Measuring Milestone's Carbon Impact


Committed to updating the 2020 collaborative carbon sequestration analysis (the “Analysis”), Milestone and SCS Engineers (SCS) focused on expanding the sampling protocol and developing a robust dataset to reliably estimate the carbon impact of E&P wastes by waste type.

The 2021 Analysis provides methods and calculations for estimating the gross carbon sequestered per waste type and provides Milestone the ability to attribute sequestered carbon to customers. The data evaluation performed by SCS included a review of laboratory analytical reports for samples collected in 2021 from incoming streams across Milestone’s network of facilities and the Total Petroleum Hydrocarbon (TPH) content of typical slurry and water waste types (as defined below) injected at our slurry facilities. Through a sequence of supportive formulas and calculations, SCS derived both the total Carbon Dioxide Equivalent (CO2e) per injected barrel per waste type and the gross/net GHG emissions from light, medium and heavy hydrocarbon chain NMVOCs under two scenarios: slurry injection and land farming. We present these estimates in the Milestone’s Negative Carbon Footprint section of this report.

Sampling and Lab Testing

To promote homogeneity and quality control, composite samples were collected directly from incoming trucks prior to unloading at the facility. Hundreds of total samples were collected from across Milestone slurry facilities providing insight into the characteristics of 12 different waste categories - including Milestone’s internally generated solids - and submitted to a National Environmental Laboratory Accreditation Program (NELAP) accredited third-party laboratory (Eurofins-Xenco in Midland, Texas) for the analysis of TPH via Texas Method 1005 (TX1005). This method examines the speciation of TPH into light (C6-C12), medium (C12-C28), and heavy (C28-C35) hydrocarbon chains. The laboratory provided analytical reports and SCS derived GHG equivalent (in CO2e) estimates of NMVOCs in each of Milestone’s types of slurry and waters injected at our slurry facilities.

Measuring TPH Carbon Content

SCS categorized each waste type sample as “slurries” (i.e., drilling mud, pit waste, tank bottoms and other slurries) or “water” (i.e., produced saltwater or flowback). SCS calculated the average measures of the carbon content (percent of total sample mass) and light hydrocarbon chain mix (percent of total carbon content) by sample category. SCS estimates that our average blended injection stream is approximately 4.5% carbon (C) by mass – versus 6% from 2020 - in the form of petroleum hydrocarbons, implying an average 28 kg CO2e/bbl injected – versus 29 kg CO2e/bbl injected in 2020; 35% of this carbon content consists of light hydrocarbon chains, 60% consisting of medium hydrocarbon chains, and the remaining 5% of carbon attributable to heavy hydrocarbon chains.1,2

Emissions and Sequestration: Scenario Analysis

SCS then derived the emissions and sequestration effects of two scenarios, which illustrate the differences between handling slurry via traditional land application disposal and our state-of-the-art slurry injection processes. For purposes of the Analysis, SCS used a mass balance model that sums the incoming carbon (i.e., customer wastes brought to Milestone for disposal), subtracts the outgoing carbon (i.e. carbon from internally generated solids hauled to landfills and the carbon from oil sales), providing Milestone with the estimated carbon injected downhole. This approach allows Milestone to estimate the carbon impact on a per-customer basis.3

The various hydrocarbon chains of TPH react to the processes within these two scenarios differently. Light hydrocarbon chains volatilize into CO2 and non-CO2 GHG emissions after limited atmospheric exposure (i.e., weathering), while medium and heavy hydrocarbon chains degrade through aerobic anaerobic processes over time. SCS cited a recent academic article that estimates 70% of the remaining TPH (medium and heavy hydrocarbon chains) could reduce and become CO2 within approximately one year of landfarming activity.4

Key Findings

Based on the Analysis, SCS estimates that one barrel of slurry waste disposed of through land application generates approximately 39 kg CO2e of gross emissions per barrel of slurry waste in the first year or less from light hydrocarbon chain volatilization and aerobic/ anaerobic degradation of the remaining TPH. Under the slurry injection scenario, the same barrel of slurry waste would generate no further emissions once in the injection pipeline, resulting in sequestration of all 49 kg CO2e/bbl contained in each barrel of slurry waste on average.

1 Based on (i) the carbon content and light carbon mix factors calculated by SCS, (ii) volume-to-mass conversion factors by waste stream per Milestone, and (iii) a weighted average waste type mix during 2021.

2 Carbon mass converted to CO2e using a standard relative molecular weight ratio of 3.67 CO2/C.

3 Both scenarios assume all water volumes injected into Class II UIC wells.

4 SCS cited the recent academic article by Guarino et al. “Assessment of three approaches of bioremediation (Natural Attenuation, Landfarming and Bioaugmentation - Assisted Landfarming) for a petroleum hydrocarbons contaminated soil.” Chemosphere vol. 170 (2017): 10-16. doi:10.1016/j.chemosphere.2016.11.165, which estimates that 70% of the remaining TPH (medium and heavy chains) becomes CO2 through bioremediation in the first year.