pollutant values were derived from zero hour steady state emissions tests performed by the manufacturer on 3512C engines operating at nominal power and speed.
Slightly more conservative than CARB, the USEPA also publishes emission standards that may be used as factors for this particular engine make and model.These values constitute allowable emissions when factors such as engine deterioration and less than nominal operational conditions are taken into consideration.These values may be used in lieu of the CARB certificate values but are generally much more conservative.
Next there are the USEPA (1996) AP-42 which publish much generalized factors for engines greater than 750 horse power.The AP-42 divides the values into controlled and uncontrolled factors for oxides of nitrogen or NOx.Controlled factors account for associated emission controls on large engines, while uncontrolled factors make the assumption that the engine has no emission controls for NOx (ie tier zero).
Table 1 gives the range of various emission factors and standards that are allowable for use when conducting an emission inventory with the aforementioned engine type.Note that both USEPA emission standards and CARB emission factors combine the NOx and volatile organic compounds (VOCs) into a single number which is referred to as non-methane hydrocarbon plus NOx (NMHC+NOx) in Table 1.The CARB Air Quality Management District guidelines outlined in Moyer (2005) were used to separate the two values into NOx and VOC which states that emission factors for NOx equals 95% of the total sum NMHC+NOx.
The VOC values for AP-42 NOx controlled and uncontrolled engines were obtained from an USEPA total organic carbon (TOC) value which according to the USEPA (1996) is 9% methane and 91% non-methane by weight.Therefore, the original TOC values of 0.43 were adjusted for both controlled and non-controlled engines by multiplying 0.91.The remainder of criteria pollutants (VOCs, CO and PM) are the same for both controlled and uncontrolled engines because the "controls" in USEPA (1996) refers to NOx only., 1988).By applying this conversion factor the calculated fuel use average of an electric rig is 385 lb/hr.
Engine data used was for a 2008, Tier 2, Diesel Compression-Ignition off-road engine listed as engine family: 8CPXL58.6T2X(CARB, 2007).Based on interviews with drilling engineers, 50% load for Brake Specific Fuel Consumption (BSFC) of No. 2 Diesel on the 3512C drilling rig generator sets was considered typical for a drilling operation in the Eagle Ford.
The CARB (2007) certificate emission factor for the 3512C Tier 2 engine used most often by the energy developers, who participated in this study, is the NMHC+NOx CARB emission factor.As previously described, the CARB emission factor for NOx in table 1 was derived from the NMHC+NOx CARB emission factor.
Calculation of NOx emissions using the fuel consumption method with the CARB emission factor and field data proceeded as follows: The emission factor was converted from g/kW-hr to lb/hp-hr; Emission Factor for NOx = 5.04 g/kW-hr x (lb/453.59g x kW/1.3405hp) = 0.008280 lb = 0.008 lb (3) Carbon Monoxide (CO), VOC, and Particulate Matter (PM) were also calculated and are mentioned in the results section of this report.

Calculation using Fuel Consumption Method and Default Data
Emission standards from the USEPA for the 1,476 horsepower 3512C Land Drilling Generator Sets were chosen as a default standard (USEPA, 1996).This would be the factor most likely chosen in an emission inventory if the engine make and model was known but little else about the operation.Standards for NOx, CO, VOC and PM were calculated based on the fuel input factors provided from AP-42 (USEPA, 1996).
The No. 2 diesel fuel usage value of 69.5 gallons per hour was provided on the Caterpillar technical data sheet for the 3512C Land Drilling Generator Set as a "nominal" or best guess value (Caterpillar, 2013).This value represents a possible default when field data was unavailable, and was used to calculate the default fuel consumption emissions.
Calculating NOx emissions using the fuel consumption method with USEPA 3512C Emission Standard listed in table 1 and default data proceeded as follows: The emission standard was converted from g/kW-hr to lb/hp-hr: (5) CO, VOC, and PM were also calculated and are mentioned in the results section of this report.

Calculation using the Traditional Horse Power Method
The horse power method is the traditional approach to air emission inventories, derived by multiplying the emission factor, the total available horsepower, and the engine load together to achieve pollutant emissions in pounds per hour.Since engine load can fluctuate dramatically during a drilling operation, collecting engine load field data is typically not an option and is therefore conservatively estimated at 100%.Hourly emission rates were calculated using conservative default emission factors for NOx controlled diesel engines found in the AP-42 (USEPA 1996).
In order to make an effective comparison, the same diesel operated 1,476 horsepower Caterpillar, Tier 2, 3512C drilling generator sets were used in examining both the horse power and fuel consumption methods.There are three of these generator sets on site for a typical drilling operation in the Eagle Ford so that total horsepower is calculated as: 1,476 hp x 3 engines = 4,428 hp total. (6) The emission equation for NOx follows: where E NOx = NO x Emissions (lb/hr); EF NOx = NO x Emission Factor (lb NO x /hp-hr); HP total = Total potential power output (hp); LF = Load factor (assumed to be 100%).
Calculating NOx emissions using the horsepower method with the default AP-42 NOx Controlled Emission Factor listed in table 1 proceeded as follows: The emission factor was converted from g/kW-hr to lb/hp-hr; Emission Factor for NOx = 7.91 g/kW-hr x (lb/453.59g x kW/1.3405hp) = 0.013 lb NOx /hp-hr (8) then NOx emissions were calculated using the horse power method, yielding; E NOx = 0.01 lb/hp-hr (100%) 4,428 Hp = 57.64lbs Nox /hr (9) CO, VOC, and PM were also calculated and are located in the results section of this report.Note that the AP-42 uncontrolled factor for NOx is twice as high as it is for controlled engines.Therefore, if we were to substitute the uncontrolled engine factor into the equation, the emissions estimation for NOx would increase significantly.

Making the Comparisons
Calculations were performed for each of the criteria pollutants using each of the protocols described in the sections above.Results as listed in Table 2 indicate that pounds of pollutants reported for the same operation could vary as much 97.21pounds depending on the protocol chosen.

Table 1 .
Allowable Emission Factors and Standards (g/kWh) for Emission Inventories of Caterpillar 3512C engines as listed by USEPA, CARB, and AP-42 Sources Calculation using Fuel Consumption Method with Field Data The amount of No. 2 diesel (gallons) used to drill a well was averaged on a per hour basis for 41 electric rigs operating in the Eagle Ford Shale play throughout 2012.The resulting average was 55 gallons of diesel used per hour for a typical diesel Tier 2, 3512C Land Drilling Generator Set Engine.The gallon per hour (gal/hr) average was converted to pounds per hour (lb/hr) using the average density for No. 2 diesel of 7 pounds per gallon (lb/gal) (American Petroleum Institute

Table 2 .
Emissions Results of Two Estimation Methods Using Available Factors and Standards(lb/hp-hr)